Stabilized sewage sludge: Sanitary aspects and potential for conversion to biosolids

The improvement of wastewater treatment techniques is of crucial importance for effluent quality, but it also results in an increased amount of waste sludge. Dehydrated sludge contains organic matter and nutrients, and therefore it can be used in agriculture and bioremediation, but it is considered a potential source of environmental pollution. As the sludge analyzed in the research does not contain impermissible levels of organic and inorganic pollutants, the aim of the research was to examine microbiological, particularly sanitary, aspects and potential for its further use. Microbial diversity was determined by the standard serial dilution technique and selective media, and sanitary quality indicators (total coliforms, fecal coliforms, Escherichia coli and Salmonella spp.) were determined by the MPN method. The abundance of fungi, actinomycetes, and bacteria (ammonifiers, spore-forming bacteria and Pseudomonas spp.) indicate possibilities for further use of the sludge. The chemical analysis included the following parameters: total nitrogen (N), phosphorus in the form of P2O5 (available P), organic carbon (C), C/N ratio, pH, and water content. The chemical composition indicates the potential of sewage sludge to be used as a soil fertilizer, but its C/N ratio is not adequate to enable successful conversion to biosolids by the composting process. The obtained results indicate a significant level of microbiological contamination, which was most pronounced in the centre of the stabilized sludge pile. The research showed the necessity to conduct further studies on the microbial diversity and sanitary aspects of sewage sludge for proper waste sludge management.