Anaerobic digestion of grass silage in batch leach bed processes for methane production

Anaerobic digestion of grass silage in batch leach bed reactors, with and without a second stage upflow anaerobic sludge blanket (UASB) reactor, was evaluated. Sixty six percent of the methane potential in grass was obtained within the 55 days solids retention time in the leach bed-UASB process without pH adjustment, whereas in the one-stage leach bed process 20% of the methane potential in grass was extracted. In two-stage operation, adjustment of the pH of influent to the leach bed reactor to 6 with HCl led to inhibition of both hydrolysis/acidogenesis and methanogenesis. In the leach bed-UASB process 39% of the carbohydrates and 58% of the acid soluble lignin were solubilised within the 49 days of operation, whereas Klason lignin was most recalcitrant. The methane potential of the digestates varied from 0.141 to 0.204 m3 CH4 kg(-1) added volatile solids.     

The effect of enzyme addition on anaerobic digestion of JoseTall Wheat Grass

The effects of the addition of enzyme products containing cellulase, hemicellulase, and β-glucosidase to anaerobic digestion systems were studied using JoseTall Wheat Grass (wheat grass) as a model substrate. Anaerobic digestion tests were performed using batch reactors operated at 50 °C. The application of enzyme products in three digestion configurations were simulated and investigated: (1) enzyme addition to a single-stage digester, (2) pre-treatment of wheat grass with enzymes followed by a single-stage anaerobic digestion, and (3) enzyme addition to the first stage (hydrolysis and acidification) of a two-stage digestion system. The enzyme products showed positive effects on the solubilization of wheat grass when used alone to treat the wheat grass. However, no significant differences in biogas and methane yields, and volatile solids reduction resulted when the enzyme products were tested in the anaerobic digestion systems. This reveals that the microorganisms present in the inoculum were effective in carrying out the digestion of wheat grass. The types of microorganisms present in the inoculum were identified using 16S rRNA sequence analysis. A comparison of the sequences between the different inocula revealed that the prevalent operational taxonomic units were similar, but that the acidified inoculum contained a higher percentage of the species Thermotogae.     

Two-stage anaerobic digestion of tomato, cucumber, common reed and grass silage in leach-bed reactors and upflow anaerobic sludge blanket reactors

Anaerobic digestion of tomato, cucumber, common reed and grass silage was studied in four separate two-stage reactor configuration consisting of leach bed reactor (LBR) and upflow anaerobic sludge blanket reactor (UASB). LBR studies showed that COD solubilization for cucumber and grass silage was higher (50%) than tomato (35%) and common reed (15%). Results also showed that 31-39% of initial TKN present in tomato and cucumber was solubilized in the leachates and 47-54% of the solubilized TKN was converted to NH₄-N. The corresponding values for common reed and grass silage were 38-50% and 18-36%, respectively. Biomethanation of the leachates in UASB reactors resulted in methane yields of 0.03-0.14 m³ CH₄ kg⁻¹VS_fed for the studied crop materials. Thus, high COD solubilization, high nitrogen mineralization and solubilization rates were feasible during anaerobic digestion of lignocellulosic materials in a two-stage LBR-UASB reactor system.     

Prediction of methane yield at optimum pH for anaerobic digestion of organic fraction of municipal solid waste

A concept of methane yield at optimum pH was advanced and subsequently a mathematical model that simulates the optimal pH of a batch process for anaerobic digestion of organic fraction of municipal solid waste (MSW) was developed and validated. The model was developed on the basis of the microbial growth kinetics and was divided into three processes: hydrolysis of substrates by hydrolytic bacteria, consumption of soluble substrate by acidogenic bacteria, and finally consumption of acetate and methane generated by methanogenic bacteria. Material balance and liquid phase equilibrium chemistry were used in this study. A series of experiments were conducted to validate the model. The model simulation results agreed reasonably with experimental data in different temperatures and total solid (TS) concentrations under uncontrolled pH. A computer circulation program was used to predict the optimal pH in different conditions. Experiments in different temperatures and TS were run under optimal pH which predicted by the model. The model was succeeded in increasing the methane production and the cumulative methane production had an average increment about 35% in optimal pH of different temperatures and TS.     

Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion

This work studied the hydrolysis kinetics and the solubilization of waste activated sludge under a medium range temperature (50-90 degrees C) and pH in the alkaline region (8-11), as a pretreatment stage for anaerobic digestion. The hydrolysis rate for the solubilization of volatile suspended solids (VSS) followed a first-order rate. A linear polynomial hydrolysis model was derived from the experimental results leading to a satisfactory correlation between the hydrolysis rate coefficient, pH, and temperature. At pH 11 and a temperature of 90 degrees C the concentration of the VSS was 6.82%, the VSS reduction reached 45% within ten hours and at the same time the soluble COD was 70.000 mg/l and the total efficiency for methane production 0.28 l of CH4 per g of VSS loading.     

Bioaugmentation of a two-stage thermophilic (68 degrees C/55 degrees C) anaerobic digestion concept for improvement of the methane yield from cattle manure

The possibility of improving a two-stage (68 degrees C/55 degrees C) anaerobic digestion concept for treatment of cattle manure was studied. In batch experiments, a 10-24% increase of the specific methane yield from cattle manure and its fractions was obtained, when the substrates were inoculated with bacteria of the genus Caldicellusiruptor and Dictyoglomus. In a reactor experiment inoculation of a 68 degrees C pretreatment reactor with Caldicellusiruptor resulted in a 93% increase in the methane yield of the pretreatment reactor for a period of 18 days, but gave only a slight increase in the overall methane yield of the two-stage setup.     

Effects of different nitrogen sources on the biogas production - a lab-scale investigation

For anaerobic digestion processes nitrogen sources are poorly investigated although they are known as possible process limiting factors (in the hydrolysis phase) but also as a source for fermentations for subsequent methane production by methanogenic archaea. In the present study different complex and defined nitrogen sources were investigated in a lab-scale experiment in order to study their potential to build up methane. The outcome of the study can be summarised as follows: from complex nitrogen sources yeast extract and casamino acids showed the highest methane production with approximately 600ml methane per mole of nitrogen, whereas by the use of skim milk no methane production could be observed. From defined nitrogen sources l-arginine showed the highest methane production with almost 1400ml methane per mole of nitrogen. Moreover it could be demonstrated that the carbon content and therefore C/N-ratio has only minor influence for the methane production from the used substrates.     

Biogas production and microbial community change during the Co-digestion of food waste with chinese silver grass in a single-stage anaerobic reactor

Chinese silver grass (CSG), a potential subtropical energy crop, was investigated as a co-substrate to enhance the anaerobic digestion of food waste for municipal solid waste treatment. Results showed that 88.1% of food wastes were degraded using CSG as a co-substrate with 45 days of digestion, where the food waste, CSG, and sludge on VS/TS/working volume was 93.14 g/111.55 g/1 L, in which the average biogas production was at 429.3 L/kg solids, and the average methane content was around 60%. During the digestion, the concentrations of ammonium and free ammonia gradually increased to 1448.2 and 265.2 mg/L respectively, without any significant inhibitory effects on biogas production, which is probably due to the buffering effects of CSG. Microbial community analysis showed that microorganisms from the class of Firmicutes and Bacteroidetes were dominant during digestion, and that the microbial community diversity increased with active methanogenesis, suggesting that the addition of substrates contribute to the increase of microbial diversity, and could be beneficial for biogas production. Therefore, using CSG as a co-substrate in the single-stage food waste anaerobic digestion system is a potential simple method to convert CSG into renewable energy and to simultaneously improve food waste treatment.     

Long-Term Enrichment on Cellulose or Xylan Causes Functional and Taxonomic Convergence of Microbial Communities from Anaerobic Digesters

Cellulose and xylan are two major components of lignocellulosic biomass, which represents a potentially important energy source, as it is abundant and can be converted to methane by microbial action. However, it is recalcitrant to hydrolysis, and the establishment of a complete anaerobic digestion system requires a specific repertoire of microbial functions. In this study, we maintained 2-year enrichment cultures of anaerobic digestion sludge amended with cellulose or xylan to investigate whether a cellulose- or xylan-digesting microbial system could be assembled from sludge previously used to treat neither of them. While efficient methane-producing communities developed under mesophilic (35°C) incubation, they did not under thermophilic (55°C) conditions. Illumina amplicon sequencing results of the archaeal and bacterial 16S rRNA genes revealed that the mature cultures were much lower in richness than the inocula and were dominated by single archaeal (genus Methanobacterium) and bacterial (order Clostridiales) groups, although at finer taxonomic levels the bacteria were differentiated by substrates. Methanogenesis was primarily via the hydrogenotrophic pathway under all conditions, although the identity and growth requirements of syntrophic acetate-oxidizing bacteria were unclear. Incubation conditions (substrate and temperature) had a much greater effect than inoculum source in shaping the mature microbial community, although analysis based on unweighted UniFrac distance found that the inoculum still determined the pool from which microbes could be enriched. Overall, this study confirmed that anaerobic digestion sludge treating nonlignocellulosic material is a potential source of microbial cellulose- and xylan-digesting functions given appropriate enrichment conditions.     

High-Rate two-phase process for the anaerobic degradation of cellulose, employing rumen microorganisms for an efficient acidogenesis

A novel two-stage anaerobic process for the microbial conversion of cellulose into biogas has been developed. In the first phase, a mixed population of rumen bacteria and ciliates was used in the hydrolysis and fermentation of cellulose. The volatile fatty acids (VFA) produced in this acidogenic reactor were subsequently converted into biogas in a UASB-type methanogenic reactor.A stepwise increase of the loading rate from 11.9 to 25.8 g volatile solids/L reactor volume/day (g VS/L/day) did not affect the degradation efficiency in the acidogenic reactor, whereas the methanogenic reactor appeared to be overloaded at the highest loading rate. Cellulose digestion was almost complete at all loading rates applied. The two-stage anaerobic process was also tested with a closed fluid circuit. In this instance total methane production was 0.438 L CH(4)g VS added, which is equivalent to 98% of the theoretical value. The application of rumen microorganisms in combination with a high-rate methane reactor is proposed as a means of efficient anaerobic degradation of cellulosic residues to methane. Because this newly developed two-phase system is based on processes and microorganisms from the ruminant, it will be referred to as "Rumen Derived Anaerobic Digestion" (RUDAD-) process.     

Cellulose hydrolysis by a methanogenic culture enriched from landfill waste in a semi-continuous reactor

This study investigates the hydrolysis of cellulose by a mixed culture enriched from landfill waste in a continuous reactor operated under prolonged residence times to accommodate methanogenic conditions. Chemostat studies of hydrolysis under balance methanogenic conditions are rarely reported, despite the importance of hydrolysis under these conditions in waste management and renewable energy industries. Continuous digestion was studied in a 1.25l digester, fed with a 1% (w/v) slurry of 50mum cellulose in sterilized leachate drawn from a 220l digester operated on a feedstock of mixed municipal solid waste. Unsterilized leachate was used as the inoculum. Stable and rapid hydrolytic conditions were established at residence time of 2.5, 3.5 and 5d with a 1st order hydrolysis rate 0.45+/-0.07d(-1) and high methane yields ranging from 57% to 62% of solubilised cellulose on a COD basis. Biomass yields were between 32% and 35% of solubilised cellulose on a COD basis, over three times that observed with fermentative cultures. This is attributed to the diversity of the microbial population which fully converted solubilised COD to methane, as evident by VFA yields of less than 8% on a COD basis.     

Plasmalogens of microbial communities associated with barley straw and clover in the rumen

Abstract: Microbial plasmalogen aldehydes (detected as dimethyl acetals, DMA) have been used to compare microbial populations associated with clover and barley straw incubated in nylon mesh bags in the rumen of a cow. The results suggest that the populations involved in the digestion of these substrates differ substantially and that population changes occur as digestion proceeds: these conclusions were supported by electron-microscopic observations. Analysis of DMA suggested that populations associated with the particles of straw and clover differed more markedly than the corresponding populations in the liquid phase. When straw was pre-incubated with the rumen cellulolytic bacterium Ruminococcus flavefaciens strain 17, the DMA characteristic of this bacterium were present at increased levels during subsequent incubation of the straw in the rumen, though the R. flavefaciens DMA tended to contribute a smaller proportion of the total DMA as the incubation time in the rumen was increased from 24 to 72h.     

Modeling solid waste decomposition

The hydrolysis rate coefficients of sorted municipal waste were evaluated from the biochemical methane potential tests using non-linear regression. A distributed mathematical model of anaerobic digestion of rich (food) and lean (non-food) solid wastes with greatly different rates of polymer hydrolysis/acidogenesis was developed to describe the balance between the rates of hydrolysis/acidogenesis and methanogenesis. The model was calibrated using previously published experimental data [Biores. Technol. 52 (1995) 245] obtained upon various initial food waste loadings. Simulations of one- and two-stage digestion systems were carried out. The results showed that initial spatial separation of food waste and inoculum enhances methane production and waste degradation in a one-stage solid-bed digester at high waste loading. A negative effect of vigorously mixing at high waste loading reported in some papers was discussed. It was hypothesized that the initiation methanogenic centers developing in time and expanding in space under minimal mixing conditions might be a key factor for efficient anaerobic conversion of solid waste into methane.     

Ruminal large and small particle kinetics in dairy cows fed red clover and grass silages harvested at two stages of growth

Passage, comminution and digestion rates of large and small particles were estimated using a rumen evacuation technique and total faecal collection with five lactating dairy cows in a 5 × 5 Latin square experiment. Two grass and two red clover silages harvested at early and late primary growth stages and a 1:1 mixture of late harvest grass and early harvest red clover were the dietary treatments. Cows received 9.0 kg supplementary concentrate per day. Ruminal contents and faeces were divided into large (>1.25 mm) and small (1.25–0.038 mm) particles by wet sieving. Indigestible neutral detergent fibre (iNDF) was determined by 12 days ruminal in situ incubation followed by neutral detergent extraction. Plant species did not affect ruminal particle size distribution, whereas advancing forage maturity decreased the proportion of large particles for both grass and red clover silage diets. Ruminal pool size of iNDF was higher (P<0.001) with red clover compared to grass silage diets. Ruminal passage rates of iNDF and potentially digestible NDF (pdNDF) increased with decreasing particle size (P<0.01). Passage rate of iNDF for small particles was slower (P<0.01) when red clover compared to grass silage diets were fed. Particle comminution rate in the rumen was slower (P<0.001) with red clover compared to grass silage diets and it increased (P<0.01) with advancing forage maturity. The contribution of particle comminution to ruminal mean retention time of iNDF in the ruminal large particle pool was smaller (P<0.01) in red clover compared to grass silage diets and it increased (P<0.05) with the mixed silage compared to the separate silages. Passage rate of pdNDF for both large and small particles was not affected by dietary treatments. Digestion rate of pdNDF for large particles was faster (P<0.001) with red clover compared to grass silage diets. Differences in ruminal passage and digestion rates of the large and small particles, in addition to differences in the passage and digestion rates of red clover compared to grass silage diets, emphasize the need to consider particle size and forage type in metabolic models predicting feed intake and fibre digestibility in ruminants.     

Effect of Increasing Total Solids Contents on Anaerobic Digestion of Food Waste under Mesophilic Conditions: Performance and Microbial Characteristics Analysis

The total solids content of feedstocks affects the performances of anaerobic digestion and the change of total solids content will lead the change of microbial morphology in systems. In order to increase the efficiency of anaerobic digestion, it is necessary to understand the role of the total solids content on the behavior of the microbial communities involved in anaerobic digestion of organic matter from wet to dry technology. The performances of mesophilic anaerobic digestion of food waste with different total solids contents from 5% to 20% were compared and the microbial communities in reactors were investigated using 454 pyrosequencing technology. Three stable anaerobic digestion processes were achieved for food waste biodegradation and methane generation. Better performances mainly including volatile solids reduction and methane yield were obtained in the reactors with higher total solids content. Pyrosequencing results revealed significant shifts in bacterial community with increasing total solids contents. The proportion of phylum Chloroflexi decreased obviously with increasing total solids contents while other functional bacteria showed increasing trend. Methanosarcina absolutely dominated in archaeal communities in three reactors and the relative abundance of this group showed increasing trend with increasing total solids contents. These results revealed the effects of the total solids content on the performance parameters and the behavior of the microbial communities involved in the anaerobic digestion of food waste from wet to dry technologies.     

秸秆蓝藻混合发酵产沼气过程中微生物群落分析

采用实验室自制秸秆蓝藻混合厌氧反应装置进行沼气发酵实验,利用16S rRNA基因克隆文库的方法,对不同产气阶段的细菌和古菌的优势菌群进行多样性研究。结果表明:(1)不同产气阶段细菌优势种类存在差异,供试秸秆沼气反应器阶段细菌种类较为丰富,分属于6个门:产气初始阶段优势菌群为变形菌门(Proteobacteria),相对丰度为51.76%;产气高峰阶段优势菌群为厚壁菌门(Firmicutes),相对丰度为47.13%;产气结束阶段优势菌群为厚壁菌门(Firmicutes),相对丰度为28.89%;此外,还包括绿弯菌门、螺旋体门、绿菌门的细菌。(2)古菌种类明显少于细菌,均属于甲烷微菌纲(Methanomicrobia)、甲烷杆菌纲(Methanobacteria)和热原体纲(Thermoplasmata)。秸秆蓝藻沼气系统微生物群落结构的阐明具有一定的意义,可为秸秆沼气工程调控提供科学依据。     

Microbial communities from Arctic marine sediments respond slowly to methane addition during ex situ enrichments

Anaerobic methanotrophic archaea (ANME) consume methane in marine sediments, limiting its release to the water column, but their responses to changes in methane and sulfate supplies remain poorly constrained. To address how methane exposure may affect microbial communities and methane- and sulfur-cycling gene abundances in Arctic marine sediments, we collected sediments from offshore Svalbard that represent geochemical horizons where anaerobic methanotrophy is expected to be active, previously active, and long-inactive based on reaction-transport biogeochemical modelling of porewater sulfate profiles. Sediment slurries were incubated at in situ temperature and pressure with different added methane concentrations. Sediments from an active area of seepage began to reduce sulfate in a methane-dependent manner within months, preceding increased relative abundances of anaerobic methanotrophs ANME-1 within communities. In previously active and long-inactive sediments, sulfur-cycling Deltaproteobacteria became more dominant after 30 days, though these communities showed no evidence of methanotrophy after nearly 8 months of enrichment. Overall, enrichment conditions, but not methane, broadly altered microbial community structure across different enrichment times and sediment types. These results suggest that active ANME populations may require years to develop, and consequently microbial community composition may affect methanotrophic responses to potential large-scale seafloor methane releases in ways that provide insight for future modelling studies.     

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.     

The influence of the energy absorbed from microwave pretreatment on biogas production from secondary wastewater sludge

In this study, microwave treatment is analyzed as a way to accelerate the hydrolysis in anaerobic digestion of municipal wastewater sludge. The influence of the absorbed energy, power and athermal microwave effect on organic matter solubilization and biogas production has been studied. In addition, a novel method that considers the absorbed energy in the microwave system is proposed, in order to obtain comparable experimental results. The absorbed energy is calculated from an energy balance.The highest solubilization was achieved using 0.54 kJ/ml at 1000 W, where an increment of 7.1% was observed in methane production, compared to the untreated sample. Using a higher energy value (0.83 kJ/ml), methane production further increased (to 15.4%), but solubilization decreased. No power influence was found when 0.54 kJ/ml was applied at 1000, 600 and 440 W. Microwave heating was compared to conventional heating in two different experimental setups, providing similar methane yields in all cases.     

Ecophysiological adaptations of anaerobic bacteria to low pH: analysis of anaerobic digestion in acidic bog sediments.

The dynamics of anaerobic digestion were examined in the low-pH sediments of Crystal Bog in Wisconsin. The sediments (pH 4.9) contained 71% organic matter and the following concentrations of dissolved gases (micromoles per liter): CO2, 1,140; CH4, 490; and H2, 0.01. The rate of methane production was 6.2 mumol/liter of sediment per h, which is slower than eutrophic, neutral sediments. Microbial metabolic processes displayed the following pH optima: hydrolysis reactions, between 4.2 and 5.6; aceticlastic methanogenesis, 5.2; and hydrogen-consuming reactions, 5.6. The turnover rate constants for key intermediary metabolites were (h-1): glucose, 1.10; lactate, 0.277; acetate, 0.118; and ethanol, 0.089. The populations of anaerobes were low, with hydrolytic groups (10(6)/ml) several orders of magnitude higher than methanogens (10(2)/ml). The addition of carbon electron donors to the sediment resulted in the accumulation of hydrogen, whereas the addition of hydrogen resulted in the accumulation of fatty acids and the inhibition of hydrogen-producing acetogenic reactions. Strains of Lactobacillus, Clostridium, and Sarcina ventriculi were isolated from the bog, and their physiological attributes were characterized in relation to hydrolytic process functions in the sediments. The present studies provide evidence that the pH present in the bog sediments alter anaerobic digestion processes so that total biocatalytic activity is lower but the general carbon and electron flow pathways are similar to those of neutral anoxic sediments.