Sewage treatment

Sewage treatment is the process of removing contaminants from municipal wastewater, containing mainly household sewage plus some industrial wastewater. Physical, chemical, and biological processes are used to remove contaminants and produce treated wastewater (or treated effluent) that is safe enough for release into the environment. A by-product of sewage treatment is a semi-solid waste or slurry, called sewage sludge. The sludge has to undergo further treatment before it is suitable for disposal or application to land.

Process

Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment.

Primary treatment

In primary treatment, sewage is stored in a basin where solids can sink to the bottom and oil, grease and lighter substances can float to the top. The collected solids are then disposed of and the remaining liquid is subjected to secondary treatment.

Secondary treatment

Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment.

Tertiary treatment

Tertiary treatment is designed to further improve the quality of the water before it is discharged to the natural environment. Tertiary treatment can remove more than 99 percent of all the impurities from sewage, producing an effluent of almost drinking-water quality.

Sludge treatment

The sludge that is produced in large quantities during sewage treatment has to be treated and disposed of in a safe and effective manner. This is achieved through sludge treatment which includes thickening, dewatering, and stabilisation processes.

Environmental considerations

The discharge of untreated sewage is a major cause of water pollution. This is particularly evident in developing countries where the investment in sewage treatment infrastructure is lacking. In many developed countries, sewage treatment has significantly improved the quality of rivers and coastal waters.

See also

• Water treatment
• Wastewater
• Sanitation

References

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