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Article

Methane and Hydrogen Sulfide Production from Co-Digestion of Gummy Waste with a Food Waste, Grease Waste, and Dairy Manure Mixture

by
Abhinav Choudhury
and
Stephanie Lansing
*
Department of Environmental Science and Technology, University of Maryland, University of Maryland Energy Research Center, 1429 Animal Science/Ag Engineering Bldg., College Park, MD 20742, USA
*
Author to whom correspondence should be addressed.
Energies 2019, 12(23), 4464; https://doi.org/10.3390/en12234464
Submission received: 1 November 2019 / Revised: 18 November 2019 / Accepted: 19 November 2019 / Published: 23 November 2019
(This article belongs to the Special Issue Biogas for Rural Areas)
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Abstract

:
Co-digestion of dairy manure with waste organic substrates has been shown to increase the methane (CH4) yield of farm-scale anaerobic digestion (AD). A gummy vitamin waste (GVW) product was evaluated as an AD co-digestion substrate using batch AD testing. The GVW product was added at four inclusion levels (0%, 5%, 9%, and 23% on a wet mass basis) to a co-digestion substrate mixture of dairy manure (DM), food-waste (FW), and grease-waste (GW) and compared to mono-digestion of the GVW, DM, FW, and GW substrates. All GVW co-digestion treatments significantly increased CH4 yield by 126–151% (336–374 mL CH4/g volatile solids (VS)) compared to DM-only treatment (149 mL CH4/g VS). The GVW co-digestion treatments also significantly decreased the hydrogen sulfide (H2S) content in the biogas by 66–83% (35.1–71.9 mL H2S/kg VS) compared to DM-only (212 mL H2S/kg VS) due to the low sulfur (S) content in GVW waste. The study showed that GVW is a potentially valuable co-digestion substrate for dairy manure. The high density of VS and low moisture and S content of GVW resulted in higher CH4 yields and lower H2S concentrations, which could be economically beneficial for dairy farmers.

Graphical Abstract

1. Introduction

Anaerobic digestion (AD) of organic substrates with dairy manure, also known as co-digestion, can increase biogas production and result in higher return on investment for dairy farmers [1]. Biogas produced from AD is a combination of 50–75% methane (CH4) and 25–50% carbon dioxide (CO2), with trace levels (0.01–1%) of hydrogen sulfide (H2S) that can be used as a source of renewable energy for heat and power generation [2]. Limitations from mono-digestion of organic materials arise from substrate properties, such as unbalanced C:N ratios, recalcitrance in the feedstock, high concentrations of long chain fatty acids, and deficiency in trace minerals required for the growth of methanogens [1,3]. These limitations can lead to unfavorable economics for dairy farmers using AD to generate energy on-farm [1,4]. Furthermore, positive synergy from co-digestion of a mixture of substrates can lead to more CH4 production than the addition of CH4 produced from mono-digestion of each individual substrate. A review by Mata-Alvarez et al. (2014) reported that co-digestion of carbon (C)-rich organic matter with cattle and poultry manure resulted in up to 3.5 times more CH4 production than the CH4 potential of the individual substrates [3]. Lisboa and Lansing (2013) reported a maximum of 29.4 times more CH4 yield when dairy manure was co-digested with chicken processing waste compared to mono-digestion of dairy manure [5]. Moody et al. (2011) determined the biomethane potential of a wide range of food waste substrates and concluded that co-digestion of manure and organic waste has the potential to increase biogas production, and in turn, increase energy generation from AD [6]. However, often studies are only applied to individual substrates due to differences in organic waste composition and collection.
Previous research on co-digestion of food waste and dairy manure has primarily focused on the CH4 production potential of co-substrates [7,8,9], with limited data on the effects of co-digestion substrate selection on the production of H2S [10]. The production H2S in biogas occurs when sulfur-containing compounds, such as sulfates, sulfites, and thiosulfate, in AD substrates are reduced by sulfate-reducing bacteria (SRB) under anaerobic conditions [11]. High H2S concentrations in biogas (0.05–1% by vol.) can become a major problem when utilizing the biogas due to health concerns and corrosion of biogas equipment [12]. Combined heat and power (CHP) systems usually require H2S concentrations to not exceed 500 ppm to prevent reduced performance from corrosion, and H2S concentrations over 100 ppm can lead to severe adverse human health impacts [10]. Most dairy farms use CHP systems to generate energy for on-farm use and lower H2S concentrations can lead to improved energy generation efficiencies and reduced maintenance. Corro et al. (2013) observed a reduction in H2S concentrations when coffee waste was co-digested with dairy manure compared to digestion of dairy manure only, but there was no discussion of the cause for the observed H2S differences [13]. Research has shown that co-digestion of organic matter with higher C:N ratios in manure-based digesters can reduce ammonia inhibition and enhance methane production [3]. Co-digestion of carbon-rich organic matter with a low sulfur (S) content may also reduce the H2S concentration in the biogas when compared to the mono-digestion of dairy manure and prevent sulfide inhibition.
Industrial food waste comprises 5% of the total food waste generated globally [14]. Although the fraction of industrial food waste is significantly less than food waste from other sources, it has logistical and economic advantages due to its high-volume generation at specific points and homogenous nature. Valorization of these industrial food waste streams can help mitigate disposal costs in landfills, while providing a source of tipping fees for dairy farmers with AD systems. The waste produced from gummy vitamin industries is high in degradable C compared to dairy manure. Production of gummy vitamin waste (GVW) from a single manufacturing facility can be up to 10% of the total weight of the product produced [15]. For example, one multi-national gummy vitamin manufacturing company produces approximately 100 million gummy vitamins daily, with a daily production of 500 tons of gummy product (5 g per gummy vitamin), resulting in approximately 50 tons/day of GVW produced [16]. Most of this waste product is landfilled, with some composting and incineration being practiced in the EU [15,17]. The GVW material can contain up to 70% sugar and gelatin, with starch or pectin-based gels that create the unique structure that is characteristic of gummy candies [18]. Due to its high sugar content, GVW can be a valuable resource for AD, yet the dense jelly-like consistency may lead to issues, such as a slow degradation rate, increased hydraulic retention time, or possible pipe clogging within the AD system. It is also possible that GVW with a high C:S ratio could reduce the H2S concentration in the biogas when co-digested with dairy manure.
The main goal of the project was to evaluate a GVW product as a co-digestion substrate for AD. The specific objective was to evaluate the CH4 and H2S production and VS degradation of a GVW substrate when co-digested with a dairy manure (DM), food waste (FW), and grease waste (GW) mixtures (DM.FW.GW). A co-digestion mixture was used for testing, as many on-farm digesters incorporate multiple waste streams and to highlight the benefits of testing co-substrates as both mixtures and single substrates. Co-digestion of the tested mixtures was expected to produce a significantly higher amount of CH4 and lower H2S compared to the mono-digestion of DM.

2. Materials and Methods

2.1. Sample Collection

Anaerobic digester effluent (inoculum source) and the GVW product were collected from a Northeastern US farm. The farm co-digested dairy manure from heifers with gummy vitamin waste, food waste, and grease waste at a 64% DM, 9% GVW, 16% FW, and 11% GW ratio, by mass. The AD effluent sample was utilized as an inoculum source, as it had been pre-acclimated to the GVW material used at the farm. The GW and FW were collected from a local supermarket. Un-separated dairy manure from the USDA Beltsville Agricultural Research Center (BARC) in Beltsville, MD, USA, was utilized as the DM substrate. Field samples were collected and brought back to lab on ice. The mean total solids (TS) and volatile solids (VS) data for the substrates used in the experiment are shown in Table 1.

2.2. Experimental Design

The GVW product was added to individual batch digesters at four inclusion levels (0%, 5%, 9%, and 23% on a wet mass basis) to a co-digestion substrate mixture of dairy manure (DM), food-waste (FW), and grease-waste (GW) and compared to mono-digestion of the GVW, DM, FW, and GW substrates, with an inoculum control. The 9% GVW treatment (64% DM, 16% FW, 11% GW by mass) represented the mixture that was used at the farm during the time of AD effluent collection. An inoculum-to-substrate ratio (ISR) of 1:1 (VS basis) was used for the experiment. Table 2 shows the experimental design and the descriptions of the treatment levels for the experiment, with each treatment conducted using triplicate AD reactors. All mass data are expressed on a wet mass basis.

2.3. Biochemical Methane Potential (BMP) Test Procedures

The batch laboratory testing followed the biochemical methane potential (BMP) protocol, which is a laboratory batch study used to characterize CH4 production potential [6]. Substrate and inoculum were added into 300 mL serum bottles, purged with N2 gas to establish anaerobic conditions, capped, and incubated at 35 °C in an environmental chamber. Biogas, CH4, and H2S concentrations were monitored at regular intervals for 67 days, at which point the daily biogas production was less than 1% of the cumulative biogas production for the treatments, indicating biogas production had largely ceased. The mass of substrate and inoculum in each bottle ranged from 31.4 to 58.8 g (Table 2) to keep the ISR at 1:1 for all treatments.
The quantity of biogas produced was measured using a graduated, gas-tight, wet-tipped 50 mL glass syringe inserted through the septa of the digestion reactors and equilibrated to atmospheric pressure. Biogas samples were collected in 0.5 mL syringes and tested on a gas chromatograph (Agilent 7890) using a thermal conductivity detector (TCD) at a detector temperature of 250 °C for CH4 and H2S concentrations. The average CH4 and H2S production in the triplicates from the inoculum control was subtracted from the other treatments to present the total CH4 production from the waste substrates only.

2.4. Analytical Methods

The treatment mixtures were analyzed for pH before and after digestion using an Accumet AB15 pH meter. Triplicate samples were tested for TS and VS, according to Standard Methods for the Examination of Water and Wastewater (APHA-AWWA-WEF, 2005) within 24 h of collection. For TS analysis, triplicate 10.0 mL samples were pipetted into pre-weighed porcelain crucibles. The samples were then dried at 105 °C until a constant mass was obtained for the TS concentration. The crucibles were then placed in a furnace at 550 °C until a constant weight was obtained to determine VS concentration. The gummy waste, dairy manure, and inoculum (digester effluent) were tested for total metals (iron, zinc) and sulfur using ICP-MS (inductively coupled plasma mass spectrometry), and total nitrogen using A3769 Methods for Manure Analysis at Agrolabs Inc., Harrington, DE, USA, [19]. The C:N ratio was calculated using the conversion factor from Adams et al. (1951) stating that 55% of the VS content is carbon [20]. The calculated C value and the measured N value were used to derive the C:N ratio.

2.5. Statistical Analysis

Collected data were reviewed in accordance with QA/QC procedures and analyzed for significant differences in biogas quantity, CH4, H2S, TS, VS, and pH using ANOVA and Tukey-Kramer post-hoc multiple mean comparison tests of the reviewed data using SAS ® statistical software package. Tests of significance were conducted with an alpha value set at 0.05. Data are reported as averages with standard errors (SE).

3. Results

3.1. Methane (CH4) Production

The co-digestion mixtures 0–23% GVW.DM.FW.GW had a significantly higher percent CH4 in the biogas compared to the mono-DM digestion (p-value < 0.0001; Table 3). However, there were no significant differences in the percent CH4 among the co-digestion mixtures, with a non-significant trend in increasing percent CH4 as the percent of GVW increased (Table 3). The cumulative CH4 production over the 67 day AD period was normalized using two methods: (1) the total mass of the substrate added (mL CH4/g substrate), as this normalization provides an estimate of CH4 production that can be readily used by farmers, and (2) the VS of the substrate (mL CH4/g VS added) for comparison with other studies [5].
As expected, the co-digestion treatments (with and without GVW addition) produced 359–524% more CH4 compared to mono-DM digestion, when normalized by the mass of substrate added (Table 3). Normalized CH4 production in co-digestion without GVW (DM.FW.GW-only) was 11.6% lower than the 5% GVW.DM.FW.GW mixture, 14.5% lower than 9% GVW.DM.FW.GW mixture, and 36.3% lower than the 23% GVW.DM.FW.GW mixture (Table 3; Figure 1). The CH4 production in the 23% GVW.DM.FW.GW mixture was the highest among all treatments, as expected. The total normalized volume of CH4 increased linearly with the mass percent of GVW added (r2 = 0.9866) (Figure 2).
When the total CH4 produced was normalized by the quantity of organic material added (mL CH4/g VS), the 23% GVW.DM.FW.GW mixture was significantly lower than the DM.FW.GW mixture with 0% GVW (p-value = 0.0156) and 5% GVW.DM.FW.GW mixtures (p-value = 0.0122) (Table 3), with no significant differences between the other co-digestion treatment groups. Mono-GVW digestion resulted in negligible CH4 production (0 mL CH4/g VS) over 67 days of digestion due to subtraction of inoculum CH4 production from each treatment, and higher CH4 production values in the triplicate inoculum reactors compared to the triplicate GVW-only AD reactors. Both treatments with negligible CH4 production (mono-GVW and mono-FW) had low final pH levels in the digestion vessels (under pH 7) (Table 4).

3.2. Hydrogen Sulfide (H2S) Production

The DM treatment produced biogas with a peak concentration of 2145 ppm H2S after 3 days of digestion (Figure 3). After this time, H2S levels decreased and no H2S was detected in the biogas by the 60th day of the experiment. The treatment with the next highest peak H2S concentration in the biogas was the 9% GVW.DM.FW.GW mixture (804 ppm H2S), which was 63% less than the DM treatment and 23% greater than the next highest treatment (DM.FW.GW-only mixture with 0% GVW) at 576 ppm H2S. The peak H2S concentrations for all treatments were observed within the first 2–3 days before peak CH4 production. The 23% GVW.DM.FW.GW treatment, DM and FW had detectable H2S concentrations in the biogas for the longest period (51 days). The mono-GVW treatment did not produce a measurable amount of CH4, but it had the shortest period of detectable levels of H2S (5 days). This is likely due to lowered microbiological activity within the digester due to the low pH levels, which led to low biogas production.
The quantity of H2S produced showed an increasing trend with increases in the percent of GVW inclusion (0–23%) when normalized by kilograms of substrate addition (5.3–15.5 mL H2S/kg substrate; Table 3, Figure 4). The H2S production in the DM treatment (17.4 mL H2S/kg substrate) was significantly higher than the treatments co-digested with GVW (p-value = 0.0046). However, in the DM.FW.GW treatment (0% GVW), the normalized H2S production was the lowest among the co-digested treatments (5.3 mL H2S/kg substrate), and significantly lower than 23% GVW.DM.FW.GW (p-value = 0.0106) and DM (p-value = 0.0023) treatments. However, there were no significant differences for normalized H2S production between the 5–23% GVW inclusion (p-value = 0.633) treatments.
When the total H2S was normalized by the amount of VS added, the DM treatment (212 mL H2S/kg VS) produced a significantly larger amount of H2S compared to all co-digestion treatments (p-value < 0.0001) (Table 3). The addition of GVW (68–72 mL H2S/kg VS) showed a significant increase in H2S production compared to the DM.FW.GW (0% GVW) treatment (35 mL H2S/kg VS; p-value = 0.0003). However, there were no significant differences within the 5–23% GVW.DM.FW.GW treatments (p-value = 1.000).

3.3. Effect of Retention Time and Solids Degradation

The percentage of CH4 in the biogas of the DM treatments rose above 25% on the 11th day of digestion, while the treatments containing additional substrates (FW, GW, and GVW) had a longer lag phase and started producing higher quantities of CH4 after 20 days of digestion (Figure 1), which is a relatively long lag-time for BMP analyses. The DM treatment produced 43% of its total cumulative CH4 within the first 20 days, while all other treatments had less than 10% of the total cumulative CH4 production during this time (Table 5). By the 41st day of the experiment, 89% of the total cumulative CH4 from the mono-DM treatment had been produced, but the percent of total cumulative CH4 from the GVW.DM.FW.GW and DM.FW.GW treatments by Day 41 varied from 57–80% of the cumulative CH4 after 67 days of digestion. The effect of the longer retention times on GVW degradation was seen, as the CH4 production rate for co-digestion was highest when no GVW was added (DM.FW.GW), with a maximum CH4 production rate of 16.8 mL CH4/VS.day). The maximum CH4 production rate decreased with increasing GVW inclusion (10.6–11.6 mL CH4/VS.day). The maximum CH4 production rate was the lowest for DM (6.0 mL CH4/VS.day) for the treatments with CH4 generation.
The C:N ratios of the GVW (196:1) was high due to the high C (255 g C/kg GVW) and low N content (1.3 g N/kg GVW), which was much higher than the dairy manure (7.7:1) and inoculum (8.0:1) utilized. The TS and VS concentrations of the GVW showed that the VS comprised 99.7% of the total solids content (46.4% of the wet GVW). While a high percentage of the GVW was degradable, there was only a 34–35.2% degradation of VS during digestion (Table 4). While there was no CH4 production from the mono-FW and mono-GW treatments, there was a decrease of >30% of the initial VS content, which can be attributed to the initial breakdown of the organic matter, resulting in CO2-enriched biogas production. Biogas volume for these treatments was over 200 mL during the first two days, with less than 0.5% CH4 and over 35% CO2 for mono-FW and over 50% CO2 for mono-GVW treatments.

4. Discussion

Increasing the amount of GVW during digestion did increase CH4 production, as expected. The GVW appeared to completely hydrolyze during digestion, with no visible trace of solid GVW in the post-BMP samples after 67 days of digestion. The GVW accounted for 5–23% of the total mass of substrate added, corresponding to 15–50% of the VS inclusion. The GVW product could be beneficial for farmers interested in co-digestion waste substrates that increase CH4 production, but the longer retention time of the GVW compared to DM digestion should be taken into consideration.
The negligible CH4 production and low pH values in the mono-GVW, FW, and GW treatments compared to the higher CH4 production (336–374 mL CH4/g VS) and pH range (7.88–7.95) in treatments that co-digested GVW, FW, GW, and DM showed that the buffering capacity of the added co-substrates is important to mitigate accumulation of volatile fatty acids (VFA) and lowered pH [3,21]. Carbon-rich substrates can have a poor buffering capability, leading to an increased rate of VFA production and methanogenesis inhibition [3]. The mono-GW treatment had an initial pH of 7.79 but did not produce significant amounts of CH4, possibly due to the slow degradation rate of lipids in the grease waste. Previous studies have also shown that digestion of lipids without co-digestion required the use of lime as a pH stabilizer [22]. The use of a buffer for pH control in the experiment was avoided since the study was originally conducted to emulate the source farm conditions. The AD system on farm did not use any pH stabilizers, as the manure provided sufficient buffering capacity for the digestion process. Generally, the high alkalinity of manure increases digester resistance to acidification for high-fat and sugar content wastes and adds a nitrogen source for micro-organisms [23]. Another important parameter that likely resulted in negligible CH4 production in the mono-GVW treatment was the high C:N ratio of GVW (196:1). High C:N ratios have been shown to result in low pH values during the digestion process and high VFA production [24]. As DM had a C:N ratio of 7.7:1 in this study, which is typical for DM, the resulting mixture in the co-digestion treatments likely increased the C:N ratio within the ideal range of 20–30 for AD, resulting in large increases in CH4 yield for the co-digestion mixtures compared to the mono-digestion treatments [25].
All treatments produced large amounts of biogas during the first two days of digestion (ranging from 39 mL for DM to 379 mL for 23% GVW.DM.FW.GW), mostly composed of CO2. The biogas volume dropped sharply for all treatments (<10 mL per day) after Day 2, and the mono-DM treatment recovered the earliest (Day 11) and started producing > 50 mL biogas per day. The reduction in VS in the treatments with negligible CH4 production for FW, GVW, and GW (Table 4) can be attributed to this initial burst of CO2 enriched biogas production due to the initial breakdown of complex organic molecules. Bujoczek et al. (2000) showed that high organic loading rates may initially lead to large amounts of biogas, composed mainly of CO2, after which biogas production slows down [26]. In their study, the biogas production recovered after 30 days of digestion with CH4 as the main component, similar to the results seen in this experiment. The authors also reported that the highest TS content for feasibility of digestion was 10%, while the shortest lag phase was obtained for 2.7% TS. The TS content in our experiment varied from 7.1% for DM to 11.6% for FW and showed similar CH4 production trends to their study. The longer lag phase associated with a high TS content could be due to either high VFA concentrations or high ammonia concentrations or a combination of the two factors [26]. The CH4 production in this study recovered after the lag phase, indicating acclimatization of the methanogenic bacteria to the initial inhibitory conditions, but the quantity of CH4 generated from the DM treatment (149 ± 11 mL CH4/g VS) was 38–44% lower than the results obtained by Moody et al. (2011) for dairy manure (239–264 mL CH4/g VS) [6]. Witarsa and Lansing (2015) showed that the normalized CH4 production on a VS basis is often lower for unseparated dairy manure due to the recalcitrant nature of the manure solids, leading to lower VS conversion efficiency [27].
It was expected that CH4 production normalized by VS in the GVW co-digested treatments would be similar, but a decreasing trend with increasing percent GVW was observed. Normalization by VS illustrates the efficiency of organic material conversion to CH4. As GVW is a dense substrate in terms of grams of VS per gram of substrate, the increase in GVW inclusion decreased the efficiency and rate of converting the VS to CH4. The longer lag phase and the larger CH4 production rates in the GVW treatments compared to DM.FW.GW and DM-only, from Days 41 to 67, suggests that long retention times would be needed to receive the full increase in expected CH4 production. This effect was also seen by Kaparaju et al. (2002) when black candy, chocolate, and confectionary by-products were digested with dairy manure for 160 days in order to obtain a complete cumulative CH4 value, with similar normalized CH4 production for the confectionary waste (320–390 mL CH4/g VS) compared to the GVW.DM.FW.GW treatments (336–374 mL CH4/g VS) [28].
In all treatments, the VS degradation was low compared to studies conducted by Lisboa and Lansing (2013) and Li et al. (2013), where the VS degradation rates ranged from 48–93% [5,29]. Only 19.3% of the initial VS content of the mono-DM treatment was degraded at the end of the experiment, illustrating recalcitrance in the manure feed. The VS degradation was consistent with co-digestion studies of forage radish and dairy manure by Belle et al. (2015b), which used the same manure source as this study with a 21.3% reduction in VS concentration in the mono-DM treatment [30]. The VS degradation of our study (30.9–35.2%) was also comparable to the aforementioned study (30.8–39.7%), with 50–80% co-digestion substrate with dairy manure.
In a review conducted by Xie et al. (2018), it was reported that addition of a carbon-rich substrate to sewage sludge digestion may lower the H2S concentration due to a dilution effect [31]. This dilution effect can be attributed to a proportionally higher biogas yield compared to the additional H2S produced from the co-digested substrates. The S concentration for GVW (212 ppm S) was lower than the inoculum source (368 ppm S), and unseparated dairy manure slurries with a TS content of 7% (average 400 ppm S) [32]. The low sulfur concentrations combined with the high VS content (46.3%) of GVW, in comparison to DM (8.2 % VS), provide more evidence to the dilution effect observed in the study, as previously hypothesized. Since more biogas was produced in the GVW treatments compared to the DM treatments, the relative percent of the biogas attributed to manure in the mixed substrate treatments was lowered, and thus, the relative contribution of H2S from the manure substrate also decreased. Furthermore, the contribution of H2S from GVW was comparatively lower due to its low sulfur content, leading to the overall decrease in H2S concentrations in the biogas. However, it should be noted that the GVW addition as a co-digestion substrate increased total normalized H2S production when compared to co-digestion with 0% GVW addition (DM.FW.GW). A co-digestion substrate with negligible S content could have led to further decreases in H2S concentrations and total yield. Some gummy vitamins are fortified with Fe, but the concentrations seen in this study (4.3 ppm Fe) was lower than the Fe concentrations in food waste (4800 ppm) and unlikely to have affected H2S production in our study [33].
The sulfurous compounds in the feedstock were primarily utilized during the initial phase of digestion as most of the H2S was produced within the first 20 days, after which the CH4 percentage started rising for all treatments. Similar results were also observed by Belle et al. (2015b) when co-digesting different mass fractions of forage radish with dairy manure in BMP experiments [30]. Forage radish has a high sulfur content and increasing the forage radish percentage led to an expected increase in H2S production initially, but all the treatments had lowered and similar H2S production by the end of the study. Belle et al. (2015a) also conducted a pilot-scale study on the same substrates and showed an increased rate of H2S production during the first two weeks of digestion, after which, the concentration decreased by >75% of the maximum H2S concentration for the remainder of the digestion period (33 days total) [10]. These observations can be attributed to increased SRB activity during the initial digestion phase, as SRBs can outcompete methanogens when the availability of biodegradable sulfur is higher.

5. Conclusions

Results from the BMP study suggested that gummy waste is a potentially valuable co-digestion substrate with dairy manure. The mixture of substrates containing gummy waste, food waste, grease waste, and dairy manure enhanced CH4 yields compared to digestion of dairy manure alone. The high density of VS and low moisture content of the gummy waste results in high CH4 yields per gram of the substrate, but due to the slower degradation rate of the GVW, higher retention times may be needed to yield these higher CH4 potentials. Co-digestion of GVW with dairy manure lowered the H2S yield and maximum H2S concentration compared to mono-digestion of dairy manure due to its low sulfur content. The research highlighted the significance of testing co-digestion mixtures in conjunction with single substrates for both CH4 and H2S to provide beneficial information for researchers and AD practitioners. Co-digestion of industrial byproducts and food waste mixtures in farm-scale biogas digesters could provide economic incentives for farmers through tipping fees and increased biogas production while redirecting valuable waste products from the landfills.

Author Contributions

Conceptualization, A.C. and S.L.; Data curation, A.C.; Formal analysis, A.C.; Funding acquisition, S.L.; Investigation, A.C.; Methodology, A.C. and S.L.; Project administration, S.L.; Resources, S.L.; Software, A.C.; Supervision, S.L.; Validation, A.C.; Visualization, S.L.; Writing—original draft, A.C.; Writing—review & editing, A.C. and S.L.

Funding

This material is based upon work supported by the National Institute of Food and Agriculture at the U.S. Department of Agriculture through a Northeast Sustainable Agriculture Research and Education (SARE) grant (# LNE15-341).

Acknowledgments

The authors would also like to thank the participating farmer for his enthusiasm and assistance with the project.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Methane (CH4) production normalized by gram of substrate (mL CH4/g substrate) ((A), top) and by gram of volatile solids (mL CH4/g VS) ((B), bottom) in the batch digestion testing of gummy vitamin waste (GVW), grease waste (GW), food waste (FW), and dairy manure (DM) digested singularly and as a mixture (DM.FW.GW), with the percent inclusion of GVW shown for the co-digestion mixtures.
Figure 1. Methane (CH4) production normalized by gram of substrate (mL CH4/g substrate) ((A), top) and by gram of volatile solids (mL CH4/g VS) ((B), bottom) in the batch digestion testing of gummy vitamin waste (GVW), grease waste (GW), food waste (FW), and dairy manure (DM) digested singularly and as a mixture (DM.FW.GW), with the percent inclusion of GVW shown for the co-digestion mixtures.
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Figure 2. Linear regression of normalized methane (CH4) production per gram of added substrate and percent gummy vitamin waste (GVW) within the co-digestion mixture.
Figure 2. Linear regression of normalized methane (CH4) production per gram of added substrate and percent gummy vitamin waste (GVW) within the co-digestion mixture.
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Figure 3. Hydrogen sulfide (H2S) concentration (ppm) in the biogas over time in the batch digestion testing of gummy vitamin waste (GVW), grease waste (GW), food waste (FW), and dairy manure (DM) digested singularly and as a mixture, with the GVW inclusion shown for each co-digestion mixture tested.
Figure 3. Hydrogen sulfide (H2S) concentration (ppm) in the biogas over time in the batch digestion testing of gummy vitamin waste (GVW), grease waste (GW), food waste (FW), and dairy manure (DM) digested singularly and as a mixture, with the GVW inclusion shown for each co-digestion mixture tested.
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Figure 4. Normalized hydrogen sulfide (H2S) production per kilogram of added substrate and percent gummy vitamin waste (GVW) within the co-digestion mixture.
Figure 4. Normalized hydrogen sulfide (H2S) production per kilogram of added substrate and percent gummy vitamin waste (GVW) within the co-digestion mixture.
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Table
Table 1. Total and volatile solids content of the individual substrates (gummy vitamin waste, food waste, grease waste, dairy manure) and digester effluent (inoculum) used for the experiment.
Table 1. Total and volatile solids content of the individual substrates (gummy vitamin waste, food waste, grease waste, dairy manure) and digester effluent (inoculum) used for the experiment.
ParametersGummy Vitamin WasteFood WasteGrease WasteDairy ManureInoculum
Total Solids (g/kg)464 ± 2.091.0 ± 1.0673 ± 4.594.5 ± 3.664.8 ± 0.9
Volatile Solids (g/kg)463 ± 2.183.1 ± 1.1645 ± 1.581.7 ± 3.647.5 ± 0.8
Table
Table 2. Experimental design using a 1:1 inoculum-to-substrate ratio, with the calculated initial total solids (TS) and volatile solids (VS) of the treatment mixtures. The percent of gummy vitamin waste (GVW) inclusion was based on mass. All treatments were conducted in triplicate.
Table 2. Experimental design using a 1:1 inoculum-to-substrate ratio, with the calculated initial total solids (TS) and volatile solids (VS) of the treatment mixtures. The percent of gummy vitamin waste (GVW) inclusion was based on mass. All treatments were conducted in triplicate.
Digestion Substrate and InoculumInoculum (g)DM (g)FW (g)GW (g)GVW (g)TS (g/L)VS (g/L)
Inoculum control31.9----64.147.0
Dairy manure (DM)31.918.3---71.759.5
Food waste (FW)31.9-18.1--74.260.0
Grease waste (GW)31.9--2.3-10587.6
Gummy vitamin waste (GVW)31.9---3.210185.5
DM.FW.GW (0% GVW)23.95.21.40.9-86.371.5
GVW.DM.FW.GW (5% GVW)28.15.21.40.90.488.273.5
GVW.DM.FW.GW (9% GVW)31.95.21.40.90.889.574.5
GVW.DM.FW.GW (23% GVW)47.95.21.40.92.493.178.0
Table
Table 3. Methane (CH4) and hydrogen sulfide (H2S) production data from the batch digestion testing.
Table 3. Methane (CH4) and hydrogen sulfide (H2S) production data from the batch digestion testing.
TreatmentCH4 (%) *mL CH4/g VSmL CH4/g SubstratemL H2S/kg VSmL H2S/kg Substrate
Dairy manure (DM)53.7 ± 0.5149 ± 1112.2 ± 0.1212 ± 1717.4 ± 1.4
Food waste (FW)14.8 ± 1.10 #0 #99.7 ± 8.88.3 ± 0.7
Grease waste (GW)25.7 ± 3.010 ± 4.56.3 ± 2.933.1 ± 30.421.4 ± 19.6
Gummy vitamin waste (GVW)6.98 ± 0.90 #0 #7.0 ± 0.13.2 ± 0.1
DM.FW.GW (0% GVW)67.4 ± 0.2373 ± 656.0 ± 0.835.1 ± 2.25.3 ± 0.3
GVW.DM.FW.GW (5% GVW)66.6 ± 1.6374 ± 1262.5 ± 271.9 ± 13.712.0 ± 2.3
GVW.DM.FW.GW (9% GVW)68.3 ± 1.2355 ± 364.1 ± 0.570.4 ± 5.212.7 ± 0.9
GVW.DM.FW.GW (23% GVW)71.1 ± 1.0336 ± 1276.3 ± 2.768.3 ± 16.615.5 ± 3.8
* The % CH4 shown is the average value from Days 53–67 of the experiment. # The CH4 production from the inoculum was subtracted from all treatments, resulting in zero values when the inoculum outperformed the treatment.
Table
Table 4. Average pH and volatile solids (VS) in all treatment mixtures pre-digestion (initial) and post-digestion (final). Initial VS data was calculated theoretically, and final VS data was determined experimentally.
Table 4. Average pH and volatile solids (VS) in all treatment mixtures pre-digestion (initial) and post-digestion (final). Initial VS data was calculated theoretically, and final VS data was determined experimentally.
TreatmentInitial VS (g/L) Final VS (g/L) Decrease in VS (%)Initial pHFinal pH
Dairy manure (DM)59.548.0 ± 1.819.3%7.647.75
Food waste (FW)60.042.0 ± 2.530.0%7.116.24
Grease Waste (GW)87.579.5 ± 1.19.1%7.797.21
Gummy vitamin waste (GVW)85.553.0 ± 0.538.0%7.756.24
DM.FW.GW (0% GVW)71.549.4 ± 0.830.9%7.927.97
GVW.DM.FW.GW (5% GVW)73.547.6 ± 3.035.2%7.847.95
GVW.DM.FW.GW (9% GVW)74.549.2 ± 1.334.0%7.877.95
GVW.DM.FW.GW (23% GVW)78.051.0 ± 2.634.6%7.777.88
Table
Table 5. Normalized methane production (mL CH4/g VS) after 20, 46, and 67 days, with the percentage of the cumulative CH4 (Day 67) by Days 20 and 46 shown in parentheses.
Table 5. Normalized methane production (mL CH4/g VS) after 20, 46, and 67 days, with the percentage of the cumulative CH4 (Day 67) by Days 20 and 46 shown in parentheses.
TreatmentDay 20
(mL CH4/g VS)		Day 46
(mL CH4/g VS)		Day 67
(mL CH4/g VS)
Dairy manure (DM)64 (43%)133 (89%)149
DM.FW.GW (0% GVW)7 (2%)299 (80%)373
GVW.DM.FW.GW (5% GVW)30 (8%)268 (72%)374
GVW.DM.FW.GW (9% GVW)29 (8%)245 (69%)355
GVW.DM.FW.GW (23% GVW)10 (3%)193 (57%)336

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Choudhury, A.; Lansing, S. Methane and Hydrogen Sulfide Production from Co-Digestion of Gummy Waste with a Food Waste, Grease Waste, and Dairy Manure Mixture. Energies 2019, 12, 4464. https://doi.org/10.3390/en12234464

AMA Style

Choudhury A, Lansing S. Methane and Hydrogen Sulfide Production from Co-Digestion of Gummy Waste with a Food Waste, Grease Waste, and Dairy Manure Mixture. Energies. 2019; 12(23):4464. https://doi.org/10.3390/en12234464

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Choudhury, Abhinav, and Stephanie Lansing. 2019. "Methane and Hydrogen Sulfide Production from Co-Digestion of Gummy Waste with a Food Waste, Grease Waste, and Dairy Manure Mixture" Energies 12, no. 23: 4464. https://doi.org/10.3390/en12234464

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