Comparison of methods for evaluating stability and maturity of co-composting of municipal solid wastes and sewage sludge in semi-arid pedo-climatic condition

The step of maturation (around 60 days) appeared less active as compared to the first step of pre-fermentation (around 90 days) since the values of temperature, recorded during the second step of maturation, are found generally less important than those recorded during the first step. A similar tendency of C/N ratio values decrease is generally observed in the two piles of wastes during the two steps of composting and these C/N ratio values determined in the pile of wastes free of sewage sludge (W1) are generally slightly higher than those observed in the pile added with dry sewage sludge (W2). The amount of total heavy metals (order of content: Zn > Pb > Ni > Cu > Cr > Cd) appeared very heterogeneous and showed a large variation in the two piles of wastes. The use of sewage sludge in the pile W2 showed generally no apparent impact on the whole amount of total heavy metals recorded in the finished product and the values recorded are usually lower than the metal concentration limits imposed by several countries. Microbial inventory and total DNA extracted from composting materials followed during all the two steps of composting showed a net variation over time, and revealed specifically a good parallel progress according to the ambient temperature recorded inside the waste materials. It appeared also from this study that the microbial diversity is much nuanced in the case of windrow W2 added with sewage sludge as compared to that observed in the case of windrow W1 free of sewage sludge.