GAS-PRODUCTION AND DIGESTION EFFICIENCY OF SEWAGE-SLUDGE CONTAINING ELEVATED TOXIC METALS

The present project was aimed at studying the effects of increasing lo ading of heavy metals on the anaerobic digestion of sewage sludge. The first experiment indicated that a sludge sample collected from Taipo Sewage Treatment Plant had a higher gas yield than one from Shatin Sew age Treatment Plant, which was possibly due to the higher content of h eavy metals including Cr Cu, Ni and Zn, in Shatin sludge. Adding pig m anure to both sludges raised their biogas yields. The second experimen t of dosing different concentrations of the four metals to Taipo sludg e showed that gas yields were severely inhibited at the highest concen tration of each metal tested. The degree of toxicity of the four metal s tested was in the order of Cr>Ni>Cu>Zn. The third experiment used wa ter extracts of the digested materials for the cultivation of a microa lga, Chlorella pyrenoidosa, and the results showed that 5% extracts of both sludges alone gave higher algal growth than the control (Bristol 's medium). Extracts obtained from the digested materials containing p ig manure gave poorer algal growth.