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The principal objective of wastewater treatment is to allow human and industrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment. Wastewater from households and industry, commonly together with water runoff from urban areas, is conveyed by the sewerage system to sewage treatment plants for safe and economic treatment of sewage, and treatment and disposal of the resulting sludge.

Conventional wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter and, sometimes, nutrients from wastewater. The concept of all biological methods of wastewater treatment is to introduce contact with bacteria (cells), which feed on the organic materials in the wastewater, thereby reducing its BOD (Biochemical Oxygen Demand) content. In other words, the purpose of biological treatment is BOD reduction. Typically, wastewater enters the treatment plant with a BOD higher than 200 mg/L, but primary settling has already reduced it to about 150 mg/L by the time it enters the biological component of the system. It needs to exit with a BOD content no higher than about 20-30 mg/L, so that after dilution in the nearby receiving water body (river, lake), the BOD is less than 2-3 mg/L. Thus, the biological treatment needs to accomplish a 6-fold decrease in BOD. Simple bacteria (cells) eat the organic material present in the wastewater. Through their metabolism, the organic material is transformed into cellular mass, which is no longer in solution but can be precipitated at the bottom of a settling tank or retained as slime on solid surfaces or vegetation in the system. The water exiting the system is then much clearer than it entered it.

A key factor in the operation of any biological process is an adequate supply of oxygen. Indeed, cells need not only organic material as food but also oxygen to breathe, just like humans. Without an adequate supply of oxygen, the biological degradation of the waste is slowed down, thereby requiring a longer residency time of the water in the system. For a given flowrate of water to be treated, this translates into a system with a larger volume and thus taking more space.

General terms used to describe different degrees of treatment, in order of increasing treatment level, are preliminary, primary, secondary, and tertiary and/or advanced wastewater treatment. The biological wastewater treatment is achieved in three stages; primary, secondary and tertiary treatments.

1 Primary Treatment

The first stage of treatment is known as primary treatment which also includes certain preliminary operations such as flow equalization, grease removal, flow measurement, etc. The unit operations in primary treatment are screening to remove larger floating objects, grit removal for removing inert sand and inorganic particles, and settling for removing settleable suspended organic solids. The main purpose of the primary treatment is to produce a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed.

2 Secondary Treatment

The next stage of treatment is secondary treatment, which is designed to remove soluble organics from the wastewater. Secondary treatment consists of a biological process and secondary settling. Secondary treatment is designed to substantially degrade the biological content of the sewage such as derived from human waste, food waste, soaps and detergent. The final step in the secondary treatment stage is to settle out the biological floc or filter material in a secondary sedimentation tank (SST) or secondary clarifier and produce sewage water containing very low levels of organic material and suspended matter.

3 Tertiary Treatment

Third stage treatment is referred to as tertiary treatment or advanced treatment. More commonly used advanced systems are adsorption to activated carbon, filtration through sand and other media, ion exchange, various membrane processes, nitrification-denitrification, coagulation-flocculation, and micro-screening.