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published online 24 June 2014Waste Manag ResAndrea Hublin, Daniel Rolph Schneider and Janko Dzodan

environmental effectsUtilization of biogas produced by anaerobic digestion of agro-industrial waste: Energy, economic and

  

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Introduction

Renewable energy sources, mainly the utilization of organic waste materials as a source of energy, will become attractive substitutes in the near future. The most important property of alternative energy sources is their environmental compatibility (Kothari et al., 2010). New environmentally friendly technolo-gies will take advantage of any renewable energy source to pro-duce energy and will treat waste in a less costly way, in terms of energy consumption and environmental impact (Spachos and Stamatis, 2011).

The production of biogas through anaerobic digestion has been evaluated as one of the most energy-efficient and environ-mentally beneficial technologies for bioenergy production (Insam and Wett, 2008; Leke et al., 2013; Panwar et al., 2011). Biogas from wastes is a versatile renewable energy source which can be used for replacement of fossil fuels in power and heat production, and it can also be used as gaseous vehicle fuel. Methane-rich biogas (biomethane) can replace natural gas as a feedstock for producing chemicals and materials or simply be injected into the gas grid. It can significantly reduce greenhouse gas (GHG) emis-sions compared to fossil fuels (Liebetrau et al., 2013; Rehl and Müller, 2013; Scholz et al., 2011; Weiland, 2010).

Methane content in a biogas is generally in the range 45%–70%, carbon dioxide from 30% to 45%, nitrogen from <1% to 15%, and other gases are only trace components (Rasi et al.,

2007). Both CO2 and CH4 are potent GHGs (CH4 has 21 times more global warming potential than CO2, over the 100-year time horizon) (Eggleston et al., 2006). CO2 released through natural mineralization is considered neutral in GHG terms as the carbon has been recently removed from the atmosphere by plant uptake, to be released again as part of the carbon cycle. Controlled anaerobic digestion of organic material is therefore environmentally beneficial in two ways: (1) by containing the decomposition processes in a sealed environment, potentially damaging CH4 is prevented from entering the atmosphere, and subsequent burning of the gas will release carbon-neutral CO2 back to the carbon cycle; (2) the energy gained from combus-tion of CH4 will displace fossil fuels, reducing the production of CO2 that is not part of the recent carbon cycle (Ward et al., 2008).

Utilization of biogas produced by anaerobic digestion of agro-industrial waste: Energy, economic and environmental effects

Andrea Hublin1, Daniel Rolph Schneider2 and Janko Džodan1

AbstractAnaerobic digestion of agro-industrial waste is of significant interest in order to facilitate a sustainable development of energy supply. Using of material and energy potentials of agro-industrial waste, in the framework of technical, economic, and ecological possibilities, contributes in increasing the share of energy generated from renewable energy sources. The paper deals with the benefits arising from the utilization of biogas produced by co-digestion of whey and cow manure. The advantages of this process are the profitability of the plant and the convenience in realizing an anaerobic digestion plant to produce biogas that is enabled by the benefits from the sale of electric energy at favorable prices. Economic aspects are related to the capital cost (€ 2,250,000) of anaerobic digestion treatment in a biogas plant with a 300 kW power and 510 kW heating unit in a medium size farm (450 livestock units). Considering the optimum biogas yield of 20.7 dm3 kg−1 of wet substrate and methane content in the biogas obtained of 79%, the anaerobic process results in a daily methane production of 2,500 kg, with the maximum power generation of 2,160,000 kWh y−1 and heat generation of 2,400,000 kWh y−1. The net present value (NPV), internal rate of return (IRR) and payback period for implementation of profitable anaerobic digestion process is evaluated. Ecological aspects related to carbon dioxide (CO2) and methane (CH4) emission reduction are assessed.

KeywordsAnaerobic digestion, agro-industrial waste, biogas, energy, economic, environmental

1 EKONERG – Energy and Environmental Protection Institute, Ltd., Zagreb, Croatia

2 Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Croatia

Corresponding author:Andrea Hublin, EKONERG – Energy and Environmental Protection Institute, Ltd., Koranska 5, HR-10000 Zagreb, Croatia. Email: andrea.hublin@ekonerg.hr

539789WMR0010.1177/0734242X14539789Waste Management & ResearchHublin et al.research-article2014

Original Article

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2 Waste Management & Research

The use of anaerobic digestion has become widespread across all European countries, thanks to specific legislative tools aimed to increase production of biogas in the various economic sectors (Comino et al., 2009). Dairy wastewater containing whey has a very strong polluting potential, approximately one hundred times greater than that caused by an equivalent volume of domestic wastewater (Mockaitis et al., 2006) because of high organic content (Demirel et al., 2005; Gannoun et al., 2008). Despite a high biodeg-radability and different possibilities of whey utilization, a large amount of whey production is not treated and is simply discarded as waste effluent (Galegenis et al., 2007; Göblös et al., 2008). This represents a significant loss of resources and causes serious pollu-tion problems. Dairy manure, which has too much suspended solids content, presents low anaerobic biodegradability (Rico et al., 2011). Consequently, co-digestion of whey with dairy manure increases specific methane yields when compared to manure-only digestion (Labatut et al., 2011; Ogejo and Li, 2010) in various types of anaer-obic bioreactors (Kavacik and Topaloglu, 2010).

Along with benefits such as pollution control, odor and patho-gen level reduction, nutrient recovery, and energy production (Amon et al., 2007; Ergüder et al., 2001; Hartmann and Ahring, 2005; Insam and Wett, 2008; Karim et al., 2005; Saddoud et al., 2007), anaerobic digestion of organic wastes to produce energy, in the form of biogas, is the most likely option to be of commer-cial interest, provided that the economics are favorable. Applying the proper process temperature conditions, an increase in biogas production and methane content in a biogas mixture could be achieved (Cavinato et al., 2010). Economic efficiency depends on investment and operating costs of the biogas plant, and on the optimum methane production (Walla and Schneeberger, 2005). The advantages of this process are the profitability of the plant, which become feasible when installation costs are amortized within a period of 4–7 years. This is possible if one has benefited from the sale of electric energy at favorable prices (Tricase and Lombardi, 2009).

Biogas utilization depends on its different commodity quality, deriving from the type of chemical refining it undergoes, at vari-ous levels, to eliminate contaminants (nitrogen, oxygen, hydro-gen sulfide, carbon dioxide, or water). Naturally, these treatments affect production costs and consequently the final price of the electricity generated (Intelligent Energy, 2012; Persson et al., 2006).

The aim of this study is to address the technology, feedstock efficiency, investment costs, and labor requirement of small and medium-scale biogas plants relevant for Croatian conditions. The results of the study will provide information for farmers and deci-sion makers.

Materials and methodsOptimization of biogas production from co-digestion of agro-industrial waste

The optimization of anaerobic co-digestion process of whey and cow manure was performed with the aim of achieving the

maximum biogas production in laboratory batch tests (Hublin et al., 2012). A lab-scale study was compared to the studies pre-sented in the literature, where experiments were performed in specially designed anaerobic reactors (Bolzonella et al., 2012; Kardos et al., 2011; Singh and Prerna, 2009; Vindis et al., 2009). Comparable results for biogas production, CH4 content in a biogas mixture, and removal efficiencies of chemical oxygen demand (COD) were obtained by comparing the laboratory batch tests and literature data.

Modeling of the co-digestion process of agro-industrial waste

A mathematical model has been developed to simulate methane generation during the co-digestion process of whey and cow manure (Hublin and Zelić, 2013). Efficient biogas production and methane content in a biogas mixture as renewable energy is the primary advantage of the whey and cow manure co-digestion process in a one-stage batch reactor at thermophilic conditions compared to other treatments of these substrates. Model applica-bility is reflected in a prediction of methane generation for differ-ent initial substrate concentrations, in a short (several days) and long (several weeks) term periods.

A lab-scale study performed in a batch system was used to estimate the parameters of the full-scale biogas plant. Results of model prediction were used for the analysis of energy, economic, and environmental issues of the presumed biogas plant.

Energy, economic and environmental effects of biogas produced by co-digestion of agro-industrial wasteTechno-economic assessment. For the purpose of techno- economic analysis of the process of co-digesting whey (10% v/v in the initial reaction mixture) and cow manure (90% v/v in the initial reaction mixture) it is assumed that a biogas plant with a 300 kW power and 510 kW heat unit is installed, in which the excrement of 450 livestock units will be treated in a batch system at 55 °C. The biogas power plant consists of two basic parts: biogas plant and power plant. The biogas plant consists of several units: raw material storage, raw material dosing and storage sys-tem, digester, post-digestion, biogas storage tank, digestate stor-age, and technology for processing/purification of biogas. Units for power and heat generation, a power station, and a unit for control and monitoring of technological processes are the basic parts of the power plant. A schematic diagram of a biogas power plant is presented in Figure 1.

Conversion of biogas into two energy forms (heat and/or elec-tricity by means of a cogeneration system) could be calculated taking into account that 1 m3 of biogas produces 1.9 kWh of elec-tricity and 3.8 kWh of heat, considering that the lower heating value of biogas is 23 MJ kg−1 (Tricase and Lombardi, 2009). After experimental evaluation of the optimum working parame-ters (concentration of substrates, temperature, and pH of reaction mixture), the economic aspects of the anaerobic co-digestion pro-cess were considered in terms of capital costs. Techno-economic

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Hublin et al. 3

analysis of the process of co-digesting whey and cow manure was made using a model which includes mass balance, energy balance, reaction kinetics, biogas utilization, capital cost esti-mate, and profitability analysis. Results were compared to litera-ture data (Baldwin et al., 2009).

The Croatian Tariff System for Electricity Production from Renewable Energy Sources and Cogeneration (Official Gazette, 2013) determines the right of eligible electricity producer on the subsidized price of electricity by the market operator. The Tariff System determines the amount of incentives (tariffs) for electric-ity produced from plants using renewable energy sources and cogeneration plants, depending on the source, power, and other elements of supplied electricity. The investment project refers to two different kinds of products that will be marketed: (1) electric-ity from a power purchase agreement, concluded with the Croatian Energy Market Operator (HROTE) for a period of four-teen years; (2) heat will be used to heat greenhouses for growing vegetables or for other purposes. Justification of the investment project was analyzed using the basic criteria of investment decision-making such as the net present value (NPV), internal rate of return (IRR), and the payback period of investment (Brealey and Myers, 2003). The analysis was conducted for an investment period of two years and various of timescales for investment projects, i.e. 12, 15, and 20 years.

NPV is the basic criterion for the financial decision-making in general and can be defined as the difference between the sum of discounted net cash flows and the amount of investment, through-out the period of the project. The following equation was applied to calculate the NPV:

NPVC

r

I

rt

Tt

tt

Tt

t00 01 1

= ∑+

∑+( )= =( )

− (1)

where NPV0 is a net present value, Ct is a net annual cash flow, It is a capital investment, t is a year in analyzed time period, T is a number of analyzed years, and r is a discount rate.

IRR, the second fundamental criterion of financial decision-making, is the discount rate that reduces the net cash flows of the project to the value of its investment throughout the period of effectuation. It is the rate of profitability of investment in the pro-ject. The following equation was used to calculate the IRR:

ti

Titi

tite

Tete

te

I

IRR

C

IRR∑

+∑

+( ) ( )1 1= (2)

where It is a capital investment, Ct is a net annual cash flow, ti is a year of investment, Ti is a total period of investment, te is a year of effectuation, Te is a total period of effectuation, and IRR is an internal rate of return.

A payback period is the simplest criterion for financial deci-sions regarding the real investments. A payback period of the project is found by calculating the number of years it takes before the cumulative forecasted cash flow equals the initial investment. The following equation was used to calculate the payback period:

I Ct

tp

t= ∑=1

(3)

where I is a capital investment, Ct is a net annual cash flow, t is a year in analyzed time period, and tp is the payback period of the investment.

Ecological aspects of biogas utilization. A methodology pro-posed by the 2006 IPCC Guidelines was used for the calculation of the GHG emission reduction potential (expressed in ktonnes CO2-eq) (Eggleston et al., 2006). Positive effects could be achieved by utilization of biogas produced by anaerobic diges-tion of agro-industrial waste as follows: (1) reducing emissions of CO2 as a product of burning fossil fuels – using biogas instead of fossil fuels directly affects the reduction of CO2 emissions since the biogas is a fuel neutral with respect to CO2; (2) reducing CH4 emissions resulting from anaerobic decomposition of biode-gradable organic waste in landfills – the use of biodegradable organic waste to produce biogas reduces the amount of waste

Figure 1. Schematic diagram of biogas power plant.

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4 Waste Management & Research

entering landfills, which is in accordance with the requirements of EU Waste Framework Directive 2008/98/EC.

Results and discussion

The efficiency of the whey and cow manure co-digestion process in comparison with the cow manure digestion is reflected by increasing the volume of produced biogas for 12% in laboratory batch-tests. The maximum methane content (79%) in a biogas mixture was achieved on the 19th day of co-digesting. Good methane content in a biogas mixture was realized from the 10th to the 45th day (from 67 to 72%). The maximum removal efficiencies of COD that is 55% indicate that whey could be efficiently degraded to biogas in a one-stage batch process when co-digested with cow manure (Hublin et al., 2012). Waste such as whey produced in dairy industries which carries a high organic load in terms of COD, instead of requiring a lot of energy to be treated in an aerobic plant, can produce energy itself. A compari-son of anaerobic treatment performance level for whey and cow manure with some other biodegradable waste is presented in Table 1.

Mathematical models (Biswas et al., 2007; Frigon et al., 2009; Galegenis et al., 2007; Hublin and Zelić, 2013; López and Borzacconi, 2010) which have been developed to simulate meth-ane generation during the co-digestion processes of agro- industrial waste include different inhibitory effects that affect the production of biogas and methane. Results obtained by simula-tion could be considered as a powerful tool for predicting the behavior of anaerobic digesters (Derbal et al., 2009; Parker, 2005).

Techno-economic aspects of biogas production from agro-industrial waste

Energy recovery from biogas has taken a leap forward in the European Union. The primary energy generation grew by 15.7% in 2012 compared to 2011, which is a 1.6 Mtoe increase (12 Mtoe produced in 2012). The electricity generation from biogas in

2012, with the growth rate of 22.2% reached 46.3 TWh, and 64.9% of this was from cogeneration plants (EurObserv’ER, 2013). Germany is Europe’s biggest biogas producer and the market leader in biogas technology. Primary biogas energy out-put increased by 1.2 Mtoe between 2011 and 2012 to reach 6.4 Mtoe, which was essentially picked up by electricity generation which rose 28.6% year-on-year (by 6.1 TWh) to reach 27.2 TWh by the end of 2012. In 2013, the number of biogas plants reached 9,200, including 107 units producing biomethane (AEBIOM, 2012; GreenGasGreeds, 2013).

In Croatia, 11 biogas plants for agro-industrial wastes with a total installed power of 11.135 MW are connected to the power grid, within the system of eligible power producers. Additionally, another six biogas plants, with total installed power of 4.548 MW, have signed power purchase agreements with the HROTE. The Croatian Tariff System makes the size of the biogas plant of ≤300 kW advantageous for investors. There is a problem of the lack of substrate on most medium and small size farms, which could be overcome by establishing centralized biogas plants.

According to the results of laboratory batch tests based on the work of Hublin et al. (2012) (biogas yield in the amount of 20.7 dm3 kg−1 of wet substrate and maximum volume fraction of methane in the produced biogas which is 79%, during the 45 days of the co-digestion process) parameters of the full-scale biogas plant are estimated. It is presumed that methane production rate is 686,830 m3 y−1, which leads to electricity generation of 2,160,000 kWh y−1 and heat generation of 2,448,000 kWh y−1. Mass and energy balance of the presumed biogas plant are pro-vided in Table 2.

Results are compared with a techno-economic analysis of the process of co-digesting cow manure and different types of agro-industrial waste (Baldwin et al., 2009) which involves different digester performance, composition of the reaction mixture, and the hydraulic retention time (HRT) of the substrate in a reactor. Among all configurations involved in the simulations provided by Baldwin et al. (2009), the modified plug flow system with HRT of 25 days had the best system performance. Waste by 450 cows was used. The reaction mixture consisted of 80% v/v dairy

Table 1. Comparison of anaerobic treatment performance level for whey and cow manure with some other biodegradable waste.

Waste type Reactor type

Retention time (day)

Temp. (ºC)

COD removal efficiency (%)

Biogas production

CH4 content in biogas (%)

Appl. status

Ref.

Whey + cow manure

B 45 55 55 20.7 dm3 kg OM−1 (wet)

79 laboratory scale

(1)

Whey + cow manure

SFB 42 35 82 0.62 m3 kg VS−1

55 laboratory scale

(2)

Maize B 35 55 NS 0.61 m3 kg VS−1

60 laboratory scale

(3)

Waste activate sludge

SFB 20 55 45 0.45 m3 kg VS−1

64 pilot scale (4)

Sewage sludge B 50 55 NS 990 dm3 kg OM−1

59 pilot scale (5)

B – batch reactor; SFB – stirred fed-batch reactor; OM – organic material; VS – volatile solids; NS – not specified. (1) Hublin et al., 2012; (2) Comino et al., 2012; (3) Vindis et al., 2009; (4) Bolzonella et al., 2012; (5) Kardos et al., 2011.

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Hublin et al. 5

manure and 20% v/v food waste. This system could produce about 1,833,000 kWh y-1 of electricity and 1,981,000 kWh y-1 of heat in a co-generation plant.

Annual efficiency is one of the basic conditions for acquiring the status of an eligible electricity producer, since the Croatian Tariff System stipulates that it must be at least 50%. By taking into account total consumed biogas, net electricity generated, and sup-plied to the power distribution system and useful heat consump-tion, the average annual efficiency amounts to 83% (Table 2).

Initial assumptions for economic analysis. Assumptions used in the economic analysis were as follows:

(1) Cost of substrate (cow manure and whey) will involve trans-portation costs; whey will be procured in agreement with the dairies, at market price.

(2) Produced biogas will be used as fuel in a cogeneration plant based on gas engines.

(3) Digestate will not be sold, but will spread across farmland.(4) All electricity generated, reduced for own consumption, will be

distributed to the electric power distribution system by stimulat-ing the purchase price. Incentive purchase price of electricity will be achieved on the basis of the signed contract with the HROTE for a period of fourteen years. For the purpose of the analysis, it is assumed that in the 15th year the price of electricity will no longer be stimulated but it will be market-based.

(5) One part of the total heat generated will be used to heat the digester. The second part of generated heat will be sold to nearby greenhouses for growing vegetables or for other purposes.

(6) Analysis was conducted for the investment period of two years and various effectuation periods of investment, i.e. 12, 15, and 20 years.

(7) Discount rate of 6% is used in NPV calculation.

An economic analysis of the investment project is presented in Table 3.

The economic analysis indicates that the presumed biogas plant is profitable for 12 and 15 year effectuation periods. The positive NPV of the investment project, which amounts to € 87,861 in the 12th year and € 183,047 in the 15th year, as well as the IRR, which is higher than the discount rate for 12 and 15 year effectuation periods, are indicators of plant profitability.

Sensitivity analysis. For the purpose of sensitivity analysis, the efficiency of the investment project was measured. Key variables that affected most the efficiency of the project are as follows:

(1) Capital investment – the impact of the percentage change of investment in land, biogas and cogeneration plant, operating and maintenance cost, etc. on the efficiency of the invest-ment project.

(2) Electricity price – the impact of the percentage change of the purchase price of electricity on the efficiency of the invest-ment project.

NPV, IRR, and payback period were used as indicators of the efficiency of the investment project. The analysis was conducted for an investment period of two years and a 15 year effectuation period. Results of the sensitivity analysis are given in Table 4.

The results of the sensitivity analysis indicate that the invest-ment project is most sensitive to the change in the purchase price of electricity regarding NPV. The change was only applied to subsidized price according to the Croatian Tariff System valid for the first 14 years of effectuation. It was adjusted annually in rela-tion to the increase in the inflation index. The NPV increases significantly in relation to increase of the purchase price of elec-tricity. For example, a 5% increase in electricity price doubles the NPV. Approximately three times higher NPV is achieved regard-ing the increase of electricity price for 10%. On the other hand, a 10% decrease in electricity price causes decrease of NPV by half.

Table 2. Mass and energy balance of the biogas power plant.

Initial mixtureCow manure t y−1 4,860Whey t y−1 540Total initial mixture t y−1 5,400Biogas productionLower heating value of biogas kWh m−3 6.4Produced biogas m3 y−1 869,400Produced methane m3 y−1 686,830Energy value of biogas kWh y−1 5,554,500DigestateTotal digestate t y−1 4,590ElectricityElectrical power kW 300Supplied electricity to the power grid kWh y−1 2,160,000HeatHeat power kW 510Delivered heat kWh y−1 2,448,000Annual efficiencyAnnual efficiency (min 50%) % 83

Table 3. Economic analysis of the investment project.

CostsCapital cost € 2,250,000Operating and maintenance cost € y−1 252,500RevenuesElectricity delivered € y−1 445,766Heat delivered € y−1 58,752ProfitabilityNPVTwelve years effectuation period € 87,861Fifteen years effectuation period € 183,047Twenty years effectuation period € −214,343IRRTwelve years effectuation period % 6.6Fifteen years effectuation period % 7.1Twenty years effectuation period % 4.2Payback period of investmentPayback period y 9.9

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Decrease of capital investment causes increase of NPV. About 2.5 times higher NPV is achieved in relation to a 10% decrease in capital investment. On the contrary, a 10% increase in capital investment decreases NPV by approximately 1.5 times.

The cost of capital investment is the key parameter related to other financial indicators, i.e. IRR and payback period. Increase in electricity price and decrease in capital investment causes increasing in IRR. Finally, increase in electricity price and decrease in capital investment causes decreasing of payback period.

The influence of parameter variations on financial indicators is presented in Figure 2.

Ecological aspects of biogas production from agro-industrial waste

According to literature data (Eggleston et al., 2006), the use of coal for power/heat generation emits 0.7–0.8 kg CO2 kWh−1, the use of oil fuel emits 0.6–0.7 kg CO2 kWh−1, whereas the use of natural gas emits 0.35–0.4 kg CO2 kWh−1. Assuming that coal is used for the power/heat generation in a power plant, the use of biogas for the equivalent amount of generated energy (Table 2) directly reduces emissions by about 1.7 ktonnes CO2 kWh−1 y−1. The use of biogas for the equivalent amount of heat generation directly reduces emissions by about 1.8 ktonnes CO2 kWh−1 y−1.

Indirect reduction of CH4 emissions by using the biodegrada-ble organic waste to produce biogas is achieved by reducing the amount of waste entering landfills. Assuming that one tonne of biodegradable waste (67% of degradable organic carbon), within the period of 20 years emitted 0.05 tonne of CH4, the use of 5,400 tonne y−1 of biodegradable waste could reduce emissions by 0.27 ktonnes CH4, corresponding to 5.7 ktonnes CO2-eq y−1.

Conclusion

This study has shown the advantages of the co-digestion process of whey and cow manure regarding the profitability of the plant and the convenience in realizing an anaerobic digestion plant to produce biogas that is enabled by the benefits from the sale of electric energy at favorable prices.

A methane production rate of 686,830 m3 y−1 leads to electric-ity generation of 2,160,000 kWh y−1 and heat generation of 2,448,000 kWh y−1 in the presumed biogas plant.

Economic analysis of the investment project shows that the presumed medium-scale biogas plant is profitable for 12 and 15 year effectuation periods, according to the NPV, IRR, and pay-back period of the investment project. The results of sensitivity analysis indicate that the investment project is most sensitive to the change in the purchase price of electricity regarding NPV. The cost of capital investment is the key parameter related to IRR and payback period as financial indicators.

Table 4. Results of sensitivity analysis.

Percent change Profitability indicator Unit Capital investment Electricity price

10.0% NPV15 € −98,554 543,544IRR15 % 5.4 9.1Payback period y 10.7 8.8

7.5% NPV15 € −27,926 453,420IRR15 % 5.8 8.6Payback period y 10.5 9.1

5.0% NPV15 € 42,550 363,296IRR15 % 6.2 8.1Payback period y 10.3 9.3

2.5% NPV15 € 112,875 273,171IRR15 % 6.7 7.6Payback period y 10.1 9.6

0.0% NPV15 € 183,047 183,047IRR15 % 7.1 7.1Payback period y 9.9 9.9

−2.5% NPV15 € 253,068 92,923IRR15 % 7.5 6.6Payback period y 9.8 10.3

−5.0% NPV15 € 322,938 2,799IRR15 % 7.9 6.0Payback period y 9.6 10.6

−7.5% NPV15 € 392,655 −87,325IRR15 % 8.4 5.5Payback period y 9.4 11.0

−10.0% NPV15 € 462,221 −177,449IRR15 % 8.8 4.9Payback period y 9.2 11.4

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Hublin et al. 7

Positive ecological effects could be achieved by utilization of biogas produced by co-digestion of whey and cow manure. Direct reduction of CO2 emission is estimated to be about 1.7 ktonnes CO2 kWh-1 y−1 regarding the electricity and 1.8 ktonnes CO2 kWh−1 y−1 regarding the heat. Indirect reduction of CH4 emission is estimated to be about 5.7 ktonnes CO2-eq y−1.

Declaration of conflicting interestsThe authors declare that there is no conflict of interest.

FundingThis research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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