Gas production and digestion efficiency of sewage sludge containing elevated toxic metals

The present project was aimed at studying the effects of increasing loading 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 Sewage Treatment Plant, which was possibly due to the higher content of heavy metals, including Cr, Cu, Ni and Zn, in Shatin sludge. Adding pig manure to both sludges raised their biogas yields. The second experiment of dosing different concentrations of the four metals to Taipo sludge showed that gas yields were severely inhibited at the highest concentration of each metal tested. The degree of toxicity of the four metals tested was in the order of Cr > Ni- > Cu > Zn. The third experiment used water extracts of the digested materials for the cultivation of a microalga, 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 pig manure gave poorer algal growth.