Assessing the potential for the uptake of on-farm anaerobic digestion for energy production in England

We report on an assessment of the potential for energy production from on-farm anaerobic digestion (AD) in England based on findings from a survey of farmers where it was found that around 40% of 381 respondents might install AD on their farms. These ‘possible adopters’ tended to have large farms and might together utilise some 6560 ha of land for feedstock production along with the wastes from some 12,000 beef and dairy cattle and 9000 pigs. When raised to the national level, such a level of AD activity would produce around 3.5 GWh of electricity. This approximates to just 0.001% of national electricity generation. Further, there are considerable perceived barriers to the widespread adoption of AD on farms in England; these include the high capital costs of installing AD and doubts about the economic returns being high enough.     

Influence of UK energy policy on the deployment of anaerobic digestion

Anaerobic digestion (AD) has the potential to contribute to greenhouse gas emissions reductions, improve energy security, increase generation of decentralised renewable electrical and thermal energy, produce low-impact fertiliser and enhance adherence to the principles of proximity as well as self-sufficiency in waste treatment, in energy generation and in resource use. Financial viability is scrutinised investigating optimal logistic pre-conditions such as catchment area or plant size. Given that a breakthrough in deployment does not only depend on technical aspects, the relative importance and magnitude of the necessary incentives is discussed. The influence of policy instruments is studied by devising different incentive scenarios for the United Kingdom. Substantial and predictable rewards for renewable electricity and heat are essential to harness the full potential of AD in addition to the current emphasis on landfill tax. A possible configuration of energy supply companies as a crucial vehicle to bring anaerobic digestion to market is highlighted.     

Biohythane production in two-stage anaerobic digestion system

Hydrogen (H₂) and methane (CH₄) are the potential alternative energy carriers with autonomous extensive and viable importance. These fuels could complement the advantages, and discard the disadvantages of each other, if produced simultaneously. Considering their complementary properties, co-production of a mixture of H₂ and CH₄ in the form of biohythane in two-stage anaerobic digestion (AD) process is gaining more interest than their individual production. Biohythane is a better transportation fuel than compressed natural gas (CNG) in terms of high range of flammability, reduced ignition temperature as well as time, without nitrous oxide (NOx) emissions, improved engine performance without specific modification, etc. Other than production of biohythane, performing two-stage AD is advantageous over one-stage AD due to short HRT, high energy recovery, high COD removal, higher H₂ and CH₄ yields, and reduced carbon dioxide (CO₂) in biogas. For improved biohythane production, various aspects of two-stage AD need to be emphasized. Keeping the facts in mind, the process of two-stage AD along with microbial diversity in comparison to one-stage AD has been discussed in the previous sections of this review. For large scale commercial production, and utilization of biohythane in automobile sector, its execution needs evaluation of process parameters, and problems associated with two-stage AD. Hence, the later part of this review describes the production process of biohythane, concerned microbial diversity, operational process parameters, major challenges and their solutions, applications, and economic evaluation for enhanced production of biohythane.     

Siting analysis of farm-based centralized anaerobic digester systems for distributed generation using GIS

There is growing interest in installing anaerobic digesters (ADs) on farms to use animal wastes as a biomass resource for both economic value and environmental benefit. This potential expansion prompts the need for land suitability assessment. In this paper, a GIS model is proposed for land-suitability assessment of potential energy systems featuring an AD coupled with an energy generator. A variety of environmental and social constraints, as well as economic factors are integrated in the model to help determine the optimal sites for installing such systems. The analytic hierarchy process (AHP) method is employed to estimate the factors’ weights in order to establish their relative importance in site selection. The model is then applied to Tompkins County, New York as a case study for demonstration. A siting suitability map was produced to identify those areas that are most suitable for distributed bio-energy systems using dairy manure. The results showed that this GIS-based model, by integrating both spatial data and non-spatial information, was capable of providing a broad-scale and multidimensional view on the potential bio-energy systems development in the area of study to account for environmental and social constraints as well as economic factors. The model can be modified for evaluating other biomass resources.     

Anaerobic digestion of molasses by means of a vibrating and non-vibrating submerged anaerobic membrane bioreactor

Bio-refineries produce large volumes of waste streams with high organic content, which are potentially interesting for further processing. Anaerobic digestion (AD) can be a key technology for treatment of these sidestreams, such as molasses. However, the high concentration of salts in molasses can cause inhibition of methanogenesis. In this research, concentrated and diluted molasses were subjected to biomethanation in two types of submerged anaerobic membrane bioreactors (AnMBRs): one with biogas recirculation and one with a vibrating membrane. Both reactors were compared in terms of methane production and membrane fouling. Biogas recirculation seemed to be a good way to avoid membrane fouling, while the trans membrane pressures in the vibrating MBR increased over time, due to cake layer formation and the absence of a mixing system. Stable methane production, up to 2.05 L L⁻¹ d⁻¹ and a concomitant COD removal of 94.4%, was obtained only when diluted molasses were used, since concentrated molasses caused a decrease in methane production and an increase in volatile fatty acids (VFA), indicating an inhibiting effect of concentrated molasses on AD. Real-time PCR results revealed a clear dominance of Methanosaetaceae over Methanosarcinaceae as the main acetoclastic methanogens in both AnMBRs.     

Kinetics of anaerobic digestion

The basic chemical kinetic equations for anaerobic digestion have been solved to get closed form solutions for the substrate concentration and the concentration of anaerobes. Numerical calculations have been performed to obtain quantitative estimates for their time behaviour.     

Principles and potential of the anaerobic digestion of waste-activated sludge

When treating municipal wastewater, the disposal of sludge is a problem of growing importance, representing up to 50% of the current operating costs of a wastewater treatment plant. Although different disposal routes are possible, anaerobic digestion plays an important role for its abilities to further transform organic matter into biogas (60–70 vol% of methane, CH₄), as thereby it also reduces the amount of final sludge solids for disposal whilst destroying most of the pathogens present in the sludge and limiting odour problems associated with residual putrescible matter. Anaerobic digestion thus optimises WWTP costs, its environmental footprint and is considered a major and essential part of a modern WWTP. The potential of using the biogas as energy source has long been widely recognised and current techniques are being developed to upgrade quality and to enhance energy use. The present paper extensively reviews the principles of anaerobic digestion, the process parameters and their interaction, the design methods, the biogas utilisation, the possible problems and potential pro-active cures, and the recent developments to reduce the impact of the problems. After having reviewed the basic principles and techniques of the anaerobic digestion process, modelling concepts will be assessed to delineate the dominant parameters. Hydrolysis is recognised as rate-limiting step in the complex digestion process. The microbiology of anaerobic digestion is complex and delicate, involving several bacterial groups, each of them having their own optimum working conditions. As will be shown, these groups are sensitive to and possibly inhibited by several process parameters such as pH, alkalinity, concentration of free ammonia, hydrogen, sodium, potassium, heavy metals, volatile fatty acids and others. To accelerate the digestion and enhance the production of biogas, various pre-treatments can be used to improve the rate-limiting hydrolysis. These treatments include mechanical, thermal, chemical and biological interventions to the feedstock. All pre-treatments result in a lysis or disintegration of sludge cells, thus releasing and solubilising intracellular material into the water phase and transforming refractory organic material into biodegradable species. Possible techniques to upgrade the biogas formed by removing CO₂, H₂S and excess moisture will be summarised. Special attention will be paid to the problems associated with siloxanes (SX) possibly present in the sludge and biogas, together with the techniques to either reduce their concentration in sludge by preventive actions such as peroxidation, or eliminate the SX from the biogas by adsorption or other techniques. The reader will finally be guided to extensive publications concerning the operation, control, maintenance and troubleshooting of anaerobic digestion plants.     

Evaluation of parameters monitored in the process of food waste anaerobic digestion

In this article, the anaerobic digestion of food waste was studied on a laboratory scale continuously stirred tank reactor, operating at 37°C under quasi-continuous conditions. The change of organic loading rate led to performance failure in the reactor. The aim of this work is to determine a reliable parameter, which could be used as an indicator of process imbalance during anaerobic digestion of food waste in the continuously stirred tank reactor anaerobic reactor. The results showed that intermediate alkalinity/partial alkalinity ratio can predicate the failure in the food waste anaerobic digestion.     

A review of simple to scientific models for anaerobic digestion

To fully model the anaerobic digestion process, biological and physico-chemical background, the kinetics of bacterial growth, substrate degradation and product formation have to be taken into account. The presented approaches differ depending on the requirements and the developer of the model. Important parameters affecting the process such as temperature, which can cause great inaccuracy, are rarely included in the models. Simple calculators are also available that estimate the applicability of the process to a specific farm and provide information to a farmer or a decision maker. Six simple calculators are presented in this study: AD decision support software, Anaerobic Digestion Economic Assessment Tool, BEAT₂, BioGC, FarmWare and GasTheo. The simpler calculators mainly use the relation that exists between volatile solids and biogas production. A tested case of 100 dairy cows and 50 sows was applied to the simple calculators to compare the results.     

Heat and energy requirements in thermophilic anaerobic sludge digestion

The heating requirements of the thermophilic anaerobic digestion process were studied. Biogas production was studied in laboratory experiments at retention times from 1 to 10 days. The data gathered in the experiments was then used to perform a heat and energy analysis. The source of heat was a conventional CHP unit system. The results showed that thermophilic digestion is much faster than mesophilic digestion and therefore produces more biogas in a shorter time or at smaller digester volumes. The major part of the heating requirements consisted of sludge heating. The heat losses of the digester were only 2–8% of the sludge heating requirements. The heating requirements in thermophilic digestion are about twice those of mesophilic digestion. Therefore a CHP unit system cannot cover all of the needs for successful operation of thermophilic digestion. Heat regeneration was introduced as a solution. Heat is regenerated from the sludge outflow at a temperature of 50–55 °C and transferred to the cold inflow sludge at a temperature of 11 °C. Enough heat is regenerated in a conventional counter flow heat exchanger to bring the thermophilic process to the same level as the mesophilic one. Considering the smaller digester volumes and the relatively small investment in the regenerative equipment, the construction of thermophilic digestion systems may be a very good alternative to conventional mesophilic sludge digestion systems.     

Modelling an off-grid integrated renewable energy system for rural electrification in India using photovoltaics and anaerobic digestion

This work describes the design optimisation and techno-economic analysis of an off-grid Integrated Renewable Energy System (IRES) designed to meet the electrical demand of a rural village location in West Bengal – India with an overall electrical requirement equivalent to 22 MWh year⁻¹. The investigation involved the modelling of seven scenarios, each containing a different combination of electricity generation (anaerobic digestion with biogas combined heat and power (CHP) and photovoltaics) and storage elements (Vanadium redox batteries, water electrolyser and hydrogen storage with fuel cell). Micro-grid modelling software HOMER, was combined with additional modelling of anaerobic digestion, to scale each component in each scenario considering the systems' ability to give a good quality electricity supply to a rural community. The integrated system which contained all of the possible elements – except hydrogen production and storage presented the lowest capital ($US 71 k) and energy cost ($US 0.289 kWh⁻¹) compared to the scenarios with a single energy source. The biogas CHP was able to meet the electrical load peaks and variations and produced 61% of the total electricity in the optimised system, while the photovoltaics met the daytime load and allowed the charging of the battery which was subsequently used to meet base load at night.     

A comparison of the drivers influencing adoption of on-farm anaerobic digestion in Germany and Australia

This review examines the drivers behind the adoption of on-farm anaerobic digestion in Germany where there were more than 4000 plants operating in 2009. In Australia, only one plant is operating, at a piggery in the State of Victoria. Germany’s generous feed-in-tariffs for renewable energy are typically given the credit for promoting investment in on-farm anaerobic digestion. But the particular biophysical and socio-economic character of farming in the country provided the fertile ground for these financial incentives to take root. Energy security has also been a major driver for the promotion of renewable energy in Germany since it imports over 60% of its energy needs. In contrast, Australia is a net energy exporter, exporting about two-thirds of its domestic energy. Although it has considerable potential for application in Australia, anaerobic digestion is unlikely to be widely adopted unless new incentives emerge to strongly encourage investment. Stronger Australian regulation of manures and effluent may serve as an incentive to a limited extent in the future. Yet the experience in Germany suggests that regulation on its own was not sufficient to encourage large numbers of farmers to invest in anaerobic digestion. Even with generous incentives from the German government, increasing construction costs and the rising cost of energy crops can put the financial viability of anaerobic digestion plants at risk. Unless improvements in efficiency are found and implemented, these pressures could lead to unsustainable rises in the cost of the incentive schemes that underpin the development of renewable energy technologies.     

Inclined-plug-flow type reactor for anaerobic digestion of semi-solid waste

Preliminary experimental results obtained from the treatment of semi-solid wastes generated from wholesale fruit and vegetable markets, supermarkets, etc. mixed together with sewage sludge, are reported.     

Anaerobic digestion of sewage sludge in an anaerobic fixed bed digester

The anaerobic digestion of sewage sludge in conventional reactors requires a hydraulic retention time of around 20 days. By using a fixed bed reactor the hydraulic retention time was reduced to 3–7 days. Recirculation increased the removal of organic matter in this system.     

Application of nano-scale transition metal carbides as accelerants in anaerobic digestion

Four types of nano-scale transition metal carbides (HfC, SiC, TiC, and WC), used as accelerants in anaerobic digestion (AD) with cattle manure, were investigated through batch experiments under mesophilic conditions (37 ± 1 °C). The AD system with four carbide accelerants showed a higher biogas yield (463–499 mL/g TS), chemical oxygen demand (COD) degradation rate (58.62–78.90%) and total Kjeldahl nitrogen (TKN) concentrations (905.0–1077.0 mg/L) as compared with control check (CK, 294 mL/g TS, 46.99%, 290 mg/L). All of the digestate samples from the AD systems using four carbide accelerants showed not only higher degradation of organic compounds during thermal analysis, but also stronger fertilizer values. The use of transition metal compounds (TMCs) as accelerants in AD can efficiently improve the conversion of waste resources into biogas and fertilizers, which can potentially open new avenues for the use of TMCs in upcoming research on biomass energy.     

An integrated approach to energy production and nutrient recovery through anaerobic digestion of Vetiveria zizanoides

This paper reports on experimental results used to verify the applicability of Vetiveria zizanoides (VZ) as a virtuous energetic crop. VZ produces biogas through its anaerobic digestion, and its nutrient content can be recovered through reuse, after digestion, as an agricultural amendment. Biomethanation tests were conducted with fresh and pretreated VZ, and the results of these tests were compared with those from the anaerobic degradation of common garden grass. Specific methane production was found to be around 650 Nm³ per ton of total organic carbon (TOC) for Vetiveria zizanoides, and around 510 Nm³ per ton of TOC for common grass, with no significant improvement after thermal pretreatment. Germination tests conducted with the digested VZ showed that the produced digestate fulfills the requirements of a fertilizer.     

Modeling of coupling gasification and anaerobic digestion processes for maize bioenergy conversion

This work estimates the advantages of using maize as fuel in a power plant composed of an anaerobic digester, a gasifier and an Internal Combustion (IC) engine. The digester is fed with maize grains, while, the remaining part of the plant, the stover, is gasified. Then biogas and syngas streams are both used as fuel into the engine. The performance of this plant was evaluated coupling gasification and anaerobic digestion mathematical models. Results of the proposed solution are compared with the performance of a 100 kW biogas power plant fed with the whole crop silaged. Results show that the overall energy yield of the improved solution is 39% higher than the conventional one fed with maize silage. This method will lead to the design of small and cheap digesters as a result of the increased conversion rate. In fact, the solution proposed fully converts the high cellulose-fiber parts of the maize plant that were tough to degrade in anaerobic digesters.     

Comprehensive study on a two-stage anaerobic digestion process for the sequential production of hydrogen and methane from cost-effective molasses

Two-stage anaerobic digestion process has been frequently applied to the sequential production of hydrogen and methane from various organic substrates/wastes. In this study, a cost-effective byproduct of food industry, molasses, was used as a sole carbon source for the two-stage biogas-producing process. The two-stage process consisted of two reactor parts named as the first-stage hydrogenic reactor (HR) operated at pH 5.5 and 35°C and the second-stage methanogenic reactor (MR) at pH 7.0 and 35°C. Microbial community analysis revealed that Clostridium butyricum was the major hydrogen-producing bacteria and methanogens consisted of hydrotrophic bacteria like Methanobacterium beijingense and acetotrophic bacteria like Methanothrix soehngenii. In the first-stage process, hydrogen could be efficiently produced from diluted molasses with the highest production rate of 2.8 (±0.22) L-H₂/L-reactor/d at the optimum HRT of 6 h. In the second-stage process, methane could be also produced from residual sugars and VFAs with a production rate of 1.48 (±0.09) L-CH₄/L-reactor/d at the optimum HRT of 6 d, at which overall COD removal efficiency of the two-stage process was determined to be 79.8%. Finally, economic assessment supported that cost-effective molasses was a potent carbon source for the sequential production of hydrogen and methane by two-stage anaerobic digestion process.     

Bio-hythane production from residual biomass of Chlorella sp. biomass through a two-stage anaerobic digestion

Residual Chlorella sp. biomass obtained after anaerobic solid-state fermentation was used to produce bio-hythane. The residual biomass was pretreated using acid, thermal, and acid-thermal methods before their respective hydrolysates were used in dark fermentation followed by the methanogenesis of anaerobic digestion to produce hydrogen and methane, respectively. Pretreatment of the residual biomass using acid and thermal methods did not significantly increase reducing sugar production. However, a maximum reducing sugar content of 28.9 mg-reducing-sugar·g-biomass⁻¹ was attained using an acid-thermal method, resulting in the highest hydrogen and methane yields of 12.5 and 81 mL·g-volatile-solid⁻¹, respectively. This was equivalent to the total energy yield of 3.03 kJ·g-VS⁻¹ or 4.6% energy recovery, based on the heating value of the residual biomass.