WASTEWATER TREATMENT

(SEWAGE, EFFLUENT TREATMENT)Pr H.A.Foster October 2013

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Sources and Sinks of Oxygen in Streams

Tributary flow Photosynthesis

Plants Algae cyanobacteria

Reaeration (weirs)

Oxidation Carbonaceous Material Ammonia Nitrogen Hydrogen sulphide

Benthic Layer (Bottom)

Respiration

SourcesSources SinksSinks

Based on a presentation byLeonard W. Casson, Ph.D., P.E., DEEBased on a presentation byLeonard W. Casson, Ph.D., P.E., DEE2

OXYGEN SAG CURVE

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The Beginnings of wastewater (sewage) treatment

(But remember water from washing, body and clothes, preparing vegetables, industrial use, rainwater runoff etc.)

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Composition of Wastewaters

Wastewater of domestic origin is usually >99% water with up to 1% solids, both suspended and in solution.

Industrial wastewater streams vary greatly depending on the industry. Abattoirs and food processing plants can produce as much BOD as a small town.

Industrial effluent can also contain toxins.

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Septic Tanks

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Preliminary treatment Removal of dead dogs, foetuses, plastic

bottles and other things you cannot pump Screening through bar screens or

perforated plates. Strainings may be passed through a

comminuter (mincer). Grit Removal – flow slowed to allow grit to

settle. In times of high water flow excess

wastewater will pass over weirs and be stored in storm tanks.

Pumping takes energy, sites often sloping to utilise gravity.

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Bar screens

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Grit removal

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•Primary settlement = removal of suspended organic matter through sedimentation•Flow of effluent slowed in circular or rectangular sedimentation tanks•Settled material = sludge. This is removed periodically together with any surface scum•Liquid is now termed primary effluent

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Rectangular tanks

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Primary treatment -settlement

Settlement may be enhanced by addition of polyelectrolytes.

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Secondary Treatment of Primary Effluent

Activated Sludge Deep Shaft Trickling filter

(Biological filter) RBC (Biodisc) Aerated Lagoons Integrated Ponds

Secondary Sedimentation

Biological Biological ProcessProcesseses SedimentationSedimentation

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Aerobic wastewater treatment

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Conventional Activated Sludge Process

PEPE

Mixed Liquor Measured HereMixed Liquor Measured Here

SESE

RASRASWASWAS

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Activated Sludge

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Diffuse aeration

Mechanical aeration

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Conventional Wastewater Treatment Unit Processes

Pre TreatmentPre TreatmentPrimary Primary

Sed.Sed.BiologicalBiologicalTreatmentTreatment

SecondarySecondarySed.Sed.

DisinfectionDisinfection

Biosolids Treatment Biosolids Treatment Solids Solids Disposal/ReclamationDisposal/Reclamation

Raw

Was

tew

ater

Raw

Was

tew

ater

Eluent TreatmentEluent Treatment

PretreatmentPretreatment PrimaryPrimary Secondary TreatmentSecondary Treatment

PEPE SESE

PE = Primary EffluentPE = Primary EffluentSE = Secondary EffluentSE = Secondary Effluent

RASRASWASWAS

RAS = Return Activated SludgeRAS = Return Activated SludgeWAS = Waste Activated SludgeWAS = Waste Activated Sludge

Effl

uent

Dis

posa

l/Rec

lam

atio

nE

fflue

nt D

ispo

sal/R

ecla

mat

ion

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Activated Sludge

Variations Completely mixed (often required nitrifying filter

beds) Extended aeration (>15 h) Plug-flow (typically 7-8 h residence time)

Organisms grow as ‘flocs’ – macroscopic aggregates of organisms.

Good floc structure is essential for later settlement. Overgrowth of filamentous fungi can cause ‘bulking’

Production of surfactants can cause foaming or moussing (? Nocardia spp.)

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Zooglea ramigera

Activated sludge

Settled flocs

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Bulking activated sludge

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Aerobic Biological Process Description

Leon

ard

W. C

asso

n, P

h.D

., P.

E., D

EELe

onar

d W

. Cas

son,

Ph.

D.,

P.E.

, DEE

Organic Organic WasteWaste

New CellsNew Cells

End ProductsEnd ProductsCOCO22, H, H22OO

EndogenousEndogenousRespirationRespiration

NonbiodegradableNonbiodegradableResidueResidue

Energy for CellEnergy for CellMaintenanceMaintenance

Energy for CellEnergy for CellSynthesisSynthesis

Yield: Greatest for Aerobic Systems;Yield: Greatest for Aerobic Systems; Less for Anaerobic SystemsLess for Anaerobic Systems

Note: Energy is available fromNote: Energy is available fromThe conversion of organic The conversion of organic Carbon to COCarbon to CO22

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Expressed in terms of pounds of BOD used per day for each pound of Mean Liquor Suspended Solids (MLSS) in the aeration tank.MLVSS (mean liquor volatile suspended solids) is also used. F/M does not consider BOD in the Return F/M does not consider BOD in the Return Sludge Sludge Sludge age (measured in days) is also controlled

.

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DEEP SHAFT (ICI)

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DEEP SHAFT (ICI)

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DEEP SHAFT OPERATION

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ADVANTAGES OF DEEP SHAFT

• Mechanical simplicity. • Low capital and operating costs.• Low land area requirements. • Low environmental impact (low

odour, mist and noise). Also largely underground.

• High energy efficiency of 2-4kg BOD/kWh.

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Deep shaft advantages

Primary treatment not required. High oxygen transfer rate (up to 3

Kg/m3/hr compared to 0.1 to 0.3 for conventional processes).

High efficiency of oxygen utilisation. High BOD removal rates. Process is unaffected by climatic

changes.

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Can operate at higher MLSS concentration (3-10 g/l compared to 2-5 g/l) for industrial effluents.

Design sludge loading (kg BOD per day/kg of MLSS) is higher, (0.7 to 1.8 compared with 0.1 to 0.2), and this reduces reactor size.

Limited growth of filamentous organisms means improved sludge settling and smaller clarifiers

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Less sludge mass/volume produced per Kg BOD removed.

No moving parts with low maintenance costs.

Overall cost effective high performance.

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Fixed Film Processes

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Trickling (Biological) Filter

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Biological filter

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Sample plastic packing material for biological filters

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Grazing fauna

Fixed Film Processes

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BIODISCS

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Rotating Biological Contactor

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RBC System small scale

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Typical RBC Schematic

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RBC Operation

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Power requirements

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Role of Microorganisms in the Removal of Organic Matter

Adsorption Absorption Respiration (oxidation of substrates for

energy) Synthesis (leading to an increase in

biomass)

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Adsorbed particleEnzymes

Small molecules

Absorption

Oxygen

RespirationEnergy

Synthesis Structural molecules and enzymes

Adsorption to extracellular matrix (capsule and slime)

Waste products

CO2, NH3, H2O48

Organisms found in Activated Sludge Bacteria

Zooglea Pseudomonas, Nitrosomonas, Nitrobacter Beggiatoa Achromobacter, Flavobacterium,

Arthrobacter Mycobacterium, Nocardia, Herpetosiphon Escherichia Leucothrix, Azotobacter, Bacillus (After Hawkes, 1975)

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Protozoa

Important in maintaining flocs and clear effluent 11 genera of phytoflagellates 7 genera of zooflagellates 13 genera of amoebae 4 genera of actinopods 59 genera of ciliates.

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Fungi and Rotifers

Fungi Arthrobotrys, Cephalosporium (Acremonium) Geotrichum, Pullularia, penicillium Cladosporium, Aternaria, Candida, Trichosporium, Leptomitus

5 genera of rotifers, mainly Bdelloidea

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Trickle Filter Slime

>20 genera of bacteria e.g Zooglea, Pseudomonas.

>10 genera of cyanobacteria e.g. Oscillatoria.

>10 genera of algae e.g. Chlorella, Nitzchia. >20 genera of fungi e.g. Fusarium,

Geotrichium. >120 genera of protozoa e.g. Paramecium. >25 genera of rotifers e.g. Habrotrocha.

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Trickle Filter Slime

>25 genera of nematodes e.g. Rhabditoides. >10 genera of worms e.g Lumbricillus. >15 genera of insects mainly diptera and

collembola e.g. Metriocnemus. Molluscs e.g. snails. Overgrowth of the film can lead to ponding

causing anaerobic conditions and blocking the flow.

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Secondary Settlement

Usually radial flow. Removes activated sludge from aerated

processes. Removes solids (humus, much of which is

insect cuticle) from Biological Filters.

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Secondary clarifier center feed with suction

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Land Application of Wastewater

Objective: To utilize the natural soil properties and associated biological conditions to remove undesirable constituents from wastewater.

Benefits: Reclamation/Reuse of water and use of nutrients for plant growth.

Areas of Concern: Metals Accumulation Nutrient Uptake Pathogens

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Land Application Hydraulic Pathway

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Leonard W. Casson, Ph.D., P.E., DEELeonard W. Casson, Ph.D., P.E., DEE59

Oxidation Pond

SUNSUN

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Oxidation Ponds

Low Cost Treatment Option. Used Primarily in rural areas. Assume these are completely mixed

biological reactors without solids return. Mixing provided by heat, wind, and

fermentation

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Aerated lagoons/oxidation ditches

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Secondary treatment of sludge -Anaerobic Digestion

Used for the degradation and breakdown of sludge from primary settlement, waste activated sludge and septic tank contents.

Organic matter is converted to CH4 and CO2

The Biological Process is thought to occur as either a two or a three step process.

Residual matter remains (treated sludge).

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Step 1 - Liquefaction

Complex carbohydrates → simple sugarsComplex carbohydrates → simple sugarsProteins → peptides and amino acidsProteins → peptides and amino acidsAmino acids and fats → glycerol and fatty Amino acids and fats → glycerol and fatty acidsacids

End Products of this process are primarily volatileEnd Products of this process are primarily volatileorganic acids. The microorganisms responsible fororganic acids. The microorganisms responsible forthis conversion are non-methanogenic this conversion are non-methanogenic microorganisms consisting of facultative and microorganisms consisting of facultative and obobliligate anaerobes.gate anaerobes.

““Acid Formers”Acid Formers”68

Step 2 - GasificationPrimarily CHPrimarily CH44 and CO and CO22 but some H but some H22S and N S and N

are produced.are produced. Bacteria for this conversion are strict anaerobesBacteria for this conversion are strict anaerobes

““Methanogens”Methanogens” These bacteria have a very slow growth rate since These bacteria have a very slow growth rate since only a small portion of the degradable waste is being only a small portion of the degradable waste is being converted to new cells.converted to new cells. A High A High TemperatureTemperature (35 (35ooC) is required for these C) is required for these bacteriabacteriaAcetic acid and Acetic acid and propionicpropionic acid are converted to CH acid are converted to CH44 and COand CO22 (“acid regression”)(“acid regression”) 69

Three Step Model

Step 1 - Acid Fermentation StageStep 1 - Acid Fermentation Stage Carbohydrates → Low MW fatty acids Carbohydrates → Low MW fatty acids

(acetic, butyric, proprionic)(acetic, butyric, proprionic) Operating Conditions:Operating Conditions: pH Drop, Increasing Odours, Increasing pH Drop, Increasing Odours, Increasing

Volatile AcidsVolatile Acids

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Three Step Model

Step 2 – Acid Regression StageStep 2 – Acid Regression Stage Organic acids + soluble nitrogen compounds →Organic acids + soluble nitrogen compounds →acetic acetic

and proprionic acidsand proprionic acids, NH, NH44, amines, carbonates, some , amines, carbonates, some COCO2 2 NN22, CH, CH44 and H and H22SS

Operating Conditions:Operating Conditions: pH Increase, Odors Increase, Gas ProductionpH Increase, Odors Increase, Gas Production

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Three Step Model

Step 3 – Alkaline Fermentation Step 3 – Alkaline Fermentation (acid (acid regression) regression) StageStage

Fermentation of low molecular weight organic Fermentation of low molecular weight organic acidsacids

Methane producing bacteria produce gasMethane producing bacteria produce gas 70 % of the methane is produced in this 70 % of the methane is produced in this

stagestage Due to the slow growth rate of the methane Due to the slow growth rate of the methane

producing organisms, sufficient time must be producing organisms, sufficient time must be provided to permit growth of these organismsprovided to permit growth of these organisms

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Acids to COAcids to CO22 and CH and CH44

Anaerobic Digesters

Single-Stage Digester with a Floating Cover (Standard Rate) Contents Unmixed and Unheated Td = 30 to 60 days

Single-Stage High Rate Digestion System Contents Mixed and Heated Td = 15 days or less

Two-Stage Anaerobic Digestion System Second Stage Can Function as a Solids

Separation Phase Additional Digestion Can also occur in this Second

Stage73

Conventional Standard Rate Anaerobic Digester

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Standard Rate Anaerobic Digester

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Standard Rate Anaerobic Digester

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High-Rate Anaerobic Digester

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Two-Stage Anaerobic Digester

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Aerobic Digestion

Similar to Activated Sludge only with higher Concentrations.

ADVANTAGES: Easy to Implement (Even use an

Activated Sludge Tank) DISADVANTAGES:

Energy Costs Product Not Easy to Dewater

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Disposal of residual sludgeDry weight of sludge produced Tonnes

(approximates to 20 kg per person per annum)

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Sludge disposal mt dry wt1992 1999-2000 2009-10

England 0.84 0.95

Northern Ireland 0.032 0.03

Scotland 0.087 0.11

Wales 0.033 0.023

Total 0.997 1.11 1.6

1993 1999

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Tertiary Treatment

Phosphorus Removal (coagulation) Nitrification/Denitrification Ammonia stripping. Carbon Adsorption. Ion exchange. Filtration. Reverse osmosis. Disinfection.

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Removal of phosphate

Coagulation Add lime or ferrous aluminium sulphate -

precipitates phosphates and residual suspended solids.

Polyphosphate accumulating bacteria Nitrification and denitrification

Anoxic zone can lead to denitrification as organisms use NO3 as a terminal electron acceptor

Additional oxidation may be needed to convert NH3 to NO3

•

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3° Treatment continued

Ammonia Stripping Vigorous aeration or stripping tower. pH affects effectiveness.

Reverse osmosis Passage through a membrane under pressure Choice of membrane affects solute passage Appropriate membranes just allow water to

pass, hence everything is removed.

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Ammonia stripping tower

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Further 3° treatments

Activated carbon Removes residual (recalcitrant?) organic

compounds. Can remove 0.3 to 0.6 kg COD per kg of

carbon

Ion Exchange Used to remove metal ions Used resin must be regenerated

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BAFF plant

Bacterially Activated Flooded Filter Uses plastic pellets “Polishes” effluent – reduces BOD Reduces NH3

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Removal of Pathogens

Most pathogens are removed by secondary treatment: They are not usually present in high concentrations

and are greatly diluted by the water in sewage (often only 25% of the volume is domestic sewage).

Pathogens often complete poorly for nutrients with the varied flora present and are eliminated by competition.

They are removed by predation by e.g. protozoa.

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Removal of Pathogens Some pathogens e.g. enterobacteria can

survive the process. Our own evidence suggests that coliforms and E. coli can be present in treated effluent at relatively high concentrations (10-50,000 ml-1).

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• Chlorination - introduction of Cl2 gas

• Cl2 + H2O → HCl + HOCl-

hydrochloric acid + hypochlorous acid

• HOCl- is actually agent that kills microorganisms • Maintain a chlorine residual in water (0.06ppm)

but not too much.• Chlorination is a balance between cost and kill potential.

Disinfection of Secondary Effluent

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• BUT • chlorine can react with certain organic trace chemicals to form carcinogens e.g. chloroform, chloramines.• Chlorination does not eliminate some viruses, Cryptosporidium or Giardia.• Alternatives are:

• Ozone• U.V. irradiation is becoming more common as new WWTP are built but effluent must be low in particulates

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Special problems – industrial waste Industry creates either high concentrations of

naturally occurring compounds or wholly man-made or xenobiotic compounds.

These may be only slowly degraded or may be recalcitrant (difficult or impossible to degrade).

All waste must have a discharge consent. Many industries treat waste before disposal.

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Manufacture of coke

TARCOAL

HEAT GAS COKE

PITCHCOOLING WATER

NH3

(150ºC)

NAPHTHA (150-220 ºC)

CREOSOTE 220-280ºC (COOLS TO NAPHTHALINE)

ANTHRACINE OIL 280-350 ºC (COOLS TO ANTHRACINE

BENZENE TOLUENE SOLVENT NAPHTHA (XYLENE) PHENOLS, CRESOLS, CN-, CNS- 94

VITOX system (BOC)

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Advantages of the Vitox system (aeration with pure oxygen)

Supports higher biomass. Less surplus sludge produced. Reduced costs. Reduced power consumption. Eliminates need for antifoams. Minimises VOC emissions, odour and

noise. Compact, higher throughput per unit size.

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Rossendale

WASTEWATER TREATMENT

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