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Sewer manholes should preferably be designed to be watertight owing to the following reasons:
(i) Water-tightness is important in sewer system. Otherwise, leakage of sewage from manholes would seriously contaminate the environment.
(ii) In the event of high water table, water would infiltrate into sewer manholes. Therefore, a higher volume of sewage would be delivered to sewage treatment plant for treatment and this essentially increases the cost for treating this additional volume of water.