Reduction of indicator organisms during biological solubilization of metals: A pilot study

Laboratory and pilot scale studies on the biological solubilization of meta ls from undigested raw sludge were carried out using elemental sulphur as t he energy source. Metals (Cu, Zn Ni and Cd), pH, sulfate and indicator bact eria [total coliforms (TC) faecal coliforms (FC) and faecal streptococci (F S)] were used to evaluate the system effectiveness. This paper does not rep ort the detailed design and operational data of the pilot plant (available elsewhere [1]) but focuses a the biological tests carried out at the Main W astewater Treatment Plant in Toronto from July to December. 1997. Since the indicator bacteria are injured from exposure to low pH and potentially tox ic metal concentrations, improved alternative methods for their enumeration and their ability to repair acid-metal induced injury in a resuscitation m edium (CASO tryptone soya broth) were also investigated. Resuscitation was not effective in repairing cells injured during bacterial leaching. The MPN technique (using lauryl tryptose broth) for enumerating indicator bacteria (total coliforms) gave higher counts and was therefore superior to the spr ead plate technique (using m-Endo agar) in recovering bacteria from acidic leached sludge(biosolids). However, the coliform bacterial counts from raw sludge were similar by the two methods. This study indicated that the biolo gical solubilization process could significantly reduce the pathogenic indi cators. Concentrations of TC, FC and FS in the leached biosolids from the s olubilization tank were lower than concentrations in the raw sludge by 4 to 6 orders of magnitude and the finished product (biosolids) met U.S.EPA req uirements for pathogen and metal concentrations for Class A biosolids to be used on agricultural land.