Evaluation of Cow Manure Effect as Bulking Agent on Concentration of Heavy Metals in Municipal Sewage Sludge Vermicomposting

Background and Sewage sludge can contain heavy metals. This study aimed at investigating the effect of cow manure as bulking agent on concentration of heavy metals (cadmium, chromium, copper and lead) during municipal sewage sludge vermicomposting and compares that with standard levels. In an experimental research, vermicompost was produced by mixing sewage sludge and cow manure using earthworms (Eisenia Foetida species). Composting process was conducted in 27 pilots for three months. The heavy metals concentrations were measured by atomic absorption spectroscopy in the vermicomposting process. Then, data was analyzed by descriptive statistics and statistical test of Repeated Measured ANOVA. In all pilots the heavy metals concentration decreased with increasing vermicomposting time. Reduction in Cd, Cr, Cu and Pb concentration values in the vermicompost produced from mixture of sewage sludge and cow manure was found 50% higher than other pilots. The concentration of heavy metals of in terms of kind of composted material decreased significantly in all pilots (P<0.05). Concentration of heavy metals for the number of worms also decreased in all pilots but this decrease was not found to be significant (P>0.05). Therefore, a compost produced by a mixture of 50% sewage sludge, 50% cow manure and 40 Eisenia Foetida in 1.2 Kg could have a higher quality in terms of heavy metals. The concentration of heavy metals was lower than the standard levels set by Iranian national (class I and II), WHO and US Environmental Protection Agency. The use of cow dung as a cheap and available bulking agent is recommended in vermicomposting of sewage sludge.