Pilot scale study of anaerobic treatment of food waste using ambient and solar heated digesters

ABSTRACT

Food waste (FW) is high in nutrients and has gained global attention as an ideal substrate for bioenergy recovery through anaerobic digestion (AD). Ambient digesters have been widely used because of their ease of installation, low cost, and low energy input. However, to improve biogas production sustainably, there is a need to consider reactor heating using renewable energy such as solar. This study sought to apply psychrophilic and mesophilic biodigester temperatures for FW treatment. For ambient digestion, a complete-mix flexible biodigester, named STH-1000A, covered in a greenhouse structure was operated between 24 and 32 °C. A prototype complete-mix tank biodigester, named VUT-1000C, was designed and operated at mesophilic conditions of 37 °C through solar geyser heating. VUT-1000C produced 1200 L of biogas per day while STH-1000A 150 L/day. VUT-1000C and STH-1000A generated up to 1.8 and 0.4 kWh of electricity, respectively. The power balance showed that VUT-1000C used 68% of its power production and STH-1000A consumed 398%. Digester heating using solar geyser is a novel and promising technique for achieving mesophilic condition leading to improved biogas production.

KEYWORDS:

• Ambient
• bioenergy
• solar-heating
• pilot scale
• mesophilic

Nomenclature

AD	=	Anaerobic digestion
FW	=	Food waste
CD	=	Cow dung
Sa	=	South Africa
VUT-1000C	=	Vaal University of Technology 1000 L controlled temperature digester
STH-1000A	=	Stonehaven 1000 L ambient temperature digester
OLR	=	Organic loading rate
TS	=	Total solids
VS	=	Volatile solids
MC	=	Moisture content
CH4	=	Methane
CO2	=	Carbon dioxide
H2S	=	Hydrogen sulfide
GHG	=	Green-house-gases
HDPE	=	High-density polyethene
PVC	=	Polyvinyl chloride
LLDPE	=	Linear low-density polyethylene

1 . Introduction

Throughout the world’s food supply chain, 1.3 billion tonnes of food is wasted yearly, causing environmental, economic, and social problems (los Mozos, Badurdeen, and Dossou 2020; Blakeney 2019). In developing countries, many sectors particularly energy supply and waste management are strained due to the growing population (Godfrey et al. 2019). Current methods of organic waste disposal are incineration, landfills, composting and anaerobic digestion. Incineration and composting require energy, while landfilling has been considered the most practical and cheapest disposal method (Pramanik et al. 2019). However, landfills are filling up quickly, creating the need for more sites farther away from waste generation, thereby increasing costs (Oelofse and Nahman 2013). More than 50% of the waste disposed of in landfills and on street corners is organic and rots under uncontrolled anaerobic conditions, releasing landfill gas into the atmosphere and leachate into underground water (DEA 2012; Lange and Nahman 2015). Landfill gas, also known as biogas, comprises primarily 55–70% methane (CH₄), 30–45% carbon dioxide (CO₂), trace amounts of hydrogen sulphide (H₂S) (0–2000ppm), moisture (depending on temperature), and siloxanes (Priebe et al. 2016). Methane is 25 times more potent than carbon dioxide as a greenhouse gas. Landfilling is greenhouse gas (GHG)-intensive, with an emission rate per tonne of organic waste of almost 400 kg CO₂e (Nordahl et al. 2020).

The controlled anaerobic digestion (AD) of food waste (FW) (and other organic wastes) is considered a key element in organic waste management due to its positive impact on the environment, economy, and energy (Kang and Yuan 2017). FW is high in moisture content (74–90%) and has a high organic fraction (80–90%), making it a suitable feedstock for AD treatment (Gallipoli et al. 2020). AD is a net energy-producing process with up to 100–150 kWh generated per ton of waste (Braber 1995). Methane, the main constituent in biogas, is a fuel constituting up to 99% of natural gas. Methane has a calorific value of 36 MJ/m³; thus, biogas has a calorific value of 22 MJ/m³ at 60% methane composition (Karne et al. 2023).

The AD treatment method reduces the emissions of greenhouse gases going into the atmosphere while producing carbon-neutral renewable energy and biofertilizer. AD is a biological degradation treatment of organic substrate undertaken by microorganisms in an aqueous environment in the absence of oxygen (Appels et al. 2011; Sawatdeenarunat et al. 2015). At the household level, AD has been mostly applied at ambient conditions due to the low energy input requirement. However, enhanced biogas production is available at elevated temperatures. Increased digester temperatures mean high input energy requirements and increased operations costs (Dev et al. 2019). With the aid of solar technology, this problem can be combated and excess biogas made available. The generation of electricity from biogas is one of the most dominant future renewable energy sources because continuous power generation from organic waste can be guaranteed (Appels et al. 2011).

Considering the challenges faced with FW management, energy supply, meeting the Green Deal requirements, and the little uptake of the AD technology particularly in South Africa (SA); this study sought to design a solar-heated mesophilic biodigester for FW treatment. Innovative solar heating was considered to attain mesophilic conditions while minimizing the input energy costs. A commercial biodgester was used for ambient digestion. Furthermore, the study sought to generate electricity from the produced biogas and to conduct energy analyses on the two pilot digesters. This study is derived from Khune (2021) master’s dissertation.

2 . Materials and methods

2.1 . Digester assembly materials

A 1 m³ tank made from linear low-density polyethylene (LLDPE), clear pipe of 8 mm internal diameter, high-density polyethylene (HDPE) pipe, polyvinyl chloride (PVC) tarpaulin, fittings, solar geyser, insulation material, biogas compressor, submersible grinder pump, pH meter (Hanna model: HI 9813-5), slaked lime, and plexiglass floating drum for the construction of a complete-mix biogas pilot plant were purchased at local hardware stores in Vanderbijlpark, SA. A 1.5 kW biogas generator, handheld biogas analyser and Ritter biogas meter were imported. The ambient anaerobic digester (1 m³) was purchased from Puxin Technology in China.

2.2 . Inoculum and substrates

Inoculum was collected from a local cattle farm in Vanderbijlpark and incubated in the pilot plants until biogas production was insignificant. Kitchen FW mainly cooked rice, slap chips, buns, bread, porridge, grease, raw dough, chicken, meat, cooked and raw vegetables, and fruits were collected from the Vaal University of Technology (VUT) cafeteria and a local restaurant, in Vanderbijlpark. All foods were thoroughly homogenised with a fine grinding FW disposer before treatment.

2.3 . Design and assembly of the anaerobic plants

The controlled temperature 1000 L complete-mix biogas pilot plant in Figure 1a was designed, assembled, and commissioned and operated at the Vaal University of Technology (VUT-1000C). The ambient 1000 L complete-mix digester flexible depicted in Figure 1b was installed as purchased, commissioned and operated at Stonehaven on Vaal restaurant (STH-1000A) and operated at the manufacturer’s specified optimal OLR. VUT-1000C was chosen as an above-ground complete-mix digester since it was the most suitable and offered flexibility during experiments and ease of maintenance.

Figure 1. Schematic representation of (a) VUT-1000C biogas pilot plant coupled to an electricity generator and (b) STH-1000A digester (Khune 2021).

2.3.1 . Heating methods

The VUT-1000C digester operated at mesophilic temperatures while STH-1000A operated at ambient temperatures. The VUT-1000C design was equipped with an external boiler, and a solar-heated geyser (100 L, Energy Rating for Standard Day of 14.2 MJ, and Aperture Area of 0.975 m²), with no electrical backup. The boiler was connected to a heating coil placed on the outside wall of the tank. The heating coil was made from 15 mm HDPE, class 4 piping with good low heat resistance, making it excellent for transferring heat to the reactor (Omnexus 2016). The digester tank and the heating coil were covered with two layers of insulation; the inner layer was a 40 mm glass-wool thermal insulator, and the outer layer was a reflective aluminium insulative sheet. A temperature control system consisting of a thermocouple, digital module, and hot water circulation pump was installed to monitor temperature variations in the VUT-1000C digester. The hot water was circulated at a rate of 38 L/min. The thermocouple was placed inside the digester towards the centre to give an accurate reading of the internal temperature, away from the sidewalls of the tank where the heat was received. The controller was connected to a hot water circulation pump that implemented the temperature adjustment commands.

The STH-1000A digester was enclosed in a hoop-like greenhouse structure to enhance solar heating during sunny days and provide some insulation during cold periods (PUXIN 2015). The greenhouse was made of galvanized steel square tubes covered with polycarbonate sheeting and had a width of 0.83 m, a length of 1.21 m, and an arc extending 0.20 m above the vertical wall, giving an overall height of 1.30 m. Polycarbonate allows sun rays to pass through it and converts them into heat energy, which is retained for a prolonged period, thus resulting in an increased internal temperature (Taki, Rohani, and Rahmati-Joneidabad 2018). Thus, the digester operated at enhanced ambient conditions and had no temperature control system.

2.3.2 . Mixing

The VUT-1000C digester used jet mixing using a submersible grinder pump to circulate the digestate internally. The pump sucked in the slurry from the bottom of the digester and jetted it in two portions. The first portion of the fluid was sprayed at the brink to break the solid layer that typically forms due to light particles floating up, and the second was jetted at the bottom of the tank to agitate settling particles. The mixing pump had a timer that turned it on hourly for 15 min and operated at a flow rate of 200 L/min. The jet mixing was also used in STH-1000A; however, the digestate was circulated using an external pump (0.45 kW with 120 L/min flow rate). The external pump recirculated digestate from the digestate outlet pipe and poured it into the feed basin. The use of an external pump was due to the flexibility of the digester membrane material requiring minimum in-basin moving parts, which made maintenance more manageable. The mixing pump, likewise, had a 15-minute timer.

2.3.3 . Feeding

VUT-1000C was fitted with a 100 mm diameter feeding tube. The tube was positioned at the center and extended two-thirds towards the bottom of the tank. At this level, the tube was submerged in the digestate creating a water seal so that no biogas escaped during feeding. FW was crushed before feeding, which was achieved by using a waste food grinder of 373 W. Unlike the VUT-1000C feed tube, STH-1000A used a feed basin located on the front end of the digester.

2.4 . Start-up and operation of the biodigesters

Anaerobic digestion in VUT-1000C was initiated by inoculating the digester with 200 kg of cow dung (CD). The obtained CD was screened to remove large particles and then fed to the digester, which contained 600 L of water to get a working volume of 800 L as recommended by Forster-Carneiro, Pérez, and Romero (2008). At the beginning of the startup and after 24 h of stabilization, biogas volume, biogas composition, digestate pH, and temperature were monitored daily. Incubation of inoculum was allowed to proceed until the difference in the cumulative biogas was less than 1%. At the end of the inoculation, the organic loading rate (OLR) was varied. The optimal OLR was determined by feeding the digester batch-wise with different organic loads of 1, 2, 3, 5, and 7 kgVS/m³/day. STH-1000A was started and operated at the manufacturer’s optimal OLR using the same feeding technique as VUT-1000C.

2.5 . Biogas conversion to electricity and energy analysis

A small-sized generator with a maximum power output of 1.8 kW was used to convert the generated biogas to electricity. A compressor connected to the gas holder pressurized the biogas to 2–6 kPa before supplying it to the generator. A wattmeter was connected between the generator and appliances to measure power output. The input energy required to run the biodigester was compared to the output energy.

2.6 . Chemical and physical analyses

Samples of the blended feed and digestate were removed and measured for total solids (TS) and volatile solids (VS) using the standard method of analysis. The substrate was dried at 105 °C to constant weight to determine TS and then ignited at 550 ^oC to determine VS (APHA 1999). Alkalinity in the digester was analysed by measuring digestate pH. The digester temperature was measured using a digital STC-1000 temperature controller connected to a thermocouple. Biogas composition was analysed with an online natural diffusion hand-held biogas analyser model SAZQ manufactured by Beijing Shi’An Technology Instrument Co., Ltd. SAZQ uses Infrared spectroscopy which is a powerful qualitative and quantitative gas analysis tool based on the interaction between infrared radiation and organic molecules (Köhler et al. 2017).

3. Results and discussion

3.1. Characterisation of inoculum and food waste

The biodegradability of the FW and inoculum was measured by determining the moisture content (MC), TS, and VS, as shown in Table 1. A high amount of moisture in a substance makes it suitable for anaerobic digestion. The FW had a MC of 85%, while the inoculum had an average MC of 53%. The average VS (%) contents obtained for FW and inoculum were 14% and 16%, respectively, similar to those obtained by Kuczman et al. (2018) for FW and Dhamodharan, Kumar, and Kalamdhad (2015) for inoculum. VS/TS ratio is an indicator for evaluating the suitability of a substrate for biogas production. Substrates with a higher VS/TS ratio (80%) contain higher organic matter and are thus more suitable for biogas production (Liu et al. 2017a). The VS/TS for FW was found to be 95%, an amount similar to that of Zhang, Lee, and Jahng (2011) of 94%. The high value indicated that the FW was rich in biodegradable matter and thus excellent for biomethanation (Zhang et al. 2013; Illmer and Gstraunthaler 2009). The VS/TS ratio for the inoculum was 35%, demonstrating that a small fraction of organic matter was to be digested. Substances with VS/TS ratio below 17.4% are considered inorganic (Kuczman et al. 2018).

Table 1. Characteristics of CD and FW (Khune 2021).

Parameter	Cow dung	Food waste
MC (%)	53	85
TS (%)	47	15
VS (%)	16	14
VS/TS %	35	95
pH	7.3	4

3.2. VUT-1000C biodigester startup

The CD was pre-incubated as inoculum in the prototype biodigesters for 55 days to create a suitable environment for FW digestion (Dhamodharan, Kumar, and Kalamdhad 2015; Pandey et al. 2011). No biogas production was observed during the first 14 days of inoculum incubation. The liquid surface in the digester had a solid scum, which was caused by the lack of anaerobic conditions. On the 20th day, bubbles started to form, which, according to Parajuli (2011), indicated the start of biogas production. On day 22 and subsequently, the biogas meter recorded biogas production as given in Figure 2a and b. From day 20, a maximum daily biogas production rate of 420 L was obtained, followed by a gradual decline in daily biogas production. The cumulative biogas production curve was allowed to plateau to a point where the difference in daily biogas production was less than 1% before introducing FW (Elbeshbishy, Nakhla, and Hafez 2012). The flammable biogas contained an average of 54% CH₄, 24% CO_2, and 17 ppm of H₂S, confirming a successful inoculation of the biodigester (Figure 2c). The pH (Figure 2d) remained stable throughout the inoculation period and within the required range of 6.8–8.2 for methanogenic bacteria. The digester pH level was self-regulated, ensuring adequate buffering capacity.

Figure 2. Daily (a) and cumulative (b) biogas production, biogas composition (c), and digester pH (d) during VUT-1000C biodigester start-up (Khune 2021).

3.3. Effect of organic loading rate during VUT-1000c biodigester operation

After the startup, to determine the optimum OLR of FW for VUT-1000C, different OLRs were monitored by evaluating pH, biogas composition, and biogas and methane yields. From Figure 3a, biogas production occurred in a series of peaks, starting with two main peaks, the second being the highest. These peaks occurred between hours 2 and 13, with the highest peak occurring between hours 8 and 13. This illustrated intense biogas production during the initial hours after digester feeding. A similar observation was made by Koch, Helmreich, and Drewes (2015), who reported that the intense biogas production indicated the presence of readily degradable compounds. The highest biogas production rate peak was 131 L/hour for OLR 7. Liu et al., (2017) obtained the highest peak of 0.8 L/day at OLR 7.5 and 10 kgVS/m³, while El-Mashad and Zhang (2010) obtained 59 L/LkgVS/day at 2 kgVS/m³ within the first day of digestion.

Figure 3. (a) Biogas and (b) methane hourly production rate, and cumulative (c) biogas and (d) methane production during anaerobic digestion of food waste at different organic loading rates (kgVS/m³/day) (Khune 2021).

Cumulative biogas production (Figure 3b) increased with an increase in loading, with OLRs of 1, 2, 3, 5, and 7 kgVS/m³ producing 544, 931, 1746, 2334, and 2796 L of biogas, respectively. The hourly production rate dropped significantly within 40 h for OLR 1, 2, and 3 kgVS/m³, whereas OLR 5 and 7 continued at significant production rates averaging 20 and 50 L/hour, respectively. Approximately 80% of biogas was obtained within 40 h of digestion for OLR 1, 2, and 3, whereas up to 50% was obtained for OLR 5 and 7 within the same period. This indicated that continued daily organic loading at high rates might result in system overload due to the accumulation of substrate and thus hindering microbial activity.

Similar to biogas production, the methane production profile comprised a series of peaks, as shown in Figure 3c. OLR 3 and 5 observed a similar trend in their first 100 h of digestion of FW. There was an increase in methane production with an increase in organic load to a limit of 3 kgVS/m³/day. OLRs of 5 and 7 kgVS/m³/day showed no clear trend in methane production, indicating instability of the anaerobic digester. Figure 3d shows that OLR 3, 5, and 7 produced almost similar amounts of methane within the first 40 h after digester feeding. These results show methane inhibition beyond OLR 3, which confirms digester overload, thereby hindering microbial activity (Liu et al. 2017). Furthermore, after day 28, methane production for the OLR of 7 kgVS/m³day increased from 18.6 to 30.4 L/hour by day 40. This trend indicated the reduction in initial overload leading to enhanced microbial activity and improved methane production.

The specific biogas and methane yields at different OLRs are represented in Figure 4. The graph shows biogas and methane yield decrease beyond OLR of 3 kgVS/m³/day. OLR of 1 and 3 kgVS/m³/day gave the highest biogas and methane yields of 544 and 582 L/kgVS and 348 and 332 L/kgVS, respectively. OLR of 7 kgVS/m³/day gave the lowest specific biogas and methane yield of 399 and 158 L/kgVS, respectively. The OLR 3 kgVS/m³/day showed the highest biogas conversion from FW and thus was optimal. The optimal OLR produced three times more biogas and methane than the OLR of 1 kgVS/m³/day in the same period, thus making it desirable. Babaee and Shayegan (2011) obtained the optimal OLR to be 1.4 kgVS/m³/day which yielded 250 LCH₄/kg VS_added. El-Mashad and Zhang (2010) obtained 657 L/kgVS from FW after 30 days of digestion, 79.1% of which was produced after 20 days. After 20 days of digestion, the methane yield accounted for 72.5% of 353 L/kgVS obtained in 30 days (El-Mashad and Zhang 2010).

Figure 4. Specific biogas and methane yields during food waste at different organic loadings in VUT-1000C (Khune 2021).

3.4. Operation of VUT-1000c and STH-1000a biodigesters at optimum conditions

The biodigesters were operated at their optimal OLR of 3 and 0.446 kgVS/m³/day for VUT-1000C and STH-1000A, respectively, for 35 days. Biogas and methane daily production rate for VUT-1000C increased gradually over the digestion period due to residual substrate from previous feeds (see Figure 5). The trend of gradual increase in daily biogas production at a constant OLR was also observed by Mu et al. (2018), Nagao et al. (2012), and Otieno (2020). A gradual increase from 341 L/day on day 1 to 1174 L/day on day 29 was observed. After day 29, the biogas production remained constant at around 1200 L/day; this indicated the achievement of a steady state. On the first day, 341 L of biogas and 187 L of methane were produced and increased daily to a maximum average of 1319 and 791 L/day, respectively, on day 34. Furthermore, the graph consists of a series of peaks occurring on days 4, 12, 26, and 34; the reduction in biogas production post peak was due to the fresh FW introduced to the digester after the prepared bulk batch had been depleted. This is because feeding fermented food makes volatile fatty acids readily available to the microorganism in the digester; hence, fermented FW is more favourable than fresh FW as it improves biogas production (Baldi, Pecorini, and Iannelli 2019).

Figure 5. Daily biogas and methane production during food waste treatment using VUT-1000C (a) and STH-1000A (b) bioreactors at optimum conditions (Khune 2021).

During the operation of STH-1000A, the biogas and methane production similarly increased gradually with time due to the build-up of the residual substrate and the growth of the bacterial population. Biogas production increased from 37 L/day on day 2 to 150 L/day on day 27 (Figure 5a). The digester’s steady state was obtained on day 27, where biogas production remained around 150 L/day. On day two, 37 and 21 L of biogas and methane, respectively, were produced, and on day 31, a maximum of 164 L of biogas and 110 L of methane was produced (Figure 5 a and b). The ever-changing ambient temperature did not cause the digestion process to be unstable; this was attributed to the insulative greenhouse structure covering the digester.

On average, VUT-1000C produced 901 L/day of biogas and 551 L/day of methane over the 35 days, while STH-1000A produced an average of 108 L/day of biogas and 69 L/day of methane. This significant difference in biogas and methane production could be due to the difference in digester design. First, the STH-1000A design operated at ambient temperature, and biogas production thrives in elevated temperatures. Secondly, the digester did not allow maximum use of the reactor volume; it only allowed for a working volume of half its reactor volume, instead of 80%, as did VUT-1000C. These constraints, in turn, reduced the digester’s optimal OLR to a sixth of VUT-1000C, ultimately leading to low biogas production.

Figure 6a outlines the temperature profile throughout the 35 days digestion period. The temperature profile for VUT-1000C ranged between 36.2 and 37.9 °C, maintained on solar energy only. The temperature deviated from the setpoint by a maximum of 1.2 °C. Solar heating of a biodigester proved very effective. STH-1000A was operated at ambient conditions (Figure 6b). Ambient temperature is typically unstable and fluctuating, and as a result, there was no clear trend in the temperature profile for STH-1000A. The ambient temperature reached its lowest value of 22 °C on day 17 and highest of 32.9 °C on day 32, and on the same days, the digester temperature was at its lowest of 24.1 °C and highest of 32.3 °C.

Figure 6. Digester temperature during food waste digestion using VUT-1000C (a) and STH-1000A (b) (Khune 2021).

3.5. Biogas conversion to electricity and energy balance

A biogas generator connected to a combustion biogas engine was used to generate electricity. A specific volume (1000 L) of biogas was used to perform the test work and a total of 2.3 kW was produced. The electrical output of the engine was 220 V, 50 Hz and 1.8 kWh. A maximum of 1.8 kWh was generated with an overall conversion efficiency of 22% as indicated in the generators manual. The biogas consumption of the generator was 650 L/hour. On a national scale, this amount of electricity can light up to 300 6 W energy saver light bulbs for an hour. Furthermore, in a rural setting where there is no electricity, this biogas pilot plant can provide electricity and allow the users to perform short-term energy-requiring tasks. By running the generator, a family would have a total of 2.3 kW for 1 h 30 min (1.5–1.8 kWh) of electricity per day from 1000 L of biogas. From 2 kWh of electricity, the family can perform one of the following tasks or a balanced mixture of these tasks for their living requirements: microwave (700 W) 32 meals, toast (800 W) 48 slices of bread, vacuum clean (1400 W) 16 rooms, run three laundry loads (800 W), iron (150 W) full laundry, bake one cake (1300 W), blend food (400 W), charge devices (36 W) and drill (650 W) and grind (650 W) for 3 h. For the power balance of the two digesters, the total energy output was compared to the total energy input. The hot water circulation pump and solar geyser water level controller for VUT-1000C were operated every hour for 15 min during the day for 12 h totaling a run time of 3 h per day, both consumed a combined total of 0.024 kWh per day. The digestate circulation pump and food blender were the highest energy-consuming components. The digestate circulation pump was operated for 15 min per hour daily; thus, it consumed 1.080 kWh. The food blender had the highest power rating; however, it was used only for 10 min daily, consuming 0.060 kWh. The hot water circulation pump ran continuously at start up for two days, and then it would turn off when the set digester temperature was reached; finally it ran for approximately 3 h in the morning. On average, the hot water circulation pump used 0.024 kWh. When running the generator, the blower was used to pressurize biogas, consuming 0.032 kWh. The power consumption of the equipment used to operate the digesters is listed in Table 2 where the power rating indicates the highest power input allowed to flow through particular equipment and the power usage indicates the total power the equipment used in a day.

Table 2. Power consumption of equipment used in biogas production and biogas use.

Component	Power rating (kW)	Hours running per day	Usage (kWh)
VUT-1000C
Digestate circulation pump	0.18	6	1.080
Hot water circulation pump	0.008	3	0.024
Solar geyser water level controller	0.005	3	0.015
Hot water circulation controller	0.003	3	0.009
Food blender	0.373	0.16	0.060
Biogas blower	0.016	2	0.032
Total			1.220
STH-1000A
Digestate circulation pump	0.25	6	1.500
Food blender	0.373	0.16	0.060
Biogas blower	0.016	2	0.032
Total			1.592

In total, the energy input needed to run the VUT-1000C pilot plant per day was 1.220 kWh compared to a power output of 1.8 kWh, translating to a 32% net energy output of 0.58 kWh. Overall the heating of the digester required 0.048 kWh of the total power input of 1.220 kWh which is 4% of the total power input and almost 3% of the total energy output in a day. Thus it can be confirmed that this is not a significant increase in the plant’s power requirement for added heating. The use of the digestate mixing pump and food blender may be re-evaluated in an attempt to reduce the power requirements of the plant. STH-1000A had a daily total power consumption of 1.592 kWh. With an average of 150 L of biogas per day, only 0.4 kWh of electricity could be generated from STH-1000A, translating to 398% power consumption (1.192 kWh power loss). This indicates that running an ambient anaerobic digester with electrical devices is extremely energy intensive.

3.6. Technology application and recommendations

The two digesters are useful for household applications and offer free energy to end users. The choice of each may depend on a combination of the following factors; the user’s ground space available, budget constraints, energy requirements as per lifestyle and number of family members and most importantly, available feedstock. The user may install multiple units or a single unit depending on these factors. Mesophilic digesters produce biogas at a faster rate than psychrophilic digesters thus allowing for higher organic loads at a given time (Wei and Guo 2018). With higher volumes of feedstock available, employing the VUT-1000C may lead to ground-space-saving because with STH-1000A a bigger size reactor may be required to treat the same amount of feed. Wei and Guo (2018) found that psychrophilic digestion took 90 days to complete producing biogas most of which was recovered in the first 45 days, whereas during mesophilic digestion biogas production was complete in 30 days and most of which was produced in the first 10 days. So psychrophilic digestion is 3 times slower than mesophilic digestion.

Mesophilic digestion is always discouraged due to its energy requirements and psychrophilic digestion is always praised for not requiring additional energy for heating (Akindolire, Rama, and Roopnarain 2022). VUT-1000C gives mesophilic digestion the same advantage through a novel design by harnessing solar energy for heating. This is beneficial to the end user, as they cannot produce more biogas without the expense of additional energy for heating. The only disadvantage to this approach is the increased initial purchase cost, but the increased biogas production soon outweighs the disadvantage. The final use of biogas in terms of efficiency and getting the most from the biogas depends mostly on the amount of biogas available to the user per day. With excess biogas available low efficiency conversion uses might not be a significant drawback. In future studies, the most efficient utilisation of biogas at a pilot scale may be investigated. This can be used as a guide for users to know what to expect from using biogas in certain applications.

4. Conclusions

The prototype VUT-1000C mesophilic pilot biogas plant was successfully designed, constructed, and commissioned for biogas production and electricity generation. The digester temperature test work results showed that a 1 m³ digester could be well heated to between 36.2 and 37.9 °C using a 100 L solar-heated geyser without electricity backup. The ambient STH-1000A operated within psychrophilic temperatures between 24 and 32 °C. VUT-1000C generated 1.8 kWh of electricity while STH-1000A produced 0.4 kWh. The novel prototype pilot digester showed that operating at mesophilic temperatures does not require significantly higher input energy and can effectively be achieved through solar heating. In evaluating the digesters’ power requirements, VUT-1000C used 68% of its energy output while STH-1000A used 398%. At the end of the 35 days operation at optimum conditions, a total of 31 535 and 19282 L of biogas and methane, respectively, was produced from VUT-1000C and 3790 and 2409 L of biogas and methane from STH-1000A. The results show a significant improvement in biogas and methane production in the prototype design, VUT-1000C, over STH-1000A, proving the advantage of adding solar heating to obtain mesophilic conditions. Future research work may focus on attaining thermophilic conditions using solar heating, perform modeling simulation for upscaling and determine the most efficient utilization of biogas at pilot scale.

Declarations

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

The research of this article was supported by the National Research Foundation (Grant number. 140335) under the German Academic Exchange Service (DAAD) within the framework of the climapAfrica programme with funds from the Federal Ministry of Education and Research, and the Water Research Commission (WRC, Project number. C2020/2021-00426) of South Africa. The authors are fully responsible for the contents.