Wastewater Management

Step 1: Understanding waste stabilization ponds.
Waste stabilization ponds are large, shallow basins used for wastewater treatment where biological processes occur naturally with the help of algae and bacteria. Mixing is mainly achieved by wind action and thermal currents.
Step 2: Reactor behavior of ponds.
Due to continuous mixing by wind and convection, the contents of a waste stabilization pond tend to have nearly uniform concentration throughout the pond volume.
Step 3: Analyzing the options.
(A) Plug flow: Incorrect, as plug flow assumes no mixing in the flow direction, which is not true for ponds.
(B) Complete-mix: Correct, because waste stabilization ponds behave approximately as completely mixed reactors.
(C) Packed bed: Incorrect, this is used in fixed media reactors.
(D) Fluidized bed: Incorrect, this involves suspended media under high velocity flow.
Step 4: Conclusion.
Waste stabilization ponds are best represented by a complete-mix reactor.

Step 1: Understanding manhole design.
Manholes provide access to underground sewer lines for inspection and maintenance. The cover design is crucial for safety and functionality.
Step 2: Reason for circular shape.
A circular cover cannot fall through its opening regardless of its orientation, unlike square or rectangular covers which can slip diagonally.
Step 3: Analyzing the options.
(A) Incorrect, strength is not the main reason.
(B) Incorrect, entry convenience is unrelated to shape.
(C) Incorrect, architectural aesthetics are not the priority.
(D) Correct, circular shape prevents accidental falling into the manhole.
Step 4: Conclusion.
Manhole covers are circular primarily to prevent falling of the cover into the manhole.

Step 1: Understanding BOD.
Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen required by microorganisms to decompose organic matter present in sewage. It is calculated based on the depletion of dissolved oxygen after incubation and corrected for dilution.
Step 2: Calculating oxygen depletion.
Initial DO = 10 mg/L
Final DO = 2.5 mg/L
Oxygen consumed = 10 − 2.5 = 7.5 mg/L
Step 3: Applying dilution factor.
Dilution = 2% = 0.02
BOD is calculated using the formula:

Step 4: Conclusion.
The Biochemical Oxygen Demand of the given sewage sample is 375 mg/L.

Step 1: Understanding self-cleansing velocity.
Self-cleansing velocity refers to the minimum velocity required in a sewer so that solid particles do not settle and any deposited matter is carried forward by the flowing sewage.
Step 2: Analyzing the purpose of self-cleansing.
The main objective is to prevent deposition of solids, which can lead to blockages, foul odour, and reduced carrying capacity of the sewer.
Step 3: Evaluating the options.
(A) Incorrect, as it does not address removal of deposited solids.
(B) Incorrect, since self-cleansing velocity is not related to maximum velocity.
(C) Correct, because it directly defines the velocity needed to flush out deposited solids.
(D) Incorrect, as this relates more to detention time rather than solid transport.
Step 4: Conclusion.
The correct definition of self-cleansing velocity is (C), which ensures removal of deposited solids in sewers.