Energy in the urban water cycle: heat recovery from wastewater treatment

Municipal wastewater may be considered an important thermal energy source, in account of the heat contained in it during its transfer to the final treatment. Wastewater temperature in households outflow may be quite high, until to 25° - 30°C, and on the basis of a specific flow-rate of 250L/ab*d, an amount of 10 - 15 MJ/ab*d of unemployed heat may be estimated. The heat recovery potential from a specific wastewater network, and therefore the feasibility of its valorization, should be evaluated considering first of all the characteristics of the collection network, with particular attention to the selection of higher flow-rate pipes, and to the evaluation of wastewater temperature, that may vary between 12°C and 25 - 30°C along the year in European climate. Afterwards the potential users of heat and their distance from the source should be outlined; finally the structural components and the construction and operation costs should be defined and compared with the economic and environmental benefits arising from thermal recovery from wastewater. In the present paper a general evaluation of the technological possibility is presented, the implementation problems are discussed, and the economical and environmental advantages are summarized. The specific application of these aspects to particular situations can start from this approach.