Initial Methane Production Rate Research Articles

Effect of micro-aeration on syntrophic and methanogenic activity in anaerobic sludge

Micro-aeration was shown to improve anaerobic digestion (AD) processes, although oxygen is known to inhibit obligate anaerobes, such as syntrophic communities of bacteria and methanogens. The effect of micro-aeration on the activity and microbial interaction in syntrophic communities, as well as on the potential establishment of synergetic relationships with facultative anaerobic bacteria (FAB) or aerobic bacteria (AB), was investigated. Anaerobic sludge was incubated with ethanol and increasing oxygen concentrations (0–5% in the headspace). Assays with acetate or H2/CO2 (direct substrates for methanogens) were also performed. When compared with the controls (0% O2), oxygen significantly decreased substrate consumption and initial methane production rate (MPR) from acetate or H2/CO2. At 0.5% O2, MPR from these substrates was inhibited 30–40%, and close to 100% at 5% O2. With ethanol, significant inhibition (>36%) was only observed for oxygen concentrations higher than 2.5%. Oxygen was consumed in the assays, pointing to the stimulation of AB/FAB by ethanol, which helped to protect the syntrophic consortia under micro-aerobic conditions. This highlights the importance of AB/FAB in maintaining functional and resilient syntrophic communities, which is relevant for real AD systems (in which vestigial O2 amounts are frequently present), as well as for AD systems using micro-aeration as a process strategy.Key points•Micro-aeration impacts syntrophic communities of bacteria and methanogens.•Oxygen stimulates AB/FAB, maintaining functional and resilient consortia.•Micro-aeration studies are critical for systems using micro-aeration as a process strategy.

Pre-Treatment of Separately Collected Biowaste as a Way to Increase Methane Production and Digestate Stability

To produce a valuable final product from anaerobic digestion (AD), one of the preferred methods of organic recycling, high quality feedstock must be ensured. In this study, separately collected real biowaste (B) was used, consisting of 90% food waste and 10% green waste. The priority issues of AD are both high methane production (MP) and high organics removal efficiency (as organic matter, OM and dissolved organics, and DCOD), which may be improved after pre-treatment. In this study, the effect of hydrothermal pre-treatment (BHT) and enzymatic additives (BE) on MP and organics removal from biowaste in mesophilic (37 °C) conditions was analyzed. To assess the adequacy of pre-treatment application, biowaste without treatment (BWT) was used. Pre-treatment of biowaste prior to AD affected the maximal MP, the removal effectiveness of both OM and DCOD, and the kinetic parameters of these processes. For BWT, the maximal cumulative MP reached 239.40 ± 1.27 NL/kg OM; the kinetic coefficient of MP (kCH4) and the initial MP rate (rCH4) were 0.32 ± 0.02 d−1 and 76.80 ± 1.10 NL/(kg OM·d), respectively. After hydrothermal pre-treatment, the MP of BHT (253.60 ± 1.83 NL/kg OM) was 6.3% higher than BWT. However, the highest MP was found for BE, 268.20 ± 1.37 NL/kg OM; to compare, it increased by 12.1% and 5.5% with BWT and BHT, respectively. However, the kinetic parameters of MP were highest with BHT:kCH4 0.56 ± 0.02 d−1 vs. 0.32 ± 0.02 d−1 (BWT) and 0.34 ± 0.02 d−1 (BE); rCH4 141.80 ± 0.02 NL/(kg OM·d) (BHT) vs. 76.80 ± 1.10 NL/(kg OM·d) (BWT) and 89.80 ± 0.50 NL/(kg OM·d) (BE). The effectiveness of OM removal was highest with BE, similarly to the MP with the use of an enzymatic additive. The kinetics of OM removal (rOM, kOM) were highest with BHT, similarly to the kinetics of MP (rCH4, kCH4). The highest effectiveness of OM and, consequently, its lowest final content obtained with BE means that the organics were used most efficiently, which, in turn, may result in obtaining a more stable digestive system.

Multi-Walled Carbon Nanotubes Enhance Methanogenesis from Diverse Organic Compounds in Anaerobic Sludge and River Sediments

Conductive nanomaterials affect anaerobic digestion (AD) processes usually by improving methane production. Nevertheless, their effect on anaerobic communities, and particularly on specific trophic groups such as syntrophic bacteria or methanogens, is not extensively reported. In this work, we evaluate the effect of multi-walled carbon nanotubes (MWCNT) on the activity of two different anaerobic microbial communities: an anaerobic sludge and a river sediment. Methane production by anaerobic sludge was assessed in the presence of different MWCNT concentrations, with direct methanogenic substrates (acetate, hydrogen) and with typical syntrophic substrates (ethanol, butyrate). MWCNT accelerated the initial specific methane production rate (SMPR) from all compounds, with a more pronounced effect on the assays with acetate and butyrate, i.e., 2.1 and 2.6 times, respectively. In the incubations with hydrogen and ethanol, SMPR increased 1.1 and 1.2 times. Experiments with the river sediment were performed in the presence of MWCNT and MWCNT impregnated with 2% iron (MWCNT-Fe). Cumulative methane production was 10.2 and 4.5 times higher in the assays with MWCNT-Fe and MWCNT, respectively, than in the assays without MWCNT. This shows the high potential of MWCNT toward bioenergy production, in waste/wastewater treatment or ex situ bioremediation in anaerobic digesters.

Experimental study on anaerobic co-digestion of the individual component of biomass with sewage sludge: methane production and microbial community

The co-digestion of the individual component of biomass with sewage sludge was investigated, concerning the cumulative methane yield, variation of pH, chemical oxygen demand (COD), volatile fatty acids (VFAs), and total ammonia nitrogen (TAN) during 50-day digestion. The microbial community at the initial stage (stage I) and maximum methane production rate stage (stage II) of the digestion process were identified by Illumina MiSeq sequencing. The cumulative methane yield from co-digestion of cellulose and sludge was increased by 33.33% over the calculated value from that of mono-digestion of cellulose and sludge, while that from co-digestion of hemicellulose and sludge was achieved as 259.32% compared with the calculated value from that of mono-digestion of hemicellulose and sludge. The Firmicutes to Bacteroidetes (F/B) ratio at the stage II of co-digestion of hemicellulose and sludge was notably decreased from 46.53 to that of 1.39 for mono-digestion of hemicellulose. The percentage of Methanosarcina in acetoclastic methanogens at the stage II of mono-digestion of hemicellulose was largely increased from 1.06% to that of 97.03% for co-digestion of hemicellulose and sludge.

Does sugarcane vinasse composition variability affect the bioenergy yield in anaerobic systems? A dual kinetic-energetic assessment

Abstract The anaerobic processing of sugarcane vinasse has been recently highlighted as an effective approach to both mitigate negative impacts of its inadequate management and recover renewable energy through biogas production. However, basic aspects of the process still require further investigations, such as understanding the impacts of the compositional variability of vinasse on the overall performance of anaerobic systems. In this context, the impacts of different types of sugarcane vinasse, i.e., from annexed (VinAn) and autonomous (VinAu) ethanol plants and spirit production (VinAl), on methane production and organic matter removal were assessed under mesophilic conditions (37 °C) from a kinetic perspective. The energetic potential of the biogas was also investigated regarding micro (methane yield) and macro (contributions to the Brazilian energy matrix) scales. The availability of biodegradable organic matter and sulfate had a direct impact on the kinetics of methane production, associating a 2-fold higher initial methane production rate with VinAl (10.12 NmL h−1) compared to VinAn (5.95 NmL h−1) and VinAu (4.25 NmL h−1). However, the methane yield was not affected by the compositional variations (328–339 NmL-CH4 g−1COD), which indicates that vinasse may be efficiently used as substrate for bioenergy generation, regardless of its origin. The global energy production estimated for VinAn (24,816 GWh) was over 6- and 7-fold higher compared to VinAu (3984 GWh) and VinAl (3035 GWh) due to discrepancies in vinasse generation flow rates, potentially increasing the available renewable power supply in Brazil by 2.2% or over 1400% of the current bioenergy produced from biogas. Overall the electric energetic potential of sugarcane vinasses (14,486 GWh) could replace over 20% of natural gas or 90% of coal use in Brazil, satisfactorily decreasing the dependence on non-renewable energy sources.

Enhancing methane production from lignocellulosic biomass by combined steam-explosion pretreatment and bioaugmentation with cellulolytic bacterium Caldicellulosiruptor bescii

BackgroundBiogas production from lignocellulosic biomass is generally considered to be challenging due to the recalcitrant nature of this biomass. In this study, the recalcitrance of birch was reduced by applying steam-explosion (SE) pretreatment (210 °C and 10 min). Moreover, bioaugmentation with the cellulolytic bacterium Caldicellulosiruptor bescii was applied to possibly enhance the methane production from steam-exploded birch in an anaerobic digestion (AD) process under thermophilic conditions (62 °C).ResultsOverall, the combined SE and bioaugmentation enhanced the methane yield up to 140% compared to untreated birch, while SE alone contributed to the major share of methane enhancement by 118%. The best methane improvement of 140% on day 50 was observed in bottles fed with pretreated birch and bioaugmentation with lower dosages of C. bescii (2 and 5% of inoculum volume). The maximum methane production rate also increased from 4-mL CH4/g VS (volatile solids)/day for untreated birch to 9–14-mL CH4/g VS/day for steam-exploded birch with applied bioaugmentation. Bioaugmentation was particularly effective for increasing the initial methane production rate of the pretreated birch yielding 21–44% more methane than the pretreated birch without applied bioaugmentation. The extent of solubilization of the organic matter was increased by more than twofold when combined SE pretreatment and bioaugmentation was used in comparison with the methane production from untreated birch. The beneficial effects of SE and bioaugmentation on methane yield indicated that biomass recalcitrance and hydrolysis step are the limiting factors for efficient AD of lignocellulosic biomass. Microbial community analysis by 16S rRNA amplicon sequencing showed that the microbial community composition was altered by the pretreatment and bioaugmentation processes. Notably, the enhanced methane production by pretreatment and bioaugmentation was well correlated with the increase in abundance of key bacterial and archaeal communities, particularly the hydrolytic bacterium Caldicoprobacter, several members of syntrophic acetate oxidizing bacteria and the hydrogenotrophic Methanothermobacter.ConclusionOur findings demonstrate the potential of combined SE and bioaugmentation for enhancing methane production from lignocellulosic biomass.

Carbon nanotubes accelerate methane production in pure cultures of methanogens and in a syntrophic coculture.

Carbon materials have been reported to facilitate direct interspecies electron transfer (DIET) between bacteria and methanogens improving methane production in anaerobic processes. In this work, the effect of increasing concentrations of carbon nanotubes (CNT) on the activity of pure cultures of methanogens and on typical fatty acid-degrading syntrophic methanogenic coculture was evaluated. CNT affected methane production by methanogenic cultures, although acceleration was higher for hydrogenotrophic methanogens than for acetoclastic methanogens or syntrophic coculture. Interestingly, the initial methane production rate (IMPR) by Methanobacterium formicicum cultures increased 17 times with 5 g·L-1 CNT. Butyrate conversion to methane by Syntrophomonas wolfei and Methanospirillum hungatei was enhanced (∼1.5 times) in the presence of CNT (5 g·L-1 ), but indications of DIET were not obtained. Increasing CNT concentrations resulted in more negative redox potentials in the anaerobic microcosms. Remarkably, without a reducing agent but in the presence of CNT, the IMPR was higher than in incubations with reducing agent. No growth was observed without reducing agent and without CNT. This finding is important to re-frame discussions and re-interpret data on the role of conductive materials as mediators of DIET in anaerobic communities. It also opens new challenges to improve methane production in engineered methanogenic processes.

Deciphering pretreatment-induced repartition among stratified extracellular biopolymers and its effect on anaerobic biodegradability and dewaterability of waste activated sludge

The effectiveness and optimization strategy of various pretreatment methods to enhance the anaerobic biodegradability and dewaterability of waste activated sludge are still inconclusive. In order to explore the intrinsic triggers determining the apparent biodegradability and dewaterability, sludges pretreated by freeze-thawing, acid, alkali, sonication, thermal autoclaving, and ultraviolet radiation were fractionized according to the cohesion of extracellular polymeric substances (EPS). The fractionized EPS and cells were characterized for microscopic properties by biochemical and elemental composition, Fourier transform infrared spectroscopy, and surface-enhanced Raman spectroscopy. Multivariate statistical analysis was used to mining the key microscopic factors affecting the apparent biodegradability or dewaterability of sludge. Results show that, solubilization degree significantly determined the dewaterability of the pretreated sludge, but did not affect the biodegradability. The properties of slime and lightly-bound EPS were different from other fractions, and inclined to be altered by various pretreatments. However, only the H content, H/C ratio and some functional groups of tightly-bound EPS have significant correlation with the ultimate methane production yield, while the N/C ratio of lightly-bound EPS was important for the initial methane production rate. Free-thawing improved sludge’s biodegradability and dewaterability through dehydration in micro-zones. Autoclaving and ultraviolet weakened sludge’s biodegradability and dewaterability by accelerating the denaturation of some collagen proteins in tightly-bound EPS, transforming it into a viscous gel-like network with higher water-holding capacity. Sonication acted similarly but at a micro-zone level. Acid and alkali pretreatments improved biodegradability by accelerating the release of cell endocytes including pigments carotene and carotenoids.

Effect of humic acids on the activity of pure and mixed methanogenic cultures

The impact of humic acid (HA) on methanogenic activity was investigated. Methanogenic crushed granular sludge and pure cultures of mesophilic methanogens were incubated in batch cultures with HA. Initial methane production rates and substrate consumption rates were quantified. In the presence of 1 kg m−3 HA, the methane production rate of all hydrogenotrophic methanogens was inhibited by more than 75%, except Methanospirillum hungatei that was not inhibited up to 5 kg m−3 HA. The acetoclastic Methanosarcina barkeri was completely inhibited by HA ≥1 kg m−3. However, Methanosaeta concilii was only slightly affected by HA up to 3 kg m−3. When methanogenic granular sludge was incubated with HA, the specific methanogenic activity (SMA) tests showed less inhibition, when compared to the pure cultures of methanogens. The SMA test with H2/CO2, formate and acetate showed reduced initial methane production rate of 42%, 23% and 40%, respectively. Differences in HA susceptibility were explained by differences in cell wall structure.

Effects of acclimated sludge used as seeding material in the start-up of anaerobic digestion of glycerol

Methods for improving the anaerobic digestion of glycerol (propane-1,2,3-triol) were investigated, particularly the effects of using acclimated sludge as seeding material during start-up. Glycerol was supplied to the anaerobic digester at an organic loading rate of 2.5 g-COD L−1 day−1. Four experimental runs were carried out with varying mixing ratios of acclimated sludge to unacclimated sludge (0, 10, 20, and 33%). Calculations were performed by employing a numerical model, whose parameters were determined by experimental measurements. Methane production rate (MPR) for all runs attained similar stable values around 21.4 mmol L−1 day−1, though more time was required for attaining stable state of methane production with lower mixing ratios of acclimated sludge. The initial MPR calculated was proportional to the mixing ratio of acclimated sludge. Furthermore, molecular biological methods showed that the types of microorganisms observed in all runs were similar. These results indicate that the seeding with different mixing ratios of acclimated sludge did not affect the microbial consortia in the anaerobic digestion approaching stable state, but did affect the cell density of the useful microorganisms at the start of methane fermentation. Consequently, it was confirmed that at a higher mixing ratio of acclimated sludge, the start of methane production became more vigorous.

Impact of trace element additives on anaerobic digestion of sewage sludge with in-situ carbon dioxide sequestration

Anaerobic digestion (AD) of sludge at wastewater treatment plants can benefit from addition of essential trace metals such as iron, nickel and cobalt to increase biogas production for utilization in combined heat and power systems, fed into natural gas pipelines or as a vehicle fuel. This study evaluated the impact and benefits of Ni/Co and olivine addition to the digester at mesophilic temperatures. These additions supplement previously reported research in which iron-rich olivine (MgSiO4) was added to sequester CO2 in-situ during batch AD of sludge. Trace element addition has been shown to stimulate and stabilize biogas production and have a synergistic effect on the mineral carbonation process. AD with 5% w/v olivine and 1.5mg/L Ni/Co addition had a 17.3% increase in methane volume, a 6% increase in initial exponential methane production rate and a 56% increase in methane yield (mL CH4/g CODdegraded) compared to the control due to synergistic trace element and olivine addition while maintaining 17.7% CO2 sequestration from olivine addition. Both first-order kinetic modeling and response surface methodology modeling confirmed the combined benefit of the trace elements and olivine addition. These results were significantly higher than previously reported results with olivine addition alone [1] (Linville et al., 2016).

Improving Methane Production During the Anaerobic Digestion of Waste Activated Sludge: Cao-ultrasonic Pretreatment and Using Different Seed Sludges

Three individual seed sludges, which domesticated by filter paper (SS1), food waste (SS2) and grease (SS3), respectively, were used for enhancing the methane production of waste activated sludge (WAS). Also CaO-ultrasonic pretreatment was performed on WAS to evaluate the effectiveness on improving efficient anaerobic digestion (AD). The results showed that WAS being acidated for 24h after CaO-ultrasonic pretreatment was an effective method for increasing initial methane production rate. The daily concentration of volatile fatty acids (VFAs) during the AD course showed that the propionic was easier to be reduced after adding seed sludge. The optimum seed sludge for improving methane production and biodegradability of WAS was SS3, which led to an increase in the methane production of 68.92% and VS reduction of 69.20% higher than the control. This pretreatment combined with adding optimum seed sludge can greatly improve clean energy generation from WAS.

Anaerobic degradation of dairy wastewater in intermittent UASB reactors: influence of effluent recirculation

This work studied the influence of effluent recirculation upon the kinetics of anaerobic degradation of dairy wastewater in the feedless phase of intermittent upflow anaerobic sludge bed (UASB) reactors. Several laboratory-scale tests were performed with different organic loads in closed circuit UASB reactors inoculated with adapted flocculent sludge. The data obtained were used for determination of specific substrate removal rates and specific methane production rates, and adjusted to kinetic models. A high initial substrate removal was observed in all tests due to adsorption of organic matter onto the anaerobic biomass which was not accompanied by biological substrate degradation as measured by methane production. Initial methane production rate was about 45% of initial soluble and colloidal substrate removal rate. This discrepancy between methane production rate and substrate removal rate was observed mainly on the first day of all experiments and was attenuated on the second day, suggesting that the feedless period of intermittent UASB reactors treating dairy wastewater should be longer than one day. Effluent recirculation expressively raised the rate of removal of soluble and colloidal substrate and methane productivity, as compared with results for similar assays in batch reactors without recirculation. The observed bed expansion was due to the biogas production and the application of effluent recirculation led to a sludge bed contraction after all the substrates were degraded. The settleability of the anaerobic sludge improved by the introduction of effluent recirculation this effect being more pronounced for the higher loads.

Monitoring the dynamics of syntrophic β-oxidizing bacteria during anaerobic degradation of oleic acid by quantitative PCR.

The ecophysiology of long-chain fatty acid-degrading syntrophic β-oxidizing bacteria has been poorly understood due to a lack of quantitative abundance data. Here, TaqMan quantitative PCR (qPCR) assays targeting the 16S rRNA gene of the known mesophilic syntrophic β-oxidizing bacterial genera Syntrophomonas and Syntrophus were developed and validated. Microbial community dynamics were followed using qPCR and Illumina-based high-throughput amplicon sequencing in triplicate methanogenic bioreactors subjected to five consecutive batch feedings of oleic acid. With repeated oleic acid feeding, the initial specific methane production rate significantly increased along with the relative abundances of Syntrophomonas and methanogenic archaea in the bioreactor communities. The novel qPCR assays showed that Syntrophomonas increased from 7 to 31% of the bacterial community 16S rRNA gene concentration, whereas that of Syntrophus decreased from 0.02 to less than 0.005%. High-throughput amplicon sequencing also revealed that Syntrophomonas became the dominant genus within the bioreactor microbiomes. These results suggest that increased specific mineralization rates of oleic acid were attributed to quantitative shifts within the microbial communities toward higher abundances of syntrophic β-oxidizing bacteria and methanogenic archaea. The novel qPCR assays targeting syntrophic β-oxidizing bacteria may thus serve as monitoring tools to indicate the fatty acid β-oxidization potential of anaerobic digester communities.

Effect of total solids content on methane and volatile fatty acid production in anaerobic digestion of food waste.

This work investigates the role of the moisture content on anaerobic digestion of food waste, as representative of rapidly biodegradable substrates, analysing the role of volatile fatty acid production on process kinetics. A range of total solids from 4.5% to 19.2% is considered in order to compare methane yields and kinetics of reactors operated under wet to dry conditions. The experimental results show a reduction of the specific final methane yield of 4.3% and 40.8% in semi-dry and dry conditions compared with wet conditions. A decreasing trend of the specific initial methane production rate is observed when increasing the total solids concentration. Because of lack of water, volatile fatty acids accumulation occurs during the first step of the process at semi-dry and dry conditions, which is considered to be responsible for the reduction of process kinetic rates. The total volatile fatty acids concentration and speciation are proposed as indicators of process development at different total solids content.

Impact of low temperature pretreatment on the anaerobic digestion of microalgal biomass

The aim of this study was to investigate the effect of low temperature pretreatment on the anaerobic digestion of microalgal biomass grown in wastewater. To this end, microalgae were pretreated at low temperatures (55, 75 and 95°C) for 5, 10 and 15 h. Biomass solubilisation was enhanced with the pretreatment temperature and exposure time up to 10h. The methane yield was improved by 14%, 53% and 62% at 55, 75 and 95°C, respectively; and was correlated with the solubilisation increase. The pretreatment at 95°C for 10h increased VS solubilisation by 1188%, the initial methane production rate by 90% and final methane yield by 60% compared to untreated microalgae. With diluted biomass (∼1% VS) positive energy balance was not likely to be attained. However, with concentrated biomass (>2% VS) energy requirements may be covered and even surplus energy generated.

Pretreatment of oil palm empty fruit bunch (OPEFB) by N-methylmorpholine-N-oxide (NMMO) for biogas production: Structural changes and digestion improvement

Pretreatment of OPEFB (oil palm empty fruit bunch) by NMMO (N-methylmorpholine-N-oxide) on its subsequent digestions was investigated. The pretreatments were carried out at 90 and 120°C for 1, 3, and 5h in three different modes of dissolution (by 85% NMMO solution), ballooning (79% NMMO solution), and swelling (73% NMMO solution). The total solid recovery after the pretreatment was 89–94%. The pretreatment process did not have a major impact on the composition of OPEFB, other than a reduction of ash from 5.4% up to 1.3%. The best improvement in biogas production was achieved by a dissolution mode pretreatment of OPEFB, using conditions of 85% NMMO, 3h, and 120°C. It resulted in 0.408Nm3/kg VS methane yield and 0.032Nm3CH4/kg VS/day initial methane production rate, which correspond in improving by 48% and 167% compared to the untreated OPEFB, respectively.

Improving methane production during the codigestion of waste-activated sludge and fatty wastewater: Impact of thermo-alkaline pretreatment on batch and semi-continuous processes

With the objective of maximising methane production from waste-activated sludge (WAS), codigestion with fatty wastewater (FW) collected from restaurants was studied and the application of both a 170°C thermal pretreatment and thermo-alkaline/saponification to mixed WAS/FW waste was evaluated. Batch anaerobic digestion tests showed that recalcitrant compounds were formed during the 170°C pretreatment of fatty wastewater and thermo-alkaline pretreatments (80°C or 120°C, pH 8, 9 or 10) led to an increased initial methane production rate and to a very slight impact on the methane potential of the mixed waste (+4–7%). Semi-continuous anaerobic digesters were fed over 4months with WAS, two mixed substrates WAS/FW 90/10 and WAS/FW 60/40, and with WAS/FW 90/10 pretreated at softer conditions (80°C and pH=8, 0.14gKOHgVS-1). This pretreatment led to a significant increase in the methane production of semi-continuous reactors (+58%). Finally, this study showed the feasibility of the codigestion of waste activated sludge with highly concentrated fatty wastewater (40% in volume, 49% in VS and 73% in COD, equivalent to 7gL−1 of lipids). It resulted in specific methane production of 362mLCH4g−1 VS whereas WAS alone produced only 116mLCH4g−1 VS.

Batch and semi-continuous anaerobic digestion of Palmaria palmata: Comparison with Saccharina latissima and inhibition studies

Macroalgae are biomass resources that can represent a valuable feedstock to produce renewable energy via methane production. In order to diversify the feedstock of raw materials for anaerobic digestion new kind of macroalgae are investigated. Composition of the algae Palmaria palmata is suggested to be a good substrate for anaerobic digestion due to its high content in volatile solids and low content of cations. In this study, P. palmata was investigated and compared to Saccharina latissima for biogas production in both batch and semi-continuous reactors. P. palmata permitted a better production of methane than for S. latissima either in batch (279mLCH4gVS−1 vs 210mLCH4gVS−1) or in semi-continuous reactor (320mLCH4gVS−1 vs 270mLCH4gVS−1). In semi-continuous experiments, after 182days of operation, a decrease in reactor performances such as methane yields occurred above a specific loading rate in the order of 408gVSkgVSS−1d−1. Inhibition by potassium and sodium observed in semi-continuous mode was confirmed by batch experiments. The initial methane production rate was reduced by 53% when potassium and sodium concentrations were equivalent to the ones observed at semi-continuous reactor failure (13.0gL−1 for potassium and 4.1gL−1 for sodium). The lower performances of semi-continuous anaerobic digestion of S. latissima than the ones obtained with P. palmata, may be increased by a higher accumulation of cations.

Biogenic methane potential for Surat Basin, Queensland coal seams

Produced water samples collected from Surat Basin coal seams in eastern Queensland, Australia were shown to contain viable microbial consortia with the ability to convert native Walloon coal into methane. Methane generation rates of up to 1.0 m3/t coal/day (30 scf/ton/day) were observed, with overall yields up to 6.5 m3/t coal (210 scf/ton); in comparison, total methane reserves for the Surat Basin are typically 4 to 8 m3/t. This is the first direct evidence of real-time biogenic coal-to-methane potential for an Australian coal seam sample. Six of the eight Surat Basin water samples tested positive for biomethane production when H2–CO2 was provided as the sole methanogenic substrate; five produced methane from methanol, but acetoclastic methanogenesis was not observed. An active H2–CO2 pathway could have implications for the conversion of sequestered CO2, if an in-situ source of reduced hydrogen is present in excess. In a simple experiment, biomethane production from a Walloon coal was not enhanced when the bicarbonate concentration in the medium was doubled. It is anticipated that the low surface area/solubility of coals may limit their bioavailability; coal-to-methane rates and final yields increased in proportion to the coal particle surface area. Similarly, when a Zonyl FSN surfactant was added to improve coal bioavailability, the initial methane production rate increased by 240%, and the final methane yield increased by 180% in comparison to a no-surfactant control. Overall, these results suggest an opportunity to enhance Surat Basin coalbed methane reserves via in-situ stimulation of indigenous microbial consortia.