Digestate Recirculation in Agricultural Biogas Plants: Effects on Process Stability, Methane Production, Water Demand, and Inhibitor Accumulation

Digestate recirculation is increasingly considered a strategy for improving resource efficiency in agricultural biogas plants by reducing process-water demand, retaining active biomass, and enhancing the internal reuse of nutrients and residual organic matter. This review critically examines the effects of digestate recirculation on anaerobic digestion stability, methane production, water balance, inhibitor accumulation, and microbial community dynamics. Previous reviews have largely addressed digestate management, nutrient recovery, fertilizer use, and broader anaerobic digestion optimization, while digestate recirculation itself has received less attention as an operational process-management strategy in agricultural biogas plants. This review addresses this gap by integrating technological, physicochemical, microbiological, and operational evidence and by emphasizing that the effects of recirculation are system-specific rather than governed by a single universal optimum recirculation ratio. Particular attention is given to the influence of recirculation ratio, digestate fraction, solids content, reactor configuration, and feedstock characteristics. Available studies show that recirculation can increase volumetric methane production, support buffering capacity, improve microbial retention, and reduce the need for external dilution water. However, the response is generally non-linear, and excessive recirculation may promote the accumulation of total ammonia nitrogen, volatile fatty acids, soluble salts, sulphides, recalcitrant compounds, and suspended solids. These changes can increase viscosity, impair mass transfer and mixing, and shift methanogenic pathways toward more stress-tolerant microbial consortia. Effective implementation requires integrated control of chemical, physical, and microbiological parameters. Digestate recirculation should therefore be treated as a flexible process-management tool whose benefits depend on system-specific operating conditions and long-term mass-balance control. Future research should define robust monitoring indicators for full-scale operation.