In situ methane enrichment in anaerobic digestion

A major cost consideration in the use of anaerobic digestion to convert biomass and waste to utility-grade gas is the expense of separating CO(2) from the product gas. Anaerobic digestion has a number of inherent properties that can be exploited to increase the methane content of the gas directly produced by the digester, the most important of which is the high solubility of CO(2)(40-60 times that of methane) in water under digestion conditions. The methane enrichment concept examined in this study involved the recirculation of a liquid stream from the digester through a CO(2) desorption process and the return of the liquid stream back to the digester for absorption of additional CO(2) produced by the conversion of organic materials. A steady-state equilibrium model predicted that a digester gas methane content exceeding 94% could be achieved with this scheme using modest recirculation rates provided a desorption process could be designed to achieve a 60+% CO(2) removal efficiency in the degassing of the liquid recycle stream. Using fixed-film laboratory digesters operated on synthetic feedstocks, the technique of methane enrichment was tested under pressurized and unpressurized conditions. A 93 + 2% methane gas stream was produced from a volatile-acid-fed bench-scale digester simulating the methanogenic stage of two-phase digestion under conditions of (1) a pH swing achieved without caustic addition that allowed digestion at pH 7. 5 and air stripping at pH 6. 5-7. 0, (2) digester pressurization to 30 psig, and (3) a recycle rate of 0. 33 L/L reactor/day. Significant but lower levels of methane enrichment were achieved with the single-stage digester at the low experimental recycle rate. However, the narrow range among all experiments of CO(2) desorption efficiencies achieved in air stripping the recycle stream (35-60% CO(2) removal) suggests that comparable methane enrichment-may be achieved with unpressurized single-stage digestion using greater recycle rates. A materials balance analysis of data from an unpressurized, single-stage digester employing no chemical addition and using laboratory degassing efficiencies indicated that 94% methane could be produced at recycle rates of less than 1. 4 L/L reactor/day with a methane loss of less than 2%.     

Analysis of the performance of an anaerobic digestion system at the Regina Wastewater Treatment Plant

From the performance analysis of the anaerobic digestion system at the Regina Wastewater Treatment Plant, it was found that the anaerobic digestion system at the Regina plant was generally operated in a stable condition as indicated by pH, volatile acids and alkalinity levels. The operation of the anaerobic digestion system was not optimal because of the low volatile solids concentration and low volatile solids loading rate, especially because of high HRT. Two options, thickening the primary sludge and increasing the volatile solids loading rate, were recommended for the optimal operation of the digestion system. After examining a number of kinetic models, it was found that the Chen-Hashimoto model could be used to predict the volumetric methane production rate and the first-order model could be used to predict the efficiency of volatile solids reduction. The study showed that utilization of digester gas for power production was the best alternative for the excess digester gas. 13.3% of the electrical demand and 35.5% of the plant's total energy could be met based on digester gas wasted, assuming 25% as the conversion efficiency.     

Application of the IWA ADM1 model to simulate anaerobic co-digestion of organic waste with waste activated sludge in mesophilic condition

Anaerobic digestion model no. 1 model of international water association was applied to a full scale anaerobic co-digestion process for the treatment of the organic fraction of municipal solid wastes along with activated sludge wastes originating from a municipal wastewater treatment plant. This operation was carried out in a digester of 2000 m(3) in volume. It is operates at an average hydraulic retention time of 26.9 days with an average organic loading rate of 1.01 kg TVS/m(3) day, at a temperature of 37 degrees C with an average gas production rate of 0.296 m(3)/m(3) day. The aim of the present study is to compare the results obtained from the simulation with the experimental values. The simulated results showed a good fit for pH, methane and carbon dioxide percentages, biogas volume, chemical oxygen demand, total volatile fatty acids, inorganic nitrogen and inorganic carbon.     

Mathematical modelling of the anaerobic digestion process: Application of dynamic mass-energy balance

The method of mass and energy balance was used in the design of a dynamic model of anaerobic digestion of complex organic substrates with production of methane. Distribution of mass flow, represented by the most abundant elements (C, H, N, O), and energy flow, represented by redoxons (available electrons), into gas and liquid output streams is influenced by environmental conditions in a continuous flow digester. Two pathways of methane generation,via cleavage of acetate andvia carbon dioxide reduction by hydrogen, are described in the model. The model was compared with experimental data from laboratory and pilot-plant experiments     

Conversion of glucose to fatty acids and methane: roles of two mycoplasmal agents.

Two species of obligately anaerobic mycoplasmas were the major components of a methanogenic glucose-limited enrichment culture. In pure culture, one of these organisms, tentatively named Anaeroplasma sp. strain London, was shown to be responsible for the fermentation of glucose to fatty acids, hydrogen, and carbon dioxide; the other mycoplasma was shown to produce methane from hydrogen and carbon dioxide and was named Methanoplasma elizabethii. This same methanogenic mycoplasma contained a low-molecular-weight fluorescent cofactor which had a maximum light absorbance at 430 nm. When both species of mycoplasmas were grown together on glucose, fermentation products included fatty acids and methane. For the first time, mycoplasmas are implicated as agents of anaerobic degradation and methanogenesis in a sewage sludge digester.     

Hydrogenase activity and methanogenesis in anaerobic sewage sludge,in rumen liquid,and in freshwater sediments

Summary The applicability of hydrogenase determinations to the evaluation of hydrogen transfer reactions occurring within methanogenic environments was investigated. Enzymatic hydrogen production was determined in digester sludge, river sediments, and rumen liquid using reduced methyl viologen, formate, and pyruvate as hydrogen donors. Hydrogenase determinations turned out not to be inhibited by toxic compounds present in sediments of the polluted river Saar. Comparative kinetic studies of the conversion of acetate and of hydrogen to methane support the assumption that carbon dioxide reduction by hydrogen accounts for the major part of methane formed in river sediments. In rumen liquid and in river sediments similar enzyme patterns were observed which were different from that found in digester sludge. The rates of methanogenesis correlated well with hydrogenase activities in all ecosystems studied: Correlation coefficients ranged from 0.84 to 0.95. Rumen liquid and river sediments exhibited higher hydrogenase activities than digester sludge when compared at identical rates of methane production. According to these results, the hydrogenase determination is applicable to the evaluation of the hydrogen transfer, occurring within the microbial biomass of anaerobic ecosystems.     

Anaerobic Methane Oxidation: Occurrence and Ecology

Anoxic sediments and digested sewage sludge anaerobically oxidized methane to carbon dioxide while producing methane. This strictly anaerobic process showed a temperature optimum between 25 and 37°C, indicating an active microbial participation in this reaction. Methane oxidation in these anaerobic habitats was inhibited by oxygen. The rate of the oxidation followed the rate of methane production. The observed anoxic methane oxidation in Lake Mendota and digested sewage sludge was more sensitive to 2-bromoethanesulfonic acid than the simultaneous methane formation. Sulfate diminished methane formation as well as methane oxidation. However, in the presence of iron and sulfate the ratio of methane oxidized to methane formed increased markedly. Manganese dioxide and higher partial pressures of methane also stimulated the oxidation. The rate of methane oxidation in untreated samples was approximately 2% of the CH₄ production rate in Lake Mendota sediments and 8% of that in digested sludge. This percentage could be increased up to 90% in sludge in the presence of 10 mM ferrous sulfate and at a partial pressure of methane of 20 atm (2,027 kPa).     

Rumen methanogens: a review

The Methanogens are a diverse group of organisms found in anaerobic environments such as anaerobic sludge digester, wet wood of trees, sewage, rumen, black mud, black sea sediments, etc which utilize carbon dioxide and hydrogen and produce methane. They are nutritionally fastidious anaerobes with the redox potential below −300 mV and usually grow at pH range of 6.0–8.0 [1]. Substrates utilized for growth and methane production include hydrogen, formate, methanol, methylamine, acetate, etc. They metabolize only restricted range of substrates and are poorly characterized with respect to other metabolic, biochemical and molecular properties.     

Development of an efficient process for the treatment of residual sludge discharged from an anaerobic digester in a sewage treatment plant

In order to reduce the discharge of residual sludge from an anaerobic digester, pre-treatment methods including low-pressure wet-oxidation, Fenton oxidation, alkali treatment, ozone oxidation, mechanical destruction and enzymatic treatment were evaluated and compared. VSS removal efficiencies of greater than 50% were achieved in cases of low-pressure wet-oxidation, Fenton oxidation and alkali treatment. Residual sludge from an anaerobic digester was pre-treated and subjected to thermophilic anaerobic digestion. As a result, the process of low-pressure wet-oxidation followed by anaerobic digestion achieved the highest VSS removal efficiency of 83%. The total efficiency of VSS removal of sewage sludge consisting of primary and surplus sludge would be approximately 92%, assuming that the VSS removal efficiency of sewage sludge is 50% in the anaerobic digester of the sewage treatment plant.     

Anaerobic digestion of the vegetable fraction of municipal refuses: mesophilic versus thermophilic conditions

The phenomena limiting the anaerobic digestion of vegetable refuses are studied through batch tests carried out using anaerobic sludge previously selected under either mesophilic (37v°C) or thermophilic (55v°C) conditions. The compositions of the hydrolysed cellulosic and hemicellulosic fractions of these materials are simulated by starch and hemicellulose hydrolysates, respectively. Non-hydrolysed mixtures of vegetable waste with sewage sludge are used to ascertain whether the hydrolysis of these polymeric materials is the limiting step of the digestion process or not. The experimental data of methane production are then worked out by a first-order equation derived from the Monod's model to estimate the kinetic rate constant and methane production yield for each material. Comparison of these results shows that passing from mesophilic to thermophilic conditions is responsible for a slight deceleration of methane production but remarkably enhances both methanation yield and methane content of biogas. The final part of the study deals with the fed-batch digestion of the same residues in static digester. Working under thermophilic conditions at a loading rate threshold of 6.0 g_COD/l · d, the hemicellulose hydrolysate ensures the highest methane productivity (60 mmol_CH4/l · d) and methane content of biogas (60%), while unbalance towards the acidogenic phase takes place under the same conditions for the starch hydrolysate. The intermediate behaviour of the non-hydrolysed mixture of vegetable waste with sewage sludge demonstrates that hemicellulose hydrolysis is the limiting step of digestion and suggests the occurrence of ligninic by products inhibition on methane productivity.     

Modeling anaerobic digestion of microalgae using ADM1

The coupling between a microalgal pond and an anaerobic digester is a promising alternative for sustainable energy production by transforming carbon dioxide into methane using solar energy. In this paper, we demonstrate the ability of the original ADM1 model and a modified version (based on Contois kinetics for the hydrolysis steps) to represent microalgae anaerobic digestion. Simulations were compared to experimental data of an anaerobic digester fed with Chlorella vulgaris. The modified ADM1 fits adequately the data for the considered 140 day experiment encompassing a variety of influent load and flow rates. It turns out to be a reliable predictive tool for optimising the coupling of microalgae with anaerobic digestion processes.     

Methanogenic Degradation of Poly(3-Hydroxyalkanoates)

Poly(3-hydroxybutyrate) and the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) were fermented to methane and carbon dioxide within 16 days by an anaerobic sewage sludge consortium. The cultures adapted quickly to metabolize these polymeric compounds, and between 83 and 96% of the substrate carbon was transformed to methane and carbon dioxide.     

Dual anaerobic co-digestion of sewage sludge and confectionery waste

Three configurations for a dual digestion system were examined. The units were based on three 5 l completely stirred tank reactors (CSTR). A first-stage thermophilic digester was used to provide the feed to each of the two second-stage mesophilic (35°C) digesters. Using a mixture of sewage sludge and strong confectionery waste, the thermophilic digester was operated at 55°C with a hydraulic retention time of 4 h. The mesophilic digesters were operated at hydraulic retention times of 8, 12 and 15 days. In terms of the reduction of volatile solids (VS), the three dual digestion configurations were similar but were more effective than the single-stage reactor which was used as a control. However, based on the specific methane yield (m³ CH₄/kg VS removed), the configuration with a first stage operating at 55°C and a secondary digester at 35°C with a hydraulic retention time of 12 days was the most effective. This configuration also maintained a more stable pH, irrespective of the quality of the feed sludge.     

Two-phase anaerobic digestion for production of hydrogen-methane mixtures

An anaerobic digestion process to produce hydrogen and methane in two sequential stages was investigated, using two bioreactors of 2 and 15 L working volume, respectively. This relative volume ratio (and shorter retention time in the second, CH(4)-producing reactor) was selected, in part, to test the assumption that separation of phase can enhance metabolism in the second methane producing reactor. The reactor system was seeded with conventional anaerobic digester sludge, fed with a glucose-yeast extract--peptone medium and operated under conditions of relatively low mixing, to simulate full scale operation. A total of nine steady states were investigated, spanning a range of feed concentrations, dilution rates, feed carbon to nitrogen ratios and degree of integration of the two stages. The performance of this two-stage process and potential practical applications for the production of clean-burning hydrogen-methane mixtures are discussed.     

Liquid-to-Gas Mass Transfer in Anaerobic Processes: Inevitable Transfer Limitations of Methane and Hydrogen in the Biomethanation Process

Liquid-to-gas mass transfer in anaerobic processes was investigated theoretically and experimentally. By using the classical definition of k(L)a, the global volumetric mass transfer coefficient, theoretical development of mass balances in such processes demonstrates that the mass transfer of highly soluble gases is not limited in the usual conditions occurring in anaerobic fermentors (low-intensity mixing). Conversely, the limitation is important for poorly soluble gases, such as methane and hydrogen. The latter could be overconcentrated to as much as 80 times the value at thermodynamic equilibrium. Such overconcentrations bring into question the biological interpretations that have been deduced solely from gaseous measurements. Experimental results obtained in three different methanogenic reactors for a wide range of conditions of mixing and gas production confirmed the general existence of low mass transfer coefficients and consequently of large overconcentrations of dissolved methane and hydrogen (up to 12 and 70 times the equilibrium values, respectively). Hydrogen mass transfer coefficients were obtained from the direct measurements of dissolved and gaseous concentrations, while carbon dioxide coefficients were calculated from gas phase composition and calculation of related dissolved concentration. Methane transfer coefficients were based on calculations from the carbon dioxide coefficients. From mass balances performed on a gas bubble during its simulated growth and ascent to the surface of the liquid, the methane and carbon dioxide contents in the gas bubble appeared to be controlled by the bubble growth process, while the bubble ascent was largely responsible for a slight enrichment in hydrogen.     

Anaerobic and complementary treatment of domestic sewage in regions with hot climates--a review

This study presents a literature review on the treatment of domestic sewage in controlled environments having the anaerobic process and specifically the upflow anaerobic sludge blanket (UASB) concept as the core, under natural hot conditions. The UASB process application is however beset by the preponderance of suspended solids, and the paper looks at its optimization via pre- and post-treatments to curb the prevailing problems, in the light of possible discharge and re-use/recycling/resource recovery, leading to efficient environmental protection. Pre-treatment clarification could be done with ferric chloride/polyelectrolyte, so that phosphate precipitates during the process. The pre-treated liquid phase can be submitted to a high rate anaerobic process, using the simple and robust UASB technology. In a subsequent post-treatment step, ammonium can be removed by ion exchange using a zeolite column through which the wastewater percolates after leaving the anaerobic digester. The various stages can also eliminate a large fraction of the pathogens present in the raw wastewater, mainly through the pre-treatment sedimentation and the ion exchange filtration. The sludge produced in the precipitation stage can be stabilized in a conventional anaerobic digester. Integration of the different treatment steps provides a sustainable technology to treat domestic sewage under hot climate conditions.     

Effects of nickel and lead and a support material on the methanogenesis from sewage sludge

The effects of the addition of two heavy metals (nickel and lead) and a support material (purified sepiolite) on the methanogenesis have been evaluated in two types of domestic sewage sludges. A higher toxic effect of the metals was observed on the production of methane from loading sludge than from the anaerobic digester sludge, nickel being more toxic than lead in all cases studied. Antagonistic effects between both heavy metals were obtained when the loading sludge was supplemented with several concentrations of these metals. The addition of sepiolite to the loading sludge reduced toxic effects of both metals, which contrasts with the results obtained using sludges from anaerobic digester.     

Fermentation of cellulose and fatty acids with enrichments from sewage sludge

Summary A mixed culture enriched from sewage sludge and anaerobic digestor effluent was able to degrade cellulose and acetate rapidly and quantitatively to methane and carbon dioxide. The maximum specific rate of gas production was 87 ml/gm cell-h, corresponding to a rate of cellulose utilization of 0.1 g/g cells-h. Acetate, an intermediate in cellulose degradation, was fermented much more rapidly than butyrate or propionate; its maximum utilization rate was first order with a rate constant of 0.34 h^–1. Addition of 2-¹⁴C-acetate to a digestor fed cellulose showed tht 2% of the methyl groups were oxidized to carbon dioxide. When 1-¹⁴C-acetate was added to a similar digestor, 51% of the carboxyl groups were reduced to methane, suggesting that not all the carbon dioxide during simultaneous cellulose and acetate utilization is treated equally. The pulse addition of large amounts of acetate, propionate and butyrate to a cellulose fed digestor was also examined.     

Complete Genome Sequence of the Hydrogenotrophic, Methanogenic Archaeon Methanoculleus bourgensis Strain MS2T, Isolated from a Sewage Sludge Digester

Methanoculleus bourgensis, of the order Methanomicrobiales, is a dominant methanogenic archaeon in many biogas-producing reactor systems fed with renewable primary products. It is capable of synthesizing methane via the hydrogenotrophic pathway utilizing hydrogen and carbon dioxide or formate as the substrates. Here we report the complete and finished genome sequence of M. bourgensis strain MS2(T), isolated from a sewage sludge digester.