The sewage treatment plant

Untreated sewage spreads disease, uses up oxygen (producing foul smells as anaerobic microbes take over), kills fish, and spoils water. Treatment aims to kill pathogens, prevent the loss of oxygen, and cut the organic content. It happens in stages.

Primary treatment (physical):

• Screening removes bulky objects.
• Grit chambers and sedimentation tanks (2-10 hours) let solids settle out as primary sludge.

This lowers the BOD by 30-40%.

Secondary treatment (biological):

• In the activated sludge process, air is pumped through the sewage, and aerobic microbes oxidise the organic matter to CO₂ and water. The sludge that settles is rich in well-adapted bacteria, so part of it is recycled as a starter. About 90% of the organic matter is digested.
• Trickling filters spray the sewage over beds of stones coated with a slime of microbes, which feed on it as it trickles down.
• Oxidation ponds (up to 5 feet deep) rely on algae and wind to supply oxygen.

The settled sludge goes to anaerobic digesters, where, over 30-40 days, microbes break it down and produce biogas: 60-70% methane and 20-30% CO₂, a useful fuel. The remaining sludge is dried and used as fertiliser (one such product is called Milorganite). Primary and secondary treatment together cut the BOD by up to 90%.

Tertiary treatment removes what is left, especially nitrogen and phosphorus. These nutrients cause eutrophication: they feed algal blooms, which then die and decay, using up the water's oxygen.

• Lime precipitates phosphate as calcium phosphate.
• Ammonia is stripped out by blowing air through the water at high pH.
• Activated carbon removes remaining dissolved organic matter (carbon polishing).
• Finally, chlorination with sodium hypochlorite (NaOCl) or bleaching powder disinfects the water.

The water is now clean enough to release, but it is still not drinking water.