Air-drying and storage of digested sludge (aerobic, anaerobic or lagoon-based treatment) is the principal method of sludge management in the State of Victoria. Current Victorian State Guidelines prescribe digestion with a minimum storage period of dewatered biosolids for three years as one method for producing Treatment Grade T1 material. T1 graded products can be applied as fertilisers to land without restrictions related to microbiological safety. A number of key nutrients (nitrogen, phosphate and organic carbon) are, however, substantially lost from stockpiles during this storage period. This is a contributory factor restricting opportunities for recycling biosolids in Victoria.
A primary issue in the current regulation for application of biosolids to land is microbial safety. This has resulted in conservative regulation, due to the absence of applicable data regarding microbial safety for standard treatment processes. In addition, although air drying for three years is listed as a T1 process, there are no recommendations in the Guidelines for air-drying and storage for restricted grade products (Treatment Grades T2 and T3).
The project consisted of four parts; following the die-off of indicator pathogens during air-drying and stockpiling treatment processes, investigating the potential for regrowth of pathogens in biosolids, observing the loss of nitrogen nutrient compounds during storage and reviewing the literature on the effects of air-drying and storage on sludge properties, indicators, pathogens and nutrients.