Conventional and Advanced Thermal Hydrolysis in Sewage Sludge Pre-treatment

Abstract

The thermal hydrolysis process (THP), a method that employs heat (at temperatures surpassing those of autoclaves, reaching up to 200 ℃) and pressure (reaching up to 25 bar) in the absence of oxygen, holds promise as a technology for pre-treating sludge before anaerobic digestion (AD) to enhance biogas production. Operating at elevated temperatures, THP also offers sludge disinfection for safe utilisation as fertiliser and improves sludge dewaterability. However, the requirement for high reaction temperatures presents drawbacks to the technology. At temperatures exceeding 150 ℃, the colour of the sludge darkens. This dark brown hue could impede the UV disinfection phase of wastewater treatment when the post-dewatering digestate wastewater is reintroduced into the wastewater stream. Furthermore, the harsh reaction conditions of THP not only lead to high energy consumption but also increase the formation of ammonia nitrogen and refractory compounds in the treated sludge. These compounds act as inhibitors of methanogenesis, significantly impacting methane production in anaerobic digestion processes. Advanced thermal hydrolysis (ATHP), an oxidative thermal hydrolysis process, has been introduced to overcome the challenges of THP. By incorporating Fenton's peroxidation, ATHP can be operated under milder conditions. This approach helps alleviate energy consumption, mitigate the impact on sludge colour, and minimise the production of refractory dissolved organic substances. In this study, a mixture comprising 55 % primary sludge and 45 % waste activated sludge from the Mangere Wastewater treatment plant in Auckland, New Zealand was used. For THP treatment, 600 mL of sewage sludge was fed into the reactor in each batch. The sludge was subjected to temperatures of 145, 160, 175, and 190 ℃, with varying processing times ranging from 5 to 30 min prior to anaerobic digestion. The vessel was pressurised with 6 bar of nitrogen (N₂) gas. The working pressure varied from 10.2 to 14.1 bar, depending on the set temperature. The influence of a sudden decompression (flash phase) on sludge solubilisation was also examined to elucidate the solubilisation mechanism. The research findings indicate hydrolysis constitutes the primary solubilisation process (accounting for approximately 76 % to 87 % of sludge solubilisation). However, the contribution of sudden decompression through flashing from working pressure to atmospheric pressure at the end of the process is substantial (ranging from 24 % to 13 %, depending on the treatment conditions). THP treatment at 190 ℃ for 10 min resulted in the highest degree of sludge solubilisation, approximately 45%, while the maximum biogas yield (around 389 mL biogas/g VS added) was achieved with sludge treated at 145 ℃ for 30 min. Operating THP at a lower temperature of 145 ℃ can minimise the release of ammonia nitrogen (NH₃-N) and reduce the colour of the sludge significantly. For the ATHP experiments, the sludge mixture was treated at 100, 115, 130, and 145 ℃, with a processing time varied from 5 to 30 min and oxygen pressure from 10 to 30 bar before AD. The highest biogas production (439.6 mL biogas/g VS added) was observed in the sludge treated at 145 ℃ for 15 min with an oxygen pressure of 20 bar. Conversely, ATHP treatment at 145 ℃ for 30 min with an oxygen pressure of 30 bar yielded a higher degree of solubilisation (25 %) in the treated sludge. The results demonstrated that under ATHP conditions of 145 ℃ for 15 min with an oxygen pressure of 20 bar, there was no increase in NH₃-N or refractory compounds that could inhibit methanogenesis. Furthermore, ATHP led to a 13% increase in biogas yield with a shorter reaction time compared to THP.