Question

Explain the process of secondary treatment given to the primary effluent upto the point it shows significant change in the level of biological oxygen demand(BOD) in it.

Answer 1

Secondary treatment is a method of treating wastewater or sewage to reach a certain degree of effluent quality by using a sewage treatment plant for physical phase separation to extract settleable solids and a biological process to eliminate dissolved and suspended organic compounds. After this type of treatment, the wastewater can be called secondary-treated wastewater.

Complete answer:
Secondary treatment is the portion of a series of wastewater treatment that eliminates dissolved and colloidal compounds calculated as the need for biochemical oxygen (BOD). Secondary treatment is typically applied to the liquid part of the sewage after the removal of settleable solids and floating material by primary treatment.

In a controlled aerobic habitat, secondary treatment is normally carried out by indigenous, aquatic microorganisms. The biodegradable soluble organic contaminants (e.g. carbohydrates, fats and organic short-chain carbon molecules from human waste, food waste, soaps and detergent) are absorbed by bacteria and protozoa when reproducing to form cells of biological solids.

1.During this treatment, large aeration tanks containing primary effluent are continuously stirred with a continuous air supply.
2. Aerobic microbes are thus vigorously growing, consuming a large portion of the organic effluent that decreases the effluent's Biological Oxygen Demand (BOD).
3. Then the sewage is moved to the settling tank where bacteria settle and it is now referred to as activated sludge.
4. Part of the activated sludge is moved to the aeration tank again to serve as an inoculum, while the remainder of it enters large tanks called digesters of anaerobic sludge.
5. Anaerobic bacteria are developing here to create a mixture of biogas-forming gases such as methane, hydrogen sulphide and carbon dioxide. It is then possible to discharge the effluent into natural bodies of water.

Note: With a monthly average of less than 30 mg/l BOD and less than 30 mg/l suspended solids, secondary treated sewage is required to generate effluent. There could be weekly averages up to 50 percent higher. At least 85% of the BOD and suspended solids are required to be removed from domestic sewage by a sewage treatment plant delivering both primary and secondary treatment.