Sewage sludge disposal with energy recovery: a review

Wastewater must be treated, and it is part of the development process of a given country to extend sewer networks and to build wastewater treatment plants. Wastewater treatment inevitably generates sewage sludge, and a trend for extending wastewater treatment infrastructures implies increasing quantities of sewage sludge produced. The fate of such a residue is a key environmental issue. The existing European environmental legislation is becoming more and more restrictive, namely in terms of the obligation to reduce environmental impacts resulting from the disposal of this particular waste, where land spreading is still the most common technique to do so. There are some interesting alternative processes relating to sewage sludge treatment, aiming at avoiding its disposal on land or in landfill and to allow it to be more environmentally friendly. This paper reviews recent research to make sewage sludge disposal safer, more environmentally friendly and still to obtain some benefit from it, namely energy, from its organic content. Topics covered include incineration, anaerobic digestion, pyrolysis and gasification, with a view to contributing to an integrated response to the problem of sewage sludge treatment and disposal.