lecture 03.08 (1) introdn to process selection.docx

8/14/2019 Lecture 03.08 (1) Introdn to Process selection.docx

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Process Selection in wastewater treatment

Nairobi, 03/08/2009

Technology Selection

Aspects to be considered for selection of technologies

for wastewater treatment

Economical

Technical

Ecological

Social


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Pre-initial questions

Is wastewater treatment a priority inprotecting public or environmental health?

Can pollution be minimised by recoverytechnologies or public awareness?

Is treatment most feasible at centralised ordecentralised facilities?

Can the intrinsic value of resources in

domestic sewage be recovered by reuse?


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Key factors for process selection

Wastewater production rate The characteristics of the sewer system (combined,

separate, small-bore)

The sources of wastewater (domestic, industrial,

stormwater, infiltration) The future opportunities to minimise pollution loads

The discharge standards for treated effluents

The availability of local skills for design, constructionand O&M

Environmental conditions such as land availability,geography and climate


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Technology Selection

Wastewater Source Treatment Objectives


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Source characteristics

Wastewater Flow Production

daily min, average, max flow

temporal variations

Wastewater Pollutantsclass (matter)

quantity (concentration)

Location


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Wastewater flows

Domestic wastewaterproduction depends on

water supply service

climatewater availability

Industrial wastewaterproduction depends on

cooling

processingcleaning

transportation products

technological operations


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Wastewater ingredientsMajor classes of wastewater contaminants

source: Metcalf & Eddy Inc., 2003


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Groups of pollutants

Group of pollutants

Toxic

compoundsSuspended

Solids

Organic

matter

Inorganic

matter

Pathogens

microorganisms

Coarse Fine BiodegradableNon-

biodegradable

Heavy

Metals

Nitrogen &

PhosphorousDissolved

Easy Hard Dissolved Emulsions


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Treatment objectives

Effluent quality

public health protection

preservation of the oxygen content in the water

prevention of eutrophication

prevention of sedimentation

preventing toxic compounds from entering the

food chainspromotion of water reuse


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Effluent standardsTypical treated effluent standards

source: Ayers and Westcot, 1985; WHO, 1989

*SAR Sodium adsorption ratio


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Levels of wastewater treatment

Classification of common wastewater treatment processes

after S. Veenstra, G.J. Alaerts and M. Bijlsma, WHO/UNEP, 1997

* UASB - Upflow Anaerobic Sludge Blanket


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Process Selection Criteria

Process applicability

Applicable flow range and flow variations

Average, or typical, efficiency and performance

Land availability (occupation space)

Chemical requirements

Inhibiting constituents

Energy consumptions


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Selection criteria

Reliability of the technology

Institutional manageability

Financial sustainability

Regulatory determinants

Climatic constrains

Compatibility

Adaptability


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Reliability

PPrroovviiddeeaacccceeppttaabblleeeefffflluueennttuunnddeerruunnuussuuaallccoonnddiittiioonnss::./changing in wastewater characteristics

occasional illegal discharge

variation in flow and concentrations

high or low temperatures

./troubles in operations

power failure

pump failure

poor maintenance0 Easy repair and restarting


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Institutional manageability

Personal requirements

availability of professionals of appropriate level of

skills

providing of necessary trainingTechnical expertise

access to a local network of research for scientific

support

good quality laboratories


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Financial Factor

Capital costs Operation and

maintenance costs


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Capital costs

Land acquisition

Legal fees

Permitting

Administrative costs

Engineering costs

Construction costs


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Operation and maintenance

Operation and maintenance requires:

Careful exhaustive planning

Qualified and trained staff devoted to its assignment

An extensive and operational system providing spare

parts and O&M utilities A maintenance and repair schedule, crew and facility

A management atmosphere that aims at ensuring areliable service with a minimum of interruptions

A substantial annual budget that is uniquely devotedto O&M and service improvement


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Operation and maintenance costs

Operator salaries, benefits and training

Energy use

Treatment or disposal of sludge

Equipment repair and replacement

Laboratory analysis


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Total costs of treatment

Typical total unit costs for wastewater treatment

Total unit cost

(investment plus

running costs)

US$ per m3

Removal, %

Preliminary

Primary

treatment

Secondary

treatment

Tertiary

treatment

based on experiencegained in EU and USA

(after Somlyody, 1993)


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Regulatory Factors

Effluent quality requirements for surface water discharge

Effluent disposal requirements for land treatment

Requirements governing the disposal of residuals for disposal of sludge resulting from treatment

Operator certification requirements for operators of wastewater treatment facilities

Local/regional restrictions special requirements of local origin


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Climate

Humidity

Precipitation

Evaporation rates

Seasonal variations

Average annual temperatures

Probability of flooding or elevated groundwater


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Removal of suspended solids

Coarse, mediocre Fine, colloidal

Grit removal

Screening

SSeeddiimmeennttaattiioonn

FFlloottaattiioonn

Coagulation

Flocculation

Depth Filtration

Surface Filtration

Membrane Filtration


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Sedimentation aspects

The design of the settling tank is based on

several factors:

influent TSS concentration

effluent TSS concentration

surface loading

detention time

sludge generation


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Flotation

Advantage Disadvantage

+ Grease and light solids to the top

and grit and heavy solids settling

to the bottom are all removed in

one unit

+ Decreased space requirements and

tank size due to high overflow rate

and short detention period

+ Odour nuisance is minimized

because of presence of dissolved

oxygen in the effluent

- Higher capital costs tanks to

additional equipment

- Less efficiency of removal

- The pressure type has highpower requirements, which

increase operating costs

- The vacuum type requires a

relatively expensive airtight

structure

- More skilled maintenance is

required


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Primary treatment

Remove: sand

grit

fibers,

floating objects

suspended particles Efficiency:

Physicochemical processesis applied for enhance removal efficiency

adjust the pH

remove any toxic or

inhibitory compounds

approximately 50-75 per cent of suspended matter

30-50 % of BOD

15-25 % of Kjeldahl-N and total P

Moderate cost


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Removal of organic dissolved solids

Biodegradable Non-biodegradable Lagoons

Activated Sludge Treatment

Aeration

Wet Combustion Oxidation Ditch

Trickling Filtration

Anaerobic Digestion

Rotated Biological Contactors Ozonation

Adsorption (Carbon)

Reverse Osmosis

Chemical Precipitation

Chemical Oxidation Electrodialysis

Distillation


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Secondary treatment

Classification of secondary treatment technology:

source: S. Veenstra, G.J. Alaerts and M. Bijlsma, WHO/UNEP, 1997


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Anaerobic treatment


+ Low sludge production

+ Effective for concentrated

sewage of organic contaminants

+ Processing at high hydraulic

loading rate

+ Reliability of power supply

+ Effluent have higher potential

for reuse (contains N,P,K)+ Local potential for selling biogas

- Low removal efficiency

- Effective only at high

sewage temperature

- Applicable only for highly

biodegradable contaminants


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Aerobic treatment

Mechanised Non-mechanised Use equipment to accelerate

the conversion process

High removal efficiency of

biodegradable contaminants Need for disinfection

increase treatment cost and

operational complexity

Requirement of skilledpersonnel for regular supply

o Low cost

o High land requirement

o Less removal efficiency

o Longer retention time


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Bio-film and suspended growth processes

Comparative analysis of the performance of the trickling filter andthe activated sludge process for secondary wastewater treatment

(not including BOD removal in primary treatment steps)

Bio-film based Suspended growth

source: S. Veenstra, G.J. Alaerts and M. Bijlsma, WHO/UNEP, 1997


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Removal of inorganic dissolved solids

Evaporation

Dialysis

Electroflotation

Electrodialysis Ion Exchange

Reverse Osmosis

Chemical Precipitation

Distillation


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Examples of physicochemical processes

Chemical oxidation with, for example, O2, O3 or Cl2(cyanide removal and oxidation of refractory organiccompounds).

Chemical reduction (for example, H2S assisted conversion

of Cr (VI) into Cr (III)). Desorption (stripping) (NH3 and odorous gas removal).

Adsorption on activated carbon (removal of refractoryorganics and heavy metals).

Ultra- and micro-filtration (separation of colloidal anddissolved compounds).


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Physicochemical treatment


+ Compact technology

with low area needs

+ Good removal ofmicro-pollutants and P

+ Fast start-up

+ Insensitivity to toxiccompounds

- Chemical dosing islabour intensive due

to fluctuating sewageload and composition

- Generation ofchemical sludges

- High unit cost per m3

of water treated


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Membrane Filtration

Application Limitation

+ Relative small volume ofprimarily twocontaminants

+ One component is quitevaluable

+ Components are enoughdifferent in molecular s

+ Non-corrosive tomembranes

- Life of membrane

- Loss in flux rate

- Small amount of effluent- Limited type of materials

which can be removed


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Evaporation

Application Limitation

+ High solids content

+ Non-corrosive and

non-scale-formingwaste

+ Inexpensive source of

heating is available

- Requires source ofenergy input

- Wastewater should bevery high in solidscontent

f


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References

Water Pollution Control A Guide to the Use of Water QualityManagement Principles, 1997 WHO/UNEP Chapter 3 Technology Selection, S. Veenstra, G.J. Alaerts and M. Bijlsma

Planning and Management of Lakes and Reservoirs: AnIntegrated Approach to Eutrophication, 2001,John M. Melack(Editor)

Wastewater Engineering: treatment and reuse, 2003, Metcalf &Eddy, Inc. 4th ed., editors: Tchobanoglous, G., Burton F.L.,Stensel H.D

Industrial and hazardous waste treatment, 1991, EnvironmentalEngineering Series, Van Nostrand Reihold, New York,Nemerow,N.L., Dasgupta, A.

Water and Wastewater Technology, 1998, Prentice Hall IndiaPrivate Ltd. 3rd ed.,Hammer, M.J., Hammer, M.J., Jr.

Introduction to Environmental Management, 1988, Elsevier,Amsterdam, The Netherlands, Hansen, S.J. and S.E. Jorgensen.

lecture 03.08 (1) introdn to process selection.docx

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