Coal ash residues, including fly ash (FA) and lagoon ash (LA), were co-composted with dewatered sludge for 100 days in a bench-scale composting system and biological parameters were monitored to evaluate the effect of ash residues on the composting process. Coal ash residue amendment caused a rise in alkalinity and salinity, but had no adverse effect on the population of thermophilic bacteria except for 25% FA amendment. All enzyme activities decreased with composting time. beta-glucosidase and alkaline phosphatase activities were reduced with an increase in ash residue amendment, while no significant reduction was noted for urease and dehydrogenase activities after 20 days of incubation. Positive correlations were obtained among beta-glucosidase, alkaline phosphatase and urease activities, while no correlation was found between thermophilic bacteria and enzyme activities. With the exception of 25% FA-amended sludge compost, addition of coal ash residues in sewage sludge is feasible and showed little effects on metabolic activity during composting process.