Hydrothermal carbonisation (HTC) has been identified as a potential route for digestate enhancement producing a solid hydrochar and a process water rich in organic carbon. This study compares the treatment of four dissimilar digestates from anaerobic digestion (AD) of agricultural residue (AGR); sewage sludge (SS); residual municipal solid waste (MSW), and vegetable, garden, and fruit waste (VGF). HTC experiments were performed at 150, 200 and 250 °C for 1 h using 10%, 20%, and 30% solid loadings of a fixed water mass. The effect of temperature and solid loading to the properties of biocoal and biochemical methane potential (BMP) of process waters are investigated. Results show that the behaviour of digestate during HTC is feedstock dependent and the hydrochar produced is a poor-quality solid fuel. The AGR digestate produced the greatest higher heating value (HHV) of 24 MJ/kg, however its biocoal properties are poor due to slagging and fouling propensities. The SS digestate process water produced the highest amount of biogas at 200 °C and 30% solid loading. This study concludes that solely treating digestate via HTC enhances biogas production and that hydrochar be investigated for its use as a soil amender.

The preparation of low-cost carbonaceous adsorbents for nitrogen recovery is of interest from agricultural and waste management perspectives. In this study, the gaseous ammonia (NH₃) and aqueous ammonium (NH₄⁺) sorption capacities have been measured for different types of carbonaceous chars produced under different conditions. The study includes a comparison of an oak-based hydrochar produced from hydrothermal carbonisation (HTC) at 250 °C with two biochars produced from slow pyrolysis at 450 °C and 650 °C, respectively. The chars were also chemically modified with H₂SO₄, H₃PO₄, H₂O₂, and KOH to investigate the potential for sorption enhancement. The highest sorption capacities for NH₃ were observed for the hydrochars with typical uptake capacities ranging from 18–28 mg g⁻¹ NH₃. Sorption capacity for oak biochars is significantly lower and ranges from 4–8 mg g⁻¹ for biochars produced at 450 °C and 650 °C, respectively. Hydrochar showed a substantially higher sorption capacity for NH₃ despite its lower surface area. The CaCl₂ extractable NH₄⁺ following ammonia adsorption is incomplete. Typically, only 30–40% of the N is released upon washing with CaCl₂ in form of NH₄⁺. Post chemical modification of the chars resulted in only limited enhancement of char NH₃ and NH₄⁺ sorption. H₃PO₄ treatment showed the greatest potential for increasing NH₃/NH₄⁺ sorption in biochars, while KOH and H₂O₂ treatment increased NH₃ sorption in the hydrochar. As only marginal increases to char surface area were observed following char treatment, these findings suggest that char surface functionality is more influential than surface area in terms of char NH₃/NH₄⁺ sorption.

Hydrothermal carbonisation (HTC) can be integrated with anaerobic digestion (AD) for the treatment of digestate, resulting in a solid hydrochar or bio-coal and a process water, which can be recirculated back into AD to produce biogas. The properties of digestate-derived hydrochars do not lend themselves to producing high quality bio-coal and blending with lignocellulosic feedstocks can improve its properties. This study investigates the co-processing of sewage sludge (SS) digestate with three lignocellulosic biomass (grass, privet hedge, and woodchip). The calorific value of the resulting bio-coal is increased following co-processing, although feedstock interactions result in non-additive behaviour. The largest increase in calorific value was observed for co-processing with woodchip. There is evidence for non-additive partitioning of metals during co-processing resulting in only moderate improvements in ash chemistry during combustion. Co-processing also effects the composition of process waters, influencing the potential for biogas production. Experimental biomethane potential (BMP) tests indicate that grass clippings are the most suitable co-feedstock for maintaining both calorific value and biogas production. However, above 200 °C, BMP yields appear to decrease, suggesting the process water may become more inhibitory. Co-processing with wood waste and privet hedge produce the higher CV bio-coal but significantly reduced BMP.

A number of countries have introduced energy policies to reduce the emission of carbon dioxide which, in the case of bio-heat, has resulted in increased use of small wood burning stoves and boilers, particularly in Europe. There are issues surrounding the supply of sustainable wood feedstock, prompting a desire to utilise local biomass resources. This includes biomass generated through the management of natural woodlands in nature reserves and conservation areas. These management practices can also extend to other areas, such as raised bog wildernesses and estuary Reed beds. We term the biomass from this resource as conservation biomass. This study is concerned with the viability of this resource as a fuel within the United Kingdom, and combustion tests were carried out using a small domestic stove. It was concluded that there is as much as 500kty(-1) that could be used in this way.