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ExxonMobil ProprietaryWATER POLLUTION CONTROL Section Page

GUIDELINES FOR SELECTINGWASTEWATER TREATMENT SYSTEMS

XIX-A 1 of 35

DESIGN PRACTICES PROPRIETARY INFORMATION - For Authorized Company Use Only December, 2002

ExxonMobil Research and Engineering Company – Fairfax, VA

CONTENTS

Section Page

SCOPE / BACKGROUND ...............................................................................................................................3

REFERENCES ................................................................................................................................................4

DEFINITIONS ..................................................................................................................................................4CONTAMINANT/POLLUTANT ................................................................................................................4CONCENTRATION AND MASS UNITS..................................................................................................4OIL AND GREASE ..................................................................................................................................4OXYGEN DEMAND.................................................................................................................................5BIOX........................................................................................................................................................6DISSOLVED OXYGEN (DO)...................................................................................................................6PH ...........................................................................................................................................................6“TOXIC" COMPOUNDS/TOXICITY .........................................................................................................6NUTRIENTS............................................................................................................................................7NITRIFICATION/DENITRIFICATION ......................................................................................................7

OVERALL WASTEWATER SOURCE MANAGEMENT AND TREATMENT SELECTION.............................7OVERALL WASTEWATER SOURCE MANAGEMENT ..........................................................................7WATER REUSE......................................................................................................................................7WATER CHEMISTRY .............................................................................................................................8SEWER...................................................................................................................................................8DECISION TREES FOR TREATMENT UNIT SELECTION ....................................................................9

TYPICAL EFFLUENT QUALITY CRITERIA ...................................................................................................9OIL AND PH............................................................................................................................................9GENERAL POLLUTANT INDICATORS ................................................................................................10OTHER TYPICAL PETROLEUM/PETROCHEMICAL CONTAMINANTS..............................................10EXAMPLES OF TOXIC COMPOUNDS.................................................................................................10

TREATMENT UNITS .....................................................................................................................................10TYPICAL REFINERY/PETROCHEMICAL WASTEWATER TREATING FLOW PLAN .........................10HOW TO ESTIMATE WASTEWATER FLOW AND QUALITY WITHOUT DATA .................................11TYPICAL REMOVAL EFFICIENCIES FOR SELECTED WASTEWATER TREATING UNITS..............11EQUALIZATION/PH CONTROL (FLOW AND CONTAMINANT LOAD)................................................11PRIMARY OIL AND SUSPENDED SOLIDS REMOVAL (PRIMARY TREATMENT).............................12

API SEPARATOR...............................................................................................................................12PLATE INTERCEPTOR/SEPARATORS ............................................................................................12SKIM PONDS.....................................................................................................................................12ENHANCEMENTS/FUTURE TECHNOLOGY ....................................................................................12

SECONDARY OIL AND SUSPENDED SOLIDS REMOVAL (ADVANCED PRIMARY TREATMENT)..13INDUCED AIR/GAS/STATIC FLOTATION (IAF/IGF/ISF)...................................................................13DISSOLVED AIR FLOTATION (DAF) ................................................................................................13MEDIA FILTRATION ..........................................................................................................................13CHEMICAL FLOCCULATION ............................................................................................................14

Changes shown by ➧➧➧➧

ExxonMobil ProprietarySection Page WATER POLLUTION CONTROL

XIX-A 2 of 35 GUIDELINES FOR SELECTINGWASTEWATER TREATMENT SYSTEMS

December, 2002 PROPRIETARY INFORMATION - For Authorized Company Use Only DESIGN PRACTICES


ENHANCEMENTS/FUTURE TECHNOLOGY ....................................................................................14OXYGEN DEMAND/DISSOLVED ORGANICS REDUCTION (SECONDARY TREATMENT)...............14

biological treatment ............................................................................................................................14non-biological treatment .....................................................................................................................16enhancements/future technology .......................................................................................................17

NUTRIENTS/AMMONIA REMOVAL/NITRIFICATION...........................................................................17SUSPENDED SOLIDS REDUCTION (TSS)..........................................................................................18

Media Filtration ...................................................................................................................................18Gravity Settling ...................................................................................................................................18Enhancements / Future Technology...................................................................................................18

TOXICITY REDUCTION (ACUTE AND CHRONIC) ..............................................................................18Enhanced Biological Treatment..........................................................................................................18Activated Carbon ................................................................................................................................19Enhancements / Future Technology...................................................................................................19

DISSOLVED SOLIDS/METALS REDUCTION ......................................................................................19Chemical Precipitation and Flocculation ............................................................................................19Biological Removal .............................................................................................................................19Ion Exchange......................................................................................................................................20Activated Carbon ................................................................................................................................20Membranes.........................................................................................................................................20Evaporation ........................................................................................................................................20

WASTEWATER SLUDGE HANDLING (VOLUME REDUCTION).........................................................20Sludge Thickening..............................................................................................................................20Dewatering Equipment .......................................................................................................................21

REFERENCES...............................................................................................................................................22

TABLESTable 1 Typical Removal Efficiencies for Selected Wastewater Treating Units .............................26

FIGURESFigure 1 Wastewater Treatment Assessment Decision Tree .........................................................28Figure 2 Free Oil Removal Decision Tree ......................................................................................29Figure 3 Oxygen Demand/Dissolved Organic Reduction Decision Tree ........................................30Figure 4 Suspended Solids Removal Decision Tree ......................................................................31Figure 5 Wastewater Effluent Toxicity Reduction Decision Tree....................................................32Figure 6 Wastewater Sludge Management Volume Reduction Decision Tree...............................33Figure 7 Typical Wastewater Treatment Flow Plan........................................................................34

Revision Memo

12/02 General technical/editorial update to the entire practice. Significant changesinclude the following:1. More complete coverage of equalization, diversion tankage and

stormwater management.

2. More coverage of heavy metals removal technology3. Corrected COD/TOD to TOC ratios



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SCOPE/BACKGROUNDThis introduction to wastewater treatment processes and water pollution control systems provides background information onhow to select the appropriate treatment technology to solve specific and general contaminant removal requirements. Detailscan be found in other Design Practice sub-sections that follow this introduction, EMRE reports, and technical/professionalsociety, vendor and other, third party literature sources.Wastewaters from the petroleum/petrochemical industry are a special category that does not conform to “sanitary" or sewagetreatment practice. Many designs and selection criteria are not published in the literature; they reside with industry experts andspecialty equipment vendors. Selecting specific equipment that is cost effective is also frequently complicated by government

➧ guidelines and regulations on effluent quality requirements, which can vary greatly around the world. It is very important to fullyunderstand the effluent quality requirements, including analytical methods used for contaminant analyses, to identify thepreferred wastewater treatment technology and equipment for a particular location.Petrochemical plant wastewater treatment can be more involved than refinery treatment because of the wider variety ofchemicals and processes used. Extra caution should be used in applying rules-of-thumb and specific selection criteria fortreatment of specific chemical plant wastewaters.The contaminated water or wastewater sources that require treatment facilities can vary depending upon governmentrequirements on effluent quality or on specific definitions/designations for these streams. For example, stormwater sources fromclean areas of the plant may still require monitoring and permitting in some locations, even though they do not contact petroleumor petrochemical process streams in the plant. Sometimes cooling tower blowdown streams, stormwater, contaminatedgroundwater, desalination plant blowdown waters and specialty streams such as spent caustics are restricted in the locationswhere they can be commingled with other wastewaters and subsequently treated. Hence, the person selecting the treatmentsystem should check government guidelines/requirements and seek information on standard industry practice in the area theyare located, since these requirements can limit treatment system flexibility, wastewater source segregation, reuse opportunities,even though they are logical from technical or cost standpoints. In some cases, the impacts of these restrictions are large andthe person selecting the treatment system should go back to the governmental authorities to seek clarification or adjustment tothe restrictions based on positive technical or environmental impact assessments.This section of the Design Practices deals primarily with “end-of-pipe" wastewater treatment process selection. The removal ofhigh concentrations of contaminants coming directly from process areas may be covered as part of the specific plant process.For example, highly contaminated wastewater streams with sulfides and ammonia, called “sour waters" are typically treated atthe process block in a sour water stripper. Design information on these units can be found in the Design Practice XIX-A10.Oil/water knockout or three phase disengaging drums, unit separators, desalters, wet gas scrubbers, causticregeneration/minimization processes and similar equipment are usually specified for the particular process or operation. Arough rule-of-thumb is if the concentration of the contaminant of concern is less than 0.1 weight percent (1,000 weight parts-per-million, ppm or mg/L), it is covered in the following Design Practice sections on “end-of-pipe" treatment technology. Oil is anexception to this rule-of-thumb. The rule-of-thumb is just a guideline and should not be used as a fixed number. For example,certain organic contaminants whose concentration is about 0.2 weight percent have been successfully removed by “end-of-pipe”treatment. Water or wastewater streams with much higher concentrations are generally handled at the process where they aregenerated.This section and the following ones do not directly deal with “water" treatment; the treatment of raw water sources for use in

➧ plant utilities for boilers, cooling, etc. Refer to the Offsite Design Practice sections for raw water treatment (Offsites DP Sectionno. XXVI and XXVII). Water reuse, potential to replace raw water to plant utilities is, however, covered in this section.The design of water pollution control systems or wastewater treatment facilities should be viewed as any other processtechnology in that proper design data are needed to meet requirements in cost effective ways. Frequently, a particular locationwill need to embark on a sampling and laboratory analysis program to define contaminant concentrations in the wastewaters aswell as flow studies to define efficient facilities, pollution prevention options, wastewater reuse opportunities and to minimize thecost of facility upgrades. The collection and analysis of these data into contaminant material balances must be thoroughlyplanned, similar to designing other facilities in the plant process block. In some instances when biological treatment isemployed or needs to be upgraded, these data collection efforts can become quite extensive, particularly when the end-of-pipetreatment facilities must meet stringent government effluent discharge standards.Subsection entitled, “Enhancements/Future Technology" is included in the treatment unit section of this Design Practice toreflect vendor or other technology that may be encountered by the reader. In some cases, Exxon Mobil or industry groups areevaluating the applicability of these technologies for use, and some of the technology is developed far enough to be consideredas an alternative to conventional technology. However, more scrutiny is needed in evaluating this evolving technology versusconventional treatment units. In other cases, the information is provided to let the reader know developments are occurring inthese areas, and future updates to this Design Practice will provide guidelines on their applicability to Exxon Mobilfacilities.





REFERENCESDue to the length of this section, the references have been placed at the end of this Design Practice. The references have alsobeen grouped to match the major sections of this Design Practice, so information on specific topics can be located easily.

DEFINITIONS

CONTAMINANT/POLLUTANTDistinctions are sometimes made between these terms; The United States Environmental Protection Agency (EPA) definitionsgiven below illustrate how regulatory agencies view the terms:Contaminant - The U.S. EPA definition: “Any physical, chemical, biological, or radiological substance or matter that has anadverse effect on air, water, or soil."A more realistic definition is any substance, not naturally occurring, that has the potential to adversely affect the use of thereceiving water body.Pollutant - The U.S. EPA definition: “Any element, substance, compound, or mixture, including disease causing agents, whichafter release into the environment and upon exposure, ingestion, inhalation, or assimilation into any organism, either directlyfrom the environment or indirectly by ingesting through food chains, will or may reasonably be anticipated to cause death,disease, behavioral abnormalities, cancer, genetic mutation, physiological malfunctions, or physical deformation in suchorganism or their offspring."A more realistic definition is any substance added to a water body that has an adverse impact on human health or theenvironment.

CONCENTRATION AND MASS UNITS➧ Wastewater contaminant concentrations can generally be expressed in the physical unit of milligrams per liter (mg/L), weight

parts-per-million (wppm or simply ppm), or weight percent. Concentrations expressed as micrograms per liter (µg/L) and parts-per-billion (ppb) are also commonly used. When contaminant concentrations in wastewater are expressed as parts-per-millionor billion, the concentration is on a weight basis and is sometimes abbreviated as wppm or wppb. For most calculations, units ofmg/L and ppm and µg/L and ppb can be interchanged since the specific gravity of the wastewater is very nearly equal to that ofwater. For example, contaminant concentrations in salt water systems or special waste stream like spent caustic should strictlybe expressed as mg/L since the specific gravity of salt water and spent caustic is greater than 1.0.Effluent limits can be measured and reported on a concentration or mass basis. Mass units are generally expressed as poundsor kilograms of contaminant per unit time (lbs/day or kg/day). When a facility has mass limits on their effluent, the facility has

➧ greater flexibility to meet the limits than those with concentration limits, since the effluent flow rate can many times be adjustedto meet the daily mass limits.

OIL AND GREASEOil and grease is a key indicator test for effluent compliance in the petroleum industry and is reported in units of ppm or mg/L. Itmeasures the total of dissolved and suspended “oil" fractions extracted from a wastewater sample by an organic solvent,typically n-Hexane (freon is no longer used) or other similar solvents. Caution should be used in interpreting the results of thistest since it can measure naturally occurring organic compounds, which may give artificially high readings in the oil test.1

Suspended Oil - Suspended or “free" oil is the amount of total oil and grease that can be removed by filtration, and a portion ofwhich can be removed by physical separation, such as an API Separator or Dissolved Air Flotation (DAF) unit.Dissolved Oil - Dissolved or soluble oil is the portion of total oil and grease that is not removed by filtration. Dissolved oil canbe removed by treatment processes such as biological treatment, chemical oxidation, or activated carbon adsorption.

➧ The analytical procedure for distinguishing suspended oil from dissolved oil is a glass fiber filter, which has the nominal rejectionrate of 1 micron size particle. Hence, in practice, dissolved oil is considered to be those hydrocarbons or organic compoundsthat are truly soluble in the wastewater stream, plus those finely dispersed oil droplets that are not filtered, and are less than 1micron. Similarly, suspended oil in wastewater samples can be considered to be those compounds or hydrocarbons meausedin the test, whose oil droplet size particles are greater than 1 micron.Solids

➧ Total Solids (TS) - Total solids is the term applied to the residue after evaporation of a sample and its subsequent drying in anincubator or oven at 217 to 221°F (103 to 105°C). Total solids includes total suspended solids and total dissolved solids(organic and inorganic) and is reported in units of ppm or mg/L.1

➧ Total Suspended Solids (TSS) - Total suspended solids consists of the amount of suspended matter removed by a glass fiberfilter (nominally > 1µm particle size) when a wastewater sample is dried at 217°F (103°C) and is reported in units of ppm or



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mg/L. Inorganic particles such as clay or grit as well as organic particles (biological solids including algae,most bacteria andother wastewater microorganisms) contribute to a waste streams suspended solids concentration.1

Total Dissolved Solids (TDS) - Total Dissolved Solids is a measure of all dissolved material in a solution, including inorganicsalts (e.g., NaCl, MgCl2 , etc.) that typically make up the bulk of the TDS measured in the standard lab test. TDS is used todetermine the salt levels of wastewater. Measurement for TDS consists of passing a sample through a standard glass fiberfilter, and the filtrate is evaporated to dryness in a weighed dish and dried to a constant weight at 356°F (180°C). The materialremaining on the filter paper is the total dissolved solids and is reported in units of ppm or mg/L. Conductivity can be used for aquick substitute measurement for TDS (reported in units of micromhos or microsiemens). As a rule-of-thumb, for wastewaterstreams at pH 7, the TDS of that stream in ppm (mg/L) can be approximated by multiplying the conductivity in units ofmicromhos or microsiemens by 0.7.Mixed Liquor Suspended Solids (MLSS) - Mixed liquor suspended solids is the concentration of total suspended solids in theaerated section of a biological treatment unit or lagoon. If Powdered Activated Carbon (PAC) is used, it will be included in

➧ MLSS and MLVSS measurements, unless special test methods are employed to estimate the amount of PAC, and allow adistinction between PAC and microbial mass. MLSS is normally expressed in units of ppm or mg/L.

➧ Mixed Liquor Volatile Suspended Solids (MLVSS) - Mixed liquor volatile suspended solids is the portion of the MLSS whichvolatilizes at 1022°F (550°C). Biological solids (microorganisms) are the main contributor to MLVSS. MLVSS can be used toroughly measure the amount of biomass in lagoon or in biological treatment system, but should not be used to measure theactivity of the biomass nor the ability to remove contaminants. MLVSS is normally expressed in units of ppm or mg/L.

OXYGEN DEMANDOxygen demand is a key process parameter in a biological treatment system. It usually refers to oxidation of organic matter inthe biological system. Other contaminants such as sulfides and ammonia also contribute to the oxygen demand. Therefore, theoxygen demand parameter should be used for monitoring purposes. For an estimate of total oxygen demand calculation,

➧ use the stoichiometric guidelines below by adding the following individual oxygen demands, depending upon which ones arepresent:For every ppm (mg/l) of: Required O2 demand

TOC 3.4 ppm (mg/l)NH4-N (for biological nitrifying systems) 4.6 ppm (mg/l)H2S 1.9 ppm (mg/l)

Biochemical Oxygen Demand (BOD) - Biochemical Oxygen Demand is a general measure of organic material in wastewatersamples that can be biologically degraded. It is the quantity of oxygen consumed during the biological decomposition(oxidation) of material in water. Certain inorganic compounds that exert an immediate oxygen demand (e.g., sulfite) will bedetected in the BOD test. The lab test must be run carefully and repeatedly to obtain an accurate measurement. BOD is usuallymeasured over a specific time period; a five-day period is commonly used, with the result expressed as BOD5. If the biologicaldecomposition is allowed to proceed to completion, the quantity of oxygen consumed is termed the ultimate BOD, oftendesignated BODult and is normally measured over 20 days. In this case, some nitrogen compounds can be oxidized, a

➧ process called nitrification. BOD5 is normally expressed as ppm or mg/L and can roughly be estimated at 0.6-0.7 of BODult orCOD. 1

Chemical Oxygen Demand (COD) - Chemical Oxygen Demand (COD) is a measure of the amount of organic or reducedinorganic compounds in a sample that can be oxidized by a strong oxidizer, usually potassium dichromate and sometimespotassium permanganate. The COD test does not oxidize ammonia. COD of a wastewater is generally greater than the BODsince the wastewater may contain material that cannot be biologically degraded. The COD test is simpler and faster than the

➧ BOD test. Caution must be used when analyzing the COD in brackish or high salt (chloride) wastewater streams ( Chlorideconcentrations over 2,000 mg/l or sodium chloride concentrations of 3,300 mg/l) since the chlorides will interfere with the testresults. For reference, sea water has about 33,000 mg/l of salt, mostly sodium chloride, with chloride content of 18,000-20,000mg/l. COD is expressed as ppm or mg/L.1

Theoretical Oxygen Demand (ThOD) - The Theoretical Oxygen Demand of a wastewater is the oxygen required to oxidize➧ stoichiometrically all organics or reduced inorganic compounds to CO2, SO4, NO3, etc. end-products. For most organics, with

the exception of some aromatics, the ThOD is equal to COD and TOD. ThOD is normally expressed as ppm or mg/L.Total Organic Carbon (TOC) - Total Organic Carbon is used to measure the quantity of organically bound carbon in a sample.A special analytical instrument breaks down the organic molecules into their carbon equivalent before measuring the amount ofcarbon present in the sample. TOC is commonly used as a replacement for BOD since the test for TOC is significantly fasterthan the 5-day test for BOD, and the BOD test can sometimes give variable results. TOC is normally expressed as ppm ormg/L.1





Total Oxygen Demand (TOD) - The total oxygen demand test measures the amount of oxygen required to oxidize all oxidizablesubstances in a sample, including the biodegradable organic matter. It is measured using a special analytical instrument. TODis normally expressed as ppm or mg/L.

BIOX➧ Abbreviation for BIological OXidation and commonly used to describe the activated sludge type biological treatment system but

can be used in reference to other biological oxidation processes used to treat wastewater.

DISSOLVED OXYGEN (DO)Dissolved oxygen level, measured in ppm or mg/L, is an important monitoring parameter for biological systems and receivingwater bodies. It indicates whether a biological treatment unit can sustain a healthy microbial population or whether the receivingwater body can sustain microbial, aquatic fish, or plant life. A minimum DO value for healthy biological treatment systems isbetween 1 and 2 ppm (mg/L). The maximum concentration in water under normal conditions (saturation concentration) isbetween 8 and 10 ppm (mg/L).1

pHpH is a measurement of the acidic or basic character of a solution at a given temperature. It is defined as the negative logarithm(to the base 10) of the hydrogen ion concentration (-log[H+]). Pure water is slightly ionized with a pH of 7, and at equilibrium theion product, Kw , is [H+][OH-] = 1.01 x 10-14 at 25°C. Generally, quality of receiving bodies of water need pH ranges between 6.5to 8.5 for healthy aquatic life. Wastewaters outside the 5.5 to 9.5 pH range (before being commingled in the mixing zone of thewastewater effluent and the receiving water body) can potentially cause harm to the receiving water

➧ aquatic life or biological treatment microorganisms.1

“TOXIC" COMPOUNDS/TOXICITYToxic compounds are those specific chemicals that have been shown to cause death or adverse effects to organisms at certainconcentrations. “Toxic," “Hazardous," “Dangerous," “Carcinogenic," “Mutagenic," and several other terms are used to describechemical effects in water systems. However, this is a complicated area since many “toxic" compounds can be essentialnutrients to human and aquatic life at low concentration, such as ammonia, cobalt, zinc, and copper.Wastewater effluent “toxicity" refers to laboratory tests that are made on the effluent where aquatic organisms, like fish orshrimp, are exposed to different percentages of the effluent for a certain period of time. These tests are frequently referred to asbiomonitoring or bioassays, and the test procedures differ for determining acute versus chronic toxicity. For acute toxicity, thetest results are commonly expressed as LC50 or lethal concentration at which 50 percent of the organisms are expected to diein a set period of time. Typically, the LC50 is based on either a 24, 48, or 96-hour test period. For chronic toxicity, the testresults are expressed as a no-observed-effect concentration (NOEC) and is the effluent concentration where there is nosignificant difference between the test organisms and the control organisms. Survival, growth, and reproduction of the testorganisms is compared to the control organisms for the chronic toxicity test. Typically, the duration of chronic toxicity test is 7

➧ days. The procedures and interpretation of these aquatic test methods can be quite complex. In situations where there is anydoubt about the test methods, the lab performing the tests, the interpretation of results, or a biomonitoring test is included intoan effluent permit for the first time, EMBSI (ExxonMobil Biomedical Sciences, Inc, Clinton, NJ) personnel should be consulted.

➧ HEAVY METALSRecent regulatory trends include effluent quality standards for heavy metals that are more stringent and presciptive than in thepast. In general, biologically treated petroleum wastewaters do not contain significant concentrations of heavy metals. Thereare some metals in crude oils, raw water sources, trace constituents in industrial inorganic chemicals such as caustic or limeand some that come from corrosion products of the materials of construction in refineries and petrochemical plants. In somelocations, specialized treatment may be needed at the end of the wastewater treatment system to remove these metals toregulatory driven effluent concentration or mass limits. Metals such as selenium, mercury, arsenic, vanadium and zinc aretypical ones found in trace amounts in refinery wastewaters. Before end- of -pipe treatment facilities are considered, a samplingand lab analysis program should be undertaken to identify the sources, to see if they can be reduced or substituted, to avoid thecost of end -of -pipe facilities. Typical sources and treatment technology for heavy metals are provided below:



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Metal Sources Treatment Technology OptionsArsenic Desalter brine, tank water draws Co-precipitation (Al and Fe salts)Copper Utility blowdowns, corrosion, tank draws Co-precipitationMercury Desalter brines, sour waters Activated carbon, ion exchangeNickel Desalter brines, sour waters, corrosion, Co-precipitation

utility bd, tank drawsSelenium Desalter brine, soure waters, tank draws Co-precipitation

, raw waterZinc Utility bd, tank draws, raw water, lube Co-precipitation

processing areas

NUTRIENTSNutrients are chemical elements such as nitrogen, potassium, phosphorous, sulfur, cobalt, zinc, and copper, which are

➧ essential for plant or animal growth and effective biological treatment. As a rule-of-thumb, a ratio of BOD5, nitrogen, andphosphorus (BOD5:N:P) = (100:5:1) is needed for effective biological activity in wastewater treatment systems. Typically 0.05 to5 ppm of micronutrient metals are needed for healthy biotreatment ( Fe, Cu, Mn, An, Mo, Co, Se) for petroleum andpetrochemcial wastewaters feed to a biox system. Other metals are required higher concentrations, tens to hundreds of ppm(Ca, Mg, K, Na). In most instances, all of these metals are in the mixtures of wastewaters treated at a refinery. If some locationsneed to add them, they need to be done so as to avoid exceeding any mass or concentration limits in the effluent dischargepermit or license.

NITRIFICATION/DENITRIFICATIONNitrification is a biological process where ammonia is converted/oxidized to nitrate (NO3- ). The process involves onemicroorganism species converting ammonia to nitrite (NO2- )and then a different microorganism species converting the nitrite tonitrate. This term should not be confused with “denitrification," which refers to the biological process of convertingnitrites/nitrates to nitrogen gas (N2).40,46

OVERALL WASTEWATER SOURCE MANAGEMENT AND TREATMENT SELECTION

OVERALL WASTEWATER SOURCE MANAGEMENTAs part of an overall water/wastewater management strategy, several issues must be considered, including wastewater reuseopportunities, government restrictions on types of wastewaters allowed to go to sewers, safety, “pollution prevention" goals, andstormwater management. Normally, a comprehensive review of wastewater sources, including flow and source assessment andlab analysis, is needed to define a cost effective overall water management strategy. A considerable effort should be plannedand made in this area before facilities are designed. Considerable savings can be realized in facilities if a complete watermanagement material balance and strategy are prepared.Stormwater handling can be an important factor, especially when considering new treatment facilities. Treatment facilities aresized on removal performance and flow rate. Frequently, the equalization or diversion of stormwater can have a large impact inreducing the size of these facilities.5-17

WATER REUSE➧ Water reuse can result in a significant reduction in the volume and cost of water (raw and discharged wastewater)

used/generated at a refinery or chemical plant. The three major types of reuse are: reduction, recycle within the unit, and reuse(direct or cascade reuse, segregation and reuse, and treatment and reuse).Reduction of the amount of water used in a process generally results in an increase in the contaminant level and thetemperature of the effluent wastewater. If this is acceptable from a process, safety, and materials standpoint, it can be used tolower raw water usage. A typical large reduction in raw water usage is obtained by increasing the cooling tower cycles.Recycle refers to when wastewater is reused in the process from which it originated. Examples of this are return of samplecoolers to the cooling tower or a pumparound on a water scrubber tower. A purge stream off of the recycle loop is needed tocontrol the level of contaminants. Other measures, such as heat removal, chemical treatment, or contaminant removal may alsobe required.Reuse refers to when wastewater is reused by routing or cascading it from one unit to another. For example, stripped sourwater (SSW) may be reused in the desalter to reduce raw water makeup and contaminant concentration, such as phenol,





originating in the SSW. The advantage of reuse over recycle is that contaminant buildup is less likely. Each process hasminimum water quality requirements. If the quality of the wastewater from a process meets the minimum water qualityrequirements of another process, it can be reused directly. If not, segregation of certain wastewaters from a combined stream ortreatment of the wastewater before reuse may be necessary.Guidelines for developing a water reuse program can be found in Design Practice XIX-B, Water Reuse. The section also givesexamples of reuse by process unit along with critical parameters for each case.

WATER CHEMISTRYUnderstanding the water chemistry is an integral part of wastewater management. The ionic and molecular (both organic andinorganic) species in the water and their interaction affect water usage and wastewater treatment. For example:• Dissolved solids may precipitate out as a scale if the water temperature or pH is changed or if the water is concentrated by

recycle or evaporation. Scale reduces heat transfer efficiency in exchangers.• pH or chlorides level in the water affect the corrosivity of the water.

➧ • High pH may cause or increase emulsification resulting in poor oil/ water separation.• Organics in water, if combined with nitrogen at the right conditions, will result in biological growth and potentially biological

fouling.• Ammonia and hydrogen sulfides in water may need to be stripped before reuse or treatment. If not, they can evolve when

the pH is changed.• A narrow pH band (7 to 8.5) is required to obtain healthy biomass in biological treatment system.

SEWERGenerally, there are two major types of sewer systems: combined and segregated sewer systems. The segregated sewersystem can be divided as follows:

➧ • Industrial/Process Sewers or Oily Water Sewers which collect process wastewater and hydrocarbon contaminatedwastewater including rainwater or firewater runoff from areas normally subject to hydrocarbon/oil contamination andcondensate/blowdown with hydrocarbon/oil or chemical contaminants.

• Chemical Sewers which collect acid, caustic, or other chemical wastewaters which require neutralization or treatment priorto discharge to industrial sewers.

• Storm Water Sewers or Clean Water Sewers which collect rainwater runoff, firewater, and condensate/blowdown that isnot normally subject to hydrocarbon/oil, or toxic chemical contamination.

The Combined Sewer Systems are those systems which combine two or more types of sewers as defined above. Thecombined sewer system is the most common sewer type in ExxonMobil. However, new projects, government regulations, and

➧ costs of end-of -pipe treatment are making many locations to go to more segregated systems.There are advantages and disadvantages of segregating stormwaters. In almost all cases, it is cost effective to divert cleanstormwater (those stormwaters that do not contact process areas or other contaminated locations) to a separate ditch or sewersystem. In some locations, this water is collected in an impoundment basin and used as a fire water supply. Studies haveshown that excessive stormwater mixed with contaminated process waters results in excessive sludge generation in thewastewater treatment equipment and solubilizing of hydrocarbons that would otherwise stay in the oil phase. However, thereare special cases where segregating stormwater can be costly and can increase the concentration of contaminants to thewastewater treatment equipment, resulting in the potential for exceeding permit limits for individual contaminants or toxicity tests.Hence, several factors need to be considered in integrating stormwater handling in the overall wastewater management plan.Stormwater that is from contaminated areas within the plant can also be managed properly to reduce downstream impact ontreatment facilities by using the first flush principle. The diversion of the dirty, first 30 to 60 minute flush of stormwater to adiversion tank, if allowed by regulatory authorities, for eventual work off into the treatment system can be an effective strategy.The water after the first 30 to 60 minutes can be clean and diverted to clean stormwater conveyances or impoundment basinsfor eventual discharge.Sewers can be constructed underground or aboveground. The aboveground sewer has the advantages of easier maintenanceand detection for leaks or infrastructure damage. However, it needs to be pressurized. Economical and operating factors forsegregated/combined and aboveground/underground sewer system should be considered when building a new sewer system.Segregated Sewer System - As mentioned above, it is frequently cost effective to segregate “clean” from “dirty” wastewaters inseparate sewer systems. Segregated sewer systems are also frequently needed when hazardous or volatile contaminants arepresent in high concentration.



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DECISION TREES FOR TREATMENT UNIT SELECTIONThis section consists of decision trees that will give general guidance to select wastewater treatment technology for differenttypes of contaminant removal. The decision trees are presented in Figures 1 through 6 and are listed below. For thosesituations that do not fit into the general decision trees, contact ER&E environmental specialists for assistance.• Wastewater Treatment Assessment (Figure 1)• Free Oil Removal (Figure 2)• Oxygen Demand/Dissolved Organics Reduction (Figure 3)• Suspended Solids Removal (Figure 4)• Wastewater Effluent Toxicity Reduction (Figure 5)• Wastewater Sludge Management Volume Reduction (Figure 6)

TYPICAL EFFLUENT QUALITY CRITERIAGovernment regulations and guidelines on the quality of wastewaters discharged from industrial facilities can vary greatlydepending upon the location in the world. In some cases, town/cities or regions impose restrictions in addition to states andcentralized country requirements. A key factor affecting the water quality criteria imposed on the treated wastewater effluents isreceiving water use and location. Much tighter restrictions are placed on facilities that discharge into fresh water bodies thathave downstream drinking water or other recreational uses involving the public or important aquatic plant and animal life. Lessstringent requirements are sometimes placed on effluents that go into government owned treatment works prior to discharge intothe aquatic environment.

➧ Effluent quality limits are generally placed on the the effluent wastewater without dilution, but mixing zones allow the effluentconcentrations of most contaminants to be higher than that needed in the receiving water body to protect aquatic resources.Near the discharge point, a mixing zone exists where constituent concentrations are between the effluent and fully mixed values.The characteristics of the mixing zone are important because they can reduce the effect of toxic chemicals by speeding theirdilution. The mixing zone is not significant for dissolved oxygen analyses because BOD exertion is slow compared to the traveltime within the mixing zone. The mixing zone size will vary depending on the mode of discharge e.g., pipe, multiport diffusers,etc. Because the mixing zone factor may be significant, i.e., up to a dilution factor of 40, it is important to include its effect whendetermining the effluent quality and when negotiating with the authorities for the site specific regulations on the wastewatereffluent discharge quality.The general requirements for industrial plant facilities are a definition of effluent contaminant parameters in terms ofconcentrations (weight parts-per-million, ppm (mg/L), or mass limits (lbs or kg per day) or both. Quality requirements forindustrial wastewater effluents have changed significantly over the past decade. Past regulations have focused on conventionalcontaminants or indicators of pollution such as oil and grease, biochemical oxygen demand, pH, temperature and suspendedsolids. In some locations, regulatory authorities are continuing to reduce the allowable concentrations and mass limits of theseconventional indicators of pollution. In other areas, the emphasis now is on the control of “toxic" or “hazardous" compounds.Specific effluent concentration and/or quantity limits are being imposed on industrial facilities for compounds

➧ such as phenolics, benzene, cyanide, ammonia, hydrogen sulfide and heavy metals. In some locations, regulatory agencies areplacing limits on nitrogen or phosphorus discharges which can potentially cause uncontrolled vegetative growth in receivingbodies of water (called eutrophication), reducing the receiving water available oxygen. Some environmental agencies arefocusing on environmental compliance or enforcement of existing limits, which means more frequent monitoring and reporting.The capability of new analytical methods to measure very low concentrations is creating the need for increased emphasis onreducing specific “toxic" constituents, such as mercury. However, artificially low limits should be challenged if they do not have aclear adverse environmental impact, rather than just their capability to be measured at low concentrations. In locations thatrequire maximum water reuse, concerns should be focused on avoiding excessive concentration of the contaminants that needto be treated prior to discharge. Also, new projects in petroleum facilities and chemical plants are changing the types andquantities of compounds entering the wastewater system. Some of these compounds may require monitoring and reporting togovernment authorities. Other emerging issues that can potentially impact effluent limits are more sophisticated biomonitoringmethods, outfall sediment quality, mixing zone modeling, and watershed overall quality.18-23

OIL AND pHPetroleum and petrochemical facilities almost always have “oil" (oil and grease) and pH limits. A key limitation is that this oil willnot cause a visible sheen on the surface of the receiving body of water. Quantitative limits generally range from 5 to 50 ppm inthe effluent for land-based facilities. Because acid and base materials are used, pH limits are generally applied and arecommonly between 6 and 9 for operating temperature between 50 and 100°F (10 and 38°C). Exceptions are made for particular





locations such as stormwater which may have a lower pH due to localized affects of acid rain, outside of the control of thefacility.

GENERAL POLLUTANT INDICATORSA listing of general pollutant indicators that frequently are required by regulatory agencies of most industrial facilities are givenbelow. This listing is provided to guide the reader in developing sampling and analytical requirements for treatment facilities.• Temperature• Dissolved Oxygen• Oxygen Demand (BOD, COD, TOC)• Odor• Suspended Solids• Total Dissolved Solids• Toxicity or Bioassays

OTHER TYPICAL PETROLEUM/PETROCHEMICAL CONTAMINANTSFrequently, government regulations require sampling and analysis of plant wastewater effluents for known compounds presentin petroleum/petrochemical wastewaters, listed below.• Phenol/Phenolics• Hydrogen Sulfide (Sulfides)• Ammonia (and other nitrogen compounds in effluents)

➧ • Selected Heavy Metals (e.g., Lead, Mercury)

EXAMPLES OF TOXIC COMPOUNDS“Toxic," “Hazardous," “Priority," “Black Listed," “Dangerous" and several other terms have been used to describe organic

➧ compounds and heavy metals that governments are particularly interested in regulating because of their human or aquatictoxicity impacts. Some governments have not required measuring these, since the treatment systems in certain locations areknown to remove these compounds, so measuring some of the general indicator compounds like BOD is sufficient to monitorthe “toxic" compounds as well. In other locations, government agencies have required sampling and measuring thesecompounds (such as benzene or lead) to ensure that the specific compounds are low enough in the effluent. Hence, there areno clear cut guidelines on these compounds because of the location specific impacts that must be considered as well as mixingzone influence of the particular water body. For example, the concentration in the effluent can be much higher than that allowedin the water body, since the effluent will mix with the water body near its outfall. Check with local regulations on thesecompounds as well as the general and other typical compounds listed above.

TREATMENT UNITS

TYPICAL REFINERY/PETROCHEMICAL WASTEWATER TREATING FLOW PLANA flow plan illustrating a typical wastewater treatment system is presented in Figure 7. The purpose of this figure is to provide ageneral idea of how the treatment units fit together, for what purposes, and where the various waste streams enter and leave thetreatment system. The flow plan also illustrates that there are several alternative technologies that can be selected to removethe targeted compound(s). Selection of the treatment units and layout of the wastewater treatment plant will depend primarilyupon the type of processes in the refinery/chemical plant and the effluent criteria (concentrations or tests) required by thegovernment. An important element of preparing a figure like Figure 7 is collecting sufficient flow and contaminant data from allthe major wastewater sources and preparing material balances, with some statistical analysis of the data.

➧ OIL REMOVAL/SEPARATION AT SOURCES- No/Low Oil to Sewer and More Equalization TankageSeveral locations have implemented a program of reducing major oil sources to the sewer and wastewater treatment systems.The result has been reducing oil losses, gaining value for the recovered oil, improving wastewater treatment, reducing treatmentchemical costs and dramatically reducing waste disposal volumes. Guidelines are available on the Wastewater and WasteBestNet to assist in planning for an effective program.



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In recent years, refineries have been processing heavier and sour crudes. These crudes have caused increased separationproblems at the desalter and other downstream equipment such as API separators and flotation units. The result has beenmore oil going into the biox systems, during upset or stormwater conditions, causing foam and poor settling biomass in theclarifer. Some locations have put dedicated tankage to improve oil water separation for the desalter washwater, and at theWWT system to protect the downstream biological systems, and to recover more oil. Even in cases where the desalter is thelatest technology, including bielectrics, AGAR type oil water interface detectors, new generation demulsifiers are used, andEMRE designed mudwash systems, some sort of settling tankage or further oil/water separation may be needed, when the typeof oil does not separate well or binds with mud to form stable emulsions. This trend will likely continue in the future.

HOW TO ESTIMATE WASTEWATER FLOW AND QUALITY WITHOUT DATAIn most cases, a focused and well-planned wastewater flow and quality study should be conducted at the plant site. Thisincludes sampling and laboratory analysis of key wastewater streams to provide a basis for a cost effective treatment strategy.In situations where new petroleum or petrochemical process units are added to an existing plant, or where rough, earlyscreening estimates are needed for a plant that does not exist, prior data from ExxonMobil studies can be used for estimatingpurposes. Information sources given in the selected references refer to refinery process units only.24-26

TYPICAL REMOVAL EFFICIENCIES FOR SELECTED WASTEWATER TREATING UNITSContaminants can be removed from wastewater using many different types of treating units. Selection of the proper treatmentunit will depend upon the contaminant that must be removed and the effluent limit for that contaminant. Table 1 provides typicalremoval efficiencies for the more common wastewater treating units. The information contained on this table should be used asa guideline only since actual removal rates vary with the influent wastewater quality and several other factors that must beconsidered prior to equipment design and specification.

TREATMENT UNITS (Cont)

EQUALIZATION/pH CONTROL (FLOW AND CONTAMINANT LOAD)Equalization facilities are provided to equalize flow and contaminant loads to downstream treatment units. For flow, theequalization facility will dampen out normal and peak flow variations that come from the process blocks or during rain events.Equalization provides a nearly constant flowrate to downstream treatment units, thus reducing the impact of flowrate on processcontrol and improving treatment unit reliability. Downstream treatment unit size and cost can be minimized since the units canbe designed based on the equalized flowrate and not on peak flow. Equalization also dampens out variations in contaminantconcentrations, which occur in daily operations and during upsets. This provides a more uniform feed concentration todownstream treatment units which in turn ensures a more consistent performance from each downstream unit and a consistentoverall effluent quality.

➧ Ideally, a wastewater treatment plant should have three different equalization facilities; one for the diversion and management ofprocess rainwater, one for high contamination off-spec wastewaters, and one to equalize the wastewater contaminantconcentration and flowrate upstream of the advanced primary treatment units such as flotation units and biological treatmentunits. If equalization and diversion tankage are sized adequately, then the three types of equalization can be combined into towtanks, one for stormwater management and the other for equalization of contaminants and flow. Equalization is especiallyimportant for biological treatment systems. Diversion of rainwater generally takes place either upstream of the oil/waterseparation facilities or is incorporated into the operation of the oil/water separation facilities. For example, the forebay of an APIseparator can be designed to divert the incoming rainwater, based on flowrate, water level, or time, to a sump for pumping to adiversion tank or basin. Once the rain event is over, the contents of the diversion tank or basin can then be bled back to thetreatment plant at a controlled rate.Under normal dry weather flow conditions, wastewater passing through the oil/water separation facilities will typically flow into anequalization tank before going to the biological treatment unit. The equalization tank is generally located downstream of the

➧ oil/water separation facilities to minimize the amount of oil and solids that enter the tank. However, as mentioned previously, theprocessing of sour and heavy crudes that cause oil/water separation problems, particularly at the desalter, may require separatetankage and oil removal. Equalization upstream of a biological unit is necessary to minimize the variability in the contaminantloading that can upset the performance of a biological unit and other treatment units.Care must be taken when selecting the type of equalization facilities. Some locations may be allowed to use an open basinwhereas others may be required to use closed tanks or covered basins. In the U.S., consideration must be given to the volatileorganic content, particularly benzene, of the streams entering sewers, impoundments, and other facilities to determine if specialequipment features are needed to minimize vapor emissions.It is essential to have a pH control system upstream or integrated with biological treatment units. A biological treatment unitoperates within a narrow pH range (approximately 7 to 8.5), and performance is very sensitive to wide swings in pH. AdditionalpH adjustment facilities may also be necessary to ensure the plant effluent falls within the regulated pH range.





PRIMARY OIL AND SUSPENDED SOLIDS REMOVAL (PRIMARY TREATMENT)Primary treatment of plant wastewaters is needed to remove gross amounts of oil and suspended solids in process wastewaters.Typical oil content can be in the few hundred ppm (mg/L) range to several thousand ppm (mg/L). The intent is to reduce the oilcontent to below about 50 to 100 ppm (mg/L) consistently and to recover the oil for recycle to the slop system. No typicalconcentration ranges are available for suspended solids content, but some guidance is provided in Table 1 and Figure 2.

API Separator

An API (American Petroleum Institute) Separator is a gravity settling basin for the separation of gross free oil and suspendedsolid from wastewater. It is normally the first end-of-pipe wastewater treating unit on oily wastewaters. Influent water maytypically contain maximums of 1,000 to 10,000 ppm (mg/L) oil. An API separator removes large oil droplets that are greater than150 microns and can reduce the free oil content to about 50 - 100 ppm (mg/L). Effluent oil and suspended solids contentsdepend upon particle size, specific gravities of the oil and solids, the specific gravity and viscosity of the wastewater, the degreeof emulsification of the oil, and the amount of soluble organic compounds present. The separator has little, if any, effect onsoluble BOD or soluble contaminant removal.29-31, 80

Plate Interceptor/Separators

A plate separator is a gravity separator containing a number of plates to enhance settling and removal of oil and suspendedsolids by reducing the distance the oil droplets and solid particles must travel to separate from the water. The plates may alsoreduce turbulence and enhance coalescence of the oil droplets into larger ones that separate easier. There is a wide variety ofplate separators and typically the design and placement of the plate packing make each separator unique. Plate separators areoften referred to by the following names: Parallel Plate Separator (PPS), Corrugated Plate Interceptor (CPI), Corrugated PlateSeparator (CPS), Tilted Plate Interceptor (TPI), etc. In general, the different plate separators serve the same function forremoving gross oil and suspended solids, and their performance are relatively similar. 29-31

Plate separators are generally designed to remove oil droplets larger than 60 microns compared to the 150 micron droplet sizethat can be removed by an API. As a result, lower effluent oil concentrations can generally be achieved with plate separators.Plate separators occupy less area than other gravity separators performing the same service, but they do not provide theequalization/diversion benefits like API separators. There are packaged units available in gravity and pressurized flow and canbe covered or enclosed to contain vapors or operate as a liquid filled unit.

Skim Ponds

A skim pond is a pond in which wastewater is retained to allow separation of oil. A skim pond may be preceded by an APIseparator and used for incremental removal of free oil. It may also be used instead of an API separator but is difficult to cover, ifneeded to contain vapors. Skim ponds generally are not designed to reduce significant amounts of soluble BOD or any othersoluble contaminants. Skim ponds generally have barriers or baffles to allow for oil containment and collection, via skimmers.In most situations, skim ponds are not recommended for gross oil and suspended solids removal. Installation of API or plateseparators is preferred for oily wastewaters.

Enhancements/Future Technology

This subsection is included in this Design Practice to reflect vendor or other technology that may be encountered by the reader.Pilot testing is recommended prior to a particular application.1. Daburt Separator - The separator operates by using hydraulic energy to bring 90 to 95 percent of free and emulsified oils

to the surface on a broad crested weir before the wastewater flows into the sludge pack part of the unit which is located atthe bottom. Further separation is said to occur in the sludge pack where “biological" activity partially digest and releasesany remaining oil which has been absorbed by the suspended solids. The ability to remove oil from solids is one featurethat the Daburt separator may potentially offer over competitive oil/water separation processes. The unit has no movingparts except for the tilted disc skimmer used to remove the separated oil from the unit. The Daburt separator has beenevaluated at the Fife Ethylene Plant on two wastewaters: 1) the combined feed to the corrugated plate interceptor (CPI)and 2) the steam cracker quench water. The Daburt performed very well on the quench water stream, consistentlyremoving particles > 25 microns. However, oil removal from the CPI feed was less than expected. Solids entrained in thefeed were too fine to be removed and may have carried oil with them.35

2. Cyclone Separators - Hydrocyclones achieve oil/water separation by generating high centrifugal forces, thereby separatingthe oil and water phases. Suspended solids are generally not separated during this process and most exit the unit in thewater phase. However, a small fraction of solids will leave with the oil. The wastewater leaving the hydrocyclone will likelyrequire additional treatment to remove the suspended solids. The hydrocyclones used for oil/water separation do not haveany moving parts; instead, they use incoming flow energy and directed flow patterns to generate the centrifugal forces. Useof hydrocyclones for oil/water separation is a new technology for refining and petrochemicals

➧ but this technology has been tested and installed at onshore and offshore production locations.



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SECONDARY OIL AND SUSPENDED SOLIDS REMOVAL (ADVANCED PRIMARY TREATMENT)The goal of “secondary" suspended oil and solids (TSS) removal equipment (frequently called advanced primary treatment) is tofurther reduce the oil and TSS to low concentrations. This treatment will either allow for the effluent to be directly discharged orprevent operating problems with downstream treatment equipment that is aimed at the removal of soluble contaminants.Generally, DAF or dual media filters are required if the effluent is discharged directly, because effluent limits typically require thesuspended oil content be below 15 to 20 ppm (mg/L). IAF or nutshell filters would be considered when the effluent will havesome downstream treatment prior to discharge. (See Figure 2 and Table 1 for details.)

Induced Air/Gas/Static Flotation (IAF/IGF/ISF)

This treating unit operates by using air/gas bubbles to remove oil droplets and suspended solids not removed by primary oil andsolids separation. Air or gas can be introduced into the system either by driving it into the wastewater with an impeller or bybubbling it up from the bottom of the unit. Air is preferred since it oxygenates the water, but other gases, such as nitrogen andfuel gas, can be used. The bubbles in the wastewater capture the oil and suspended solids while rising to the surface. The oilyfroth is removed by a skimmer and is further treated and dewatered as a waste sludge. Wastewater leaves the units with greatlyreduced levels of oil and suspended solids. These units can be covered to reduce vapors and odors, if needed.This type of system offers the advantage of lower capital cost and smaller space requirements over that of a dissolved airflotation (DAF) unit. Performance data indicate that the air or gas flotation systems can effectively remove free oil andsuspended solids and minor amounts of dissolved highly volatile compounds but not to the same degree as a DAF unit.The disadvantages compared to DAF include higher connected power requirements, performance is dependent on stricthydraulic control, less chemical addition flexibility, and relatively high volumes of float as a function of wastewater throughput (2to 5 percent of the incoming flow for induced air systems is common compared to 0.25 to 3 percent for the dissolved airsystems).81

These units are usually recommended when there is downstream treatment, such as biological treatment or effluent oil andsuspended solids requirements are not too stringent.

Dissolved Air Flotation (DAF)This process operates similar to Induced Air/Gas Flotation Units, except the air is pressurized prior to contact with thewastewater. The pressure is released in the unit resulting in fine air bubbles in the wastewater that improve separation of oil andsuspended solids over the IAF unit. In many cases, an induced air or gas flotation unit will provide comparable effluent quality tothat from a DAF unit but at a lower cost. DAF units are generally specified for wastewater streams that must meet a

➧ stringent (< 15-25 ppm) effluent suspended oil limit, and there are no downstream treatment facilities after the DAF unit.81

➧ In recent years the oil in the wastewaters going to the DAF has shown poorer settling characteristics than in prior years, so theperformance of the DAF units has not been as effective, resulting in oil effluent concentrations over 25 ppm on occasions. Thiseffluent quality is not desirable, but as long as it does not adversely affect the downstream biox or final effluent quality limits, itcan be considered acceptable performance.

Media Filtration

Filtration of wastewater is used to remove suspended oil and solids not removed by primary treatment processes. Granular-media filters are supported beds of granular material (sand, anthracite, or garnet) through which wastewater is passed toremove suspended material. These filters effectively remove suspended oil and solids from wastewater streams by theprocesses of straining, sedimentation, and adsorption of the oil and solids within the filter bed. The filters must be periodicallycleaned by backwashing to remove the filtered material thereby reducing the pressure drop across the filter bed. Backwashwater is generally returned to the front end of the treatment plant.There are several types of media filters: downflow pressure filters, downflow gravity filters, and upflow sand filters. Typically, adual media filter contains a bottom layer of fine sand and a top layer of coarse anthracite.Nutshell filters are a special type of media filter, generally made from ground walnut shells and sometimes a combination ofwalnut and pecan shells. The nutshells are better than sand or anthracite for suspended oil removal (particularly “sticky" oil)since they are preferentially water wetted and thus readily adsorb and desorb the oil. Nutshell filters are cleaned bybackwashing and by passing the media through a scrubber to remove oil and solids. They use significantly less backwash waterthan dual media filters.A granular-media filter can remove greater than 80 percent of the suspended oil and solids in the influent and produce awastewater effluent of < 15 ppm (mg/L) suspended oil. Effluent oil levels as low as 5 ppm (mg/L) have been observed. Noremoval in soluble BOD, phenol, or other dissolved substances takes place. Walnut/pecan shell filters have reduced backwashrequirements and increased flux rates but do not remove suspended solids to the same effluent quality as the sand or dualmedia filters.56, 57, 82





Chemical Flocculation

The chemical flocculation process uses inorganics or organic chemicals to remove suspended oil and solids. pH adjustment isalso required in most cases. In general, this process is not recommended since it gives poorer performance than IAF or DAF,and it generates a greater volume of sludge for disposal. It can be a useful alternative to IAF or DAF when dissolved solids ormetals require removal. See Subsection on “Dissolved Solids/Metals Reduction" of this Design Practice.


Daburt Separator - Daburt separators can potentially be used for secondary oil and suspended solids removal. See Subsectionon “Preliminary Oil and Suspended Solids Removal" of this Design Practice for details on this unit.

OXYGEN DEMAND/DISSOLVED ORGANICS REDUCTION (SECONDARY TREATMENT)

Biological Treatment

Biological treatment is typically used to reduce oxygen demand and remove dissolved organics. The term BIOX is anabbreviation for BIological OXidation and is commonly used to describe the activated sludge process as well as any otherbiological oxidation process used to treat wastewater. Biological treatment is done in a variety of ways: activated sludge,extended aeration, aerated lagoons, fixed film bioreactors, and anaerobic treatment. (See Figure 3 and Table 1 for details.)The usual form of treatment is either activated sludge, extended aeration, or aerated lagoons; however, there are specific caseswhere other types of biological systems are more economical and produce a more stable system. Below is a brief description ofthe types of systems and general guidelines that indicate what form may be the best method of treatment for a particular case.1. Activated Sludge (BIOX) - Activated sludge treatment is a controlled biological oxidation process that involves the growth

of suspended, “activated" microorganisms capable of removing soluble organic contamination to low concentrations. Theactivated sludge process unit typically consists of aeration and clarification. The aeration step takes place in either a basinor tank where the oxygen is supplied with either mechanical or diffused aerators. In the case of diffused aerators, either airor pure oxygen can be introduced into the aeration tank/basin, but most systems are based on air, due to lower cost fortypical applications. The aerators are also used to mix the microorganisms with the wastewater to ensure completecontacting and treatment. While in the aeration basin/tank, the microorganisms use the dissolved organic material as afood and energy source by converting it, in the presence of oxygen and nutrients, to CO2, H2O, and more microorganisms.Nitrogen and phosphorus, as well as trace amounts of other micronutrients and metals, are needed for microbialmetabolism. Nitrification, the process of oxidizing ammonia and organic nitrogen to nitrate, can also take place in theaeration basin/tank. After a specified amount of time [the hydraulic residence time (HRT) of the basin/tank], the mixture ofmicroorganisms in the aeration basin/tank flow into a clarifier where suspended solids settle to the bottom and effluent iseither discharged or routed for further treatment. Most or all of the settled activated bio-sludge is recycled back to theaeration basin/tank (referred to as return activated sludge), while excess bio-sludge is “wasted," typically to facilities forthickening and dewatering, prior to disposal.Biological oxidation processes are sensitive to gross variations in influent characteristics such as organic concentration, pH,temperature, oil, and inorganics, particularly salts that can be present in ballast water or salt water cooling tower blowdown.The influent concentration of total organics is measured by either TOC, BOD, COD, or all of these parameters to obtain anaccurate loading basis for design and operation. In order to produce a stable activated sludge system, the BOD5 influentconcentration should be greater than 100 ppm (mg/L). Influent oil concentrations should be below 50 ppm (mg/L), whileinlet inorganic ions should also be controlled (i.e., sulfides, ammonia, and salt). Salt concentration variations should notexceed 5000 ppm (mg/L). The activated sludge process can be expected to remove greater than 90 percent BOD5 and 95+percent phenol. Metals removal can also be achieved across the BIOX through adsorption onto the biomass.2-4, 47, 84

A variation of the Activated Sludge process for smaller facilities is the Sequencing Batch Reactor (SBR). The SBR is a fill-and-draw activated sludge treatment system. Aeration and sedimentation/clarification are carried out sequentially in thesame tank. There is no return activated sludge system because all the biological treatment and settling occur in the sametank. SBR units have not been used within Exxon because they were developed subsequent to installation of mostbiological systems. They operate in a semi-batch mode versus the more familiar continuous operation, and their bio-sludge(TSS) level in the effluent may not be as low as with activated sludge. SBR's are also generally more appropriate for highconcentration wastewaters and lower flowrate applications.47, 49

2. Extended Aeration - Extended Aeration is a form of the activated sludge process in which the HRT and SRT are largeenough to allow significant biological oxidation of some hard to degrade organics, ammonia, and the biological solidsthemselves thus reducing the amount of bio-sludge for disposal. Characteristics of the Extended Aeration process includelong hydraulic retention times (18 hours to a few days), low food to mass (F/M) ratio (less than 0.1 pounds of BOD5 perday/pound of MLVSS) and a high sludge retention time (SRT) (longer than 20 days).



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Extended Aeration requires the most oxygen and produces the least excess biological solids of all the variations of activatedsludge processes. Extended Aeration can be designed to remove greater than 95 percent of BOD5 to a minimum of 15 ppm(mg/L) in the effluent and 98+ percent removal of phenolic compounds. 47, 84

3. Biological Oxidation Lagoons/Ponds - An aerated lagoon or pond is a biological system where wastewater is treated on aflow through basis without solids recycle. Sometimes a series of lagoons are used to obtain improved treatment and toallow settling of biological solids. Some aerated lagoons have been partitioned with a barrier(s) so that a portion of thelagoon is used for aeration and the remaining portion is used for settling. Periodically, the settled biomass must be

➧ removed from the lagoon to maintain effective flow distribution, residence or biological reaction time. In some cases thebiomass is returned to the front section of the lagoon to improve treatment and reduce biomass sludge amounts. Lagoonsrequire more surface area than other biological oxidation methods and are used only when space is available, the cost ofland area is not prohibitive, and there are no restrictions on un-lined impoundments (lining can be expensive).Aerated lagoons are less efficient than either Activated Sludge or Extended Aeration since there is no sludge recycle. Theycan be designed to reduce incoming BOD5 by 60 to 90 percent and phenol by 80 to 95 percent. Effluent characteristicsinclude < 15 ppm (mg/L) of oil, and a suspended solids content of about one-half the incoming BOD5, but not less thanabout 50 to 150 ppm (mg/L). See Figure 3 and Table 1 for details. 47, 84

4. Fixed Film Reactors - A fixed film bioreactor or attached growth bioreactor is a process similar to the suspended microbialaerobic and anaerobic treatment systems. However, the microorganisms primarily adhere to fixed media rather than beingsuspended in the wastewater. There are various fixed film aerobic processes: trickling filter, rotating biological contactor(RBC), and fixed-film packed bed or fluidized bed reactor.a. Trickling Filter - The trickling filter consists of a bed of large rocks or specially designed synthetic media for the

microorganisms to grow on while wastewater is percolated or trickled through the bed. As the microorganisms grow,they form a slime layer on the media, which increases in thickness as the microorganisms grow. Diffused oxygen isconsumed by the outer portion of the slime layer before it can penetrate the full depth of the slime layer. For thisreason, the slime layer near the surface of the media acts anaerobically (without oxygen). The permeable media canbe rock/slag in the size range of 1 to 4 inches (2.5 to 10 cm) in diameter or plastic packing material. Trickling filters areconstructed with an underdrain system to collect the treated wastewater. The effluent from the trickling filter goes to asettling tank where bio-solids are separated from the treated wastewater. In some cases, a portion of the treatedwastewater from the supernatant of the clarifier is recycled back to the influent of the trickling filter to maintain a moistbiological slime layer.Trickling filters remove 40 to 80 percent of BOD5 providing that wastewater BOD5 influent is above 100 ppm (mg/L).Trickling filters with high hydraulic loading rates are sometimes called roughing filters. They are usually used to reduceorganic contamination prior to downstream treatment processes. Trickling filters are less efficient than an activatedsludge process and are normally not used within Exxon.2, 47, 84

b. Rotating Biological Contactors (RBC) - The rotating biological contactor (also called bio-disks) is another form of afixed film reactor that consists of large-diameter plastic media disks mounted on a horizontal shaft in a trough. Influentwastewater enters at one end of the trough and effluent leaves at the other end of the trough. Flow can be parallel orperpendicular to the shaft. The horizontal shaft slowly rotates so that approximately 40 percent of the surface area issubmerged and 60 percent is exposed to the air to provide oxygen to the microorganisms that attach to the disk. Therotation of the disks also serves as a means for removing excess solids by shearing forces. The sloughed bio-solids inthe effluent go to a clarifier to separate the sloughed solids and treated effluent. The RBC units can be designed to bestaged in parallel, series or both.Another variation is the submerged type RBC equipped with air capture cups that use air to rotate and aerate the bio-disks. Covers are sometimes provided to control odors and temperature changes. Removal efficiencies range from 70to 95 percent depending on the hydraulic loading and oxygen demand. In the past few years, mechanical modificationswere made to the RBCs to enhance its reliability and performance. They are simpler to operate and sometimes lowerin cost versus activated sludge processes. 2, 42

c. Fixed Film Packed and Fluidized Bed Reactors - These systems are able to treat more dilute wastewater streams(with BOD5 < 100 ppm (mg/L) concentration) than activated sludge systems. These systems include: GranularActivated Carbon “Fluidized" Bed, Sand “Fluidized" Bed, Fixed Bed with Expanded Clay, Fixed Film Bio-tower, Biostyr,Moving Bed Biofilm Reactor (MBBR), and Captor Media Activated Sludge Process. All these systems use the sameprinciple as the other attached growth biological treatment systems. Each system varies in the type of media that thebiomass adheres to. Not all these systems have been tested by Exxon. The MBBR technology is being applied at theBaton Rouge Chemical Plant and most other applications have been designed to treat contaminated groundwater.39

5. Anaerobic Treatment - The anaerobic treatment process is a biological treatment process that occurs in the absence ofdissolved oxygen. The suspended or attached growth microorganisms “consume" the soluble organic material byconverting it, in the presence of nutrients, to a gas mixture consisting of CH4, CO2, H2S, water vapor, and moremicroorganisms. The microorganisms obtain their energy from organic (i.e., alcohols and ketones) or inorganic (i.e.,carbonates and sulfates) compounds in the feed. In this biological process, wastewater is mixed with recycled bio-sludge





and then digested in a reactor sealed off from the entry of air. The contents of the digester are mixed completely. Theanaerobic sludge is recycled back to mix with the influent wastewater while the effluent is discharged for further treatment.The anaerobic process has a low microorganism growth rate, and therefore, the disposal of excess bio-sludge is minimized.There are several types of anaerobic reactors: fixed bed, fluidized bed, expanded bed, conventional digester, recycled bed,recycle flocs, and upflow anaerobic sludge blanket reactor. Some of these reactors contain fixed films that supply a surfacearea for the microorganisms to grow.The anaerobic treatment process becomes stable and economical when the influent BOD5 wastewater concentration isgreater than 3000 ppm (mg/L) and is mainly soluble organics. However, this process is sensitive to excessive oil, pH, andsalt variations. The BOD5 removal efficiency ranges from 60 to 90 percent. The effluent from the anaerobic digester is notsuitable for discharge to a receiving water body since the BOD5 is typically greater than the discharge limit and thereforemust be treated further.43, 46

Non-Biological Treatment

1. Air/Gas Stripping - Stripping is a process where a wastewater, containing volatile organics, is contacted with either gas,air, or steam to transfer the volatile organics into the vapor phase. Stripping occurs in a tower where the liquid and vaporflow is countercurrent (liquid flow is downward and vapor flow is upward). A stripping tower is equipped with a liquid andgas distributor and either trays or packing to enhance the mass transfer between the vapor and liquid. Henry's Law can beused to determine the affinity the volatile organics have for the vapor phase and thus determining if stripping is feasible.There are several areas where stripping to remove volatile organics might be considered over biological treatment. Forexample, current U.S. regulations require special treatment of wastewater streams with > 10 ppm (mg/L) benzene prior toentering the wastewater treatment plant. Gas and steam strippers are currently being used to remove the benzene.Another example is air stripping of groundwater that contains low concentrations of volatile aromatics.Selection of the vapor medium will depend upon what is available in the refinery or chemical plant and on the quality of thewater. When using steam, scaling in the tower can be a potential problem if the system temperature approaches thesolubility of compounds in solution (i.e., calcium, magnesium, etc.). Using steam allows for the organics to be recovered bycondensing the overhead and separating the organic phase. When using gas, it may be possible to return the gas to theplant gas system or send it to the flare. For air stripping, special consideration must be given to the handling the overheadair stream, particularly if air emissions are of concern.

2. Granular Activated Carbon (GAC) - Granular activated carbon [diameter greater than 0.004 in. (0.1 mm)] can be used toremove dissolved organics from waste streams by adsorption. GAC is usually considered a polishing treatment process forwastewaters that have been biologically treated because of the high carbon replacement costs, but it can also be used totreat low BOD or COD (BOD5 < 100 mg/L) wastewater streams directly. The surface area, pore size distribution, andregeneration characteristics of the carbon are a result of the materials used to produce the carbon and specific preparationprocedure.A fixed bed column is typically used to contact wastewater with GAC and is operated in series or in parallel with influentwastewater introduced at the top of the column and withdrawn from the bottom. Carbon is held in the column with an

➧ underdrain at the bottom of the column. Other types of activated carbon beds have been used, such as expanded bed andmoving bed carbon contactors have also been developed for GAC, but are not generally recommended. In the expandedbed process, the influent is introduced at the bottom of the column. In the moving bed system, spent carbon is replacedcontinuously with fresh carbon. Activated carbon beds should be preceded by a granular media filtration step to preventbed plugging. Aside from the moving bed, spent GAC will have to be periodically removed from the column and returned

➧ to the manufacturer for regeneration. The preferred configuration of fixed bed activated carbon systems is downflow servicemode and upflow air and water backwashing.A wastewater treating system that includes an activated carbon unit can remove 80 to 95 percent of the BOD5 in wastewaterand can produce an effluent containing 1 ppm (mg/L) or less of oil and less than 5 ppm (mg/L) suspended solids. However,activated carbon units do not effectively remove low molecular weight oxygenates such as alcohols, ethers, organic acidsalts, etc. See Decision Tree Figure 3 for details.47, 51, 54, 85

3. Chemical Oxidationa. Chemical Oxidation (Peroxide, Chlorine Dioxide, Ultraviolet, Ozone) - Chemical oxidation is a process to reduce

➧ the oxygen demand and/or dissolved organics (phenolics), and odorous compounds in in wastewaters. In general, itshould only be considered versus other treatment technologies when the contamination is at low concentrations, lessthan 50 ppm (mg/L) on a BOD5 basis, due to the high cost of chemicals. The process works similar to biologicaloxidation in that organics are chemically oxidized to CO2 or carbonates and inorganics are oxidized to their higheststate (e.g., SO3 and SO4).Hydrogen peroxide, , chlorine dioxideand ozone are preferred versus other oxidizing chemicals, such as chlorine,hypochlorite, bromine, and permanganate. Chlorinated and halogenated chemicals can be used for inorganic oxygendemand reduction but are not recommended for organics contaminants due to the potential of forming chlorinated or



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halogenated organics that are more toxic than the original organics. Permanganate is not usually recommended sinceit adds manganese to the water effluent and imparts a color which may be restricted by governmental regulation.Ultraviolet (UV) light is frequently coupled with ozone and/or hydrogen peroxide, since it accelerates the oxidation andlowers the chemical dose. UV, however, requires wastewaters with little turbidity, suspended solids, and oil.87

b. Wet Air Oxidation - Wet air oxidation is a high temperature, high pressure air oxidation process that reduces oxygendemand and changes some organics into compounds that are more easily biodegraded. Hence, its application is onlyfor low flow wastewater streams with BOD5 values in thousands of ppms (mg/L). Minimum temperature and pressuresare generally about 300°F (150°C) and 300 psi (2,070 kPa). The reaction takes place in a pressurized vessel that hasa residence time of 30 to 60 minutes based on stream flow.


1. Additivesa. Powdered Activated Carbon (PAC) - Powdered activated carbon has been used to enhance the activated sludge

process. The carbon adsorbs the dissolved organics that may be toxic or resistant to biological treatment and providesa solid surface for microbes to attach to in suspension. PAC has been used for spot treatment of organic surges andmay help in meeting effluent toxicity requirements.

b. Zeolites - The addition of zeolites into a biological treatment unit is a future technology that is being evaluated forapplication. Some prior research has shown that specialized zeolites can adsorb ammonia from the wastewaterinfluent in the activated sludge unit. The zeolites decrease the peak ammonia load and can help in biological

➧ nitrification of ammonia to nitrate. Zeolites or other ion exchangers have the disadvantage of producing a concentratedliquid regenerant stream that can be difficult to disposed of , or if used for single application, generates a significantvolume of waste material that needs disposal.

c. Fixed Film/Attached Growth - The addition of fixed film or attached growth media into the activated sludge system isa future technology enhancement that has been selectively applied in sanitary treatment. Research has shown that theaddition of fixed film media can increase the concentration of biomass in the system as well as keep the biomass in thesystem (particularly slow growing nitrifying microorganisms) longer to prevent washout.

d. Cultured Microbes - The addition of cultured microbes (microorganisms) into the biological treatment unit is to bestudied further for its application. Some research has shown that special microbes can be added to improve theremoval of specific compounds. Also, these microbial products (solid or liquid cultures) can assist in starting up orrecovering from upsets in biological systems. They are not recommended for routine use.

e. Enzymes/Vitamins/Specialized Additives - The addition of enzymes/vitamins and other specialized additives intoBIOX are claimed to improve the removal efficiencies of existing BIOX units. Limited evaluation of these has beendone. Pilot testing is recommended prior to applications.

2. Clarifier Replacement With Ultrafiltration (UF) Membranes - BIOX clarifier replacement with ultrafiltration membranes isa relatively new technology that has not been fully tested by Exxon. Some research has shown that ultrafiltrationmembranes achieve lower TSS effluent concentrations than conventional clarifiers. It has the potential to replace a clarifierplus dual media filter setup to meet stringent TSS effluent limits. However, membrane plugging, replacement frequency,and potential high initial cost require thorough analysis prior to installation.

NUTRIENTS/AMMONIA REMOVAL/NITRIFICATIONWastewater streams with high concentrations of ammonia are generally first treated in sour water strippers (SWS). SWSbottoms and other plant wastewater streams with low ammonia concentrations (usually below 50 ppm (mg/L) as nitrogen) aregenerally treated with biological treatment, activated sludge or lagoons, if needed to meet regulatory requirements. Nitrogencompounds can be both transformed and removed (in microbe mass as it grows) in these systems. Nitrogen compounds areregulated like phosphorus (though generally not in petroleum/petrochemical wastewaters) because they can aggravateuncontrolled vegetation growth (eutrophication) in receiving water bodies. This area of nutrient and ammonia removal can bequite complex and expert advice should be consulted. For example, organic nitrogen compounds, such as MEA(monoethanolamine), are biologically converted to ammonia in addition to some of the nitrogen being consumed to grow moremicrobes.Nitrification is a biological process where ammonia is converted/oxidized to nitrite and then nitrate via two separate stepsinvolving different microorganism species. Typically, biological treatment units for normal operating temperature (70 - 90°F or21 and 32°C) are designed with large enough hydraulic (greater than 12 hours) and bio-sludge retention times (greater than 15days) to allow nitrification to occur at the same time other microorganisms are removing oxygen demand and organiccontamination. Nitrification requires significant amounts of oxygen and reduces the pH of the wastewaters so these factors

➧ must be considered in an aeration system and pH control system design.41In recent years, some locations have had to reducetotal nitrogen in their wastewater effluents. Nitrate can be further removed in the total biological system by installingdenitrification sections, that operate under no-free dissolved oxygen. The denitrifying microbes use NO3 as their oxygen source





and require the proper organic carbon to nitrogen ratios. System employing nitrification and denitrification can be complicatedand EMRE environmental engineers should be consulted on potential applications.

SUSPENDED SOLIDS REDUCTION (TSS)In many plant situations, wastewater treatment equipment installed for removal of suspended oil or other contaminants will alsoremove suspended solids to regulated levels (oil/water separators, flotation units, and clarifiers on biological units). However,some locations may not have some of these treatment units and are confronted with the need to reduce effluent suspendedsolids. Hence, the addition of gravity sedimentation or filtration processes may be needed.56, 57

Media Filtration

See discussion on media filtration under Subsection on “Secondary Oil and Suspended Solids Removal" of this Design Practice.Other types of filters (cartridge and microscreen), not discussed in the Subsection on “Secondary Oil and Suspended SolidsRemoval," have been used for specialized applications but generally are not cost effective for end-of-pipe wastewater treatment.

Gravity Settling

Sedimentation - Sedimentation is a common process employed for the removal of suspended solids from wastewaters either ina piece of equipment, such as a clarifier, or a lagoon/pond. Additional information on clarifier design is in Design PracticeSection XIX-A4.83


Membrane Replacement For Media Filtration - Ultrafiltration (UF) can be used for separating organic species above amolecular weight of about 200 and for removing particles larger than about 1 micron. However, UF has not been demonstratedfor use in petroleum/petrochemical wastewater applications. Research & Development work may show that these membranesystems have merit in the future. (DG 11-2-2)

TOXICITY REDUCTION (ACUTE AND CHRONIC)Some government regulations in certain plant locations may require monitoring wastewater for effluent toxicity (calledbiomonitoring) in addition to specific compounds and quality parameters. The monitoring usually takes the form of bioassaytesting where either fresh water or sea water aquatic organisms are exposed to several percentages of the wastewater effluent.The bioassay test result for acute toxicity is reported as the percent of effluent that resulted in death to a certain percent of thetest organisms. Typically, a LC50 is reported or lethal concentration that caused death to 50 percent of the organisms in aspecific time period. Acute toxicity tests are generally conducted either for 24, 48, or 72 hours.Some locations are required to perform these tests continually and other locations are required to perform new, moresophisticated “chronic" toxicity tests that assess test organism reproduction and growth as well as mortality or survivability. Thespecific procedures for performing these tests and their accurate interpretation, which can have costly implications on treatmentfacilities, are complicated. Contact ExxonMobil Biomedical Sciences, Inc. (EMBSI) or EMRE Environmental EngineeringSpecialists for advice in resolving technical issues associated with toxicity testing (biomonitoring) or their impacts on wastewatertreatment technology.In general, wastewaters treated with well operated biological treatment systems are of a high quality and have low effluenttoxicity and contaminant levels. For typical refinery/petrochemical plant wastewaters, biological treatment is the first choice inmeeting effluent toxicity because it removes several classes of organic and inorganic compounds that can cause toxicity to

➧ aquatic organisms. Activated carbon, whether in powered form or in fixed bed adsorbers can be used to meet low toxicityeffluent requirements, if needed, after biological treatment.

Enhanced Biological Treatment

Extended Aeration (EA) is an enhancement to the biological treatment process to reduce the toxicity of wastewater effluents.The EA process is an activated sludge process with long sludge residence time (SRT > 20 days) and longer hydraulic retentiontime (HRT > 18 hours) that may be used to degrade certain residual organics and ammonia that are not removed in lower HRT(< 18 hours) activated sludge systems.Not all microorganisms are equally effective at degrading particular pollutants. If a stable microbial community can beestablished which is capable of reducing certain toxic contaminants, additional toxicity reduction may result. This may requireseeding the system, possibly with special additives or microorganisms. Bioaugmentation is the addition of cultured microbeproducts containing different strains of microorganisms to wastewater with the purpose of providing a sufficient quantity anddiversity of microorganisms to improve the treatment performance of the wastewater treatment system.The effectiveness and cost of establishing a new biological population or activated sludge process enhancement should becarefully investigated in bench-scale or pilot-scale prior to implementation.



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Activated Carbon

Effluent polishing may be needed to reduce toxicants in the treated wastewater to meet strict effluent requirements. Activatedcarbon would not normally be needed at locations with well operated biological systems. However, the presence of “toxic," non-polar organics, and low concentrations of inorganics (such as metals) can cause toxicity in the effluent. These compounds inthe wastewater may be reduced with activated carbon treatment. Activated carbon is supplied either in granular (GAC) orpowdered (PAC) form.PAC is typically added directly to aerated basins in biological systems for spot treatment of organic surges. GAC is used as aseparate piece of treatment equipment to remove specific organic compounds in an equipment configuration that resemblesmedia filtration. GAC is also an excellent filter, but accumulation of suspended solids on the carbon will rapidly deteriorate theperformance for the adsorption of organics. Hence, some sort of pretreatment with filters is needed prior to GAC. Bench-scaletests often determine the type and grade of carbon and how much to use.


1. Membranes - Membranes can potentially be used for removal of organic or inorganic compounds that contribute to effluenttoxicity. Although membrane technology is not new, the use of membranes for toxicity reduction is in the developmentstage, and its costs are high compared with other conventional technologies. Ultra filtration may be an option for removinglarge organic molecules that are not fully degraded in a biological unit. Reverse Osmosis (RO) or “nano" membranes maybe an option for removal of metals or other inorganics that are in the ion form such as nitrite instead of using ion exchangetechnologies.The reject stream from the membrane unit that is concentrated with the toxic compound(s) will likely require specialhandling for disposal. Depending upon the feed quality, pre-treatment may be required since the membranes are

➧ susceptible to plugging from oil and suspended solids. Generally, membrane technology has not been employed inrefineries or petrochemical plants for end-of -pipe treatment because of the high initial capital cost and the uncertainty inlong term reliability, without frequent replacements.

2. Wetlands/Aquatic Plants - Wetlands or aquatic plants are being used in some locations to remove residual BOD, TSS,nitrogen, and metals from municipal wastewater and from industrial wastewaters as a final polishing step to reduce effluenttoxicity. Most of the wetlands are artificially constructed but some facilities use natural wetlands. Design of the artificialwetland is critical to the performance of the system and must consider weather conditions, plant and soil type, physical size,flowrate and method of flow (subsurface or surface).The advantage of using wetlands is low capital and maintenance costs, and the system has a high resistance tocontaminant shocking. The large land area required for operation is a disadvantage and obtaining a permit to operate thefacility can potentially be difficult.

DISSOLVED SOLIDS/METALS REDUCTION

Chemical Precipitation and Flocculation

Dissolved solids and metals can be removed from wastewater by adding chemicals that cause the compound to precipitate outof solution (converts contaminant from liquid to solid form). Flocculation is the agglomeration of the precipitate into largercolloidal particles that will settle. Filtration after settling may be required to reduce the precipitated suspended solids in thewastewater.Chemicals typically used for the precipitation of dissolved solids include aluminum sulfate, ferric chloride, ferric sulfate, ferroussulfate, lime, and activated aluminum. Sludges generated from aluminum and iron salts contain insoluble gelatinous aluminumor ferric hydroxide and are difficult to dewater. The sludge generated from the use of lime consists mainly of calcium carbonateand magnesium hydroxide and the volume of sludge generated is significantly larger than that generated with the other metalsalts.Metals precipitation is generally done by altering the wastewater pH with either lime or caustic to where the metal(s) of interesthave minimum solubility and precipitate in the hydroxide form. The presence of cyanide and ammonia can inhibit the removal ofmany metals. Concentrated metals in the sludge may make disposal difficult.

➧ Iron co-precipitation is the likely choice for end-of -pipe treatment for selected metals. EMRE wastewater specialists should beconsulted for potential applications.

Biological Removal

Biological treatment, using processes described in the Subsection on “Secondary Oil and Suspended Solids Removal" of thisDesign Practice, can be used to remove metals. This process works through either the adsorption of some metals into themicroorganism cell wall, or by the uptake of some metals by the microorganisms to satisfy their micronutrient requirements.64





Ion Exchange

Ion exchange involves the use of specially prepared resins which substitute either cations or anions with other ions in solution.This process replaces the undesirable ions with ones that are more desirable or less harmful (i.e., calcium can be exchanged forsodium to reduce scaling). Ion exchange beds are generally used for making either softened or deionized boiler feedwater.When used for softening, there is no net decrease in the total dissolved solids. Resins exist specifically for the exchange ofselected metals. However, these resins are expensive and other alternatives for removing metals should be evaluated.Each resin has a specific exchange capacity which must be regenerated when the capacity is exhausted. Cationic resins areregenerated with strong acids such as sulfuric or hydrochloric and sodium hydroxide is typically used to regenerate anionicresins. The spent regenerant may require special disposal.Suspended solids can plug the ion exchange beds, resulting in high headloss and inefficient operation. Oil can foul the resinresulting in reduced capacity. Hence, pretreatment with filtration is generally needed.

Activated Carbon

Activated carbon is normally used to remove low concentrations of dissolved organic contaminants from wastewater streams.However, it can, in very special cases, be used to remove heavy metals such as arsenic and mercury that may be difficult toremove by other methods, such as precipitation. Consult ER&E environmental engineering specialists prior to considering thisoption compared with other alternatives, such as biological treatment, ion exchange or precipitation.

Membranes

Reverse Osmosis (RO) membrane technology may be an option for removing metals from a wastewater stream. The use of thistechnology for metals removal from wastewater is relatively new. RO treatment of biologically treated refinery wastewater hasbeen commercialized outside of Exxon. RO treatment of raw (non-biologically treated) wastewater was investigated by Exxon onstripped sour water. However, fouling issues prevented progression of this technology beyond the pilot phase. For either raw ortreated wastewater, pretreatment of the water prior to the RO unit is critical to the success of the application and pilot testing isrecommended.

Evaporation

Evaporation is an option to remove dissolved salts or metals. However, it would be rarely considered in petroleum/petrochemical plants due to its high cost. It may have an application to concentrate particularly high strength waste streams fordisposal by other means.

WASTEWATER SLUDGE HANDLING (VOLUME REDUCTION)Almost all facilities that treat wastewaters will generate sludges. Hence, equipment to remove, store, and reduce the volume ofthese sludges for further treatment/destruction or disposal must be considered in wastewater treatment facility design. Becauseof the high cost of sludge treatment/disposal, volume reduction by dewatering is generally very cost effective.In most cases, it is cost effective to provide thickening followed by dewatering equipment for wastewater treatment type sludges.In some cases, the solid content is high enough that the sludge can be dewatered directly without thickening. See Figure 6.65-79

Sludge Thickening

1. Gravity Thickeners - The operation of a gravity thickener is similar to that of a clarifier, but its purpose is to thickensuspended solids removed from wastewater treatment units as sludges (i.e., API sludges), for volume reduction. Theconcentrated solids are collected in the underflow of the thickener, while the aqueous effluent flows over a weir at the top.This overflow is usually recycled back to the front end of the wastewater treatment plant. Chemical conditioners, such ascoagulant aids or polyelectrolytes, are sometimes added to the feed to improve solid/liquid separation.A well-operated thickener will recover at least 95 percent of the incoming solids in the underflow. The concentration of thesolids in the underflow are typically 2 to 3 wt% for biological solids, about 5 wt% for filter backwash, and 10 to 15 wt% forAPI separator bottoms.

2. Dissolved Air Flotation Thickeners - Dissolved Air Flotation (DAF) thickeners are used to concentrate sludges which aredifficult to settle in a gravity thickener, such as biological solids from the activated sludge process, or media filter backwashwater. Most of the DAF thickener effluent water is generally returned to the upstream part of the wastewater plant, but somemay be recycled to be pressurized in the DAF air saturation drum.Higher loading can be used with flotation thickeners than are permissible with gravity thickeners because of the rapidseparation of solids from the wastewater. Float concentration can vary with the speed of the float skimmer, amount ofentrained water, and type of feed. Chemical conditioning is almost always necessary for effective flotation thickening.Biological sludges may be concentrated to about 4 to 5 wt% with chemical conditioners. Oily sludges can be concentrated



XIX-A 21 of 35



to 10 wt% or more. See Subsection on “Secondary Oil and Suspended Solid Removal" of this Design Practice and DesignPractice XIX-A2.81

Dewatering Equipment

1. Plate and Frame (Pressure) Filter Presses - A plate and frame filter press consists of a series of parallel plates, eachfitted with a filter cloth and rigidly held together in a structural frame. The solids are captured between the filter cloth whileexcess water passes through the cloth. At the end of the pressure filtration cycle, the plates are separated, and the solidsdrop into a container. The water filtrate is normally returned to the upstream part of the treatment plant.There are several advantages of plate and frame filter presses over other filters and simple dewatering devices. This typeof press is relatively simple and durable, and a limited number of plates may often be added to accommodate throughputincreases even after the press has been installed. It can normally achieve the highest solids content in the filter cake (bestdewatering) except for dryers, and the recovery of oil may be possible.The disadvantages associated with a plate and frame filter press include mechanical complexity, chemical costs, andlimitation on filter cloth life. Also, if solids conditioning is inadequate, the filter cake may not drop from the filter cloth whenthe plates are opened resulting in the need for manpower to remove the cake. The operation is not continuous, and,therefore, sufficient feed storage must be provided. Plate and frame filter presses are normally housed in a building whichincreases the cost of the treatment facility. If the sludges are easy to dewater, this equipment has comparable manpowerrequirement to other dewatering equipment. Otherwise, manpower is higher than with other equipment.

2. Gravity Belt Filter Presses (GBFP) - These units are continuous, mechanical, sludge dewatering devices. They use singleor double moving belts to dewater sludges continuously through one or more stages of dewatering. All belt press filtrationprocesses operate with three basic stages - chemical conditioning of the feed, gravity drainage of the sludge anddewatering of the drained sludge by shear and compression.Exxon locations generally use GBFP for dewatering dissolved air flotation (DAF) oily float and biological sludges. However,certain sites dewater API separator bottoms or mixtures of WWTP sludges and other high-water-content wastes. Little oilrecovery is accomplished in GBFP when oily sludges are dewatered because the GBFP operating pressure is too low toexpel the oil.The advantages associated with the GBFP are effective water removal compared to other dewatering devices (althoughlower than plate and frame filters), and the costs are sometimes lower compared to other types of filters and centrifuges.The belt weave is more porous than the cloths on plate and frame or vacuum filters and is, therefore, less likely to plug.

3. Thermal Dryers - Thermal dryers are used to remove moisture through evaporation. Wastes, such as dewateredwastewater sludges, slop oil emulsion solids, tank bottoms, and contaminated soils, can be dried to achieve heating valueincrease, volume reduction, or removal of some specific constituents. The dried residues can be shipped to cement kilns,incinerators, or secure land farms/landfills for final disposal, depending on circumstances. New, high temperature dryersmay remove significant amounts of volatile and mid-boiling range hydrocarbons to allow disposal of the material as anindustrial “non-hazardous” sludge.The advantages of the thermal dryers are, first, that significant volume reductions are achievable (75 to 80 percentreduction for dewatered DAF and BIOX sludges) and units can either be purchased or rented from a contractor. Theheating value of the dried sludge (residue) makes it more attractive for disposition to a cement kiln.The disadvantages of the thermal dryer relate to the disposition of the condensed off-gas, and the mechanical reliability ofthe dryer's feed and discharge conveyors.

4. Centrifuges - Centrifuges can be used in thickening service as well as dewatering. The process of centrifugation is anacceleration of the process of sedimentation by the application of centrifugal forces. There are two principle types ofcentrifuges generally used for thickening and dewatering of wastewater sludges: solids-scroll and disc centrifuges. The disccentrifuge depends on magnifying the tendency of particles to settle by using a high gravitational force (> 1,000 timesgravity). Scroll centrifuges can potentially achieve the highest solids concentration in a product cake whereas disccentrifuges can provide three products, oil, water, and a solids slurry stream. Scroll centrifuges are more commonly usedthan disc centrifuges within Exxon.Centrifuges have the following advantages: they occupy a small space for a large throughput, and unlike filters, they do notrequire a building if adequate cold weather protection is provided. They are also continuous devices, which can minimizethe feed storage capacity required. The system can be almost entirely closed to reduce emissions, if needed.Disadvantages with centrifuges include: wear, fatigue, and erosion of the high speed rotation machinery; capital, costmaintenance requirements, and power costs; and the extent of dewatering is lower than other dewatering devices. Ingeneral, centrifuges may only be cost effective where relatively high volumes of solids are being generated or where plotspace is limited.





REFERENCES

DEFINITIONS

1. Clesceri, L. S., A. E. Greenberg, R. R. Trussell, (ed), “Standard Methods for the Examination of Water and Wastewater,"American Public Health Association, Washington (1993), 23rd Edition.

2. Eckenfelder Jr., W. W., “Industrial Water Pollution Control," 2nd Edition, McGraw-Hill, New York (1989).3. Grady Jr., C. P. L., H. C. Lim, “Biological Wastewater Treatment, Theory, and Applications," Marcel Dekker, Inc., New York

(1980).4. Water Pollution Control Federation, Manuals of Practice.

WASTEWATER SOURCE MANAGEMENT AND REUSE

5. Balmer, R. G., “Reuse of Wastewater as Cooling Tower Makeup-An Interim Report," EE.054E.84, (June 1984).6. Balmer, R. G., K. J. Fetcho, “Reuse of Wastewater as Cooling Tower Makeup-A Final Report," EE.058E.85, (September

1985).7. Balmer, R. G., R. S. Lewis, “Economics of Dissolved Solids Removal for In-Plant Wastewater Reuse," EE.17E.84, (February

1984).8. Batch, R. C., “Treatment of Concentrated Wastewater Streams for Reuse," EE.83E.82, (July 1982).9. Batch, R. C., “Reverse Osmosis Treatment of Chemical Plant Process Water for Reuse," EE.082E.82, (July 1982).10. Beychok, Milton R., “Aqueous Wastes from Petroleum and Petrochemical Plants," Great Britain, The Gresham Press, 1973.11. Biemel, G. D., “Practical Methods for Determining Stormwater Runoff Rates and Quantities at Industrial Sites," Proceedings

of the 46th Industrial Wastes Conference, Lewis Publishers, Chelsea, MI, pp. 893, (May 1992).12. Creighton, P. J., “Wastewater Reuse in Chemical Plants," EE.068E.82, (September 1982).13. Goodrich, Jr., R. R., et al., “Guideline for Reducing Waste Treatment Costs," EE.48E.85, (July 1985).14. Goodrich, Jr., R. R., G. M. Grey, S. A. Kaczmarek, “Bayway Refinery Wastewater Flow and Solids Reduction Project,"

EE.001DC.86, (May 1986).15. Kaczmarek, S. A., P. S. Shah, “The Need for Neutralization and Equalization in Sarnia WETS," EE.003E-T.85, (April 4,

1985).16. Penman, B. R., “Reverse Osmosis Treatment of Refinery Process Wastewater Streams for Reuse," EE.121E.81,

(November 1981).17. Penman, B. R., “Screening of Process Wastewaters for Reuse," EE.073E.80, (August 1980).

EFFLUENT QUALITY

18. Peskin, H. M., E. P. Seskin, “Cost Benefit Analysis and Water Pollution Policy," The Urban Institute (Washington, DC,1975).

19. Quinn, James J., “Water Quality Management; Policy and Practice in Selected Countries," Discussion Paper No. 072,American Petroleum Institute, (March 1992).

20. Shah, P. S., “Exxon Company International Affiliates' Environmental Assessment," 91 EEEL.577, (August 1991).21. The Clean Water Act of 1987, Summary of U.S. EPA regulation, Water Pollution Control Federation Publication No.

P0070R, (July 1987).22. World Health Organization, International Health Regulations (1969), Third Annotated Edition, Geneva, 1983.23. Water Quality Criteria, U.S. Environmental Protection Agency.



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REFERENCES (Cont)

ESTIMATING CONTAMINANT CONCENTRATIONS/MASS LOADS

24. Devine, F.A., et al, “Onsite Process Units Wastewater Source Load Study," Report No. 50012. (February 21, 1975).25. Urban, D. B., R. R. Goodrich, Jr., “Refinery Process Unit Wastewater Load Factors-Final Report," EE.086E.86, (October

1986).26. Urban, D. B., “Refinery Process Unit Wastewater Load Factors-Interim Report," EE.014E.84, (February 1984).

PRIMARY/SECONDARY OIL REMOVAL (OIL/WATER SEPARATION)

27. Bennet, G. F., “The Removal of Oil From Wastewater by Air Flotation-A Review," CRC Critical Review in EnvironmentalControl, Environmental Progress, AIChE, New York.

28. Cozenwith, C., “Oil Removal for Wastewater-A State of the Art Review," EE.166E.72, (December 1972).29. Green, G., R. R. Goodrich, Jr., “Package Oil-Water Separators for Oily Water Applications," EE.9E.87, (February 1987).30. “Manual on Disposal of Refinery Wastes," Volume on Liquid Wastes (Chapters 5 and 6), American Petroleum Institute

(1969).31. “Monographs on Refinery Environmental Control-Management of Water Discharges-Design and Operation of Oil-Water

Separators," API Publication 421, First Edition, (February 1990).32. Nocella, C. A., J. B. Wilkinson, “Economic Evaluation of Advanced Wastewater Pretreatment Processes," EE.073E.83,

(October 1983).33. Nocella, C. A., “Field Evaluation of Induced Air Flotation versus Dissolved Air Flotation," EE.021D.80, (March 1980).34. Thibault, G. T., “Improvement of Pretreat Dissolved Air Flotation," EE.17E.78, (January 1978).

➧ 35a.Feerick, C.P., P. Edwards, A. Boone, “Evaluation of the Daburt Oil/Water Separator At the Fife Ethylene Plant”, EE.20E.94,(February 1994).

OXYGEN DEMAND REDUCTION-BIOLOGICAL TREATMENT

➧ 35b. Baird, Rodger B. adn Roy-Keith Smith, Water Environment Federation, "Third Century of Biochmical Oxygen Demand",copyright, ISBN-1-57278-171-8, 2002

36. Churchman, J. W., “The Effects of Temperature on Biological Treatment Systems," EE.023E.80, (May 1980).37. Dennis, R. W., “Screening Potential Problematic Pollutants in Biological Wastewater Treatment," EE.093E.81, (September

1981).38. Dennis, R. W., L. A. Lau, “Effect of Powdered Activated Carbon and Solid Residence Time on Biological Wastewater,"

EE.098E.81, (October 1981).39. Fort, L. R., “Enhanced Water Effluent Treatment System," 92 ECS2 104, (November 23, 1992).40. Kaczmarek, S. A., “Biological Treatment for Upstream Concentrated Wastewaters," EE.046E.84, (June 1984).41. Kaczmarek, S. A., “Nitrification in Refinery Wastewater Treatment," EE.062E.85, (October 1985).42. Lindquist, L.A., “Attached Growth Performance of RBC during Cyanide Shock Load," EE.86E.80, (July 1980).43. Mattress, R. A., “Waste Minimization Via Anaerobic Filtering-A Pilot Plant Study,” EE.79E.91.44. Palis, J. C., “Effects of Solid Retention Time and Powdered Activated Carbon on BIOX Performance," EE.103E.82,

(September 1982).45. Shah, P. S., “Effectiveness of Phenobac Demonstrated in Activated Sludge Test at Benicia Refinery," EE.34E.78, (March

1978).46. Stein, L. B., “Waste Minimization Via Anaerobic/Membrane Treatment for Source Control," EE.13E.90, (February 1990).47. Tchobanoglous G., F. L. Burton (edited for Metcalf & Eddy, Inc.), “Wastewater Engineering Treatment, Disposal, and

Reuse," 3rd Edition, McGraw-Hill, New York (1991).48. Thibault, G. T., “Wastewater Treatment by Aerobic Biological Oxidation Alternatives to Activated Sludge," EE.62E.76, (June

1976).49. Goodrich, R.R., A.M. Vincitore, “Exxon Co. USA, Philadelphia Marketing Terminal Tank Draw-off Water Treatment Project,”

93 ECS2 015 (March 1993).





REFERENCES (Cont)

DISSOLVED ORGANICS REDUCTION

50. Altemoeller, P. H., “In-Plant, Packaged Wastewater Treatment Technologies, Removal of Volatile Compounds,"EE.046E.90, (July 1990).

51. Balmer, R. G., P. B. Thibault, “Tertiary Carbon Adsorption of Convential and Toxic Pollutants from PetrochemicalWastewater," EE.104E.79, (October 1979).

52. Churchman, “The Use of Hydrogen Peroxide in Wastewater Treatment," EE.109E.79, (December 1979).53. Douchand, M. N., “Evaluation of Wet Air Oxidation by Bench Scale Testing," EE.056E.82, (April 1982).54. Fernbacher, J. M., “Activated Carbon Adsorption versus BIOX for Wastewater," EE.3RA.71, (1971).55. “Removal of Benzene from Refinery Wastewater," Health and Environmental Affairs Departmental Report, API, (April 1991).

SUSPENDED SOLIDS REDUCTION/FILTRATION

56. Nocella, C. A., “End-of-Pipe Granular Media Filtration of Process Plant Wastewater," EE.99E.81, (October 1981).57. Shah, P. S., “Novel Wastewater Filters: A State of the Art Review," EE.004E.85, (January 1985).

TOXICITY REDUCTION

58. Altemoeller, P. H., J. H. Higinbotham, “Packaged Activated Carbon Adsorption for Toxicity Reduction/EmergencyApplications," EE.42E.91, (October 1991).

59. Kaczmarek, S. A., R. Kilpert, “The Biomonitoring of Final Effluents," EE.004E.81, (January 1981).60. Kaczmarek, S. A., “Update on Toxics in Wastewater," EE.013E.81, (September 1981).61. Kaczmarek, S. A., “Toxics in Effluents Assessments of Environmental Impacts," EE.074E.81, (June 1981).62. Kaczmarek, S. A., R. R. Goodrich, Jr., “Wastewater Toxicity Troubleshooting," EE.84E.86, (October 1986).63. Kilpert, R., “Toxics in Wastewater Studies," EE.036E.80, (May 1980).

➧ HEAVY METALS REDUCTION63b. API, Publication No. 4676, Arsenic: Chemistry, Fate, Toxicity, and Wastewater Treatment Options, Oct. 1998

DISSOLVED SOLIDS/METALS REDUCTION

64. Goodrich, Jr., R. R., “Metals in Wastewater-BIOX Impact and Water Quality Criteria," 88 ESC2 42, (April 11, 1988).

WASTEWATER SLUDGE HANDLING

65. Altemoeller, P. H., R. R. Goodrich, Jr., “Guidelines for Sizing Gravity Thickeners-Wastewater Treatment SludgeApplication," EE.24E.89, (October 1989).

66. Archibald, B. B., P. S. Shah, “Solvent Extraction for Sludge Deoiling," EE.12E.90, (March 1990).67. Boyer, G. T., R. R. Goodrich, Jr., “Fundamentals of Wastewater Sludge Separation," EE.004DE.83, (May 1983).68. Cashion, B. R., “Waste Activated Sludge Reduction," EE.16E.84, (February 1984).69. Ebihara, T., R. R. Goodrich, Jr., “Biological Decontamination for Oil Contaminated Soils and Oily Sludges," EE.59E.89,

(November 1989).70. Nocella, C. A., “Oil Recovery from Dissolved and Induced Air Flotation Floats," EE.031E.81, (April 1981).71. Palis, J. C., J. L. Duncan, “An Evaluation of BP Zero Sludge Effluent Treatment Process," EE.070E.82, (June 1982).72. Palis, J. C., “Hazardous Wastewater Treatment Sludge Reduction: Separator Bottoms Solids Identification and DAF Float

Recycle Feasibility," EE.1DE.84, (December 1984).73. Powell, R. W., “Aerobic Digestion of Mixtures of Oily and Biological Wastewater Treatment Sludges," EE.81E.79, (August

1979).74. Schinner, C. M., “Refinery Sludge Thermal Reuse in CO Boilers," EE.32E.93, (February 1, 1993).75. Shah, P. S., J. C. Palis, “Secondary Oily Sludge Recycle to API Separators," EE.47E.85, (July 1985).



XIX-A 25 of 35



REFERENCES (Cont)

76. Stone, W. A., “An Economic Comparison of Aerobic and Anaerobic Sludge Digestion, " EE.140E.82, (December 1982).77. Wang, J. S., “Guidelines for Evaluating Thermal Reuse Options For Disposal of Plant Wastes," EE.67E.89, (December

1989).78. Wang, J. S., “Application of BREAXIT OEB-9 to Breakup Emulsion and Reduce Waste Disposal Cost," EE.18E.93,

(February 1993).79. Wang, J. S., “Guidelines for Evaluating Thermal Reuse Options for Disposal of Plant Wastes," EE.67E.89, (December

1989).

EXXONMOBIL ENGINEERING DESIGN PRACTICES

80. Section XIX-A1 Primary Oil / Water Seperators (Rev. 1999)81. Section XIX-A2 Flotation Units (Rev. 1999)82. Section XIX-A3 Media Filtration (Rev. 1998)83. Section XIX-A4 Chemical Flocculation / Specific Ion Removal and Clarification of Wastewater (Rev. 2001)84. Section XIX-A5 Biological Treatment of Wastewater (Rev. 2001)85. Section XIX-A8 Activated Carbon Treatment (Rev. 2001)86. Section XIX-A9 Water/Wastewater Chemical Feed Systems (Rev. 2001)87. Section XIX-A11 Chemical Oxidation (Rev. 1997)88. Section XIX-B Water Reuse (Rev. 1997)

EXXONMOBIL ENGINEERING GLOBAL PRACTICES

89. GP 3-2-1 Sanitary and Industrial Sewer Systems

EXXONMOBIL ENGINEERING WATER AND WASTEWATER DESIGN GUIDE

90. DG 11-1-1 Granular Media Filters91. DG 11-2-1 Fixed Bed ION Exchange Water Treating Units92. DG 11-6-3 Chemical Feeders for Wastewater Treating93. DG 11-7-1 Wastewater Dissolved-Air Flotation System94. DG 11-8-1 Gravity Belt Filter Press System





➧ TABLE 1TYPICAL REMOVAL EFFICIENCIES FOR SELECTED WASTEWATER TREATING UNITS(1)

WASTEWATER TREATING UNIT PARAMETER REMOVED TYPICAL % REMOVAL

PRIMARY OIL /SUSPENDED SOLIDS REMOVAL (PRIMARY TREATMENT)

API Separator OilSuspended Solids

25 to 200 ppm residual(2, 3)

80%

Plate Separators OilSuspended Solids


80%

Skim Pond OilSuspended Solids


80%

SECONDARY OIL/SUSPENDED SOLIDS REMOVAL (ADVANCED PRIMARY TREATMENT)

Induced Air/Gas Flotation OilSuspended SolidsHydrogen Sulfide


40 to 80%10 to 30%(5)

Dissolved Air Flotation OilSuspended SolidsHydrogen Sulfide5-day BOD


50 to 90%10 to 30%10 to 20%

Media Filtration Oil 10 to 15 ppm residual(2, 4)

(Sand or Mixed Media) Suspended Solids 80 to 95% (<15 ppm residual)

Chemical Flocculation Unit OilSuspended SolidsHydrogen Sulfide5-day BOD

< 40 ppm residual(2, 4)

50 to 60%70 to 80%(6)

10%

OXYGEN DEMAND/DISSOLVED ORGANICS REDUCTION

Activated Sludge OilSuspended SolidsHydrogen SulfidePhenol5-day BOD


30 to 50 ppm residual90%(7)

95 to 99%> 90%

Extended Aeration OilSuspended SolidsHydrogen SulfidePhenol5-day BOD


30 to 50 ppm residual90%(7)

98%> 95%



XIX-A 27 of 35



TABLE 1 (Cont)TYPICAL REMOVAL EFFICIENCIES FOR SELECTED WASTEWATER TREATING UNITS(1)

WASTEWATER TREATING UNIT PARAMETER REMOVED TYPICAL % REMOVAL

OXYGEN DEMAND/DISSOLVED ORGANICS REDUCTION

Biological Oxidation Lagoons OilSuspended SolidsHydrogen SulfidePhenol5-day BOD


50 to 150 ppm residual70 to 90%(7)

70 to 95%60 to 90%

Trickling Filter OilSuspended SolidsHydrogen SulfidePhenol5-day BOD


50 to 150 ppm residual50 to 95%(7)

50 to 75%40 to 80%

Anaerobic Treatment OilSuspended SolidsHydrogen Sulfide5-day BOD

50%(4)

Net increaseNet increase60 to 80%

Granular Activated Carbon Oil < 1 ppm residual(2, 4)

(with some biological activity) Suspended SolidsHydrogen SulfidePhenol5-day BOD

< 5 ppm residual> 90%(8)

> 98%80 to 95%

Notes:(1) These values are given only to provide some guidance. They represent average performances, based on typical refinery

effluent data and are not representative of every case. For example, the presence of material of high solubility can result incontaminant levels higher than those given here.

(2) Measured by n-Hexane extraction of organics in wastewater samples using gravimetric or infrared analysis on the extract. Theoil measurement includes the concentrations of both suspended and dissolved oil.

(3) This is a gross oil removal unit that is not designed on the basis of a residual oil content. The residual oil content will dependupon the size of the oil droplets, the presence of emulsifying agents, etc. Data from existing separators at various Exxonlocations indicate effluent oil contents from 10 to over 500 ppm (mg/L) and considerable variability for a particular location. Thevariation is attributed to hydraulic underload or overload, emulsions, accumulations of silt, etc.

(4) For suitably pretreated feed.➧ (5) Applies only to air flotation units. Addition of iron chloride coagulant or hydrogen peroxide can greatly improve H2S removal.

(6) Chemical flocculation will remove significant amounts of sulfides using iron coagulants but will result in an iron sulfide sludgethat can be pyrophric.

(7) Sufficient aeration required to achieve this removal level.➧ (8) With inlet hydrogen sulfide concentrations up to 50 ppm (mg/L), the hydrogen sulfide will be removed to less than 0.5 ppm if

conditions exist for aerobic biological activity including sufficient dissolved oxygen levels (over 2 ppm).





FIGURE 1WASTEWATER TREATMENT ASSESSMENT DECISION TREE (A)

Notes:(A) Yes/No decisions that are provided are not absolute. They are meant to provide a rough guideline and it is expected that there will be exceptions. Contact ER&E Environmental Specialists for assistance.(B) Regulatory expectations/intepretation may be determined in joint effort with the plant personnel, by discussing issues such asmixing zone effect, abmormal concentration, etc. Then, discussions should be held with Government officials to negotiate finalregulatory limits, to ensure they are clear , not excessive, and have a sound scientfic and technologybasis.(C) Need to account for data variability. 1. If wastewater treatment facilities do not exist or data are not available see: a) References (24 - 26) for characterization of refinery wastewater stream load values or, b) Process designers for estimated flowrates and contaminant concentrations. 2. If facilities already exist a) Generate probability plots for the individual contaminants. b) Select basis that accounts for upset conditions.(D) At source treatment refers to treatment of a single wastewater stream prior to being discharged to the sewer.(E) End-of-Pipe treatment refers to treatment of combined wastewater streams in a stand alone treatment facility.

Identify And Characterize AllMajor Wastewater Stream (C)

Determine Effluent Limits(Regulatory Expections) (B)Start

Determine Amount OfContaminant Reduction

Required ToMeet The Limits

Consider UpstreamReduction, Reuse,and/or Segregation

End-Of-Pipe TreatmentRequired For

All/Remaining WastewaterStreams (E)

At Source Treatment MayReduce End-of-Pipe

Treatment Requirements (E)

Determine TreatmentTechnology To Reduce The

Identified Contaminant

Compare Cost For At SourceTreatment Versus End-of-Pipe

Treatment

AfterTreatment,

Does WastewaterStream Meet All

RegulatoryLimits?

Free Oil Removal - Figure 2Oxygen Demand Reduction - Figure 3Suspended Solids Removal - Figure 4Effluent Toxicity Reduction - Figure 5

Sludge Management - Figure 6

No

Yes

No

No

Yes

Yes

Is At SourceTreatment An Option

For Any Of The Streams?(e.g., Contaminant> 1,000 ppm) (D)

Determine TreatmentTechnology Required ToMeet Regulatory Limits

No Further TreatmentRequired For That

Stream

Is At SourceTreatment Cost

Effective?

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XIX-A 29 of 35



FIGURE 2FREE OIL REMOVAL DECISION TREE (A, B, C)

Start

No Oil RemovalRequired

Is TheOil Limit< 50 ppm

API SeparatorRequired. See Design

Practices SectionXIX-A1 For Details

No Oil Or SuspendedSolids Removal

Required (E)

See Suspended SolidsRemoval Decision Tree(Figure 4) For Details

Primary And SecondaryOil Removal Required

Is InletOil > 1000 ppm?

Is EffluentOil Limit < 15 ppm?

Induced Air FlotationUnit Required. See

Design PracticesSection XIX-A2

For Details

Plate SeparatorRequired (i.e. Parallel,

Tilted, Corrugated,etc.). See DesignPractices SectionXIX-A1 For Details

NoYes

No

Yes

Yes

DoesFacility HaveAn Effluent Oil

Limit? (D)

DoesWastewater

Exceed The OilLimit?

DoesFacility Have A

SuspendedSolids Limit?

Oil Can Be RemovedBy Gravity Settling

Is InletOil > 1000 ppm?

API Separator Required.See Design Practices

Section XIX-A1 For Details.

Plate Separator Required(i.e. Parallel, Tilted,

Corrugated, etc.). SeeDesign Practices Section

XIX-A1 For Details

Consider Dissolved AirFlotation Or Nutshell

Media Filters. See DesignPractices Section XIX-A2

or XIX-A3 For Details

Yes

Yes

Yes

Yes

NoNo

No

No

No

Notes:(A) Yes/No decisions that are provided are not absolute. They are meant to provide a rough guideline and it is expected that there

will be exceptions.(B) For situations that do not fit into this general decision tree, contact ER&E Environmental Specialists for assistance.(C) Selection and design of units are based on free oil removal. However, each treatment unit will remove some degree of

suspended solids.(D) Most regulatory oil limits are based on total oil. Since the treatment units remove free oil, it is suggested the wastewater

samples be analyzed for both free and dissolved oil. 1. If dissolved oil is less than the oil limit, physical separation of the oil will be sufficient. 2. If dissolved oil is greater than the oil limit, additional treatment such as biological treatment or activated carbon may be necessary.(E) It is prudent to provide oil/water separator as prevention against abnormal spill.

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FIGURE 3OXYGEN DEMAND/DISSOLVED ORGANIC REDUCTION DECISION TREE (A, B, C)

Start

Consider AnaerobicBiological

Treatment (E)

No Oxygen DemandReduction Required

Does BOD5Need To Be Reduced

By > 95%?

Does BOD5Need To Be Reduced

By > 90%?

Consider GranularActivated Carbon (DesignPractices Section XIX-A8)

Or Fixed Film FluidizedBiological Reactor (E)

No

Yes

DoesFacility Need

To Reduce OxygenDemand In The

Wastewater (e.g.,BOD, TOC, or

COD)?

Is TheIncoming BOD5> 3000 ppm (D)

Consider BiologicalExtended Aeration. See


For Details

Consider BiologicalActivated Sludge. See


For Details

Consider A BiologicalOxidation Lagoon (F) . SeeDesign Practices Section

XIX-A5For Details

Yes

Yes

No

No

Yes

No

Is TheInlet BOD5

> 100 ppm (D)

Is ThereA Restriction On Land

Availability?

Yes

Consider Biological FixedFilm Treatment Units(e.g., Trickling Filters,

Rotating Biological Contactor,etc.). See Design PracticesSection XIX-A5 For Details

Yes

No

Is ThereA Restriction ForBuilding Earthen

Basins?

No

Yes

Notes:(A) Yes/No decisions that are provided are not absolute. They are meant to provide a rough guideline and it is expected that there

will be exceptions.(B) It is assumed that each biological treatment unit will have clarification facilities for handling the biological solids.(C) Alternatives to the treatment technologies presented exist. Contact ER&E Environmental Specialists for more information.(D) BOD5 is typically the parameter monitored for wastewater oxygen demand. This decision tree uses BOD 5 as a guideline.

Contact ER&E Environmental Specialists if effluent limit is not based on BOD 5.(E) Design Practices section for this technology does not exist, contact ER&E Environmental Specialists for assistance.(F) Use of oxidation lagoons is not a common practice in the U.S. and Europe.

Is TheInlet BOD5

< 500 ppm (D)

No

Yes

No

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XIX-A 31 of 35



FIGURE 4SUSPENDED SOLIDS REMOVAL DECISION TREE (A, B)

Start

DoesFacility Have A

Suspended SolidsLimit?

Suspended Solids RemovalNot Required

DoesFacility Have

Oil/Water SeparationFacilities?

Oil/Water SeparationUnits Can Remove Up To 80% of The Influent

Suspended Solids

DoesFacility Have A

Biological Treatment Unit(e.g., Activated Sludge,

Oxidation Lagoon, etc.) (C) ?

Is TheStream

Non-Process Or CleanRainwater?

A Settling Pond May BeUsed To Reduce

Suspended Solids (D)

Biological Solids WillContribute To Effluent

Suspended Solids

No Biological SolidsTo Handle

Consider Media Filtration,Dissolved Air Flotation or

Clarification (DesignPractices Sections XIX-A3,

XIX-A2, or XIX-A4)

IsClarification

Sufficient To MeetSuspended Solids

Limit?

WillAddition Of

Polyelectrolyte In TheClarifier Improve

Suspended SolidsRemoval?

Consider Media Filtration(Design PracticesSection XIX-A3)

Install PolyelectrolyteAddition Facilities(D)

Further Suspended SolidsRemoval Not Required

Notes:(A) Yes/No decisions that are provided are not absolute. They are meant to provide a rough guideline and it is expected that there will be exceptions.(B) For situations that do not fit into this decision tree contact ER&E Environmental Specialists for assistance.(C) Decision tree assumes clarification is part of a biological treatment unit.(D) Design Practices section for this technology does not exist, contact ER&E Environmental Specialists for assistance.

No

Yes

YesYes

Yes

Yes

Yes

No

No

No

NoNo

DP19AF4





FIGURE 5WASTEWATER EFFLUENT TOXICITY REDUCTION DECISION TREE (A, B, C)

Start

DoesFacility Have A

Toxicity Limit (Acute OrChronic)?

Can TheExisting TreatmentSystem Meet The

Toxicity Limit?

No Action Required

No Action Required

DoesFacility Have A

Biological ActivatedSludge Or ExtendedAeration Treatment

Unit?

WillBIOX Facility Aid

In ReducingToxicity?

Consider ActivatedCarbon (Design Practices

Section XIX-A8),Membranes, Or Other

Alternatives.(D)

CanThe Source of

Toxicity Be RemovedBy BiologicalTreatment?

Consider BiologicalEnhancements. See

Design Practices SectionXIX-A5 For Details.

Install A BIOX Facility.See Design Practices

Section XIX-A5For Details.

No

Yes

Yes

Yes

YesYes

No

No

No No

Notes:(A) Yes/No decisions that are provided are not absolute. They are meant to provide a rough guideline and it is expected that there will be exceptions.(B) Contact ER&E Environmental Specialists for guidance in selecting the appropriate method for reducing toxicity.(C) Toxicity reduction is currently a focus in the U.S. only.(D) Design Practices section does not exist for this technology. See ER&E Environmental Specialists for assistance.

Conduct RiskAssessment. IsToxicityLimit Realistic, Science

Based?

Negotiate withAuthorities No

Yes

DP19AF5



XIX-A 33 of 35



FIGURE 6WASTEWATER SLUDGE MANAGEMENT VOLUME REDUCTION DECISION TREE (A, B, C)

Start

Is SuspendedSolids ConcentrationOf The Waste Stream> 500 - 1000 mg/L?

Is SuspendedSolids Concentration

> 10%?

Consider Thickening To At Least10% Solids. See Design

Practices Sections XIX-A2and XX-A2 For Details.

Is SuspendedSolids Concentration

> 20 - 30%?

Consider Dewatering Processes.See Design Practices Section

XX-A3 For Details.

Sludge Contains Water And Oil ButCannot Be Pumped. ConsiderThermal Treatment Processes.

See Design Practices Section XX-A5and Water and Wastewater Design

Guide DG 11-8-2 For Details.

Route StreamTo Sewer

Notes:(A) Yes/No decisions that are provided are not absolute. They are meant to provide a rough guideline and it is expected that there will be exceptions.(B) For this section 1% = 10,000 mg/L.(C) For situations that do not fit into this general decision tree, contact ER&E Environmental Specialist for assistance.

No

Yes

Yes

Yes

No

No

DP19AF6





FIGURE 7TYPICAL WASTEWATER TREATMENT FLOW PLAN

Trickling FilterOr Rotating

Biological Contactor

GravitySettling

Oil ToSlop

Oil To Slop

API or PlateSeparation

Equalization

ChemicalFlocculation

Nut ShellMedia

Filtration

GranularActivatedCarbon*

OxidationLagoon

Equalization/Diversion ForStormwater/Off-Spec

Wastewater

Step 2Pre-

Treatment

Sludge

Oily Storm Sewer

Oily Water Sewer

Chemical Sewer

Boiler Blowdown

Cooling Tower Blowdown

Clean Storm WaterStep 1Collection

ContaminatedStormwaterFirst Flush

Ballast Water

Sour Water Streams

BallastWaterTank

SourWater

Stripper

Step 3GravitySettling

Step 4Further Oil

andSuspended

SolidsRemoval

Step 5Oxygen

DemandReduction

BiologicalSludge

BiologicalSludge

Floated Sludge

Activated SludgeOr

Extended Aeration

Water To Recycle(e.g., Desalters)

Flotation Units(Induced Or

Dissolved Air)

MembraneActivatedCarbon Evaporization Ion

Exchange

ToOutfall

Sludge

Step 6AdvancedTreatment

* Technology used for special cases where the dissolved organics in the wastewater are low.** If segregated sewer system exists, "clean" and "contaminated" storm water may be separated.

Typical Flow PathAlternative Flow Path

KEY

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