Determination of Carbon/Nitrogen Ratio and Heavy Metals in Bulking Agents Used for Sewage Composting

Abstract

Improving the soil quality with organic matter content and presence of elements such as N, P and K are some of sewage sludge benefits. Adjusting carbonaceous materials such as plant wastes to dewatered sludge compost increases its moisture and improves C/N ratio. So dewatered sludge has low C/N ratio and should be mixed with bulking agents. This study was performed in a three months period of the year 2002, in laboratory of Chemistry of Water and Wastewater in the School of Public Health of Isfahan University of Medical Science, to determine the C/N ratio and heavy metals concentration of bulking agents in sawdust, leaves, rice hulls and dewatered sewage sludge. The results showed that concentration of chromium and cadmium in the mixture of dewatered sewage sludge and bulking agents was lower than the standard level. Means of cobalt(115.44 mg/kg),nickel(57.44 mg/kg)and zinc(273.48 mg/kg) concentrations were maximum in dewatered sludge but mean concentration of cobalt (25.66 mg/kg) in rice hull samples and mean zinc( 8.99 mg/kg) and nickel (5.106 mg/kg) concentrations in sawdust samples were minimum. The optimal conditions sewage sludge composting, each kilogram of sludge needs 350 grams of saw dust, 470 grams of leaves and 388 grams of rice hull. Amount of heavy metals present in the bulking agents is lower than the amount mentioned for the compost.