Changes and variability in concentration of heavy metals in sewage sludge during composting

Abstract

The heavy metal concentration of sewage sludge increased during composting for cadmium, chromium, lead and zinc and decreased for copper and nickel. Cadmium and lead were the most variable of the metals, copper and zinc the least. Variability within a batch was small (variation of 4–12%). Variability attributable to batch‐to‐batch differences was generally larger (variation of 4–37%). The variability of sludge entering the batch process and of compost exiting the process were similar for each metal.