**2. Methodology**

The anaerobic digestion system employed in this chapter includes three CSTR tanks: balance tank (BAL), facultative tank (FAC), anaerobic digester (AD), and a final batch sedimentation 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 hydrolysis would be favored.

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

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


\* OLR and HRT in S1 and S2 were calculated based on digester only (considering recycle); OLR and HRT in S3–S5 were calculated based on digester + sedimentation tank (not considering recycle).

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

system is centrally focused on the digester design and operation, although it has been stated

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, approxi-

substantial amount of energy could be used in different ways. Also, the environmental effect

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

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

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 anaerobic digestion system employed in this chapter includes three CSTR tanks: balance tank (BAL), facultative tank (FAC), anaerobic digester (AD), and a final batch sedimentation

of methane could be generated from these grease wastes [6, 7]. This

that the feedstock is as important as the digester technology.

relationship between feed rate and reactor size, and so on.

would be minimized compared to the effect of dumping to landfills [8].

mately 14 million m3

98 Energy Systems and Environment

process investment [15, 16].

the process to get the optimal value.

**2. Methodology**


a In BG, COD, TS, and VS are measured as mg/kg.

b pH of BG was measured by suspending 100 g BG in 1 L tap water. Tap water has pH of 8.05 and alkalinity of 55 mg L−1 as CaCO3 .

400 mg L−1 as HAc except S5 when the VFA level is somewhat elevated up to 630 mg L−1 as HAc. TN and TP concentration in system is 230–600 mg L−1 as N and 1–4 mg L−1 as P respectively, which indicates enough nitrogen but slightly lower in phosphorus concentration.

Kg-VS−1) 0.99 0.66 0.52 0.48 0.45 0.30–1.00

**Operating periods S1 S2 S3 S4 S5 Typical range** *p*H 7.34 ± 0.05 / 7.12 ± 0.08 7.10 ± 0.07 7.01 ± 0.17 6.5–8.5 T(°C) 36.0 ± 0.7 36.3 ± 0.7 34.3 ± 1.8 34.3 ± 2.1 37.9 ± 1.0 35–40 DO (mg L−1) 0.01 ± 0.00 / 0.06 ± 0.04 0.15 ± 0.05 0.10 ± 0.03 /

ORP (mV) −209 ± 14 −228 ± 24 −243 ± 40 −247 ± 37 −263 ± 23 −400 to −150 TN (mg L−1) 591 ± 83 409 ± 37 237 ± 74 314 ± 50 306 ± 46 60–1000 TP (mg L−1) 3.4 ± 2.4 1.5 ± 0.4 0.9 ± 0.4 2.3 ± 1.1 2.2 ± 0.4 6–50

VFA (mg L−1 as HAc) 274 ± 97 / 199 ± 76 394 ± 84 629 ± 378 <1800 COD removal efficiency (%) 42.1 ± 6.7 50.6 ± 5.8 82.3 ± 11.0 61.7 ± 12.3 53.5 ± 8.7 / VS removal efficiency (%) 26.8 ± 7.9 37.1 ± 4.3 70.1 ± 8.4 65.6 ± 7.0 62.3 ± 7.2 /

content (%) 74.3 ± 2.0 74.6 ± 1.0 75.9 ± 1.9 74.6 ± 1.8 75,4 ± 1.0 /

content (%) 22.3 ± 1.3 / 23.9 ± 1.9 25.2 ± 1.8 24.2 ± 1.0 /

S content (ppm) 38.2 ± 4.1 / 147.2 ± 34.8 185.2 ± 28.1 371.7 ± 127.6 /

) 3087 ± 282 / 1455 ± 457 2478 ± 291 2204 ± 222 1500–5000

Biogas Production from Brown Grease and the Kinetic Studies

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

101

**Figure 1** shows the COD and VS variation and removal efficiency during each operating period. During S1 and S2, the ST has not been introduced to system yet, the effluent from AD was considered as final effluent and some of the sludge from AD was recycled to FAC manually that results for the higher effluent COD concentration (20,000–30,000 mg L−1) compared with other stages (~10,000 mg L−1). The COD removal efficiency in these periods is relatively lower than other periods, about 30–60% (**Figure 1a**). After ST was added (S3–S5), the effluent COD was kept in a relatively stable range (~10,000 mg L−1) even if the influent COD was varied from 15,000 to 80,000 mg L−1 (**Figure 1a**). This implies that sedimentation tank was efficiently in the elimination of a substantial amount of COD and polishing the quality of final effluent. With the stable effluent COD, during S3, FC was added as a cosubstrate and the initial COD loading was increased; thus, the COD removal efficiency was increased (70–95%, **Figure 1a**)

During each operating period, VS variation has a similar trend with COD; the VS removal efficiency during S3–S5 did not change too much, in the range of 40–70% (**Figure 1b**), while the effluent VS concentration in S4 seems higher that may be due to the higher influent VS concentration. After added the ST, the VS removal efficiency was also improved from 20–40%

to the highest value in the overall process.

to 40–70% (**Figure 1b**).

Alkalinity (mg L−1 as CaCO3

CH4

CO<sup>2</sup>

H2

CH4

yield (m3


**Table 3.** Operating conditions in each period.

**Table 2.** Substrate characteristics.

During S3 and S5, instead of mixing with tap water, FC and SPL were introduced as cosubstrate, respectively. Compared with BG, FC and SPL have a relatively low COD concentration and solids content (see **Table 2**). Also, since BG contains enough amount of total nitrogen (TN) and total phosphorous (TP) for anaerobic digestion, no additional nutrients were added to the batch.
