77. Waste Water Treatment

1. Sources of Waste Water

• Domestic: Household waste from toilets, sinks, and showers. It contains organic matter and harmful microbes.

• Agricultural: Nutrient runoff from fields and animal waste from farms.

• Industrial: Factories that produce chemicals. This water often contains harmful chemicals and requires extra treatment steps.

2. The Sewage Treatment Process

The process of treating sewage generally involves three main stages:

1. Screening: The sewage is passed through a mesh to remove large objects like twigs, grit, and plastic bottles.

2. Sedimentation: The sewage stands in a settlement tank.

   • The heavier solids sink to the bottom to form sludge.

   • The lighter liquid, called effluent, floats on top.

3. Biological Breakdown: Both parts are treated using microorganisms, but under different conditions:

   • Effluent (Aerobic Digestion): Air is pumped into the tank to provide oxygen. Aerobic bacteria break down the organic matter and harmful microbes. Once treated, the water can be safely released back into rivers.

   • Sludge (Anaerobic Digestion): The tank is sealed to exclude oxygen. Anaerobic bacteria break down the organic matter. This process produces methane gas (which can be used as an energy source) and the remaining waste can be used as fertilizer.

3. Additional Treatment

If the waste water contains toxic substances (like heavy metals), additional steps are needed:

• Adding chemicals to precipitate out metals.

• Using membranes or ultraviolet (UV) radiation.

Summary of Digestion Types