A study on recirculating process water from hydrothermal carbonisation of spent brewing grains: Impact on hydrochar characterisation and process water composition

Spent brewing grain is a by-product of the brewing industry that is frequently underutilised been either landfilled or used as animal feed. Hydrothermal carbonisation (HTC) is a process in which wet biomass is valorised into carbonaceous material (hydrochar) at moderate temperature and pressure. The process also results in a liquid by-product, known as process water (PW), that includes various chemical compositions. PW is considered on the burdens of commercialising the HTC process. Therefore, PW treatment and/or recycling is an important step to minimise the HTC waste streams. Recently, PW has been integrated with anaerobic digestion process to convert the dissolved organic compounds into biogas. This study is designed to investigate the feasibility of recirculating the PW to a new HTC reaction and impact on the produced hydrochar properties and PW composition. Spent brewing grains were processed in a HTC reaction at 200 ˚C for 1 h. The produced hydrochar from the first run (HC-A) was filtered, dried and characterised (proximate analysis, SEM/EDX, FTIR), achieving a hydrochar yield of 50.9%. While the process water (PW-A) was separated and analysed via SPME GC-MS. The PW-A was recirculated to valorise a fresh sample of spent brewing grains at same conditions resulting in solid (HC-B) and liquid (PW-B) products, with the second hydrochar yield recorded at 54.26%. The product characterisation results indicated a minimal difference between HC-A and HC-B where fixed carbon was reported as 9.23% and 6.40%, respectively. Similarly, volatile matter, ash content, morphology. On the other hand, there has been a clear difference between the SPME GC-MS results of PW-A and PW-B, where PW-B exhibited a higher abundance of volatile organic compounds, specifically including significantly increased concentrations of urea, 1-butanol, phenol, 4-ethyl-2-methoxyphenol, benzenediamine, 2,5-dimethoxy-α-methyl and several pyrazine derivatives including 2-methylpropyl and 2,5-dimethyl-3-(3-methylbutyl). Furthermore, new compounds such as 3-allyl-6-methoxyphenol and phenethyl acetate were detected with notably higher concentrations in PW-B compared to PW-A. Results suggest that the recycled PW retains the essential chemical characteristics necessary for effective HTC, with some chemical composition variations observed for PW-B when compared to PW-A. Additionally, the HC-B exhibited yields and physicochemical properties that were comparable to HC-A, thereby demonstrating that PW recirculation does not compromise the overall efficiency of the hydrochar production process. This integrated strategy not only minimises waste and resource consumption but also addresses the pressing environmental and economic challenges inherent in commercialising the HTC process. Future research will focus on process optimisation, impact of repeated recirculation on the hydrochar and PW properties.