WASTEWATER TREATMENT
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(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
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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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Deep shaft advantages
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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Deep shaft advantages
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
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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
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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
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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
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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
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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
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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
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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
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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