Preliminary research into the digestion of post-coagulation sludge

An increasingly higher number of water treatment plants are currently searching for a technology which would make it possible to dispose post-coagulation sludge from water treatment processes. Due to the fact that sludge contains a substantial amount of organic compounds, some tests were conducted to see how such sludge can be stabilised in anaerobic conditions. Sludge supplied by two water treatment plants was tested. Sludge 1 was a mixture of sludge from the treatment of filter backwash water and backwash water separated from water after coagulation. Sludge 2 was the product of the treatment of backwash water from carbon and contact filters as well as wastewater produced when primary settling tanks are cleaned. Prior to digestion, post-coagulation sludge was inoculated with the sewage sludge collected from the anaerobic digester of a municipal wastewater treatment plant. To determine the effectiveness of the tests, the sewage sludge used as an inoculate was also digested. The digestion process was conducted for 35 days at a temperature of 37°C. Compared to sewage sludge, a small amount of the sludge-digestion gas produced by the post-coagulation sludge was observed during the process. For the sewage sludge, the sludge-digestion gas evolution rate per volatile solids (VS) input was 0.19 m³/kg VS, for post-coagulation sludge 1-0.09 m ³/kg VS and for post-coagulation sludge 2-0.05 m³/kg VS. The sludge mineralization rate expressed as a percentage loss of dry volatile solids was the highest for the sewage sludge alone and it was 19%, whereas for the post-coagulation sludge it was 11.8% (sludge 1) and 5.8% (sludge 2). The digestion process substantially enhanced the filtration properties of the tested sludge. The post-coagulation sludge produced small volumes of sludgedigestion gas, thus it can be stated that the digestion of this sludge alone would not be technically nor economically profitable. Therefore, research into the digestion of post-coagulation sludge with a considerably higher share of sewage sludge should be done to identify the potential for their costabilization. Such a solution would provide a potential opportunity for resolving the problem of post-coagulation sludge through its disposal in wastewater treatment plants.