Abstract
This work aims to evaluate the microbiological contamination of sewage sludge (SS) collected in urban wastewater treatment plants (WWTP) from Portugal. Two types of SS were considered: urban mixed (UM) and from anaerobic digestion (AD). The two types of samples were characterized in relation to the main physical and chemical parameters, as well as the microbiological contamination (Escherichia coli and Salmonella spp). Then, sanitation tests were conducted through thermal drying and chemical treatments. Towards a circular economy, industrial alkaline wastes (green liquor dregs - GLD, lime mud, coal fly ash, eggshell) were tested as alternatives to lime. Only six out of nineteen samples complied with the legal limits for both microorganisms. However, drying at 130 °C sanitized selected samples below the E. coli limit, regardless of the initial moisture or contamination. Additionally, CaO (obtained from eggshell) led to the complete elimination of E. coli at any dosage studied (0.05–0.15 g/g SSwet basis). GLD evidenced the ability to reduce E. coli contamination at room temperature, but not enough to comply with the legal limit. In general, this work highlights the need to sanitize the SS before its application to the soil, and the positive role of some wastes on this goal.
Highlights
The circular economy framework encourages the recycling of organic wastes as a source of renewable energy and/or for increasing the organic matter in the soil after a stabilization process
The European Union (EU) policies have recommended a drastic reduction in landfill disposal, while a significant increase in incineration with energy recovery has been observed in some countries
This study aims to assess the regulated pathogens of sewage sludge (SS) from Portuguese wastewater treatment plants (WWTP), through the quantification of E. coli and the evaluation of presence/absence of Salmonella spp
Summary
The circular economy framework encourages the recycling of organic wastes as a source of renewable energy and/or for increasing the organic matter in the soil after a stabilization process. The options for its proper management have been agriculture reuse, composting followed by soil application, incineration, landfill, or others [3]. The EU policies have recommended a drastic reduction in landfill disposal, while a significant increase in incineration with energy recovery has been observed in some countries (e.g., in Germany and Slovenia). Regarding the SS recycling, in 2015, agriculture applications (directly or after composting) represented the main final disposal, whereas the prevailing technology can be very distinct in each Member State [1,4]
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