Aims Of Wastewater Treatment 3y3y35

Aims of wastewater treatment Protect ecological health of the receiving environment  Disposal is often via fresh and marine ecosystems  Remove material which exerts a oxygen demand through biological and chemical process  Prevent the receiving environment turning anoxic  Prevent Eutrophication o Increase in N and P in ecosystems stimulates the growth of primary producers like algae o N and P are termed growth limiting according to the Redfield Ratio o Dark respiration (form of respiration in plants where carbon dioxide is released without the aid of sunlight) causes oxygen sag at night o Impact on ecology as the conditions favours the primary producers (algae) and the competition will lead to a decrease in diversity o Algae on the surface will restrict light penetration changing the light climate of the water. Impacts benthic aquatic plants that require lights for photosynthesis o Epiphyte and biofilm grow over the aquatic plants create a shade and restrict photosynthesis o Loss of aquatic plants due to the anoxic conditions leads to mineralization → release of bound nutrients from sediment causing more algae growth

Protect public health of humans that comes into with effluent  Exposure through ocean fallout/recreational waters  Water reclamation (recycle water)  Avoid spread of waterborne diseases  Chemicals found in wastewater can have detrimental impacts to human health o Originate from industrial chemicals (heavy metal, pesticides), household chemicals (detergent, pesticides), excreted by people (disposing unused drugs), formed during wastewater treatment (disinfection by-products) o Present chronic health risk – long term exposure to small doses can have a cumulative effect, e.g.  Endocrine disruption compounds (EDCs)  May result in disruptive endocrine activities such as modulating the effects of hormones that control sexual development and reproduction  Xenoestrogens (synthetic or natural chemicals that imitates estrogen)  Found in personal care products (cosmetics), industrial chemicals and insecticides Recycle and Recover the valuable components of wastewater  Water (water reclamation)  Energy recovery (Methane production for power generation) o Focus convert WWTP into net energy producers  Nutrient recovery → phosphorous required by all life forms for growth however a limited resource

Wastewater treatment process Preliminary Treatment  Targets the removal of gross pollutants o Size distribution 1 – 100mm  Disposed via land fill  Screening o Rotating screen belts, screw screens  Grit Chambers o Velocity of the incoming sewage is adjust to allow the settlement of gross pollutants o Circular/vortex chambers creates circular flow pattern causing grit to settle while lighter organic material in suspension for treatment down stream o Aerated girt chamber creates air flow in a circular manner Primary treatment  Physical treatment aimed at removing the solids using sedimentation tank  ~ 30% removal of BOD and organic N  Results in 2 waste streams o Water (primary effluent) → high in nutrient (N and P) o Sludge → high in organic carbon

Secondary Treatment  Targets constituents that exert an oxygen demand and eutrophication (BOD/COD and N/P)  Converts nutrient into: o Biomass that can be separated from water by sedimentation, filtration, etc → assimilation of N and P into living biomass o Environmentally benign inorganic substances → biological oxidation and reduction  Achieved using biological reactors o Highly engineered, artificial systems creating favourable environments to concentrate specific organisms and enhance their degradation capabilities o Process utilize the natural degrading capabilities of microorganisms Classification of microorganism  Carbon source o Heterotrophic – respiration of organic carbon (sugars, alcohols, organic acids) o Autotrophic – fix inorganic carbon o Mixtrophic – Both carbon source  Energy source o Chemotrophic – energy from chemical reaction  Chemoautotroph – energy from oxidation of inorganic molecules  Chemoheterotroph – energy from respiration of organic molecules o Fermentation – energy from converting carbohydrates o Phototrophic – energy from sunlight  Oxygen requirement o Aerobic – dissolved oxygen o Anaerobic – bonded oxygen in the form of NO3 or NO2 o Anaerobic (e.g. fermentation)

Degradation of Organic substance  Heterotrophic process  Most of the organic substance (BOD, COD or TOC) are biologically degradable  Biological degradation (respiration) results in o Growth of microorganisms o Oxygen consumption  Aerobic (dissolved oxygen)  Anoxic (NO3 or NO2)  End products o Carbon dioxide (when NO3 and N2 are reduced) o Water o Synthesis of biomass (new cell growth)

Denitrification  Returns nitrogen to atmosphere by reducing nitrate to nitrogen gas NO3- → NO2- →NO + N2O → N2  Performed by a diverse range of Heterotrophic bacteria that have nitrate reductase, Nitrite reductase, N2O reductase  Anoxic conditions → bacteria require NO3 as a substitute electron acceptor  Requirements o Anaerobic conditions → inhibited by oxygen however require NO3 and/or NO2 o Electron donor → organic carbon source (BOD/COD)

Nitrification  Two steps process o Oxidation of ammonia to nitrite using ammonia oxidising bacteria such as Nitrosomans (aerobic condition) NH4 +O2 → NO2- + H2O o Oxidation of nitrite to nitrate using nitrite oxidising bacteria such as Nitrobacter spp (aerobic condition) NO2- + O2 → NO3 Autotrophic process as AOB and NOB fix CO2 into biomass using energy yielded from their chemical reactions  Slow growing  Requirements o Aerobic : DO (4.5mh per 1 mg of NH4) o pH > 6 (optimal between 7.3 – 8) o Temperature > 5 (optimal between 25-300C o Inorganic carbon (CO2 or CaCO3) o Low BOD concentrations – in high BOD concentration, conditions will promote the growth of heterotrophic organisms and dominate the slow growing AOB and NOB ANAMMOX (deammonifcation)  NH4 is oxidised to N2 under anaerobic conditions using NO2 as the electron acceptor  Three steps o Reduction of nitrite to nitric oxide by nitrite reductase o Condensation of ammonium and nitric oxide to form hydrazine (N2H4) by a hydrazine synthase o Oxidation of hydrazine into dinitrogen gas by a hydrazine oxidoreductase  Autotrophic process  Very slow growing  Requirements o NH4 to NO2 ratio is 1 : 1 o Anoxic and Mesophilic conditions o Alkaline solution o Low BOD

Phosphorous removal  All bacteria contain ~1-2% of P in their biomass while others accumulate larger quantities of P (5-7%)  Fermentation of BOD/COD produce volatile fatty acid (VFA)  Anaerobic conditions required to assimilate VFA into carbon storage  Aerobic conditions are then required for the internal/stored carbon source to be oxidized providing energy to grow and form/store polyphosphate

BNR reactors Suspended growth Attached growth  Biological floc exists in  Biofilm system suspension via aeration and  Biological slime attached to a mixing surface  Complex consortia of  Group of microbes that adhere microorganisms together other on a surface  Filamentous bacteria play an  Embedded within a selfimportant role in floc formation produced matrix of  Biomass / water separated by extracellular polymeric sedimentation substances (EPS) – “Glue”

The Bathing Medium  Biofilms depend on bathing medium (sewage) for supply of nutrients  Transportation is termed mass transport or mass flux o Diffusion → rate determined by the concentration gradient o Attached to a surface means they benefit from convection  Bulk movement / flow  Shear force  Thickness of the biofilm is related to the flow rate of the bathing medium  Shear stress caused by viscosity & turbulence on the biofilm causes detachment (“sloughing”)

Biofilms  Variations in pH, oxygen and nutrient availability, it creates a wide range of ecological niches  The distribution of species within the community are not spatially uniform o This is because the aerobic bacteria takes up the O2 reducing the O2 and creates an anaerobic conditions for the anaerobic bacteria, resulting in the stratification of aerobes and anaerobes o This also results in the stratification of slow (aerobes) and fast (anaerobes) growing microbes and gradient of nutrients  Results in highly diverse ecosystems which can perform more functions

Attached growth system – Trickling filters Components  Infiltrating water (primary effluent) percolates over highly permeable media, high in surface area  Height is typically 1.5-5 m tall  Media s a community of microorganisms  ive aeration of infiltration water  Media bed (highly permeable) o Rock bed  SA of 50-90m2 , Void of 50-60%  Good BOD removal  Small void percentage leads to ponding creating anaerobic dead zones, results in poor nitrification  Ponding eventually causes odours and files o Plastic media  Corrugated → high surface area (up to 240 m2)  Uniformed → 95% void space, prevents clogging  Promotes a high aerobic environment → good nitrification  Underdrains and ventilation ports → collect water to take away from m media and also allow air to enter and aerate the system Set up of trickling filter  Singular tank → mainly for BOD removal o Recycle to return nitrified effluent (high in NO3) to the top of the filter o Utilize BOD in the influent to drive denitrifcation o Occurs within anaerobic micro-environments within the biofilm  Operate in series o First tank → meant for BOD removal Second tank → meant for nitrification (exposed to low con. of BOD)

Design criteria  Based on loading o Flow per surface area per day o Organic loading o Nitrogen loading  BOD removal o High flow rate → high BOD load = poor nitrification o Because fast growing heterotroph outcompete slow growing nitrifiers for oxygen and space  Nitrification o Low flow rate → low BOD load = good nitrification o Plastic media to improve oxygen transfer into the biofilm o Nitrification can occur only once BOD has exhausted

Advantages  Easy and robust process to operate  Stable operation  No aeration system required  No sludge return  Low energy consumption as the rotating arm is hydraulically driven by the water coming out from the nozzle

Disadvantages  No simultaneous phosphorus removal  Difficult to achieve total N removal  Risk of clogging and odour problems with rock media  Grazing snails consume and dominate biomass  Temperature problems in cold climate cause the bed to ice up  Large physical footprint

Suspended growth reactor – activated sludge Clarifier → sludge (biomass) separation using a big sedimentation tank Operational parameters of clarifier  Mixed liquor suspended solids (MLSS) → crude estimates of biomass concentrations within the reactor o Adjust according to season as the rate of reaction depends on temperature  Winter → colder → reaction falls  Sludge retention time / Sludge age o Calculated by dividing the total amount of solids in the system by the rate of loss o Controlled by sludge waste rate (WAS)  Waste more → young sludge  Waste less → older sludge o Controls the microbial population in the activated sludge  Old sludge age encourage the slow-growing nitrifiers to grow as it need at least 8 days old sludge to have nitrifiers  Sludge loading o Food to microorganism ration (F/M)  Dissolved oxygen concentration → has to be maintained to achieve BOD removal  Recycle/return rates  Settleability → measure how well the biomass settles o Poor settling sludge (bulking sludge) → caused by filamentous bacteria o Results in high effluent SS concentration which impacts the environment or downstream treatment process o Washout of biomass reduces sludge age  Sludge Volume Index (SVI) =

Denitrifcation process (issues)  Nitrate is produced during nitrification and denitrification needs to follow to reduce the NO3 to N2.  As denitrification is heterotrophic process, it requires an organic carbon source, however during nitrification, the organic substrate (BOD) required for denitrification is concerted  Problem can be solved by : o Dosing of external carbon source  Post Denitrification o Special reactor layout  Modified Lutzack Ettinger (MLE)  Simultaneous Nitrification – Denitrification MLE Pros  Small reactor volume  Make use of the incoming BOD Cons  High process control, high variability = complicated  Common for bigger plants (large footprints)  Increased pumping

Simultaneous Nitrification – Denitrification Pros  Stable, easy operation, very good effluent quality  Common for smaller plants or very low effluent targets Cons  More reactor volume, more expensive during construction

Post Denitrification Pros  Very good effluent quality, no internal recirculation Cons  External carbon source = high operational cost

Other reactors Attached growth system Rotating biological ors  Aeration achieved by rotation of disc  Biofilm grows on the disc Pros  Low energy consumption  Stable and easy process  Good for small WWTPs Cons  Less specific surface than trickling filter  Eventual odour problems when it becomes anaerobic  Housing required (cold conditions)

Suspended growth system Integrated fixed-film activated sludge (IFAS)  Moving/suspended bed of plastic media increase surface area  Hybird floc and biofilm system  Long biomass retention time and thus s slow growing nitrifiers Pros  Small physical footprint Cons  High energy demand as high aeration rate needed to suspend the bed  Plastic becomes brittle and breaks away, blocking the screens

Suspended growth system Sequencing batch reactor  Not continuous flow and can alter sewage concentration to increase carbon content  Single reactor o Feed o React o Settle o Decant  Nitrification and Denitrification occur in 1 reactor → time orientated Pros  Small physical footprint

Suspended growth system Membrane bioreactor (MBR)  Combination of activated sludge with membrane filtration  High concentration of MLSS is retained  Coarse aeration to limit membrane fouling Pros  High removal of pathogens  Good quality effluent, suitable for reuse  Compact, small footprint Cons  Periodic backwashing / chemical cleaning of membrane to remove fouling layers increase maintenance cost  Energy intensive

Attached growth vs Suspended growth Attached growth Pros   Cons  

No aeration (ive aeration by ventilation) reduce cost Simple design and operation Large physical footprint Poor total N and P removal

Suspended growth Pros   Cons  

High nutrient removal Compact design High energy requirement Process control

Waste stabilisation pond Operates like a „natural system‟ consisting of co-metabolic activity (anaerobic and aerobic process) Water column  Algae photosynthesize producing O2 and it creates aerobic conditions  Aerobic respiration of organic carbon produces CO2  Algae uptake of NH4 and PO4 through assimilation  Nitrification of NH4 to get NO3  Algae uptake of NO3  Ammonium volatilisation (pH increase during photosynthesis) Sediment  Bound nutrients (algae dies and settle to the bottom)  Anaerobic respiration (Denitrification of NO3 to N2)  Mineralisation leads to release of NH4 and PO4  Nitrification in sediment – water interface Shallow  Light penetration has solar radiation which is a power disinfectant  Critical barrier in water reuse schemes for pathogen removal Desluge

Pros  Low capital and operation cost  Energy efficient (low greenhouse footprint) Cons  Low rate compared to the amount of space used  Difficult to separate the algae as it doesn‟t settle well  Expensive to remove the algae Classification  Based on organic loading  Primary lagoons o Deep, dark, anaerobic pot o Can be covered or uncovered (capture CH4) o Sedimentation and BOD removal  Secondary o Facultative (anaerobic and aerobic degradation) o > 1-2 m deep  Tertiary (maturation pond) o Shallow (<1.5m) o Polishing of nutrients, solids and pathogen inactivatin o Aerobic

Sludge Treatment Aims  Stabilisation o Reduction of the organic load → CO2 and CH4 o Prevention of odour disturbance o Pathogen reduction o Exhaust organic material to avoid GHG emission during dewater or storage  Reduction of sludge volume o Storage space o Reduction of transport and removal costs o Separation of the sludge liquor by  Thickening  Dewatering

Anaerobic digestion  Complex microbial community  Aim is to break down solid waste through biological degradation  Without O2 – prevented from entering the system through physical containment  Mesophilic conditions o Most common form of digestion o Retention time 2 – 4 weeks  Production o biogas (CH4) which can be used to generate power  Mixing occurs via gas production and mechanical means o Mechanical mixers o Pumping o Gas  Heating to maintain mesophilic tempeartures o Heat exchange → recover heat from cooling power generators / boiler o Heat incoming sludge

Sludge Stabilization  Biological processes o Anaerobic Digestion  Fermentation / Methanogenesis create CH4  Pychrophilic (low rate) → T = 10 – 200C  Mesophilic (high rate) → T = 30 – 370C  Thermophilic → T = ca. 550C  More insulation / heating required  High rate / smaller footprint  Good pathogen ki o Aerobic digestion  Aerobic respiration of organic carbon create CO2  Simultaneous aerobic stabilisation (extended aeration)  Composting  Thermal process o Incineration Simplified microbial pathways  Anaerobic (without oxygen)  Hydrolysis using microbial enzymes o Breakdown of log chained polymer (carbohydrates, fats, protein) into monomers (simple sugars, fatty acids, amino acids)  Fermentation of products from hydrolysis o Produces alcohols or volatile acid (acetate) + CO2  Methanogenesis of products from fermentation o Acetate or CO2 are converted to CH4

Sludge co-digestion  Increase biogas production by o Co-digest wastewater sludge with organic rich industry waste  Use existing mesophilic digesters  Tanker in organic trade waste from industry o Animal waste o Fish and seafood production o Fruit, vegetable, algae o Oil and fats, carbohydrate and whey o High in BOD/COD  Waste that would otherwise be discharged into sewer as trade waste and thus by- the sewer network  Combination of good quality gas and efficient turbines can double or triple power production  Industry waste can provide an energy and income source  Benefits o Increase energy production o By- sewer network  Waste that would otherwise be discharged into sewer as trade waste → industry saves on permits  Reduce odours, blockages, corrosion of sewer network  By- primary and secondary treatment as there is a lower oxygen demand for the secondary treatment processes  Cheap for industry as it does not need to fund aeration process in secondary treatment  Also reduce disposal cost

Mechanical Dewatering Aims  Reduction of sludge volume  Making storage possible  Reduction of disposal costs Processes  Mechanical processes o Centrifuge  Natural processes o Sludge drying beds Conditioning of sludge prior to mechanical dewatering necessary Sludge stabilisation lagoons  Essential that bioavailable carbon is exhausted and dried to minimise CH4 emissions during storage  Reduce carbon tax  Reduce odours

Water reuse Risk Analysis of waste reuse schemes  Assessment of the water recycling system o Identification of source water (stormwater or sewage) o Intended end-use → purpose o Identify major hazards → pathogen of concern  Identify preventative measures o Selection of barriers – critical control points o Depends on the end-use (opportunity for exposures) o Exposure Assessment  Operation procedures o Identify monitoring parameters o Parameters for the detection of process failres  Validation o To ensure health-based targets are met  Emphasis on “knowing your system” o Identify vulnerabilities and risk of the system which need to be managed Health-based targets  Performance targets for treatment process in of o Required log removal value of pathogens via treatment o On-site controls aimed at reducing exposure  Defined by : o Concentrations of pathogens in sewage → guidelines provide default values o Does response data  Relationship between the dose of pathogen and the likelihood of illness  Tolerable does o Exposure  Depends on end-use (opportunity for exposure)  Frequency & duration of exposure will influence dose  More likely to be exposed = higher health based target

Index pathogens  Impractical to set human health-based targets for all pathogens  Campylobacter for bacteria  Rotavirus and Adenovirus for viruses  Cryptosporidium for protozoa  Chosen because o Represent 3 key pathogens groups o All associated with faecal contamination → abundant in storage o Good does response data available  Information of the relationship between dose of these pathogens and likelihood of illness  Data available from investigations of outbreaks or experimental studies o Reasonably infective

Validation of health-based targets  Measure log removal value (LRV) o Demonstrate LRV of individual process units to assess the contribution of individual process units to pathogen removal o Treatment process train as total entity cannot be validated from influent to effluent  Based on removal of microbial indicators (surrogates) o Indicators should mimic pathogen behaviour/fate of pathogen so that  Have same removal mechanisms  Removed by the treatment process equivalent or lesser extent than the pathogen