Flexible biogas production for demand-driven energy supply--feeding strategies and types of substrates

Nitrogen-doped activated carbon-based steel slag composite material as an accelerant for enhancing the resilience of flexible biogas production process against shock loads: Performance, mechanism and modified ADM1 modeling

Anaerobic digestion for flexible biogas production can lead to digestion inhibition under high shock loads. While steel slag addition has shown promise in enhancing system buffering, its limitations necessitate innovation. This study synthesized the nitrogen-doped activated carbon composite from steel slag to mitigate intermediate product accumulation during flexible biogas production. Material characterization preceded experiments introducing the composite into anaerobic digestion systems, evaluating its impact on methane production efficiency under hydraulic and concentration sudden shocks. Mechanistic insights were derived from microbial community and metagenomic analyses, facilitating the construction of the modified Anaerobic Digestion Model No. 1 (ADM1) to quantitatively assess the material's effects. Results indicate superior resistance to concentration shocks with substantial increment of methane production rate up to 33.45% compared with control group, which is mediated by direct interspecies electron transfer, though diminishing with increasing shock intensity. This study contributes theoretical foundations for stable flexible biogas production and offers an effective predictive tool for conductor material reinforcement processes.

Evaluating the sustainability of demand oriented biogas supply programs under different flexible hierarchies: A suggested approach based on the triple bottom line principle

In this paper, a decision-making approach based on the triple bottom line concept is presented for evaluating the sustainability of demand-oriented biogas supply (DOBS) programs with regard to their environmental, economic, and social impacts. For the assessment, an indicator system was developed, whose main parameters were quantified by integrating emergy analysis, economic benefit assessment, and a proposed social risk accounting method. The Charnes-Cooper-Wei-Huang (CCWH) model with constrained cone was adopted to calculate the comprehensive sustainability via the synthesis of the economic, environmental, and social indicators, in which eight scenarios were set according to the flexibility hierarchy of biogas supplied for load demand, biogas production mode, and feeding substrates. The evaluation results show that the DOBS scenario of supplying for real-time varying power demand by using straw and livestock manure has the highest sustainability score in our case study. Based on the results, corresponding managerial implications are proposed.

Demand-driven biogas production from Upflow Anaerobic Sludge Blanket (UASB) reactors to balance the power grid

Future energy systems necessitate dispatchable renewable energy to balance electrical grids with high shares of intermittent renewables. Biogas from anaerobic digestion (AD) can generate electricity on-demand. High-rate methanogenic reactors, such as the Upflow Anaerobic Sludge Blanket (UASB), can react quicker to variations in feeding as compared to traditional AD systems. In this study, experimental trials validated the feasibility of operating the UASB in a demand-driven manner. The UASB was operated with leachate produced from a hydrolysis reactor treating grass silage. The UASB demonstrated a high degree of flexibility in responding to variable feeding regimes. The intra-day biogas production rate could be increased by up to 123% under 4 hours in demand-driven operation, without significant deterioration in performance. A model based on kinetic analysis was developed to help align demand-driven operation with the grid. The findings suggest significant opportunities for UASBs to provide positive and negative balance to the electricity grid.

Metagenomic Analysis of Anaerobic Microbial Communities Degrading Short-Chain Fatty Acids as Sole Carbon Sources

Analyzing microbial communities using metagenomes is a powerful approach to understand compositional structures and functional connections in anaerobic digestion (AD) microbiomes. Whereas short-read sequencing approaches based on the Illumina platform result in highly fragmented metagenomes, long-read sequencing leads to more contiguous assemblies. To evaluate the performance of a hybrid approach of these two sequencing approaches we compared the metagenome-assembled genomes (MAGs) resulting from five AD microbiome samples. The samples were taken from reactors fed with short-chain fatty acids at different feeding regimes (continuous and discontinuous) and organic loading rates (OLR). Methanothrix showed a high relative abundance at all feeding regimes but was strongly reduced in abundance at higher OLR, when Methanosarcina took over. The bacterial community composition differed strongly between reactors of different feeding regimes and OLRs. However, the functional potential was similar regardless of feeding regime and OLR. The hybrid sequencing approach using Nanopore long-reads and Illumina MiSeq reads improved assembly statistics, including an increase of the N50 value (on average from 32 to 1740 kbp) and an increased length of the longest contig (on average from 94 to 1898 kbp). The hybrid approach also led to a higher share of high-quality MAGs and generated five potentially circular genomes while none were generated using MiSeq-based contigs only. Finally, 27 hybrid MAGs were reconstructed of which 18 represent potentially new species-15 of them bacterial species. During pathway analysis, selected MAGs revealed similar gene patterns of butyrate degradation and might represent new butyrate-degrading bacteria. The demonstrated advantages of adding long reads to metagenomic analyses make the hybrid approach the preferable option when dealing with complex microbiomes.

Prequalification of flotation sludge for a sustainable increase in biogas production and in regard of demand-driven feeding strategy

Co-substrates can increase gas production in a digester significantly. The characteristic properties of substrates, depending on the amounts added, influence the processes in the digester reactor. As a consequence, they can have an impact on the buffer capacity, pH value, C:N ratio, dewaterability of the digested sludge and introduce contaminants to the digester among others. In the future, a discontinuous digester feeding could contribute to the demand-driven energy supply by WRRFs. Due to the increasing instability caused by fluctuating organic load, higher demands are placed on the selection of co-substrates. This study examined to what extent flotation sludge from dairy companies is suitable for a sustainable co-digestion. In addition, it should be evaluated whether flotation sludge is applicable for demand-driven feeding strategies. It was shown that flotation sludge has positive effects on the reactor as well as a significant increase in biogas production and a high degree of degradation of at least 80%. Even at high organic loads pH remained at a high level at around 7.5 due to the high alkalinity of the substrate. Nonetheless, addition of more than 20 w-% flotation sludge lead to a significant decrease of the dewaterability of the digested sludge.

Demand-oriented biogas production to cover residual load of an electricity self-sufficient community using a simple kinetic model

Flexible biogas production can enable demand-oriented energy supply without the need for expensive gas storage expansions, but poses challenges to the stability of the anaerobic digestion (AD) process. In this work, biogas production of laboratory-scale AD of maize silage and sugar beets was optimized to cover the residual load of an electricity self-sufficient community using a simple process model based on first-order kinetics. Experiments show a good agreement between biogas demand, predicted, and measured biogas production. By optimizing biogas conversion schedules based on the measured gas production, a gas storage capacity of 7-8 h was identified for maximum flexibility, which corresponds to typical gas storage sizes at industrial biogas plants in Germany. Various stability indicators were continuously monitored and proved resilient process conditions. These results demonstrate that demand-oriented biogas production using model predictive control is a promising approach to enable existing biogas plants to provide balancing energy.

Two-stage anaerobic digestion: State of technology and perspective roles in future energy systems

Two-stage anaerobic digestion (TSAD) systems have been studied on a laboratory scale for about 50 years. However, they have not yet reached industrial scale despite their potential for future energy systems. This review provides an analysis of the TSAD technology, including the influence of process parameters on biomass conversion rates. The most common substrate (35.2% of the 38 selected studies) used in the analysed data was in the category of rapidly hydrolysable industrial waste with an average dry matter content of 7.24%. The highest methane content of 85% was reached when digesting food waste in a combination of two mesophilic continuously stirred tank reactors with an acidic (pH 5.5) first stage and alkaline (pH 7) second stage. Therefore, the review shows the limitations of the TSAD technology, future research directions, and the effect of integration of TSAD systems into the current strategy to reduce greenhouse gas emissions.

Methane production of banana plant: Yield, kinetics and prediction models influenced by morphological parts, cultivars and ripening stages

Banana trees and fruits with three ripening stages, including green, ripe, and overripe, of two cultivars, namely Nam wa and Hom were separated into different morphological parts for biogas yield determination. Specific methane yields (SMY) were significant different among banana parts (p ≤ 0.05). High non-structural carbohydrates and high non-lignocellulosic residual in substrates promoted high SMY. Pseudostem showed the highest share of energy yields among farm wastes which Nam wa cultivar provided higher energy potential than Hom. Peel presented the major energy source from fruit wastes which ripening stages did not have a significant effect on its SMY. Modified Gompertz model presented the best fit for methane production of most substrates. The SMY prediction models based on chemical constituents were developed to obtain conveniently used methane estimating tool which showed that a combination of lignin, hemicellulose, non-lignocellulosic residual, and crude fiber contents presented the highest performance for banana substrates.

Anaerobic Digestion of Pig Slurry in Fixed-Bed and Expanded Granular Sludge Bed Reactors

Anaerobic digestion of animal manure is a potential bioenergy resource that avoids greenhouse gas emissions. However, the conventional approach is to use continuously stirred tank reactors (CSTRs) with hydraulic retention times (HRTs) of greater than 30 d. Reactors with biomass retention were investigated in this study in order to increase the efficiency of the digestion process. Filtered pig slurry was used as a substrate in an expanded granular sludge bed (EGSB) reactor and fixed-bed (FB) reactor. The highest degradation efficiency (ηCOD) and methane yield (MY) relative to the chemical oxygen demand (COD) were observed at the minimum loading rates, with MY = 262 L/kgCOD and ηCOD = 73% for the FB reactor and MY = 292 L/kgCOD and ηCOD = 76% for the EGSB reactor. The highest daily methane production rate (MPR) was observed at the maximum loading rate, with MPR = 3.00 m3/m3/d at HRT = 2 d for the FB reactor and MPR = 2.16 m3/m3/d at HRT = 3 d for the EGSB reactor. For both reactors, a reduction in HRT was possible compared to conventionally driven CSTRs, with the EGSB reactor offering a higher methane yield and production rate at a shorter HRT.

Effect of a variable organic loading rate on process kinetics and volatile solids destruction in synthetic food waste-fed anaerobic digesters

With the increasing installation of weather-dependent renewable sources such as solar and wind power, the ability to produce electricity on demand to balance any shortfall in supply is becoming more important. Anaerobic digestion is a low-carbon energy source with the potential to be flexible to meet this need. An investigation was conducted into the response of two laboratory-scale anaerobic digesters at loading rate of 2.5 gVS L^-1 day^-1 over five months using a synthetic food waste as a substrate. One digester was consistently fed at the same rate, whereas the other digester was fed with periods of varying organic loading rate, from 0.1 to 7 gVS L^-1 day^-1, using a feed pattern derived from a record of restaurant food waste. The digester that had been fed at a variable rate showed a pronounced increase in biogas production after feed events and a 9.6% higher VS breakdown than the steady-feed digester (81% compared to 74%), with no effect on digester stability, volatile fatty acid concentration, overall biogas output or biogas quality. These findings support and encourage the use of variable-rate feeding to balance the electricity demand.

Demand-oriented biogas production and biogas storage in digestate by flexibly feeding a full-scale biogas plant

This work studied the demand-oriented biogas production and the biogas storage in digestate by flexibly feeding a full-scale research biogas plant. The investigated continuous stirred tank reactor (CSTR) was equipped with a fast-moving submersible motor mixer and a slow-moving inclined shaft agitator. A model for the biogas storage in digestate was introduced and tested in full scale using temporally highly resolved volume flow measurements. An increase in mixing time led to a faster biogas production: A two to five hours reduction of the time to reach the maximum biogas production after feeding occurred in our experiments. However, no influence of the rheology and of the mixing regime on the methane yield could be derived from the measurements. Further, a 30% reduction of the stored biogas in the digestate occurred when the viscosity was lowered by 66%. This knowledge can be used to enhance the existing biogas formation models.

Empirical Validation of a Biogas Plant Simulation Model and Analysis of Biogas Upgrading Potentials

Biogas plants may support the transformation towards renewable-based and integrated energy systems by providing dispatchable co-generation as well as opportunities for biogas upgrading or power-to-X conversion. In this paper, a simulation model that comprises the main dynamics of the internal processes of a biogas plant is developed. Based on first-order kinetics of the anaerobic digestion process, the biogas production of an input feeding schedule of raw material can be estimated. The output of the plant in terms of electrical and thermal energy is validated against empirical data from a 3-MW biogas plant on the Danish island of Bornholm. The results show that the model provides an accurate representation of the processes within a biogas plant. The paper further provides insights on the functioning of the biogas plant on Bornholm as well as discusses upgrading potentials of biogas to biomethane at the plant from an energy perspective.

Optimizing the co-digestion supply chain of sewage sludge and food waste by the demand oriented biogas supplying mechanism

Co-digestion of sewage sludge with food waste is a beneficial pathway for sewage plants to enhance their biogas yield. This paper employs hybrid programming with system dynamics simulation to optimize such a co-digestion system from the perspective of demand-oriented biogas supply chain, thus to improve the efficiency of the biogas utilization. The optimum operational parameters of the co-digestion system are derived from the simulation model. It is demonstrated that the demand-oriented biogas supply mechanism can be effectively driven under market-oriented incentive policy. For better compensation of the external cost to assist the operations of the co-digestion supply chain, it is suggested that the substrate collection and transportation subsidy should be combined with the renewables portfolio standard to be implemented as the optimum incentives. The limitations of the study are discussed to lay the foundation for future improvements.

Dynamic biogas production from anaerobic digestion of sewage sludge for on-demand electricity generation

The aim of this work was to study the potentials and benefits of dynamic biogas production from Anaerobic Digestion (AD) of sewage sludge. The biogas production rate was aimed to match the flexible demand for electricity generation and so appropriate feeding regimes were calculated and tested in both pilot and demonstration scale. The results demonstrate that flexibilization capability exists for both conventional AD and advanced AD using Thermal Hydrolysis Process (THP) as pre-treatment. Whilst the former provides lower capability, flexible biogas production was achieved by the latter, as it provides a quick response. In all scenarios, the value of the biogas converted into electricity is higher than with a steady operational regime, increasing by 3.6% on average (up to 5.0%) in conventional and by 4.8% on average (up to 7.1%) with THP. The process has proven scalable up to 18 m³ digester capacity in operational conditions like those in full scale.

Beneficial effects of intermittent feedstock management on biogas and methane production

Intermittent supply of easily degradable carbohydrates can be used for on-demand biogas production. The study tested the effects of splitting feeding portions of sugar beet silage (S) on biogas production rates and total yield, respectively and if methane production rates follow those ones of biogas. Four experimental AD reactors were operated for 117 days at organic loading rates of 2.0 kg_VS m^-3 d^-1 and VS ratios of maize silage (M) to S of 3:1. While M was supplied hourly (h₀-h₁₂), reactors differed only regarding the intermittent S supply, provided at once (h₀), twice (h₀, h₁) and three times (h₀, h₁, h₂) per twelve-hour observation period. Biogas and methane production rates rose simultaneously after S supply and lasted depending on S intakes. Biogas and methane yields were significantly increased at S given once and twice per period. Appropriate feedstock management can thus influence production rates and increase biogas and methane yields.

Residues from the dairy industry as co-substrate for the flexibilization of digester operation

Water resource recovery facilities (WRRF) can make an important contribution to increase the share of renewable energies in Germany. In this context, it is important to utilize unused digester capacities on WRRF. In addition, a demand-orientated biogas production could synchronize electricity demand and electricity generation and improve the overall energetic balance of the WRRF. As part of the project "Water Resource Recovery Facilities in interaction with the waste and energy industry: A German-Austrian Dialogue - COMITO," the influence of residues from the dairy industry on the digestion process was examined as well as the suitability for the flexibilization of digester gas production. Four reactors were fed with different amounts of flotation sludge from the dairy industry for several months. The difference in the feed resulted in organic loading (OLR) rates between 3.2 kg COD/(m³  day) and 6 kg COD/(m³  day). The reactors were fed with a daily shock load. The investigations showed that volumetric loads up to 4.4 kg COD/(m³  day) did not lead to an accumulation of organic acids. Organic loading rate of 6 kg COD/(m³  day) showed a significant accumulation of organic acids higher than 2,500 mg/L oHAc. Nevertheless, the reactor could be operated with a degradation rate of 71% with a corresponding biogas yield with a methane content of 71%. With increasing flotation sludge content, a higher concentration in ammonium of up to 2.000 mg/L NH₄ -N could be detected in the effluent of the digester. Despite higher phosphorus concentration in the flotation sludge, the concentration of PO₄ -P remained constant for all reactors fluctuating between 20 and 40 mg/L PO₄ -P. Dewatering worsened significantly with increasing levels of flotation sludge. PRACTITIONER POINTS: Main purpose of the research is to flexibilize digester operation on WRRF using flotation sludges from the dairy industry. Flexibilization of the digester using flotation sludge is possible up to an organic load of 6 kg COD/(m³  day). Higher NH₄ -N concentration in the effluent of the digester must be accepted when using higher amounts of flotation sludge. Phosphate concentration in the effluent of the digester remained on a low level despite higher phosphorus content in the flotation sludge. High levels of organic acids (mainly acetic acid) can be tolerated and can be recovered within a short time after reducing the load.

Investigating flexible feeding effects on the biogas quality in full-scale anaerobic digestion by high resolution, photoacoustic-based NDIR sensing

In future energy systems based on renewable energies, biogas plants can make a significant contribution to stabilizing the electricity grids. However, this requires load-flexible and demand-oriented electricity production by means of flexible feed management. However, these flexible feeding strategies using greatly oscillating, temporally varying high mass loads may lead to critical process failures of the anaerobic digestion process. Currently there is no online, high resolution gas quality measurement technique to detect and prevent biological process failures available. In this contribution, we present a miniaturized, low-cost biogas quality measurement system providing data with high precision and high temporal resolution to overcome this technology gap. To highlight the capabilities of the system we have installed it using a bypass to the main biogas duct after hydrogen sulfide removal at a full-scale research biogas plant. During a three-month field trial, the effect of flexible feeding on the biogas quality has been monitored. The results demonstrate long-term stability of the sensor solution and reveal the effects of changing feeding frequency and composition on gas quantity and quality, which cannot be detected with commercially available state-of-the-art sensing systems.

Model-based control for a demand-driven biogas production to cover residual load rises

The development of systems for energy storage and demand-driven energy production will be essential to enable the switch from fossil to renewable energy sources in future. To cover the residual load rises, a rigorous dynamic process model based on the Anaerobic Digestion Model No. 1 (ADM1) was applied to analyse the flexible operation of biogas plants. For this, the model was optimised and an operational concept for a demand-driven energy production was worked out. Different substrates were analysed, both by batch fermentation and Weende analysis with van Soest method, to determine the input data of the model. The lab results show that the substrates have got different degradation kinetics and biogas potentials. Finally, the ADM1 was extended with a feeding algorithm which is based on a PI controller. Essential feeding times and quantities of available substrates were calculated so that a biogas plant can cover a defined energy demand. The results prove that a flexible operation of biogas plants with a feeding strategy is possible.

Utilization of Food and Agricultural Residues for a Flexible Biogas Production: Process Stability and Effects on Needed Biogas Storage Capacities

Biogas plants can contribute to future energy systems’ stability through flexible power generation. To provide power flexibly, a demand-oriented biogas supply is necessary, which may be ensured by applying flexible feeding strategies. In this study, the impacts of applying three different feeding strategies (1x, 3x and 9x feeding per day) on the biogas and methane production and process stability parameters were determined for a biogas plant with a focus on waste treatment. Two feedstocks that differed in (1) high fat and (2) higher carbohydrate content were investigated during semi-continuous fermentation tests. Measurements of the short chain fatty acids concentration, pH value, TVA/TIC ratio and total ammonium and ammonia content along with a molecular biology analysis were conducted to assess the effects on process stability. The results show that flexible biogas production can be obtained without negative impacts on the process performance and that production peaks in biogas and methane can be significantly shifted to another time by changing feeding intervals. Implementing the fermentation tests’ results into a biogas plant simulation model and an assessment of power generation scenarios focusing on peak-time power generation revealed a considerable reduction potential for the needed biogas storage capacity of up to 73.7%.

Representative Sampling Implementation in Online VFA/TIC Monitoring for Anaerobic Digestion

This paper describes an automatic sampling system for anaerobic reactors that allows taking representative samples following the guidelines of Gy’s (1998) theory of sampling. Due to the high heterogeneity degree in a digester the sampling errors are larger than the analysis error, making representative sampling a prerequisite for successful process control. In our system, samples are automatically processed, generating a higher density of data and avoiding human error by sample manipulation. The combination of a representative sampling system with a commercial automate titration unit generates a robust online monitoring system for biogas plants. The system was successfully implemented in an operating biogas plant to control a feeding-on-demand biogas system.

Environmental sustainability of integrating the organic Rankin cycle with anaerobic digestion and combined heat and power generation

Given the growing scarcity of primary energy resources, increasing the efficiency of energy conversion is one of the key challenges for optimising energy use. For this reason, low-grade or waste heat from various processes is becoming increasingly more attractive as an energy source. This study considers anaerobic digestion (AD) coupled with a combined heat and power plant (CHP) as a source of low-grade heat for electricity generation utilising an organic Rankine cycle (ORC) system. The aim is to evaluate the environmental sustainability of such a system relative to the AD-CHP system without heat recovery. Ten real AD-CHP plants using cereal silage and animal slurry as feedstocks are considered for these purposes and their impacts have been estimated through life cycle assessment. The results suggest that systems with the ORC have lower impacts than those without it, but the average reductions are relatively small (1.6-5.8%). However, for the smaller plants fed mainly with animal slurry, climate change increases significantly (up to 27 times). The reduction in impacts is greater for the bigger plants where the surplus heat available for the ORC is higher. The impacts from the ORC plant are insignificant, with its electrical efficiency and lifespan showing little effect on the results. Small slurry-fed plants without the ORC have lower environmental impacts than the bigger silage-fed plants fitted with an ORC system for nine out of 13 categories considered; climate change is up to 32 times lower. They are only slightly worse than the bigger plants with the ORC for ozone depletion and human toxicity due to the economies of scale. Therefore, while there are clear benefits of fitting an ORC system to an AD-CHP plant, greater benefits can still be achieved by utilising waste feedstocks, such as animal slurry, instead of fitting an ORC to a plant utilising cereal silage.

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.

Intermittent fasting for microbes: how discontinuous feeding increases functional stability in anaerobic digestion

BACKGROUND

Demand-driven biogas production could play an important role for future sustainable energy supply. However, feeding a biogas reactor according to energy demand may lead to organic overloading and, thus, to process failures. To minimize this risk, digesters need to be actively steered towards containing more robust microbial communities. This study focuses on acetogenesis and methanogenesis as crucial process steps for avoiding acidification. We fed lab-scale anaerobic digesters with volatile fatty acids under various feeding regimes and disturbances. The resulting microbial communities were analyzed on DNA and RNA level by terminal restriction fragment length polymorphism of the mcrA gene, 16S rRNA gene amplicon sequencing, and a [2-13C]-acetate assay. A modified Anaerobic Digestion Model 1 (ADM1) that distinguishes between the acetoclastic methanogens Methanosaeta and Methanosarcina was developed and fitted using experimental abiotic and biotic process parameters.

RESULTS

Discontinuous feeding led to more functional resilience than continuous feeding, without loss in process efficiency. This was attributed to a different microbial community composition. Methanosaeta dominated the continuously fed reactors, while its competitor Methanosarcina was washed out. With discontinuous feeding, however, the fluctuating acetic acid concentrations provided niches to grow and co-exist for both organisms as shown by transcription analysis of the mcrA gene. Our model confirmed the higher functional resilience due to the higher abundance of Methanosarcina based on its higher substrate uptake rate and higher resistance to low pH values. Finally, we applied our model to maize silage as a more complex and practically relevant substrate and showed that our model is likely transferable to the complete AD process.

CONCLUSIONS

The composition of the microbial community determined the AD functional resilience against organic overloading in our experiments. In particular, communities with higher share of Methanosarcina showed higher process stability. The share of these microorganisms can be purposefully increased by discontinuous feeding. A model was developed that enables derivation of the necessary feeding regime for a more robust community with higher share of Methanosarcina.

Substrate-Induced Response in Biogas Process Performance and Microbial Community Relates Back to Inoculum Source

This study investigated whether biogas reactor performance, including microbial community development, in response to a change in substrate composition is influenced by initial inoculum source. For the study, reactors previously operated with the same grass–manure mixture for more than 120 days and started with two different inocula were used. These reactors initially showed great differences depending on inoculum source, but eventually showed similar performance and overall microbial community structure. At the start of the present experiment, the substrate was complemented with milled feed wheat, added all at once or divided into two portions. The starting hypothesis was that process performance depends on initial inoculum source and microbial diversity, and thus that reactor performance is influenced by the feeding regime. In response to the substrate change, all reactors showed increases and decreases in volumetric and specific methane production, respectively. However, specific methane yield and development of the microbial community showed differences related to the initial inoculum source, confirming the hypothesis. However, the different feeding regimes had only minor effects on process performance and overall community structure, but still induced differences in the cellulose-degrading community and in cellulose degradation.

Intrinsic gas production kinetics of selected intermediates in anaerobic filters for demand-orientated energy supply

As a consequence of a growing share of solar and wind power, recent research on biogas production highlighted a need for demand-orientated, flexible gas production to provide grid services and enable a decentralized stabilization of the electricity infrastructure. Two-staged anaerobic digestion is particularly suitable for shifting the methane production into times of higher demand due to the spatio-temporal separation of hydrolysis and methanogenesis. To provide a basis for predicting gas production in an anaerobic filter, kinetic parameters of gas production have been determined experimentally in this study. A new methodology is used, enabling their determination during continuous operation. An order in methane production rate could be established by comparing the half lives of methane production. The order was beginning with the fastest: acetic acid>ethanol>butyric acid>iso-butyric acid>valeric acid>propionic acid>1,2propanediol>lactic acid. However, the mixture of a natural hydrolysate from the acidification tank appeared to produce methane faster than all single components tested.

Sugar beet silage as highly flexible feedstock for on demand biogas production

On demand biogas production is a great option to complement solar and wind power for the energy revolution. Alternatives like feedstock management are important in order to avoid expensive and complex adjustments for gas storage systems. The use of sugar beet silage (S) is a good option because it mainly contains carbohydrates that are easily degradable. Anaerobic digestion was performed for 63 days in four completely stirred tank reactors (CSTR) with different ratios of maize silage (M) and S. M given every hour was used as a base load for the fermentation and S was given two times a day every 12h. Biogas and methane production rates were measured every 5min in order to achieve data with high resolution. Also, pH value, VFA/TIC values and volatile fatty acids were measured during the experiment. The process remained stable in CSTR1 (M:S1:0), CSTR2 (M:S6:1) and CSTR3 (M:S3:1). Instabilities occurred in CSTR4 (M:S1:3) after an operation time of 33 days. Nevertheless, methane yields more than doubled for CSTR3 within 5min after the input of S. Use of sugar beet as a feedstock for biogas production is a further application for this agricultural commodity.

Demand-driven biogas production from sugar beet silage in a novel fixed bed disc reactor under mesophilic and thermophilic conditions

A newly developed fixed bed disc reactor (FBDR) which combines biofilm formation on biofilm carriers and reactor agitation in one single system was assessed for its applicability to demand-driven biogas production by variable feeding of sugar beet silage. Five different feeding patterns were studied at an organic loading of 4g_VSL^-1d^-1 under mesophilic and thermophilic conditions. High methane yields of 449-462L_Nkg_VS were reached. Feeding variable punctual loadings caused immediate response with 1.2- to 3.5-fold increase in biogas production rates within 15min. Although variable feeding did not induce process instability, a temporary decrease in pH-value and methane concentration below 40% occurred. Thermophilic temperature was advantageous as it resulted in a more rapid, higher methane production and less pronounced decrease in methane content after feeding. The FBDR was demonstrated to be well-suited for flexible biogas production, but further research and comparison with traditional reactor systems are required.

Characteristics of on-demand biogas production by using sugar beet silage

On-demand electricity generation can be achieved by just-in-time biogas production instantly utilized in co-generation units. For this goal, easily degradable substrates like sugar beet silage have a high potential. Potential for on-demand biogas production from co-digestion of sugar beet silage (SS) with grass silage (GS) was evaluated in two experiments at organic loading rates (OLRs) of 1.5 kgVS m^-3 day^-1 and 2.5 kgVS m^-3 day^-1, respectively. Each experiment was fed with intermittent feeding system at 8 hrs interval at the same feedstock ratios (volatile solids based) of GS:SS-1:0, 3:1 and 1:3, respectively. Modelling by Gaussian equation was performed in order to understand the effects of SS on biogas production. Addition of sugar beet silage led to maximum biogas production within a short time, but it differed significantly depending on feedstock ratios and OLRs, respectively. At OLR 1.5 kgVS m^-3 day^-1, during mono fermentation of grass silage maximum biogas production rate of 0.27 l_N hr^-1 was reached at 2.74 hrs. Production rate did not change at feedstock ratio of GS:SS-3:1 but increased to 0.64 l_N hr^-1 at GS:SS-1:3 within a shorter time span (1.58 hrs). On the contrary, at OLR of 2.5 kgVS m^-3 day^-1 time span between feedstock input and maximum biogas production did not differ significantly (p > 0.05) among the reactors. Biogas production rates were 0.60 l_N hr^-1 within 2.27 hrs and 0.82 l_N hr^-1 within 2.30 hrs at GS:SS-3:1 and GS:SS-1:3, respectively. Surprisingly, there was no time lag between maximum biogas and methane production rates, irrespectively of OLR. This implies that once the whole microbial community is adapted to intermittent substrate input, the metabolic products are instantly utilized through the all steps of anaerobic substrate degradation. Applying this finding opens new perspectives for on-demand biogas energy production.

Influence of process parameters on the extraction of soluble substances from OFMSW and methane production

Microorganisms involved in anaerobic digestion require dissolved substrates to transport them through the cell wall to different processing units and finally to be disposed as waste, such as methane and carbon dioxide. In order to increase methane production, this work proposes to separate the soluble substances from OFMSW and analyse methane production from extracts and OFMSW. Using water as solvent, four extraction parameters were proposed: (1) Number of consecutive extractions, (2) Duration of mixing for every consecutive extraction, (3) OFMSW to water mass ratios 1:1, 1:2, and 1:3 and, (4) The influence of temperature on the extraction process. Results indicated that is possible to separate 40% of VS from OFMSW with only three consecutive extraction with mixing of 30min in every extraction using ambient temperature water. For every OFMSW to water combination, the first three consecutive extracts were analysed for biochemical methane potential test during 21days at 35°C; OFMSW was also tested as reference. Methane production from all substrates is highest during the first day and then it slowly decreases to increase again during a second stage. This was identified as diauxic behaviour. Specific methane production at day 21 increased with increasing water content of the extracts where OFMSW methane production was the lowest of all with 535NL/kgVS. These results indicate that it is feasible to rapidly produce methane from extracted substances.

Fueling the future with biomass: Processes and pathways for a sustainable supply of hydrocarbon fuels and biogas

Global economic growth, wealth and security rely upon the availability of cheap, mostly fossil-derived energy and chemical compounds. The replacement by sustainable resources is widely discussed. However, the current state of biotechnological processes usually restricts them to be used as a true alternative in terms of economic feasibility and even sustainability. Among the rare examples of bioprocesses applied for the energetic use of biomass are biogas and bioethanol production. Usually, these processes lack in efficiency and they cannot be operated without the support of legislation. Although they represent a first step towards a greater share of bio-based processes for energy provision, there is no doubt that tremendous improvements in strain and process development, feedstock and process flexibility as well as in the integration of these processes into broader supply and production networks, in this review called smart bioproduction grids, are required to make them economically attractive, robust enough, and wider acceptance by society. All this requires an interdisciplinary approach, which includes the use of residues in closed carbon cycles and issues concerning the process safety. This short review aims to depict some of the promising strategies to achieve an improved process performance as a basis for future application.

Life cycle assessment of flexibly fed biogas processes for an improved demand-oriented biogas supply

This paper analyses concepts to facilitate a demand oriented biogas supply at an agricultural biogas plant of a capacity of 500kWhel, operated with the co-digestion of maize, grass, rye silage and chicken manure. In contrast to previous studies, environmental impacts of flexible and the traditional baseload operation are compared. Life Cycle Assessment (LCA) was performed to detect the environmental impacts of: (i) variety of feedstock co-digestion scenarios by substitution of maize and (ii) loading rate scenarios with a focus on flexible feedstock utilization. Demand-driven biogas production is critical for an overall balanced power supply to the electrical grid. It results in lower amounts of emissions; feedstock loading rate scenarios resulted in 48%, 20%, 11% lower global warming (GWP), acidification (AP) and eutrophication potentials, and a 16% higher cumulative energy demand. Substitution of maize with biogenic-waste regarding to feedstock substitution scenarios could create 10% lower GWP and AP.

Modelling a demand driven biogas system for production of electricity at peak demand and for production of biomethane at other times

Four feedstocks were assessed for use in a demand driven biogas system. Biomethane potential (BMP) assays were conducted for grass silage, food waste, Laminaria digitata and dairy cow slurry. Semi-continuous trials were undertaken for all feedstocks, assessing biogas and biomethane production. Three kinetic models of the semi-continuous trials were compared. A first order model most accurately correlated with gas production in the pulse fed semi-continuous system. This model was developed for production of electricity on demand, and biomethane upgrading. The model examined a theoretical grass silage digester that would produce 435kWe in a continuous fed system. Adaptation to demand driven biogas required 187min to produce sufficient methane to run a 2MWe combined heat and power (CHP) unit for 60min. The upgrading system was dispatched 71min following CHP shutdown. Of the biogas produced 21% was used in the CHP and 79% was used in the upgrading system.

Evaluation of anaerobic degradation, biogas and digestate production of cereal silages using nylon-bags

In this study, the degradation efficiency and the biogas and digestate production during anaerobic digestion were evaluated for the cereal silages most used to feed biogas plants. To this purpose, silages of: maize from the whole plant, maize from the ear, triticale and wheat were digested, inside of nylon bags, in laboratory scale digesters, for 75days. Overall, the test involved 288 nylon bags. After 75days of digestion, the maize ear silage shows the highest degradation efficiency (about 98%) while wheat silage the lowest (about 83%). The biogas production ranges from 438 to 852Nm(3)/t of dry matter for wheat and ear maize silage, respectively. For all the cereal silages, the degradation as well as the biogas production are faster at the beginning of the digestion time. Digestate mass, expressed as percentage of the fresh matter, ranges from 38% to 84% for wheat and maize ear silage, respectively.

Biogas Production: Microbiology and Technology

Biogas, containing energy-rich methane, is produced by microbial decomposition of organic material under anaerobic conditions. Under controlled conditions, this process can be used for the production of energy and a nutrient-rich residue suitable for use as a fertilising agent. The biogas can be used for production of heat, electricity or vehicle fuel. Different substrates can be used in the process and, depending on substrate character, various reactor technologies are available. The microbiological process leading to methane production is complex and involves many different types of microorganisms, often operating in close relationships because of the limited amount of energy available for growth. The microbial community structure is shaped by the incoming material, but also by operating parameters such as process temperature. Factors leading to an imbalance in the microbial community can result in process instability or even complete process failure. To ensure stable operation, different key parameters, such as levels of degradation intermediates and gas quality, are often monitored. Despite the fact that the anaerobic digestion process has long been used for industrial production of biogas, many questions need still to be resolved to achieve optimal management and gas yields and to exploit the great energy and nutrient potential available in waste material. This chapter discusses the different aspects that need to be taken into consideration to achieve optimal degradation and gas production, with particular focus on operation management and microbiology.

Kinetics of biogas production in Anaerobic Filters

This study investigates methane production kinetics from individual volatile fatty acids (VFA) in an Upflow Anaerobic Filter (AF). 1gCOD in the form of acetic (HAc), propionic (HPr) or butyric acid (HBu) was injected into the AF while operating at an organic loading rate (OLRCOD) of 3.5gL(-1)d(-1). A new method is introduced to separate gas production of the baseload from the product formation of VFA degradation after the injection. The lag phase, fractional rate of gas production and half-life has been determined for the methane production of the three VFAs. The half-lives were in the order HAc<HBu<HPr showing a slower gas production from the C3 acid than from the C4 acid. The results can be used for prediction models for on-demand biogas production, a vital approach that provides the transforming energy market with balancing power.

Changing Feeding Regimes To Demonstrate Flexible Biogas Production: Effects on Process Performance, Microbial Community Structure, and Methanogenesis Pathways

Flexible biogas production that adapts biogas output to energy demand can be regulated by changing feeding regimes. In this study, the effect of changes in feeding intervals on process performance, microbial community structure, and the methanogenesis pathway was investigated. Three different feeding regimes (once daily, every second day, and every 2 h) at the same organic loading rate were studied in continuously stirred tank reactors treating distiller's dried grains with solubles. A larger amount of biogas was produced after feeding in the reactors fed less frequently (once per day and every second day), whereas the amount remained constant in the reactor fed more frequently (every 2 h), indicating the suitability of the former for the flexible production of biogas. Compared to the conventional more frequent feeding regimes, a methane yield that was up to 14% higher and an improved stability of the process against organic overloading were achieved by employing less frequent feeding regimes. The community structures of bacteria and methanogenic archaea were monitored by terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA and mcrA genes, respectively. The results showed that the composition of the bacterial community varied under the different feeding regimes, and the observed T-RFLP patterns were best explained by the differences in the total ammonia nitrogen concentrations, H2 levels, and pH values. However, the methanogenic community remained stable under all feeding regimes, with the dominance of the Methanosarcina genus followed by that of the Methanobacterium genus. Stable isotope analysis showed that the average amount of methane produced during each feeding event by acetoclastic and hydrogenotrophic methanogenesis was not influenced by the three different feeding regimes.

Assessment of the influence of energy density and feedstock transport distance on the environmental performance of methane from maize silages

In Europe, thanks to public subsidy, the production of electricity from anaerobic digestion (AD) of agricultural feedstock has considerably grown and several AD plants were built. When AD plants are concentrated in specific areas (e.g., Northern Italy), increases of feedstock' prices and transport distances can be observed. In this context, as regards low-energy density feedstock, the present research was designed to estimate the influence of the related long-distance transport on the environmental performances of the biogas-to-electricity process. For this purpose the following transport systems were considered: farm trailers and trucks. For small distances (<5 km), the whole plant silage shows the lowest impact; however, when distances increase, silages with higher energy density (even though characterised by lower methane production per hectare) become more environmentally sustainable. The transport by trucks achieves better environmental performances especially for distances greater than 25 km.