Waste Water Engineering
If the flame of a miner's safety lamp in the upper layers of the sewer forms an explosive, the sewer certainly contains

Methane
Hydrogen sulphide
Carbon dioxide
Oxygen

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Waste Water Engineering
Nitrogen cycle of sewage, is

Liberation of nitrogen-formation of nitrates-formation of nitrites-liberation of ammonia
Liberation of ammonia-formation of nitrites-formation of nitrates-liberation of nitrogen
Formation of nitrates-formation of nitrites-liberation of nitrates-liberation of nitrates
Liberation of nitrogen-liberation of ammonia-formation of nitrites- formation of nitrates

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