A method and apparatus for treating sewage sludge by heating in a sludge pasteurisation process in which small bubbles of steam are injected into a continuous flow of sludge to cause highly turbulent sludge flow conditions during which the steam condenses to heat the sludge. Incoming, untreated sludge is initially pre-heated as required and as available heat sources such as independent power plant or excess capacity of the steam generating boiler. Thus, a suitable treating temperature is more readily achieved. The heated sludge is retained in an insulated tank to allow the pasteurisation process to be completed. Heat dumping may be provided to regulate treatment temperatures.

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