Effects of acetate, propionate, and butyrate on the thermophilic anaerobic degradation of propionate by methanogenic sludge and defined cultures

Complexity of temperature dependence in methanogenic microbial environments

There is virtually no environmental process that is not dependent on temperature. This includes the microbial processes that result in the production of CH₄, an important greenhouse gas. Microbial CH₄ production is the result of a combination of many different microorganisms and microbial processes, which together achieve the mineralization of organic matter to CO₂ and CH₄. Temperature dependence applies to each individual step and each individual microbe. This review will discuss the different aspects of temperature dependence including temperature affecting the kinetics and thermodynamics of the various microbial processes, affecting the pathways of organic matter degradation and CH₄ production, and affecting the composition of the microbial communities involved. For example, it was found that increasing temperature results in a change of the methanogenic pathway with increasing contribution from mainly acetate to mainly H₂/CO₂ as immediate CH₄ precursor, and with replacement of aceticlastic methanogenic archaea by thermophilic syntrophic acetate-oxidizing bacteria plus thermophilic hydrogenotrophic methanogenic archaea. This shift is consistent with reaction energetics, but it is not obligatory, since high temperature environments exist in which acetate is consumed by thermophilic aceticlastic archaea. Many studies have shown that CH₄ production rates increase with temperature displaying a temperature optimum and a characteristic apparent activation energy (E_a). Interestingly, CH₄ release from defined microbial cultures, from environmental samples and from wetland field sites all show similar E_a values around 100 kJ mol^-1 indicating that CH₄ production rates are limited by the methanogenic archaea rather than by hydrolysis of organic matter. Hence, the final rather than the initial step controls the methanogenic degradation of organic matter, which apparently is rarely in steady state.

Biodegradation of willow sawdust by novel cellulase-producing bacterial consortium from wood-feeding termites for enhancing methane production

This study was designed to develop a cellulase-producing bacterial consortium (CBC) from wood-feeding termites that could effectively degrade willow sawdust (WSD) and consequently enhance methane production. The bacterial strains Shewanella sp. SSA-1557, Bacillus cereus SSA-1558, and Pseudomonas mosselii SSA-1568 exhibited significant cellulolytic activity. Their CBC consortium showed positive effects on cellulose bioconversion, resulting in accelerated WSD degradation. After nine days of pretreatment, the WSD had lost 63%, 50%, and 28% of its cellulose, hemicellulose, and lignin, respectively. The hydrolysis rate of treated WSD (352 mg/g) was much higher than that of untreated WSD (15.2 mg/g). The highest biogas production (66.1 NL/kg VS) with 66% methane was observed in the anaerobic digester M-2, which contained a combination of pretreated WSD and cattle dung in a 50/50 ratio. The findings will enrich knowledge for the development of cellulolytic bacterial consortia from termite guts for biological wood pretreatment in lignocellulosic anaerobic digestion biorefineries.

Syntrophic propionate-oxidizing bacteria in methanogenic systems

The mutual nutritional cooperation underpinning syntrophic propionate degradation provides a scant amount of energy for the microorganisms involved, so propionate degradation often acts as a bottleneck in methanogenic systems. Understanding the ecology, physiology and metabolic capacities of syntrophic propionate-oxidizing bacteria (SPOB) is of interest in both engineered and natural ecosystems, as it offers prospects to guide further development of technologies for biogas production and biomass-derived chemicals, and is important in forecasting contributions by biogenic methane emissions to climate change. SPOB are distributed across different phyla. They can exhibit broad metabolic capabilities in addition to syntrophy (e.g. fermentative, sulfidogenic and acetogenic metabolism) and demonstrate variations in interplay with cooperating partners, indicating nuances in their syntrophic lifestyle. In this review, we discuss distinctions in gene repertoire and organization for the methylmalonyl-CoA pathway, hydrogenases and formate dehydrogenases, and emerging facets of (formate/hydrogen/direct) electron transfer mechanisms. We also use information from cultivations, thermodynamic calculations and omic analyses as the basis for identifying environmental conditions governing propionate oxidation in various ecosystems. Overall, this review improves basic and applied understanding of SPOB and highlights knowledge gaps, hopefully encouraging future research and engineering on propionate metabolism in biotechnological processes.

The efficiencies and capacities of carbon conversion in fruit and vegetable waste two-phase anaerobic digestion: Ethanol-path vs. butyrate-path

To rapidly treat and stably utilize great quantities of fruit and vegetable waste (FVW), the strategies in anaerobic digestion pattern have been constantly improved. In this work, the efficiencies and capacities of carbon conversion in different FVW anaerobic digestion systems were studied. Compared to butyrate-path (BD) two-phase and single-phase anaerobic digestion (SD), the ethanol-path two-phase anaerobic digestion (ED) system showed the highest rate of converting insoluble into soluble carbon formation (82.2%) and methane yield conversed from soluble carbon which is 0.14 gCOD_CH4 (gVSS d)^-1. It was also found that the coexistence of Bacillus and Methanococcus in the methanogenic phase maintained fatty acids and methane generation. The advantage of carbon conversion efficiency in ED can be elucidated from the highest acetification rate (704.10 mgCOD (L h)^-1) which means more converted acetate can be smoothly used for methane generation. Compared to methanogenesis converted from butyrate and propionate, the thermodynamic condition of methanogenesis converted from ethanol was more feasible. Also, the highest capacity of max methane production (197773.7 mL) of ED was simulated. ED might be an efficient and advantageous option for FVW methane digestion. Furthermore, comparison of acidogenic product and methane in conversion efficiency revealed that fatty acids should think as ideal anaerobic product rather than methane.

Electron transfer and mechanism of energy production among syntrophic bacteria during acidogenic fermentation: A review

Volatile fatty acids (VFAs) production plays an important role in the process of anaerobic digestion (AD), which is often the critical factor determining the metabolic pathways and energy recovery efficiency. Fermenting bacteria and acetogenic bacteria are in syntrophic relations during AD. Thus, clear elucidation of the interspecies electron transfer and energetic mechanisms among syntrophic bacteria is essential for optimization of acidogenic. This review aims to discuss the electron transfer and energetic mechanism in syntrophic processes between fermenting bacteria and acetogenic bacteria during VFAs production. Homoacetogenesis also plays a role in the syntrophic system by converting H₂ and CO₂ to acetate. Potential applications of these syntrophic activities in bioelectrochemical system and value-added product recovery from AD of organic wastes are also discussed. The study of acidogenic syntrophic relations is in its early stages, and additional investigation is required to better understand the mechanism of syntrophic relations.

Long-term enrichment of anaerobic propionate-oxidizing consortia: Syntrophic culture development and growth optimization

Propionate is a quantitatively important methanogenic intermediate in anaerobic digesters and only limited number of microbes can utilize it under syntrophic association with methanogens. The syntrophic propionate oxidizing bacterias are known to be slow growers due to the low energy yield. Thus, propionate get accumulated frequently in anaerobic digesters and it negatively affect digester performance. In this study, propionate degrading consortia from four different seeding sources were enriched in sequential bath mode in two phases; first adaption phase with 1 g/L of propionate concentration and later, high-strength phase with 3 g/L. From 16s rRNA gene based metagenomics analysis of the former phase, four syntrophic microbial groups, Syntrophaceae, Syntrophomonadaceae, Methanobacterium and Methanosaeta were found to be dominant with complete degradation of propionate. The substrate accelerated microbial shifts were observed at high-strength phase with significant decrease of Syntrophaceae up to 26.9 %. Using Response Surface Methodology, pH 6.8-6.9 and temperature 34.5-34.9 °C were found to be optimum growth conditions for the propionate degradation culture. Observed results could be useful to improve degradation efficiencies and obtained enriched culture can be used to recover propionate-accumulated digesters by bio-augmentation.

Insights into Ammonia Adaptation and Methanogenic Precursor Oxidation by Genome-Centric Analysis

Ammonia released from the degradation of protein and/or urea usually leads to suboptimal anaerobic digestion (AD) when N-rich organic waste is used. However, the insights behind the differential ammonia tolerance of anaerobic microbiomes remain an enigma. In this study, the cultivation in synthetic medium with different carbon sources (acetate, methanol, formate, and H2/CO2) shaped a common initial inoculum into four unique ammonia-tolerant syntrophic populations. Specifically, various levels of ammonia tolerance were observed: consortia fed with methanol and H2/CO2 could grow at ammonia levels up to 7.25 g NH+-N/L, whereas the other two groups (formate and acetate) only thrived at 5.25 and 4.25 g NH+-N/L, respectively. Metabolic reconstruction highlighted that this divergent microbiome might be achieved by complementary metabolisms to maximize biomethane recovery from carbon sources, thus indicating the importance of the syntrophic community in the AD of N-rich substrates. Besides, sodium/proton antiporter operon, osmoprotectant/K+ regulator, and osmoprotectant synthesis operon may function as the main drivers of adaptation to the ammonia stress. Moreover, energy from the substrate-level phosphorylation and multiple energy-converting hydrogenases (e.g., Ech and Eha) could aid methanogens to balance the energy request for anabolic activities and contribute to thriving when exposed to high ammonia levels.

Anaerobic Conversion of Saline Phenol-Containing Wastewater Under Thermophilic Conditions in a Membrane Bioreactor

Closing water loops in chemical industries result in hot and highly saline residual streams, often characterized by high strength and the presence of refractory or toxic compounds. These streams are attractive for anaerobic technologies, provided the chemical compounds are biodegradable. However, under such harsh conditions, effective biomass immobilization is difficult, limiting the use of the commonly applied sludge bed reactors. In this study, we assessed the long-term phenol conversion capacity of a lab-scale anaerobic membrane bioreactor (AnMBR) operated at 55°C, and high salinity (18 gNa^+.L^–1). Over 388 days, bioreactor performance and microbial community dynamics were monitored using specific methanogenic activity (SMA) assays, phenol conversion rate assays, volatile fatty acids permeate characterization and Illumina MiSeq analysis of 16S rRNA gene sequences. Phenol accumulation to concentrations exceeding 600 mgPh^.L^–1 in the reactor significantly reduced methanogenesis at different phases of operation, while applying a phenol volumetric loading rate of 0.12 gPh^.L^–1.d^–1. Stable AnMBR reactor performance could be attained by applying a sludge phenol loading rate of about 20 mgPh^.gVSS^–1.d^–1. In situ maximum phenol conversion rates of 21.3 mgPh^.gVSS^–1.d^–1 were achieved, whereas conversion rates of 32.8 mgPh^.gVSS^–1.d^–1 were assessed in ex situ batch tests at the end of the operation. The absence of caproate as intermediate inferred that the phenol conversion pathway likely occurred via carboxylation to benzoate. Strikingly, the hydrogenotrophic SMA of 0.34 gCOD-CH₄^.gVSS^–1.d^–1 of the AnMBR biomass significantly exceeded the acetotrophic SMA, which only reached 0.15 gCOD-CH₄^.gVSS^–1.d^–1. Our results indicated that during the course of the experiment, acetate conversion gradually changed from acetoclastic methanogenesis to acetate oxidation coupled to hydrogenotrophic methanogenesis. Correspondingly, hydrogenotrophic methanogens of the class Methanomicrobia, together with Synergistia, Thermotogae, and Clostridia classes, dominated the microbial community and were enriched during the three phases of operation, while the aceticlastic Methanosaeta species remarkably decreased. Our findings clearly showed that highly saline phenolic wastewaters could be satisfactorily treated in a thermophilic AnMBR and that the specific phenol conversion capacity was limiting the treatment process. The possibility of efficient chemical wastewater treatment under the challenging studied conditions would represent a major breakthrough for the widespread application of AnMBR technology.

Upgrading the anaerobic membrane bioreactor treatment of chicken manure by introducing in-situ ammonia stripping and hyper-thermophilic pretreatment

The slow hydrolysis rate and ammonia inhibition effects significantly limit the performance of anaerobic digestion (AD) of nitrogen rich wastes. An innovative two-stage AD was therefore investigated for chicken manure by combining hyper-thermophilic (70 °C) pretreatment and a anaerobic membrane bioreactor (AnMBR). An in-situ stripping unit was assembled into the AnMBR to remove the ammonium-N, thus alleviating the inhibition effects. Through the 120-day experiment, the hydraulic retention time was optimized at 15 days for AnMBR with a constant retention 4 days for pretreatment. The hydrolysis efficiency and methane yield reached 72.4% and 352 mL-CH₄/g-VS_in respectively. About 3000 mg/L ammonium-N was removed through stripping, attributing to methane yield increased by 139 mL-CH₄/g-VS_in and volatile fatty acids decreased by 2683 mg/L compared to the control. No significant fouling was observed for the membrane. Conclusively, the combined two-stage AD process may offer an alternative approach for the treatment of nitrogen rich organic waste.

Thermophilic Methane Production from Hydrothermally Pretreated Norway Spruce (Picea abies)

Norway spruce (Picea abies) is an industrially important softwood species available in northern Europe and can be used to produce bio-methane after proper pretreatment to overcome its recalcitrant complex structure. Hot water extraction (HWE) pretreatment at two different conditions (170 °C for 90 min (severity 4.02) and 140 °C for 300 min (severity 3.65)) was applied to extract hemicellulosic sugars from Norway spruce for thermophilic anaerobic digestion (AD) of the hydrolysate. The methane yield of hydrolysate prepared at the lower pretreatment severity was found to be 189 NmL/gCOD compared to 162 NmL/gCOD after the higher pretreatment severity suggesting higher pretreatment severity hampers the methane yield due to the presence of inhibitors formed due to sugars and lignin degradation and soluble lignin, extracted partially along with hemicellulosic sugars. Synthetic hydrolysates simulating real hydrolysates (H170syn and H140syn) had improved methane yield of 285 NmL/gCOD and 295 NmL/gCOD, respectively in the absence of both the inhibitors and soluble lignin. An effect of organic loadings (OLs) on the methane yield was observed with a negative correlation between OL and methane yield. The maximum methane yield was 290 NmL/gCOD for hydrolysate pretreated at 140 °C compared to 195 NmL/gCOD for hydrolyate pretreated at 170 °C, both at the lowest OL of 6 gCOD/L. Therefore, both pretreatment conditions and OL need to be considered for efficient methane production from extracted hydrolysate. Such substrates can be utilized in continuous flow industrial AD with well-adapted cultures with stable organic loading rates.

Bioaugmentation strategy for overcoming ammonia inhibition during biomethanation of a protein-rich substrate

High ammonia levels inhibit anaerobic digestion (AD) process and bioaugmentation with ammonia tolerant methanogenic culture is proposed to alleviate ammonia inhibition. In the current study, hydrogenotrophic Methanoculleus bourgensis was bioaugmented in an ammonia-inhibited continuous reactor fed mainly with microalgae (a protein-rich biomass), at extreme ammonia levels (i.e. 11 g NH₄⁺-N L^-1). The results showed 28% increase in methane production immediately after bioaugmentation. Moreover, volatile fatty acids decreased rapidly from more than 5 g L^-1 to around 1 g L^-1, with a fast reduction in propionate concentration. High throughput 16s rRNA gene sequencing demonstrated that the bioaugmented M. bourgensis doubled its relative abundance after bioaugmentation. "Microbiological domino effect", triggered by the bioaugmented M. bourgensis establishing a newly efficient community, was proposed as the working mechanism of the successful bioaugmentation. Additionally, a strong aceticlastic methanogenesis was found at the end of the experiment evidenced by the dominant presence of Methanosarcina soligelidi and the low abundance of syntrophic acetate oxidising bacteria at the final period. Overall, for the first time, this study proved the positive effect of bioaugmentation on ammonia inhibition alleviation of the microalgae-dominating fed reactor, paving the way of efficient utilization of other protein-rich substrates in the future.

Effects of nano-sized MnO2 on methanogenic propionate and butyrate degradation in anaerobic digestion

The responses of methanogenic propionate and butyrate degradation to nano-sized MnO₂ exposure were explored. The results showed that supplementation with 50 mg/g volatile suspended solids (VSS) of nano-sized MnO₂ significantly enhanced the production rate of CH₄ in propionate and butyrate degradation by 25.6% and 21.7%, respectively. The stimulatory effects most likely resulted from enhancements in the microbial metabolic activity based on the observed increases in the extracellular polymeric substance (EPS) secretion and activity of the electron transport system. In contrast, the CH₄ yields obtained were irreversibly inhibited by the presence of 400 mg/g VSS of nano-sized MnO₂, in which just 62.8% and 6.5%, respectively, of the yield obtained from the control. Further investigations indicated that supplementation by nano-sized MnO₂ could cause oxidative stress in microbial cells, resulting in the release of reactive oxygen species (ROS). Compared with that of the control, the amount of intracellular ROS generated in the systems increased by 28.3% (fed with propionate) and 42.5% (fed with butyrate), corresponding to approximately 43.9% and 64.8% losses in cell viability, respectively; thus, ROS generation was suggested to be the main factor responsible for the inhibitory effects of nano-sized MnO₂ on methanogenic propionate and butyrate degradation.

Feeding frequency influences process performance and microbial community composition in anaerobic digesters treating steam exploded food waste

In anaerobic digestion, studies of feeding frequency have produced conflicting results. Hence, the effect of feeding frequency on process variables and microbial community structure was investigated by comparing a laboratory-scale digester fed steam exploded food waste 10 times daily vs. one fed an equivalent amount once daily. The Frequently Fed Digester (FFD) produced on average 20% more methane and had lower effluent concentrations of long-chain fatty acids. Greater daily fluctuations in acetate, pH and biogas production rate could explain the lower specific methane yield and β-oxidation. Feeding frequency also influenced the microbial community whereby Tenericutes (42%) dominated in FFD but Firmicutes (31%) was most abundant in the Daily Fed Digester (DFD). Feeding frequency effects are therefore postulated to occur more often in digesters fed labile feedstocks at high organic loading rates.

Methanogenic capacity and robustness of hydrogenotrophic cultures based on closed nutrient recycling via microbial catabolism: Impact of temperature and microbial attachment

A biological methanation system based on nutrient recycling via mixed culture microbial catabolism was investigated at mesophilic (37 °C) and thermophilic (55 °C) temperatures. At mesophilic temperatures, the formation of biofilms on two different types of material was assessed. Results showed that with intense mixing the biofilm reactors presented methanogenic capacities (per working volume) 50% higher than the ones operated with suspended cultures. Gas feeding rates of 200 L/L/d were achieved at a H₂/CO₂ to CH₄ conversion efficiency of above 90% by linking two reactors in series. Furthermore the robustness of the cultures was assessed under a series of inhibitory conditions that simulated possible process interferences at full scale operation. Full recovery after separate intense oxygenation and long starvation periods was observed within 2-5 days.

Build-up and impact of volatile fatty acids on E. coli and A. lumbricoides during co-digestion of urine diverting dehydrating toilet (UDDT-F) faeces

This study examined the potential of Escherichia coli (E. coli) and Ascaris lumbricoides (A. lumbricoides) eggs inactivation in faecal matter coming from urine diverting dehydrating toilets (UDDT-F) by applying high concentrations of volatile fatty acids (VFAs) during anaerobic stabilization. The impact of individual VFAs on E. coli and A. lumbricoides eggs inactivation in UDDT-F was assessed by applying various concentrations of store-bought acetate, propionate and butyrate. High VFA concentrations were also obtained by performing co-digestion of UDDT-F with organic market waste (OMW) using various mixing ratios. All experiments were performed under anaerobic conditions in laboratory scale batch assays at 35±1 °C. A correlation was observed between E. coli log inactivation and VFA concentration. Store bought VFA spiked UDDT-F substrates achieved E. coli inactivation up to 4.7 log units/day compared to UDDT-F control sample that achieved 0.6 log units/day. In co-digesting UDDT-F and organic market waste (OMW), a ND-VFA concentration of 4800-6000 mg/L was needed to achieve E. coli log inactivation to below detectable levels and complete A. lumbricoides egg inactivation in less than four days. E. coli and A. lumbricoides egg inactivation was found to be related to the concentration of non-dissociated VFA (ND-VFA), increasing with an increase in the OMW fraction in the feed substrate. Highest ND-VFA concentration of 6500 mg/L was obtained at a UDDT-F:OMW ratio 1:1, below which there was a decline, attributed to product inhibition of acidogenic bacteria. Results of our present research showed the potential for E. coli and A. lumbricoides inactivation from UDDT-F up to WHO standards by allowing VFA build-up during anaerobic stabilization of faecal matter.

Performance and genome-centric metagenomics of thermophilic single and two-stage anaerobic digesters treating cheese wastes

The present research is the first comprehensive study regarding the thermophilic anaerobic degradation of cheese wastewater, which combines the evaluation of different reactor configurations (i.e. single and two-stage continuous stirred tank reactors) on the process efficiency and the in-depth characterization of the microbial community structure using genome-centric metagenomics. Both reactor configurations showed acidification problems under the tested organic loading rates (OLRs) of 3.6 and 2.4 g COD/L-reactor day and the hydraulic retention time (HRT) of 15 days. However, the two-stage design reached a methane yield equal to 95% of the theoretical value, in contrast with the single stage configuration, which reached a maximum of 33% of the theoretical methane yield. The metagenomic analysis identified 22 new population genomes and revealed that the microbial compositions between the two configurations were remarkably different, demonstrating a higher methanogenic biodiversity in the two-stage configuration. In fact, the acidogenic reactor of the serial configuration was almost solely composed by the lactose degrader Bifidobacterium crudilactis UC0001. The predictive functional analyses of the main population genomes highlighted specific metabolic pathways responsible for the AD process and the mechanisms of main intermediates production. Particularly, the acetate accumulation experienced by the single stage configuration was mainly correlated to the low abundant syntrophic acetate oxidizer Tepidanaerobacter acetatoxydans UC0018 and to the absence of aceticlastic methanogens.

Optimization of simultaneous production of volatile fatty acids and bio-hydrogen from food waste using response surface methodology

Anaerobic digestion of food waste (FW) is commonly considered an effective and green technology to convert solid waste into valuable feedstock including volatile fatty acids (VFAs) and hydrogen. Response surface methodology (RSM) was selected to analyze the production of VFAs and hydrogen from food waste in a batch process. The effect of the three variables i.e. total solid content (TS), pH, and reaction time under each variable at three levels on VFAs and hydrogen production was assessed. The optimum conditions determined via RSM were pH = 7.0, TS = 100 g L⁻¹, and reaction time = 3 d. The maximum VFA and hydrogen production was 26.17 g L⁻¹ and 46.03 mL g⁻¹ volatile solids added, respectively. The ratio of observed hydrogen (H_o) to predicted hydrogen (H_p) was x < 1.0 because of inhibition of hydrogen production by VFA accumulation. The subsequent microbial community analysis result was also consistent with the abovementioned results. The evolution of Bacteroidetes, which facilitate VFA production, has been enriched by about 16.1-times at pH 7.0 followed by 10.2-times at pH 6.0 as compared to that in the uncontrolled pH batch.

Response surface methodology was applied to optimal VFA production from food waste, which could evaluate the interactive effect of each parameter as compared to the traditional approach about just one variable a time on VFA production.

Feasibility of thermophilic anaerobic processes for treating waste activated sludge under low HRT and intermittent mixing

Thermophilic anaerobic digestion (AD) arises as an optimized solution for the waste activated sludge (WAS) management. However, there are few feasibility studies using low solids content typically found in the WAS, and that consider uncommon operational conditions such as intermittent mixing and low hydraulic retention time (HRT). In this investigation, a single-stage pilot reactor was used to treat WAS at low HRT (13, 9, 6 and 5 days) and intermittent mixing (withholding mixing 2 h prior feeding). Thermophilic anaerobic digestion (55 °C) was initiated from a mesophilic digester (35 °C) by the one-step startup strategy. Although instabilities on partial alkalinity (1245-3000 mgCaCO₃/L), volatile fatty acids (1774-6421 mg/L acetic acid) and biogas production (0.21-0.09 m³/m³_reactor.d) were observed, methanogenesis started to recover in 18 days. The thermophilic treatment of WAS at 13 and 9 days HRT efficiently converted VS into biogas (22 and 21%, respectively) and achieved high biogas yield (0.24 and 0.22 m³/kgVS_fed, respectively). Intermittent mixing improved the retention of methanogens inside the reactor and reduced the washout effect even at low HRT (<9 days). The negative thermal balance found was influenced by the low solids content in the WAS (2.1% TS) and by the heat losses from the digester walls. The energy balance and economic analyses demonstrated the feasibility of thermophilic AD of WAS in a hypothetical full-scale system, when the heat energy could be recovered from methane in a scenario of higher solids concentration in the substrate (>5% TS).

Application of the Initial Rate Method in Anaerobic Digestion of Kitchen Waste

This article proposes a methane production approach through sequenced anaerobic digestion of kitchen waste, determines the hydrolysis constants and reaction orders at both low total solid (TS) concentrations and high TS concentrations using the initial rate method, and examines the population growth model and first-order hydrolysis model. The findings indicate that the first-order hydrolysis model better reflects the kinetic process of gas production. During the experiment, all the influential factors of anaerobic fermentation retained their optimal values. The hydrolysis constants and reaction orders at low TS concentrations are then employed to demonstrate that the first-order gas production model can describe the kinetics of the gas production process. At low TS concentrations, the hydrolysis constants and reaction orders demonstrated opposite trends, with both stabilizing after 24 days at 0.99 and 1.1252, respectively. At high TS concentrations, the hydrolysis constants and the reaction orders stabilized at 0.98 (after 18 days) and 0.3507 (after 14 days), respectively. Given sufficient reaction time, the hydrolysis involved in anaerobic fermentation of kitchen waste can be regarded as a first-order reaction in terms of reaction kinetics. This study serves as a good reference for future studies regarding the kinetics of anaerobic digestion of kitchen waste.

Experimental and Artificial Neural Network Modeling of a Upflow Anaerobic Contactor (UAC) for Biogas Production from Vinasse

A pilot scale Upflow Anaerobic Contactor (UAC), based on upflow sludge blanket principle, was designed to treat vinasse waste obtained from beet molasses fermentation. An assessment of the anaerobic digestion of vinasse was carried out for the production of biogas as a source of energy. Average Organic loading rate (OLR) was around 7.5 gCOD/m3/day in steady state, increasing upto 8.1 gCOD/m3/day. The anaerobic digestion was conducted at mesophilic (30–37 °C) temperature and a stable operating condition was achieved after 81 days with average production of 65 % methane which corresponded to a maximum biogas production of 85 l/day. The optimal performance of UAC was obtained at 87 % COD removal, which corresponded to a hydraulic retention time of 16.67 days. The biogas production increased gradually with OLR, corresponding to a maximum 6.54 gCOD/m3/day (7.4 % increase from initial target). A coupled Artificial Neural Network-Differential Evolution (ANN-DE) methodology was formulated to predict chemical oxygen demand (COD), total suspended solids (TSS) and volatile fatty acids (VFA) of the effluent along with the biogas production. The method incorporated a DE approach for the efficient tuning of ANN meta-parameters such as number of nodes in hidden layer, input and output activation function and learning rate. The model prediction indicated that it can learn the nonlinear complex relationship between the parameters and able to predict the output of the contactor with reasonable accuracy. The utilization of the coupled ANN-DE model provided significant improvement to the study and helps to study the parametric effect of influential parameters on the reactor output.

Effect of increased load of high-strength food wastewater in thermophilic and mesophilic anaerobic co-digestion of waste activated sludge on bacterial community structure

In recent years, anaerobic co-digestion (AcoD) has been widely used to improve reactor performance, especially methane production. In this study, we applied two different operating temperatures (thermophilic and mesophilic) and gradually increased the load of food wastewater (FWW) to investigate the bacterial communities during the AcoD of waste activated sludge (WAS) and FWW. As the load of FWW was increased, methane production rate (MPR; L CH4/L d) and methane content (%) in both Thermophilic AcoD (TAcoD) and Mesophilic AcoD (MAcoD) increased significantly; the highest MPR and methane content in TAcoD (1.423 L CH4/L d and 68.24%) and MAcoD (1.233 L CH4/L d and 65.21%) were observed when the FWW mixing ratio was 75%. However, MPR and methane yield in both reactors decreased markedly and methane production in TAcoD ceased completely when only FWW was fed into the reactor, resulting from acidification of the reactor caused by accumulation of organic acids. Pyrosequencing analysis revealed a decrease in bacterial diversity in TAcoD and a markedly different composition of bacterial communities between TAcoD and MAcoD with an increase in FWW load. For example, Bacterial members belonging to two genera Petrotoga (assigned to phylum Thermotogae) and Petrimonas (assigned to phylum Bacteroidetes) became dominant in TAcoD and MAcoD with an increase in FWW load, respectively. In addition, quantitative real-time PCR (qPCR) results showed higher bacterial and archaeal populations (expressed as 16S rRNA gene concentration) in TAcoD than MAcoD with an increase in FWW load and showed maximum population when the FWW mixing ratio was 75% in both reactors. Collectively, this study demonstrated the dynamics of key bacterial communities in TAcoD and MAcoD, which were highly affected by the load of FWW.

Piezo-tolerant natural gas-producing microbes under accumulating pCO2

BACKGROUND

It is known that a part of natural gas is produced by biogenic degradation of organic matter, but the microbial pathways resulting in the formation of pressurized gas fields remain unknown. Autogeneration of biogas pressure of up to 20 bar has been shown to improve the quality of biogas to the level of biogenic natural gas as the fraction of CO₂ decreased. Still, the pCO₂ is higher compared to atmospheric digestion and this may affect the process in several ways. In this work, we investigated the effect of elevated pCO₂ of up to 0.5 MPa on Gibbs free energy, microbial community composition and substrate utilization kinetics in autogenerative high-pressure digestion.

RESULTS

In this study, biogas pressure (up to 2.0 MPa) was batch-wise autogenerated for 268 days at 303 K in an 8-L bioreactor, resulting in a population dominated by archaeal Methanosaeta concilii, Methanobacterium formicicum and Mtb. beijingense and bacterial Kosmotoga-like (31% of total bacterial species), Propioniferax-like (25%) and Treponema-like (12%) species. Related microorganisms have also been detected in gas, oil and abandoned coal-bed reservoirs, where elevated pressure prevails. After 107 days autogeneration of biogas pressure up to 0.50 MPa of pCO₂, propionate accumulated whilst CH₄ formation declined. Alongside the Propioniferax-like organism, a putative propionate producer, increased in relative abundance in the period of propionate accumulation. Complementary experiments showed that specific propionate conversion rates decreased linearly from 30.3 mg g^-1 VS_added day^-1 by more than 90% to 2.2 mg g^-1 VS_added day^-1 after elevating pCO₂ from 0.10 to 0.50 MPa. Neither thermodynamic limitations, especially due to elevated pH₂, nor pH inhibition could sufficiently explain this phenomenon. The reduced propionate conversion could therefore be attributed to reversible CO₂-toxicity.

CONCLUSIONS

The results of this study suggest a generic role of the detected bacterial and archaeal species in biogenic methane formation at elevated pressure. The propionate conversion rate and subsequent methane production rate were inhibited by up to 90% by the accumulating pCO₂ up to 0.5 MPa in the pressure reactor, which opens opportunities for steering carboxylate production using reversible CO₂-toxicity in mixed-culture microbial electrosynthesis and fermentation.Graphical abstractThe role of pCO₂ in steering product formation in autogenerative high pressure digestion.

Acidogenic fermentation of food waste for volatile fatty acid production with co-generation of biohydrogen

Fermentation experiments were designed to elucidate the functional role of the redox microenvironment on volatile fatty acid (VFA, short chain carboxylic acid) production and co-generation of biohydrogen (H2). Higher VFA productivity was observed at pH 10 operation (6.3g/l) followed by pH 9, pH 6, pH 5, pH 7, pH 8 and pH 11 (3.5 g/l). High degree of acidification, good system buffering capacity along with co-generation of higher H2 production from food waste was also noticed at alkaline condition. Experiments illustrated the role of initial pH on carboxylic acids synthesis. Alkaline redox conditions assist solubilization of carbohydrates, protein and fats and also suppress the growth of methanogens. Among the carboxylic acids, acetate fraction was higher at alkaline condition than corresponding neutral or acidic operations. Integrated process of VFA production from waste with co-generation of H2 can be considered as a green and sustainable platform for value-addition.

Kinetic characterization of thermophilic and mesophilic anaerobic digestion for coffee grounds and waste activated sludge

This study was conducted to characterize the kinetics of an anaerobic process (hydrolysis, acetogenesis, acidogenesis and methanogenesis) under thermophilic (55 °C) and mesophilic (35 °C) conditions with coffee grounds and waste activated sludge (WAS) as the substrates. Special focus was given to the kinetics of propionic acid degradation to elucidate the accumulation of VFAs. Under the thermophilic condition, the methane production rate of all substrates (WAS, ground coffee and raw coffee) was about 1.5 times higher than that under the mesophilic condition. However, the effects on methane production of each substrate under the thermophilic condition differed: WAS increased by 35.8-48.2%, raw coffee decreased by 76.3-64.5% and ground coffee decreased by 74.0-57.9%. Based on the maximum reaction rate (Rmax) of each anaerobic stage obtained from the modified Gompertz model, acetogenesis was found to be the rate-limiting step for coffee grounds and WAS. This can be explained by the kinetics of propionate degradation under thermophilic condition in which a long lag-phase (more than 18 days) was observed, although the propionate concentration was only 500 mg/L. Under the mesophilic condition, acidogenesis and hydrolysis were found to be the rate-limiting step for coffee grounds and WAS, respectively. Even though reducing the particle size accelerated the methane production rate of coffee grounds, but did not change the rate-limiting step: acetogenesis in thermophilic and acidogenesis in mesophilic.

Mathematical modelling of methanogenic reactor start-up: Importance of volatile fatty acids degrading population

Development of balanced community of microorganisms is one of the obligatory for stable anaerobic digestion. Application of mathematical models might be helpful in development of reliable procedures during the process start-up period. Yet, the accuracy of forecast depends on the quality of input and parameters. In this study, the specific anaerobic activity (SAA) tests were applied in order to estimate microbial community structure. Obtained data was applied as input conditions for mathematical model of anaerobic digestion. The initial values of variables describing the amount of acetate and propionate utilizing microorganisms could be calculated on the basis of SAA results. The modelling based on those optimized variables could successfully reproduce the behavior of a real system during the continuous fermentation.

Hydrolytic and acidogenic fermentation potential of food waste with source segregated feces-without-urine as co-substrate

This study explored a new approach to enhance VFA productivity from anaerobic co-digestion of food-waste (FW) with source-segregated brown-water (BW) [feces-without-urine]. Effort was made to separate urine and BW from the source (using no-mix-toilet) mainly to expedite further treatment and resource-recovery. Effect of alkaline-pH [B] and acclimatized acidogenic inoculum [C] on acidification efficiency was investigated and compared with raw FW+BW co-digestion [A]. Batch-assay results indicated that VFA productivity persists for 144-h with about 615%, 522% and 376% increase in VFA-COD, respectively for 3-conditions [A-C]; which accounted for 70%, 49% and 58% of CODs input, respectively. High butyric-acid was observed in [A] and [C], followed by acetic, propionic-acids; whereas, abundant acetic-acid (86% of TVFA) was observed in [B], which are the most favorable-forms for methane production or other value-added-products. For 144-h of acidification, this study validated the feasibility of maximizing VFA-yield by 7-12 times compared to FW or BW as a sole-substrate.

Improving the performance of thermophilic anaerobic digester through recirculation of low hydrogen biogas

Biogas recirculation was conducted to improve the performance of two thermophilic anaerobic sequenced batch reactors (ASBRs), in which high concentrations of volatile fatty acids (VFAs) were accumulated. To accelerate degradation of VFAs, facilitating acetate consumption via syntrophic acetate oxidation coupled with hydrogenotrophic methanogenesis (SAO-HM) was expected to be effective. Hence, to promote the SAO-HM pathway, hydrogen was removed to create low hydrogen partial pressure (pH2) in reactor RH, yet in reactor RB, hydrogen was not treated. The performance of RB and RH on VFAs degradation and methane production processes was compared at steady stage; the VFAs and soluble microbial products (SMP) in the effluents were monitored. The results showed that low pH2 intensified the SAO reaction, thereby accelerating conversion of acetate to methane, as well as acetate production from glucose and VFAs. Glucose fermentation type was also influenced. VFAs and SMP in the effluents were reduced after the introduction of biogas mixing, which proceeded much faster in RH with low pH2. Recirculation of low hydrogen biogas with SAO-HM pathway being promoted should be more effective to alleviate high acid level stress and to improve the reactor performance.

Effect of inoculum to substrate ratio on the hydrolysis and acidification of food waste in leach bed reactor

The aim of present study was to determine an appropriate ISR (inoculum to substrate ratio) to enhance the hydrolysis rate and reduce the solid retention time of food waste in hydrolytic-acidogenesis leach bed reactor (LBR). LBR 1-4 were inoculated with 0%, 5%, 20% and 80% (w/w basis) of anaerobically digested sludge, respectively, using artificial food waste as substrate. Experiments were conducted in batch mode at mesophilic condition (35 °C) for 17 days. Higher ISR resulted in 4.3-fold increase in protein hydrolysis; whereas, only a modest increase in the decomposition of carbohydrate. Two kinetic models for carbohydrate and protein degradation were proposed and evaluated. The differences among four ISRs in volatile solids removal efficiencies were marginal, i.e. 52.4%, 62.8%, 63.2% and 71.7% for LBR 1-4, respectively; indicating that higher ISR was insignificant in enhancing the overall hydrolysis rate in LBR. Therefore, a lower ISR of 20% was recommended in the hydrolytic-acidogenic process.

Performance of a commercial-scale DiCOM demonstration facility treating mixed municipal solid waste in comparison with laboratory-scale data

The current paper describes the performance of a commercial-scale (20,000 tpa) demonstration facility of the DiCOM process, a biological treatment for the organic fraction of municipal solid waste (OFMSW). The 21-d process combines aerobic composting and high-solids (30%DM), thermophilic (55 °C) anaerobic digestion (AD), within a single vessel. Mechanically sorted OFMSW, derived from mixed household MSW (324 t), was exposed to sequential aerobic/anaerobic/aerobic treatment. The AD, initiated by adding anaerobic inoculum from a previous trial, was stable (without pH intervention) and the onset of methanogenesis, rapid (<3 h). Volatile fatty acids formed during AD (including propionate) were exhausted prior to reuse of the inoculum. As measured by an electron flux from solids to gaseous end-products, AD accounted for the greatest portion of solids degradation (86%=160 m(3)CH(4)/drytOFMSW). However, unlike laboratory trials, limited degradation occurred during initial aerobic treatment. The discharged solids were classified as a composted soil conditioner.

Enhancement of bioenergy production from organic wastes by two-stage anaerobic hydrogen and methane production process

The present study investigated a two-stage anaerobic hydrogen and methane process for increasing bioenergy production from organic wastes. A two-stage process with hydraulic retention time (HRT) 3d for hydrogen reactor and 12d for methane reactor, obtained 11% higher energy compared to a single-stage methanogenic process (HRT 15 d) under organic loading rate (OLR) 3 gVS/(L d). The two-stage process was still stable when the OLR was increased to 4.5 gVS/(Ld), while the single-stage process failed. The study further revealed that by changing the HRT(hydrogen):HRT(methane) ratio of the two-stage process from 3:12 to 1:14, 6.7%, more energy could be obtained. Microbial community analysis indicated that the dominant bacterial species were different in the hydrogen reactors (Thermoanaerobacterium thermosaccharolyticum-like species) and methane reactors (Clostridium thermocellum-like species). The changes of substrates and HRT did not change the dominant species. The archaeal community structures in methane reactors were similar both in single- and two- stage reactors, with acetoclastic methanogens Methanosarcina acetivorans-like organisms as the dominant species.

Start-Up Strategy to Achieve Excellent Efficiency of Degradation Acetate,Ethanol and Propionate in UASB

The performance of upflow anaerobic sludge blanket (UASB) fed with three metabolic intermediate (acetate, ethanol, and propionate) respectively was studied. The degradation of metabolic intermediate were investigated to discuss the reason for propionate inhibition problem in anaerobic treatment. The hydraulic retention time (HRT) in the reactors started with 8.0h.The yield rate of biogas were 237ml/gCOD, 242ml/gCOD, 218ml/gCOD for acetate, ethanol and propionate, respectively when finishing start-up under OLR of 5.0 kgCOD/(m3·d) (HRT=9.6h).The HRT remained constant 9.6h,and the substrate concentration was gradually increased from 1,000 to 16,000mg/L as COD,and the organic loading rates(OLR) was from 3.0 to 40.0 kgCOD/(m3·d).The maximum propionate concentration was 41.6 gHPr-COD/L at the organic loading rate of 43.9 kgCOD/(m3·d) (HRT, 9.6h) as well as acetate and ethanol.

Using enriched cultures for elevation of anaerobic syntrophic interactions between acetogens and methanogens in a high-load continuous digester

Volatile fatty acids (VFAs) are key intermediates in anaerobic digestion. Enriched acetogenic and methanogenic cultures were used for syntrophic anaerobic digestion of VFAs in a high-load continuous reactor fed with acetic (HAc), propionic (HPr) and butyric (HBu) acids at maximum concentrations of 5, 3 and 4 g/L, respectively. Interactive effects of HPr, HBu and HAc were analyzed. Furthermore, hydraulic retention time (HRT) and methanogen to acetogen population ratio (M/A) were investigated as key microbiological and operating variables of VFA anaerobic degradations. Optimum conditions were found to be HPr=1125.0mg/L, HBu=1833.4 mg/L, HAc=1727.4 mg/L, HRT=21 h and M/A=2.5 (corresponding to maximum VFA removal and biogas production rate (BPR)). Results of verification experiments and predicted values from fitted correlations were in close agreement at 95% confidence interval. HRT and M/A had positive effects on VFA removal and BPR. M/A was the most important factor that affected BPR. All VFAs inhibited VFA removals.

Syntrophic degradation of cadaverine by a defined methanogenic coculture

A novel, strictly anaerobic, cadaverine-oxidizing, defined coculture was isolated from an anoxic freshwater sediment sample. The coculture oxidized cadaverine (1,5-diaminopentane) with sulfate as the electron acceptor. The sulfate-reducing partner could be replaced by a hydrogenotrophic methanogenic partner. The defined coculture fermented cadaverine to acetate, butyrate, and glutarate plus sulfide or methane. The key enzymes involved in cadaverine degradation were identified in cell extracts. A pathway of cadaverine fermentation via 5-aminovaleraldehyde and crotonyl-coenzyme A with subsequent dismutation to acetate and butyrate is suggested. Comparative 16S rRNA gene analysis indicated that the fermenting part of the coculture belongs to the subphylum Firmicutes but that this part is distant from any described genus. The closest known relative was Clostridium aminobutyricum, with 95% similarity.

Pentachlorophenol dechlorination by an acidogenic sludge

This study reports the feasibility of removing pentachlorophenol (PCP) by an acidogenic process in batch reactors. When the acidogenic sludge was first acclimated with 2,4,6-trichlorophenol (2,4,6-TCP) and developed 2,4,6-TCP dechlorinating activity, PCP could be ortho-dechlorinated to 3,4,5,-trichlorophenol (3,4,5-TCP) via 2,3,4,5-tetrachlorophenol as the intermediary. However, due to PCP's higher hydrophobicity and its higher expected Gibbs free energy yield, it was adsorbed to the sludge and dechlorinated preferentially to 2,4,6-TCP. This resulted in the inhibition of 2,4,6-TCP dechlorination. PCP removal under acidogenic condition was attributed to both reductive dechlorination and adsorption. At low PCP loads of 0.48micromoles/gMLVSS.d, dechlorination was the dominant removal mechanism (69% of total removal), while at the higher PCP load of 9.3micromoles/gMLVSS.d, adsorption was the main mechanism (82% of total removal). Attempts to induce meta or para position dechlorination of PCP failed when using meta position chlorophenols such as 2,3,6-TCP, 3,4,5-TCP and 3,5-DCP as the initial substrates. Overall, acidogenic biotreatment was an effective process in reducing PCP loads prior to downstream biological treatment.

Methanogenesis from acetate and propionate by thermophilic down-flow anaerobic packed-bed reactor

The maximum propionate removal rate was 13.7 g/L-reactor/day at the organic loading rate of 66.4 kg-CODcr/m3-reactor/day (HRT, 4.75 h); however, the removal efficiency was very low. Clone library analysis and quantification by real-time PCR using 16S rRNA gene revealed that the population of methanogenic archaea in the biofilm fraction that developed on the packed bed was higher than that in the liquid fraction. The clone, which is related to Methanosarcina, was detected only in the biofilm fraction. The clones closely related to Pelotomaculum, which is capable of degrading propionate, and the hydrogenotrophic methanogen Methanothermobactor were also detected only in the biofilm fraction in the acetate and propionate-fed reactor. The experimental results indicate that the packed-bed design can maintain a sufficiently high density of methanogenic microorganisms within the system even at reduced HRTs as well as facilitate an efficient degradation of propionate and acetate, possibly through syntrophic reactions.

Methanol utilizing Desulfotomaculum species utilizes hydrogen in a methanol-fed sulfate-reducing bioreactor

A sulfate-reducing bacterium, strain WW1, was isolated from a thermophilic bioreactor operated at 65 degrees C with methanol as sole energy source in the presence of sulfate. Growth of strain WW1 on methanol or acetate was inhibited at a sulfide concentration of 200 mg l(-1), while on H2/CO2, no apparent inhibition occurred up to a concentration of 500 mg l(-1). When strain WW1 was co-cultured under the same conditions with the methanol-utilizing, non-sulfate-reducing bacteria, Thermotoga lettingae and Moorella mulderi, both originating from the same bioreactor, growth and sulfide formation were observed up to 430 mg l(-1). These results indicated that in the co-cultures, a major part of the electron flow was directed from methanol via H2/CO2 to the reduction of sulfate to sulfide. Besides methanol, acetate, and hydrogen, strain WW1 was also able to use formate, malate, fumarate, propionate, succinate, butyrate, ethanol, propanol, butanol, isobutanol, with concomitant reduction of sulfate to sulfide. In the absence of sulfate, strain WW1 grew only on pyruvate and lactate. On the basis of 16S rRNA analysis, strain WW1 was most closely related to Desulfotomaculum thermocisternum and Desulfotomaculum australicum. However, physiological properties of strain WW1 differed in some aspects from those of the two related bacteria.

Thermophilic anaerobic digestion of source-sorted organic fraction of household municipal solid waste: start-up procedure for continuously stirred tank reactor

Two feeding strategies for start-up of continuously stirred tank reactors (CSTR) treating source-sorted organic fraction of household municipal solid waste (SS-OFMSW) at 55 degrees C were evaluated. Two reactors were started up separately with a limited amount of initial inoculum (i.e. 10% of the final volume of 3.5l) and operated in a fed batch mode until the reactors were filled (30 days). A reference reactor was filled up with 3.5l of inoculum and fed at a constant rate (11.4 g volatile solids (VS)/d). Loading at progressively increasing rate (from 1.7 to 15 gVS/d), as calculated based on an activated biomass concept, showed superior process performance compared to a fixed feed rate (5.7 gVS/d). Methane yield of 0.32 m(3)/kg VS was produced during the start-up in reactor filled at progressively increasing rate and was comparable to the reference reactor. On the contrary, significant inhibition due to volatile fatty acid (VFA) build-up, mainly due to butyrate, was noticed in the reactor filled at constant rate. Thus, low initial and progressive increasing inoculum loading rate could be used as a strategy for a successful start-up of CSTR treating SS-OFMSW as it allowed a gradual acclimation of the biomass. Lab-scale results were further reaffirmed from the start-up of a full-scale plant (7000 m(3) total capacity) which was supplied with inoculum corresponding to approx. 16% of final volume and operated in a fed batch mode until the reactors were filled (58 days). Stable biogas production with low VFA (<3 g/L; based on titration method) were noticed during the start-up period when fed at progressively increasing rate. Thus, a controlled and reliable start-up procedure was found essential, which could allow rapid process stabilization and time to focus on other technical aspects of plant operation. In addition, the influence of substrate to inoculum amount (1.5-30% TS) and temperature (5-65 degrees C) on anaerobic degradation and methane production of SS-OFMSW was investigated in batch assays as a protocol for start-up procedure.

Microbial community in methanogenic packed-bed reactor successfully operating at short hydraulic retention time

The microbial community in a thermophilic anaerobic packed-bed reactor, which had been successfully operated to convert acetic and butyric acids to methane at a short hydraulic retention time (from 24 h to 1.9 h), was investigated. Archaea closely related to known methanogens were detected by 16S rRNA gene analyses of the effluents, together with diverse types of unidentified bacteria.

Decolorization and toxicity of reactive anthraquinone textile dyes under methanogenic conditions

Reductive decolorization of two anthraquinone reactive dyes (Reactive Blue 4, RB4; Reactive Blue 19, RB19) under methanogenic conditions was performed using a mixed, methanogenic culture. Decolorization of the two anthraquinone dyes was investigated to evaluate the rate and extent of color removal as well as to assess possible toxic effects of the dyes and their decolorization product(s) on the methanogenic culture as a function of initial dye concentration ranging from 50 to 300 mg x L(-1). A dextrin/peptone mixture was used as the carbon and electron source. A high rate and extent of color removal was achieved ranging from 4.3 to 29.9 mg x L(-1)h(-1) and 73-91% for RB4, and 13.0-74.4 mg x L(-1)h(-1) and 90-95% for RB19. Initial RB4 concentrations up to 100 mg x L(-1) did not result in any significant inhibition. Both the 200 and 300 mg x L(-1) RB4-amended cultures, and all RB19-amended cultures resulted in severe inhibition of both acidogenesis and methanogenesis. Sequential dye addition at 300 mg x L(-1) for both RB4 and RB19 resulted in accumulation of volatile fatty acids (VFAs) and a very low methane production at the end of the first dye addition after 44 days of incubation. However, at the end of the second dye addition, after a relatively long incubation (384 days), recovery of methanogens in the RB4-amended culture was observed in contrast to the complete inhibition of methanogenesis in the RB19-amended culture. Therefore, RB19 resulted in a higher degree of inhibition of both acidogenesis and methanogenesis than RB4. Addition of dextrin/peptone to dye-inhibited cultures resulted in acidogenesis and a gradual recovery of methanogenesis (mainly aceticlastic methanogenesis) in the RB4-inhibited culture, and a slow recovery of acidogenesis but no recovery of methanogenesis in the RB19-inhibited culture. In contrast, addition of 80% H(2)-20% CO(2) gas to dye-inhibited cultures resulted in recovery of hydrogenotrophic methanogenesis in both the RB4- and RB19-inhibited cultures. In spite of the relatively severe inhibition of the two anthraquinone dyes on the mixed, methanogenic culture, a high extent of color removal was achieved.

Comparison of two-stage thermophilic (68°C/55°C) anaerobic digestion with one-stage thermophilic (55°C) digestion of cattle manure

A two-stage 68 degrees C/55 degrees C anaerobic degradation process for treatment of cattle manure was studied. In batch experiments, an increase of the specific methane yield, ranging from 24% to 56%, was obtained when cattle manure and its fractions (fibers and liquid) were pretreated at 68 degrees C for periods of 36, 108, and 168 h, and subsequently digested at 55 degrees C. In a lab-scale experiment, the performance of a two-stage reactor system, consisting of a digester operating at 68 degrees C with a hydraulic retention time (HRT) of 3 days, connected to a 55 degrees C reactor with 12-day HRT, was compared with a conventional single-stage reactor running at 55 degrees C with 15-days HRT. When an organic loading of 3 g volatile solids (VS) per liter per day was applied, the two-stage setup had a 6% to 8% higher specific methane yield and a 9% more effective VS-removal than the conventional single-stage reactor. The 68 degrees C reactor generated 7% to 9% of the total amount of methane of the two-stage system and maintained a volatile fatty acids (VFA) concentration of 4.0 to 4.4 g acetate per liter. Population size and activity of aceticlastic methanogens, syntrophic bacteria, and hydrolytic/fermentative bacteria were significantly lower in the 68 degrees C reactor than in the 55 degrees C reactors. The density levels of methanogens utilizing H2/CO2 or formate were, however, in the same range for all reactors, although the degradation of these substrates was significantly lower in the 68 degrees C reactor than in the 55 degrees C reactors. Temporal temperature gradient electrophoresis profiles (TTGE) of the 68 degrees C reactor demonstrated a stable bacterial community along with a less divergent community of archaeal species.

Interspecies electron transfer in methanogenic propionate degrading consortia

Propionate is a key intermediate in the conversion of complex organic matter under methanogenic conditions. Oxidation of this compound requires obligate syntrophic consortia of acetogenic proton- and bicarbonate reducing bacteria and methanogenic archaea. Although H(2) acts as an electron-carrier in these consortia, evidence accumulates that formate plays an even more important role. To make energy yield from propionate oxidation energetically feasible for the bacteria and archaea involved, the concentrations of H(2) and formate have to be extremely low. On the other hand, the diffusion distance of these carriers has to be small to allow high propionate conversion rates. Accordingly, the high conversion rates observed in methanogenic bioreactors are due to the fact that the propionate-oxidizing bacteria and their methanogenic partners form micro-colonies within the densely packed granules.

Anaerobic granular sludge and biofilm reactors

The long retention time of the active biomass in the high-rate anaerobic digesters is the key factor for the successful application of the high rate anaerobic wastewater treatment. The long solids retention time is achieved due to the specific reactor configuration and it is enhanced by the immobilization of the biomass, which forms static biofilms, particle-supported biofilms, or granules depending on the reactor's operational conditions. The advantages of the high-rate anaerobic digestion over the conventional aerobic wastewater treatment methods has created a clear trend for the change of the role of the anaerobic digestion in the wastewater treatment plants from a pre-treatment method to the main biological treatment method. The application of staged high-rate anaerobic digesters has shown the larger potential among the recent developments in this direction. The most common high-rate anaerobic treatment systems based on anaerobic granular sludge and biofilm are described in this chapter. Emphasis is given to a) the Up-flow Anaerobic Sludge Blanket (UASB) systems, b) the main characteristics of the anaerobic granular sludge, and c) the factors that control the granulation process. Finally, the most innovative staged anaerobic digesters are also presented.