Anaerobic digestion (AD) processes can face operational challenges or flaws such as substrate structure and characteristics complexity, process complexity, low productivity, inefficient biodegradability, and poor stability, which suppresses or reduces biogas and biomethane production. As a result of the need to overcome these challenges/shortcomings and improve or enhance biogas and biomethane yield, process intensification methods have gained attention. There is some literature review on pretreatment and co-digestion as a means of improving AD performance; however, there is no systematic information on the various strategies required for improving AD performance and, in turn, increasing biogas/biomethane yield. The AD process produces biogas, a valuable renewable biofuel. Biogas is composed primarily of biomethane and other undesirable components such as carbon dioxide, oxygen, hydrogen sulphide, water vapour, ammonia, siloxanes, nitrogen, hydrocarbons, and carbon monoxide, which act as impurities or contaminants and tend to reduce the biogas specific calorific value while also causing various problems with machine operation. As a result, various technologies are used to improve raw biogas quality by removing contaminants during biogas transformation to biomethane. As a result, this paper provides a comprehensive review of the various systematic process intensification strategies used to overcome AD process challenges/shortfalls, improve or enhance biogas and biomethane production, and conventional and emerging or advanced technologies for biogas purification, cleaning, and upgrading.

The continual generation and discharge of waste are currently considered two of the main environmental problems worldwide. There are several waste management options that can be applied, though anaerobic digestion (AD) process technology seems to be one of the best, most reliable, and feasible technological options that have attracted remarkable attention due to its benefits, including the generation of renewable energy in the form of biogas and biomethane. There is a large amount of literature available on AD; however, with the continuous, progressive, and innovative technological development and implementation, as well as the inclusion of increasingly complex systems, it is necessary to update current knowledge on AD process technologies, process variables and their role on AD performance, and the kinetic models that are most commonly used to describe the process-reaction kinetics. This paper, therefore, reviewed the AD process technologies for treating or processing organic biomass waste with regard to its classification, the mechanisms involved in the process, process variables that affect the performance, and the process kinetics. Gazing into the future, research studies on reduced MS-AD operational cost, integrated or hybrid AD-biorefinery technology, integrated or hybrid AD-thermochemical process, novel thermochemical reactor development, nutrient recovery from integrated AD-thermochemical process, and solid and liquid residual disposal techniques are more likely to receive increased attention for AD process technology of biomass wastes.

The brewery industry generates a huge quantity of brewers’ spent grain (BSG) which can pose waste disposal and pollution problems. Anaerobic digestion of BSG, a recalcitrant lignocellulosic waste, is slow but can be enhanced by bioaugmentation, biostimulation and co-digestion to obtain a higher biogas yield. Hence, the effect of inoculum from brewery wastewater sludge (BWWS), iron (III) chloride (FeCl₃) and co-digestion with poultry manure (PM) on the production of biogas from BSG was investigated. Cumulative biogas and biomethane yields of 588.19 NL/kgVS and 400.34 NL_CH4/kgVS, respectively, were obtained from a slurry consisting of a blend of 60% BSG and 40% PM plus 15 mg L^-1 FeCl₃ in BWWS, after 30 day retention time at 37 ^oC. However, mono-digestion of BSG in only water yielded 402.17 NL_biogas/kgVS and 262.86 NL_CH4/kgVS. The synergistic effects of inoculum from BWWS, FeCl₃ and poultry manure on anaerobic digestion of BSG resulted in 46% and 52% increases in biogas and methane yields, respectively, compared with BSG mono-digestion. The biogas and biomethane production kinetics were well described by the dual pooled first order, logistic and modified Gompertz models.

The stalk of banana is an abundant biomass that can be sustainably converted into bioenergy, biofuels, biosorbents, animal feeds and fibers. The moisture content of freshly harvested banana stalk is high, so drying is essential before its storage and prior to some of the conversion processes. Hence, solar and open sun drying characteristics of banana stalk chips were investigated. Untreated (5, 10 and 15 mm thick) and pretreated (hot water, salt water and sulphite) banana stalk chips were dehydrated in a solar dryer and directly in sunlight. The data were fitted to twelve drying models. The moisture diffusivities were also evaluated. Banana stalk dried mainly in the falling-rate phase and the rate of drying increased with decreasing chip thickness. However, the chips dried slower in direct sunlight compared to the solar dryer. The pretreatments significantly (P < 0.05) enhanced the rate of dehydration of the chips in the solar dryer. Diffusivities for the solar and direct sunlight drying of banana stalk chips were 1.28 × 10^–9–5.32 × 10^–9 m² s⁻¹ and 1.08 × 10^–9–3.65 × 10^–9 m² s⁻¹, respectively. The Weibull model aptly described the solar drying of the chips while the Midilli-Kucuk and Weibull models most properly depicted the open sun dehydration of untreated and pretreated chips, respectively. Solar energy may be more efficiently utilized for drying banana stalk chips biomass by using a solar dryer as well as a very thin layer of chips and by pretreating the biomass prior to the drying operation. The effective use of solar energy for drying banana stalk biomass can reduce its processing cost and enhance the sustainability of biomass utilization.