Influence of bulking agents on organic matter evolution during sewage sludge composting; consequences on compost organic matter stability and N availability

The influence of bulking agents on organic matter (OM) stability and nitrogen (N) availability in sludge composts was investigated. Seven mixtures were composted over a 12-week period in 170 L-reactors using the same sludge with different bulking agent mixtures. The OM evolution was characterised by carbon (C) and N mass balances in biochemical fractions. The evolution of OM stability and N potential availability were measured during soil incubations. The type of bulking agent had little influence on the intensity of OM stabilisation and the N availability in final composts. However, they influenced the time to reach similar OM stability and the biochemical evolution of OM. Depending on their ligno-cellulosic characteristics and a careful control of aeration, initial mixtures with high C:N ratio and rather biodegradable carbonaceous materials could favour the organisation of initially present mineral N or easily mineralisable sludge N and therefore limit NH3 volatilisation responsible for the low compost N availability measured.