Effects of acid-acclimated inoculum on solid-state anaerobic digestion of spent mushroom substrate

Solid-state anaerobic digestion (SS-AD) generally operates at solid content between 20%-40%. Due to the low water content, mass transfer in the digestion material is slow and the intermediate acids are apt to accumulate locally. Excessive acid accumulation might inhibit the activity of methanogens, reduce bigas yield, and even result in failure of the SS-AD process. Acetic acid is the most important intermediate in anaerobic digestion, and its concentration directly affects the activity of methanogens. Spent mushroom substrate (SMS) is a feasible feedstock for anaerobic digestion, because it has high organic content and the fiber and lignin have been greatly degraded during mushroom growth. However, using spent mushroom substrate as feedstock for SS-AD is easy to result in acid inhibition due to the rapid hydrolysis and acidification of the organic matters. In order to avoid acid inhibition and improve the stability of SS-AD process, the inoculum was acclimated by gradually increasing the concentration of acetic acid in this study. The acclimation process was divided into four stages at the pH value of 7.2, 6.7, 6.2 and 6.0, respectively. During acclimation stage, the pH value of the inoculum was adapt to the set value with 6 mol/L acetic acid solution and the biogas production, methane content, additive ammout of acetic acid and conversion of actic acid were measured every day. The activities of dehydrogenase, CMCase and Co F420 were measured at the last day of each acclimation stage. The acclimation lasted for 40 days and a acid-acclimated inoculum that can prouduce biogas quickly under the condition of acetic acid concentration of 10 200 mg/kg and pH value 6.0 was obtained. Compared to the original inoculum, the diversity of methanogens in the acid-acclimated inoculum decreased and the abundance of acetic acid diauxotrophic Methanosarcina was obviously enriched. The activity of dehydrogenase increased 101.5% in the first acclimation stage (pH value 7.2) and then kept decreasing during the next 3 stages. The activity of dehydrogenase reflects the metabolic capacity of all the microorganisms in the digestion system. With the decrease of pH value during the acclimation, the activity of some microorganisms was inhibited. Coenzyme F420 is present in various methanogens. It often be used to reflect the methane production activity of sludge. The activty of coenzyme F420 also reached the highest in the first acclimation stage because the pH is optimum for most methanogenes. Although the abundance of the acetic acid diauxotrophic Methanosarcina was significantly enriched in the final acid-acclimated inoculum, the diversity of the archaea communities was reduced due to the low pH value. The activity of cellulase in the acclimation process reached the highest in the third stage (pH value 6.7). To investigate the effects of acid-acclimated inoculum, SS-AD experiments of SMS were carried out by adding different proportion of acid-acclimated inoculum. The results showed that adding acid-acclimated inoculum could avoid acid inhibition and accelerate the start-up of SS-AD. The methane yield increased 56.1% when acid-tolerant inoculum accounted to 75%. The results can provide some theoretical guidance for effectively solving acid inhibition in SS-AD process.