Thermodynamic method for analyzing and optimizing pretreatment/anaerobic digestion systems

This paper builds a quantitative thermodynamic model for the microbial hydrolysis process (MHP, which uses Caldicellulosiruptor bescii at 75°C for pre-digestion) for producing biogas from a 5-10% aqueous suspension of dairy manure (naturally buffered near pH 7.8 by ammonium bicarbonate) by anaerobic digestion with a mix of acetoclastic and syntrophic methanogenesis. Standard Gibbs energy changes were calculated for the major reactions in pre-digestion, for reactions producing H2, acetate, and CO2 in the digester, and for methanogenesis reactions in the digester. The available data limit the study to analyzing reactions in the digester to reactions of short-chain volatile fatty acids anions. Results are presented as curves of ΔrxnG (Gibbs energy change) vs. acetate concentration. The H2(aq) concentration must be above 1.2×10-9 M to get significant syntrophic methanogenesis, i.e., for ΔrxnG to be negative. The results show syntrophic methanogenesis of propionate, butyrate, and valerate slows as acetate concentration increases because hydrogen production also decreases, and consequently, biogas production from syntrophic methanogenesis slows as acetate increases. Bicarbonate also inhibits both acetoclastic and syntrophic methanogenesis but is necessary to prevent acidification (souring) of the digester. At identical steady-state conditions, acetoclastic methanogenesis runs about 1.4 times faster than syntrophic methanogenesis. Because syntrophic methanogenesis produces acetate catabolized by acetoclastic methanogens, both types of methanogens are necessary to maximize biogas production. The culture in the digester is predicted to evolve to optimize the ratio of acetoclastic methanogens to syntrophic methanogens, a condition signaled by a constant, low acetate concentration in the digester effluent. Obtaining volatile solids reduction as high as 75% with MHP requires a feedstock with less than 25% lignin and a culture of acetoclastic methanogens and syntrophic methanogens and their symbiotic bacteria

Cellular adhesiveness and cellulolytic capacity in Anaerolineae revealed by omics-based genome interpretation

The Anaerolineae lineage of Chloroflexi had been identified as one of the core microbial populations in anaerobic digesters; however, the ecological role of the Anaerolineae remains uncertain due to the scarcity of isolates and annotated genome sequences. Our previous metatranscriptional analysis revealed this prevalent population that showed minimum involvement in the main pathways of cellulose hydrolysis and subsequent methanogenesis in the thermophilic cellulose fermentative consortium (TCF).published_or_final_versio

Enhancing methane production from food waste fermentate using biochar. The added value of electrochemical testing in pre-selecting the most effective type of biochar

Background: Recent studies have suggested that addition of electrically conductive biochar particles is an effective strategy to improve the methanogenic conversion of waste organic substrates, by promoting syntrophic associations between acetogenic and methanogenic organisms based on interspecies electron transfer processes. However, the underlying fundamentals of the process are still largely speculative and, therefore, a priori identification, screening, and even design of suitable biochar materials for a given biotechnological process are not yet possible. Results: Here, three charcoal-like products (i.e., biochars) obtained from the pyrolysis of different lignocellulosic materials, (i.e., wheat bran pellets, coppiced woodlands, and orchard pruning) were tested for their capacity to enhance methane production from a food waste fermentate. In all biochar-supplemented (25 g/L) batch experiments, the complete methanogenic conversion of fermentate volatile fatty acids proceeded at a rate that was up to 5 times higher than that observed in the unamended (or sand-supplemented) controls. Fluorescent in situ hybridization analysis coupled with confocal laser scanning microscopy revealed an intimate association between archaea and bacteria around the biochar particles and provided a clear indication that biochar also shaped the composition of the microbial consortium. Based on the application of a suite of physico-chemical and electrochemical characterization techniques, we demonstrated that the positive effect of biochar is directly related to the electron-donating capacity (EDC) of the material, but is independent of its bulk electrical conductivity and specific surface area. The latter properties were all previously hypothesized to play a major role in the biochar-mediated interspecies electron transfer process in methanogenic consortia. Conclusions: Collectively, these results of this study suggest that for biochar addition in anaerobic digester operation, the screening and identification of the most suitable biochar material should be based on EDC determination, via simple electrochemical tests. © 2017 The Author(s)

Effect of proton pump inhibitor on microbial community, function, and kinetics in anaerobic digestion with ammonia stress

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 beta-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

Role and Potential of Direct Interspecies Electron Transfer in Anaerobic Digestion

Anaerobic digestion (AD) is an effective biological treatment for stabilizing organic compounds in waste/wastewater and in simultaneously producing biogas. However, it is often limited by the slow reaction rates of different microorganisms' syntrophic biological metabolisms. Stable and fast interspecies electron transfer (IET) between volatile fatty acid-oxidizing bacteria and hydrogenotrophic methanogens is crucial for efficient methanogenesis. In this syntrophic interaction, electrons are exchanged via redox mediators such as hydrogen and formate. Recently, direct IET (DIET) has been revealed as an important IET route for AD. Microorganisms undergoing DIET form interspecies electrical connections via membrane-associated cytochromes and conductive pili; thus, redox mediators are not required for electron exchange. This indicates that DIET is more thermodynamically favorable than indirect IET. Recent studies have shown that conductive materials (e.g., iron oxides, activated carbon, biochar, and carbon fibers) can mediate direct electrical connections for DIET. Microorganisms attach to conductive materials' surfaces or vice versa according to particle size, and form conductive biofilms or aggregates. Different conductive materials promote DIET and improve AD performance in digesters treating different feedstocks, potentially suggesting a new approach to enhancing AD performance. This review discusses the role and potential of DIET in methanogenic systems, especially with conductive materials for promoting DIET

Field-Scale Observations of a Transient Geobattery Resulting from Natural Attenuation of a Crude Oil Spill

We present evidence of a geobattery associated with microbial degradation of a mature crude oil spill. Self-potential measurements were collected using a vertical array of nonpolarizing electrodes, starting at the land surface and passing through the smear zone where seasonal water table fluctuations have resulted in the coating of hydrocarbons on the aquifer solids. These passive electrical potential measurements exhibit a dipolar pattern associated with a current source. The anodic and cathodic reactions of this natural battery occur below and above the smear zone, respectively. The smear zone is characterized by high magnetic susceptibility values associated with the precipitation of semiconductive magnetic iron phase minerals as a by-product of biodegradation, facilitating electron transfer between the anode and the cathode. This geobattery response appears to have a transient nature, changing on a monthly scale, probably resulting from chemical and physical changes in subsurface conditions such as water table fluctuations

Inhibition of bioactivity of UASB biogranules by electroplating metals

Among the high-rate anaerobic wastewater treatment processes, the upflow anaerobic sludge blanket (UASB) reactor has attracted most commercial and research interests. In this study, the toxicity of five heavy metals commonly found in the electroplating effluent on the activity of anaerobic biogranules was investigated. Biogranules were sampled from UASB reactors treating four types of wastewaters containing, individually, acetate, propionate, starch and benzoate as the sole substrate. All reactors were operated at 37T for over six months at a loading rate of 10 g C0D-ld-l. The methanogenic activity of biogranules treating each type of wastewater decreased with the increase of metal concentration. The toxicity of each metal to the biogranules was measured by the reduction of methanogenic activity of biogranules in serum vials, as compared to the controls. Results show that, in general, acetate- and propionate-degrading biogranules exhibited higher resistance to electroplating metals than the starch- and benzoate-degrading ones. Acetate conversion is a one-step process conducted by acetotrophic methanogens alone. Propionate degradation, on the other hand, is a two-step process conducted by acetogens and methanogens; whereas, benzoate and starch degradations are multi-step processes conducted jointly by fermentative/acidogenic bacteria, acetogens and methanogens. Results of this study imply that heavy metals from electroplating effluent, in general, inhibit the bioactivities of fermentative/acidogenic bacteria more than those of methanogens and acetogens. For the latter two groups of bacteria, the toxicity of the heavy metals were in the following descending order: zinc > nickel > copper > cadmium > chromium.published_or_final_versio