FEASIBILITY OF USING COAL ASH RESIDUES AS CO-COMPOSTING MATERIALS FORSEWAGE-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 meta l 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 b atch reactors. The changes in pH, electrical conductivity (EC), CO2 pr oduction, microbial population, soluble and extractable heavy metal co ntents were measured during the composting period. Following an initia l increase, pH started to decrease from day 7 onward till the end of t he composting period for all treatments. Sludge with coal fly ash amen dment had a higher pH and EC than those of the control and LA-sludge c omposts. Increasing fly ash amendment levels resulted in a significant reduction in DTPA-extractable Cd, Cu, Zn, Mn and Pb contents of the F A-sludge composts while the reduction was less obvious in the LA-sludg e 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 a nd CO2 production. The inhibition which was possibly due to the high F H of coal fly ash, decreased with an increase in composting time. It c an be concluded that the co-composting of coal ash residues with sewag e sludge was able to reduce the availability of metals but did not exe rt a significant inhibition on the biological process of composting, e xcept for 25% FA-sludge compost.