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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:

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Wastewater Treatment Goals

Protect public health from contamination of water supplies

Reliable and economic operation

Minimum capital cost

Aims

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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

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Typical Characteristics of Wastewater

Oxygen demand, BOD5 mg/L 200 - 400

Total suspended solids mg/L 200 - 300

Nitrogen mg/L 20 - 30 as NH3

30 - 70 total

Phosphorus, mg/L 8 - 16 total

Total dissolved solids, mg/l 400 - 600

Toxins eg metals, organics

Grease and oil

Total coliform number/mL 105 - 106

Fecal coliform, number/mL 104 - 105

Fecal streptococci number/mL 103 - 104

Enteric virus number/mL 10 - 102

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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

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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

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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

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Mechanical bar screen

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Rotating drum screen

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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

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Comminutor

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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

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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

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Constant Velocity Grit Chamber

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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

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Aerated or Spiral Flow Grit Chamber

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Vortex Flow Grit Chamber

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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

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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

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Primary settling % removed vs time

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Types of Primary Settling tanks

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

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Rectangular horizontal-flow

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Types of Primary Settling tanks

Square with 60o sludge hopper

No moving parts as sludge is removed hydrostatically

Some possible particle carry over

Up-flow tank

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Up-flow settling tank

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Types of Primary Settling tanks

Inflow to a central stilling box

Radial-horizontal flow

Uses radial scrapers to remove sludge

Circular radial flow tank

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Circular Radial Flow Tank

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Circular Radial Flow

Tank

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Circular Radial Flow Tank

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Pulteney Bridge and Weir, City of

Bath

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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

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Typical microorganisms in activated sludge

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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

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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

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Interaction of biofilm

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Trickling Filter

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Trickling filters at Wetalla

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A rotating biological

contact unit

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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

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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

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Activated sludge process with alternative wasting locations

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Surface aerators

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Final sedimentation tank

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Final clarifier

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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 High

Land requirement High Low

Operator control Limited More

Shock loads Rapid recovery Very slow

Foaming None Often

Odour Yes Minimal

Filter flies Yes None

Noise Minimal Moderate

Hydraulic washout No Yes

Plugging Yes No

Drying of media Yes No

Output of sludge moderate High

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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

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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

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Low rate single-stage sludge digester

.

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High rate two-stage sludge digester

.

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Anaerobic sludge digester

.

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Aerobic sludge digester

.

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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

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Nano-membrane filtration

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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

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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

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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

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Biological phosphorus removal

Modified Bardenpho process

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End of Module 18