Odorous gas emissions from sewage sludge composting windrows affected by the turning operation and associated health risks

Abstract

The treatment and disposal of sewage sludge (SL) has long been a challenging task in China. Open windrow composting, coupled with mechanical turning, is preferred in small cities and rural areas, due to low costs and ease of operation. However, the emission of odorous volatile organic compounds (VOCs) from open composting windrows, as well as related health risks, has aroused strong protests from surrounding populations. This study investigated VOC emissions (including hydrogen sulphide) from five open SL composting windrows at a single site, before, during and after turning operations, and across different seasons. As expected, the highest VOC concentration (6676 μg m−3) was measured while turning the windrows, whilst an additional emission peak was observed at all windrows at different times after turning, which was determined by the raw material mixing ratio (SL: woodchips), as well as ambient and windrow temperatures. In general, higher VOCs emissions and odour concentrations were measured in summer, and odour pollution was mainly caused by sulphur and oxygenated compounds, due to their high odour activity values (OAVs). Methyl mercaptan, dimethyl disulphide, dimethyl sulphide, diethyl sulphide, acetaldehyde and ethyl acetate were identified as the odour pollution indicators for the composting facility. The results from a health risk assessment showed that acetaldehyde was the most hazardous compound, with both non-carcinogenic and carcinogenic risks exceeding acceptable levels. The carcinogenic risks of benzene and naphthalene were also above acceptable levels; however, their risks were insignificant at the studied site due to the low concentrations.