Microbiological quality of sewage sludge after digestion treatment: A pilot scale case of study

In the frame of the water circular economy, the reuse of the treated sewage sludge supposes a good action. Nevertheless, the reuse might suppose a human health risk because of the presence of pathogen germs, concentrated in the generated sludge during urban wastewater treatment. For this reason, sludge treatments, conventional or advanced, should be focused on the reduction of fermentation capacity and microorganisms presence. This research assess the microbiological quality of the treated sludge, based on the use of faecal pollution indicators such as total coliforms, Escherichia coli, Enterococcus sp., Pseudomonas sp., Staphylococcus aureus and total mesophilic bacteria, after the application of different digestion treatments, and their final aptitude to be reused. Three different pilot scale plants were used: two anaerobic digestion plants with a volume of 2.75 m3 working in mesophilic (35-37 °C) and thermophilic (55-57 °C) conditions, and hydraulic retention times (HRT) ranging from 5 to 20 days; and a autothermal thermophilic aerobic digestion plant (ATAD) with a volume of treatment of 3.9 m3 working at 55-57 °C for an HRT of 5–15 days. The raw sludge to be treated came from a full-operative WWTP located in the region of Navarra (Spain), laid within Ebro River Basin. It was concluded that both treatments studied obeyed the current European legislation in therms of E. coli concentration and absence of Salmonella spp., being available for agricultural purposes. Considering other microbiological parameters, the anaerobic thermophilic treatment had the best conditions to reduce the Enterococcus sp. concentrations and the aerobic thermophilic treatment was the best one reducing E. coli, being non-significant the influence of HRT to microbial reduction. For this reason, it is strongly recommended the use of both faecal indicators to assess the microbial quality of the treated sludge.