The presence of pathogens and high concentrations of heavy metals in sewage sludges constitutes a major constraint to the use of these biosolids as agricultural fertilizers. In this regard, the research being conducted by Prof. Henry and coworkers on sewage sludge detoxification is interesting and promising. Previous studies by his group (Henry et al. 1988, 1991) have demonstrated that the microbial leaching process, which makes use of Thiobacillus ferrooxidans without any exogenic substances, has only a limited capacity for the destruction of indicator bacteria. This restricted efficiency warrants additional studies on the development of a sludge disinfection technique during, or prior to, the metal leaching process. The concept of using copper sulphate, a known bactericide, for pathogen destruction in sludges is hence appealing. The major factors controlling the bactericidal operation in this work are (a) the copper concentration added, (b) the time of copper contact with the sludge, (c) the pH and the redox potential (ORP) of the sludge, and (d) the sludge solids concentration. Tests on coliform inactivation at varying sludge solids concentration is an important aspect, since it reflects the actual sludge conditions, even though it is not clear why 3% solids concentrations was used as a threshold limit in the results presented. Due to lack of certain experimental details in this paper, however, it becomes difficult to assess the complete scope of this work. For example, the anaerobic conditions under which the experiments were conducted are not detailed. If this work was done in the absence of oxygen, the sludge ORP should have dropped significantly, changing the copper speciation in the medium and should have resulted in a changed experimental condition. However, certain other details presented in the paper indicate that the work may have been done under aerobic conditions. In this discussion, it is assumed that by the term anaerobic condition, the authors mean the initial conditions in which the anaerobic sludges are normally found. For coliform destruction, the most important aspect is the ionic c u 2 + toxicity. Speciation of copper in any medium depends upon its pH and ORP conditions. Copper carbonate could be the major inorganic species at neutral pH and around +200 mV ORP, the usual situation for raw or aerobically digested sludges. At low ORP values, which is usually the case in anaerobically digested sludges, copper tends to be in the cuprous sulphide precipitate form (Theis and Hayes 1978). In both cases, very little copper is present in the ionic form. The actual percentage of available c u 2 +