**1. Introduction**

Biogas as an alternative source of energy is gaining more traction throughout several nations of the world [1]. Researchers have been conducting massive experiments on evaluating the conversion of miscellaneous wastes like animal manure, municipal solid waste, energy crops, municipal biosolids, food waste, and so on to biogas [2–4]. Biogas as an end-product could be produced from either artificially engineering anaerobic digester processing or naturally through the organic waste decaying process. Both the artificial and natural processes will include the main steps in anaerobic digestion; these are: hydrolysis, acidogenesis, and methanogenesis. Optimization of biogas generation from artificially engineering the anaerobic

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

system is centrally focused on the digester design and operation, although it has been stated that the feedstock is as important as the digester technology.

tank (ST). The raw substrate streams were equalized in BAL and then been fed to FAC for bioaugmentation and predigestion, then the predigested substrates were pumped continuously into AD for digestion. During the treatment process, the mixing condition in AD vessel was checked periodically to confirm the accordance of sample. The BAL and FAC tanks were always operating under an internal mixing condition so that the *p*H adjusting and grease

Biogas Production from Brown Grease and the Kinetic Studies

http://dx.doi.org/10.5772/intechopen.74083

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The whole evaluation process takes 343 days. Due to system maintenance recovery and feeding transition issues during the operation, five periods (S1–S5) of stable system performance were selected for assessment. **Table 1** summarizes the divided evaluation periods and correlated operating parameters. During each operating period, a steady-state operation was selected for further analysis. The settling tank was introduced at the 196th day. In S3 and S5,

The characteristics of BG, FC, and SPL were listed in **Table 2**. BG used in this study was obtained from a food waste plant in Houston, TX. Before feeding to the BAL, the stream was prescreened and the rest was dehydrated; lime was introduced to increased pH to neutral (see **Table 2**) in order to maintain the minimum microorganism activity; afterward, the BG was

Date 4/13/11–7/26/11 7/27/11–8/7/11 8/8/11–10/24/11 10/25/11–12/7/11 12/8/11–2/29/12 3/1/12–3/21/12

Settling tank No No No Yes Yes Yes Feeding BG BG BG BG + FC BG BG + SPL

OLR\* / 2.0 ± 0.2 2.0 ± 0.2 0.8 ± 0.2 0.6 ± 0.2 0.9 ± 0.3 HRT\* / 7.3 ± 0.6 11.9 ± 1.1 15.2 ± 1.1 15.8 ± 1.9 11.0 ± 0.1

> Reduce flow rate

OLR and HRT in S1 and S2 were calculated based on digester only (considering recycle); OLR and HRT in S3–S5 were

Establish BG steady state

calculated based on digester + sedimentation tank (not considering recycle).

/ 1–12 13–90 91–135 136–217 218–238

/ 1–12 34–43 107–133 184–217 218–238

/ 19,208 ± 1579 26,205 ± 2685 26,570 ± 6264 33,881 ± 9176 30,200 ± 1503

/ 10,367 ± 662 12,802 ± 925 10,139 ± 754 13,224 ± 3236 13,225 ± 1891

Establish BG + FC steady Maintenance and recovery

Establish BG + SPL steady state

state

**S1 S2 S3 S4 S5**

FC and SPL was fed as a cosubstrate to investigate their impact for the system.

shattered by a beater to ensure complete emulsification with tap water.

hydrolysis would be favored.

**System start-up**

Days of operation (d)

Days with consistent data (d)

Influent COD (mg L−1)

Influent VS (mg L−1)

\*

Activity Seeding and

initiating

**Table 1.** Feeding schedule and operating periods.

A type of food waste, brown grease (BG) was selected here for investigation. In the United States, there are 3800 million pounds of trap grease produced every year (per National Renewable Energy Laboratory Report [5]). Considering 100% conversion efficiency, approximately 14 million m3 of methane could be generated from these grease wastes [6, 7]. This substantial amount of energy could be used in different ways. Also, the environmental effect would be minimized compared to the effect of dumping to landfills [8].

BG is a mixture of fat, oil and grease from animal fat, vegetable oil, and other grease typically found in grease interceptors in restaurants and food industries [9]. Most of the time BG will be disposed to landfill, as the landfill treatment cost is only 5 cents per pound of brown grease [5]. However, landfills will generate some side effects such as soil and water pollution; these side effects will make the soil sterile and unusable to support plant life [5] and then garner ever-growing environmental concerns. These problems have a significant negative impact on the industrial cost and environmental effect of the BG treatment processes [10]. Anaerobic digestion technique is a viable option of the BG treatment [11]. The benefits of using anaerobic digestion include that the technique requires less reactor size, eliminates off-gas air pollution, produces less sludge, and generates substantial amounts of biogas as energy recovery; the generated biogas could then support the plant operation after converted to mechanical energy [12–14]. However, nowadays, only a small portion of BG has been treated by anaerobic digestion process, mainly because the biogas energy benefit cannot meet the anaerobic process investment [15, 16].

Kinetic simulation models of the anaerobic digestion process have been used to predict the digestion patterns and help to optimize design parameters of digestion reactors. As we have known that anaerobic digestion is a complex microorganism reaction process, to simplify the model, a pseudo first-order kinetic model can be applied to provide information, such as the relationship between feed rate and reactor size, and so on.

In this chapter, the biogas forming potential of BG as well as its COD and solids removal efficiency during the anaerobic digestion process was illustrated; moreover, system kinetic study has been performed to estimate the effects of input variations and substrate composition to the overall stability of the process. To make the investigation more comprehensive, various substrates from paper mill including foul condensate (FC) and screw press liquor (SPL) have also been introduced. The process parameters including substrate composition, hydraulic retention time (HRT), organic loading rate (OLR), and others have been studied in the process to get the optimal value.
