Optimised ammonia sanitation of sewage sludge

Abstract: The usage of sewage sludge as an organic fertiliser offers a way to recycle nutrients and organic matter for a sustainable agriculture. Sewage sludge is rich in plant nutrient like nitrogen, phosphorus, potassium and micronutrients. Concerning the depletion of phosphorus resources, the reuse of sewage sludge can contribute in reducing the production of artificial fertiliser and satisfy agricultural nitrogen and phosphorus demands. Heavy metals and organic contaminants contained in sewage sludge hampered the image of sewage sludge application in agriculture, although reduced since the last decades. Another concern is pathogens in the sludge, posing a risk on human health and the food chain when applied to land. Pathogens in sewage sludge have not been considered as limiting for the use as heavy metals, thus regulations on pathogens putting less restrictions. This has changed over the last years, especially in the EU member states, lowering pathogen limits for sewage sludge (biosolids) when applied to land. Therefore, proper sanitisation of sewage sludge is required and the need for cost‐effective, simple and quick treatment methods for sludge stabilisation increased. In this study, an optimum ammonia treatment for sanitising sewage sludge was attempted. Pathogen inactivation was monitored by using indicator and model organisms. The small‐scale experiment comprised two phases, an aerobic composting phase (<40°C) and a subsequent anaerobic chemical treatment phase using urea solely or combined with carbonate or ECOX (sodium percarbonate). Urea addition releases ammonia, which has shown to effectively reduce pathogens. This antimicrobial effect is combined with the biological heat development in compost, also reducing pathogens. All tested chemical treatments reached a 5 log10 reduction for total thermotolerant coliforms and an 8 log10 reduction for Salmonella spp. within the two weeks experiment, with the best effect using 1.5% urea. Somatic coliphages reached a 1‐2 log10 reduction in treatments and control. The treatments had no significant effect on enterococci and only a 1 log10 reduction was achieved. The viability of Ascaris suum eggs was reduced in all treatments, but without consistency. No viable eggs were detected with 1.5% urea and 1.5% urea+3% CaCO3, when temperature was maintained over 35°C during four consecutive days. All chemical treatments used in this study increased the pH to around 9. The highest values were reached with 1% urea +2% ECOX. Considering treatment efficiency and cost‐effectiveness, the best sanitisation result in this study was achieved with 1.5% urea.