Effect of coal ash residues on the microbiology of sewage sludge composting

Alkaline coal ash residues (fly ash, FA, and lagoon ash, LA) which have been used as co-composting materials for sewage sludge to reduce the availability of trace metals, may have an adverse impact on the composting process. It is therefore the aim of the present study to evaluate the effect of FA and LA on the microbial activities of sewage sludge composting. Fly ash and LA were mixed with dewatered sludge at 10 and 25% (w/w dry weight basis) and the mixtures were composted for 100 days. The changes in temperature, pH, EC, CO₂ and O₂ concentration in exhaust, and microbial population were measured during the composting period. The amounts of CO₂ produced and O₂ consumed were about the same for all treatments except that with 25% of FA which had a significantly lower CO₂ production and higher O₂ concentration. Addition of ash residues did not significantly inhibit the growth of thermophilic bacteria except for 25% FA amendment, but significantly inhibited the growth of thermophilic actinomycetes and fungi throughout the composting period. The inhibition was possibly due to the high pH and salinity of the ash residues, but inhibition decreased with composting time. The present results support the use of both FA and LA to co-compost with sewage sludge without exerting a significant inhibition on the microbial population of composting except for 25% FA.