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Waste water engineering:
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Overview: Water Pollution
Wastewater Characteristics
Wastewater Treatment Considerations
Wastewater Treatment Plant Design
Physical, Chemical and Biological
Processes
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Water Pollution
Water is essential for life. Over 97% of water is
present in the oceans but being unfit for human
consumption. Only a limited amount of this is
available for human use. Triggered by enormous
increase in population and shrinking resources per
capita availability of water is decreasing.
Added to the lower availability is the highincidence of pollution of water supplies in the
developing world.
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Sources of water pollution
Point sources: pollutants are discharged from
sewage plants, meat and dairy industries through
pipes at specific points into the discharge waters
(stream, pond, lake or ocean)
Non-point sources: pollutants present in soil are
carried away as run offs by rain water intostreams, rivers or oceans at several points
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Types of water pollution sources
Agricultural wastes- Rain water carries them off as runoffs from fields and animal farms.
Industrial wastes- Waste water discharges from food,
paper, leather, and distillery industries are heavy in terms oforganic load.
Domestic sewage-It is 99% water with 1% solids(drymatter). The solids are made up of 70% organic and 30%inorganic chemicals( grits, salt metal oxides)
Natural sources / run offs- Run offs from virginforests(free of human activities) or those due to
earthquakes or cyclones are not considered as pollution.
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Types of pollutants in water
1. Oxygen demanding pollutants
2. Pathogens
3. Nutrients
4. Salts5. Thermal pollution
6. Heavy metals
7. Pesticides8. Volatile organic compounds
9. Radionuclides
10.Oil pollutants
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Effects of water pollution1.Organic matter causes a decrease in O2 affecting aquatic life.2. Release of nutrients into waters leads to eutrophication and algal
blooms. Shell fish feeding on toxic algae have caused paralytic
shell-fish poisoning.3.Pathogens as pollutants lead to gastroenteritis and dysentery.4.Heavy metals cause nephro toxicity.5. Presence of pesticides,VOC and radio nuclides are harmful; in
drinking water supplies and aquatic life.
6. Pollutants impart colour, off-odour and off-taste to water
rendering it unfit for consumption.
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Sewage treatment-IntroductionWater after use becomes sewage or waste water.Sewage includes all waste waters- from toilets, kitchenand laundry washings, rain water flowing into municipal
drains, industrial wastes from citi drains etc.Municipal wastewater is 99.9 % water , balance made
up of suspended and dissolved solids:Total solids = Total suspended solids + Total dissolved
solids (suspended solids can be removed by filtration,
while total dissolved solids cannot be removed)
In large metropolitan cities this small percentage ofsolids
May account for more than 1000tons of solids per day.
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Overview: WastewaterEngineering
Wastewater Treatment
Wastewateris the water used by a
community Collected and treated prior to discharge
Standards of treatment vary but aregenerally becoming more stringent
Treatment is a process of removal ofsmaller and smaller particles
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Overview: WastewaterEngineering
Sludge Treatment Sludgesare the product of biological
treatment of wastewater Sludges comprise solids found in
wastewater plus organisms used in thetreatment process
Disposal is a major issueVarious disposal techniques are used but
each has advantages and disadvantages
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Overview: WastewaterEngineering
Water Disposal and Re-Use
Wastewater treatment generates a Cleaned
Water Stream Cleaned water is often discharged to a
larger body of water for dilution
Alternatively, cleaned water my be re-usedfor irrigation or rarely drinking waterproduction
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Wastewater Characteristics
Characteristics are normally estimated byempirical methods
Wastewater Flows Flows arise from households, industry,
infiltration and storm flows
Flows are considered in both hydraulic and
process design Levels of treatment may vary for different
flowrates
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Wastewater Characteristics
Wastewater Flows
There is a variety of methods for
estimating flowrates However there may be great variability in
the factors which affect flowrates fromregion to region
Different multiples of the Dry WeatherFlow will receive each level of treatment
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Wastewater Characteristics
Wastewater Composition
Key design issues:
Solids: density, particle size, level of VolatileSuspended Solids
Biochemical Oxygen Demand
Temperature
Ammonia
Nutrient levels
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Wastewater Characteristics
Wastewater Treatment Considerations
Objective: to maintain or improve the
quality of the receiving body of water Treatment stages:
Preliminary
Primary
Secondary
Tertiary
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Primary treatments
Raw sewage
Screening
Grit chamber
Primary settling tank
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Objective of secondary treatment is to further remove BOD
and suspended solids beyond what is achievable by primary
settling tank.Secondary treatment follows three approaches all of which
take advantage of microorganisms to convert organic waste
into stable low energy compounds.Effluent from primary treatment is treated in a) Trickling
filters or Rotating Biological Contactors (RBC) and b)
Activated sludge processSewage untreated can also be directly taken to oxidation
ponds or lagoons for BOD removal.
Secondary treatments
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Tertiary treatment
Primary and secondary treatments do not remove all the biologically
degradable organic matter.
Tertiary treatment is given to remove nitrogen and phosphorus
nutrients from the effluent.To avoid these O2 depletion and eutrophication problems, treatment
plants have to be provided with additional facilities to achieve higher
rates of N2 removal.
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Wastewater Characteristics
Classification of Methods
Physical
Processes
Chemical
Processes
Biological
Processes
Screening
Sedimentation
Filtration
Precipitation
Chlorination
Aerobic
Anaerobic
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Wastewater Characteristics
Applicability and Selection of Methods
Different processes are used to treat
wastewater depending on thecontaminants present
Similarly, different processes are used totreat sludges, depending on the objective
of treatment
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Overview: Wastewater TreatmentPlant Design
Flowrate and Mass Loading Wastewater feed is not specified, therefore the
plant must be able to treat whatever thewastewater contains
Plant design will take account of historical flowsand loadings, but must also be able to deal withexpansion
Plant must also be able to deal with a range offlow and load conditions, plus peak upstream flowconditions (eg storms)
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Overview: Wastewater TreatmentPlant Design
Selection of Design Flowrates Domestic: based on a flow per head.Varies between countries
Industrial: Based on records of dischargeconsents and of metered supplies
Infiltration/Exfiltration: based on a % of
the domestic flow Peaking Factors: Depends on size of
community
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Overview: Wastewater TreatmentPlant Design
Selection of Design Flowrates
Upstream Flow Equalisation
Plants are not normally designed to treat totalpeak arisings
Instead, hydraulic controls will direct flowsabove plant capabilities to storm system on
larger works
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Overview: Wastewater TreatmentPlant Design
Selection of Design Flowrates Upstream Flow Equalisation
UK maximum flow to treatment plant is determined byFormula A:
Formula A = PG+E+I+1360P+2E l/d
P=population
G=average daily consumption per head l/h/dE=Industrial & commercial discharges to sewer l/d
I=infiltration l/d
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Overview: Wastewater TreatmentPlant Design
Selection of Design Flowrates Upstream Flow Equalisation
For larger works with storm tanks, only FullTreatment Flow (FTF) will receive fulltreatment.
FTF=3PG+3E+I
Formula A-FTF will pass to storm tanks
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Overview: Wastewater TreatmentPlant Design
Selection of Design Mass LoadingsAverage mass loadings for BOD and
Suspended Solids are commonlydetermined by Population Equivalent (PE)
Design mass loadings are more complexand must take account of seasonal, diurnaland industrial load variations
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Manufacturing process: ordinary packaged
drinking water1.Clarification: to remove particles, colloids and contaminant2.Prefiltration : to protect final filter
3.Final Filtration/Sterilization: to remove particles, mould,
algae and protozoa; water sterile filtration to remove bacteria
and other microorganisms
4.Carbonation: to bring CO2 levels in water to original levels
in water. Carbonation. It is used to produce sparkling water.
CO2 injected in water must be free of particles and
microorganisms.5 Filling
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