The paradox of composting; a solution for sewage sludge management and also a cause for odour nuisances

Waste management is a growing concern for public authorities, local authorities and industry alike. With the development of sanitation, the management of wastewater treatment plant (WWTP) sludge has become a major issue for those involved in water treatment. In 2006, according to the French Environment Institute (IFEN), 40% of national waste management expenditure was devoted to wastewater management. Since Decree 97-1133 of 1997, WWTP sludge has been considered as waste – within the meaning of the law of July 15, 1975 –, which means that it must be traceable, and the producer is responsible for its disposal. However, some of its characteristics make sludge a potentially valuable product, particularly after appropriate treatment and subject to strict quality criteria. Composting has proved to be an economically and ecologically interesting alternative for promoting the agricultural recovery of sewage sludge, as part of a sustainable development policy. However, the odor nuisance generated by industrial composting facilities is the source of complaints from local residents, and can even lead to the closure of the site. As one of the main obstacles to the development of composting, odor nuisance needs to be addressed. Numerous physical, chemical and biological processes are now available to control odors, in line with the discharge standards imposed by regulations.

The aim of this article is to present composting from its two a priori antagonistic angles: as an interesting alternative for problematic sludge management on the one hand, and as a source of secondary air pollution on the other. The article therefore begins by putting composting in context, addressing the problem of sludge management and presenting the strategic orientations governing the distribution of sludge in the various possible recovery and/or disposal channels. The article focuses on the composting process, underlining the advantages that make it a choice alternative for sludge recovery, in energy, environmental and economic terms. Finally, the focus is on odor emissions linked to composting, which it is important to know, understand and characterize in order to be able to deal with them. This is the most sensitive aspect of composting, and therefore the main obstacle to the industrial development of sewage sludge composting. In any case, regulations impose strict discharge standards, which are outlined at the end of the article.