A review of process parameters influence in solid-state anaerobic digestion: Focus on performance stability thresholds

Process parameters such as pH, ammonia, and volatile fatty acids are physico-chemical indicators of stress level, reactor performance, and methane yield in anaerobic digestion. Unlike liquid-state anaerobic digestion, there are no established and specified thresholds of these parameters to ensure balance among anaerobic microbes and effective reactor performance in solid-state anaerobic digestion (SSAD). Thus, this review examines SSAD literature from the past twenty-one years and draws out process parameter thresholds that apply to SSAD. It further discusses the strength and limitations of these parameters in actualizing this task. Summarily, to minimize acid inhibition in the SSAD reactor, avoid VFA/Alk >1.5. This is in addition to maintaining a pH within 7.5–8.3 to optimize methanogens activities in SSAD and avoiding ammonia concentration >9.0 g/L which could completely disrupt the archaea activities. In addition, SSAD with an initial C/N ratio between 10 and 29 is likely to avoid inhibition and be successful.