Digestate Storage Research Articles

The potential use of shear viscosity to monitor polymer conditioning of sewage sludge digestates

The work assessed the use of shear viscosity at 0.1 s−1 (η[0.1s−1]) as a parameter to detect changes in the conditioning and dewatering of digestates. Total and soluble fractions of organic matter of digestate samples before and after storage were also assessed in regards to their conditioning and dewatering performance. Digestate from a conventional mesophilic anaerobic digestion (CMAD) and advanced anaerobic digestion (AAD) plants were used. Linear regression and correlation analysis of 29 different parameters showed that soluble and total fractions of organic matter (Norg, Sc, Sp, Tp, TKN/COD, tCOD and sCOD) during plant operation and storage conditions correlated (r between 0.80 and 0.99) with the variation in polymer dose, floc strength and CST of conditioned digestate samples. The variations occurred within the content of soluble and total fractions of organic matter, and showed to correlate with both conditioning requirements and the variation in η[0.1s−1]. The work concluded that η[0.1s−1] measurements of unconditioned digestate samples have the potential to be used as a parameter to monitor conditioning requirements during digestate storage or during process changes. It was found important to analyse soluble and total fractions of organic matter in order to understand the changes in η[0.1s−1] within specific process conditions.

Agricultural anaerobic digestion plants: What LCA studies pointed out and what can be done to make them more environmentally sustainable

Anaerobic Digestion (AD) has been recognized as a viable solution to produce renewable energy and to reduce global warming especially when secondary feedstock and/or wastes are used. Several LCA studies analysed the environmental performances of biogas production systems. The results of this review highlight that the goal, scope, life cycle impact assessment (LCIA) methodology, feedstocks and geographical regions covered by the studies vary widely. Most studies are based in Europe, several in China and few in South and North America and in Africa.To better highlight how the choices on the feeding mix, the digestate storage, the surplus heat valorisation as well as the plant size can affect the environmental performances of agricultural AD plants four plants have been analysed in this study. The results suggest that the energy crops production and the operation of anaerobic digesters, including digestate emission from open tanks, are the main contributors to the impacts from biogas electricity. This entails that it is environmentally better to have smaller plants using slurry and waste rather than bigger plants fed with energy crops. Recovering heat waste as well as covering of digestate tank would improve significantly the environmental sustainability of biogas electricity, and particularly the global warming category.

Ammonia emissions from an anaerobic digestion plant estimated using atmospheric measurements and dispersion modelling

Anaerobic digestion (AD) is becoming increasingly implemented within organic waste treatment operations. The storage and processing of large volumes of organic wastes through AD has been identified as a significant source of ammonia (NH3) emissions, however the totality of ammonia emissions from an AD plant have not been previously quantified. The emissions from an AD plant processing food waste were estimated through integrating ambient NH3 concentration measurements, atmospheric dispersion modelling, and comparison with published emission factors (EFs). Two dispersion models (ADMS and a backwards Lagrangian stochastic (bLS) model) were applied to calculate emission estimates. The bLS model (WindTrax) was used to back-calculate a total (top-down) emission rate for the AD plant from a point of continuous NH3 measurement downwind from the plant. The back-calculated emission rates were then input to the ADMS forward dispersion model to make predictions of air NH3 concentrations around the site, and evaluated against weekly passive sampler NH3 measurements.As an alternative approach emission rates from individual sources within the plant were initially estimated by applying literature EFs to the available site parameters concerning the chemical composition of waste materials, room air concentrations, ventilation rates, etc. The individual emission rates were input to ADMS and later tuned by fitting the simulated ambient concentrations to the observed (passive sampler) concentration field, which gave an excellent match to measurements after an iterative process. The total emission from the AD plant thus estimated by a bottom-up approach was 16.8±1.8mgs−1, which was significantly higher than the back-calculated top-down estimate (7.4±0.78mgs−1). The bottom-up approach offered a more realistic treatment of the source distribution within the plant area, while the complexity of the site was not ideally suited to the bLS method, thus the bottom-up method is believed to give a better estimate of emissions. The storage of solid digestate and the aerobic treatment of liquid effluents at the site were the greatest sources of NH3 emissions.

Cover Chem. Eng. Technol. 4/2016

AbstractCover: BIOGAS PILOT PLANT at the DBFZThe cover picture shows the biogas pilot plant at the DBFZ (German Biomass Research Center). This extends the range of application‐oriented research being carried out at the DBFZ to enhance process understanding and improve the efficiency of biogas production. The dimensioning of the fermenters allows experiments to be conducted on full scale, thereby ensuring good transferability of results into practice. The facility features two independent lines with identical capacity which can be operated as a single‐ or two‐stage system, with optional hydrolysis. The first line is a wet fermentation system with a main fermenter in the form of a continuous stirred tank reactor with a central agitator. The second line can optionally be run with a main fermenter of identical design or with a plug‐flow fermenter. A post‐digester with a flexible rubber gas dome collects the digestate from both lines and after post digestion, they are carried over to the digestate storage tank. The biogas is used in a 75 kWel combined heat and power unit (CHP) to cover the facility's own energy demand. Surplus power can be fed into the DBFZ grid. For substrate supply, small amounts of self‐produced silage can be stored on site. For a precise measurement of the gas production rate the primary fermenters have solid roofs. Terminal units in the pipeline system and at the gas capture point permit sampling and the installation of additional sensors.

Life Cycle Environmental Impacts of Electricity from Biogas Produced by Anaerobic Digestion.

The aim of this study was to evaluate life cycle environmental impacts associated with the generation of electricity from biogas produced by the anaerobic digestion (AD) of agricultural products and waste. Five real plants in Italy were considered, using maize silage, slurry, and tomato waste as feedstocks and cogenerating electricity and heat; the latter is not utilized. The results suggest that maize silage and the operation of anaerobic digesters, including open storage of digestate, are the main contributors to the impacts of biogas electricity. The system that uses animal slurry is the best option, except for the marine and terrestrial ecotoxicity. The results also suggest that it is environmentally better to have smaller plants using slurry and waste rather than bigger installations, which require maize silage to operate efficiently. Electricity from biogas is environmentally more sustainable than grid electricity for seven out of 11 impacts considered. However, in comparison with natural gas, biogas electricity is worse for seven out of 11 impacts. It also has mostly higher impacts than other renewables, with a few exceptions, notably solar photovoltaics. Thus, for the AD systems and mesophilic operating conditions considered in this study, biogas electricity can help reduce greenhouse gas (GHG) emissions relative to a fossil-intensive electricity mix; however, some other impacts increase. If mitigation of climate change is the main aim, other renewables have a greater potential to reduce GHG emissions. If, in addition to this, other impacts are considered, then hydro, wind, and geothermal power are better alternatives to biogas electricity. However, utilization of heat would improve significantly its environmental sustainability, particularly global warming potential, summer smog, and the depletion of abiotic resources and the ozone layer. Further improvements can be achieved by banning open digestate storage to prevent methane emissions and regulating digestate spreading onto land to minimize emissions of ammonia and related environmental impacts.

Economics of GHG emissions mitigation via biogas production from Sorghum, maize and dairy farm manure digestion in the Po valley

The Greenhouse gas (GHG) emissions and economic feasibility of electricity production from the anaerobic digestion of different substrates are studied in this paper. Three realistic substrate options for the climatic and soil conditions of a modelled farm in the Po Valley in Italy are analysed: manure from a dairy farm, Sorghum and maize.A detailed cost analysis is performed with field data provided by farmers and suppliers and literature sources. The capital costs (CAPEX) and the operational costs (OPEX), disaggregated by their components, are presented. Investment payback time is then calculated for the different substrates and technologies, while taking into account the Italian government feed-in tariff scheme for biogas plants implemented in 2013.In the specific conditions assumed, electricity production via anaerobic digestion of manure and co-digestion of manure with at most 30% Sorghum (no till) provide both GHG savings (in comparison to the Italian electricity mix) and profit for economic operators.The anaerobic digestion of silage maize or Sorghum alone, instead, provides no (or very limited) GHG savings, and, with the current feed-in tariffs, generates economic losses.Both economic and environmental performance are improved by the following practices: cultivating Sorghum instead of maize; implementing no till agriculture; and installing gas-tight tanks for digestate storage. A tool allowing a customised calculation of the economic performances of biogas plants is provided.

Laboratory testing on the removal of the veterinary antibiotic doxycycline during long-term liquid pig manure and digestate storage

The veterinary antibiotic doxycycline (DOXY) is today frequently applied in conventional pig husbandry for the control of respiratory diseases. After the treatment, pigs excrete major amounts of DOXY as the unchanged active substance. Thus, DOXY residues were found in liquid manures and digestates of biogas plants at concentrations of mg kg−1 dry weight. In order to assess the impact of field applications of contaminated manures and digestates on the entry of DOXY residues into arable and grassland soils, thorough information about the removal of DOXY during long-term storage of farm fertilizers is required. Since this aspect has been only less investigated for manures but not for digestates, first long-term storage simulation tests were performed at laboratory scale. Within the 170-d incubation periods under strictly anaerobic conditions, doxycycline was removed in liquid pig manure by 61% and in digestate by 76%. The calculated half-lives of 120 d and 91 d thus emphasized the persistence of doxycycline in both matrices. Due to the substance specific properties of DOXY, this removal was caused neither by mineralization, epimerization nor biotransformation. According to the high affinity of DOXY to manure and digestate solids, however, the formation of non-extractable residues has to be taken into account as the predominant concentration determining process. This was indicated by the sequential extraction procedure applied. Hence, these results confirmed that a full removal capacity for doxycycline cannot be reached through the long-term storage of farm fertilizers.

Environmental assessment of electricity generation from an Italian anaerobic digestion plant

A standard ISO Life Cycle Assessment study was carried out to evaluate the environmental sustainability of electricity production from an anaerobic digestion (AD) plant using a mixture of dedicated energy crops, agricultural residues and livestock effluents as input materials. The functional unit was 1 MJ of electricity. System boundaries were from cradle to grave and covered all the phases from energy crops cultivation to the production of biogas and its use in a Combined Heat and Power plant to produce electricity. Liquid and solid digestate storage and spreading on agricultural land were included. Primary data were collected from the AD plant for all the above phases. Since heat produced is used only internally, no allocation was applied in the study. As regards digestate management, CH4 emissions were calculated from literature, whereas four literature methods were applied for calculation of nitrogen emissions with the goal to perform a sensitivity analysis on LCA results. ILCD Handbook impact assessment methodologies were used. Results show that the main hotspots are energy crops cultivation and the management of digestate, mainly because of both nitrogen and methane emissions, affecting Global Warming, Acidification, Marine and Freshwater Eutrophication. Finally, a detailed Monte Carlo analysis, was carried out to evaluate the results uncertainty. The study represents the state of the art about the environmental performance of the AD plant with the use of sensitivity and uncertainty analysis, which both improve the reliability of results, and allows drawing general conclusions on how to mitigate the environmental impacts of AD process.

Lifecycle Greenhouse Gas Analysis of an Anaerobic Codigestion Facility Processing Dairy Manure and Industrial Food Waste.

Anaerobic codigestion (AcoD) can address food waste disposal and manure management issues while delivering clean, renewable energy. Quantifying greenhouse gas (GHG) emissions due to implementation of AcoD is important to achieve this goal. A lifecycle analysis was performed on the basis of data from an on-farm AcoD in New York, resulting in a 71% reduction in GHG, or net reduction of 37.5 kg CO2e/t influent relative to conventional treatment of manure and food waste. Displacement of grid electricity provided the largest reduction, followed by avoidance of alternative food waste disposal options and reduced impacts associated with storage of digestate vs undigested manure. These reductions offset digester emissions and the net increase in emissions associated with land application in the AcoD case relative to the reference case. Sensitivity analysis showed that using feedstock diverted from high impact disposal pathways, control of digester emissions, and managing digestate storage emissions were opportunities to improve the AcoD GHG benefits. Regional and parametrized emissions factors for the storage emissions and land application phases would reduce uncertainty.

Environmentally Sustainable Biogas? The Key Role of Manure Co-Digestion with Energy Crops

We analysed the environmental impacts of three biogas systems based on dairy manure, sorghum and maize. The geographical scope of the analysis is the Po valley, in Italy. The anaerobic digestion of manure guarantees high GHG (Green House Gases) savings thanks to the avoided emissions from the traditional storage and management of raw manure as organic fertiliser. GHG emissions for maize and sorghum-based systems, on the other hand, are similar to those of the Italian electricity mix. In crop-based systems, the plants with open-tank storage of digestate emit 50% more GHG than those with gas-tight tanks. In all the environmental impact categories analysed (acidification, particulate matter emissions, and eutrophication), energy crops based systems have much higher impacts than the Italian electricity mix. Maize-based systems cause higher impacts than sorghum, due to more intensive cultivation. Manure-based pathways have always lower impacts than the energy crops based pathways, however, all biogas systems cause much higher impacts than the current Italian electricity mix. We conclude that manure digestion is the most efficient way to reduce GHG emissions; although there are trade-offs with other local environmental impacts. Biogas production from crops; although not providing environmental benefits per se; may be regarded as an option to facilitate the deployment of manure digestion.

Determining methane emissions from biogas plants – Operational and meteorological aspects

A micrometeorological method, combining an inverse dispersion technique with path-integrated concentration measurements, was applied on an Austrian biogas plant over the period of more than one year to determine emissions of the whole plant. Measurement campaigns were conducted to characterize the emission response to operational activities (e.g. digestate management) and meteorological changes. When digestate storage tanks were filled, an average emission rate of 7.2kg CH4/h (approx. 4% of the calculated CH4 production) was determined, while 5.4kg CH4/h of emissions (approx. 3% of the calculated CH4 production) were quantified after the tanks had been emptied. It could be observed that besides the operation mode (e.g. filling level or agitation of the openly stored digestate, maintenance), the meteorological conditions such as wind speed and solar radiation (e.g. heat flux) can also affect the emission rate.

Consequential life cycle assessment of biogas, biofuel and biomass energy options within an arable crop rotation

AbstractFeed in tariffs (FiTs) and renewable heat incentives (RHIs) are driving a rapid expansion in anaerobic digestion (AD) coupled with combined heat and power (CHP) plants in the UK. Farm models were combined with consequential life cycle assessment (CLCA) to assess the net environmental balance of representative biogas, biofuel and biomass scenarios on a large arable farm, capturing crop rotation and digestate nutrient cycling effects. All bioenergy options led to avoided fossil resource depletion. Global warming potential (GWP) balances ranged from −1732 kg CO2e Mg−1 dry matter (DM) for pig slurry AD feedstock after accounting for avoided slurry storage to +2251 kg CO2e Mg−1 DM for oilseed rape biodiesel feedstock after attributing indirect land use change (iLUC) to displaced food production. Maize monoculture for AD led to net GWP increases via iLUC, but optimized integration of maize into an arable rotation resulted in negligible food crop displacement and iLUC. However, even under best‐case assumptions such as full use of heat output from AD‐CHP, crop–biogas achieved low GWP reductions per hectare compared with Miscanthus heating pellets under default estimates of iLUC. Ecosystem services (ES) assessment highlighted soil and water quality risks for maize cultivation. All bioenergy crop options led to net increases in eutrophication after displaced food production was accounted for. The environmental balance of AD is sensitive to design and management factors such as digestate storage and application techniques, which are not well regulated in the UK. Currently, FiT payments are not dependent on compliance with sustainability criteria. We conclude that CLCA and ES effects should be integrated into sustainability criteria for FiTs and RHIs, to direct public money towards resource‐efficient renewable energy options that achieve genuine climate protection without degrading soil, air or water quality.

Digestion of bio-waste - GHG emissions and mitigation potential

For a precise description of the emission situation of the anaerobic digestion (AD) of the separately collected organic fraction of household waste (bio-waste), only a few data are available. The paper presents the greenhouse gas (GHG) emissions measured at 12 representative AD plants treating bio-waste. The results of the emission measurements were used to assess the ecological impact of bio-waste digestion and to describe possible mitigation measures to reduce the occurring GHG emissions. With respect to the climate protection, a quantitative assessment of the emissions of energy generation from biomass and biological waste treatment is important. Biogas plants need to be operated in a way that negative environmental effects are avoided and human health is not compromised. GHG balances were calculated based on the measured emissions of the gases methane, nitrous oxide, and ammonia of bio-waste AD plants. The emission analysis supports the reduction of GHGs in biogas production and contributes to a climate-efficient technology. The results show that GHG emissions can be minimized, if the technology and operation of the plant are adjusted accordingly. The open storage of active material (e.g., insufficient fermented residues from batch fermentation systems), open digestate storage tanks, missing acidic scrubbers in front of bio-filters, or insufficient air supply during the post-composting of digestate can cause relevant GHG emissions. Consequently avoiding open storage of insufficient fermented residues and using aerated post-composting with short turnover periods, smaller heaps, and an optimized amount of structure (woody) material can reduce GHG emissions.

A Novel Solution for Biogas Applications in Poultry Industry: CLAMBS approach

Chicken manure has high solid and high ammonia content which dictates the dilution of feed material with huge amount of freshwater in order to minimize ammonia toxicity, if it is to be digested alone. In this study, a novel approach is presented to significantly minimize fresh water need for biogas application and to eliminate the construction of costly digestate storage tanks. For this purpose, membrane process which allows recycling of digestate after ammonia removal was integrated to the conventional biogas system. Therefore, the selection of proper membranes for the removal of ammonia from digestate was carried out by testing six different types of Nano Filtration (NF) membranes (NF270, NF245, NF200, NF90, NF-NF, NP010) and three different types of Reverse Osmosis (RO) membranes (BW30, SW30, X20) in cross flow microfiltration experiments. The performance of the membranes was tested by monitoring parameters such as pH, conductivity, COD, TN, NO2-N, NO3-N, TP in the permeate under various operating conditions. NF90 (83%) and X20 (92%) type membranes were found to be most effective for the removal of TN from the digestate. Moreover, COD (Chemical Oxygen Demand) removal efficiency reached up to 99% for both NF90 and X20 membrane.

Evaluation of anaerobic digestate for greenhouse gas emissions at various stages of its management

Digestate has certain amount of plant nutrients and organic matter and can be used as organic fertilizer. However, it has significant amount of methane formation potential, and could contribute to the emission of greenhouse gases (GHGs) and hence need to be managed carefully. The main purpose of this study was to find environmentally suitable options for digestate management. In this study digestate from a continuously operated pilot-scale thermophilic dry anaerobic digester was continuously monitored for its characteristics like TS, VS, C and N. It was found that the C/N ratio of digestate was 15–20 for most of the study period, which is safe range for its application to agricultural land without further treatment. Based on the characteristics, GHG emission potential of digestate was determined at its different phases of handling/management (raw digestate, stored digestate, stored-cured digestate) and net GHG emission was found for five proposed scenarios of digestate management. Results showed that raw digestate had maximum GHG emission potential of 139 g CO2-eq/kg waste compared to stored digestate and stored-cured digestate having 125 and 80 g CO2-eq/kg waste respectively. Similarly, storage of digestate for 2 months was found to emit 10% of its total GHG potential. Moreover, net GHG emission was found minimum in the digestate management scenarios having land application of digestate.

Cattle feed or bioenergy? Consequential life cycle assessment of biogas feedstock options on dairy farms

AbstractOn‐farm anaerobic digestion (AD) of wastes and crops can potentially avoid greenhouse gas (GHG) emissions, but incurs extensive environmental effects via carbon and nitrogen cycles and substitution of multiple processes within and outside farm system boundaries. Farm models were combined with consequential life cycle assessment (CLCA) to assess plausible biogas and miscanthus heating pellet scenarios on dairy farms. On the large dairy farm, the introduction of slurry‐only AD led to reductions in global warming potential (GWP) and resource depletion burdens of 14% and 67%, respectively, but eutrophication and acidification burden increases of 9% and 10%, respectively, assuming open tank digestate storage. Marginal GWP burdens per Mg dry matter (DM) feedstock codigested with slurry ranged from –637 kg CO2e for food waste to +509 kg CO2e for maize. Codigestion of grass and maize led to increased imports of concentrate feed to the farm, negating the GWP benefits of grid electricity substitution. Attributing grass‐to‐arable land use change (LUC) to marginal wheat feed production led to net GWP burdens exceeding 900 kg CO2e Mg−1 maize DM codigested. Converting the medium‐sized dairy farm to a beef‐plus‐AD farm led to a minor reduction in GWP when grass‐to‐arable LUC was excluded, but a 38% GWP increase when such LUC was attributed to marginal maize and wheat feed required for intensive compensatory milk production. If marginal animal feed is derived from soybeans cultivated on recently converted cropland in South America, the net GWP burden increases to 4099 kg CO2e Mg−1 maize DM codigested – equivalent to 55 Mg CO2e yr−1 per hectare used for AD‐maize cultivation. We conclude that AD of slurry and food waste on dairy farms is an effective GHG mitigation option, but that the quantity of codigested crops should be strictly limited to avoid potentially large international carbon leakage via animal feed displacement.

Multisource emission retrieval within a biogas plant based on inverse dispersion calculations--a real-life example.

Open digestate storage tanks were identified as one of the main methane (CH4) emitters of a biogas plant. The main purpose of this paper is to determine these emission rates using an inverse dispersion technique in conjunction with open-path tunable diode laser spectroscopy (OP-TDLS) concentration measurements for multisource reconstruction. Since the condition number, a measure of "ill-conditioned" matrices, strongly influences the accuracy of source reconstruction, it is used as a diagnostic of error sensitivity. The investigations demonstrate that the condition number for a given source-sensor configuration in the highly disturbed flow field within the plant significantly depends on the meteorological conditions (e.g., wind speed, stratification, wind direction, etc.). The CH₄ emissions are retrieved by removing unrepresentative periods with high condition numbers, which indicate uncertainty in recovering the individual sources. In a final step, the CH₄ emissions are compared with the maximum biological methane potential (BMP) in the digestate analyzed under laboratory conditions. The retrieved methane emission rates represent an average of 50% of the maximum BMP of the stored digestate in the winter months, while they comprised an average of 85% during the measurement campaigns in the summer months. The results indicate that the open tanks have the potential to represent a substantial emission source even during colder periods.

Life cycle environmental impacts of generating electricity and heat from biogas produced by anaerobic digestion

Financial incentives in many European countries have led to a surge in anaerobic digestion (AD) installations to produce heat and/or electricity from biogas. This paper presents the life cycle environmental impacts of a system producing biogas from agricultural wastes by AD and co-generating heat and electricity in a combined heat and power (CHP) plant. The results suggest that this can lead to significant reductions in most impacts compared to fossil-fuel alternatives, including the global warming potential (GWP) which can be reduced by up to 50%. However, the acidification and eutrophication potentials are respectively 25 and 12 times higher than for natural gas CHP. The impacts are influenced by the type and source of feedstock, digestate storage and its application on land. Using energy crops such as maize instead of waste reduces the GWP owing to higher biogas yields, but eight out of 11 impacts increase compared to using waste feedstocks. If digestate is not used to displace artificial fertilisers, the majority of impacts are higher than from natural gas CHP. Some other bioenergy options have lower GWP than energy from biogas, including woodchip CHP plants. Implications for policy are discussed based on the results of the study.

Life Cycle Assessment of electricity production in Italy from anaerobic co-digestion of pig slurry and energy crops

This study aims to evaluate the environmental consequences and energy requirements of a biogas production system and its further conversion into bioenergy by means of the Life Cycle Assessment (LCA) methodology. To do so, an Italian biogas plant operating with pig slurry and two energy crops (maize and triticale silages) as feedstock was assessed in detail in order to identify the environmental hotspots. The environmental profile was estimated through six impact categories: abiotic depletion potential (ADP), acidification potential (AP), eutrophication potential (EP), global warming potential (GWP), ozone layer depletion potential (ODP) and photochemical oxidation potential (POFP). An energy analysis related to the cumulative non-renewable fossil and nuclear energy demand (CED) was also performed, considering this indicator as an additional impact category.According to the results, the biomass production subsystem was identified as the main environmental key issue in terms of ADP, AP, EP, ODP and CED, with contributions ranging from 26% to 61% of the total impact. Regarding ADP, ODP and CED, these results are mainly related with diesel requirements in agricultural machinery, derived combustion emissions and mineral fertilizers production. Concerning AP and EP the production field emissions derived from fertilizers application was observed as the main contributor. Concerning GWP, this step presents an environmental credit due to the uptake of CO2 during crop growth, which contributes to offset the GHG emissions. The bioenergy production plant significantly contributes to the environmental impact in categories such as GWP (43%) and POFP (59%), mostly related with emissions produced in the gas engine and biogas losses. Emissions derived from digestate storage contribute to AP (52%) and EP (41%). The use of the digestate as an organic fertilizer has a beneficial role because this action avoids the production and use of mineral fertilizers.A sensitivity analysis was also conducted to assess the influence of variations in important parameters of biogas systems. The environmental profile of the biogas system turned out to be highly dependent on the selection of system boundaries and the allocation method.To sum up, this study aims to assess the environmental performance of a biogas technology available not only in Italy but also in other European countries. The environmental analysis of the process under study highlights the environmental benefits of the co-digestion processes, which not only produces biofuel but also reduces the disposal of solid wastes and produces digestate, with special value in the fertilization of agricultural soil.

Assessment of GHG emissions of biomethane from energy cereal crops in Umbria, Italy

Biomethane from energy crops is a renewable energy carrier and therefore it potentially contributes to climate change mitigation. However, significant greenhouse gas (GHG) emissions resulting from cultivation and processing must be considered. Among those, the production and use of nitrogen fertilizers, the resulting nitrous oxide (N2O) emissions, the methane emissions from digestate storage and the energy consumption of the biogas plant are crucial factors. In the present paper an integrated life cycle assessment (LCA) of GHG emissions from biomethane production is carried out, taking into account own measurements and experience data from a modern biogas plant located in Umbria, Italy. The study is also focused on the electricity consumption of the biogas plant, assessing the specific absorption power of each machinery. The analysis is based on the methodology defined by the European Union Renewable Energy Directive 2009/28/EC (RED). The main result is that the biomethane chain exceeds the minimum value of GHG saving (35%) mainly due to the open storage of digestate. However by varying the system, using heat and electricity from a biogas CHP plant and covering digestate storage tank, a reduction of 68.9% could be obtained.