Coal fly ash as a composting material for sewage sludge: Effects on microbial activities

Coal fly ash has a strong alkaline buffering capacity which may render heavy metals in sewage sludge unavailable and the aim of this project is to study the potential of coal fly ash as a co- composting material for sewage sludge by evaluating its effect on the microbial decomposition of sewage sludge. Lagoon ash and fly ash at 0, 2.5, 5, 10, 20, and 50% w/w were mixed with dewatered sewage sludge and incubated at 25°C for 3 months. Carbon dioxide evolution for all treatments followed the same trend except 50% w/w amendment for both ash residues. When compared with the control (sludge only), higher levels of ash amendments had a lower CO2 evolution which may be due to the high entrained salt content and pH in both ash residues. The present results indicate that fly ash has a more pronounced inhibition on sewage sludge decomposition while 2.5% w/w and 5% w/w lagoon ash amendments demonstrate no significant inhibition. Therefore, lagoon ash would be a more suitable co-composting candidate for sewage sludge and the suitable amendment rate should be < 10%.