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Wastewater Treatment On completion of this segment you should be:

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1 Wastewater Treatment On completion of this segment you should be:
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

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

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

4 Typical Characteristics of Wastewater

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 Primary Treatment Usually the first stage of wastewater treatment comprises largely physical processes. A well-designed primary treatment should remove about % of TSS and about % BOD5 A possible pre-treatment is the injection of air, O2, H2O2 and pre-chlorination if the influent is 'stale’ Processes include screening, grit removal and primary settling

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

8 Mechanical bar screen

9 Rotating drum screen

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 Comminutor

12 Grit Chambers Purpose is to remove inorganic grit/sand 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 Constant Velocity Grit Chamber
Class I settling - horizontal flow Uniform velocity at m/s Ideal parabolic shape or approximation Width:depth ratio 1:1 Length  18 x max. depth

14 Constant Velocity Grit Chamber

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 > ep HRT of about 3 min at PWWF

16 Aerated or Spiral Flow Grit Chamber

17 Vortex Flow Grit Chamber

18 Primary Sedimentation
Solids separation by gravity 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

19 Some Features of Primary Settling
Design to accept 2 to 3 x ADWF Removal of % suspended solids Some incidental BOD5 reduction % Hydraulic loading Q/A  30 m3/m2.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 Primary settling % removed vs time

21 Types of Primary Settling tanks
Rectangular horizontal-flow Tanks use less space Forward velocity mm/s Weir loading rate < 300 m3/m.d Length:width ratio 3:1

22 Rectangular horizontal-flow

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

24 Up-flow settling tank

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

26 Circular Radial Flow Tank

27 Circular Radial Flow Tank

28 Circular Radial Flow Tank

29 Pulteney Bridge and Weir, City of Bath

30 Secondary Treatment Removal of dissolved solids through microbial action 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)

31 Typical microorganisms in activated sludge

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 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 CO2 and H2O 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 Interaction of biofilm

35 Trickling Filter

36 Trickling filters at Wetalla

37 A rotating biological contact unit

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 CO2, H2O 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 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 Activated sludge process with alternative wasting locations

41 Surface aerators

42 Final sedimentation tank

43 Final clarifier

44 Comparison between attached film and suspended growth systems
Parameter Trickling filter Activated sludge BOD removal 85 – 90% > 95% Lower limit of BOD effluent 15 mg/L < 10 mg/L Capital cost High Moderate Operating cost Minimal Land requirement Low Operator control Limited More Shock loads Rapid recovery Very slow Foaming None Often Odour Yes Filter flies Noise Hydraulic washout No Plugging Drying of media Output of sludge moderate

45 Wastewater Disinfection
Some microorganisms (105 – 107/100 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 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 Low rate single-stage sludge digester
. Low rate single-stage sludge digester

48 High rate two-stage sludge digester
. High rate two-stage sludge digester

49 Anaerobic sludge digester
. Anaerobic sludge digester

50 Aerobic sludge digester
. Aerobic sludge digester

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 Nano-membrane filtration

53 Nutrient Removal The major components of nutrients in wastewater are nitrates and phosphates. They contribute to the eutrophication of receiving water 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

54 Nitrogen Removal Involves two stages of microbial action under different conditions 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

55 Phosphorus Removal Process may be through chemical precipitation or by preferred microbial action 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

56 Biological phosphorus removal
Modified Bardenpho process

57 End of Module 18

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