Feasibility of using coal ash residues as co-composting materials for sewage sludge

Alkaline coal ash residues produced from a coal-fired power plant were co-composted with sewage sludge to evaluate it's effect on heavy metal availability and the biological process of composting. Coal fly ash (FA) and lagoon ash (LA) were mixed with dewatered sludge at 0, 10 and 25% w/w, and the mixtures were composted for 100 days in laboratory batch reactors. The changes in pH, electrical conductivity (EC), CO₂ production, microbial population, soluble and extractable heavy metal contents were measured during the composting period. Following an initial increase, pH started to decrease from day 7 onward till the end of the composting period for all treatments. Sludge with coal fly ash amendment had a higher pH and EC than those of the control and LA-sludge composts. Increasing fly ash amendment levels resulted in a significant reduction in DTPA-extractable Cd, Cu, Zn, Mn and Pb contents of the FA-sludge composts while the reduction was less obvious in the LA-sludge composts. No significant difference in CO₂ production and the number of thermophilic bacteria were noted for all treatments except for 25% FA-sludge compost which had a reduced thermophilic bacterial growth and CO₂ production. The inhibition which was possibly due to the high pH of coal fly ash, decreased with an increase in composting time. It can be concluded that the co-composting of coal ash residues with sewage sludge was able to reduce the availability of metals but did not exert a significant inhibition on the biological process of composting, except for 25% FA-sludge compost.