Wastewater Treatment

1. Wastewater Treatment The effective treatment of wastewater to meet water quality objectives for water reuse applications and to protect public health is a critical element of water reuse systems. Municipal wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter, pathogens, metals, and sometimes nutrients from wastewater.

2. Wastewater Treatment General terms used to describe different degrees of treatment, in order of increasing treatment level are, preliminary, primary, secondary, tertiary, and advanced treatment. A disinfection step for control of pathogenic organisms is often the final treatment step prior to distribution or storage of reclaimed wastewater.

3. Wastewater Treatment The goal in designing a wastewater reclamation and reuse system is to develop an integrated cost-effective treatment scheme that is capable of reliably meeting water quality objectives. The degree of treatment required in individual water treatment and wastewater reclamation facilities varies according to: Specific reuse application Associated water quality requirements

6. Generalized flow diagram for municipal wastewater treatment

7. Preliminary Treatment The objective of preliminary treatment is the removal of coarse solids and other large materials often found in raw wastewater. Removal of these materials is necessary to enhance the operation and maintenance of subsequent treatment units. It typically include coarse screening, grit removal and , in some cases, comminution of large objects.

8. Preliminary Treatment In grit chambers, the velocity of water through the chamber is maintained sufficiently high, or air is used, so as to prevent the settling of most organic solids. Grit removal is not included as a preliminary treatment in most small wastewater treatment plants. Flow measurement devices, often standing-wave flumes, are always included at the preliminary treatment stage.

9. Primary Treatment The objective of primary treatment is the removal of settable organic and inorganic solids by sedimentation, and by removal of materials that will float (scum) by skimming. In conventional wastewater treatment facilities, primary treatment includes: Screening and comminuting for removal of large solids Grit removal Sedimentation

10. Primary Treatment Conventional primary treatment is effective in removing of: Particulate matter larger than about 50 μm from wastewater. In general, about 50% of the suspended solids and 25to 50% of the BOD5 are removed from untreated wastewater. Nutrients, hydrophobic constituents, metals, and microorganisms that are associated with particulates in wastewater. About 10 to 20% of the organic nitrogen and about 10% of the phosphorous are removed.

12. Primary Treatment Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units. Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units.

13. Primary Treatment Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units. Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units.

14. Primary Treatment In large sewage treatment plants, primary sludge is not commonly processed biologically by anaerobic digestion. In the digestion process, anaerobic and facultative bacteria metabolize the organic material in sludge, thereby reducing the volume requiring ultimate disposal, making the sludge stable and improving the dewatering characteristics.

15. Primary Treatment Digestion is carried out in covered tanks (anaerobic digesters), typically 7 to 14 m deep. The residence time may vary from a minimum of about 10 days for high rate digesters to 60 days or more. Gas containing about 60-65% methane ie produced during digestion and can be recovered as an energy source.

16. Primary Treatment In small sewage treatment plants, sludge is processed in a Varity of ways including: Aerobic digestion Storage in sludge lagoons Direct application to sludge drying beds In-process storage (as in stabilization ponds), and Land application

18. Primary Treatment The removal efficiency of primary treatment processes can be increased by incorporating coagulation/flocculation upstream of gravity sedimentation and /or by using filtration downstream of gravity sedimentation. For most wastewater reuse applications, primary treatment alone does not provide adequate treatment to meet water quality objectives.

19. Secondary Treatment The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual and suspended solids. In most cases, secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes.

20. Secondary Treatment Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-product (principally CO2, NH3, and H2O).

21. Secondary Treatment Secondary treatment systems include an array of biological treatment processes coupled with solid/liquid separation. Biological processes are engineered to provide effective microbiological metabolism of organic substrates dissolved or suspended in wastewater. The microbial biomass interacts with wastewater using a suspended growth or a fixed film process.

22. Secondary Treatment Several aerobic biological processes are used for secondary treatment differing primarily in: Manner in which oxygen is supplied to microorganisms, and Rate at which organisms metabolize the organic matter, which can be : High rate biological processes Low rate biological processes

23. Secondary Treatment Common high-rate processes include: Activated sludge processes Trickling filters or biofilters Oxidation ditches Rotating biological contractors

24. Secondary Treatment Common low-rate processes include: Waste Stabilization Ponds Overland Treatment of Wastewater Macrophyte Treatment Nutrient Film Technique

25. Secondary Treatment High rate biological processes are characterized: By relatively small reactor volumes and high concentration of microorganisms compared with low rate processes. The growth rate of new organisms is much greater in high rate systems because of the well controlled environment

26. Secondary Treatment A combination of two of these processes in series (e.g., biofilter followed by activated sludge) is sometimes used to treat municipal wastewater containing a high concentration of organic material from industrial sources.

27. Secondary Treatment/ Activated Sludge In the activated sludge process, the dispersed-growth reactor is an aeration tank or basin containing a suspension of wastewater and microorganisms, the mixed liquor. The contents of the aeration tank are mixed vigorously by aeration device (submerged diffusers that release compressed air or mechanical surface aerators that introduce air by agitating the liquid surface) which also supply oxygen to the biological suspension.

28. Secondary Treatment/ Activated Sludge Hydraulic retention time in the aeration tanks usually ranges from 3 to 8 hours but can be higher with high BOD5 wastewaters. Following the aeration step, the microorganisms are separated from the liquid by sedimentation and the clarified liquid is secondary effluent.

29. Secondary Treatment/ Trickling Filters A trickling filters or biofilter consist of a basin or tower filled with support media such as stones, plastic shapes, or wooden slats. Wastewater is applied intermittently, or sometimes continuously, over the media. Microorganisms become attached to the media and form a biological layer or fixed film.

30. Secondary Treatment/ Trickling Filters The thickness of the biofilm increases as new organisms grow, and periodically a portion of the film slough off the media and it is separated from the liquid in a secondary clarifier and discharged to sludge processing.

31. Secondary Treatment/ Trickling Filters Clarified liquid from the secondary clarifier is the secondary effluent and a portion is often recycled to the biofilter to improve hydraulic distribution of the wastewater over the filter.

32. Secondary Treatment/ RBCs Rotating biological contractor (RBCs) are fixed-film reactor similar to biofilter in that organisms are attached to support media. In case of RBCs, the support media are slowly rotating discs that are partially submerged in flowing wastewater in the reactor.

33. Secondary Treatment/ RBCs Oxygen is supplied to the attached biofilm from the air when the film is out of the water and from the liquid when submerged, since oxygen is transferred to the wastewater by surface turbulence created by the discs' rotation. Sloughed pieces of biofilm are removed in the same manner described for biofilters

34. Secondary Treatment High rate biological treatment processes, in combination with primary sedimentation, typically remove 85% of the BOD5 and SS originally present in the raw wastewater and some of the heavy metals. Activated sludge generally produces an effluent of slightly higher quality, in terms of these constituents, than biofilters or RBCs.

35. Secondary Treatment When coupled with a disinfection step, these processes can provide substantial but not complete removal of bacteria and virus. They remove very little phosphorous, nitrogen, non-biodegradable organics, or dissolved minerals.

37. Secondary Treatment Low-rate biological treatment systems are available for the treatment of organic wastewaters such as municipal sewage and tend to be lower in cost and less sophisticated in operation and maintenance.

38. Secondary Treatment Although such processes tend to be land intensive by comparison with the conventional high-rate biological processes already described, they are often more effective in removing pathogens and do so reliably and continuously if properly designed and not overloaded.

39. Secondary Treatment Among the low rate biological treatment systems available, stabilization ponds and land treatment have been used widely around the world and a considerable record of experience and design practice has been documented.

40. Secondary Treatment The nutrient film technique is a fairly recent development of the hydroponics plant growth system with application in the treatment and use of wastewater. Wastewater stabilization pond systems are designed to achieve different forms of treatment in up to three stages in series, depending on the organic strength of the input waste and the effluent quality objectives.

41. Secondary Treatment For ease of maintenance and flexibility of operation, at least two trains of ponds in parallel are incorporated in any design. Strong wastewaters, with BOD5 concentration in excess of about 300 mg/l, will frequently be introduced into first-stage anaerobic ponds, which achieve a high volumetric rate of removal.

42. Secondary Treatment Weaker wastes or, where anaerobic ponds are environmentally unacceptable, even stronger wastes (say up to 1000 mg/l BOD5) may be discharged directly into primary facultative ponds. Effluent from first-stage anaerobic ponds will overflow into secondary facultative ponds which comprise the second-stage of biological treatment. Following primary or secondary facultative ponds, if further pathogen reduction is necessary maturation ponds will be introduced to provide tertiary treatment.

43. Stabilization pond configurations AN = anaerobic pond; F = facultative pond; M = maturation pond (Pescod and Mara 1988)

44. Secondary Treatment/ Anaerobic Ponds Anaerobic ponds are very cost effective for the removal of BOD, when it is present in high concentration. Normally, a single, anaerobic pond in each treatment train is sufficient if the strength of the influent wastewater is less than 1000 mg/l BOD5. For high strength industrial wastes, up to three anaerobic ponds in series might be justifiable but the retention time in any of these ponds should not be less than 1 day (McGarry and Pescod, 1970).

45. Secondary Treatment/ Anaerobic Ponds Anaerobic conditions in first-stage stabilization ponds are created by maintaining a high volumetric organic loading, certainly greater than 100g BOD5/m3 d. Very high loadings, up to 1000g BOD5/m3d, achieve efficient utilization of anaerobic pond volume but, with wastewater containing sulphate concentrations in excess of 100 mg/l, the production of H2S is likely to cause odour problems. In the case of typical municipal sewage, it is generally accepted that a maximum anaerobic pond loading of 400g BOD5/m3d will prevent odour nuisance (Meiring et al. 1968).

46. BOD REMOVALS IN ANAEROBIC PONDS LOADED AT 250 g BOD5/m3d Source: Mara (1976)

47. Secondary Treatment/ Anaerobic Ponds Anaerobic ponds normally have a depth between 2m and 5m and function as open septic tanks with gas release to the atmosphere. The biochemical reactions which take place in anaerobic ponds are the same as those occurring in anaerobic digesters, with a first phase of acidogenesis and a second slower-rate of methanogenesis.

48. Secondary Treatment/ Anaerobic Ponds Ambient temperatures in hot-climate countries are conducive to these anaerobic reactions and expected BOD5 removals for different retention times in treating sewage have been given by Mara (1976). More recently, Gambrill et al. (1986) have suggested conservative removals of BOD5 in anaerobic ponds as 40% below 10°C, at a design loading of 100 g/m3d, and 60% above 20°C, at a design loading of 300 g/m3d, with linear interpolation for operating temperature between 10 and 20°C.

49. Secondary Treatment/ Anaerobic Ponds Higher removal rates are possible with industrial wastes, particularly those containing significant quantities of organic settleable solids. Of course, other environmental conditions in the ponds, particularly pH, must be suitable for the anaerobic microorganisms bringing about the breakdown of BOD. In certain instances, anaerobic ponds become covered with a thick scum layer, which is thought to be beneficial but not essential, and may give rise to increased fly breeding.

50. Secondary Treatment/ Anaerobic Ponds Solids in the raw wastewater, as well as biomass produced, will settle out in first-stage anaerobic ponds and it is common to remove sludge when it has reached half depth in the pond. This usually occurs after two years of operation at design flow in the case of municipal sewage treatment.