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Domestic Wastes and Waste Treatment
Objectives of Wastewater Treatments
Remove organic matter
Remove pathogenic microorganisms
Remove toxic chemicals
Components of Domestic Wastes
Domestic wastewater or sewage is a combination of human fecal matter, urine, and gray water
Gray water results from washing, bathing and meal preparation (kitchen sink)
Water from industries and business establishments may also enter the system
The amount of organic matter in domestic wastes determines the degree of biological
treatments required
Three tests are used to assess to the amount of organic matter
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BOD: Biochemical oxygen demand
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101-2

Enteric virus

101-2

Sewage Treatment
Sewage: The used water supply containing domestic waste together with human excrement,
wash water and industrial waste including acids, animal matter, greases, vegetable matter and
stormwater
Treatment Principle: the basic principle in sewage treatment is that water is separated from
waste while solid organic matter is biodegraded by microorganisms to simple compounds like
nitrates, sulfates, carbonates, CO2 and methane
Cesspools
It is used to dump human wastes
It has cylindrical rings with pores in the walls
- Water passes through the pores into the surrounding sand where the solid accumulates
at the bottom
- Anaerobic bacteria, in the compact sludge later at bottom of cesspool digest the organic
matter
- The broken down products diffuse into the ground
Dried bacterial spores such as Bacillus subtilis is also used to enhance the bacterial activities
Septic Tank
It is an enclosed concrete box into which all sewage of a house flows into
The organic matter accumulates at the bottom of the tank, when water rises to the outlet pipe to
flow to a distribution box/tank/well
The tank is pumped out regularly as there is no absorption of the digested organic matter into
the soil
Small towns collect the sewage into large ponds called “Oxidation lagoons”
The sewage is left in the lagoon for a couple of months during which aerobic bacteria digest the
organic matter in the water

4 Categories of Oxidation Tanks
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3
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Aerobic ponds
Anaerobic ponds
Aerated ponds
Facultative ponds

Aerobic Ponds
They are shallow enough to allow effective light penetration to stimulate growth which promotes
subsequent O2 generation
The wastewater is detained for 3-5 days

Anaerobic Ponds
May be 1-10 meters deep and require relatively long detention (20-50 days) time and do not
require much aeration
Often retains small amount of sludge, as they serve as pre-treatment step for high BOD
(Biochemical Oxygen Demand)

Aerated Ponds
These are aerated
Mat be 1-2 meters deep
Detention time is 10 days

Facultative Ponds
These are the most common sewage treatment tanks
Sewage is treated for both aerobic and anaerobic processes

The ponds are 1-3 meters deep and have 3 zones:
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Middle facultative zone
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Primary treatment
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Tertiary treatment
Primary Treatment
It is the physical separation of large debris followed by sedimentation
It involves the removal of suspended solids and floating particles
The solid material which settles down is subjected to anaerobic digestion in landfills or
composting
The liquid portion is then passed into sludge tanks for further treatment
Flocculating materials such as iron sulfate, Al-sulfate are added to trap microbes
Secondary Treatment (Microbial Biodegradation)
In this treatment, a small portion of the dissolved organic matter is mineralized and a large
portion is converted to removable solids
BOD is reduced to 80-90%
This process relies on microbial activity, may be aerobic or anaerobic
It involves additional purification, either through filtration or chlorination
How Secondary Treatment is Achieved:
- Oxidation ponds

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Trickling filters
Bio disc system
Conventional activated sludge
Non-conventional activated sludge:
- Single sludge system
- Multi-sludge system

Oxidation Ponds (Lagoons)
Heterotrophic bacteria degrade sewage organic matter within ponds producing mineral products
Oxygen produced by algae compensate poor O2 condition created by heterotrophic bacteria
The pond should be shallow, about 10 feet deep
Bacteria and algal cells settle at the bottom of the pond
Aerobic processes
- Oxidation pond
- Activated sludge
- Trickling filter
Anaerobic processes
- Septic tank
- Anaerobic digestion

Trickling Filter
This system is simple but expensive
Sewage is distributed by revolving sprinkler suspended over a bed of porous material
The sewage slowly percolates through this porous bed and the effluent is collected the bottom
The porous material of the filter bed becomes coated with a dense, slimy bacterial growth
The slimy matrix harbors a heterogenous microbial community
The bacterial community dissolves organic nutrients in the sewage, reducing BOD of the effluent

Poros bed allows aeration

Bio Disc System
It is an advanced type of aerobic film-flow treatment system
Closely placed discs made of plastics are rotated in a trough containing sewage effluent
The discs are partially submerged and become coated with a microbial slime similar to trickling
filters
Continuous rotation of the discs keeps the slime well aerated and in contact with sewage

Conventional Activated Sludge
This process is also known as “aeration tank digestion”
After primary settling, the sewage is introduced into an aeration tanks and mixed with a
bacterial-rich slurry known as “activated sludge”
Air is injected or mechanical stirring provide aeration
Activated sludge process reduce the BOD of the effluent to 10-15% of the raw sewage

Tertiary Treatment
It is the physiochemical process that removes turbidity caused by the presence of nutrients (e
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Nitrogen), dissolved organic matter, metals or pathogens
In second treatment, organic matter in the wastewater is oxidized by microorganisms
Filtration, disinfection and removal of chemical pollutants

Land Application of WasteWater
Biosolids may be added as solid or liquid

Pathogenic microbes are removed before application with different treatments

3 Basic Methods used in application of treated wastewater:
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Overhead flow
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