The effects of clay amendment on composting of digested sludge

Sewage sludge contains significant plant nutrients and has long been recycled to farmland. Composting of the sludge can reduce the potential risks of pathogen contamination, ground water pollution and odours when applied on to land. Sewage sludge usually also contains significant heavy metals and high moisture content that may limit its land application. Composting of sewage sludge with bauxite refining residue (red mud) may overcome these problems. A laboratory experiment of composting digested sewage sludge amended with red mud was therefore carried out to investigate how red mud affected the composting process. The temperature, pH, moisture content and volatile solids were monitored during composting. A mass balance was conducted by also measuring the carbon dioxide, oxygen, ammonia and condensate in the exit air; and total organic carbon and nitrogen in the compost. The results show that the amendment with red mud improved the composting process by raising the temperature, removing moisture, and increasing the decomposition rate. The red mud adsorbed the CO2 released from the sludge during decomposition, increasing the pH buffering capacity of the compost. The alkalinity of red mud and low C-to-N ratio in digested sludge increased ammonia volatilisation during sludge composting. Addition of external organic carbon effectively increased the C-to-N ratio and decreased the ammonia volatilisation.