**3.1. Performance of anaerobic process**

The summary of anaerobic operating parameters and results was listed in **Table 3**. For comparison purpose, typical values for anaerobic digestion processes were also listed. During the entire process, the *p*H in digester has been controlled at the neutral range (7.01–7.34, optimal *p*H range for anaerobic digestion 6.9–7.6) and the digestion occurred mostly at mesophilic temperature (34.3–37.9°C). The low DO concentration (< 0.1 mg L−1) and ORP value (<−200 mV) indicate that the system is strictly anaerobic. VFA concentration is lower than


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

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.

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

) / 205 ± 50 /

The summary of anaerobic operating parameters and results was listed in **Table 3**. For comparison purpose, typical values for anaerobic digestion processes were also listed. During the entire process, the *p*H in digester has been controlled at the neutral range (7.01–7.34, optimal *p*H range for anaerobic digestion 6.9–7.6) and the digestion occurred mostly at mesophilic temperature (34.3–37.9°C). The low DO concentration (< 0.1 mg L−1) and ORP value (<−200 mV) indicate that the system is strictly anaerobic. VFA concentration is lower than

**3. Results and discussion**

**Table 2.** Substrate characteristics.

Alkalinity (mg L−1 as CaCO3

100 Energy Systems and Environment

a

b

as CaCO3 .

**3.1. Performance of anaerobic process**

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

**Parameter Brown grease (BG)a**

**(μ ± σ, n = 17)**

COD (mg L−1) 910,634 ± 229,993 2973 ± 142 4498 ± 2020 dCOD (mg L−1) / 2740 ± 125 609 ± 189 TS (mg L−1) 437,778 ± 91,348 406 ± 104 8768 ± 7957 VS (mg L−1) 372,111 ± 77,646 210 ± 14 3742 ± 1666 VS/TS ratio 0.85 ± 0.06 0.53 ± 0.1 0.5 ± 0.1 TSS (mg L−1) / 357 ± 577 4048 ± 1750 VSS (mg L−1) / 339 ± 461 1997 ± 875 VSS/TSS ratio / 0.83 ± 0.25 0.49 ± 0.06

*p*Hb 6.51 ± 0.77 9.28 ± 0.18 8.44 ± 0.83 TN (mg L−1) / 52.2 ± 4 2.3 ± 0.1 TP (mg L−1) / 0.24 ± 0.09 0.41 ± 0.04

Sulfide (mg L−1) / 52.2 ± 20.5 / Sulfate (mg L−1) / <40 / Moisture content (wt%) 56 ± 9 / /

**Foul condensate (FC) (μ ± σ, n = 11)**

**Screw press liquor (SPL)**

**(μ ± σ, n = 13)**

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.

**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**) to the highest value in the overall process.

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 40–70% (**Figure 1b**).

**Figure 1.** COD (a) and VS (b) concentration variation before and after AD, and their removal efficiency. Five stable operating periods (S1–S5) were marked.

The indicator of system organic removal is the volatile ratio (VS/TS and VSS/TSS) before and after AD. **Figure 2** shows the VS/TS ratio (**Figure 2a**) and VSS/TSS ratio (**Figure 2b**) in FAC and AD, respectively. In FAC, the volatile ratio is 0.84–0.86, and this ratio decreases to 0.66– 0.69 in AD. The reduced ratio indicates that there was organic digestion since the inorganic parts should always be consistent in the anaerobic digestion process. Compared with the dCOD reduction from FAC to AD (see **Table 3**), the decrease of volatile fraction should come

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**Figure 3** shows the scattered plot between system organic removal and organic loading rate in terms of COD (**Figure 3a**) and VS (**Figure 3b**). In S1 and S2, the OLR is higher than S3–S5 since the clarifier was not included and the OLR was calculated with considering the recycling

of hydrolysis, acidogenesis, and methanogenesis together.

**Figure 2.** Volatile fraction including VS/TS (a) and VSS/TSS (b) in both FAC and AD.

**Figure 2.** Volatile fraction including VS/TS (a) and VSS/TSS (b) in both FAC and AD.

The indicator of system organic removal is the volatile ratio (VS/TS and VSS/TSS) before and after AD. **Figure 2** shows the VS/TS ratio (**Figure 2a**) and VSS/TSS ratio (**Figure 2b**) in FAC and AD, respectively. In FAC, the volatile ratio is 0.84–0.86, and this ratio decreases to 0.66– 0.69 in AD. The reduced ratio indicates that there was organic digestion since the inorganic parts should always be consistent in the anaerobic digestion process. Compared with the

**Figure 1.** COD (a) and VS (b) concentration variation before and after AD, and their removal efficiency. Five stable

operating periods (S1–S5) were marked.

102 Energy Systems and Environment

dCOD reduction from FAC to AD (see **Table 3**), the decrease of volatile fraction should come of hydrolysis, acidogenesis, and methanogenesis together.

**Figure 3** shows the scattered plot between system organic removal and organic loading rate in terms of COD (**Figure 3a**) and VS (**Figure 3b**). In S1 and S2, the OLR is higher than S3–S5 since the clarifier was not included and the OLR was calculated with considering the recycling

**3.2. Methane yield and kinetic analysis**

gas (CO<sup>2</sup>

(~1500 ppm).

0.45–0.49 m3

The cumulative CH4


The produced biogas has a consistently high CH4

**Figure 4.** VFA variation before and after AD, and alkalinity level in AD.

mentioned hereafter have been normalized to STP.

) consisted of the other ~25% by volume (**Table 3**) and trace gases (e.g., H<sup>2</sup>

day 160–175, the system was recovered from system maintenance and the methane content built up from 40% to 75% quickly (**Figure 5**). During the entire evaluation, the average H<sup>2</sup>

ane yields of S3–S5 were calculated directly as the ratio of the two slopes. The value was reported based on VS removal because the organic content of the BG feed was mainly in the suspended solid phase. The methane yield of BG in S3–S5 was consistent in the range of

For the first two stages (S1 and S2), the apparent VS removal efficiency (25–40%, **Table 3**) was significantly lower than in S3–S5 (55–75%, **Table 3**) because ST had not been introduced to the system. Based on that, the effluent VS during S1 and S2 contains a large amount of biomass produced from the anaerobic digestion of brown grease. During S3–S5, ST was used to collect and recycle most of the generated biomass back to the AD, resulting in the higher organic removal efficiency. To estimate the BG conversion into biogas during S1 and S2, a mass bal-

ance analysis on solids before and after the AD was performed as follows:

production and digested VS in S3–S5 are shown in **Figure 6**. The meth-

Kg-VS−1 (at standard temperature and pressure, STP). All the gas volumes

concentration was 189 ppm, significantly lower than that level that may cause H<sup>2</sup>

content (~75%, see **Table 3**). The other major

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105

S). From

S toxicity

S

**Figure 3.** Scatter plots between organic loading rate and organic removal in terms of COD (a) and VS (b).

sludge. Both on COD and VS, the removal efficiency was not significantly affected by OLR variation, resulting in linear increase of COD and VS removal with respect to the applied OLR. This indicates that the system throughput could be improved within the OLR range applied during the evaluation periods to obtain higher organic removal efficiency.

VFA is also an important parameter to investigate the anaerobic process. As the source for methanogenesis, the system needs a certain amount of VFA; while VFA accumulation to greater than 1800 mg L−1 has been shown suggested to significantly decrease *p*H and *in vitro* toxicity, thus somewhat alkalinity was needed to offset the extra amount of VFA as well. **Figure 4** shows the VFA concentration in FAC and AD as well as the alkalinity in AD. The mean alkalinity during overall process is 2122 mg L−1, which is adequate for extra VFA. As shown in **Figure 4**, when substrate moves from FAC to AD, the average VFA concentration decreased from 800 to 413 mg L−1, which indicates that FAC was efficiently augmenting VFA generation and improving the methanogenesis process in AD.

**Figure 4.** VFA variation before and after AD, and alkalinity level in AD.

#### **3.2. Methane yield and kinetic analysis**

sludge. Both on COD and VS, the removal efficiency was not significantly affected by OLR variation, resulting in linear increase of COD and VS removal with respect to the applied OLR. This indicates that the system throughput could be improved within the OLR range

VFA is also an important parameter to investigate the anaerobic process. As the source for methanogenesis, the system needs a certain amount of VFA; while VFA accumulation to greater than 1800 mg L−1 has been shown suggested to significantly decrease *p*H and *in vitro* toxicity, thus somewhat alkalinity was needed to offset the extra amount of VFA as well. **Figure 4** shows the VFA concentration in FAC and AD as well as the alkalinity in AD. The mean alkalinity during overall process is 2122 mg L−1, which is adequate for extra VFA. As shown in **Figure 4**, when substrate moves from FAC to AD, the average VFA concentration decreased from 800 to 413 mg L−1, which indicates that FAC was efficiently augmenting VFA

applied during the evaluation periods to obtain higher organic removal efficiency.

**Figure 3.** Scatter plots between organic loading rate and organic removal in terms of COD (a) and VS (b).

generation and improving the methanogenesis process in AD.

104 Energy Systems and Environment

The produced biogas has a consistently high CH4 content (~75%, see **Table 3**). The other major gas (CO<sup>2</sup> ) consisted of the other ~25% by volume (**Table 3**) and trace gases (e.g., H<sup>2</sup> S). From day 160–175, the system was recovered from system maintenance and the methane content built up from 40% to 75% quickly (**Figure 5**). During the entire evaluation, the average H<sup>2</sup> S concentration was 189 ppm, significantly lower than that level that may cause H<sup>2</sup> S toxicity (~1500 ppm).

The cumulative CH4 production and digested VS in S3–S5 are shown in **Figure 6**. The methane yields of S3–S5 were calculated directly as the ratio of the two slopes. The value was reported based on VS removal because the organic content of the BG feed was mainly in the suspended solid phase. The methane yield of BG in S3–S5 was consistent in the range of 0.45–0.49 m3 -CH4 Kg-VS−1 (at standard temperature and pressure, STP). All the gas volumes mentioned hereafter have been normalized to STP.

For the first two stages (S1 and S2), the apparent VS removal efficiency (25–40%, **Table 3**) was significantly lower than in S3–S5 (55–75%, **Table 3**) because ST had not been introduced to the system. Based on that, the effluent VS during S1 and S2 contains a large amount of biomass produced from the anaerobic digestion of brown grease. During S3–S5, ST was used to collect and recycle most of the generated biomass back to the AD, resulting in the higher organic removal efficiency. To estimate the BG conversion into biogas during S1 and S2, a mass balance analysis on solids before and after the AD was performed as follows:

$$(1 - f)F = (1 - \alpha)X + (1 - \beta)Y\tag{1}$$

$$aX + \beta Y = M \tag{2}$$

A pseudo-first-order kinetic model was applied to analyze the substrate utilization. Similar approaches have been used earlier [2, 17]. The substrate concentration was calculated based

**Figure 6.** Cumulative CH4 production at STP and cumulative VS digested during five selected stages (S1–S5). The slopes of each linear stage were used to calculate corresponding CH4 yield. In S1 and S2, the mass of digested VS was corrected

zation rate constant (d−1), and *θ* is the HRT (d). The estimated *k* value is in a relatively consis-

For comparison purposes, the previous reported methane yields of food wastes and their firstorder kinetic parameters are shown in **Table 4** [18–25]. Different degradation rate constants were obtained for different substrates and bench-scale reactors. Generally, the rate constants were in the range of 0.03–0.4 d−1. The rate constant obtained in this study (0.10–0.19 d−1) has probably been adversely affected by the greater difficulty of controlling the digestion conditions (temperature and mixing) in a pilot-scale system due to the ambient temperature variation (>15°C diurnal change). It was slightly lower than that of municipal solid sludge in batch reactors (0.2–0.4 d−1), comparable to that of municipal solid sludge in CSTR (0.175 d−1), and higher than that of canary grass in CSTR (0.03–0.04 d−1). The methane yields in this study range

is the influent substrate concentration (mg L−1 VS), *k* is the first-order substrate utili-

Kg-VS−1, higher than earlier reported data (0.11–0.42 m3

**Table 4** [18–25]) from food wastes in solid digesters. The biogas quality produced by BG is excellent (~75%, **Table 3**), possibly due to the high lipid content of BG. These pilot-plant data suggest that BG can be effectively digested anaerobically for high-quality biogas production.

<sup>1</sup> <sup>+</sup> *<sup>k</sup>* (3)

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107


Kg-VS−1,

on VS. For a CSTR at a steady state, the effluent concentration (*C*) can be estimated as:

<sup>∁</sup><sup>=</sup> \_\_\_\_ *Co*

tent range of 0.10–0.19 d−1 throughout the evaluation process.

where *C0*

by biomass calculation.

from 0.45 to 0.85 m3


Eq. (1) represents the mass balance of the inorganic (fixed) solids where *f* is the volatile fraction of influent BG obtained from measurement (0.808 in S1 and 0.816 in S2), *F* is the mass flow of influent total solid (Kg d−1), *α* and *β* are the volatile fraction (VS/TS) of biomass and undigested BG substrate, respectively (α ≈ 0.80). *X* and *Y* are the mass flow of biomass and undigested BG, respectively (Kg d−1). Eq. (2) represents the VS composition in the effluent, where *M* is the mass flow of VS in the effluent (Kg d−1). Using the solid measurements, we estimated that the generated biomass constitutes 25–50 wt% in the effluent.

Based on the mass balance results, the cumulative CH4 production and the digested VS during S1–S5 were plotted in **Figure 6**. The methane yield was then calculated as the ratio of the slopes of the two lines in the respective period. The estimated methane yield in S1 (0.40–0.49 m3 -CH4 Kg-VS−1) was comparable with S3–S5 (0.45–0.49 m3 -CH4 Kg-VS−1). S2 has a higher methane yield (0.58–0.77 m3 -CH4 Kg-VS−1) at the cost of lower organic removal at higher organic loading (**Table 3**), which would require greater treatment effort for the digester effluent. During S3–S5, the organic removal was obviously higher with reduced methane yield. Therefore, the mode of process operation will depend on the treatment objective (better organic removal or higher methane yield). Also, the added cosubstrate (FC and SPL) did not adversely affect the methane yield during S3–S5 (**Table 3**).

**Figure 5.** Measured daily biogas production and CH4 /CO<sup>2</sup> content.

(1 − *f*)*F* = (1 − *α*)*X* + (1 − *β*)*Y* (1)

*X* + *Y* = *M* (2)

Eq. (1) represents the mass balance of the inorganic (fixed) solids where *f* is the volatile fraction of influent BG obtained from measurement (0.808 in S1 and 0.816 in S2), *F* is the mass flow of influent total solid (Kg d−1), *α* and *β* are the volatile fraction (VS/TS) of biomass and undigested BG substrate, respectively (α ≈ 0.80). *X* and *Y* are the mass flow of biomass and undigested BG, respectively (Kg d−1). Eq. (2) represents the VS composition in the effluent, where *M* is the mass flow of VS in the effluent (Kg d−1). Using the solid measurements, we

S1–S5 were plotted in **Figure 6**. The methane yield was then calculated as the ratio of the slopes of the two lines in the respective period. The estimated methane yield in S1 (0.40–0.49 m3

ing (**Table 3**), which would require greater treatment effort for the digester effluent. During S3–S5, the organic removal was obviously higher with reduced methane yield. Therefore, the mode of process operation will depend on the treatment objective (better organic removal or higher methane yield). Also, the added cosubstrate (FC and SPL) did not adversely affect the

/CO<sup>2</sup>

content.


Kg-VS−1) at the cost of lower organic removal at higher organic load-

production and the digested VS during

Kg-VS−1). S2 has a higher methane


estimated that the generated biomass constitutes 25–50 wt% in the effluent.

Based on the mass balance results, the cumulative CH4

Kg-VS−1) was comparable with S3–S5 (0.45–0.49 m3


methane yield during S3–S5 (**Table 3**).

**Figure 5.** Measured daily biogas production and CH4

yield (0.58–0.77 m3

106 Energy Systems and Environment

**Figure 6.** Cumulative CH4 production at STP and cumulative VS digested during five selected stages (S1–S5). The slopes of each linear stage were used to calculate corresponding CH4 yield. In S1 and S2, the mass of digested VS was corrected by biomass calculation.

A pseudo-first-order kinetic model was applied to analyze the substrate utilization. Similar approaches have been used earlier [2, 17]. The substrate concentration was calculated based on VS. For a CSTR at a steady state, the effluent concentration (*C*) can be estimated as:

$$\mathbf{C} = \frac{\mathbf{C}o}{1 + k\theta} \tag{3}$$

where *C0* is the influent substrate concentration (mg L−1 VS), *k* is the first-order substrate utilization rate constant (d−1), and *θ* is the HRT (d). The estimated *k* value is in a relatively consistent range of 0.10–0.19 d−1 throughout the evaluation process.

For comparison purposes, the previous reported methane yields of food wastes and their firstorder kinetic parameters are shown in **Table 4** [18–25]. Different degradation rate constants were obtained for different substrates and bench-scale reactors. Generally, the rate constants were in the range of 0.03–0.4 d−1. The rate constant obtained in this study (0.10–0.19 d−1) has probably been adversely affected by the greater difficulty of controlling the digestion conditions (temperature and mixing) in a pilot-scale system due to the ambient temperature variation (>15°C diurnal change). It was slightly lower than that of municipal solid sludge in batch reactors (0.2–0.4 d−1), comparable to that of municipal solid sludge in CSTR (0.175 d−1), and higher than that of canary grass in CSTR (0.03–0.04 d−1). The methane yields in this study range from 0.45 to 0.85 m3 -CH4 Kg-VS−1, higher than earlier reported data (0.11–0.42 m3 -CH4 Kg-VS−1, **Table 4** [18–25]) from food wastes in solid digesters. The biogas quality produced by BG is excellent (~75%, **Table 3**), possibly due to the high lipid content of BG. These pilot-plant data suggest that BG can be effectively digested anaerobically for high-quality biogas production.


The conclusion of this chapter is that BG has the industrial potential to be anaerobically treated as an energy feedstock and there has been ongoing commercial effort to build largescale digesters using BG as the primary substrate. Using BG for biogas production could serve

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as a profitable model for converting waste to renewable energy.

Address all correspondence to: pengchong.zhang@siemens.com Siemens Oil and Gas, Siemens Energy, Inc., Houston, TX, USA

**Author details**

Pengchong Zhang

**References**

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2012;**96**:482-486

Conference; 2002

Management. 2007;**27**:1792-1799

**Table 4.** Comparison of reported and calculated first-order degradation rate constants and methane yields [18–25].
