Waste Water Treatment: Introduction and Processing

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Water is the most useful and important resource of life. Our life depends on it. When waste water is treated to return this resource to the environment a semisolid, nutrient-rich by product called waste water sludge is produced. By Nadia Hassan. It contains 0.25 to 7% solids by weights.
When processed properly, the resulting product known as biosolids has several beneficial uses including
• Applying to cropland to improve soil fertility
• Using a soil amendment in landscaping
• Using as a daily cover

Table showing: Waste-water components

Component	Of special interest	Environmental effect
Micro-organisms	Pathogenic bacteria, virus & worm eggs	Risk when bathing and eating
Biodegradable organic material	Oxygen depletion in river	Changes in aquatic life
Nutrient	N2, P, NH3	Eutrophication
Metals	Hg, Pb, Cd, Cr, Cu, Ni	Toxic effect
Odour (taste)	H2S	Aesthetic inconvenience

Treatment:
Treatment today is often carried out by municipal treatment (sewage) plants. Once the polluted sewer water reaches the municipal plant, its treatment is generally divided in three basic stages (Primary, Secondary and Tertiary).
1) Primary treatment:
Uses physical process, especially screening and settling to remove materials. It has three basic steps.
i. A bar screen removes branches, garbage and other large materials.
ii. The grit chamber hold waste water for few minutes and other particles settle down.
iii. Primary settling tank hold water for three hours allowing finer sediments.
It removes 60% of the suspended solids & 35% of the oxygen-demanding waste.

3) Secondary treatment:
It has three basic steps:
i. Aeration tank mixes oxygen, waste-water & bacteria together. Bacteria digest the sewage.
ii. In secondary settling tank more sludge is formed which is piped back to sludge digester.
iii. Chlorination tank adds chlorine to water. It kills disease causing organisms.

3) Advanced (Tertiary) Treatment:
Advanced treatment removes
• Plant nutrients
• Toxic chemicals
• Some pathogens
• 50% N₂, 70% P
It is recommended if water have to use for drinking. Advanced treatment is often used as a part of closed loop waste-water reclamation.

Septic tanks:
In areas without a sewer system septic tanks are used. It is made of concrete. The waste is decomposed by bacteria in the tank and by soil bacteria in the absorption field.

Different mechanisms used for waste water treatment are:
1) Aerobic Biological treatment:
Primarily used to remove dissolved and colloidal organic matter from waste water. It is a natural process as organic matter in water will naturally decay as a result of microorganisms present. One of the aerobic biological treatments is the activated sludge process. Activated sludge is defined as suspension of microorganisms both living and dead in a waste-water. Process is activated by O₂ and then aeration and settling is performed.

2) Anaerobic waste-water treatment:
In contrast to aerobic treatment does not require air input and generates considerably smaller quantities of sludge. Requires heat energy, produce biogas (methane) and is promoted as a solution to energy problem. BOD test have been devised to assess the potential for a waste to be treated under anaerobic conditions.

3) Treatment of waste-water in natural system:
It includes application of waste-water to land and treating waste water through natural or artificial wetlands. These methods are satisfactory because they render waste-water as a resource rather than as a problem to be rectified. High BOD and TSS removals can be achieved in these systems.

4) Wetlands for waste-water treatments:
Wetlands are natural wet ecosystems with diverse and complex roles in nature. These are designed to remove conventional pollutants of BOD, SS and anaerobic conditions and denitrification.

Two types are designed:

• Surface flow system:

Similar to natural wetlands and free water surface is maintained.

• Subsurface flow systems:

Water flows through a permeable medium. Emergent vegetation is supported in these systems. Treatment is generally better in subsurface flow systems and there is no mosquito problem.

References:
o Theory and practice of water and waste water treatment by Ronald L. Droste (1997) page no (545-663)
o Environmental science by Daniel D. Chiras 7th edition (2006) page no (61, 246-260)
o Environmental science systems & solutions by Michael L. Mckinney, Rober M. Schoch (2003) page no (370-383)