Abstract
Hydrothermal carbonisation (HTC), a thermochemical technology used for the conversion of wet waste biomass into a carbon-neutral solid fuel, could help address the rising global demand for energy without increasing global land-sea temperatures. However, a major obstacle in the commercialisation of HTC is the production of substantial volumes of contaminated process water (PW) that need to be treated to recover any valuable nutrients and to reduce environmental impact.
This thesis outlines a proposed treatment regime for the valorisation of the PW from the HTC of wet waste biomass. The specific feed is spent coffee grounds (SCG). Through adsorption onto MN200 resin, nanofiltration (NF) and reverse osmosis (RO), 66.5 % of the PW volume achieved a 95.7 % reduction in COD loading (from 32,700 mgCOD L-1 to 1,400 mgCOD L-1). MN200 was seen to have a good maximum adsorption capacity, qmax, of 2,720 mgCOD g-1, could be regenerated to 92 % of its virgin capability, and was regenerable over 23 cycles. The retentate from NF was subjected to phosphorus recovery before combination with the retentate from RO to produce a treated PW feed for anaerobic digestion (AD). The treated PW in the AD achieved a 93.3 % reduction in COD and high average COD degradation efficiency of 93.5 %, the highest reported in the literature and a significant improvement over that of PW direct from the HTC. The AD was stable for 190 days, including 116 days of treated PW feed when biogas was produced with a specific CH4 yield of 0.21 L gCOD-1. A novel aerobic biodegradability test, an activated sludge mimic, was also utilised in the successful demonstration of comparative and absolute biodegradability of the different PW streams produced in the HTC of SCG and at various stages in the proposed treatment regime. The successful treatment of HTC PW, as in the technological proof of process shown in these results, could prove crucial to the commercialisation of HTC in the future, and associated improvement in the recirculation of resources.