question_answer

Give an overview of the process of sewage tratment.

OR

(i) (a) How is activated sludge produced during sewage treatment?

(b) Explain how this sludge is used in biogas production.

(ii) What are the differences between primary sludge and activated sludge?

 

Answer:

Wastewater treatment is necessary before wastewater is disposed of without producing any undesirable or harmful effect. Wastewater treatment process are carried out by municipal treatment plant as a community process. These are primary treatment & secondary treatment.
Primary \[(1{}^\circ )\] Treatment: It is a physical process that involves physical removal of particles from the sewage through filtration and sedimentation. So, it remove coarse solids and settle able solids in sedimentation units (tanks, basin, or mechanical devices) by which particulate material is concentrated as sludge. The sludge and liquid effluent are processed separately the effluent from the primary settling tank is taken to secondary.
Secondary \[(2{}^\circ )\]treatment: Secondary treatment involves the oxidation of organic material in effluent by microbial method. These are -
(a) Activated sludge process. In activated sludge process, aeration of sewage in large aeration and agitation tank, lead to the formation of flocs (masses of bacteria associated with fungal filaments to form mesh like structure) and floccules. When mould and protozoa. This growth is effective in oxidation of organic compound. This significantly reduces BOD (Bio chemical oxygen demand). BOD is measure of organic content in \[{{H}_{2}}O\]. Greater is BOD in \[{{H}_{2}}O\], more is its polluting potential.
(b) Filteration by intermittent sand filters and trickling filter.
The liquid sludge is sprayed over surface of bed of crushed stone, gravel, slag which saturate the liquid with oxygen. This maintains aerobic conditions in the bed. The filtering medium of tank is coated with a microbial flora - the zoological film -consisting of bacteria, fungi, protozoa and algae. As the sewage sweep over them, the micro-organism metabolize organic constituents to more stable end product. This requires repetition over a few weeks.
OR
(i) (a) Once the BOD of sewage or wastewater is reduced significantly, the effluent is passed into a settling tank where the bacterial floes undergo sedimentation and the sediment is thus, called activated sludge.
(b) A small part of activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaini major part of the activated sludge is pumped into large tanks called anaerobic sludge digesters.
The anaerobic bacteria digest the bacteria and fungi in the sludge and produce mixtue of gases like methane, hydrogen sulphide and CO which constitute biogas.
(ii) Difference between primary and activated sludge are
S. No	Primary Sludge	Activated Sludge
1	It is produced through mechanical wastewater treatment process.	The removal of dissolved organic matter and nutrients from the wastes takes place in the biological treatment step.
2	It includes the solid substance, which got settled after primary treatment.	It is the mixture of effluent from primary treatment and bacterial floes.
3	For primary productivity, sunlight is required.	For decomposition, sunlight is not required. .