How do effluent treatment plants and water treatment plants work?

A water treatment plant will usually take water from a dammed river. The water is then filtered and purified till it is potable (that is, drinkable). Depending on the quality of the water it may have to be disinfected with chlorine or other chemicals to make it safe for use.

An effluent treatment plant takes sewage runoff from toilets, factories, stormwater drains and other sources.

Screening: First the effluent is screened to remove any solids (cans, plastic, condoms, tampons etc). It then moves through a tank slowly enough so that sand, stones and rock sink to the bottom where they can be removed before the mixture starts going through pumps. (The sand is usually sent to landfill.)

Primary Treatment: The sewage then flows slowly through large tanks. Here the sludge sinks to the bottom and the oil and grease rise to the top. The sludge and the oils are then removed.

Secondary Treatment: The remaining mixture is treated using aerobic biological processes. Air is pumped through to encourage bacteria and protozoas to break down the contaminants.

Tertiary Treatment: Finally the treated water is given a final processing before it is discharged into a river, lake or ocean. Any one or more of the following methods are used:

• filtration (through sands)
• lagooning (stored in large man-made lagoons where reeds and other plants assist)
• constructed wetlands (rather like lagooning, using reedbeds and other similar methods)
• nutrient removal (to remove nitrogen and phosphorus)
• disinfection