During secondary sewage treatment,
Flocs from the settling tank are sent for their digestion
Only a part of it is sent as inoculum in the aeration tank
Then the aeration tank is agitated where this inoculum multiplies for the development of flocs

I have a doubt,
Can we not transfer all the flocs from the sediment tank to the aviation tank and not just the small inoculum? Because now even if we don't agitate the aeration tank but allow the flocs in the aeration tank to break down the sewage the same process would occur in the aeration tank in a much less time and we would also be saving the energy spent in the agitation of aeration tank.

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Solution

If the entire floc is transfered from sedimentation tank in to aeration tanks their number will be too much.They will increase in number further and will not get sufficient oxygen to respire.Due to competition they may die.Instead if small inoculum is taken ,it will grow well and clear the sewage better as they grow.The bacteria require oxygen and hence they need to be aerated any way.

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Clarified water is then passed into aeration tanks, where it is agitated or sprayed. Air may also be bubbled through to increase oxygen levels in the water. Why is this step carried out in wastewater treatment plants?

Q. Study the given differences between primary sludge and activated sludge and select the incorrect ones.

	Primary Sludge	Activated sludge
(i)	It is sludge formed during primary sewage treatment.	It is sludge formed during secondary sewage treatment.
(ii)	It possesses flocs of decomposer microbes.	It does not possess flocs of decomposer microbes.
(iii)	It does not require aeration.	Formation of activated sludge requires aeration.
(iv)	A lot of decomposition occurs during formation of primary sludge.	Very little decomposition occurs during formation of activated sludge.

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(a) Grit chamber
(b) Aeration tanks
(c) Water tank
(d) Septic tank

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Q. Refer the given flowchart of sewage treatment, accordingly match Column I with Column II and select the correct answer from the codes given below:

\begin{matrix} Column I & Column II A. & The stage in which physical treament of sewage is done & (i) & Anaerobic degestion of activated sludge and production of biogas B. & The stage in which biological treatment of sewage is done & (ii) & Activated sludge C. & Name of the sediment in primary treatment & (iii) & Aeration tanks D. & It is carried to aeration tanks from primary settling & (iv) & Primary effluent E. & Name of the sediment in secondary treatment & (v) & Primary sludge F & Site of flocs growth & (vi) & Secondary treatment G & Function of sludge digester & (vii) & Primary \end{matrix}

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