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Wastewater Characterisation and Treatment

Recommended text books:

Wastewater Engineering – Metcalf and Eddy

Standard Methods for the Examination of Water and Wastewater

Contact: Benoit Guieysse

B.J.Guieysse@massey.ac.nz

RC.2.18

Lecture block outline

The big picture: “Understanding the nature of wastewater is essential in the design and operation of collection, treatment, and reuse facilities – and in the engineering management of environmental quality”

We need to know what’s in it before we can decide what to do with it!

Characterisation

Sampling

Bio pollutants

Chemical pollutants

Physical pollutants

Treatment

Disposal

Tertiary

Secondary

Primary

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10. Wastewater Treatment

Example: City of Toronto, Canada

Typical steps in wastewater treatment: from the most cost-efficient to the least!

Preliminary treatment

Primary treatment

Secondary treatment

Tertiary treatment

Sludge treatment

Removal large debris, greaseEqualization

Removal SS (include some COD)

Removal BOD/COD & nutrients

Removal nutrient & pathogens

Sludge digestion, stabilization &

dewateringTypically:Preliminary and Primary treatments are based on physical mechanismsSecondary = biologicalTertiary = specialized

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Common processes for pollutant removalPollutants Common Processes

Debris, large solids Grid removal

Suspended solids (including VSS) Sedimentation

FOG Dissolved Air Flotation

Dissolved solids Coagulation-Flocculation

Organic pollutants* Biological treatment

N Biological nitrification-denitrification

P Chemical precipitation

Priority pollutants** Adsorption

Pathogens** Biological treatment (maturation ponds) and specific treatment (UV irradiation, chlorination)

* A fraction of organic pollutants found as suspended solids will be removed with solids.** A fraction of priority pollutants and pathogen can be indirectly removed with solids.Membrane filtration (from ultra- to nano-filtration and reverse osmosis) are gaining popularity in situations where reuse is necessary or space seriously limited. Membrane bioreactors combine size separation with biological removal.

Waste water↓

Screening↓

Grit removal↓

Flow balancing (optional) ↓

Dissolved air flotation↓

Sedimentation↓

AEROBICSuspended culture

Attached culture

Specialised Processes

Sludge

SLUDGE TREATMENT

DISPOSAL / REUSEDISPOSAL / REUSE

Sludge

PRIMARY

SECONDARY

TERTIARY

SLUDGE THICKENING/ DEWATERING

� Solids � Landfill

ANAEROBICSuspended cultureAnaerobic lagoon

UASBContact processAttached growthAnaerobic filter

PRELIMINARY

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Important definitionsPollutant loading rate = amount of pollutant reaching a tank/process per unit of

time – usually kg/d

Example: A wastewater containing 100 g COD/m3 and 250 g TS/m3 is treated in a

pond. The wastewater flow rate is 300 m3/d. The organic loading rate = 100×300 =

30,000 g COD/d = 30 kg COD/d. Similarly, the solid loading rate = 75 kg TS/d.

Hydraulic Residence Time (HRT) = amount of time a “liquid volume” introduced

into a tank/process will stay inside the tank/process before being removed

Solid Retention Time (SRT) = amount of time a solid would remain in the system.

Pollutant loading rates and the retention times are important design criteria

during WWT.

Preliminary & Primary Treatment

Objectives: Balancing of flows, screening/settling and fat

removal.

Crucial steps that reduce the pollutant load to the rest of the

facility.

Offer best value for $$

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Examples large debris removal

Need for equalization

1 3 6 9 12

Typical daily variation of municipal wastewater: water use peaks during morning and evening are seen with a 1-2 hours delay at the WWT

0 6 12 18 24

Typical yearly variation of municipal wastewater in South France: Winter peaks reflect storms and summer peaks reflect the increase of population (up to 100 times)!

Month

hours

Flow

/or

gani

clo

adin

gFl

ow/

orga

nic

load

ing

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Balancing and storage

Tank or pond used to store wastewater

Reduce impact of changes in wastewater flow-rate and strength

Adjust pH if needed

Storage capacity in case of breakdown

Useful if wastewater is used for irrigation

Fat removal – Dissolved Air Flotation

http://en.wikipedia.org/wiki/File:DAF_Unit.png

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Primary treatment

Physical separation of solids

from the wastewater by

sedimentation

Examples

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Secondary treatment

“All most all wastewaters containing biodegradable constituents can be treated biologically”. You must understand the processes to ensure the proper conditions are produced/controlled effectively.

Main processes for Secondary treatment

Aerobic

suspendedAerobic attached

Anaerobic suspended

Anaerobic attached

Trickling filterRotative disks

Activated SludgeAerobic ponds

UASBAnaerobic pond

+ multitude of hybrid (anaerobic/aerobic or suspended/attached), anoxic (e.g. NO3

- instead of O2) processes in various combinations. Process can also be classified as batch/continuous etc

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Cell

Products: CH4, oil,

NO3-, N2, NO2

-, CO2,

O2, heat, enzymes,

toxins etc

More cells (containing C, N, and P)

Nutrient sources: N, P, H, O etc

Energy source

(Organic and inorganic compounds)

Carbon source (organic compounds)

Electron acceptor: O2,NO3

-, CO2 etc

Main principle of biological removal:Food + organisms = Products + biomass

Aerobic Carbon removalOrganics compounds are used as sources of carbon and energy

C10H19O3N + O2 + N + P → C5H7NO2 + CO2 + H2O + NH4+ ...

Anaerobic carbon removalOrganics → Biomass + CO2 + CH4 + NH4

+

Organic pollutants are converted into methane (energy), CO2 and biomass (sludge)

These reactions summarize far more complex mechanisms!

Symbolize organic matter in WW =

pollutant. In fact, WW is made up of 1000s

of different compounds

Biomass that must also be removed (sludge).

Note the biomass contains N and P and

can be itself considered as organic pollutant

Nutrients

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Anoxic carbon removal

Anoxic treatment often means that oxygen is absent and replaced by nitrate of

sulfate.

Nitrate instead of oxygen:

C10H19O3N + NO3- + nutrients → C5H7NO2 + CO2 + N2 + ...

This process is also known as denitrification (see N-removal)

Sulfate instead of oxygen:

C10H19O3N + SO42- + nutrients → C5H7NO2 + CO2 + H2S + ...

H2S formation results in bad smells

N-Removal

Biological nitrogen removal is based on the conversion of N (organic or

inorganic) into N2 that escapes into the atmosphere. This is a 2-step process:

1. Nitrification (AEROBIC conditions) – conversion of NH4+ to NO3

-

NH4+ + CO2 + O2 → biomass + NO3

-

.

2. Followed by denitrification (ANOXIC) - conversion of NO3- to N2

C10H19O3N + NO3- → biomass + N2 + CO2

As seen above, biomass is made of C, N and P (C5H7NO2). N and P are therefore

removed by assimilation = uptake into cells during population growth. This is

the main mechanism for N and P removal in ponds

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Bio-P removal: relatively new (and unreliable) technology

‘Special’ organisms take up more P than is required for growth

More traditional approach is P removal by precipitation

Phosphorus Removal

Impact of N and P on NZ freshwater:http://www.mfe.govt.nz/environmental-reporting/freshwater/river/nutrients/

The Activated Sludge process

Reactor – micro-organisms are kept in suspension as flocks

Liquid/solids separator – usually a sedimentation tank

Recycle stream – for maintaining adequate biomass conc.

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Trickling Filters

The filter is a non-submerged, fixed film reactor using rock or plastic

packing (almost all new filters are constructed with plastic packing).

The wastewater is evenly distributed over the top of the bed by a rotary

distributor

The mircro-organisms grow on the packing. Treatment occurs as the

wastewater flows over the film.

Anaerobic treatment

Advantages Disadvantages Less energy required (no need for forced aeration)

Longer start-up time

Less biological sludge production May require alkalinity addition

Fewer nutrients required May require polishing

CH4 production – energy source Bio N and P removal is not possible

Elimination of off-gas air pollution (no forced aeration)

Much more sensitive to lower temps

Potential for production of odours

Potential for production of corrosive gases

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UASB (upflow anaerobic sludge blanket)

Rely on ‘granulation’ which enables very high sludge concentrations to accumulate at the base of the reactor.

Liquid and gas flow suspend granules. Baffles retains bacterial granules, separate gas/liquid.

HRT of 0.5 - 1 d

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Wastewater Ponds (Lagoons)

This is the more “low-tech” of WWT reactors: usually an earthen basin,

which might be covered with an impermeable liner.

Very common for small communities or

as maturation ponds (tertiary treatment)

Normally in series 2/3 ponds

New Zealand: 200

USA: 3,500+

France: 2,500+

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Why

pond

s?Disadvantages Advantages

Easy operation

Primary/secondary/tertiary

Low maintenance

Ready equalisation

High SS in effluent

Process modification and control difficult

Temperature effect

Large land area needed

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Types of ponds

Anaerobic

Facultative aerobic

Maturation (Aerobic pond)

Surface aerated

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Anaerobic ponds

2-5m depth, no algae, HRT of 20-50 d

Relatively small with a high organic load 125-300 kg BOD/ha-day.

40-70% BOD removal.

Problem with methane release in the atmosphere (unless the pond is covered and methane is burned).

Treatment mechanisms in a facultative pond(Source: Wastewater Engineering by Metcalf & Eddy, 1991, pg 437)

Facultative ponds

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Facultative ponds

Example of design criteria: 10-350 kg BOD/ha day, 1.0-2m depth, 17-200 d HRT

(for temperate-subtropical weather, 10-350 kg BOD/ha day, 1.5-2m depth, 33-100

d HRT)

90% BOD removal

The algae provide oxygen and capture CO2 during photosynthesis. CO2 capture can

cause the pH to increase, which improves pathogen kills, N stripping and P

precipitation

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Maturation ponds (aerobic lagoons)Look like facultative, 1-1.5m deep

Low organic loading – well oxygenated

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Aerated pondLagoon depth:1 – 3m

earthen basin

typically, mechanical aeration on floats or fixed platforms

Similar as a facultative ponds with the upper layer is aerated with surface

aerators in order to avoid odor formation!

Aeration is often intermittent (during night)

5-25 days retention times

10 g BOD/m3-d

Irrigation

Salinity: related to electrical conductivity (can use TDS as a measure)

Nutrients: provide fertilizer (phosphorus is often bound in the soil but

nitrogen can leach quite readily).

Fats and biological growth can cause the blocking of sprinkler systems; an

issue for using WW for irrigation.

Discharge areas should be rotated (approx every 20 days) to allow organic

and nutrient conversion

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Sludge disposal

‘Sludge’ = wastes from screens, primarily clarifiers and biosolids from bioreactors

Disposal: land application, landfills, incineration

Sludge processing: key process is thickening – helps transportation, digestion, drying and combustion

Sludge can often be digested anaerobically (more common) or aerobically. Digested sludge can be composted for further stabilization (pathogen removal).

ComparisonCriteria Activated

Sludge Plant

Biological

filter

Aerated

Lagoon

Waste

stabilization pond

Plant

Performance

BOD Removal F F G G

Pathogen removal P P G G

SS Removal G G F F

Economic

Factors

Simple & economic

construction

P P F G

Simple operation P F P G

Land Requirement G G F P

Maintenance cost P F P G

Energy Demand P F P G

Sludge Removal

costs

F F F G

G = Good - F = Fair - P = Poor

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General overviewProcess Applications Advantages Disadvantages Cost

Activated

sludge

Low/moderate

conc.

Proven, good control Emissions of volatile compounds

and aerosols, high sludge

production. Aeration costs

+++

Aerated

lagoons /

ponds

Low conc. Simple, low costs Emissions of volatile compounds

and aerosols, sensitive to shock

and climate, land requirment, no

control

+

Trickling

filter

Low conc.,

recalcitrant

organics

Little sludge,

biodiverse

Emissions of volatile compounds,

sensitive to shocks, clogging, odor

+

Anaerobic

process

High-strength Methane production,

low sludge

Sensitive to temperature, higher

capital costs, odor

++

Source: Environmental Biotreatment. CN Mulligan.This is given as an example only, very specific of North America

Conclusions

Most large wastewater treatment systems are based on activated sludge

variations as secondary treatment because this is the best described process.

The trickling filter is used but there are some operational problems (clogging).

Anaerobic technologies are generally recommended for effluents with high

concentrations of biodegradable organic matter. There are therefore very

commonly used for sludge digestion. They remain limited by odor and

instability issues (temperature, toxicity).

Ponds are common for primary and secondary treatment at small scales

(decentralized treatment) or for tertiary treatment as stabilization ponds.

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Example

Case study: treating dairy wastewaters

Characteristic Concentration

Biochemical oxygen demand 90 - 12,400 (mg/l)

Chemical oxygen demand 180 - 23,000 (mg/l)

Suspended solids 7 - 7,200 (mg/l)

Nitrogen 1 – 70 (mg/l)

Fat 0 – 2100 (mg/l)

Phosphorus (as PO4) 4-150 (mg/l)

pH 3 – 13

Temperature 11 – 72 (oC)

Remember wastewater properties can change in with time and location!

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Dairy wastewater

Approx 14.7 billions litres milk produced in NZ each year (2005/06)

Wastewater produced:

0.5 – 2 m3 wastewater per m3 milk received

This accounts for 7 – 29 billion litres of wastewater produced from milk

production only!

Treatment options

Characteristic Concentration

BOD 90 - 12,400 (mg/l)

COD 180 - 23,000 (mg/l)

SS 7 - 7,200 (mg/l)

N 1 – 70 (mg/l)

Fat 0 – 2100 (mg/l)

P (as PO4-) 4 - 150 (mg/l)

pH 3 – 13

Temperature 11 – 72 (oC)

High BOD and COD values + “good” BOD/COD ratio = plenty of biodegradable organic matter = excellent for anaerobic treatment!

Low concentrations of suspended solids = primary settling might not be efficient

A balance tank would be useful (neutralize pH and temperature) and primary settling would help reduce the suspended solids. Fat removal is often necessary