Wastewater Characterisation and Treatment

Transcription

1 Wastewater Characterisation and Treatment Recommended text books: Wastewater Engineering Metcalf and Eddy Standard Methods for the Examination of Water and Wastewater Contact: Benoit Guieysse RC.2.18 Lecture block outline Characterisation Physical pollutants Chemical pollutants Bio pollutants Sampling Treatment Primary Secondary Tertiary Disposal The big picture: Understanding the nature of wastewater is essential in the design and operation of collection, treatment, and reuse facilities and in the engineering management of environmental quality We need to know what s in it before we can decide what to do with it! 1

2 10. Wastewater Treatment Example: City of Toronto, Canada Typical steps in wastewater treatment: from the most cost-efficient to the least! Removal SS (include some COD) Removal BOD/COD & nutrients Removal nutrient & pathogens Preliminary treatment Primary treatment Secondary treatment Tertiary treatment Removal large debris, grease Equalization Sludge treatment Typically: Preliminary and Primary treatments are based on physical mechanisms Secondary = biological Tertiary = specialized Sludge digestion, stabilization & dewatering 2

3 Common processes for pollutant removal Pollutants Common Processes Debris, large solids Grid removal Suspended solids (including VSS) Sedimentation FOGDissolved Air Flotation Dissolved solids Coagulation-Flocculation Organic pollutants* Biological treatment N Biological nitrification-denitrification P Chemical precipitation Priority pollutants** Adsorption Pathogens** Biological treatment (maturation ponds) and specific treatment (UV irradiation, chlorination) * A fraction of organic pollutants found as suspended solids will be removed with solids. ** A fraction of priority pollutants and pathogen can be indirectly removed with solids. Membrane filtration (from ultra- to nano-filtration and reverse osmosis) are gaining popularity in situations where reuse is necessary or space seriously limited. Membrane bioreactors combine size separation with biological removal. PRELIMINARY PRIMARY Waste water Screening Grit removal Flow balancing (optional) Dissolved air flotation Sedimentation Solids Landfill Sludge SECONDARY AEROBIC Suspended culture Attached culture ANAEROBIC Suspended culture Anaerobic lagoon UASB Contact process Attached growth Anaerobic filter Sludge SLUDGE THICKENING/ DEWATERING TERTIARY Specialised Processes SLUDGE TREATMENT DISPOSAL / REUSE DISPOSAL / REUSE 3

4 Important definitions Pollutant loading rate = amount of pollutant reaching a tank/process per unit of time usually kg/d Example: A wastewater containing 100 g COD/m 3 and 250 g TS/m 3 is treated in a pond. The wastewater flow rate is 300 m 3 /d. The organic loading rate = = 30,000 g COD/d = 30 kg COD/d. Similarly, the solid loading rate = 75 kg TS/d. Hydraulic Residence Time (HRT) = amount of time a liquid volume introduced into a tank/process will stay inside the tank/process before being removed Solid Retention Time (SRT) = amount of time a solid would remain in the system. Pollutant loading rates and the retention times are important design criteria during WWT. Preliminary & Primary Treatment Objectives: Balancing of flows, screening/settling and fat removal. Crucial steps that reduce the pollutant load to the rest of the facility. Offer best value for $$ 4

5 Examples large debris removal Flow / organic loading Need for equalization Typical daily variation of municipal wastewater: water use peaks during morning and evening are seen with a 1-2 hours delay at the WWT hours Flow / organic loading Typical yearly variation of municipal wastewater in South France: Winter peaks reflect storms and summer peaks reflect the increase of population (up to 100 times)! Month 5

6 Balancing and storage Tank or pond used to store wastewater Reduce impact of changes in wastewater flow-rate and strength Adjust ph if needed Storage capacity in case of breakdown Useful if wastewater is used for irrigation Fat removal Dissolved Air Flotation 6

7 Primary treatment Physical separation of solids from the wastewater by sedimentation Examples 7

8 Secondary treatment All most all wastewaters containing biodegradable constituents can be treated biologically. You must understand the processes to ensure the proper conditions are produced/controlled effectively. Main processes for Secondary treatment Activated Sludge Aerobic ponds Aerobic suspended Aerobic attached Trickling filter Rotative disks Anaerobic pond Anaerobic suspended Anaerobic attached UASB + multitude of hybrid (anaerobic/aerobic or suspended/attached), anoxic (e.g. NO 3- instead of O 2 ) processes in various combinations. Process can also be classified as batch/continuous etc 8

9 Main principle of biological removal: Food + organisms = Products + biomass Carbon source (organic compounds) Nutrient sources: N, P, H, O etc Energy source (Organic and inorganic compounds) Electron acceptor: O 2, NO 3-, CO 2 etc Cell Products: CH 4, oil, NO 3-, N 2, NO 2-, CO 2, O 2, heat, enzymes, toxins etc More cells (containing C, N, and P) Aerobic Carbon removal Organics compounds are used as sources of carbon and energy C 10 H 19 O 3 N + O 2 + N + P C 5 H 7 NO 2 + CO 2 + H 2 O + NH Symbolize organic matter in WW = pollutant. In fact, WW is made up of 1000s of different compounds Nutrients Biomass that must also be removed (sludge). Note the biomass contains N and P and can be itself considered as organic pollutant Anaerobic carbon removal Organics Biomass + CO 2 + CH 4 + NH 4 + Organic pollutants are converted into methane (energy), CO 2 and biomass (sludge) These reactions summarize far more complex mechanisms! 9

10 Anoxic carbon removal Anoxic treatment often means that oxygen is absent and replaced by nitrate of sulfate. Nitrate instead of oxygen: C 10 H 19 O 3 N + NO 3- + nutrients C 5 H 7 NO 2 + CO 2 + N This process is also known as denitrification (see N-removal) Sulfate instead of oxygen: C 10 H 19 O 3 N + SO nutrients C 5 H 7 NO 2 + CO 2 + H 2 S +... H 2 S formation results in bad smells N-Removal As seen above, biomass is made of C, N and P (C 5 H 7 NO 2 ). N and P are therefore removed by assimilation = uptake into cells during population growth. This is the main mechanism for N and P removal in ponds Biological nitrogen removal is based on the conversion of N (organic or inorganic) into N 2 that escapes into the atmosphere. This is a 2-step process: 1. Nitrification (AEROBIC conditions) conversion of NH 4+ to NO 3 - NH 4+ + CO 2 + O 2 biomass + NO Followed by denitrification (ANOXIC) - conversion of NO 3- to N 2 C 10 H 19 O 3 N + NO 3- biomass + N 2 + CO 2 10

11 Phosphorus Removal Bio-P removal: relatively new (and unreliable) technology Special organisms take up more P than is required for growth More traditional approach is P removal by precipitation Impact of N and P on NZ freshwater: The Activated Sludge process Reactor micro-organisms are kept in suspension as flocks Liquid/solids separator usually a sedimentation tank Recycle stream for maintaining adequate biomass conc. 11

12 Trickling Filters The filter is a non-submerged, fixed film reactor using rock or plastic packing (almost all new filters are constructed with plastic packing). The wastewater is evenly distributed over the top of the bed by a rotary distributor The mircro-organisms grow on the packing. Treatment occurs as the wastewater flows over the film. Anaerobic treatment Advantages Less energy required (no need for forced aeration) Less biological sludge production Fewer nutrients required CH 4 production energy source Elimination of off-gas air pollution (no forced aeration) Disadvantages Longer start-up time May require alkalinity addition May require polishing Bio N and P removal is not possible Much more sensitive to lower temps Potential for production of odours Potential for production of corrosive gases 12

13 UASB (upflow anaerobic sludge blanket) Rely on granulation which enables very high sludge concentrations to accumulate at the base of the reactor. Liquid and gas flow suspend granules. Baffles retains bacterial granules, separate gas/liquid. HRT of d Wastewater Ponds (Lagoons) This is the more low-tech of WWT reactors: usually an earthen basin, which might be covered with an impermeable liner. Very common for small communities or as maturation ponds (tertiary treatment) Normally in series 2/3 ponds New Zealand: 200 USA: 3,500+ France: 2,

14 Disadvantages Advantages Why ponds? Large land area needed Temperature effect Process modification and control difficult High SS in effluent Ready equalisation Low maintenance Primary/secondary/ tertiary Easy operation 27 Types of ponds Anaerobic Facultative aerobic Maturation (Aerobic pond) Surface aerated 28 14

15 Anaerobic ponds 2-5m depth, no algae, HRT of d Relatively small with a high organic load kg BOD/ha-day % BOD removal. Problem with methane release in the atmosphere (unless the pond is covered and methane is burned). 29 Facultative ponds Treatment mechanisms in a facultative pond (Source: Wastewater Engineering by Metcalf & Eddy, 1991, pg 437) 15

16 Facultative ponds Example of design criteria: kg BOD/ha day, 1.0-2m depth, d HRT (for temperate-subtropical weather, kg BOD/ha day, 1.5-2m depth, d HRT) 90% BOD removal The algae provide oxygen and capture CO 2 during photosynthesis. CO 2 capture can cause the ph to increase, which improves pathogen kills, N stripping and P precipitation 31 Maturation ponds (aerobic lagoons) Look like facultative, 1-1.5m deep Low organic loading well oxygenated 32 16

17 Aerated pond typically, mechanical aeration on floats or fixed platforms earthen basin Lagoon depth: 1 3m Similar as a facultative ponds with the upper layer is aerated with surface aerators in order to avoid odor formation! Aeration is often intermittent (during night) 5-25 days retention times 10 g BOD/m 3 -d Irrigation Salinity: related to electrical conductivity (can use TDS as a measure) Nutrients: provide fertilizer (phosphorus is often bound in the soil but nitrogen can leach quite readily). Fats and biological growth can cause the blocking of sprinkler systems; an issue for using WW for irrigation. Discharge areas should be rotated (approx every 20 days) to allow organic and nutrient conversion 17

18 Sludge disposal Sludge = wastes from screens, primarily clarifiers and biosolids from bioreactors Disposal: land application, landfills, incineration Sludge processing: key process is thickening helps transportation, digestion, drying and combustion Sludge can often be digested anaerobically (more common) or aerobically. Digested sludge can be composted for further stabilization (pathogen removal). Plant Performance Criteria Activated Sludge Plant Biological filter Comparison Aerated Lagoon Waste stabilization pond BOD Removal F F GG Pathogen removal P P GG SS Removal GGF F Economic Factors Simple & economic construction P P F G Simple operation P F P G Land Requirement GGF P Maintenance cost P F P G Energy Demand P F P G Sludge Removal costs F F F G G = Good - F = Fair - P = Poor 18

19 General overview Process Applications Advantages Disadvantages Cost Activated sludge Low/moderate conc. Proven, good control Emissions of volatile compounds and aerosols, high sludge production. Aeration costs +++ Aerated lagoons / ponds Trickling filter Low conc. Simple, low costs Emissions of volatile compounds and aerosols, sensitive to shock and climate, land requirment, no control Low conc., recalcitrant organics Little sludge, biodiverse Emissions of volatile compounds, sensitive to shocks, clogging, odor + + Anaerobic process High-strength Methane production, low sludge Sensitive to temperature, higher capital costs, odor ++ Source: Environmental Biotreatment. CN Mulligan. This is given as an example only, very specific of North America Conclusions Most large wastewater treatment systems are based on activated sludge variations as secondary treatment because this is the best described process. The trickling filter is used but there are some operational problems (clogging). Anaerobic technologies are generally recommended for effluents with high concentrations of biodegradable organic matter. There are therefore very commonly used for sludge digestion. They remain limited by odor and instability issues (temperature, toxicity). Ponds are common for primary and secondary treatment at small scales (decentralized treatment) or for tertiary treatment as stabilization ponds. 19

20 Example Case study: treating dairy wastewaters Characteristic Concentration Biochemical oxygen demand 90-12,400 (mg/l) Chemical oxygen demand ,000 (mg/l) Suspended solids 7-7,200 (mg/l) Nitrogen 1 70 (mg/l) Fat (mg/l) Phosphorus (as PO 4 ) (mg/l) ph 3 13 Temperature ( o C) Remember wastewater properties can change in with time and location! 20

21 Dairy wastewater Approx 14.7 billions litres milk produced in NZ each year (2005/06) Wastewater produced: m 3 wastewater per m 3 milk received This accounts for 7 29 billion litres of wastewater produced from milk production only! Treatment options Characteristic Concentration BOD 90-12,400 (mg/l) COD ,000 (mg/l) SS 7-7,200 (mg/l) N 1 70 (mg/l) Fat (mg/l) P (as PO 4- ) (mg/l) ph 3 13 Temperature ( o C) High BOD and COD values + good BOD/COD ratio = plenty of biodegradable organic matter = excellent for anaerobic treatment! Low concentrations of suspended solids = primary settling might not be efficient A balance tank would be useful (neutralize ph and temperature) and primary settling would help reduce the suspended solids. Fat removal is often necessary 21