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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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Technology Selection
Wastewater Source Treatment Objectives
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Source characteristics
Wastewater Flow Production
daily min, average, max flow
temporal variations
Wastewater Pollutantsclass (matter)
quantity (concentration)
Location
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Wastewater ingredientsMajor classes of wastewater contaminants
source: Metcalf & Eddy Inc., 2003
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Effluent standardsTypical treated effluent standards
source: Ayers and Westcot, 1985; WHO, 1989
*SAR Sodium adsorption ratio
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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 in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Selection criteria
Reliability of the technology
Institutional manageability
Financial sustainability
Regulatory determinants
Climatic constrains
Compatibility
Adaptability
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Financial Factor
Capital costs Operation and
maintenance costs
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Capital costs
Land acquisition
Legal fees
Permitting
Administrative costs
Engineering costs
Construction costs
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Climate
Humidity
Precipitation
Evaporation rates
Seasonal variations
Average annual temperatures
Probability of flooding or elevated groundwater
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Secondary treatment
Classification of secondary treatment technology:
source: S. Veenstra, G.J. Alaerts and M. Bijlsma, WHO/UNEP, 1997
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Anaerobic treatment
Advantage Disadvantage
+ 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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Removal of inorganic dissolved solids
Evaporation
Dialysis
Electroflotation
Electrodialysis Ion Exchange
Reverse Osmosis
Chemical Precipitation
Distillation
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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
Physicochemical treatment
Advantage Disadvantage
+ 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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
Nairobi, 03/08/2009
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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Process Selection in wastewater treatment
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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.