The microbial leaching and composting of sewage sludge

The treatment and disposal of sewage sludge is currently one of the main problems faced by the UK wastewater treatment industry. The volume of sewage sludge requiring land-based treatment in the UK will increase significantly by 1999 as a result of EC directives. The aim of this study was to produce a stabilised, hygienically safe, aesthetically-acceptable, useful sewage sludge-based product with a relatively low heavy metal content. Two existing processes, microbial leaching and composting, were combined to achieve this aim.Composting has been reported by the United States Department of Agricultural to be the most technically efficient, economically sound, environmentally safe, agriculturally beneficial and politically feasible disposal option for sewage sludge. The presence of heavy metals in sewage sludge was cited as a significant barrier to composting in the 17th Report by the Royal Commission on Environmental Pollution. Microbial leaching is a heavy metal removal process which displays great potential for significantly reducing the heavy metal contamination of sewage sludge. Both processes conserve the soil conditioning and fertilising properties of sewage sludge.A successful combined process was developed and subsequently replicated through the use of laboratory scale models. A bench scale composting system was designed and constructed. Microbial leaching for the combined process was conducted in 5l capacity round bottomed flasks. Additional microbial leaching experiments were conducted in 250ml capacity flasks.A standard microbial leaching system was developed for the combined process. It was able to effect highly satisfactory levels of heavy metal removal from the anaerobically digested sludge used in the study. Copper, zinc and manganese were reduced by over 70%. The standard leaching system also significantly improved the dewatering characteristics of the sludge. The level of sludge dewaterability improvement effected by microbial leaching was much greater than that effected by polymer conditioning. This is an important finding. Microbially leached sludge produced by the standard system was composted successfully using straw as bulking agent. Straw significantly increased the potassium content of end material produced by the combined process.