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Why are there flames at the Dihua Sewage Treatment Plant’s exhaust combustion chimney?

Methane (march gas) is generated in the sludge digesting process of the Dihua Sewage Treatment Plant; this gas provides power for stirring the sludge within the digesting tank and provides a heat source for the sludge dryer; the excessive gas, if any, will be burnt away, that is why flames are seen from time to time at the top floor of the plant.