Poor management of organic waste is a key environmental and public health issue as it contributes to environmental contamination and the spread of diseases. Anaerobic digestion (AD) presents an efficient method for organic waste management while generating energy and nutrient-rich digestate. However, the AD process is limited by key factors, which include process inefficiencies from substrate-induced instability, poor quality digestate, and poor management of effluent and emissions. Lately, there has been more interest in the use of biochar for improving anaerobic digestion. Biochar can improve methane production by speeding up the methanogenesis stage, protecting microorganisms from toxic shocks, and reducing inhibition from ammonia and volatile fatty acids. It can be applied for in situ cleanup of biogas to remove carbon dioxide. Applying biochar in AD is undergoing intensive research and development; however, there are still unresolved factors and challenges, such as the influence of feedstock source and pyrolysis on the performance of biochar when it is added to the AD process. In light of these considerations, this review sheds more light on various potential uses of biochar to complement or improve the AD process. This review also considers the mechanisms through which biochar enhances methane production rate, biochar’s influence on the resulting digestate, and areas for future research.

Different strategies for sewage sludge management have been implemented to minimize the environmental impacts and benefit from the resources embedded in this waste stream. In that regard, Hydrothermal treatments (HTTs) of biomass can contribute generating high-value products (hydrochar) and enhancing biogas generation. In this work, different strategies were analysed for implementing HTTs at WWTPs considering the effect of 2 different process temperatures (160 and 250 °C) on different sewage sludge samples (primary, secondary and digestate sludge). Determination of carbon, nitrogen and phosphorus fate and mass and energy balances in hydrochar and process waters were carried out. HTT promoted solubilization of nitrogen, phosphorus and organic carbon up to 89%, 13% and 124% respectively. Moreover, biomethane yield increased up to 168% reaching methane concentrations up to 79% in the biogas. High heating values of hydrochars ranged from 12.0 to 18.2 MJ kg⁻¹. The integration of HTT as an intermediate step between two-step anaerobic digestion process showed an increase in the net energy balance of up to 124% in comparison with the conventional anaerobic digestion process. Graphical Abstract

Fast pyrolysis has been a subject of intensive research thanks to its ability to produce high yields of liquid products, known as pyrolysis oil. This is an important renewable intermediate which could be used for the subsequent production of fuels and chemicals. For fossil-based materials, pyrolysis oil can provide circular building blocks. Furthermore, direct use of pyrolysis oil in gas turbines has also been proven feasible. However, a relatively high oxygen content in raw biomass has detrimental effects on the quality of such oil. This work proposes hydrothermal carbonization as a valorization technique, beneficial from the point of view of subsequent fast pyrolysis. Within the scope of this work, the influence of the parameters of hydrothermal carbonization (HTC) on the kinetics of fast pyrolysis of agricultural biomass (miskanthus), as well as the influence of in situ use of a CaO catalyst, is investigated. Kinetics is investigated using a novel type of thermogravimetric analyzer (TGA) called Cyclonic TGA, which is able to achieve heating rates similar to a real fast pyrolysis process. Moreover, the influence of HTC on the removal of part of its inorganic constituents is determined within the scope of this work.

New regulations aimed at curbing the problem of eutrophication introduce limitations for traditional ways to use the by-product of anaerobic digestion—the digestate. Hydrothermal carbonisation (HTC) can be a viable way to valorise the digestate in an energy-efficient manner and at the same time maximise the synergy in terms of recovery of water, nutrients, followed by more efficient use of the remaining carbon. Additionally, hydrothermal treatment is a feasible way to recirculate recalcitrant process residues. Recirculation to anaerobic digestion enables recovery of a significant part of chemical energy lost in HTC by organics dissolved in the liquid effluent. Recirculating back to the HTC process can enhance nutrient recovery by making process water more acidic. However, such an effect of synergy can be exploited to its full extent only when viable separation techniques are applied to separate organic by-products of HTC and water. The results presented in this study show that using cascade membrane systems (microfiltration (MF) → ultrafiltration (UF) → nanofiltration (NF)), using polymeric membranes, can facilitate such separation. The best results were obtained by conducting sequential treatment of the liquid by-product of HTC in the following membrane sequence: MF 0.2 µm → UF PES 10 → NF NPO30P, which allowed reaching COD removal efficiency of almost 60%.