Chapter 13 — Wastewater Story

Wastewater is used water that has become dirty. It includes black-brown water rich in lather and oil that flows down drains from sinks, showers, toilets, and laundries.

Sewage is wastewater released by homes, industries, hospitals, offices, and other users. It also includes rainwater that runs off roads and rooftops during storms, carrying harmful substances with it. Sewage is a liquid waste containing dissolved and suspended impurities.

Sewage contains organic impurities (human faeces, animal waste, oil, urea, pesticides, herbicides, fruit and vegetable waste), inorganic impurities (nitrates, phosphates, metals), nutrients (phosphorus and nitrogen), bacteria such as Vibrio cholerae and Salmonella paratyphi, and other microbes such as protozoa.

Sewage is the liquid waste itself. Sewerage is the network of big and small pipes called sewers that act as a transport system, carrying sewage from the point where it is produced to the treatment plant. Manholes are located every 50–60 m along sewerage lines, at junctions, and where direction changes.

Treatment involves: (1) Bar screens remove large objects like rags, sticks, cans, plastic packets, and napkins. (2) A grit and sand removal tank slows incoming water so sand, grit, and pebbles settle. (3) A large settling tank allows solids (sludge) to settle and a skimmer removes floating oil and grease, giving clarified water. (4) Air is pumped into clarified water so aerobic bacteria can consume remaining waste. (5) Suspended microbes settle as activated sludge; treated water is removed from the top and may be disinfected with chlorine or ozone before discharge.

Bar screens are the first stage of treatment. They remove large solid objects such as rags, sticks, cans, plastic packets, and napkins from the incoming wastewater.

Sludge is the solid matter (such as faeces) that settles at the bottom of the settling tank during treatment. It is transferred to a separate tank where anaerobic bacteria decompose it, producing biogas that can be used as fuel or to generate electricity. The remaining activated sludge is about 97% water; this water is removed using sand drying beds or machines, and the dried sludge is used as manure.

Vibrio cholerae causes cholera and Salmonella paratyphi causes typhoid. Both are present in sewage as biological contaminants.

Untreated human excreta and contaminated water are a health hazard and cause waterborne diseases including cholera, typhoid, polio, meningitis, hepatitis, and dysentery.

Cooking oil and fats can harden inside pipes and cause blockages. In open drains they clog soil pores, reducing the soil's effectiveness in filtering water. They should be thrown in the dustbin instead.

After aerobic bacteria consume organic waste in the aeration tank, the suspended microbes settle at the bottom of the tank—this settled mass is called activated sludge. It is about 97% water. The water is removed, and the dried sludge is used as manure, returning organic matter and nutrients to the soil.

Sludge from the settling tank is transferred to a separate tank where anaerobic bacteria decompose it. This decomposition produces biogas, which can be used as fuel or to generate electricity.

Low-cost on-site systems include septic tanks (suitable for hospitals, isolated buildings, or clusters of 4–5 houses), chemical toilets, and composting pits. Some toilets connect to biogas plants where excreta flows through covered drains into a biogas plant. A vermi-processing toilet uses earthworms to convert human excreta completely into vermi cakes, which are useful for the soil.

Chemicals like paints, solvents, insecticides, motor oil, and medicines should not be poured down the drain because they kill the microbes that help purify water. Used tealeaves, solid food remains, soft toys, cotton, and sanitary towels must go in the dustbin—they choke drains, block free flow of oxygen, and hamper the degradation process.

Harappa and Mohenjodaro had what may be the world's first urban sanitation system. Individual houses or groups of houses had separate bathing rooms, and wastewater was directed to covered drains lining the major streets. The oldest toilet made of bricks is about 4,500 years old.

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