Question

Which of the following is present in activated sludge?
(a) Bacteria
(b) Yeast
(c) Protozoa
(d) All of the above

Answer 1

Hint: In the case of sewage treatment activated sludge is obtained after the secondary treatment of sewage. It consists of organic matter and decomposer microbes which form flocs with the help of slime and all that microbes are aerobic.

Complete answer:
Sewage treatment is mainly divided into primary, secondary, and tertiary treatment. Primary treatment involves the removal of small or large particles by filtration and sedimentation. After that, it passed into the primary settling tank where heavy particles settle down. Now remain sewage with fine organic matter passed for secondary treatment which is a biological process and needs microbial degradation. In this activated sludge passed into an aeration tank which agitates mechanically. In the activated sludge large numbers of aerobic microbes are present like bacteria, fungi, and protozoa which form flocs with the help of slime and settle down in the form of sediment in the secondary settling tank. In anaerobic sludge digester activated sludge with primary sludge broken down into organic acids. Wasted sludge is used in the form of manure and landfill. In anaerobic digestion, the pathogen of sewage gets killed. The organic product of anaerobic sludge digester like methanogen gets converted into methane, hydrogen sulfide, and carbon dioxide. The water form on the upper surface in the secondary tank was passed into the rivers and streams which Is free from any type of sewage.

So, the correct answer is ‘All of the above’.

Note: The need for the sewage treatment plant increased with the increasing population and increasing urbanization which lead to the accumulation of high wastage. Increasing water pollution increases biological oxygen demand. In secondary treatment, biological oxygen demand falls to 10-20% and after sedimentation of activated sludge, the biological oxygen demand decreases considerably.