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The effect of heavy metal concentration to the anaerobic sludge digestion process is studied by means of simple batch bioassay tests. Daily gas production rate and percentage volatile solid reduction decreased with increasing solids concentration. Analysis of the results by least square showed that gas production rate was directly proportional to the concentration of metals in sludge expressed as milliequivalent per litre. Threshold values for inhibition and toxicity limits, corresponding to 20% and 50% decrease in gas production respectively, are derived from the experimental results and can be used as a tool for diagnosis of potential toxicity.

The paper presents the conclusions of a study sponsored by the Ministry of Environment and Public Works and conducted by a research team of the National Technical University of Athens, aiming to investigate alternative management schemes for the sludge produced and expected after the operation of the second phase of the Psyttalia Sewage Treatment Works. The alternative management schemes examined include agricultural use, co-disposal of dewatered sludge with solid wastes, co-disposal of dried sludge with solid wastes, disposal of dewatered sludge at a separate landfill, disposal of dried sludge at a separate landfill, use of dried sludge as fuel for cement or electricity production and finally incineration. On the basis of the evaluation, the production of dried sludge which can be subsequently used for agricultural purposes or as a fuel stands out as the best long term option.