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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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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 sm

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