**2.2. Brown grease**

Using biogas as an alternative source of energy is gaining more attention globally in recent decades [41, 42]. There have been an increasing number of studies performed to evaluate the conversion of waste streams such as animal manure, municipal solid wastes, energy crops, municipal biosolids, and food wastes to biogas [43–46]. In Europe, there are over 50 waste treatment plants using these materials to produce biogas [16, 45, 46]. For instance, ~15% of organic wastes are being converted annually in Germany [47]. The practice of converting wastes to energy provides a two-fold benefit of environmental protection and energy recovery.

Brown grease (BG) is a mixture consisting of trapped grease, sewage grease, and black grease collected in grease interceptors (traps) of restaurants and food industries [48]. In the United States, there are 1.84 million tons of BG produced every year [49]. Most collected BG eventually ends up in landfills. The landfill cost for BG is ~5 cents per pound [50]. This results in a very high direct disposal cost. In addition, the moisture content in BG can lead to soil and water pollution, making the soil sterile and unable to support plant life [51]. Because of these drawbacks, the European Union enacted a general ban on landfilling organic waste in 2005 [52]. An earlier study suggested that 14 × 10<sup>6</sup> m3 of CH4 could be produced in the United States annually by converting the generated BG into biogas [53]. This is a substantial amount of renewable bioenergy. Recovering the energy and eliminating the waste input to landfills yields both economic and environmental benefits [54, 55].

AD is a treatment process capable of producing biogas from organic wastes. The benefits of anaerobic digestion include smaller reactor size in terms of organic loading, lower air emissions, and a smaller amount of generated sludge compared to aerobic biological treatment [55]. Greasy wastes such as BG have been added as a lipid-rich cosubstrate in earlier AD studies for sewage sludge [56–58], municipal wastewater [59–61], and the digestible fraction of municipal solid wastes [62]. Typically, it is blended at 2–50% of the primary substrate's organic loading to improve the biogas yield and methane content [56–62]. However, higher lipid loading (>50% of the substrate) can cause long-chain fatty acid (LCFA) inhibitions [55, 61, 62], scum and foam formation, and fat clogging problems [56]. To our knowledge, there are few studies devoted to investigating the degradability and biogas production using BG alone.
