Pure oxygen or a gas containing substantially more oxygen than air is injected under pressure into sewage held in or flowing through a rising or pumping sewer. This injection may be used to prevent the concentration of dissolved oxygen in the sewer falling to a level at which there occurs bacterial reduction to hydrogen sulphide of sulphate present in the sewage and/or to reduce the Biochemical Oxygen Demand of the sewage. The injection can also be used to oxidize to sulphur any sulphide dissolved in the sewage.
