5 Sludge Treatment & Disposal
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Transcript of 5 Sludge Treatment & Disposal
Jae K. (Jim) Park Page 1
WASTEWATER TREATMENT PLANT DESIGN
11/30/2008 1
Sludge TreatmentSludge TreatmentWhat goes here depends on the method of final disposal:
• Solids concentration• Pathogen reduction
Sludge DisposalSludge Disposal• Land application• Landfill• Incineration
Processedsludge
Raw sludge, grit, cleanings
Influent EffluentWastewater Treatment
Sludge treatment and disposal may account for up to 80% of the total wastewater treatment cost.
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Primary Treatment 2º TreatmentPreliminary Disinfection
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WASTEWATER TREATMENT PLANT DESIGN
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Sludge•A mixture of organic and inorganic solids in water•Color : from brown to black
Treated sludge•Incineration, landfilling•Contain low levels of toxic compounds
12-50Dewatered sludge4-10Digested sludge1-5Waste trickling filter sludge0.5-1.5Waste activated sludge
2-7Primary sludge, without thickening
Typical concentration, %
Source
Typical Sludge Concentration
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Sludge TypesPrimary sludge
3 to 8% solidsAbout 70% organic material
Secondary sludgeConsists of wasted microorganisms and inert materialsAbout 90% organic materialWAS: 0.5 to 2% solidsTrickling filter sludge: 2-5% solids
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WASTEWATER TREATMENT PLANT DESIGN
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Sludge Types
Tertiary sludgeIf secondary clarifier is used to remove phosphate, this sludge will also contain chemical precipitates (more difficult to treat)Denitrification sludges - similar to WAS sludge
Sources of sludgeSources of sludge
Primary sedimentation tankAeration basin or secondary clarifierScreening and grinderFilter backwash water
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Sludge Treatment
Sludge Thicken Condition Dewater SanitaryLandfill
Stabilize Condition Dewater SoilIncorporation
ReductionAsh
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WASTEWATER TREATMENT PLANT DESIGN
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Sludge ThickeningReducing the water in primary and secondary sludges.Methods
Gravity thickening : using an additional clarifier to remove more waterDissolved air flotation (DAF)
Used to concentrate secondary sludgesSludge is pressurized and injected with air releasing into a settling tank (when the pressure is released, the extra air comes our of solution, attaching to the sludge particles as microscopic bubbles) the sludge is floated to the surface in a concentrated form the underflow returns to the head of the plant.
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Gravity ThickenerFlotation
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WASTEWATER TREATMENT PLANT DESIGN
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Thickening
FlotationEspecially effective on activated sludgeIncreases solids content from 0.5 - 1% to 3-6%
Gravity thickeningBest with primary sludgeIncreases solids content from 1-3% to 10%
PrimarySludge
Gravity Thickening
SecondarySludge
FlotationFurther processing
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Gravity ThickeningAccomplished in circular sedimentation basinsDegree of thickening: 2~5 times the incoming solids conc.Max. achievable solids concentration: < 10%Chemical and waste activated sludges are difficult to thicken under gravity.
10
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WASTEWATER TREATMENT PLANT DESIGN
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Gravity Thickener Design Criteria
85~92
60~8580~9285~98
Solids captured
, %
25~80
10~3535~50
90~144
Solids loading, m3/m2·d
200~1,0002~62~61~4Trickling filter
300~8004~104~60.5~2Combined primary + WAS
200~1,0002~42~40.2~1.5WAS
300~1,00024~335~101~7Primary
Overflow TSS, mg/L
Hydraulic loading, m3/m2·d
Thickened conc.,
%
Inf. solids conc., %Type of sludge
m3/m2·d × 24.57 = gal/ft2 ·dkg/m2 ·d × 0.2048 = 1b/ft2·d
• Gravity thickener side water depth: 3 m (10 ft)• Detention period: 24 hrs• Hydraulic loading rate: 10~30 m3/m2·d = 250~740 gpd/ft2
• To achieve the hydraulic loading rate, secondary effluent is often blended with the sludge fed into the thickener.
• The sludge-blending tank may utilize mechanical mixing or air mixing.
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Gravity Thickener Equipment
Generally circular concrete tanks with bottom sloping toward the center.
Equipment
Rotating bottom scraper armVertical picketsRotating scum-collection mechanism with scum baffle platesOverflow weir
Other configurations
Circular steel tank: Generally cheaper because of simplicity of construction, equipment installation, and operation and maintenance
Rectangular concrete and steel tanks
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WASTEWATER TREATMENT PLANT DESIGN
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Gravity Thickener
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Dissolved Air Flotation (DAF)Primarily used to thicken the solids in chemical and WASSeparation of solids is achieved by introducing fine air bubbles created under pressure of several atmosphere into the liquid, attaching to solids to cause flotation of solidsDegree of thickening: 2~8 times the incoming solids concentrationMax. solids concentration: 4~5%DAF Variations
Pressurize total or only a small portion of the incoming sludgePressurize the recycled flow from the flotation thickener – preferred because it eliminates the need for high-pressure sludge pumps.
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WASTEWATER TREATMENT PLANT DESIGN
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Dissolved Air Flotation (DAF)The released air bubbles become attached to the suspended particles by one of the following mechanisms:1.Condensation2.Collision3.Entrapment
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Advantages of DAF
Space requirements are minimal.Capability to treat a wide variety of organic and inorganic solids and dissolved waste streams.Low retention time from wastewater stream to effluent ejection.Superior clarification of most waste streams.Easy to clean and maintain.Higher density sludge with low water content.Installation cost is low for low flows. The unit is typically delivered fully prefabricated. Normal concrete pad installation.
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WASTEWATER TREATMENT PLANT DESIGN
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Design ParametersHydraulic Loading: Effective design ranges from 1.0 to 2.5 gpm/ft2, depending upon the application.Solids Load: Design points for solid loadings range from 0.5 to 3.5 lbs/hr/ft2, depending on the application and the type of solids involved. It should be noted that any chemical additives used to promote coagulation and flocculation are generally included as solids determining the surface loading since the chemicals used are removed with the float from the system.Air-to-Solid Ratio: Generally, air is injected in a range of two percent to eight percent (2% to 8%) by volume. Depending upon the type of solids and application, the air-to-solids ratio ranges from 0.020 to 0.1.
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Air/solids ratio, A/S
sa= solubility of air at the required temperature, mg/L;Sa = solids in incoming sludges, mg/L;f = fraction of air dissolved at pressure P, usually 0.5~0.8P = pressure in atmosphere = (p + 101.35)/101.35 (SI units)
= (p + 14.7)/14.7 (US customary units);p = gauge pressure, kPa (lb/in2);q = recycle flow or a portion of incoming flow pressurized, m3/d; andQ = sludge flow to the thickener, m3/day.
90~95
80~9590~9885~95
Solids captured
, %
1000~4000
1000~30001000~30001000~4000
Polymer added
(mg/kg)
100~60090~25050~1200.02~0.05Trickling filter
100~60090~25060~1500.02~0.05Combined primary + WAS
100~60060~18050~900.03~0.05WAS
100~60090~25090~2000.04~0.07Primary
TSS in side
stream, mg/L
Hydraulic loading, m3/m2·d
Solids loading
rate, kg/m2·d
Air/solids ratioType of sludge
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WASTEWATER TREATMENT PLANT DESIGN
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Sludge Stabilization
Aerobic DigestionExtension of activated sludgeAccomplished by aeration of sludge then followed by sedimentationSupernatant goes back to head of plant (high in BOD, TKN, total-P)Treated sludge is 3% solids
Anaerobic Digestion2 stage: acid fermentation followed by methane productionAdvantages:
produce methanedo not add oxygen
As with aerobic digestion, supernatant goes to headworks
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Sludge Stabilization
Stabilization alters the characteristics of sludge so it can be returned to the environment with a minimum of environmental and health risks.Biological Treatment
1. Aerobic Digestion2. Anaerobic digestion3. Lagoons4. CompostingChemical Treatment
1. Wet combustion2. Lime stabilization3. Chlorination4. Heat stabilization5. Irradiation
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WASTEWATER TREATMENT PLANT DESIGN
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Stabilization
Aerobic DigestionExtension of activated sludgeAccomplished by aeration of sludge then followed by sedimentationSupernatant goes back to head of plant (high in BOD, TKN, total-P)Treated sludge is 3% solids
Anaerobic Digestion2 stage: acid fermentation followed by methane productionAdvantages:
produce methanedo not add oxygen
As with aerobic digestion, supernatant goes to headworks
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Stabilization
Anaerobic DigestionAnaerobic DigestionAerobic DigestionAerobic Digestion
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WASTEWATER TREATMENT PLANT DESIGN
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Aerobic Digestion
Aerate until O2 uptake < 2 mg O2/g VS/hrSludge age = 10~20 days at 20°CLoading = 0.02~0.15 lb VS/ft3·dayThermophilic aerobic digestion: 77~122°F (25~50°C); higher removal of biodegradable fraction (up to 80%) at very short detention times (3~4 days)
Primarysedimentation
tankAeration
basinSecondary
clarifier
Aerobicdigester
RAS
Sludge
Decant
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Aerobic Digestion - continuedAdvantages: VS reduction similar to anaerobic digestion, lower BOD in supernatant liquor, production of an odorless, humus-like biologically stable end product, recovery of more of the basic fertilizer values in the sludge, relatively easy operation, and lower capital cost
Disadvantages: high power cost, poor mechanical dewatering characteristics of sludge, and sensitive to temp., location, and type of tank material, and loss of CH4recovery potential.
Process descriptionC5H7NO2 + 7O2 → 5CO2 + NO3
- + 3H2O + H+
pH drop7.14 lb of alkalinity as CaCO3 lost/lb ammonia oxidized
Primary sludge: direct oxidation of organic matter Biological sludge: endogenous oxidation of the cell tissue
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WASTEWATER TREATMENT PLANT DESIGN
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Aerobic Digestion - continued
Conventional Aerobic DigestionDigester liquid temperatures: dependent on weather conditions and thus can fluctuate extensively → use concrete instead of steel tanks, place tanks below grade instead of above grade, provide insulation, or use subsurface instead of surface aerationDesign at the lowest expected liquid operating temperature and provide the max. oxygen requirements at the max. expected liquid operating temperature.Tank volume
where Y = fraction of influent BOD5 consisting of primary sludge
Kd = reaction rate constant, 1/day; andfv = volatile fraction of digester suspended solids.
)1/θfX(K)YS(XQV
cvd
iii
++
=
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Aerobic Digestion - continued
Conventional Aerobic Digestion – continued
Oxygen requirementsBiological sludge: 2.3 lb O2/lb of cellsPrimary sludge: 1.6~1.9 lb O2/lb destroyedDO > 1 mg/L under all operating conditionsEnergy requirements for mixingMechanical aerators: 0.75~1.5 hp/103 ft3Diffused-air mixing: 20~40 ft3/103 ft3·minSolids loading: 0.1~0.3 lb VS/ft3·minVS reduction: 40~50%Process operationpH may drop to ~5.5 at long HRT → filamentous bulkingProvide decanting facilities so as to use to thicken the digested sludge solids before discharge to subsequent operations. Consider operator control and visibility of the decanting operation.
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WASTEWATER TREATMENT PLANT DESIGN
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Anaerobic DigestionAnaerobic Digestion
1. Fermentative organisms (50% of viable organisms in anaerobic digester)
Cellulose Acetate, alcohols, Strepto cocciLipids other organic acids, EnterobacteriaProteins H2, CO2, NH3, HS- Chlostridia
2. Acetogenic bacteria (acid formers)Simple organics (except acetate)
(fatty acids, sol. organics)3. Methanogenic bacteria (methane formers)
Get energy from forming CH4Need low redox potentialO2 toxicNeed temp.↑
}→
→ Acetate
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Anaerobic digestion of sludge decrease the volatile organics by 40-50% and reduce the numbers of pathogenic organisms in sludges.
Accomplished by holding the sludge in closed tanks for periods of 10 to 90 days.
Old process : unmixed, unheated, long detention time (30-90 days)
Recent process : complete mixing, heating (35-45℃), detention time 10-20 days
Anaerobic Digestion
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WASTEWATER TREATMENT PLANT DESIGN
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Complete mix, or high-rate, anaerobic digester
Heater
Digestion Separation
Digestergas
Digestergas
Gas storage Gas storage
mixer Supernatant
Scum
Settled,Digestedsludge
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Advantages
High degree of waste stabilization at high organic loading rates Very little sludge production (< 5% of biodegradable organic matter being converted to cell material) (10% of aerobic sludge production) Easy dewatering of the excess sludge Low nutrient requirement (10% of aerobic process requirement)No aeration equipmentMethane production – very low energy input (if the methane gas is used to heat the digester)
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WASTEWATER TREATMENT PLANT DESIGN
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Disadvantages
Low bacterial yield prolonged periods of biomass build-up, requiring longer start-up period (8 to 12 weeks).
Temperature, pH, toxic sensitiveInherent process instabilityHigh capital costsComplex operation requiring skilled operators
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Anaerobic Digestion - TheoryComplex organics
CH4
10%
13%
60% 15%
Intermediates Propionate
100%
20%
5%
Acetate H2
Fermentation & hydrolysis
72% 28%
Methanogenic phase Methanogenic phase
Acetogenic phase 2%50%
Rate limiting stepsConversion of propionic and acetic acid to CH4Hydrolysis of organic solids (cellulose)
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WASTEWATER TREATMENT PLANT DESIGN
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Anaerobic Digestion: Reactions
CH3COOH → CO2 + CH4CH4 + 2O2 → CO2 + 2H2O
CH3COOH + 2O2→ 2CO2 + 2H2O
* CH4 has high energy
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Anaerobic Contact Process
Recycle increases biomass levels and sludge age, thereby separating sludge age from HRT.Difficult to obtain efficient settling of the biomass.A degasifier is required to aid in solids settling.
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WASTEWATER TREATMENT PLANT DESIGN
Egg Shape Digester
35
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Mixing DeviceGas injection
Unconfined: collect gas at the top of the digesters, compress the gas, and then discharge the gas through a pattern of bottom diffusers or through a series of radially placed top-mounted lances...
Confined: collect gas at the top of the digesters, compress gas and then discharge through confined tubes...
Mechanical stirring: low speed turbine or mixers, suitable for fixed or floating covers
Mechanical pumping: propeller-type pumps mounted in internal or external draft tubes or axial flow or centrifugal pumps and piping installed externally, suitable for fixed covers
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WASTEWATER TREATMENT PLANT DESIGN
Mixer Types
Unconfined gas injectionsystems
Confined gas injectionsystems
Mechanical stirringsystems 37
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Digester HeatingTo raise the incoming sludge to digestion tank temperatures, to compensate for the heat losses through the walls, floor, and roof of the digester, and to make up the losses that might occur in the piping between the source of the heat and the tank.Internal or external heat exchangersHeat requirements
q = U A ∆Twhere
q = heat loss, BTU/h (W); U = overall coefficient of heat transfer, BTU/ft2·hr·°F (W/m2·°C); A = cross-sectional area through which the heat loss is occurring, ft2 (m2); and ∆T = temperature drop across the surface in question, °F (°C).
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WASTEWATER TREATMENT PLANT DESIGN
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Sludge Conditioning
Chemical Conditioning
Add lime, ferric chloride, or alumCan also add polymersChemicals are added just prior to de-watering stage
Heat TreatmentHigh temperatures (175-230 oC)High pressures (10 to 20 atmospheres)Advantages
bound water is released and sludge is easily dewatered
Disadvantagescomplex processhighly concentrated liquid stream
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Heating sludge under pressure to temperatures in the range of 200-300℃ for a few minutes can effectively sterilize it and convert it to a form that is easily dewatered.Disadvantages
Its high energy requirementThe production of a high-strength return liquid form the dewatering process
Heat treatment is used only in a few large POTWs
Heat Treatment
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WASTEWATER TREATMENT PLANT DESIGN
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Dewatering & Drying
Sludge Drying BedsMost popular methodSimpleLow maintenanceEffected by climate
FiltrationApply vacuum to pull out waterForce out water by essentially squeezing water between two moving filter belts
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Dewatering and Drying
By applying the sludge to sand drying bedsConsist of a layer of sand with an underdrain systemThe sludge is pumped onto the bedThe sun and wind dry the material furtherUsed at smaller POTWs
By using mechanical dewatering equipmentUsed medium size plants, larger POTWsThe sludge is applied to a metal, cloth, or systheticrubber surfaceThis equipment is automated but and experienced, well-trained operator is required.
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WASTEWATER TREATMENT PLANT DESIGN
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De-watering
Sludge Drying Beds Vacuum Filtration
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Filter PressDewatering is accomplished by pumping sludge into chamber (A) surrounded by filter cloths (B). As pumping pressure is increased, the filtrate is forced through the accumulated filter cake (C) and cloth, leaving the chambers full of solid filter cake. The chambers in HEI presses are formed by two recessed plates held together under hydraulic pressure. The hydraulic ram (D) moves the follower (E) against the stack of filter plates (F) closing the press. The ram continues to apply pressure of sufficient force to counteract the high internal compaction pressures. The head stock (G) and tail stock (H) are held in place by specially engineered side rail supports bars (I). The filtrate passes through the filter
cloth and is directed by channels in the plates and drain ports (J) to the head stock for discharge. The filtrate typically contains less than 15 ppmsuspended solids. The filter cake is easily removed by simply reversing the hydraulic ram, thus opening the press. The lightweight plates may then be moved apart permitting the compacted cake to fall from the chamber.
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WASTEWATER TREATMENT PLANT DESIGN
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45
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Centrifuges
Centrifugation is the process of separating solids from liquids by the use of centrifugal force. The centrifuge is a cylindrical drum that rotates to develop the separating force. When the slurry enters the interior of a rotatingcentrifuge, it is thrown out against the bowl wall. The denser materials are separated first, and hug the interior wall of the rotating machine.
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WASTEWATER TREATMENT PLANT DESIGN
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Centrifuges - continued
A helical screw conveyor fits inside the bowl. Rotating at a slightly lower rate that the bowl, it conveys solids from the zone of settling to the dewatering beach, where it is discharged to screw conveyors located below.
The main bowl is turned by an electric motor while the screw conveyor inside is controlled or slowed down using a hydraulic backdrive.
Prior to entering the centrifuge, sludge would have been conditioned with polymer. Larger and heavier particles are most easily captured by the centrifuge. Fine particles that cannot be settled separately must be agglomerated by chemicals (polymer) to a size that will settle.
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Sludge Volume Reduction
IncinerationComplete evaporation of water from sludgeRequires fuelSolid material is inertExhaust air must be treated prior to discharge
Wet OxidationTreated sludge is wetRequires energySolid material is inertExhaust air must be treated prior to discharge
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WASTEWATER TREATMENT PLANT DESIGN
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Volume Reduction: Fluidized Bed Incineration
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Ultimate disposal the return of the material to the environment.LandfillingLand application & Land Spreading:
• gardens• agricultural land• forest land• golf courses and other public recreational areas
IncinerationOther methods
Care must be taken in applying sludge to land, so that excessiveconcentrations of heavy metals or other toxic materials do not accumulate in the soilIncineration : lager municipalities
Maximum volume reducing, detoxification, and energy recoveryCapital and operating costs are highThere are environmental effects (air discharges, scrubber sludgegeneration), operation problems, and the continuing need for trained operating personnel.
Sludge Ultimate Disposal
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WASTEWATER TREATMENT PLANT DESIGN
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Wastewater Sludge and Biosolids
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Wastewater Sludge and Biosolids
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WASTEWATER TREATMENT PLANT DESIGN
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Wastewater Sludge and Biosolids
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Wastewater Sludge and Biosolids
+ Biosolids =+ Biosolids =
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WASTEWATER TREATMENT PLANT DESIGN
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