Bioremediation of sewage sludge by composting

Sewage sludge used as soil organic amendment can positively affect soil physical, chemical, and biological properties. This organic material may be a solution for improving the low organic matter content of Mediterranean soils. However, the addition of sewage sludge to soils have not always shown positive effects, probably due to the particular characteristics of the residues concerned: high heavy metal or phytotoxic substance content, excess of labile organic matter, pathogenic microorganisms, etc. Composting process can be a useful tool to eliminate some of these problems. In the present work, a sewage sludge mixed with wood shaving as bulking agent has been composted. During this process, a decrease of organic matter fractions due to organic matter mineralization was observed. Fresh sewage sludge contained high numbers of pathogen microorganisms but when this material was submitted to composting process microorganisms decreased to suitable level. Organic compounds such as pesticide residues can also be partially or totally degraded by microorganisms during composting process, prometrine and terbutrine being the most recalcitrant compounds. Composting process eliminated phytotoxic compounds contained in sewage sludge, permitting an adequate germination index.