In the last decade, significant growth exhibited by the organic waste management industry in the UK has begun to decline due to removal of incentives, reduced material availability due to competition and implementation of quality standards for environmental regulation. Therefore, the purpose of this thesis is to provide a proof of concept for a novel approach to integrating biomass conversion technologies for improved organic waste management, maximising energy recovery and accelerating the process of composting. Specifically, the research aim is to identify the potential opportunities for the integration of hydrothermal carbonisation (HTC) with anaerobic digestion (AD) to treat and valorise digestate and green waste materials.
AD operators competing for feedstock often accept contaminated wastes which produce digestate that does not meet the quality specification for application as a fertiliser. As a result, operational expenses increase due to storage, transport and gate fees required for the disposal of digestate. Digestate, when applied to land also gives rise to concerns about eutrophication and fugitive greenhouse gas emissions. Additionally, the emissions generated from the composting of green wastes are not captured. Furthermore, if these biomass materials are not utilised as an agricultural product the nutrients they hold are lost, unless they are recycled. Literature has highlighted several key development areas within the AD industry aimed at decreasing the operating costs of digestion facilities. Digestate enhancement technologies, such as HTC and pyrolysis thermochemical processing, have been recognised for treating and increasing the value of digestate to secure use and create new markets. However, a significant gap in the literature, which this thesis intends to fill, lies in comparing thermochemical technologies, experimentation on a range of digestate materials and assessment of the application of products generated. Synthesis of this study’s findings show that the aim and objectives of the study have been met. Opportunities for valorisation lie in enhanced biogas generation from the recirculation of HTC process waters into AD and the use of hydrochar as a soil amender. Furthermore, the integration approach also introduces alternative recalcitrant AD feedstock via HTC pre-treatment which allows for acceleration of solubilisation and the humification process, whilst mitigating greenhouse gas emissions.
Chapter-specific results show that composition analysis of the feedstock materials demonstrate digestate and green waste have considerable energy recovery potential, due to high fractions of protein and lignocellulose respectively. The application of HTC process waters via AD generates bio-methane owing to the high concentrations of soluble organic compounds, particularly from high biodegradable process waters from the HTC of sewage sludge digestate and grass clippings at 200 °C. Hydrochar, when applied in agriculture as a soil amender has the potential as a slow release fertiliser due to improvements in fresh plant mass yields owing to being humic like and exhibiting good macro and micro nutrient content, especially from sewage sludge digestate. Furthermore, hydrochar and pyrochar converted from digestate materials are not suitable as a solid fuel due to high inorganic content and low energy density, resulting in very likely slagging and fouling propensity. However, high temperature 250 °C HTC co-processing by blending with green wastes, particularly woodchip, increases calorific values (up to 20 MJ per kg), energy density (up to a factor of 1.2) and reduces inorganic concentrations (by up to 50 percent), improving solid fuel properties. It was also found that the enhancement of digestate via pyrolysis is not favourable due to requirements of dewatering digestate, low liquid yields and high slagging and fouling potential of pyrochar as a solid fuel. HTC is more suitable for waste biomass treatment as it promotes demineralisation and solubilises organic compounds.
The significance of this study will benefit the circular economy and aids in reaching climate change targets from the generation of renewable energy and production of hydrochar that locks in carbon and provides benefits to agriculture. This is especially important in terms of reducing fossil fuel use and its impacts on soil erosion and fertility. The contributions of this thesis will be of interest to researchers in the fields of microbiology and chemical processing for the production of bio-hydrogen, bio-alcohols and fine chemicals from the fermentation of HTC process water. Soil scientists will also be interested in the potential of hydrochar applied in agriculture for remediation and soil amelioration. Moreover, researchers in the field of energy and storage will be interested in the combustion and electrical storage potential of hydrochar.
