spent sodium hydroxide

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This minimum volume of effluent contains a significant level of sulphides 2 to 5 g·L–1 S2– and occasionally phenols; these contaminants require separate treatment for removal.

Desulphurisation can be effected by acidification at pH 3-3.5 and stripping the resulting H2S using a neutral gas. The effluent is stored and sent, at low flow rates, to the wastewater treatment plant, to the sulphur removal unitif existing, or upstream of the dissolved air flotation.

Phenol is removed by biological treatment after being diluted in the general wastewater stream.