Anaerobic digestion is a well-established technology commonly used to stabilize municipal wastewater sludge while reducing mass of biosolids for disposal. It is also a proven and effective process for pretreatment of high strength industrial wastewaters. The process provides the additional benefit of creating an opportunity to utilize methane (CH₄), the main product of anaerobic digestion, as a renewable energy source (McCarty et al., 2011; Totzke, 2017).

Fats, oils and greases (FOG) require special disposal and constitute a burden on sewage networks if disposed of improperly. FOG deposits weighing as much as 140 tons have been found in London sewers and removal of such FOG deposits costs millions of dollars each year (Adam, 2018). Due to its high organic matter content, FOG also represent a potential source of renewable energy which could be captured through anaerobic digestion. FOG has been shown to be amenable to co-digestion processes as the addition of wastewater treatment scum, which is made up of FOG, was shown to increase biogas production from anaerobic digestion process without any negative effects on the digestion process (Alanya et al., 2013). The authors demonstrated that scum at the highest loading rate studied (7 g COD/L.d) led to generation of 250L CH₄/kg COD fed (1 atm and 35°C). Other researchers also demonstrated the anaerobic digestion and co-digestion of FOG along with other biomass including wastewater sludge and animal manure. An excellent review by Long et al. (2012) details the previous findings on anaerobic digestion/co-digestion of FOG. The authors noted that long-term anaerobic digestion of FOG at high loading rates may potentially lead to accumulation of long chain fatty acids and digester failures.

According to the National Renewable Energy Laboratory’s (NREL) designation, FOG include yellow grease (spent cooking oil), brown grease (BG), and animal fats (US Department of Energy, 2017). BG, also referred to as trap grease, is collected from grease traps at restaurants and similar food processing facilities. Traps separate grease from water and thus concentrate grease for special disposal. Approximately 1.7 million tons of brown grease is produced each year in the United States, corresponding to 501 million gasoline gallon equivalent (GGE) potential energy, which is largely untapped (Milbrandt and Skaggs, 2016, cited in the US Department of Energy, 2017).

Although there have been numerous studies looking at the anaerobic digestion and co-digestion of FOG, which in some cases included BG (Long et al., 2012), literature on anaerobic digestion of BG alone is scarce. To the best of authors’ knowledge, only one previous research evaluated anaerobic digestion of BG at a pilot-scale study. It was shown that BG as a feedstock to a pilot-scale high-rate anaerobic digester produced a greater volume of CH₄ per kg of volatile solids (VS) removed (0.40–0.77 m³/kg VS_removed) than other feedstocks which are commonly used, such as municipal sludge (0.309 m³/kg VS_removed) or corn stover (0.239 m³/kg VS_removed) (Zhang et al., 2014) The authors reported consistently high CH₄, 75%, in the biogas from anaerobic digestion of BG.

The biogas produced through anaerobic digestion of organic matter can be utilized in a number of ways. One method is combustion to generate electricity. However, a significant portion of the potential energy is lost as heat, with only 30 to 40 percent becoming electricity. Energy recovery may be increased by using a combined heat and power (CHP) approach, utilizing the excess heat to reduce energy costs of the wastewater treatment plant itself (McCarty et al., 2011). Another way to utilize biogas is in compressed natural gas (CNG), using biomethane as fuel for vehicles. However, this method requires that biogas be upgraded to have at least an 88% CH₄ content, necessitating additional infrastructure. The approach also requires specialized vehicles which are designed for CNG (or in many cases both CNG and diesel) as well as additional monitoring to prevent oxygen contents which could lead to explosions (Ong et al., 2014).

Injecting CH₄ directly into the natural gas pipeline is a desirable alternative because it allows the gas to be converted to electricity in larger energy generation facilities, which have a greater efficiency than small-scale systems (Ong et al., 2014). The final product must be refined to at least 96 percent CH₄ in order to meet standards for pipeline usage (Shen et al., 2015). This presents an issue since anaerobic digestion normally produces a biogas made up of 50–70 percent CH₄ (Shen et al., 2015). The remaining unwanted gases include carbon dioxide, water vapor, hydrogen sulfide, nitrogen, hydrogen, and trace amounts of other gases (Tchobanoglous et al., 2013). A variety of methods may be applied to purify biogas, of which pressure swing adsorption (PSA) is the most common. In-situ removal of unwanted gases would provide considerable energy savings, not to mention the benefit of avoiding construction of PSA or other gas purification facilities. This would both improve production and cost efficiency of biomethane, increasing viability of the application of anaerobic digestion.

Previous studies have shown the potential for in situ sequestering of unwanted gases with corn stover biochar (Shen et al., 2015). Likewise, hydrochar, a type of biochar produced through hydrothermal carbonization (HTC), has also shown promise in increasing the CH₄ yield during anaerobic digestion (Mumme et al., 2014). However, this is possibly due to the digestible contents of the hydrochar. In another case hydrochar was found to be inhibitive to the anaerobic digestion process (Mumme et al., 2014; Wang et al., 2017).

The purpose of this study was twofold. The first objective was to study the anaerobic digestibility and hence biomethane potential of BG. The second objective was to investigate the effect of adding dry hydrochar product to increase CH₄ content of the biogas.

The BMP tests were conducted to determine the anaerobic digestibility and CH₄ potential of BG under different initial BG concentrations. These experiments lasted 30 days in order to ensure that the ultimate BMP of BG is reached. The cumulative CH₄ productions for various BG loadings are shown in Figure 1.

The results presented in Figure 1 show that as BG concentration increases, the cumulative CH₄ produced also increased, indicating that BG is anaerobically digestible even at the highest concentration tested, 30.97 g BG/L, or 5.51 g COD/L. It is important to note that the highly heterogenous nature of BG sample is reflected by greater deviation from the average cumulative CH₄ (error bars represent low and high measurements) as BG concentration increased.

The rate of CH₄ production for each BG loading is the slope of the linear portion of the cumulative CH₄ curve in Figure 1. The highest CH₄ production occurred within the first 4 days of BMP tests and the CH₄ was linear during this time. The net cumulative CH₄ production, CH₄ produced under each BG loading minus the CH₄ produced in seed control, are presented in Figure 2 along with the trendlines. The slopes of these trend lines are net CH₄ production rates under different BG loading.

Clearly, the heterogenous nature of the BG also impacted the specific BMP of BG, which ranged from a low of 18.5 to a high of 63.8 L CH₄/kg BG_fed (at 1 atm and 35°C). Using the average 178 mg COD/g BG, these translate to specific BMP ranging from 104 to 358 L CH₄/kg COD_fed (at 1 atm and 35°C). These values are comparable to 250 L CH₄/kg COD fed (1 atm and 35°C) reported for anaerobic co-digestion of scum from wastewater treatment plants reported by Alanya et al. (2013). Unfortunately, no direct comparison can be made to the results reported by Zhang et al. (2014) due to different units of BMP used by the authors. Regardless, 358L CH₄/kg COD_fed at the highest BG loading translate into 32 trillion BTUs potential energy annually. This 28 million GGE potential energy could easily be recovered by anaerobically digesting or co-digesting BG without any investment in infrastructure.

When the CH₄ production rate is divided by the mass of seed biomass expressed in g VS, it results in specific CH₄ production rates. When the specific CH₄ production rate is expressed in units of COD equivalents of CH₄–395 mL CH₄ produced per gram of COD removed at 1 atm and 35°C (Speece 1996)–it is equal to the specific substrate utilization rate. Figure 3 shows the specific CH₄ production rates, in COD equivalent, for different BG loadings expressed in g COD/L.

The data presented in Figure 3 show that the specific substrate utilization rate is linearly increasing even at the highest substrate concentrations tested, indicating that maximum specific substrate utilization has not been reached at 5 g COD/L loading. Regardless, 0.04 g COD/g VS. day substrate utilization rate at the highest BG loading translates into 0.8 kg COD/m³ day volumetric loading rate for typical low rate anaerobic processes, which are operated at 2% VS content. Considering the fact that typical low rate anaerobic processes with complete mixing are designed for 1–2 kg COD/m³ day loading rates for easily biodegradable soluble COD containing wastes (Totzke, 2017), 0.8 kg COD/m³. day for BG is very encouraging. In addition, FOG is typically co-digested with sludge and there is evidence in literature that co-digestion of FOG and sludge may have synergistic effects and thus higher loading rates might be possible (Long, et al., 2012).

The next iteration of BMP tests was conducted to determine the impact of increasing hydrochar concentrations on total CH₄ production, CH₄ production rate, and biogas quality. These BMP were carried out over a 14-day period as determining CH₄ potential of hydrochar was not an objective. The experimental design was explained earlier in Table 2. Cumulative CH₄ from two controls–R6 (seed only) and R7 (BG only)–along with those from four reactors with increasing concentrations of hydrochar are presented in Figure 4.

The results presented in Figure 4 show that increasing concentrations of hydrochar resulted in higher cumulative CH₄ produced. Reactor 9, 10, and 11 containing constant BG at 15.96 g/L and hydrochar concentrations of 0.52, 0.91 and 1.275 g/g TS generated 113, 140, and 164 mL of CH₄ (1 atm and 35°C), respectively. Since BG concentration was constant at 15.36 g/L in these reactors, the increase in cumulative CH₄ with increasing hydrochar concentrations can only be attributed to the presence of anaerobically biodegradable material in hydrochar.

The rate of CH₄ production for each treatment was again calculated as done with the BG trials and using the first 5 days of testing where CH₄ production increased linearly. Figure 5 shows the cumulative CH₄ production during the first 5 days of testing and the corresponding trendlines. It should be noted that the cumulative CH₄ for each hydrochar concentration reported in Figure 5 are net CH₄ produced, i.e., CH₄ from controls are subtracted.

When the rates of CH₄ production in Figure 5 are analyzed, it is noted that the reactor containing hydrochar only (R8) has the highest CH₄ production rate. When BG is introduced in the reactor (R9, R10 and R11), the rate decreases indicating that anaerobic microorganism are inhibited due to an antagonistic effect between hydrochar and BG. The cumulative CH₄ production data presented in Figure 4 indicate that this antagonistic effect lasted throughout the 14 days of incubation as reactors with both BG and hydrochar (R9, R10 and R11) produced less CH₄ than that of sum of net CH₄ from BG only (R7), 63 mL, and hydrochar only (R8), 97 mL.

One of the objectives of this study to assess if hydrochar would act as an in situ CO₂ sequestering agent leading to higher CH₄ content in the gas phase. Figure 6 shows the gas phase CH₄ content in Reactor 6 through 11 (see Table 2 for reactor contents).

One-way, single factor Anova analyses with 95% confidence level and eight degrees of freedom between the control reactors and the reactors containing increasing concentrations of hydrochar–0.52–1.275 g HC/g TS (Reactor 8 through 11 in Figure 6)–was carried out to determine if CH₄ content of biogas in these reactors was statistically different. The results showed that there was no statistically significant difference in gas phase CH₄ content of controls and hydrochar containing reactors suggesting that the food waste derived hydrochar studied is not a suitable in situ CO₂ sequestering agent.

In summary, the results presented herein indicate that BG is highly digestible under anaerobic conditions with 354 mL CH₄/g COD equivalent of BG (1 atm and 35°C) biomethane potential. The maximum substrate utilization was not reached even at the highest initial concentration of BG (30.97 g BG/L or 5.5 g COD/L) suggesting that BG can be utilized by anaerobic microbial consortia at a rate of 0.04 g COD/gVS.day or higher. At a typical 2% VS content of anaerobic digesters, this translates into 0.8 kg COD/m³ day or higher volumetric loading rate. The particular food waste derived hydrochar tested in this study did not show any capacity to sequester CO₂ in the biogas.