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

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

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

Typical Characteristics of Wastewater

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

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% 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

Screens The removal of large objects that may damage pumps or block channels 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

Mechanical bar screen

Rotating drum screen

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

Comminutor

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

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

Constant Velocity Grit Chamber

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

Aerated or Spiral Flow Grit Chamber

Vortex Flow Grit Chamber

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

Some Features of Primary Settling Design to accept 2 to 3 x ADWF Removal of 40 - 75% suspended solids Some incidental BOD5 reduction 25 - 40% 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

Primary settling % removed vs time

Types of Primary Settling tanks Rectangular horizontal-flow Tanks use less space Forward velocity 10 - 15 mm/s Weir loading rate < 300 m3/m.d Length:width ratio 3:1

Rectangular horizontal-flow

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

Up-flow settling tank

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

Circular Radial Flow Tank

Circular Radial Flow Tank

Circular Radial Flow Tank

Pulteney Bridge and Weir, City of Bath

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)

Typical microorganisms in activated sludge

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

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

Interaction of biofilm

Trickling Filter

Trickling filters at Wetalla

A rotating biological contact unit

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

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

Activated sludge process with alternative wasting locations

Surface aerators

Final sedimentation tank

Final clarifier

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

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

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

Low rate single-stage sludge digester . Low rate single-stage sludge digester

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

Anaerobic sludge digester . Anaerobic sludge digester

Aerobic sludge digester . Aerobic sludge digester

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

Nano-membrane filtration

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

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

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

Biological phosphorus removal Modified Bardenpho process

End of Module 18