Indicators and their survival during sewage sludge treatment
Creators: name: Mondal, Tania
Publisher: name: RMIT University
Year: 2024

description: Biosolids are defined as the end-products from sewage sludge treatment processes that meet specific quality requirements for land application. These products have value as nutrient sources and soil conditioners and as alternatives to chemical fertilizers for crop production. In Victoria, Australia approximately 60% of the biosolids (66,700 dry tonnes) is produced by two major metropolitan treatment plants serving the city of Melbourne. A common method used by these treatment plants is mesophilic anaerobic digestion (MAD) followed by pan drying and stockpiling. In Victoria, only a very small proportion of biosolids is used for land application, compared with other states in Australia and in other developed countries. One reason for this is the conservative interpretation of Victorian guidelines, in part due to the lack of the microbiological data on sludge treatment under Australian conditions. Furthermore, the guidelines stipulate a minimum storage period of three years which greatly diminishes the agronomic value of biosolids. This study examines the fate of indicator organisms (representing pathogens) present in sludge during treatment by MAD, pan drying and stockpiling.<br><br>The decay of indicator organisms (Escherichia coli, enterococci, coliphage) over time at different stages in the sludge treatment process was followed at one treatment plant. Biosolids produced were microbiologically safe for land application after pan drying, based on indicator organism measurements. Further storage in stockpiles for three years, as required by the current Biosolids Guidelines in Victoria, is not deemed necessary. <br>A series of controlled laboratory experiments was completed to simulate sludge treatment by MAD, pan drying and stockpiling to understand the decay of different indicators; indigenous coliphage, seeded MS2 bacteriophage and P22 bacteriophage. Decay coefficients of indigenous coliphage obtained in the field and laboratory simulations and field drying were not significantly different, thus validating the laboratory simulation with respect to bacteriophages.<br><br>Factors affecting the decay of bacterial and viral indicators during MAD and drying pan treatment were also investigated. Nutrient deprivation and the presence of indigenous flora affect bacterial decay in liquid material, while dryness and increased mixed salt concentration were important factors as drying increased. <br><br>Since the rapid decay of coliphage was observed during laboratory simulation of MAD and pan drying and also in the field, the effect of sludge extract (protease source) on coliphage die-off in MAD output and drying pan sludge was investigated. MS2 bacteriophage showed a 3-3.5 log10 reduction within 25 hours at 37°C when exposed to these extracts. This observation suggests that proteases with other enzymes produced by indigenous bacteria in sludge extracts are responsible for coliphage die-off.<br>It is concluded that numbers of potential indicators can be reduced after 8 to 11 months of pan drying without long term stockpiling. Competition with Indigenous flora, predation by protozoa, nutrient deprivation, dryness, increased mixed salt concentration together with enzyme activity are responsible for the decay of indicators during pan drying treatment of sludge. These findings may be helpful to regulatory authorities and water companies in setting guidelines and treatment parameters for wastewater treatment plants in Victoria.<br>