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Per §8.3.3 (Action of Sewage), the chief mechanism by which domestic sewage damages concrete sewers (despite low sulphate content) is:
A{'text': 'Direct corrosion by household soaps and surfactants in the wastewater stream', 'label': 'A'}
B{'text': 'Hydrogen sulphide from sludge becomes sulphuric acid that attacks concrete above the liquid', 'label': 'B'}
C{'text': 'Calcium chloride from cleaning products crystallises and disrupts the sewer wall surface', 'label': 'C'}
D{'text': 'High pH from common laundry detergents alone drives the concrete to deteriorate over time', 'label': 'D'}
Answer & Solution
Correct answer: B. {'text': 'Hydrogen sulphide from sludge becomes sulphuric acid that attacks concrete above the liquid', 'label': 'B'}
1. §8.3.3(e): 'hydrogen sulphide gas emanating from sludge digestion tank can promote the formation of sulphuric acid which can attack the concrete surface above the liquid level. Concrete sewers running full are not attacked.'
2. Hence the gas-phase mechanism (above-water region) is the critical concern in partially-filled sewers; prevention = adequate flow, ventilation, no stagnation.
_Source: IGNOU ET-522 Block-2 Unit-8 Durability of Concrete, §8.3.3(e), p. 71_
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