Question

What are ‘flocs’, formed during the secondary of sewage?

Answer 1

Hint: Removal of contamination from the wastewater is very important for the cleanliness of our rivers, for this government has set up sewage treatment plants all over the country.

Complete answer: Sewage is the waste that is generated from the industrial and residential establishment.
The waste liquids from kitchen, toilet, bath, sink, etc are sewages that are disposed of through sewer. This contaminated water gets mixed with the rivers and lacks and makes them dirty. So, to control this water contamination sewage treatment plants were established.
1. The treatment is carried out in three stages; primary treatment, secondary treatment, and tertiary treatment.
2. In the primary treatment the sewage is stored in a basin, inside that the sludge gets settled down in the bottom and the oil particles float to the top. These layers of sludge and oil particles are removed and the water that remained is sent to secondary treatment.
3. The next stage is the secondary treatment; in this stage the contaminants that are dissolved in water get treated. This treatment is done with the help of microorganisms, uses aerobic bacteria that digest the organic compounds of the sewage like sugar, fats, etc. and the suspended bacterial growth is used for the treatment of activated sludge.
4. For this flocs are developed that are essential groups of bacteria that form a structure like sludge and bind together with the use of slime.
5. This increases the rate of decomposition and is kept in an aeration tank and reduces the BOD of the sewage upto 15% before passing to the settlement tank and finally, in the tertiary treatment the water is cleaned by using effluent polishing. This treatment includes many filtration processes to disinfect the sewage.

Note:The sewage water after secondary treatment is clean enough to discharge directly to the river but tertiary treatment is carried out for some of the specific chemicals that is not digested by flocs. Some of the treated sewage water consists of ammoniacal nitrogen, this can be removed by using a nitrifying and denitrification process that converts ammoniacal nitrogen to nitrogen gas.