Waste Water Part c

7/23/2019 Waste Water Part c

http://slidepdf.com/reader/full/waste-water-part-c 1/18

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Wastewater TreatmentWastewater Treatment

CHNG 3804

Fariba Dehghani

Review of Secondary TreatmentReview of Secondary Treatment

• Secondary Treatment• Biological Removal of BOD

– Ponds

– Activated Sludge

– High Rate Processes

UASB digesters

lagoons

Removal of other ContaminantsRemoval of other Contaminants

• Activated Sludge Processes – Mass Balances – Hydraulic retention time (HRT) and Sludge retention

time (SRT)

• Biological Nutrient Removal – Nitrogen – Phosphorus

– Microbial Metabolism – Continuous and sequential batch reactor (SBR)

• Sludge Management – Anaerobic Digestion – Composting

Activated SludgeActivated Sludge -- ContinuousContinuous

• Removes BOD and Suspended Solids, typical Effluent BOD ~ 20 mg/L,SS ~ 30mg/L

• Sedimentation tank is an integral part of the activated sludge process.• Because of the variable process microbiology that is possible, it has

been found that the settling characteristics of the biological solids in themixed liquor will differ with each plant.

Influent

Return of activated sludge

Effluent

Waste Sludge

Process MicrobiologyProcess Microbiology• Pseudomonas • Zoogloea • Achromobactor • Flavobacterium • Nocardia • Bdellovibria • Mycobacterium • Nitrosomonas • Nitrobacter • Sphaerotilus • Beggiatoa • others

Bulking in Activated Sludge ProcessBulking in Activated Sludge Process

• The presence of filamentous organisms leads toformation of bulky flocs which do not settle well.

• Examples are fungi, actinomycetes , etc..

• Addition of chlorine and hydrogen peroxide tothe return waste-activated sludge, control ofoxygen are alternative way to minimize thebulking in activated sludge process.



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Aerated Ponds (Lagoons)Aerated Ponds (Lagoons)

• Evolved from facultative stabilization ponds when surface aeratorswere installed to overcome the odors from organically overloadedponds.

• The aerated lagoons process is the same as the activated sludgeprocess (20 days).

• An earthen basin is used for the reactor, oxygen is supplied from thesurface, and solid maintained in suspension.

• Aerated lagoon are used in conjunction with settling facilities torecycle the biological solids.

Sludge

Complex compounds+O2 +bacteria

CO2 +H 2O +more bacteria

Sequential Batch Reactor (SBR)Sequential Batch Reactor (SBR)

• Rather than pumping all of the wastewater from one tankto another it may be better to operate as a batch andchange the conditions

• This is termed a Sequencing Batch Reactor

• Activated sludge treatment system is a Fill-and Draw.

• Aeration and sedimentation/clarification are carried out inthe systems.

• Activated-sludge process is carried out simultaneously inseparate tank, however, in SBR the process is carriedout sequentially in the same tank.

• The steps are (1) fill, (2) react (aeration), (3) settle(sedimentation), (4) draw (decant), and (5) idle.

Sequential Batch Reactor (SBR)Sequential Batch Reactor (SBR)

Fill

Influent

React

Add Substrate

Reaction time

Settle

Clarity

Draw

Remove effluent

effluent

Idle

Waste sludge

1

2

3

4

5

Sequencing Batch Reactor (SBR)Sequencing Batch Reactor (SBR) Intermittent operation is also used for biological nutrient

removal. Instead of using different reactors, the conditions in the

reactor can be changed in the same reactor at differenttimes.

Trickling FilterTrickling Filter• Biological unit operation

• Organic material is removed by contact withattached biomass

• Some volatile pollutants are removed by transferto the gas phase

• Consist of a

– filter bed, which may be wood, plastic or mineral

– distribution system - normally rotating

– Support layer and effluent collection

Trickling FilterTrickling Filter

Effluent

Influent

Rotating Distributor

Design process, key parameter is surface hydraulic loading rate.The volume is then calculated from BOD5 loading.



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• A highly permeable medium is provided to whichmicroorganisms are attached.

• The filter media consists of either rock or variety ofplastic packing materials.

• The depth of the rock ranges from 0.9 to 2.5 m, whileplastic media from 4 to 12 m.

• Liquid wastewater is distributed over the top of the bedby rotary distributor.

• Fillers are constructed with underdrain system forcollecting the treated wastewater system.

• Air recirculates across the pores in the underdrainsystem.

Issues in Trickling FilterIssues in Trickling Filter

• High incidence of clogging• The long rest period required.

• Relatively low loading that could be used.


40-65%60-80%65-85%50-70%80-90%BOD5

removalefficiency

nonelowlowpartialhighNitrification

1-41-21-20-10RecirculationRate

5-103-101-22-32-3Depth (m)

1.5-7.50.5-1.60.5-10.25-0.50.08-0.4BOD5

loading(kg/m3 /d)

50-19012-709-383.5-9.41.2-3.5Loading

(m3 /m2 /d)

Plastic/Wood

PlasticRockRock/SlagRock/SlagFilter Media

RoughingSuper high-rate

High-rate

Intermediate-rate

Low-rate

AnaerobicAnaerobic

Lower quality Effluent thanAerobic

Can degrade chemicalswhich are not degradableaerobically

Production of hydrogen

sulfide

Elimination of off-gas

pollution (aeration stripsVOC)

Little nutrient removalLow Nutrient Requirements

Susceptible to failureLower sludge disposal costs

Longer Start-upProduces CH4 gas

DisadvantagesAdvantages

Energy Production (Anaerobic)Energy Production (Anaerobic)

• The yield of CH4 is typically 0.35 L/g ofCOD

• Calorific value of methane is 33.81 kJ/L

• Efficiency of Internal combustion enginesare typically 25%

• Can be more efficient if used in boiler

Energy Requirements (Aerobic)Energy Requirements (Aerobic)

• Aerobic processes require agitation and/oraeration – Except for Large shallow lagoons

• For large ponds, mechanical aerators arepreferred

• For fixed geometric vessels fine diffusersare preferred

• The type of diffuser has a large effect,different to agitated fermenters



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Energy Requirements (Aerobic)Energy Requirements (Aerobic)

1.2-2.4100HorizontalMechanicalAerator

1.5-2.2125VerticalMechanicalAerator

0.9-1.2100Coarse Bubblestatic diffuser

1.5-3.6200Fine bubble staticdiffuser

Energy Required(kg O2 /kWh)

Maximum AerationIntensity (g/m3 /h)

Aeration system

Anaerobic PondsAnaerobic Ponds

• Used with relatively high organic loads• Fairly Deep Ponds (3-6 m)• Slow rate of biomass formation (5-15% of Carbon in feed)• Top of pond usually covered in layer of scum

– The formation of the layer can be accelerated by covering thepond with straw

• pH needs to be 6.4-7.8• Excessive feeding causes pH to drop and Methane

formation to cease

Sludge

Scum

Complexcompounds

acid methaneH2S and NH4

CO2 + H2

Anaerobic Ponds DesignAnaerobic Ponds Design

GuidelinesGuidelines

mg/L200-500Effluent COD

mg/L1000-3000Influent COD

days6-20HRT

oC25-35Temperature

kgBOD5 /m3 /d0.1-0.2Load

kgBOD5 /ha/d300-600Load

UnitsTypical ValuesParameter

BOD removal 60-80%, odour emission, need to monitor pH at 6.4-7.8

Facultative PondsFacultative Ponds

• Shallower pond than anaerobic (1.5-4 m)

• Two zone environment

– Top section of pond is aerobic

– Lower section is anaerobic

• Medium organic load, odor free• Must be careful not to overload and turn

entire pond anaerobic

– Which in turn causes odor problems

Facultative Pond DesignFacultative Pond Design

GuidelinesGuidelines

days5-30HRT

kgBOD5 /ha/d20-40Load (T<15oC)

kgBOD5 /ha/d40-140Load (T>15oC)

UnitsTypical ValuesParameter

Oxidation/Aerobic PondsOxidation/Aerobic Ponds• Natural Oxygenation

– Wind

– Photosynthesis

• Shallow, 1-1.5 m

• Low organic loading, suitable for treating effluentfrom anaerobic ponds

• Must be careful not to overload and turn thepond anaerobic – Which in turn causes odor problems

• Design parameter 40-120 kgBOD5 /ha/d



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Exercise 2 Abattoir Waste WaterExercise 2 Abattoir Waste Water

• An Abattoir (Cow/Sheep/Pig =>Meatfactory) produces 1ML/day of effluent witha BOD of 3,000 mg/L after primarytreatment.

• What is the required volume, area anddepth of the anaerobic pond?

High Rate Aerobic TreatmentHigh Rate Aerobic Treatment

ProcessesProcesses

• The productivity of activated sludge processes islimited by the suspended solids concentration.

• This concentration is limited by the settling in thesecondary settler.

• This in turn limits the COD removal and the HRT

• The moving bed bioreactor (MBBR) removesthis constraint by retaining the biosolids in thereactor.

• This is achieved by growing the solids asbiofilms on carriers.

Moving Bed Bioreactor (MBBR)Moving Bed Bioreactor (MBBR)

• Rules of thumb

– Load: 100kg COD/m3 /day

– HRT: as low as 1-2 hrs

– COD removal up to 85%

– Advantages:• Lower N and P concentration in outlet effluent

• Smaller volume• More robust

• Less chance of odor production

High Rate Anaerobic TreatmentHigh Rate Anaerobic Treatment

ProcessesProcesses• Anaerobic is a complex process, including a number of

microbial processes. The stages can be physicallysegregated to achieve better control

• Invented in the Netherlands in 1980’s• Good for

– high strength wastewater pre-treatment – highly soluble COD

• Most common HRAT is called the Upflow Anaerobic

Sludge Blanket (UASB)• Consists of

– Acidification pretreatment – Mixing – Sludge Blanket

UASBUASB• Hydrolysis

– Breakdown of solids to soluble compounds

• Acidogenesis

– Conversion of soluble compounds to short chain fattyacids

• Acetogenesis

– Conversion of other acids into acetic acid

• Methanogenesis

– Generation of Methane from acetic acid and hydrogen

UASBUASB

SludgeBlanket

GasCollectors

MethaneTreatedEffluent

Need uniformdistributionChanneling is apotential problem

Gas is collectedbelow water levelto reduceturbulence at theoverflow



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High Rate of AnaerobicHigh Rate of Anaerobic

Treatment (HRAT)Treatment (HRAT)

• Design Rules of Thumb – Loading 4-12 kg COD/m3 /day

– HRT 4-12 hours

– 85% COD removal

Exercise 3 High Rate ProcessesExercise 3 High Rate Processes

• A brewery discharges 50 m3 /hr of wastewater with a soluble COD of 4,000mg/L

• Determine the size of a

– High rate aerobic MBBR

– High rate anaerobic UASB

Revision QuestionsRevision Questions

1. Would you select aerobic or anaerobic treatment forhigh strength waste water?

2. What are three advantages of• Aerobic over Anaerobic• Anaerobic over Aerobic

3. Name and describe the types of waste treatmentponds

4. For the same strength waste water, which pond will

have the smallest• Volume?• Area?

5. What happens if a facultative pond is overloaded withCOD? How can you tell?

Revision QuestionsRevision Questions

1. Would you select aerobic or anaerobic treatment for high strengthwaste water? Anaerobic

2. What are three advantages of• Aerobic over Anaerobic (no odor, lower volume, higher efficiency)• Anaerobic over Aerobic (produce methane, lower energy, low sludge

ad waste disposal)

3. Name and describe the types of waste treatment ponds (aerobic,facultative, anaerobic)

4. For the same strength waste water, which pond will have thesmallest

• Volume?• Area?

5. What happens if a facultative pond is overloaded with COD? Howcan you tell? (work anaerobic, and have odor)

Substrate Removal and BiomassSubstrate Removal and Biomass

Growth RateGrowth Rate• The objective of a wastewater treatment process is to remove the

substrate.

• Rate of substrate uptake

• Y is the maximum yield coefficient, mg/mg defined as the ratio of themass of cells formed to the mass of substrate consumed.

• Such a process leads to the formation of biomass

• Rate of biomass formation

( )S K

S

Y

X r

s

ms +

−= µ

X k S K

S X r

d

s

m X −

+= µ

Kd: endogenous decay coefficient, time-1

See chapter 8 Wastewater engineering, Metcalf and Eddy

Kinetic ParametersKinetic Parameters

(Domestic Wastewater)(Domestic Wastewater)

0.0025hr-10.002-0.003hr-1kd

0.6 mgVSS/mgBOD50.4-0.8mgVSS/mgBOD5

Y

60 mgBOD5 /L25-100 mgBOD5 /LKs

0.12 hr-10.1-0.5 hr-1µm

TypicalRangeCoefficient

The yield can include the effects of endogenous respiration, e.g. the cells consumingdead cells, in which case it is called the observed yield. (see chapter 8 Metcalf andEddy)

Adapted from IWES Wo rkshop for wastewater treatment



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Kinetic ParametersKinetic Parameters

(Industrial Wastewater)(Industrial Wastewater)

BasiskdYKsµm

Skimmilk

Meat

Poultry

Textile

2.5

0.9

-

0.1-7

100

150-300

500

90

BOD50.040.5

COD0.03-1.00.3-0.4

BOD50.71.3

BOD50.01-0.10.5-0.7

Adapted from IWES Workshop for wastewater treatment

Activated Sludge (HRT and SRT)Activated Sludge (HRT and SRT)

Aerobic ReactorVolume V

Influent


Effluent

Waste Sludge

Q, S0, X0 (1+α)Q, Sa, Xa (1-β)Q, Se, Xe

(α+β)QSr, Xr

βQαQ

S: soluble COD (substrate)X: particulate biomass

AssumptionsSteady-stateNo biomass in influent (X0=0)Assume no reaction occur in the clarifier

For domestic waste water α is ~1

Steady State Mass BalanceSteady State Mass Balance

( ) xar e r VX QX QX +−−−= β β 10Out Waste Growth

Aerobic ReactorVolume V

Influent


Effluent

Waste Sludge

Q, S0, X0 (1+α)Q, Sa, Xa (1-β)Q, Se, Xe

(α+β)QSr, Xr

βQαQ

Solids Retention Time (Solids Retention Time (θθθθθθθθcc))

• The solid retention time (SRT) or cellresidence time (θc) is basically a measureof how long bacteria stay in the system.

• The SRT is a very important operationalparameter as it affects: – Treatment performance

– Settling tank volume

– Sludge production

– Oxygen requirements

SRT and Treatment GoalSRT and Treatment Goal

Temp/specificpopulation/compounds

5-50Degradation of Xenobioticcompounds

Temp20-40Stabilisation of AS

Temp/specificpopulations

2-4Biological PhosphorousRemoval

Temp/specificpopulation/compounds

3-18Provide complete nitrification

Temperature1-2Develop flocculent biomass

Temperature2-4Conversion of particulateorganics

Temperature1-2Removal of Soluble BOD

Factors Affecting SRTSRT Range(days)

Treatment Goal

Metcalf and Eddy p 680

Example 1Example 1• An activated sludge plant is processing 3,800

m3 /d.

• The COD of the influent is 100 mg/L and theCOD of the effluent is 30 mg/L.

• The observed yield is 0.3 mg/mg (1 kg of Sludgehas a COD of 1.4 kg)

• What is the daily rate of sludge production?

• What is the rate of oxygen consumption?O2 consumption = (1-Yobs)*(S0-Se)



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• 3800 (m3/d) x 1000 (L/m3)x(100-30)/1000 (g/mg)/1000(kg/g)=266 Kg COD removed

• biomass produced from COD: 0.3 x 266 = 79.8 Kg

• Total sludge produced:

• 79.8 x 1/1.4 = 56 .2 Kg

• Oxygen = (1 - 0.3) x (3800 x 1000) x (100-30)/1000 =

186.2 kg/day

Example (Continued)Example (Continued)

• Adsorption of organics by activated carbon

• Filtration of solids and colloids (sand filter

and membranes)

• Biological nitrogen removal

• Biological phosphorous removal

Tertiary TreatmentTertiary Treatment

Adsorption EquilibriaAdsorption Equilibria

Is defined as follows:

M

V)CC(q o

e

−=

Whereqe : equilibrium contaminant concentration (mg contamination/gadsorbent)Co : initial contamination Conc. In solution (mg/L)C: equilibrium contamination concentration in solution (mg/L)M: Mass of adsorbent (g)

Adsorption SystemsAdsorption Systems

• Batch – stirred tank with filtration/sedimentation

– Low capacity

• Continuous

– Fluidized bed – Fixed bed in series

• Others

Ion ExchangeIon Exchange

Adapted from IWES workshop

Ion ExchangeIon Exchange




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Wetland SystemsWetland Systems

The hydraulic retention time is about 2-5 days for BOD

removal and 7-14 days for N removal. The wetland requires 0.5-2 years to establish and

harvested regularly. Expensive Used for biomass production

Effluent

influent

Seal


Free Water Surface ConstructedFree Water Surface Constructed

WetlandWetland

Is an example of wetland. Wastewater containing BOD, nutrients and solids is

treated using plants and micro-organisms Plants and micro-organisms uptake the nutrients Filtration and coagulation is used to remove solids Plants adsorb the ions Due to some reaction, contaminants precipitate.

Effluent

influent

SealAdapted from IWES worskshop

FiltrationFiltration

• Removal of solid residue and pathogens from effluentafter secondary treatment.

• Solid is removed by straining, precipitation andflocculation

• Continuous or intermittent process can be used.

• Most common is semi continuous, cleaning the filter byreverse flow (back washing).


Need for Nutrient RemovalNeed for Nutrient Removal

• Why do we need to remove nutrients suchas Carbon, Nitrogen and Phosphorus?

• Carbon - causes a reduction in dissolvedoxygen concentrations.

• Nitrogen and Phosphorus – linked togrowth of cyanobacteria and algal blooms.

• Particularly significant for inlandwaterways or enclosed water bodies.

Need for Nutrient RemovalNeed for Nutrient Removal• International legislation is now being passed to

confine N&P in effluents.

• In Australia environmental regulations typicallyrequire total N < 5mg/L and total P < 1mg/L.

• Over US$300 billion worldwide is estimated toconstruct new plant to comply with newregulations for nutrient removal.

Biological Nutrient Removal (BNR)Biological Nutrient Removal (BNR)

BNR utilises biological processes to remove C, N and Pfrom wastewater

• Reduction (anaerobic) or oxidation (aerobic) of organiccarbon

CH2O → CH4 ↑ + CO2 ↑ + H2 ↑CH2O → CO2↑

• Nitrification (oxidation) and denitrification of nitrogen

NH3 /NH4+ → NO3

- → N2↑• Intracellular storage and wastage of phosphorous

PO43- → [PO4]n↓



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Oxidation of Organic CarbonOxidation of Organic Carbon

Carbon may be oxidised with either oxygenor nitrate as electron acceptor:

• Heterotrophic carbon oxidation

CH2O + O2 → cells + CO2 + H2O

• Denitrification

CH2O + NO3- → cells + CO2 + N2

Note that many heterotrophs may switchto nitrate as electron acceptor whenoxygen is exhausted

Nitrogen Transformations in BiologicalNitrogen Transformations in Biological

Treatment ProcessesTreatment ProcessesOrganic nitrogen

(protein, Urea)

Ammonia nitrogen Organic nitrogen(Bacterial cells)

Organic nitrogen(net growth)

Nitrite (NO2-)

Nitrate (NO3-) Nitrogen gas (N2)

N i t r i f i c a t i o n

Denitrification

Organic carbon

O2

O2

Figure 8-32 Wastewater Engineering, Metcalf and Eddy

Nitrification/Nitrification/DenitrificationDenitrification

• Nitrogen can occur in many forms in wastewaterand undergo numerous transformations in wastewater treatment.

• Ammonia-nitrogen convert to product such asnitrogen gas.

• Two principle mechanisms are assimilation and

nitrification-denitrification.• Nitrogen is nutrient and can be incorporated into

the cell mass.

NitrificationNitrification• Nitrogen removal is a two step process:• Two bacteria Nitrosomonas and Nitrobacter are responsible for nitrification.

The latter convert the nitrite to nitrate.• For nitrosomonas the equation is • 109 HCO3

- + 76O2 + 55NH4+ →→→→ C5H7O2N + 57H2O+104H2CO3

• For Nitrobacter the equation is

• 195O2 + NH4+ + 400NO2

- +4H2CO3 + HCO3- →→→→ C5H7O2N + 3H2O+400NO3

-

• Approximately 4.3 mg O2 per mg ammonia-nitrogen oxidized to nitrate-nitrogen is required.

• A large amount of alkalinity is consumed 8.64 mg HCO3- per mg of

ammonia-nitrogen oxidized.• Changing ammonia does not facilitate nitrogen removal but does eliminateits oxygen demand.

• Nitrifying bacteria are sensitive to pH & temperature. A variety of organicand inorganic agents can inhibit the growth and action of these organisms.

NitrificationNitrificationNitrifiers are autotrophic bacteria.They obtain their carbon, used for cell synthesis from CO2.


DenitrificationDenitrification• Under anoxic conditions (without oxygen) the nitrate is converted to nitrogen

gas.

• Many different bacteria are involved in denitrification process: Micrococcus,lactibacillus, bacillus, alcaligenes, Pseudomonas, etc.

• These bacteria are hetereotrophs capable of dissimilatory nitrate reduction.

• NO3- →→→→ NO2

- →→→→ NO (g) →→→→ N2O (g) →→→→ N2 (g)

• Dissolved oxygen is critical in denitrification. Dissolved oxygen (DO) suppressthe enzyme system needed for denitrification.

• Microorganisms are also sensitive to pH and temperature.

• By including denitrification in a nitrifying process, the oxygen use can bereduced by up to 60% and the alkalinity depletion by up to 50%.

• The key factor affecting nitrogen removal include, pH, temperature, age ofsludge, dissolved oxygen, and feed composition.



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DenitrificationDenitrification


Phosphorous RemovalPhosphorous Removal• Both anaerobic and aerobic condition is required as shown below.

• It is based on activated sludge like nitrogen removal.• Factors affecting biological P removal include sludge age, Readily

Biodegradable Chemical Oxygen Demand (RBCOD), Nitrate(NO3-N which reduces P by 1 mg P/mg NO3-N in anaerobic zone).

Influent

COD,PAnaerobic Aerobic Sedimentation

Tank

effluent

LowCOD,P

Biomass recycling

Removal of P in bacteria

Adapted from IWES workshop Adapted from IWES workshop

Enhanced Biological PhosphorousEnhanced Biological Phosphorous

Removal (EBPR)Removal (EBPR)

• Phosphorous appears in wastewater as orthophosphate(PO4

-3), polyphosphate (P2O7), and organically boundphosphorus.

• Microbes utilize phosphorous during cell synthesis andenergy transport.

• Phosphorous is first released under anaerobicconditions. At these conditions Phosphate AccumulatingOrganism (PAO) take up more phosphorous.

• Soluble phosphate is then accumulated as anintracellular poly-phosphate under aerobic or anoxicconditions.

• The cells are then wasted as sludge.

BNR MetabolismBNR Metabolism

• Heterotrophic carbon oxidation (aerobic)

• Heterotrophic denitrification and carbonoxidation (anoxic)

• Autotrophic nitrification (aerobic)

• Heterotrophic phosphate release and carbonuptake (anaerobic)

• Heterotrophic phosphate uptake and carbonoxidation (anoxic)

• Heterotrophic phosphate uptake and carbonoxidation (aerobic)



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Chemical Phosphorus RemovalChemical Phosphorus Removal

• Chemicals such as ferric chloride (FeCl3), ferrous chloride

(FeCl2), alum (Al2(SO4)3,14.3 H2O) and lime (Ca(OH)2)are used to precipitate P.

Al3+ + HnPO43-n→ AlPO4 + nH+

• Addition of alum may• produce more sludge• Difficulty in removing water from sludge• Decrease alkalinity and require to add lime for pH control• A simple jar test is used to determine the amount of alum

required.

Wastewater TreatmentWastewater Treatment

• Primary treatment – settling

– removes BOD (COD or C)

• Secondary treatment

– COD removal (some N and P)

• Tertiary treatment

– biological nutrient removal (BNR)

– removes C, N, P

– “polishing” (wetlands, sand filtration, etc)

– disinfection

Biological Nitrogen RemovalBiological Nitrogen Removal

• Nitrogen in wastewater is usually in thefrom of organic (~30%) or ammonia(~70%)

AnoxicNO3 => N2AerobicNH3 => NO3

The recycle from the aerobic to the anoxic is large ~ 5-20 times the influent feed rate

Biological Phosphorus RemovalBiological Phosphorus Removal

• Relatively new technology and is the focus of alots of research

• Sludge must be cycled between anaerobic andaerobic conditions

• Anaerobic conditions – take up RBCOD into their cells and release ortho-

phosphates in the process

• Aerobic conditions – utilise stored RBCOD and take up ortho-phosphates

in excess of what they released

• Overall net uptake of phosphorous is removedby sludge wasting

Biological Phosphorus RemovalBiological Phosphorus Removal

Anaerobic Aerobic

Carbon, Nitrogen and PhosphorousCarbon, Nitrogen and Phosphorous

RemovalRemoval• Carbon

– aerobic/anoxic (or anaerobic)

• Nitrogen

– aerobic followed by anoxic

– carbonate required for aerobic reaction

– organic carbon required for anoxic reaction

• Phosphorous

– anaerobic followed by aerobic/anoxic

– organic carbon required for anaerobicreaction



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Carbon, Nitrogen and PhosphorousCarbon, Nitrogen and Phosphorous

RemovalRemoval

• In order to remove nitrogen andphosphorus the bacteria in a BNR requireCOD

• Rules of Thumb

• COD/P >50 for complete biological Premoval

• COD/TKN >10 for biological N removal

Biological Process ConfigurationBiological Process Configuration

(A(A22/O)/O)

• Proprietary A2 /O process

• The disadvantage is that nitrate equal to the effluentconcentration is recycled to the anaerobic zone.

• This reduces P removal

• SRT 2-4 days

Anaerobic Anoxic Aerobic

ModifiedModified BardenphoBardenpho

• Incorporates a secondary anoxic reactor toachieve greater N removal

• SRT is 10-20 days better carbon removalthan A2 /O

Anaerobic Anoxic Aerobic Anoxic Aerobic

Standard UCTStandard UCT

• Developed at the University of Cape Town

• Designed to minimise nitrates in weaker wastewaters• Differs from A2 /O in that sludge is recycle back to the

Anoxic zone, reducing the nitrate concentration in theanaerobic zone.

• This increases phosphate uptake in the anaerobic zone.

Anaerobic Anoxic Aerobic

HeuristicsHeuristics

• Batch systems are more cost effectivethan continuous systems.

• Nitrification and Enhanced BiologicalPhosphorous Removal (EBPR) arefavored by plug flow mixing patterns.

• For biological removal of C, N & P,separate sludge processes are moreeconomical and perform better.

BNR Process ConfigurationBNR Process Configuration




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Effect of Hydraulic Retention TimeEffect of Hydraulic Retention Time

(HRT)(HRT)

• Reactions go to completion as HRT is increased• Acetate uptake by PAO is fast → long anaerobic

HRT leaves time for phosphorous release fromendogenous metabolism (large stores ofpolyphosphate)

• Mixed liquor suspended solids (MLSS) is dilutedas HRT increases → smaller secondary settlingtank (shorter settling time)

Solids Retention TimeSolids Retention Time

• Typically 3-50 days• Limited by phosphorous removal

• Controlled by wasting ratio

F)of (fractionratiowastingW

:where

W

HRT

FW

VSRT

=

=⋅

=

Effect of SRTEffect of SRT

• Phosphorous removal capacity dependson PAO density.

• Nitrification depends on autotrophic nitrifierdensity and denitrifier density.

• Longer SRT → more biomass → more

nutrient removal.

• Longer SRT → accumulation of inertparticulates → larger secondary settlingtank (longer settling time).

Other StagesOther Stages

• At this point we have removed manycontaminants from the waste water stream

• But we still have to deal with the sludge we areproducing

and

• Disinfect the water before recovery/discharge

Sludge ManagementSludge Management

• Sludge - solids streams are a by-productof many primary secondary and tertiarytreatment processes.

• Organic sludges can be used for

– Digestion

– Soil amendment

– Land spreading


• Sludge - solids streams are a by-product ofmany primary secondary and tertiary treatmentprocesses.

• Organic sludges can be processed by digestionor composting to reduce solid mass. – Soil amendment – Land spreading

• Toxic sludge may require stabilisation prior todisposal.

• Sludge can be processed by chemical or organicprocess.



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• Organic – Digestion

– Soil amendment

– Animal feed

– Land spreading

• Chemical – Reuse by another industry

– Incineration

– landfill

DigestionDigestion

• The purpose of digestion is to remove volatilesolids from the sludge

• The digestion can be aerobic or anaerobic

• Anaerobic reactors tend to be the more popularas they can generate methane and henceenergy

• They are typically egg shaped and a portion ofthe biogas produced is reinjected to mix thecontents.

Anaerobic Digestion ProcessAnaerobic Digestion Process

Adapted from IWES workshop and websites listed in refs.

Anaerobic Digestion ProcessAnaerobic Digestion Process


Anaerobic DigestionAnaerobic Digestion

1. Hydrolysis and fermentation(liquefaction) of organic material

2. Acetogenesis and dehydrogenation

3. Methane fermentation

AnaerobicOrganics (CHON)→ CH4+CO2 + NH4

AerobicOrganics (CHON) + O2→ CO2 + H2O + NOx

Anaerobic DigestionAnaerobic Digestionsolid organiccompounds

VOC

acetic acid CO2

methane

H2

soluble organiccompounds

hydrolytic

enzymes

acidogenic

bacteria

acidogenic

bacteria

acetoclastic

methanogenic

bacteria

hydrogen-

utilising

methanogens

acidogenic

bacteria

acidogenic

bacteria

reduced nitrogenand sulfurcompounds

volatile aromaticcompounds, VFA

STAGE 1

STAGE 2

STAGE 3



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Two Stage DigestionTwo Stage Digestion

• Acid forming bacteria prefer pH 5-6, grow quickly andare not as sensitive to toxic components in the feed.

• Methane forming bacteria prefer a neutral pH, growslowly and are sensitive to toxic components.

• Either use two reactors, a pre-fermenter followed by adigester

• Or use one reactor at near neutral pH.

• Typically the digester is run at 30-40oC and a pH of 7-7.5.

Anaerobic DigestionAnaerobic Digestion

• Note anaerobic digesters also produce a lot ofCO2, typically the biogas is 60-70% methaneand 30-40% CO2.

• The production of this CO2 causes the pH todrop, hence you need alkalinity.

• This is of great importance due to methanogensbeing inhibited below a pH of 6.8.

Sulfate ReducersSulfate Reducers

• A problem for many anaerobic digester is thepresence of Sulfur in the feed.

• Inevitably sulfate reducing bacteria will occur inthe process.

• This results in the formation of H2S.

• The effects of this compound can be reduced by

dosing with iron to precipitate FeS or raising thepH slightly to keep it in solution.

Sludge DryingSludge Drying

• Drying the sludge reduces the volume and massthat must be handled/transported.

• It also stabilises the sludge and reduces thepathogen levels.

• Sludge can be dried by – Spreading the sludge over a field and relying on

natural convection, can load at 100-15 kg ofVS/m2 /year. 1 hectare is sufficient for 70,000 people.

– Heating

– Filtration – typically using a belt press

– Centrifuge

Sludge CompostingSludge Composting

• Composting is the biological degradation oforganic material.

• Start with dewatered sludge (~60%) moisture

• Need to ensure aeration to avoid odourproblems

• The biological activity increases the temperatureto 50-70oC.

• Thought to be sufficient to kill pathogens.

Sludge CompostingSludge Composting

• Windrow – The sludge is placed in an open pile and

mechanically turned to provide aeration.

– Requires around 30 days

• Static Pile – Air is supplied mechanically

– Requires around 30 days

• In-vessel – Compost is mechanically stirred and aerated

– Higher capital cost, but lower labour and land costs,odour problems less likely



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Sydney WaterSydney Water BiosolidsBiosolids

• Have a look at Sydney Waters web site forinformation on sludge uses

• http://www.sydneywater.com.au/EnsuringTheFuture/Biosolids/

DisinfectionDisinfection• Inactivate or remove the phathogenic micro-organisms

– Chemical (Cl2, ClO2, NaOCl, Ca(OCl)2, O3, pH)adjustment

– Physical (heat or pressure)

– Radiation (UV)

– Filtration (membranes)

• Most common is chemical treatment


Membranes in WastewaterMembranes in Wastewater

• Micro-filtration to remove residual colloidal solids

• Ultra-filtration to remove solids and large solubleparticles (sterilisation)

• Nano-filtration and reverse osmosis to remove dissolvedpollutants

http://www.mhhe.com/engcs/civil/metcalf/information/chapter1.pdf


Membranes in WastewaterMembranes in Wastewater


NoShortNoneShort/noModLongResidualTime

UnknownSludgeUnknownUnknownYesYesHazardousProducts

NoModNoSlightYesYesFish Toxicity

UnknownUnknownGoodGoodGoodPoorVirus

GoodGoodVery GoodVery GoodGoodGoodBacteria

Kills

NoneLowNoneLowMedHighSafetyConcern

ModVery ShortShortMod/LongMod/LongLongContactTime

MedHighLow/MedHighMedLowMaintenance

MedHighLowHighMedLowOperating

HighHighLowHighMedLowCapital

Cost

LargeMed/LargeAllMed/LargeSmall/MedAllSize of Plant

MembraneLimeUVO3ClO2Cl2

This table is adapted from IWES 2005, see Metcalf and Eddy p 1222 for original

Need for Nutrient RemovalNeed for Nutrient Removal• Why do we need to remove nutrients such as

Carbon, Nitrogen and Phosphorus?

• Carbon - causes a reduction in dissolved oxygenconcentrations

• Nitrogen and Phosphorus – linked to growth ofcyanobacteria and algal blooms

• Particularly significant for inland waterways orenclosed water bodies

• In Australia environmental regulations typicallyrequire total N < 5mg/L and total P < 1mg/L




SummarySummary

• This lecture we have looked at – Activated Sludge

– Tertiary Treatment

– Biological Nutrient Removal

– Sludge Management

– Disinfection

ReferencesReferences

• “Wastewater Engineering, Treatment, Disposal and

Reuse by Metcalf and Eddy” – http://www.geocities.com/RainForest/5161/water1.ht

m

– http://www.geocities.com/RainForest/5161/wwtps.htm

– http://www.gocolumbiamo.com/PublicWorks/Sewer/wwtppg_4.php

– http://en.wikipedia.org/wiki/Sewage_treatment

– http://ohioline.osu.edu/aex-fact/0768.html – http://www.mhhe.com/engcs/civil/metcalf/information/chapter1.p

df

• IWES Workshop, Principles of wastewater treatment2005.

Waste Water Part c

Documents

Transcript of Waste Water Part c