Inhibition of metabolic intermediates and enhancement of methane production in dry anaerobic digestion
Wang, Zhongzhong
Wang, Zhongzhong
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Publication Date
2022-07-15
Type
Thesis
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Abstract
Dry anaerobic digestion (AD) is advantageous over wet AD in treating high-solid organic wastes (e.g., livestock and food wastes). However, it often suffers from low methane production and metabolic intermediates inhibition (such as ammonia) due to the high solids content. In addition, there are still considerable knowledge gaps concerning the differential ammonia tolerance of acetoclastic and hydrogenotrophic methanogens (AMs and HMs). To evaluate the effects of the total solids content on the performance of co-AD of pig manure (PM) and food waste (FW), investigate the impact of different ammonia levels on methanogenesis, and enhance the methane production of dry AD of PM and FW, experiments were carried out in laboratory-scale digesters under mesophilic conditions. The results showed the specific methane yield (SMY) had no significant difference with the increase of total solids (TS) contents from 5% to 15% (278.8-291.7 NmL/g VSadded), while it was reduced at a 20% TS content (259.8 NmL/g VSadded). The analysis on the microbial community structure clearly showed that in dry AD (20%TS), there was a general shifting from the acetoclastic pathway to the mixotrophic pathway and the hydrogenotrophic pathway. The tolerance level of HMs to free ammonia (FAN, IC50=1345 mg N/L) and NH4+ (IC50=6050 mg N/L) was nearly 11 times and 3 times that of AMs (NH3, IC50=123 mg N/L; NH4+, IC50=2133 mg N/L), respectively. The HMs were more adversely affected by NH4+ when the pH was ≤8.0. A low TAN (1.0-4.0 g N/L) could cause irreversible inhibition of the AMs due to significant cell death, whereas the activity of HMs could be fully or even over recovered from severe ammonia stress (FAN≤ 0.9 g N/L or TAN≤10 g N/L; pH ≤8.0). The addition of biochars can mildly elevate the SMY in dry AD of PM and FW under mesophilic conditions by 7.9%, 9.4% and 12.0% for bamboo, rice husk and pecan shell derived biochar additions, respectively. Enhancing electron transfer might play an important part in dry AD process. These results in this research could contribute to an in-depth understanding of wet and dry AD and distinguishing responses of acetoclastic and hydrogenotrophic methanogens to ammonia exposure. Besides, these findings provide a basis for developing tailored operating strategies to reduce ammonia inhibition and offer an effective approach to improve the dry AD performance in practice.
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NUI Galway