Potential to inactivate microorganisms in sewage sludge by ammonia treatment : different temperatures and urea additions
Abstract: Sewage sludge is rich in nutrients and organic matter, both of which can be useful in the agricultural system. By recycling the nutrients from our excreta, a more sustainable use of nutrients in agriculture can be achieved and the use of mineral fertilizer can thereby be decreased. There are however risks associated with the use of sewage sludge in agriculture, and these must be minimized to guarantee the safety of the environment and general public. The dispersion of heavy metals, organic pollutants and pathogens are the main risk factors generally considered. The focus of this study was on the reduction of pathogens in anaerobically digested, dewatered sewage sludge. Combinations of different temperatures (28.0, 33.0, 38.0, 41.5 and 44.0°C) and additions of urea (0.5, 1.0 and 1.5% wet weight) resulted in 20 treatments that were studied for seven days. The effect from added urea and temperature on the reduction of Enterococcus spp., thermo tolerant coliform bacteria (TTC), somatic coliphages and Salmonella spp. was analyzed. Additionally the treatments effect on pH, total nitrogen (N-tot) and total ammonia nitrogen (TAN) concentrations was also analyzed. The efficiency for treatments to reach threshold values proposed for Swedish future legislations concerning the studied organisms was assessed. Treatment temperature had a positive effect on reduction of all studied organisms and added urea had a positive effect on Enterococcus spp. and TTC reduction. Mean log10 reduction for Enterococcus spp. was 2.28 and five of the treatments (0.5, 1.0 and 1.5% urea at 44.0°C and 1.5% urea at 38 and 41.5°C) reached proposed (2010) legislative threshold values. Mean log10 reduction of TTC was 4.22 and proposed threshold concentrations were reached using most treatments, only control treatments (no added urea) at 28, 33 and 38°C, and 0.5 and 1.0% urea treatment at 28°C failed to reach proposed concentrations. Mean log10 reduction of somatic coliphages was 1.27 and none of the treatments reached proposed threshold reductions for viruses. The overall thresholds for 2010 years regulatory proposal was reached for most treatments. Complete compliance with the 2013 proposal could not be verified in this study since parasite reduction was not studied, and since the model organism for viruses used was conservative. The efficient reduction observed for most of the studied organisms lead to the conclusion that the method has great potential in this field of use. When also considering the added value of increased nitrogen content in the final product, to be used as closed-loop based fertilizer, the benefits using the method seem even greater. To further enhance the method, facilitating the fulfillment of future regulations, further calibrating is needed.
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