1 Introduction

The decrease in Zimbabwe’s electricity supplies due to reduced electricity generation at Kariba Power Station resulting from low water levels in Kariba Dam and curtailed output from thermal power stations resulting from dilapidated infrastructure has increased load shedding. This has led the energy regulator to engage various stakeholders to work together to solve the energy shortage problem. Suggested solutions include installing rooftop solar photovoltaic systems for own consumption and net metering excess electricity to the national grid [1]. Biomass waste, abundant in the country, can also generate electricity for consumption and export to the grid. Compared to other renewable energy resources such as wind and solar, biomass has lower investment and per unit operational costs, making it more affordable [2]. Also, using biomass waste to produce biogas, a cleaner energy carrier, may prevent competition with food and feed production.

Biogas is produced from anaerobic digestion (AD) of biodegradable waste such as crop, municipal solid, animal, industrial, slaughterhouse, and sewage sludge [3,4,5,6]. It is a mixture of mainly methane (50–75) and carbon dioxide (25–45%) and some hydrogen sulphide, water, ammonia, siloxanes, oxygen, carbon monoxide, nitrogen, and volatile organic compounds [7, 8]. The energy content and applications of biogas can be increased by cleaning and upgrading the biogas using various technologies well documented in various literature [7]. The use of AD in providing modern energy services greatly contribute to meeting Sustainable Development Goals (SDGs) [9,10,11]. The potential impacts and contributions of installing a mini biogas electricity plant (MBEP) on a farm on 9 of the 17 SDGs, according to the World Biogas Association estimates, are listed in Table 1 [12].

Table 1 Using AD in achieving SDGs
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AD involves four metabolic reactions. During stage 1, hydrolysis, enzymes convert large complex molecules into small soluble molecules; in stage 2, acidogenesis, acidogenic bacteria continue to break soluble molecules into simpler compounds such as volatile fatty acids and organic acids; in stage 3, acetogenesis, products of the previous stage are further decomposed into carbon dioxide, acetate, and hydrogen; and in the final stage, methanogenesis, methanogens convert stage 3 products into methane and carbon dioxide [13, 14]. The hydrolysis stage is relatively slow, which limits the whole process. AD occurs in three temperature ranges, namely, psychrophilic (10–30 °C); mesophilic (30–40 °C), with optimum temperature between 30 and 35 °C; and thermophilic 50–70 °C [4]. The optimum pH of the AD process is between 6.8 and 7.2.

It is important to carry out a techno-economic appraisal before embarking on an AD process. The first step determines the amount of biomass waste that is sustainably available for biogas production. Different methods are used to quantify the biogas potential of the available biomass, which include estimating the theoretical biomass production from the total solid and volatile solid values of the waste [15] and using biogas yield factor (\(\mu\)), which is the biogas produced from a unit quantity of biomass waste [16, 17]. The biodigester volume is determined from the retention period, the amount of waste, and the waste-to-water mixing ratio. The biogas can be used for various applications like cooking, conversion to biomethane, and combined heat and power (CHP) generation. The energy output of a CHP generator is calculated using the amount of biogas, the methane content of the biogas, the energy content of methane, and the CHP efficiency.

Different financial indicators have been used to determine the profitability and viability of AD projects to help in decision-making. These include the cost–benefit ratio, where a project is accepted if the cost–benefit ratio is greater than 1; the average rate of return (\(ARR\)), where an AD project is accepted if \(APR\) is greater than the bank interest rate (\(r\)); internal rate of return (\(IRR\)), where a project is accepted if \(IRR > r\); simple payback period (\(PBP\)), where an AD project is very economically attractive when the simple \(PBP<2\); and net present value (\(NPV\)), where an AD project is viable when \(NPV \ge 0\) [9, 18, 19]. Regarding capital budgeting, \(r\) equals the weighted average cost of capital [18]. \(PBP\) can be less than 2 years if the country has high electricity tariffs and other renewable energy incentives. For example, under the Renewable Energy Independent Power Producers Procurement Program (REIPPPP) in South Africa, the electricity tariff is R1,475 /MWh, while companies pay for electricity at the megaflex tariff of R763.8 /MWh [18]. The sale of heat energy from the CHP generator at the megaflex tariff and the inclusion of carbon tax avoidance contribute to lowering PBP. Subsidies and the sale of carbon credits greatly increase projects’ profitability and reduce the \(PBP\) [20,21,22].

Various research listed in Table 2 has been conducted on the techno-economic analysis of the AD systems in different countries.

Table 2 Techno-economic analysis of AD systems in different countries
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The uptake of biodigesters is still low in Zimbabwe, but the country has abundant biomass waste, which can be utilised for AD [16, 17]. As of 2017, the country had 650 family biodigesters, 48 institutional biodigesters, and 13 municipal biodigesters, mainly utilising cow dung, pig waste, and municipal sewage [32]. The main types of biodigesters installed in the country by non-governmental organisations, individuals, government, and local authorities are the Chinese dome, bio latrine, the Indian floating drum, and the Carmatec digester [32, 33]. The gas is mainly used for cooking, lighting, and space heating in pig sties and chicken runs, but in some cases, the gas is flared into the atmosphere.

The Zimbabwe Energy Regulatory Authority (ZERA) was established in terms of the Energy Regulatory Authority Act [Chapter 13:23] of 2011 with a mandate to regulate the energy sector in Zimbabwe including the biogas sector [1]. The functions of ZERA include regulating and licensing energy projects, supporting energy research and development, and ensuring environmental protection. The Electricity Act allows ZERA to issue licences for generating, transmitting, distributing, and supplying electricity in excess of 100 kW [34]. Generation of electricity for own consumption requires no licence fee. The National Energy Policy provides an overall framework for the supply of modern energy services to support socio-economic development in the country [35]. The National Renewable Energy Policy emphasises using renewable energy resources such as biogas to augment the national vision of transforming Zimbabwe into an upper-middle-income economy by 2030 [36]. Using MBEPs would enhance the provision of modern energy services in remote off-grid areas and accelerate economic growth.

Most remote areas that are not grid-connected depend on agriculture for livelihood. Agricultural practices range from subsistence farming to high-level commercial farming. Although large amounts of waste are produced on the farms, farmers are still ‘energy-poor’. Lack of adequate information has been cited as one of the reasons for the low uptake of AD technology [37]. Thus, farmers need to be provided with various options for utilising the available biomass waste to access modern energy services and increase the profitability of their farms. Also, farmers would replace firewood usage with biogas as a way to conserve forests [38]. However, there is insufficient information to enable farmers to make informed decisions on AD technology uptake. A technical assessment for an MBEP for a typical farm in Zimbabwe would provide information on the amount of biogas and electricity that can be produced on the farm and explain the potential use of all AD outputs. The economic assessment would provide information on the financial viability of the system. No techno-economic information on AD of farm biomass waste in Zimbabwe has been found in the literature. Thus, publishing the first techno-economic assessment results will make the information easily accessed by researchers collaborating with entrepreneurs/farmers and consultants to provide adequate information on available options for providing modern energy services to ‘energy-poor’ communities.

The aim of this study was to analyse the techno-economic viability of the MBEP. To achieve the aim, the following objectives had to be met:

  1. 1.

    Quantify the biomass waste sustainably available for biogas production at the farm.

  2. 2.

    Compute the amount of biogas produced from the available biomass waste.

  3. 3.

    Determine the biodigester volume and the CHP generator size for the MBEP.

  4. 4.

    Determine the best MBEP technology for the farm.

  5. 5.

    Establish the economic indicators of the MBEP.

2 Materials and methods

2.1 The study area

The farm is located 300 km from the capital city of Zimbabwe. The farm is not connected to the national electricity grid. The farmer was interviewed to get information about the size of the farm, land utilisation, crop yields, animals kept, and uses of crop and animal waste. The farm has 20 ha of arable land and 50 ha of grazing land. Currently, some land is not being utilised. The farmer practices horticulture using water from a nearby river, grows seasonal crops, and keeps chickens and some animals. There are about 20 households with varying numbers of occupants on the farm compound.

2.2 Assumptions for the study

It was assumed that the properties of the farm biomass waste are similar to those found in the literature; thus, the biogas and electricity estimation results are valid. Also, the US dollar used for the calculations will remain a viable currency in Zimbabwe, and the results will be meaningful even in the distant future. Some researchers experimentally quantified total solids and volatile solids values and used them for estimating biogas yields [27], while others used chemical oxygen demand [18], but this study assumed that the use of total biomass produced per species per day and the species biogas yield factor (\({\mu }_{{\text{s}}}\)) used in other studies is sufficient to give a valid estimate of biogas produced by AD [16]. The biomass-to-water mixing ratio for crop wastes was assumed to be 1, which is consistent with the ratio used by interviewed local biodigester owners. The methane content of biogas of 62% and the energy content of methane (10 kWh/m3) were assumed to be sufficient for calculating electricity production from biogas.

2.3 Quantification of biomass

2.3.1 Biomass residues produced from crops and fruits

The total amount of sustainable crop wastes in t/y (\({T}_{{\text{s}},{\text{cw}}}\)), which comprises wastes from seasonal crops and waste produced from the management of permanent plantations such as clipping and replantation of citrus, coffee, and other trees, was calculated considering the kinds of crops and the total planted area; crop yield t/y (\({y}_{{\text{c}}}\)) and its waste-product ratio (\(\alpha\)); waste removal rate (\({\eta }_{{\text{rr}}}\)) which does not have harmful effects on the soil; and competing uses of crop waste in t/y (\({U}_{{\text{comp}}}\)) using (1) [17].

$${T}_{{\text{s}},{\text{cw}}}={y}_{{\text{c}}}\alpha {\eta }_{{\text{rr}}}-{U}_{{\text{comp}}}$$
(1)

The \(\alpha\) used for crops and fruits available at the farm is well documented in various literature, and the values used in this study are listed in Table 3, \({\eta }_{{\text{rr}}}\) is assumed to be 50% so that part of the waste is left to decompose in the fields, and all the collected waste is used for AD [16, 17, 39]

Table 3 Various factors used to determine available biomass and biodigester volume
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2.3.2 Animal waste

The total amount of sustainable animal waste (\({T}_{{\text{s}},{\text{aw}}}\)) in t/y was calculated considering the number of specific animals kept on the farm (\({N}_{{\text{a}}}\)), the amount of dry waste produced in kilograms per animal per day (\(\beta\)) and the waste collection efficiency (\({\eta }_{{\text{wc}}}\)) using (2) [17]. The \({\eta }_{{\text{wc}}}\) and \(\beta\) for the animals kept on the farm were obtained from literature and are given in Table 3.

$${T}_{{\text{s}},{\text{aw}}}=\beta {N}_{{\text{a}}}{\eta }_{{\text{wc}}}/1000$$
(2)

2.3.3 Household solid waste (HSW)

The total amount of sustainable household solid waste (\({T}_{{\text{s}},{\text{hsw}}}\)) used for AD was obtained from the farm households and comprised of biodegradable waste from kitchen wastes, sweepings, rags, paper, cardboard, plastic, bone, and metals considering the number of people on the farm (\({N}_{{\text{p}}}\)), per capita waste generation (\(\rho\)), waste collection efficiency (\({\eta }_{{\text{wc}}}\)), and fraction of biodegradable waste (\({\chi }_{{\text{bw}}}\)) using (3) [16].

$${T}_{{\text{s}},{\text{hsw}}}=\rho {N}_{{\text{p}}}{\eta }_{{\text{wc}}}{\chi }_{{\text{bw}}}$$
(3)

Some researchers estimated \(\rho\) to be 0.5 kg/d and was used for this study, while \({\chi }_{{\text{bw}}}\) was estimated to be between 10 and 45% [16].

2.3.4 Sewage

The total amount of sustainable sewage (\({T}_{{\text{s}},{\text{s}}}\)) was calculated using (4) considering \({N}_{{\text{p}}}\) and the urine and faeces produced per person per day (\({\gamma }_{{\text{uf}}}\)), which is in the range 130–520 g/person/d [45]. This study assumed \({\gamma }_{{\text{uf}}}\) to be 0.4 kg, the middle of the range.

$${T}_{{\text{s}},{\text{s}}}={N}_{{\text{p}}}{\gamma }_{{\text{uf}}}$$
(4)

2.3.5 Total sustainably available biomass for biogas production

The total sustainably available biomass waste for biogas production (\({T}_{{\text{s}},{\text{abwbp}}}\)), was obtained using (5).

$${T}_{{\text{s}},{\text{abwbp}}}={T}_{{\text{s}},{\text{cw}}}+{T}_{{\text{s}},{\text{aw}}}+{T}_{{\text{s}},{\text{hsw}}}+{T}_{{\text{s}},{\text{s}}}$$
(5)

2.4 Sizing of biogas plants

Most of the biomass is not available throughout the whole year. Thus, maximum monthly sustainable biomass waste (\({T}_{{\text{s}},{\text{mbw}}}\)) given by (6) was used to size the biodigester, which must accommodate total biomass during the month with the highest feedstock volume.

$${T}_{{\text{s}},{\text{mbw}}}={\text{Max}}\left(\mathrm{Monthly flow}!{\text{Jan}}:{\text{Dec}}\right)$$
(6)

2.4.1 Total biogas production

The daily amount of biogas produced by each species (\({B}_{{\text{s}}}\)) in m3 was determined from total biomass in tonnes, produced per species per day (\({T}_{{\text{d}}/{\text{s}}}\)) and the species biogas yield factor (\({\mu }_{{\text{s}}}\)) in m3/t given by (7) [19]. The values of \({\mu }_{{\text{s}}}\) are given in Table 3 except for sewage, which is 0.4 m3/kg waste [46].

$${B}_{{\text{s}}}={T}_{{\text{d}}/{\text{s}}}\times {\mu }_{{\text{s}}}$$
(7)

The total biogas from all species (\({B}_{{\text{tot}}}\)) was obtained the summation of \({B}_{{\text{s}}}\).

2.4.2 Biodigester volume

The contribution of each species biomass waste to digester volume (\({V}_{{\text{bd}},{\text{s}}}\)) in m3 was determined using (8).

$${V}_{{\text{bd}},{\text{s}}}={R}_{{\text{t}},{\text{s}}}\left({T}_{{\text{b}}/{\text{s}}}+{W}_{{\text{s}}}\right)$$
(8)

where \({R}_{{\text{t}},{\text{s}}}\) is the retention time for species biomass and \({W}_{{\text{s}}}\) is the amount of water mixed with the species biomass determined from the species biomass to water mixing ratio (\({MR}_{{\text{sbw}}}\)). The mixing ratios for the biomass waste listed in Table 3.

The total biodigester volume (\({V}_{{\text{b}}}\)) in m3 was found by the summation of (\({V}_{{\text{bd}},{\text{s}}}\)) using (9).

$${V}_{{\text{b}}}=\sum_{{\text{n}}=1}^{{\text{sn}}}{V}_{{\text{bd}},{\text{s}}}$$
(9)

where \({\text{sn}}\) is the total number of the species.

2.5 Mini biogas-electricity plant design (MBEP)

This study did not produce the construction engineering design. It was limited to the selection of the AD technology and its components.

2.5.1 Selection of the AD technology

This study adopted multi-criteria analysis (MCA) for selecting the AD technology [47]. The performance matrix that facilitated the decision-making is illustrated in Table 4. The overall score was calculated using (10) [47]

Table 4 Performance matrix for the AD technology decision-making
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$${\sigma }_{k}=\sum_{i=1}^{m}{\varpi }_{i}{\sigma }_{ki}={\varpi }_{1}{\sigma }_{k1}+{\varpi }_{2}{\sigma }_{k2}+{\varpi }_{3}{\sigma }_{k3}\dots .{\varpi }_{m}{\sigma }_{km}$$
(10)

The AD technologies considered were the plastic digester, the Chinese fixed dome digester, the Indian floating drum digester, the Carmatec digester, and the garage-shaped type digester. The plastic digester, the Chinese fixed dome digester, the Indian floating drum digester, and the Carmatec digesters have been used in Zimbabwe. Thus, some locals have varying levels of experience with them, while the garage-shaped digester has not been used. The parameters that were compared as performance indicators are lifespan, experiences, and lessons on design, physical structure, capital cost, and availability of unique extra features which enhance improved performance [47, 48].

2.5.2 Determining the size of the electricity generator

The energy content of methane (\({E}_{{\text{methane}}}\)) was estimated at 9.96 kWh/m3 and the methane content of biogas (\({B}_{{\text{mc}}}\)) at 62% [27]. The electricity recovery efficiency (\({\eta }_{{\text{eg}}}\)) and heat recovery efficiency (\({\eta }_{{\text{hr}}}\)) were estimated at 35% and 50%, respectively [27, 49]. The CHP generator capacity (\({E}_{{\text{CHP}}}\)) was calculated using (11).

$${E}_{{\text{CHP}}}={B}_{{\text{tot}}}\times {E}_{{\text{methane}}}\times {B}_{{\text{mc}}}\times {\eta }_{{\text{eg}}}\times {\eta }_{{\text{hr}}}$$
(11)

2.6 The cost and benefits of the MBEP

2.6.1 Cost of the MBEP

The initial investment costs for the MBEP shown in Table 5 comprise the digester and the ancillary equipment such as pumps, pipework, storage tanks, and valves; the engineering requirements such as civil, electrical, control components and services; and the digestate treatment works [27]. To increase the biogas energy content and reduce corrosion of MBEP equipment, the biogas is upgraded before it is used. Pressure swing adsorption (PSA) with 0.3 €N/m3 biogas initial investment and 0.1 €N/m3 biogas operational cost was recommended for upgrading biogas for plants that sell the gas to customers who use the gas with low methane purity level and was suggested for the MBEP [7].

Table 5 Initial investment and operational costs
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The operational and maintenance (O & M) costs listed in Table 5 comprise the internal heat and electrical energy consumption of the MBEP; raw waste treatment and loading and administrative activities; repairs and scheduled maintenance [7, 27, 47]. The plant consumes 5% of the generated electricity per day, while 0.2 kW/t of the digestate is used for separating the digestate into solid and liquid [27]. The CHP has a lifespan of 5 years, and thus, a CHP replacement cost is factored in the costing of the MBEP [47].

2.6.2 Benefits of the MBEP

The benefits of the MBEP are the electricity and heat generated by the CHP and organic fertiliser. The electricity and heat will be used to provide the farm’s heat and electrical energy needs such as heating the chicken runs and water pumping. The cost of avoided purchase of fuel and electricity will be a revenue to the farm. The biomass mass balance suggested by [47] was used to calculate the quantity of the biofertiliser. The digestate is 90 to 95% of the total biomass fed into the biodigester, the difference having been converted to \({B}_{{\text{tot}}}\). It is then composted to produce organic fertiliser. About 63% of the digestate, comprising 84% water and some volatile solids, is lost during composting, while 37% forms compost comprising water, volatile solids, and ash. The compost is sold at US$140 /t.

Production of electricity from biogas (\({E}_{{\text{MWh}}}\)) instead of fossil fuels results in avoided GHG emission (\({\delta }_{{\text{GHG}},{\text{A}}}\)) into the atmosphere calculated from (12) using a climate change indicator of 0.5 kg CO2 eq/ kWh generated [21]. This is equivalent to an avoided GHG emission factor of 0.5 t of carbon dioxide /MWh.

$${\delta }_{{\text{GHG}},{\text{A}}}=0.5\times {E}_{{\text{MWh}}}$$
(12)

2.7 Economic analysis of the MBEP

The yearly cash flows (\(CF\)) for year (\(t\)) for the MBEP were estimated for its 20 years lifespan (\(T\)) and discounted to present value using the discount factor (\(r\)). The discounted cash flows’ net present value (NPV), which is positive for a viable project, was calculated using (13) [18].

$$NPV=C{F}_{0}+\sum_{t=0}^{T}{CF}_{t}/{\left(1+r\right)}^{t}$$
(13)

The discount factor at which \(NPV\) is 0 (\(IRR\)) was determined by (14).

$$NPV=C{F}_{0}+\sum_{t=0}^{T}{CF}_{t}/{\left(1+IRR\right)}^{t}$$
(14)

3 Results and discussion

3.1 Biomass sustainably available for biogas production at the farm

The type and quantity of biomass available per year for biogas production at the farm are shown in Fig. 1. Sweet potatoes produce the most biomass waste at the farm, but they are only available from July to September. The total amount of biomass that can be used to produce biogas each year is 452 t.

Fig. 1
figure 1

Yearly biomass available for biogas production: Quantities and types of renewable organic wastes at the farm

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The biomass is not evenly distributed annually. As a result, the biodigester throughput fluctuates. The daily biomass that is available for each month of the year is shown in Table 6. The biomass quantity was highest from June to October, valued at 1887 kg/d. The least amount of biomass, 746 kg/d, is available in May when all crop biomass waste is unavailable. The methane content of biogas from different biodigesters varies significantly (up to three times) depending on the feedstock used [50]. Thus, the CHP energy output of the biodigester in this study is affected by the feedstock’s quantity and quality change due to the feedstock’s seasonality, which disturbs the digestion process stability [51]. Consequently, there is a need to carry out a detailed study to optimise the biomass throughput in terms of quantity and quality and advise farmers accordingly.

Table 6 Biomass available for each month (kg/d)
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Feedstock distribution in a biodigester is critical for ensuring efficient and stable biogas production. Good feedstock distribution maximises contact between microorganisms and organic matter. To prevent the formation of zones with varying concentrations of organic matter, it is important to feed the biodigester with well-mixed and homogenised feedstock. For example, mechanical pretreatment of the feedstock by grinding or shredding reduces the particle size and enhances biomass distribution [52]. Proper mixing of the feedstock enables the maintenance of uniform conditions such as nutrients and temperature throughout the biodigester. With biodigesters utilising different feedstock, researchers need to work on the rheological behaviour of the feedstock in the biodigester and develop techniques for optimizing mass transfer and mixing of the substrate within the biodigester.

3.2 Size of the biogas plant

3.2.1 Biodigester capacity

The biodigester volume is 283 m3. The contribution of each type of biomass to the biodigester volume is shown in Fig. 2. Sweet potatoes have the largest contribution of 36.3%, while mangoes have the least contribution of 0.5%. Sweet potatoes’ waste is available from July to September; thus, the biodigester volume is underutilised during the rest of the year. The farmer is advised to outsource some biomass waste to fully utilise the biodigester during the period.

Fig. 2
figure 2

The relative contribution of various biomass types to the overall capacity of the biodigester

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A continuous steady availability of feedstock is necessary to produce biogas of homogenous quality to fuel the CHP for stable power generation [48]. Thus, it is prudent to optimise the biodigester volume, uniformly distribute the biomass within the biodigester, and ensure stable biomass to the plant. The optimisation of biodigester volume requires multidisciplinary research, including scientific, engineering, and social science aspects [53]. Scientifically, it involves understanding the biochemical processes, microorganism interactions, feedstock characteristics, and environmental factors like temperature and pH. Engineering aspects focus on designing the biodigester for efficient biogas production, considering factors like organic loading rate, retention time, and reactor design concepts. Social science aspects are also crucial, encompassing the broader implications of biodigester optimisation on waste management, energy production, and sustainable development within communities, as well as the technology’s economic feasibility and societal acceptance. Too small biodigesters may not be able to handle the amount of feedstock, resulting in incomplete decomposition and reduced biogas production, while too large biodigesters may lead to an increase in energy required for mixing and heating the feedstock. Thus, further research is necessary to determine the optimum biodigester size.

3.2.2 Total production of gas

The production of biogas also varies throughout the year because biomass is not always available. It varies depending on the feedstock availability. It was discovered that 88,124.8 m3 of biogas was produced annually. The total annual biogas production is shown in Fig. 3 along with the contribution of each biomass feedstock source. The biogas yield per day for each month of the year is shown in Table 7. The highest amount of 253 m3 of biogas is produced in February. It is an advantage for the farmer to have high biogas production during the winter season (June to August) because the gas will be used for heating the biodigester to maintain the temperature between 35 and 37 °C and heat the chicken run. The least amount of biogas is produced in May because all crop biomass waste is not available during this month.

Fig. 3
figure 3

An overview of the yearly biogas yield and the distinctive contributions from various biomass feedstock sources

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Table 7 Biogas yield by biomass type (m3/d)
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The amount of biogas produced and its characteristics are greatly influenced by the feedstock composition. To enhance biogas yield from AD of multiple feedstocks, variation in chemical properties for the feedstocks must be considered, such as volatile matter, heating value, and carbon and nitrogen content [54]. This will enable achieving proper mixing ratios to get the required operational parameters to maximise biogas production. For example, there is a need to maintain a suitable carbon-to-nitrogen ratio in the range of 25–30 [55]. It is thus essential to do a proximate and ultimate analysis of the feedstock and determine the average structural components to know the properties of the feedstock.

Various parameters such as the temperature, pH, carbon-to-nitrogen ratio, inoculum, and organic loading rate can be optimised to control the AD process of the farm biomass waste [53]. The temperature regulates microbial intracellular enzyme activity, while the pH, which depends on the type of feedstock, influences the chemical equilibria of ammonia, hydrogen sulphide, and volatile fatty acids, affecting the metabolic activity of microorganisms and fermentation efficiency [56, 57]. Accumulation of these substances and pH fluctuations may reduce biogas production and lead to biodigester failure in extreme cases. Thus, there is a need to understand the feedstock’s characteristics to develop mechanisms for optimising the organic loading rate to enhance biogas production.

The biodigester’s optimisation and process control aspects guarantee the efficient and effective conversion of organic matter into biogas and digestate. The AD process is affected by various key parameters, and optimising process control includes management of these parameters to maximise biogas production and digestate quality while minimizing costs and resource usage. This includes optimising the feedstock mix, temperature, pH, nutrient availability, mixing intensity, and the absence of inhibitors to create ideal conditions for anaerobic microorganisms [58]. Process control refers to the monitoring and regulation of the key parameters to ensure a stable and efficient operation of the biodigester. Thus, the farmer needs to practise effective process control by periodically measuring parameters such as temperature, pH, organic loading rate, and gas production to identify deviations from optimal conditions and implement necessary corrective actions to maintain process stability and maximise biogas production.

The obtained biogas is a mixture of methane and unwanted gases such as carbon dioxide, hydrogen sulphide, water, and siloxanes which results in a reduction of the biogas energy content and corrosion of equipment that utilises the gas. This study suggests the use of a pressure swing adsorption for upgrading the biogas since it is used in a CHP which does not require very high methane purity [7]. Some studies suggest the use of plants that include microaeration for controlling [27]. Both an external upgrading and the microaeration inclusion raise the initial investment but are compensated by high purity of the gas which saves the equipment from corrosion.

3.3 The biodigester and electricity generator technology

The biodigester technology was selected based on the scoring of the considered indicators.

3.3.1 Lifespan

The lifespan for the biodigester technology given in Table 8 refers to the useful time of the biodigester. The biodigester has to be replaced after the lifespan. Maintenance activities such as removing the sludge and cleaning allows the technology to have good performance throughout its lifespan.

Table 8 Lifespan of biodigester technologies
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The lifespan was converted to MCA scores in the range 0 to 100 where 0 was assigned to lowest lifespan and 100 to the highest lifespan. Direct proportionality was applied to scores for other lifespans.

3.3.2 Experiences and lessons on design

This parameter refers to the experience curve built over time focusing on construction, operation, monitoring, maintenance, and evaluation of the technologies. Experienced personnel in these aspects can install and run the biodigesters more profitably than inexperienced personnel. The features of the biodigester technologies in terms of experiences and lessons on design are given in Table 9.

Table 9 Experiences and lessons on design features
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A score of 0 was assigned to the technology with an unclear status in Zimbabwe and creates limited job opportunities, while the technology with the highest % installation was given a score of 100. Direct proportionality was applied to scores for other % installations. Lastly, a score of 33 was given to technologies that have not been used in Zimbabwe but create many permanent jobs.

3.3.3 Physical structure

The material used for constructing the biodigester influences the environmental impact on the biodigester performance and its maintenance requirements. The physical structure features of the technologies are listed in Table 10.

Table 10 Physical structure features of the technologies
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Direct rating was used to assign scores to the technologies: 0 was given to the technology with the least favourable features, while 100 was given to the technology with the best structural features.

3.3.4 Capital cost

The capital cost given in Table 11 includes all the costs for purchasing material and corresponding labour to implement the civil, mechanical, and electrical works to set up the plant [18].

Table 11 Capital costs of different technologies
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Direct rating was used to assign scores to the technologies: 0 was given to the technology with the highest capital cost, while 100 was given to the technology with the lowest cost.

3.3.5 Unique extra features

The garage-shaped digester can allow additional structures to be added to it to improve its performance. In addition, it has an advantage of saving water by recycling effluent liquid from the biodigester and output dry digestate that can be composted to produce biofertiliser, and the feedstock does not need pretreatment [47]. Thus, it was given a score of 100, and all the other technologies were given a score of 0.

Table 12 lists the MCA scores for the different biodigesters based on Zimbabwean standards. One hundred points were equally distributed among parameters by assigning 20% of the points to each parameter. The total scores were calculated using (10) as explained in Section 2.5.1. The garage-shaped digester with a score of 66.6% was selected.

Table 12 MCA performance matrix
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A 59-kW combined heat and power (CHP) system, which simultaneously produces heat and power, was chosen. The energy output is \(468.5\ {{\text{kWh}}}_{{\text{e}}}\) and \(780.9\ {{\text{kWh}}}_{{\text{t}}}\). The thermal energy is used to heat the digester and the chicken runs [27]. The electricity will be used for the electrical energy needs of the farm, such as lighting the chicken runs and water pumping.

3.4 Cost and benefits of the MBEP

The BEP plant’s initial cost is US$72,994.5. It has a present annual operating cost of US$3,155.4. The biomass balance for the MBEP, which depicts the benefits of the plant, is given in Fig. 4. The plant produces 435,350.7 kWh/y energy which is used for heating the chicken runs and biodigester, lighting the chicken runs and outdoor, water pumping, and cell phone central charging system. The produced 105.8 t of biofertiliser can be used in horticulture and seasonal cropping. The excess fertiliser can be sold on the market.

Fig. 4
figure 4

Comprehensive biomass balance analysis for the garage-shaped MBEP: Demonstration of the overall benefits and efficient resource utilisation

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Garcia suggests mixing 50% of the digestate with fresh feedstock to reduce the amount of water added to the digester and quicken the start of the digestion process [47]. The remaining digestate is added value and flexibility by separating into liquid and solid using solid-liquid separator coupled to the digester [62]. The liquid component can be used in aquaculture, while the solid is passed on to composting where the final decomposition of the organic matter occurs under aerobic conditions to form biofertiliser. This scenario is suggested for the MBEP owners.

The AD of biomass waste has several environmental and social benefits. It helps abating climate change and improve livelihoods of rural communities by providing energy for productive work, which promotes economic and creates local jobs [27]. Figure 5 shows the monthly variation of the contribution of the MBEP to the abatement of climate change by avoiding carbon dioxide emission into the atmosphere. The change results from variation in feedstock quantity throughout the year, resulting in fluctuation of produced biogas. A study conducted in the peri-urban areas of Harare revealed that the household occupants are willing to take up AD technology [23]. They cited benefits such as reduced cleanliness problems, decreased land degradation due to deforestation, and improved indoor air quality.

Fig. 5
figure 5

Monthly variation in the contribution of the MBEP to abatement climate change through avoidance of carbon dioxide emission

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3.5 Economic analysis

The NPV for this project is − US$483,712.7; therefore, the installation of the garage-shaped digester and CHP system is not an economically viable project.

The second option for the biodigester technology was considered. The Carmatec biodigester, which is the most commonly installed biodigester in the country [33], was chosen, and the economic analysis for the MBEP was conducted. The MBEP plant’s initial investment is US$72,994.5. The major driver for low investment cost is the low installation cost for the Carmatec biodigester of US$150 per m3 [23]. Table 13 lists the economic indicators for MBEP. Based on the three economic indicators used, the project is demeed viable. The \(NPV\) is positive and \(IRR\) is greater than the \(r=14\%\) used for the economic analysis. However, the \(PBP\) of 2.6 years is slightly greater than 2 years which is recommended for business capital budgeting requirements [18]. If all other parameters remain constant, it decreases to less than 2 years if the cost of electricity is increased from the current tariff of US$0.1 to US$0.12 /kWh. However, it is in the range of past AD techno-economic feasibility evaluations both in Zimbabwe and outside Zimbabwe. For example, Mukumba et al. obtained a \(PBP\) of 2 years for AD of cattle dung, chicken manure, and human waste assuming an energy content of methane of 9 kWh/ m3. In Syria, a \(PBP\) of 2.9 years was obtained for AD digestion of animal waste in a volatile economy causing a sudden upsurge in costs and revenue increase like in Zimbabwe [9].

Table 13 Economic indicators for Carmatec MBEP
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Several initiatives can improve the project’s economics and encourage farmers to take up the technology. The government can chip in with some form of subsidy, and the farmers should have access to low-interest rate loans. Also, issuing carbon credits to farmers and facilitating their trade in the national and international arena might be effective in supporting AD uptake rather than only benefiting through improving electricity tariffs [25].

3.6 Practical applications and future research

The practical applications of the MBEP are multifaceted and hold significant potential for addressing energy and agricultural challenges. The findings of this study have several practical implications and potential future research directions. Implementing MBEPs present a practical solution for off-grid areas in Zimbabwe, where access to reliable electricity is limited. By utilising biomass waste from agricultural activities, farmers can generate electricity for their use, reducing their dependence on traditional energy sources and improving energy access in the rural communities [9]. Adopting AD technology allows for efficient management of biomass waste from agricultural activities. Farmers can mitigate environmental pollution and improve soil fertility by converting organic waste into biogas and biofertilisers and promoting sustainable agricultural practices [63]. The techno-economic assessment provides valuable insights into the financial feasibility of implementing MBEPs on typical farms [31]. This information can guide policymakers, investors, and farmers in making informed decisions regarding adopting AD technology, considering its economic benefits and potential challenges. Future research could focus on optimising biogas production from different types of biomass waste, considering variations in feedstock composition, temperature, and retention time. This could lead to the development of tailored AD systems that maximise biogas yield and energy efficiency [53, 64]. Integrating MBEPs with other renewable energy sources, such as solar photovoltaics or wind, could offer a holistic approach to off-grid electrification. Research on hybrid renewable energy systems’ technical and economic synergies would be valuable for enhancing energy reliability and resilience in rural areas [65]. It is important to investigate supportive policies, incentives, and regulatory frameworks to promote the widespread adoption of AD technology in agricultural settings. This could involve studying the potential impact of carbon credits, subsidies, and low-interest loans on the economic viability and scalability of MBEPs. Finally, the practical applications and future research directions stemming from this techno-economic analysis offer valuable insights for advancing sustainable energy solutions and agricultural practices in off-grid areas. By addressing the challenges of energy access, waste management, and economic viability, the findings of this study contribute to the broader discourse on renewable energy deployment and rural development.

4 Conclusions

In conclusion, the study highlights the significant potential and practical implications of implementing such systems. The study demonstrates that utilising biomass waste from agricultural activities to generate electricity can provide a practical solution for off-grid areas with limited access to reliable energy sources. By adopting AD technology, farmers can efficiently manage biomass waste, reduce environmental pollution, and improve soil fertility by producing biogas and biofertilisers. The findings also emphasise the financial feasibility of implementing MBEPs on typical farms, providing valuable insights for policymakers, investors, and farmers. Moreover, future research directions could focus on optimising biogas production from different types of biomass waste, integrating mini biogas-electric plants with other renewable energy sources for a holistic approach to off-grid electrification, and exploring supportive policies and incentives to promote widespread adoption of AD technology in agricultural settings. Overall, this study contributes to advancing sustainable energy solutions and agricultural practices in off-grid areas, addressing energy access, waste management, and economic viability challenges.