Anaerobic Co-Digestion of Food Waste with Livestock Manure at Ambient Temperature: A Biogas Based Circular Economy and Sustainable Development Goals

Abstract

A shift from a linear economy to a circular economy of resource consumption is vital for diverting the value from lost resources to resource-efficient products towards developing a sustainable system. Household digesters provide one opportunity to create a biogas-based circular economy. Because household digesters are typically fed a wide and variable range of substrates, it is important to determine the ideal mixing ratios for them. In this study, an anaerobic digester startup process was analyzed and an assessment of anaerobic co-digestion of food waste with different livestock manures was carried out at ambient temperatures. Food waste (FW), cow manure (CM), poultry litter (PL) and goat manure (GM) were co-digested at mixing ratios (FW:PL:CM) of 2:1:1, 2:2:1, 1:1:2, 1:1:1 (wt/wt) and FW:PL:GM at mixing ratios of 2:1:1 and 1:1:2, at an organic loading rate of 1 g volatile solid (VS)/L/day, and 8% total solids. A maximum methane yield was obtained from co-digestion of FW:PL:GM at a mixing ratio of 2:1:1 in autumn-to-winter conditions, 21–10 °C, while the mixing ratio of FW:PL:CM at 2:2:1, showed negligible methane production under the same temperature condition. This study suggests that co-digestion of food waste and poultry litter with goat manure yields more biogas than other substrate combinations. Therefore, selecting suitable co-substrates with an optimized mixing ratio can promote several key indicators of a biogas-based circular economy towards achieving sustainable development goals 2, 3, 5, 6, 7, 9, 13 and 15.

1. Introduction

About 28 percent of the world’s agricultural land area and 250 km³ of total water volume are used annually to produce food that is lost or wasted [1]. Every year, around 1.3 billion tons of food waste consisting of vegetables, cereals, dairy products, fruits, and meat is wasted or lost globally A significant amount of food is lost during the early and middle stages of our food supply chain, whereas much less food is wasted at the consumer level. In South and Southeast Asia, only 6–11 kg of food per capita per year is wasted due to consumers’ purchasing or eating habits or due to oversupply of food in the market. In contrast, 120–170 kg is lost per capita each year due to poor infrastructure, lack of technology, and inefficiency in the food supply chain. [1,2]. The average municipal solid waste (MSW) generation in Nepal is estimated to be about 0.32 kg/capita/day, of which 60% is constituted of organic waste [3]. The total organic waste from municipalities of Nepal is about 2456 tons/day [3,4]. Additionally, there are more than 103 million livestock (cattle, buffalo, sheep, goat, pigs) and poultry in Nepal. The estimated manure production from these livestock and poultry is about 187,397 tons per day [5,6]. The carbon footprint from food wastage is estimated at 3.3 billion tons of CO₂ equivalent of greenhouse gases (GHGs) released into the atmosphere per year globally [7]. In addition, all the animal production sectors are responsible for nearly 14.5% of GHG emissions, among which cattle farms are responsible for nearly 65% [7]. As reported by The Food and Agriculture Organization (FAO) of the United Nations, the greatest GHG emission reduction potential from ruminants could be achieved from Southeast Asia and Africa. On the bright side, it claims that GHG from livestock could be reduced by 30% if existing technologies like anaerobic digestion are widely practiced in these areas [7,8].

Anaerobic digestion (AD) is the decomposition of biodegradable matter initiated by combinations of Bacteria and Archaea in an absence of oxygen. The AD system is deemed the heart of the circular economy of organic wastes and a bridge connecting the circular economy and bio-economy to sustainable development goals (SDGs) [9,10]. Previously popular linear economy approaches utilize the take-make-dispose cycle, which ultimately ends in the limiting of the supply resources and diminished value of resources after use, leading to high risk of economic and social suffering in human beings. A circular economy, on the other hand, is restorative and regenerative and works towards the use of renewable resources. It includes the design and selection of materials that could be reused, repaired, or upcycled, resource optimization, and remanufacturing and recycling from consumption [9,10]. Hence, a circular economy is a continuous and sustainable cycle. Bio-economy encompasses the idea of production of renewable bio-resources and technological development for conversion of these resources into value-added products such as energy, bio-product, food, or feed with replacement of a considerable amount of fossil fuels [11]. Both the circular economy and bio-economy work towards lowering GHG footprints and increasing resource efficiency and sustainability [12]. The approach to a circular economy and bio-economy resonates with AD systems such as a waste treatment plant or biogas plant and closes the loop of these concepts. AD enables upcycling of disposable waste material into high-end outcomes such as biogas, primarily consisting of methane (55–60%) and carbon dioxide (40%), and nutrient-rich digestate that can be used as fertilizer [13,14,15]. Both biogas as clean energy and digestate as fertilizer could be the sustainable future replacement for energy sources dependent on fossil fuel and fossil fuel-intensive chemical fertilizer, and is an excellent solution for the reduction of the carbon footprint in the environment [16,17,18]. In addition, AD of organic wastes contributes towards the achievement of SDGs 2, 3, 5, 6, 7, 9, 13 and 15 [19,20], which are discussed later in the paper.

The AD process could be optimized to enhance the organic conversion of the wastes by proper start-up processes, digester designs, substrate mixture ratio, operating temperature, organic loading rate (OLR), and hydraulic retention time (HRT) [21,22,23]. Some of the well-studied methods to optimize start-up are the introduction of well-adapted inoculum, the use of controlled temperature mostly in the mesophilic range, and the use of an appropriate waste-to-inoculum ratio [24,25,26,27]. This adjustment would help to reduce the effect on the stability of the process, biogas production, and yield due to ambient temperature fluctuation [16,28].

Millions of the rural household biogas digesters operating in developing countries of Asia and Africa are working under ambient conditions and are primarily unheated. Therefore, they experience maximum fluctuation of temperature, resulting in a reduction of biogas production [29,30,31]. It is recommended that a digester’s operating temperature should be in the range of 33–37 °C or 45–55 °C during the anaerobic start-up of the digester. The failure to maintain the temperature within that range will inhibit methanogenic growth, increase the start-up time, and reduce biogas production [32,33]. In most of the studies, acclimated seed (inoculum), chemical and thermal pretreatments of feeding substrate and mesophilic temperature are maintained to speed up the start-up process along with the overall AD process [34,35,36]. However, the results obtained from these studies could not be verified for systems operating in ambient conditions without acclimated inocula at start-up, which is the situation for household digesters in many developing countries, including Nepal. Therefore, this study intends to evaluate the start-up process of an anaerobic digester fed with cow manure at ambient temperature.

Anaerobic co-digestion (ACoD) is a well-known optimization method for enhancing biogas production during the anaerobic digestion process [21]. ACoD is the process of feeding two or more organic substrates in an anaerobic digestion process. It offers a wide range of benefits in comparison to mono-digestion, such as dilution of toxic substances, balancing of C:N ratio, enhanced methane yield, the synergic effect of microorganisms, and nutrient balance [37]. ACoD not only uses mixtures of household organic wastes and animal farm wastes for generating value-added products, but also considers industrial waste such as paper pulp, press mud (wastes from sugar mills), and tea waste, which perfectly encompasses a circular economy approach [38,39,40]. Nevertheless, the selection of appropriate co-substrates, their characterization, composition, and mixing ratio should be wisely considered before applying it to biogas generation. Numerous research efforts have been devoted to studying biogas production from co-mixed substrates, each contributing to the literature and scientific community [41,42,43]. However, there is a dearth of studies on co-digestion of food waste with cow manure, poultry litter, and goat manure as co substrates in different mixing ratios at ambient temperature. These substrates are commonly available to use in rural digesters in Nepal. One study showed that co-digestion of food waste with cow manure and poultry litter in reactors acclimated with anaerobically treated inoculum and under a mesophilic temperature of $\pm 35 °$C yields methane in the range of 317–500 L CH₄/kgVS_added [44,45]. A similar study on co-digestion of food waste, cow dung, and pig manure collectively at a mixing ratio of 2:2:1 has shown the highest biogas yield of 488 L biogas/kgVS_added, with methane content of 63% [42].

As discussed before, in most developing nations, the anaerobic digester operates in ambient conditions without using acclimated inoculum before actual feeding of substrates. Hence many complexities arise, such as prolonged reactor start-up time and poor methane yield, which have been major challenges for domestic biogas plants in developing nations. In addition, single substrate (livestock manure) digestion is a usual practice that misses an opportunity to realize the synergetic benefits of co-digestion—namely, the improved AD process, stability, and optimized biogas production—even in low temperatures. Considering this practical knowledge gap, this paper attempts to contribute to the knowledge of the startup process at ambient temperatures, together with an optimization of biogas production from food waste when co-digested with different ratios of cow manure, poultry litter, and goat manure, thus mimicking the operating condition of field-based households’ biogas digesters mainly used in low-income countries. The study provides insight into realistic biogas production from carefully selected substrates and their optimized mixing ratio to provide a feasible solution for clean energy generation and waste management, while adhering to a circular economy in achieving SDGs.

2. Methodology

2.1. Feeding Materials

Fresh cow manure was collected from a cattle farm in Dhulikhel, Nepal. Food waste was obtained from a student mesh at the university. Undesirable items such as meat bones, plastics, tissues, and pickles were handpicked and discarded. The remaining food waste was mainly composed of rice, lentils, and cooked vegetables, which were homogenized using a kitchen grinder for about 1 min until they were of approximately 5 mm particle size. Poultry litter was collected from a deep litter poultry farm, while the goat droppings were collected from a local household near the university. All the collected feeding materials were stored at $4 °$C. They were stored for a maximum of 5 days.

2.2. Digester Setup and Operation

A laboratory-scale 5 L semi-continuous reactor was used, which was fitted with infusion sets for gas measurement and PVC pipe attached with a plastic funnel as the inlet for the substrate, as shown in Figure 1. The arrangement was sealed and made airtight for the AD process. The total working volume of the reactor during the AD process was 3 L, whereas 2 L of it was left as the headspace.

The experimental anaerobic reactors were initially fed with cow manure at 8% TS and the start-up of the reactors was first monitored in batch mode for a month. Since the methane content of the biogas did not increase from 20%, the start-up strategy was switched to semi-continuous mode. These digesters were again fed with cow manure at 8% Total Solids (TS) and OLR 0.3 g VS/L/day from the last week of August to the second week of October.

In the first phase, a co-digestion experiment of food waste (FW), cow manure (CM), Poultry litter (PL), and goat manure (GM) was conducted from the third week of October to the first week of February at an average ambient temperature range of 10–21 °C. Co-substrate of FW, PL and CM with mixing ratios (wt/wt) of 2:1:1 and 2:2:1 and co-substrates of FW, PL and GM with a mixing ratio of 2:1:1 were selected for the experiment, as shown in Table 1. For all the reactors, the OLR was maintained at 1 gVS/L/day and an HRT of 60 days. The mixing ratios were chosen to maintain the C:N ratio in a standard range (i.e., 20–30) and to maintain the pH of the substrate within an acceptable value of 6.5–7.5 for the AD process [46,47]. All the sets of co-digestion experiments were conducted at 8% TS with main operational parameters pH, C:N ratio, temperature, and an OLR replicating the fixed dome cattle-manure fed household biogas digesters commonly operated in Nepal.

In the second phase, from the first week of February to the third week of April, with an ambient temperature range of 11–21 °C, the co-substrates of FW:PL:CM, with mixing ratios of 1:1:2 and 1:1:1, and FW:PL:GM, with a mixing ratio of 1:1:2, were co-digested at a constant OLR of 1 g VS/L/day. All the other operational parameters were kept the same as in the previous trial. The daily record of biogas production and the ambient room temperature was monitored, and the measurement of biogas composition was recorded fortnightly. Finally, the results were analyzed for comparison in terms of biogas yield and methane composition.

2.3. Analytical Methods

The TS, VS, total organic carbon (TOC), and total organic nitrogen (TON) were analyzed using standard methods. The TS content was measured following the APHA 2540 D guideline, whereas the VS content was measured following the APHA 2540 E guideline. To measure the C:N ratio of the substrates and their mixture, TOC was measured following the American Society of Agronomy and Soil Science guideline, while their TON was measured following the APHA 4500-Norg guideline. Daily samples were taken for pH analysis and were measured using Exotech SOL 100 pH meters. The water displacement method was used for measuring daily biogas production. The biogas composition was analyzed every 10 to15 days using Sewerin Multitech 545 gas analyzer,(Gutersloh, Gremany). A temperature logger was used to record the ambient temperature every thirty minutes to account for temperature fluctuation during the AD process.

3. Result and Discussion

3.1. Physio-Chemical Properties

Understanding the feedstock properties is very important, as they influence the performance of AD. The properties of the feedstock directly affect the startup process, AD process stability, and biogas yield [22,48]. Table 2 presents the physical and chemical properties of collected raw samples and prepared feeding samples. FW has an average TS content of 20.8% (standard deviation (s.d) 2.4, number of samples, n = 12), which lies in the typical range of 18–39.5%, as reported in the literature [38,44,49,50,51]. The pH of the food waste is very low, around 4.1 (Table 2). Low pH results in the excessive production of volatile fatty acids (VFAs) [46,52]. Thus, pH needs to be adjusted within a range of 6.5–7.5, for methanogens to thrive effectively during the AD process [22,53,54]. Hence, the co-digestion of FW with different co-substrates such as PL, CM, and GM maintains the pH and the C:N ratio of the feeding material in an acceptable range of 20–30 [55].

3.2. Start-Up Process Fed with Cow Manure

AD process stability is a key factor in improving the production of biomethane and reducing the inefficiency of the process. As an important indicator, pH value is frequently used to determine the AD process′s stability [22,55]. During the start-up process, the average daily ambient temperature decreased from $22 °$C to $20 °$C from July to October.

Figure 2a,b indicates an unstable AD process, as gas production was unstable until the 35th day of operation. The gas production was first observed on the third day of operation of the digester; however, the methane composition was found to be less than 40% for over a month. There was a nearly 25% reduction in pH in just 35 days of operation of the anaerobic digester, as seen in Figure 2b. The pH less than 6.5 suggested the slurry inside the digester was acidic due to rapid accumulation of volatile fatty acids. The low methane content and pH reduction suggested the inability of methanogens to grow inside the reactor.

Since the reactors showed instability for more than a month in batch mode, as a recovery measure they were shifted to semi-continuous feeding on day 35, with a mixture of cow manure (8% TS) at OLR of 0.3 gVS/L/day and cow urine until the methane composition rose above 50% consistently (Figure 2a). After only a week of semi-continuous feeding of the biogas production, the methane content increased, until it stabilized near 50 mL of biogas production per day with a methane content of 70% at day 67 (Figure 2a). The pH of the digestate was nearly constant, around 6.8, during semi-continuous operation (35–71 days). On the 72nd day, the same reactors were started and fed with different ratios of the co-digested substrates, such as food waste, cow manure, poultry litter, and goat manure.

3.3. Co-Digestion of Different Substrates at Ambient Temperature Conditions

3.3.1. Co-Digestion for Biogas Production from Autumn to Winter

After the start-up of the reactor, the experiment was continued for co-digestion of FW, CM, PL, and GM. Figure 3 indicates that the average ambient daily temperature from autumn to winter dropped drastically from $21 °$C to nearly $10 °$C. The lowest temperature of $10 °$C was recorded in January (day 151).

Figure 4 shows cumulative biogas generation at two different mixing ratios of food waste, poultry litter, and cow manure (FW:PM:CM) and a single mixing ratio of food waste, poultry litter, and goat manure (FW:PM:GM) for the autumn-to-winter timeframe (72–181days).

The reactors C₁ and C₂ were fed with mixing ratios FW:PM:CM—2:1:1 and 2:2:1, while reactor C₃ was fed with mixing ratio FW:PM:GM—2:1:1. The OLR was kept constant at 1 gVS/L/day for all experiments. The daily average temperature fluctuation from autumn to winter was significant, from 21 °C to 10 °C, which was well below the mesophilic range, i.e., 30–40 °C. The average daily biogas yields of C₁ and C₃ while the ambient temperature was slowly declining from 21–12 °C (Figure 3) were 205.8 L/kgVS_added, and 275.3 L/kgVS_added, respectively. But as the ambient temperature reduced and remained somewhat constant near 12–10 °C, the average daily biogas yield in both reactors also dropped down to 174.2 and 247.6 L/kgVS_added,, respectively. The overall average daily biogas yield was found to be maximum in the reactor containing goat manure (C₃), about 264.4 L/kgVS_added, whereas the overall average daily biogas yield of C₁ was 190.2 L/kgVS_added. The methane (CH₄) was recorded in the range of 62 to 67% in reactor C₁ and the average digestate pH was 7. The CH₄ content in reactor C₃ was in the range 60–63%, with an average digestate pH of 7. In comparison, reactor C₂ was unstable from the beginning of the digestion process, as shown in Figure 4. The initial pH of the feedstock was alkaline (pH 8.2) due to the high concentration of nitrogen content in poultry litter. The pH remained stable at a range of 7.9–8.3 in reactor C₂, indicating highly alkaline digestate inside the reactor in comparison to reactors C₁ (7.1–7.5) and C₃ (7.1–7.6). A high pH throughout the AD process in reactor C₂ might have caused instability of the process due to the rapid conversion of (ionized) ammoniacal nitrogen into free ammonia nitrogen (unionized form). This form of ammonia at high concentration inhibits the methanogenesis process [56,57]. It’s possible the high alkalinity of feedstock in C₂ caused imbalances in the methanogenesis process due to ammonia inhibition. The reactor C₂ had an average biogas yield of only 7.4 L/kgVS_added during the whole AD process, with negligible average methane composition (8.5%).

Figure 4 clearly shows higher cumulative gas production when food waste was co-digested with poultry litter and goat manure compared to poultry litter and cow manure (C₃ > C₁). The result also suggests that the mixing ratio of FW:PL:CM—2:1:1 is more appropriate than the mixing ratio of FW:PL:CM—2:2:1, which might be due to the lower concentration of poultry manure in the mixture (less concentration of nitrogen in feed).

3.3.2. Co-Digestion for Biogas Production from Winter to Spring

The average ambient temperature from winter to spring rises from $11 °$C to nearly $20.5 °$C from February to April, as shown in Figure 3. For winter-to-spring conditions (182–258th day), the same co-substrates—cow manure, poultry litter, and goat manure—were used for anaerobic co-digestion with food waste. The experiments were carried out in the same reactor (previously used for the autumn-to-winter conditions AD process), keeping all operational parameters the same except the mixing ratio of substrates. Based on results during autumn to winter, the poultry litter was kept at a lower ratio for all subsequent tests. Figure 5 shows the cumulative biogas production from co-digestion of food waste with poultry litter and cow manure (FW:PL:CM) in two ratios: 1:1:2 (reactor B₁), and 1:1:1 (reactor B₂), and a single mixing ratio of food waste with poultry litter and goat manure (FW:PL:GM) at 1:1:2 (reactor B₃).

Figure 5 indicates that gas production from reactors B₁ and B₂ seemed to increase after nearly one month of operation, perhaps due to steadily increasing operating temperature. Similar to the previous analysis, reactors B₁ and B₂ had higher initial pH, at 7.5 and 7.7, respectively, than did reactor B₃, with an initial pH of 7.0, and concurrently demonstrated worse initial performance at low temperature. However, in this case the reactors were able to recover and mediate pH to achieve strong performance, perhaps due to increasing operating temperature (instead of lowering temperature as in the autumn-to-winter trials and high pH). The average daily biogas yield in reactors B₁ and B₂ when the average temperature range was 11–15 °C were 86.8 L/kgVS_added and 38.9 L/kgVS_added, respectively_, whereas the average daily biogas yield when the average temperature was slowly increasing from 16–20 °C, were 232.7 L/kgVS_added and 185.8 L/kgVS_added. The exponential increase of the cumulative gas production of both reactors was observed after the average daily ambient temperature increased from 15 °C. In contrast, reactor B₃ produced a significant amount of biogas with a daily average biogas yield of 245.5 L/kgVS_added after the same temperature rise (16–20 °C). Biogas production was again greater for the reactor using goat manure (B₃) than the reactor using cow manure at the same ratios (B₁). The co-digestion of FW:PL: CW at a mixing ratio 1:1:2 (B₁) seems to have had better biogas yield than the ratio 1:1:1 (B₂) of the same substrate. The CH₄ percentage of all reactors were recorded in the range of 60–65%. The overall average daily biogas yield was found to be maximum in the reactor containing goat manure (B₃), about 223.9 L/kgVS_added, whereas the overall average daily biogas yields of B₁ and B₂ were 181.5 and 131.9 L/kgVS_added, respectively. The average pH of all the reactors were also within an acceptable range of 6.9 to 7.4. The fluctuation in gas production seems more pronounced in the winter-to-spring temperature condition (Figure 5) than in the autumn-to-winter temperature condition.

Table 3 shows that the confidence interval of biogas yield during winter-to-spring operation was greater than that of autumn-to-winter operation, indicating more fluctuation of daily biogas production and less stable operation, which is perhaps due to low temperature at startup.

The detailed analysis suggests that the combination of substrates containing goat manure have notably improved the methane yield. It can also be inferred that the reactor’s efficiency was quite low during the winter to spring, which means changes in feeding composition, mixture, etc., extended the time required for microbial adaptation in winter.

This study suggests that with continuation of summer start-up without changes in feeding materials, composition, and organic loading rate, higher biogas yield would be achieved. This approach can strategically be beneficial when applied to actual household digesters in Nepal in terms of improved biogas production from currently available feeding materials.

3.4. Comparison with Other Studies

The co-substrates selected for this study and operational parameters, mainly temperature (ambient temperature conditions), have not yet been available in the literature. Similar co-digestion studies with other combinations of substrates digested at mesophilic temperature (Table 4) were used for a broad comparison. One study reported that mono digestion of PL generates biogas yield in the range of 0.245–0.372 L/gVS_added, with about 60% methane content [58]; however, this biogas production is only valid when PL has diluted below 6% TS concentration, otherwise problems like ammonia toxicity cause process instability [58]. Similarly, mono digestion of FW faces problems such as excessive accumulation of VFA, nutrient imbalances, and process instability [51,59]. A study conducted on mono digestion of FW at 6% and 10% TS and ambient temperature conditions in hilly climates of Nepal showed a very low methane content of only 13% and overall process instability after few weeks of operation, the reason being the rapid acidification of digester due to excessive accumulation of VFAs and low initial pH of the feed [60]. Similarly, a study done in a thermophilic, continuously stirred reactor showed a 33% higher methane yield when CM is co-digested with wheat straw than digested alone [61]. The implementation of co-digestion is not only applicable for increasing methane generation, but also in improving the process stability and waste management as a whole [59,62]. It was reported that when FW is co-digested with CM or PL, and CM and PL, biogas yield increased by 30–40%, indicating a favorable environment for a stable AD process [63,64]. Table 4 compares this study with various other AD studies conducted in controlled (mesophilic) temperature along with an automatic mixing arrangement. However, in the present study, no parameters were controlled; rather, the experiment was intended to replicate the actual household digester subjected to a natural environment. Despite the ambient temperature fluctuation throughout the experiment, it showed that the microbial adaption in the digester was strong, with only 20–30% less methane yield than in the controlled mesophilic temperature with acclimated inoculation (Table 4).

3.5. Co-Digestion in Achieving Sustainable Development Goals in Nepal

In general, developing countries use a linear economy model, i.e., “Take, Make and Dispose”, which works on an assumption that there are abundant available resources and lacks concern about their depletion. This assumption has led to over-exploitation of resources and fast resource depletion along with pollution and climate change [68]. Shifting from a linear economy to a circular economy is therefore essential to slow down the resource depletion rate and to increase resource productivity. A systematic adoption of a circular economy model in these countries can ensure resource productivity around 85% [68]. This presents opportunities for developing countries including Nepal to evolve their linear economy into a circular one. Nevertheless, these countries must still meet the stringent standards of a circular economy (i.e., reducing environmental footprint, increasing income generation, reducing resource dependency, and minimizing wastes) to realize the enormous benefits while also achieving sustainable development pathways [69]. More than 432,000 household biogas plants are installed in Nepal, but the circular economy of organic waste is naively practiced and is being adopted at a very slow pace in the country.

As previously discussed in Section 1, the total organic wastes generated from municipalities of Nepal is about 2456 tons/day, i.e., 896,440 tons/year. The biogas generation potential from these wastes is about 37.7 million m³/year (considered TS%: 21%, VS%: 89% and Biogas yield: 225 L/kgVS_added) [5]. With more than 103 million (cattle, buffalo, goat, pigs and sheeps) livestock and poultry head, estimated manure production was nearly 68.4 million tons per year [5,6]. Currently, wise manure management practice in Nepal is rarely adopted. Most of the farmers scrape fresh manure from the cattle shed and apply it directly to land as a fertilizer or leave the manure in uncovered heaps, which greatly risks environmental pollution. Some of the farmers use dry manure to make dung cakes for the purpose of heating and cooking animal food, while very few practice composting [69].

If all the wastes are used for resource recovery, nearly 3203 millions m³ of biogas could be generated annually, which would have potential to replace nearly 110 million cylinders of Liquid Petroleum Gas (LPG) in a year (1 LPG cylinder is 14.2 kg LPG equivalent to 29 m³ biogas). The total avoided greenhouse gas (GHG) emissions saved by replacing LPG cylinders would be around 4.5 million tons CO₂-e per year [5]. The World Bank Carbon Pricing Dashboard values of USD 1.18 per ton of CO₂-e for Kazakhstan ETS (emissions trading system) and USD 3.71 per ton of CO₂-e for Chongqing ETS were used to estimate the range of monetary value from these avoided emissions (as of 4/1/21). This projects to 5.3–16.7 million US dollars per year as part of a regional market. The offsets could be worth far more on the international market. The biogas production estimate used was only for mono digestion of each of the substrates; however, several studies, including the present study, shown in Table 4, verify the fact that anaerobic co-digestion is an excellent optimization technique for enhanced biogas production and methane yield of organic wastes. The study done in an ambient condition also showed that anaerobic co-digestion was not only stable, but also increased the methane percentage by nearly 40–50% [60,70]. Hence, the biogas generation potential and fossil fuel replacement might increase exponentially if the co-digestion process is adopted and established in countries such as Nepal [5]. Furthermore, this research will also provide diverse options to decision makers for implementing a sustainable organic waste management system along with relevant technical details to develop appropriate designs for resource recovery [71]. In doing so, A CoD contributes to a biogas-based circular economy in achieving several SDG goals. Furthermore, since 70–80% of digestate from the reactor is recovered as bio-slurry, this can be an excellent organic fertilizer (composed of nitrogen, phosphorus, potassium) for providing the soil with healthy nutrients for improving crop production. Therefore, the replacement of fossil fuels by biogas and chemical fertilizer by organic fertilizer would contribute in achieving SDG goals 2, 3, 5, 6, 7, 9, 13 and 15 [19,72]. Table 5 provides information on how sustainable production of biogas contributes to the circular economy and helps to achieve sustainable development goals.

4. Conclusions

A transition from linear to circular economies will help in the complete utilization of resources from low-valued wastes towards increased generation of value-added products such as biogas and fertilizers. In this study, the anaerobic co-digestion process was optimized to increase resource recovery from organic wastes. The anaerobic co-digestion process was performed with different mixing ratios of food waste, cow manure, goat manure, and poultry litter under ambient temperature. The mixture FW:PL:GM—2:1:1 had the highest average biogas yield of 264.4 ± 9.1 LCH₄/kgVS_added at weather conditions from autumn to winter. The mixture of FW:PL:CM—2:2:1 showed failure in biogas generation with an average pH stable at 8 during the whole process. The halt in methanogenesis process could be attributed to the increased ammonia concentration at the beginning of the digestion. During winter to spring, a mixing ratio of FW:PL:GM 1:1:2 showed the highest biogas yield at 223.9 ± 12.3 CH₄/kgVS_added. Goat manure yielded greater biogas production than cow manure when used in the same ratios with food waste and poultry litter for both temperature regimes. The results indicate that using an optimized mixture of co-substrates along with summer startup could notably improve energy generation at the household level. It can be inferred from this research that anaerobic co-digestion of food and livestock waste with an appropriate model could potentially demonstrate pathways to a biogas-based circular economy towards achieving sustainable development goals.

Author Contributions

B.D.: Experimentation, formal analysis, resources, data curation, writing original draft, editing, and visualization. S.P.L.: Conceptualization, methodology, formal analysis, resources, data curation, writing original draft, reviewing, editing, and visualization. M.M.: Reviewing, editing, and visualization. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partly funded by EnergizeNepal Project (ENEP-RENP-18-II-01).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to acknowledge the EnergizeNepal Project for the financial support of the biogas project (ENEP-RENP-II-18-01).

Conflicts of Interest

Authors would like to declare that they have no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Semi-continuous reactor setup.

Figure 2. Daily (a) and cumulative (b) gas production with methane composition during start-up of the digesters using only cow manure.

Figure 3. Ambient temperature profile during the reactor monitoring period.

Figure 4. Cumulative gas production from co-digestion of food waste from utumn to winter.

Figure 5. Cumulative biogas production from co-digestion of food waste from winter to spring.

Table 1

Feeding substrate, digestion process, and mixing ratio.

Experiment Season	Co-Digestion Substrates	Feeding Ratio	Digestion Process	ReactorAbbreviation
October to February	FW:PL:CMFW:PL:GM	2:1:12:2:12:1:1	Semi-continuous	C1C2C3
February to April	FW:PL:CMFW:PL:GM	1:1:21:1:11:1:2		B1B2B3

Table 2

Physio-Chemical properties of Samples.

Collected Raw Samples	TS%	VS% (of TS)	pH	C/N Ratio
FW	20.8	89.2	4.1	22.4
PL	38.1	63.4	8.8	18.3
CM	18.9	86.3	6.4	20.6
GM	31.9	81.4	6.7	18.8
Cow urine			8
Feedstock and Mixing Ratio	TS%	VS% of TS	pH	C/N Ratio
FW:PL:CM (2:1:1)	8	82.0	7.3	20.9
FW:PL:CM (2:2:1)	8	78.3	8.2	19.9
FW:PL:GM (2:1:1)	8	80.8	7.3	20.5
FW:PL:CM (1:1:2)	8	81.3	7.5	20.5
FW:PL:CM (1:1:1)	8	79.6	7.7	20.4
FW:PL:GM (1:1:2)	8	78.8	7.0	19.6

Table 3

Descriptive statistics of co-digestion of food waste at different seasons.

Descriptive Statistics	Autumn to Winter		Winter to Spring
	C1 (FW:PL:CM)	C3 (FW:PL:GM)	B1 (FW:PL:CM)	B2 (FW:PL:CM)	B3 (FW:PL:GM)
OLR (gVS/L/day)	1
HRT (days)	60
Temperature	21–10 °C		11–21 °C
Average biogas yield L/kgVSadded	$190.2\pm 6.3$*	$264.4\pm 9.1$*	$181.5\pm 14.7$*	$131.9\pm 12.6$*	$223.9\pm 12.3$*
Average methane composition %	63.7	61.9	64.4	62.7	65.4
Confidence Level (95.0%)L/Kg VSadded	12.5	18.2	29.3	25.1	24.6

* ±standard error (95% confidence level).

Table 4

Comparison with other studies.

Feed Substrates	Ratio	TS (%)	Temperature (°C)	Reactor Type	OLR (per day)	Methane Composition (%)	Biogas Yield	Action of Co-Digestion	Source
FW:PL: CM	2:1:1 C1	8	21–10 °C	Semi-continuous	1 gVS/L/day	63.7%	190.2 ± 6.3 L/kgVSadded	Methane yield from combination involving goat manure seems to have a higher value than with other co-substrates	This Study
	1:1:2 B1		11–20.5 °C			64.4%	181.5 ± 14.7 L/kgVSadded
	1:1:1 B2					62.7%	131.9 ± 12.6 L/kgVSadded
FW:PL:GM	2:1:1 C3		21–10 °C			61.9%	264.4 ± 9.1 L/kgVSadded
	1:1:2 B3		11–20.5 °C			65.4%	223.9 ± 12.3 L/kgVSadded
FW: Pig manure (PM)	7%:93% (PM: FW) 17%:83% (PM: FW)	NA	37 °C	Batch reactor	3.2–6.35 gCOD/L/day	NA	358 L CH4/kgVSadded for 7:93 PW: FW	The high methane yield obtained with a higher concentration of pig manure in food waste during AD	[65]
							388 LCH4/kgVSadded for 17:83 PM: FW
FW: GW (Garden waste)	FW alone	NA	36 °C	500 L Semi continuous reactor	0.34 g VS/L/day, 0.44 g VS/L/day, 0.54 g VS/L/day	Food waste only 37–60%	60–420 LCH4/kgVSadded	Methane yield increased by 25% with the addition of garden waste to the food waste, the addition of 20% food waste OLR to GW improved biogas production by 40–50%	[66]
	FW with 20% GW					Fw+GW 56–61%	140–490 LCH4/kgVSadded
FW	FW: 20% inoculum	6–10%	20–26 °C	Semi-continuous	1 gVS/L/day	13%	135 L/kgVSadded gas yield	Large pH drop recorded with overall process instability and very low methane percentage	[60]
(32% CM + 12% GM + 24%FW + 32% agro waste): CM	80:2070:3065:3560:4055:45(wet weight)	NA	35 °C	120 mL serum bottle	NA	45–55% of the single substrate45% during co-digestion	FW—406 L/kgVSadded	20% higher cumulative production during co-digestion relative to mono digestionMethane yield increased 28–177% with co-digestion	[67]
							CW—302 L/kgVSadded
							PL—232 L/kgVSadded
							GM—199 L/kgVSadded
							Agro waste—125 L/kgVSadded
							co-digestion 436–551 L/kgVSadded

Table 5

Anaerobic digestion, circular economy and sustainable development goals indicators.

Circular Economy Indicators	AD Indicators	Sustainable Development Goals	References
By-products during production become input for another product, repurposing end product, waste reduction	Restoring soils, recycling nutrients, organic matters, biofertilizers for increasing crop yields	SDGs 2: Zero hunger	[9,10]
Replacing finite resources with renewables, bio-product utilization	Replacing biomass-based fuels with biogas for reducing indoor air pollution	SDGs 3: Good health and well-being	[9,10]
	Improving lives of women and children, reducing the burden of firewood collection	SDGs 5: Gender equality	[19]
Selecting materials that can be recycled or upcycled, selection of bioresource such as sewage sludge	Treatments of wastewater, sewage, reducing odors and disease spread	SDGs 6: Clean water and sanitation	[9,10,73]
Waste or by-product becomes an input for another process or product, upcycle waste into a valuable product	Reducing dependency on fossil fuels with biogas, waste to energy production, utilizing locally produced wastes and crops for energy generation	SDGs 7: Affordable and clean energy	[10,19]
Resilient and sustainable industries	Reducing the dependency of industries on government and improving self-sufficiency for energy generation, a collaboration of industry and agricultural farms, employment opportunity	SDGs 9: Industry innovation and infrastructure	[9,10,19]
Increased climate awareness, reduction of GHG emission, mitigate climate change and increase resilience	Reducing carbon dioxide emission, replacement of chemical fertilizers in farms, reducing methane emission from landfills, control in deforestation	SDGs 13: Climate action	[9,10,19]
Optimized the raw material usage, maximize production, minimize waste, recycle discarded materials after the end of consumption, regenerative process	Waste generation and energy generation cycle continuation, recirculation of nutrients to soil after biogas generation through bio-slurry	SDGs 15: Life on Land	[19,74]

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