This paper asks how much heat could be recovered from wastewater treatment plants under UK climatic conditions, and can this heat be used effectively to reduce their carbon footprint? Four wastewater treatment sites in southern England have been investigated and the available heat quantified. Issues relating to the environmental, economic and practical constraints on how this energy can be realistically recovered and utilised are discussed. The results show there is a definite possibility for thermal energy recovery and demonstrates that the financial feasibility of three options for using the heat (either for district heating, sludge drying or thermophilic heating in sludge digestion processes) is highly dependent upon the current shadow price of carbon. Without the inclusion of the cost of carbon, the financial feasibility is significantly limited. An environmental constraint for the allowable discharge temperature of effluent after heat extraction was found to be the major limitation to the amount of energy available for recovery. The paper establishes the true potential of thermal energy recovery from wastewater in English conditions and the economic feasibility of reducing the carbon footprint of wastewater treatment operations using this approach.