2 In a municipal wastewater treatment plant what is the prin

2. In a municipal wastewater treatment plant, what is the principal objective of each of the following stages of treatment? How is this objective achieved? a) Primary treatment b) Secondary treatment c) Tertiary treatment

Solution

(a) Primary Treatment

The principal objective of primary treatment is the removal of settleable organic and inorganic solids by the process of sedimentation, and also the removal of materials which will float i.e. scum, by skimming. Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD₅), 50 to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment.

Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary treatment by sedimentation, but colloidal and dissolved constituents are not affected. Primarily treated water is the effluent with these colloidal impurities and must be treated further with secondary treatment.

(b) Secondary Treatment

Effluent of the primary treatment is treated to remove the residual organics and suspended solids. The colloidal organic matter and suspended biodegradable are treated using aerobic biological treatment processes. When a high-rate biological treatment process is used, in combination with primary sedimentation, it typically removes 85 % of the BOD₅ and SS originally present in the raw wastewater. The effluent after secondary treatment is clearer.

(c) Tertiary Treatment

Tertiary and/or advanced wastewater treatment is employed when specific wastewater constituents which cannot be removed by secondary treatment must be removed.This third and last step is mostly comprised of removing phosphates and nitrates from the water supply. Individual processes may be needed to remove the specific impurities, for example, activated sludge process is used to remove the nitrogen and phosphorus.