Pretreatment of sludge by thermal hydrolysis: impact on dewaterability and structure of digested sludge

Anaerobic digestion (AD) is the most common wastewater sludge treatment method for more than half a century. However, AD still has many challenges, especially if the circular economy's principles should be met. Production of renewable energy in the form of biogas, and sludge cake production of a quality that allows agricultural reuse or extraction of raw materials, has recently gained growing interest. Therefore, many approaches were undertaken to enhance anaerobic digestion. These are mainly: raise sludge concentration in the digester, lower hydraulic retention time, increase biodegradability, and reduce sludge's viscosity. The thermal hydrolysis process (THP) could be the answer to these challenges. High-temperature thermal hydrolysis pretreats the sludge before entering the anaerobic digestion, with temperatures between 140-175°C and pressure 6-11 bar for 30-60 minutes. Pretreatment by thermal hydrolysis significantly reduces the viscosity of sludge, allowing higher TSS concentration in digester while overcoming transport difficulties and mixing problems in AD. Sludge disintegration, destruction of extracellular polymers found in sludge, and further cell lysis reduce sludge's water content and increase its biodegradability. Simultaneously, more biogas is produced in a shorter time due to overcoming hydrolysis as rate liming step of AD. Studies have shown that sludge cake made after pretreatment by thermal hydrolysis meets the US EPA regulations for biosolids 'Class A,' allowing its use on agricultural land as a fertilizer. Besides the advantages listed, there are still some challenges to be solved. Firstly, all full-scale commercial applications of THP are in combination with mesophilic anaerobic digestion (MAD). Additional studies should focus on other AD modifications such as thermophilic anaerobic digestion (TAD), temperature phased anaerobic digestion (TPAD), or multi-stage anaerobic digestion. Therefore, further research of interactions between THP and different AD modifications is needed. Secondly, it is essential to focus on the chemical processes that lead towards the production of refractory compounds created at high temperatures in THP. These refractory compounds deteriorate the quality of wastewater treatment plant effluent. Simultaneously, it is crucial to maintain high biogas production and enhanced sludge dewaterability for the process's self-sufficiency and economic feasibility. Finally, requirements for the quality of the end product must be met according to its further use. If the application on agricultural land would be an option, the possible widespread of antibiotic resistance genes (ARG) and bacteria (ARB) must be prevented. The aim of this work is to extend our knowledge of different AD modifications combined with thermal hydrolysis. Structural changes of the sludge flocs and the dewaterability changes will be observed after the thermal hydrolysis, anaerobic digestion, and dewatering stage. Subsequently, the removal efficiency of ARG and ARB will be studied with THP upstream AD.