Relating MEC population dynamics to anode performance from DGGE and electrical data

Impact factors and novel strategies for improving biohydrogen production in microbial electrolysis cells

Microbial electrolysis cell (MEC) system is an environmentally friendly method for clean biohydrogen production from a wide range of biowastes owing to low greenhouse gas emissions. This approach has relatively higher yields and lower energy costs for biohydrogen production compared to conventional biological technologies and direct water electrolysis, respectively. However, biohydrogen production efficiency and operating costs of MEC still need further optimization to realize its large-scale application.This paper provides a unique review of impact factors influencing biohydrogen production in MECs, such as microorganisms and electrodes. Novel strategies, including inhibition of methanogens, development of novel cathode catalyst, advanced reactor design and integrated systems, to enhance low-cost biohydrogen production, are discussed based on recent publications in terms of their opportunities, bottlenecks and future directions. In addition, the current challenges, and effective future perspectives towards the practical application of MECs are described in this review.

Growth-associated catabolic potential of Acetoanaerobium sticklandii DSM 519 on gelatin and amino acids

Acetoanaerobium sticklandii DSM 519 is a hyperammonia-producing anaerobe that catabolizes proteins and amino acids into organic solvents and volatile acids via the Stickland reactions. However, the specific growth rate and metabolic capability of this organism on proteins and amino acids are not yet known. Therefore, the present study was intended to evaluate its specific growth rate and metabolic potential on gelatin and amino acids in the experimental media. We carried out metabolic assay experiments to calculate its ability to utilize pure gelatin, single amino acids, and amino acid pairs at different growth phases. The results of this study show that complete assimilation of gelatin was achieved by its log-phase culture. The subsequent fermentation of amino acids was much faster than gelatin hydrolysis. The rate of gelatin degradation was associated with the growth and catabolic rates of this organism. Many amino acids were not assimilated completely for its growth and energy conservation. A log-phase culture of this organism preferably utilized l-cysteine, l-arginine, and l-serine, and released more fraction of ammonia. As shown by our analysis, the catabolic rates of these amino acids were determined by the rates of respective enzymes involved in amino acid catabolic pathways and feedback repression of ammonia. The growth kinetic data indicated that at the initial growth stage, a metabolic shift in its solventogenesis and acidogenesis phases was associated with catabolism of certain amino acids. Thus, the results of this study provide a new insight to exploit its log-phase culture as a starter for the production of biofuel components from gelatin processing industries.

Rapid degradation of 2,4-dichloronitrobenzene in single-chamber microbial electrolysis cell with pre-acclimated bioanode: A comprehensive assessment

2,4-dichloronitrobenzene (DClNB) as a typical refractory pollutant, exists in multifarious industrial wastewater widely and poses a serious threat to the environment. An ion exchange membrane (IEM)-free microbial electrolysis cell (MEC) with pre-acclimated bioanode was built and evaluated systematically for treatment of DClNB containing wastewater. Results showed that compared with the non-acclimated or IEM-equipped MECs, the pre-acclimated IEM-free MECs had the best DClNB removal efficiency of 91.3% under COD and DClNB loading rates of nearly 1000 kg m^-3 d^-1 and 100 g m^-3 d^-1. Both of anode pre-acclimation and IEM removal reduced the electron transfer resistance by 71.1 and 194.5 Ω, respectively. Compared to the pre-acclimated IEM-equipped MEC, the cathode current efficiency of pre-acclimated IEM-free MEC increased by 13.7%. Analysis of live/dead cell staining indicated that a higher proportion of live cells was observed in the acclimated anode biofilm (66.1% vs. 47.3%), and the detoxification of DClNB in the pre-acclimated IEM-free MECs was significantly better (p < 0.05) than those of non-acclimated or IEM-equipped MECs. This study contributes to the performance improvement of the MEC process for treatment of toxic industrial wastewater.

A metabolic study to decipher amino acid catabolism-directed biofuel synthesis in Acetoanaerobium sticklandii DSM 519

Acetoanaerobium sticklandii DSM 519 is a hyper-ammonia-producing anaerobe. It has the ability to produce organic solvents and acids from protein catabolism through Stickland reactions and specialized pathways. Nevertheless, its protein catabolism-directed biofuel production has not yet been understood. The present study aimed to decipher such growth-associated metabolic potential of this organism at different growth phases using metabolic profiling. A seed culture of this organism was grown separately in metabolic assay media supplemented with gelatin and or a mixture of amino acids. The extracellular metabolites produced by this organism were qualitatively analyzed by gas chromatography-mass spectrometry platform. The residual amino acids after protein degradation and amino acids assimilation were identified and quantitatively measured by high-performance liquid chromatography (HPLC). Organic solvents and acids produced by this organism were detected and the quantity of them determined with HPLC. Metabolic profiling data confirmed the presence of amino acid catabolic products including tyramine, cadaverine, methylamine, and putrescine in fermented broth. It also found products including short-chain fatty acids and organic solvents of the Stickland reactions. It reported that amino acids were more appropriate for its growth yield compared to gelatin. Results of quantitative analysis of amino acids indicated that many amino acids either from gelatin or amino acid mixture were catabolised at a log-growth phase. Glycine and proline were poorly consumed in all growth phases. This study revealed that apart from Stickland reactions, a specialized system was established in A. sticklandii for protein catabolism-directed biofuel production. Acetone-butanol-ethanol (ABE), acetic acid, and butyric acid were the most important biofuel components produced by this organism. The production of these components was achieved much more on gelatin than amino acids. Thus, A. sticklandii is suggested herein as a potential organism to produce butyric acid along with ABE from protein-based wastes (gelatin) in bio-energy sectors.

Methane oxidation coupled to perchlorate reduction in a membrane biofilm batch reactor

A specially designed CH₄-based membrane biofilm batch reactor (MBBR) was applied to investigate anaerobic methane oxidation coupled to perchlorate reduction (AnMO-PR). The 0.21 mM ClO₄^- added in the first stage of operation was completely reduced in 28 days, 0.40 mM ClO₄^- was reduced within 23 days in stage 2, and 0.56 mM of ClO₄^- was reduced within 30 days in stage 3. Although some chlorate (ClO₃^-) accumulated, the recovery of Cl^- was over 92%. Illumina sequencing of the 16S rRNA gene documented that the bacterial community was mainly composed by perchlorate-reducing bacteria (PRB), methanotrophic bacteria, and archaea. Real-time quantitative PCR showed the archaeal 16S rRNA and mcrA genes increased as more ClO₄^- was reduced, and the predominant archaea belonged to Methanosarcina mazei, which is related to ANME-3, an archaeon able to perform reverse methanogenesis. Several pieces of evidence support that ClO₄^- reduction by the MBBR biofilm occurred via a synergism between Methanosarcina and PRB: Methanosarcina oxidized methane through reverse methanogesis and provided electron donor for PRB to reduce ClO₄^-. Because methanotrophs were present, we cannot rule out that they also were involved in AnMO-PR if they received O₂ generated by disproportionation of ClO₂^- from the PRB.

(Bio)electrochemical ammonia recovery: progress and perspectives

In recent years, (bio)electrochemical systems (B)ES have emerged as an energy efficient alternative for the recovery of TAN (total ammonia nitrogen, including ammonia and ammonium) from wastewater. In these systems, TAN is removed or concentrated from the wastewater under the influence of an electrical current and transported to the cathode. Subsequently, it can be removed or recovered through stripping, chemisorption, or forward osmosis. A crucial parameter that determines the energy required to recover TAN is the load ratio: the ratio between TAN loading and applied current. For electrochemical TAN recovery, an energy input is required, while in bioelectrochemical recovery, electric energy can be recovered together with TAN. Bioelectrochemical recovery relies on the microbial oxidation of COD for the production of electrons, which drives TAN transport. Here, the state-of-the-art of (bio)electrochemical TAN recovery is described, the performance of (B)ES for TAN recovery is analyzed, the potential of different wastewaters for BES-based TAN recovery is evaluated, the microorganisms found on bioanodes that treat wastewater high in TAN are reported, and the toxic effect of the typical conditions in such systems (e.g., high pH, TAN, and salt concentrations) are described. For future application, toxicity effects for electrochemically active bacteria need better understanding, and the technologies need to be demonstrated on larger scale.

Effects of salinity and COD/N on denitrification and bacterial community in dicyclic-type electrode based biofilm reactor

A dicyclic-type electrode based biofilm electrode reactor (BER) was developed for advanced nitrate removal from saline municipal wastewater. The denitrification efficiency was evaluated with a synthetic feed (NO₃^--N, 20 mg L^-1) under different salinity and COD to nitrogen ratios (COD/N). As the salinity increased from 0% to 1.0%, the denitrification performance of both the traditional biofilm reactor (BR) and BER was inhibited; however, the BER showed better adaptation and ability to recover. The BER achieved a high nitrate removal efficiency (≥90%) at a salinity of 1.0% and a low COD/N of 2.5 (theoretical stoichiometric 2.86 ignoring microbial growth). The abundance of Methylotenera mobilis in BR and Clostridium sticklandii in BER was higher than in the initial sludge sample used as inoculum. Likewise, the abundance of napA, nirS and nosZ genes increased as the COD/N further decreased. Under high salinity stress, the BER had a higher denitrification efficiency and the consumption of the organic carbon source (i.e., methanol) was reduced compared to BR. The cooperation between heterotrophic and autotrophic denitrifiers in the BER system provides a more efficient and feasible solution for nitrate removal from saline municipal wastewater.

Metabolic Reconstruction and Modeling Microbial Electrosynthesis

Microbial electrosynthesis is a renewable energy and chemical production platform that relies on microbial cells to capture electrons from a cathode and fix carbon. Yet despite the promise of this technology, the metabolic capacity of the microbes that inhabit the electrode surface and catalyze electron transfer in these systems remains largely unknown. We assembled thirteen draft genomes from a microbial electrosynthesis system producing primarily acetate from carbon dioxide, and their transcriptional activity was mapped to genomes from cells on the electrode surface and in the supernatant. This allowed us to create a metabolic model of the predominant community members belonging to Acetobacterium, Sulfurospirillum, and Desulfovibrio. According to the model, the Acetobacterium was the primary carbon fixer, and a keystone member of the community. Transcripts of soluble hydrogenases and ferredoxins from Acetobacterium and hydrogenases, formate dehydrogenase, and cytochromes of Desulfovibrio were found in high abundance near the electrode surface. Cytochrome c oxidases of facultative members of the community were highly expressed in the supernatant despite completely sealed reactors and constant flushing with anaerobic gases. These molecular discoveries and metabolic modeling now serve as a foundation for future examination and development of electrosynthetic microbial communities.

Microbiological mechanism of the improved nitrogen and phosphorus removal by embedding microbial fuel cell in Anaerobic-Anoxic-Oxic wastewater treatment process

Anaerobic-Anoxic-Oxic (AA/O) wastewater treatment process is a widely used wastewater treatment process for simultaneous nitrogen and phosphorus removal. Microbial fuel cell (MFC) can generate electricity and treat the organic wastewater simultaneously. Our previous research showed that embedding MFC in AA/O wastewater treatment process could enhance the pollutants removal efficiency. However, the mechanism was not clear. In this study, a lab-scale corridor-style AA/O reactor with MFC embedded was operated and both the total nitrogen and total phosphorus removal efficiencies were enhanced. DGGE and Illumina Miseq results demonstrated that both the microbial community structures on the surface of the cathode and in the suspensions of cathode chamber have been changed. The percentage of Thauera and Emticicia, identified as denitrifying bacteria, increased significantly in the suspension liquid when the MFC was embedded in the AA/O reactor. Moreover, the genus Rheinheimera were significantly enriched on the cathode surface, which might contribute to both the nitrogen removal enhancement and electricity generation.