Advancing anaerobic digestion through two-stage processes: Current developments and future trends

Two-stage biogas production is reported to overcome the drawbacks of productivity in anaerobic digestion (AD). Recent publications indicate an increase in methane yield between 10 and 30% via two-stage AD. However, the industrial acceptance is minimal due to their reliability and operational issues. This paper critically reviews the two-stage AD for biogas production. Some of the research gaps identified in two-stage AD include lack of techno-economic analysis to show the industry about the feasibility of this process. There is a clear trade-off between the increase in the methane yield vs. the cost it takes to build the second digester. Practically, building a second digester is not economically feasible due to economies of scale. Other technical challenges include the recirculation leads to ammonia accumulation in the system, and disturbance in syntrophic relationships of microbes between the two-stages. Techno-economic analysis suggests that two stage AD could be about 3% expensive than a single stage AD. Further detailed analysis is required to show clear evidence about the economics and feasibility of two stage AD. The parasitic energy demand of the two-stage system will be higher than a single stage AD due to the reason that two reactors are involved for mixing or maintaining temperature. Most of the two-stage AD, operates at a different temperature and hence the energy demand will be different for different reactors. Some of the problem in the literature includes assessing the stage wise OLR, HRT data, and TS/VS balance before and after the process. To address these issues, further work is necessary to standardize the way two-stage experiments are carried out including the parameters that are necessary to be measured for reproducibility.