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1 Wastewater Treatment Aware of the public health aspects and goals of wastewater treatment Able to describe the processes involved in primary, secondary.

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Presentation on theme: "1 Wastewater Treatment Aware of the public health aspects and goals of wastewater treatment Able to describe the processes involved in primary, secondary."— Presentation transcript:

1 1 Wastewater Treatment Aware of the public health aspects and goals of wastewater treatment Able to describe the processes involved in primary, secondary and tertiary treatment Able to compare the differences between the fixed-film and suspended growth systems in biological treatment Aware of some methods available for nutrient removal On completion of this segment you should be:

2 2 Wastewater Treatment Goals Protect public health from contamination of water supplies Reliable and economic operation Minimum capital cost Aims

3 3 Wastewater Treatment Goals (cont) Removal of floating, suspended and soluble matter Reduce BOD, COD pathogenic organisms and nutrient Maintain aesthetics of natural water bodies, ecology of water systems Outcomes

4 4 Typical Characteristics of Wastewater

5 5 Treatment Selection Wastewater treatment comprises primary, secondary and tertiary treatments The selection of appropriate treatment processes is dependent upon the nature and strength of pollutants, quantity of flow, and discharge licence conditions

6 6 Primary Treatment Usually the first stage of wastewater treatment comprises largely physical processes. A well-designed primary treatment should remove about 40 - 75% of TSS and about 25 - 40% BOD 5 A possible pre-treatment is the injection of air, O 2, H 2 O 2 and pre-chlorination if the influent is 'stale Processes include screening, grit removal and primary settling

7 7 Screens Fixed or mechanical Velocity in channels about 0.3 - 0.4 m/s Design for PWWF All screenings to be removed/buried Location of strong odour from decomposition The removal of large objects that may damage pumps or block channels

8 8 Mechanical bar screen

9 9 Rotating drum screen

10 10 Comminutors These are mechanical cutting screens that reduce the size of large objects Shredded matter are returned to the flow stream A by-pass may be included

11 11 Comminutor

12 12 Grit Chambers Purpose is to remove inorganic grit/sand 0.2 - 1 mm size through differential settling Aim is to prevent damage to pumps, blockage of channels and cementing of sludge in settling tanks Two types of grit chambers, namely constantly velocity and aerated/spiral flow tanks

13 13 Constant Velocity Grit Chamber Class I settling - horizontal flow Uniform velocity at 0.25 - 0.35 m/s Ideal parabolic shape or approximation Width:depth ratio 1:1 Length 18 x max. depth

14 14 Constant Velocity Grit Chamber

15 15 Aerated or Spiral Flow Grit Chamber Flexibility of control; more efficient grit removal and can assist pre-aeration Air supply or spiral flow controls the amount of silt removed Suitable for larger population > 10 000 ep HRT of about 3 min at PWWF

16 16 Aerated or Spiral Flow Grit Chamber

17 17 Vortex Flow Grit Chamber

18 18 Primary Sedimentation Aim is to remove gross suspended solids (organic matter) Largely class II settling of flocculent matter and natural coalescence or flocculation occurs Surface skimmers remove floating matter (scum, grease etc) The settled solids are pumped to an anaerobic digestion tank. The effluent (settled sewage) from primary treatment flows to the next stage ie. secondary treatment Solids separation by gravity

19 19 Some Features of Primary Settling Design to accept 2 to 3 x ADWF Removal of 40 - 75% suspended solids Some incidental BOD 5 reduction 25 - 40% Hydraulic loading Q/A 30 m 3 /m 2.d Hydraulic retention time (HRT) 1.5 to 3 h; depth 2.5 to 5 m Also act as flow/strength equalisation basins Sludge scrapers should not cause re-suspension

20 20 Primary settling % removed vs time

21 21 Types of Primary Settling tanks Tanks use less space Forward velocity 10 - 15 mm/s Weir loading rate < 300 m 3 /m.d Length:width ratio 3:1 Rectangular horizontal-flow

22 22 Rectangular horizontal-flow

23 23 Types of Primary Settling tanks Square with 60 o sludge hopper No moving parts as sludge is removed hydrostatically Some possible particle carry over Up-flow tank

24 24 Up-flow settling tank

25 25 Types of Primary Settling tanks Inflow to a central stilling box Radial-horizontal flow Uses radial scrapers to remove sludge Circular radial flow tank

26 26 Circular Radial Flow Tank

27 27 Circular Radial Flow Tank

28 28 Circular Radial Flow Tank

29 29 Pulteney Bridge and Weir, City of Bath

30 30 Secondary Treatment Objective is to remove the remaining suspended solids and also dissolved solids The process is mainly biological using microorganisms to convert the dissolved solids to biomass Two distinct systems are available i.e. fixed film (trickling filter) and suspended growth (activated sludge) The biomass is removed as sludge in final sedimentation tanks (clarifiers) Removal of dissolved solids through microbial action

31 31 Typical microorganisms in activated sludge

32 32 Fixed-Film Systems Land treatment, trickling and rotating biological filters are predominantly aerobic biological processes Land treatment ie. broadcasting of sewage, is one of the earliest forms of wastewater treatment

33 33 Trickling Filter Comprising an inert structure for growth of biofilm containing microorganisms (attached growth) Microorganisms in biofilm interact with wastewater and metabolise the organic matter (BOD) into CO 2 and H 2 O Natural sloughing of the biofilm when it reaches a thickness that cannot be sustained Filter medium voids (40 – 60%) promote air circulation and aerobic condition Solids in the effluent are separated in the secondary settling (humus) tank

34 34 Interaction of biofilm

35 35 Trickling Filter

36 36 Trickling filters at Wetalla

37 37 A rotating biological contact unit

38 38 Suspended Growth Systems Microorganisms are held in suspension as a high concentration flocculent, bulky matter through agitation, stirring The microorganisms interact with influent wastewater and biodegrade organic matter into CO 2, H 2 O and by- products, releasing energy for growth of new cells The activated sludge process is an example of an aerobic suspended growth system. The anaerobic digester for the break down of waste sludge is an example of an anaerobic suspended growth system

39 39 Activated Sludge Process The heart of the process is the reactor where aeration and oxidation of organic compounds occur Microorganisms are held in suspension by aeration and stirring Energy requiring process but has greater control and flexibility Return activated sludge and sludge wasting maintain the design biomass concentration (MLVSS) Final clarifier separates solids from the clear effluent and returns the settled sludge to the reactor

40 40 Activated sludge process with alternative wasting locations

41 41 Surface aerators

42 42 Final sedimentation tank

43 43 Final clarifier

44 44 Comparison between attached film and suspended growth systems ParameterTrickling filterActivated sludge BOD removal85 – 90%> 95% Lower limit of BOD effluent15 mg/L< 10 mg/L Capital costHighModerate Operating costMinimalHigh Land requirementHighLow Operator controlLimitedMore Shock loadsRapid recoveryVery slow FoamingNoneOften OdourYesMinimal Filter fliesYesNone NoiseMinimalModerate Hydraulic washoutNoYes PluggingYesNo Drying of mediaYesNo Output of sludgemoderateHigh

45 45 Wastewater Disinfection Some microorganisms (10 5 – 10 7 /100 mL) are still present in treated wastewater after secondary treatment Disinfection is required to reduce pathogenic microorganisms Chlorine is still the cost-effective disinfection, but requires minimum contact time and has adverse effects Other environmental friendly methods include UVL, ozone disinfection, membrane microfiltration and constructed wetlands

46 46 Sludge Digestion Sludge from primary and secondary settling tanks (including waste activated sludge) must be treated in digesters Sludge is thickened before passing to sludge digesters Sludge may be treated anaerobically or aerobically Anaerobic sludge digestion involves 2 sequential stages ie. acid formation and methane formation Digested sludge is dewatered before disposal

47 47 Low rate single-stage sludge digester.

48 48 High rate two-stage sludge digester.

49 49 Anaerobic sludge digester.

50 50 Aerobic sludge digester.

51 51 Tertiary Treatment Tertiary maturation ponds – an aerobic polishing process with detention time and further reduction in BOD and TSS (NFR) Nutrient removal comprising nitrification and denitrification and phosphorus removal Microfiltration and reverse osmosis

52 52 Nano-membrane filtration

53 53 Nutrient Removal Total nitrogen may be about 35 mg/L and total phosphorus 8 mg/L after secondary treatment Raw sewage composition of C:TN:TP 100:25:6 Normal plant growth only need C:TN:TP of 100:15:1 The major components of nutrients in wastewater are nitrates and phosphates. They contribute to the eutrophication of receiving water

54 54 Nitrogen Removal Ammonia is first oxidised to nitrites and nitrates through a process of nitrification by microorganisms Nitrification uses aerobic autotrophic microorganisms Dinitrification uses facultative heterotrophic microorganisms under anoxic condition where nitrates are converted to nitrogen gas Involves two stages of microbial action under different conditions

55 55 Phosphorus Removal Use of coagulants e.g. lime, aluminium sulfate, ferric chlorine will precipitate phosphorus Process is expensive and results in quantities of difficult sludge Preferred process is through microbial action with uptake of phosphorus by a select group of microorganisms Process may be through chemical precipitation or by preferred microbial action

56 56 Biological phosphorus removal Modified Bardenpho process

57 57 End of Module 18

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