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 mini-mize 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.