| Municipal wastewater sludge is the by-product of a chemical or a biological solid removal process that aims at minimizing human impact on receiving waters. Without this process, the capacity of the affected ecosystems would quickly be exceeded. Water quality would be severely affected and so would ecosystems and human health. Wastewater treatment processes implies the management of large quantities of sludge. Anaerobic digestion is a great step toward sustainable sludge reduction and stabilization. Nevertheless, digestion solves only partially the problem of sludge disposal. Unfortunately, sludge management is still the weak link of the environmental impact of wastewater treatment plants. Despite the efforts invested, two thirds of sludge produced in Quebec (Canada) still ends up in landfills or in incinerators.;This research aimed to identify and analyze an optimal energy-from-sludge approach while trying to maintain some of the fertilizing properties in the end product. A particular attention has been given to the synergy of an anaerobic digestion and gasification coupling. Priority has been put on fossil fuel reduction and on organic matter and minerals recovery. The scientific approach relies on energy and mass balances based on real conditions for anaerobic digestion (Châteauguay wastewater treatment plant in Quebec, Canada) and on pilot scale conditions for gasification.;One of the main challenges is the high moisture content and the low heating value of sludge. Yet, is has been evaluated that an excess of 7.4 and 4.1 GJ per dry ton of sludge could be recovered respectively as biogas and low temperature heat. Furthermore, 95% of total phosphorus has been maintained during the gasification step while doubling its concentration in the final product, biochar.;This technological coupling shows many advantages but also great technical challenges. It makes no doubt that this coupling could be realized and would be a significant improvement over actual disposal practices. An exergetic balance evaluation as well as a life cycle analysis would contribute to a better understanding of the place of this approach in Quebec context and elsewhere. |
