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Ammonia stress changes microbial metabolism of anaerobic digestion and decreases methane yield, where proton pump overactivated by free ammonia suggested to be the centre of the metabolism changes in anaerobic digestion under ammonia stress. The work demonstrated that proton pump inhibitor (PPI) could alleviate the overactivated proton pump and mitigate ammonia inhibition. Its impacts on iron transporter, substrates uptake, and energy conservation were investigated in anaerobic digestion treating food and kitchen waste. The PPI formed a stimuli-responsive drug delivery system driven by pH for the more inhibited microbe (p < 0.01), confirmed by FE-SEM/EDS and high throughput sequencing, implying the PPI was activated at inhibited microbe more than mixed liquor. Consistent microbial population increase observed in syntrophs and methanogens, who utilized the substrates for high yielding pathway and facilitated the energy sharing by direct interspecies electron transfer. These results demonstrated PPI could recovery methane production and could mitigate fatty-acid accumulation under high ammonia stress by delivery and activation in acetoclastic methanogen., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The proton pump is a convincing mechanism for ammonia inhibition in anaerobic digestion, which explained how the ammonia accumulated intercellularly due to diffusion of free ammonia. Proton pump inhibitor (PPI) was dosed for mitigating the accumulation in anaerobic digestion with ammonia stress, with respect to kinetics. Results show PPI inhibited β-oxidation of fatty acids by targeting ATPase in anaerobic digestion with ammonia stress. Alternatively, PPI stimulated syntrophic acetate oxidization. Random forest located key genera as syntrophic consortia. Methane increased 18.72 ± 7.39% with 20 mg/L PPI at the first peak, consistent with microbial results. The deterministic Gompertz kinetics and stochastic Gaussian processes contributed 97.63 ± 8.93% and 2.37 ± 8.93% in accumulated methane production, respectively. Thus, the use of PPI for anaerobic digestion allowed mitigate ammonia inhibition based on the mechanism of proton pump, facilitate intercellularly ammonia accumulation, stimulate syntrophic consortia, and eliminate uncertainty of process failure, which resulted in efficient methane production under ammonia stress., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Relieving from ammonia inhibition and enhancing the utilization of thermodynamically unfavorable propionate are crucial for methane harvest in the high solid anaerobic digestion (HSAD) of ammonia-rich swine manure. In this study, the potential of dosing zero-valent iron (ZVI, 150 um) for enhancing the methanogenesis to resist total ammonia (TAN) over 5.0 g-N·L -1 was investigated via batch experiments under mesophilic condition. The cumulative methane production was enhanced by 22.2% at ≥160 mM ZVI dosage and the HSAD duration was further shortened by 50.6% at ≥320 mM ZVI dosage. The enhanced methanogenesis was mainly resulted from the full utilization of propionate and the accelerated collapse of posterior-biodegradable organics which might be driven by ZVI. Results of microbial community and qPCR (mcrA) showed that ZVI might trigger the blooming of Methanosarcina (from 27.9% to 78.3%) and Syntrophomonas (0.5% to 3.7%) and attribute to their possible direct interspecies electron transfer (DIET) to enhance propionate utilization. Besides, the main methanogenesis might remain in the effective aceticlastic pathway even under free ammonia (FAN) almost 1.0 g-N·L -1 because syntrophic acetate oxidizing bacteria (SAOB) decreased to almost none at 320 mM ZVI dosage. Dosing ZVI could relieve HSAD from TAN inhibition and more dosage was required to resist FAN inhibition., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Ammonia inhibition is a prominent problem for anaerobic digestion (AD) of nitrogen-rich organic wastes. This study evaluated the effect of zero valent iron (ZVI) and its hybrid with activated carbon (AC), graphite and Fe-C material on the mitigation of ammonia inhibition under ammonia concentration over 5 g/L, according to the batch mode experiments. Results showed that ZVI (4 g/L) and its hybrid with carbon-based material preserving methane production from ammonia inhibition, with kinetics of shortening lag phase from 4.77 d to 2.62 d or even below 2 d with carbon-based material. ZVI preserved methane production with the enrichment of Methanosarcina (the relative abundance was over 80%), which was mostly derived from the activating hydrogenotrophic methanogenesis through the enhanced DIET but not the changes of ORP and FAN., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Graphical abstract Highlights • Higher average pH seized over 6% more CO 2 in stage-II. • Most CO 2 ·H 2 O converts to HCO 3 − and CO 3 2− when pH > 7.9. • Lack of CO 2 ·H 2 O limited the hydrogenotrophic methanogenesis. • Ammonia is not only an inhibitor but also a pH buffer capacity contributor. • Endogenous VFAs regulated pH cycle in stage-II. Abstract Biochemical biogas upgrading retaining more CO 2 from biogas to form biomethane opens new avenues for sustainable biofuel production. For developing this technology, maintaining sustain pH for CO 2 ·H 2 O is fundamental. This study proposes an innovative control strategy for in-situ biogas upgrading retaining and converting the CO 2 in the biogas into CH 4 , via hydrogenotrophic methanogenesis without external agent. The Biogas-pH strategy limited pH drop over 7.4 by stop feeding and maintained the methanogenesis activity by biogas flow rate over 98 ml·h−1. Low pH (7.37–7.80) decrease CO 2 ·H 2 O as a substrate in stage-I, higher pH (7.40–8.41) enhances CO 2 to CO 2 ·H 2 O transfer by 6.29 ± 2.20% in stage-II. Because of that 95% CO 2 ·H 2 O converts to HCO 3 − and CO 3 2− when pH > 7.9, higher pH > 7.9 did not further upgrading the biogas. The carbonate buffer system shown open and close system characteristics in gas and liquid phase. The biogas CH 4 was upgraded from 67.27 ± 5.21% to 73.56 ± 5.01%. [ABSTRACT FROM AUTHOR]

Buffering is requisite for anaerobic digestion (AD) to maintain suitable pH for methane production. In this study, mesophilic AD of swine manure at total solid (TS) from 4% to 14% was carried out to investigate the formation mechanism of buffer system. Results indicated that the rapid ammonia release (3.5–6.6 g-N L −1 ) greatly neutralized the accumulated volatile fatty acids (VFAs) (21.4–32.5 g L −1 ) to prevent pH from lower than 6.4. The released ammonia and accumulated VFAs established the initial stage of buffer system, ensuring suitable acid-base environment for methanogens to acclimate to high ammonia concentration at TS ≥8%. With the consumption of VFAs by acclimated methanogens, more CO 2 in biogas was seized and transformed into carbonates to remedy the decreased VFAs for buffering free ammonia from overmuch pH increase. The improved methane production of 303.1 ± 23.0 N mL g-VS added −1 was obtained at 8% TS under the convoy of ternary buffer system. Meanwhile, the ternary pH buffer system with ammonia-carbonates-VFAs that transformed obeying ionic equilibrium was visually revealed by a simulated ternary contour during overall AD process. Such understanding of buffer system may contribute to develop more effective pH adjustment measures to counter ammonia inhibition on methane production. [ABSTRACT FROM AUTHOR]