Valorisation of sewage sludge for sustainable bioresources and bioenergy recovery through hydrothermal treatment

The development of an efficient valorisation of waste-activated sludge (WAS) from wastewater treatment plants (WWTPs) is a sustainable solution to bioresource and bioenergy exploitation. The use of hydrothermal treatment (HT) integrated with anaerobic digestion (AD) to enhance the efficiency of bioresource and bioenergy generation from WAS is the focal area of this research. The HT process was designed under a central composite design, while detoxification via adsorption, precipitation of struvite and biogas production under the AD process were further exploited to enhance the recovery of nutrients and biogas yields. The study has shown improvement in the solubilisation of organic matter after HT, phosphorus recovery, and biogas generation. Consequently, this integration approach offers an optimistic perspective on developing a novel strategy for improving resource recovery from WWTPs. A temperature of 220°C and a residence time of 20 min were found as optimum operating conditions for the HT, and effectively solubilised organics with reported soluble chemical oxygen demand (SCOD) from 613 mg/L to 8474 mg/L. The adsorption using magnetic biochar in the hydrothermally treated sludge was capable of reducing phenolic compounds by 37.9% and heavy metals - copper, manganese, and nickel by 60.4, 73.5 and 56.2%, respectively. This process was also associated with a loss of phosphate and ammonium by 50.3% and 47.2%, respectively, through adsorption. The struvite precipitation process resulted in a high overall phosphorus recovery efficiency of 70.52% and moderate ammonium removal efficiency of 35.71% under the optimised condition of pH of 9.24 and the addition of 14.2 mL/L of magnesium chloride. The biogas yield was enhanced greatly in hydrothermally treated and detoxified sludge, highlighting the synergy between HT and AD with cumulative methane yields for WAS, HT, adsorption and precipitation streams of 2.3, 72.7, 58.6 and 32.4 mL/g-VS, respectively. Furthermore, the obtained hydrochar with a heating value of 17.8 MJ/kg makes it a viable biofuel source. • Hydrothermal treatment improves biogas and nutrient recovery from sewage sludge. • Struvite precipitation achieved 70.52% phosphate recovery efficiency. • Detoxification removed 37.9% of phenolics, improving waste management. • Methane production increased after hydrothermal treatment by 72.7 mL/g-VS. • Hydrochar, with a heating value of 17.8 MJ/kg, is a viable biofuel source.