Treatment of activated sludge for utilization in agriculture and for recovery of non-ferrous metals

A sample of anaerobically digested sludge from an operation for municipal wastewater treatment in Bulgaria was characterized by relatively high contents of some heavy metals (mainly of iron, zinc, copper, manganese and chromium) and of some organochemical pollutants (mainly polyaromatic hydrocarbons and oil products). Some microorganisms which are indicators for the faecal pollution of the waters being treated such as Escherihia coli, Enterococcus faecalis and Streptococcus faecalis, were found in the samples, as well as some pathogenic microorganisms (related to the genus Clostridium) and eggs of helminths. The objective of this study was not only to remove the toxic pollutants from the activated sludge in connection with the relevant sanitary requirements but also to use the pretreated sludge as an amendment in agriculture and, having in mind the contents and the current prices of some heavy metals present in the activated sludge before treatment, to use it as a source of these metals. Copper and zinc were the most promising in this respect. Different treatment techniques were tested to achieve the objective mentioned above. The best results were obtained with sludge sample initially subjected to autoclaving and then leached by a mixed culture of moderately thermophilic chemolithotrophic bacteria. The addition of iron ions (mainly in the ferric state) and sulphuric acid from outside at the start of the leaching facilitated the growth of bacteria and the solubilization of metals. Leaching at 50 oC and an initial solid content of 20 % in the suspension resulted in high extractions of copper (98.8 %) and zinc (95.7 %) within 120 hours. The dissolved metals were selectively recovered from the pregnant leach solution by means of solvent extraction plus electrowining. The residual treated sludge contained no toxic chemical and biological pollutants in concentrations higher than the relevant permissible levels. At the same time, the bioassimilable forms of C, N, P, K and some essential microelements were still present in sufficient concentrations for utilization in agriculture.