Maximizing the energy potential of urban sludge treatment: An experimental study and a scenario-based energy analysis focusing on anaerobic digestion with ultrasound pretreatment and sludge combustion

With stricter regulations and concerns over sustainability, combustion can be a beneficial handling method as dried sludge can be used as an energy source. Anaerobic digestion of sludge also provides energy in the form of biogas without the need for sludge drying. Yet, anaerobic digestion reduces the calorific value (CV) of sludge. In this study, the effect of anaerobic digestion (with or without ultrasound pretreatment) on the fuel properties of sludge were examined through laboratory experiments. Additionally, a number of sludge management strategies were evaluated to maximize energy gain. Temporal changes in fuel properties of sludge were tracked during digestion. Results showed that sonication provided a 3.4-fold increase in soluble COD, 15% increase in biogas production and 32% increase in methane production, while the CV of sludge reduced from 14.7 kJ/g to 10 kJ/g. Energy balances for a number of sludge management process trains showed that the optimal method was the combination of thickening, dewatering, thermal drying and combustion. Addition of digestion and ultrasound pretreatment to this scenario reduced the energy gained from sludge. Yet, high ambient temperatures impacted results such that the net energy gain by the scenario including digestion and combustion was close to that of combustion only.