**3.1. Anaerobic treatability of paper mill effluents**

As mentioned in Section 2.1, most of the current researches related to paper mill effluent treatment are focused on lab-scale experiments; therefore, an upscaling research is necessary to predict more comprehensive and representative results. In this study, a pilot scale sequential reactor system was introduced to evaluate the biotreatability of paper mill waste stream. Various waste streams from different paper making process were used, including liquid waste from bleaching process (DO), liquid discharge from alkaline extraction operating process (EOP), foul condensate from chemical pulping process (FC), and screw press liquor from dewatering operation process (SPL). For pH adjustment purpose, as well as improve the biodegradability, a small volume of wasted sugar water (SW) from a food processing plant was also blended in, as a co-digestion substrate used in this study.

The entire pilot system was established on a property outside of a pulp and paper mill, waste streams were obtained from the paper on a daily basis. The whole system consists of an equalization tank with a volume of 2.1 m<sup>3</sup> to blend all substrates equalized. Before sending to the packed-bed AD column, a 0.95 m<sup>3</sup> continuous stirred tank reactor was used for predigestion. The AD column is a cylindrical column with 1.07 m in diameter and 2.60 m in height, 85% of the AD column was packed with commercial ceramic bio-packing media. The discharge of the AD column will be fed to a 0.95 m<sup>3</sup> aerobic tank for final aeration, and the sample was taken on each tank on a daily basis.

The evaluation lasted for 156 days and was divided into six periods according to different feeds and operating conditions. Initially, the packed-bed column was operated as a downflow digester. From the 80th day, the flow direction was changed, and the AD column is operating as an upflow flooded-bed reactor, the HRT is about 1.7–2.4 days. The feeding and operational characteristics are summarized in **Table 1**.

The evaluation lasted for 156 days, and was divided into six periods according to different feeds and operating conditions. Initially, the packed-bed column was operated as a downflow digester, with a recirculation ratio of 5.0. Note for this stage, there is no water retention. Beginning with the 80th day, the AD column was operated upflow direction; the HRT was kept at 1.7–2.4 days. The entire operation is built on neutral pH range (6.92–7.60, see **Table 1**) and slightly mesophilic condition (T = 31.5–34.5°C, measured for effluent, see **Table 1**).

**Table 2** listed the initial characteristics of each kind of substrates. The COD concentrations for each type of substrate ranged from 2800 to 4500 mg L−1. In this study, the waste streams from paper mill are mostly in liquid phase and have relatively very low solid content (TS < 1 wt%, see **Table 2**). As mentioned above, a sugar water substrate was used to adjust the pH of the mixed substrate. The sugar water (SW) is a high organic content and slightly acidic substrate (COD = 408,000 mg L−1, pH = 3.99). In this study, the sugar water was blended for about 0.5 wt%.

**Figure 1** shows the plots between cumulative CH4 production and the cumulative COD digested (mass basis) against the time axis. Note the system start-up and recovery during substrate changes were not included in the figure. There are totally six linear stages (Stage I–Stage VI, see **Figure 1**) that the system has a stable and consistent CH4 production rate; these six periods were considered as steady state periods. The CH4 yield was calculated as the ratio of the slopes of the two curves in **Figure 1**. The values range from 0.22 to 0.34 m<sup>3</sup> -CH4 kg-COD-1 for the substrates evaluated.

Based on the treatability study listed above, all waste streams are readily treatable. The anaerobic treatment removed 50–65% of substrate COD. Coupled with the aerobic treatment using a CSTR ASP, the overall COD removal efficiency was 55–70%. The application of anaerobic treatment has the potential of significantly improving the energy footprints of the pulp and paper industry.


1 Based on the volume of packing media.

Stillage handling is the most energy consuming process in the life cycle of corn to ethanol process. The drying and evaporation of stillage will take more than 35% of the total energy consumption [65], which makes the stillage treatment technique a main limitation of bioethanol making process [66]. Except for energy consumption, thin stillage is also a kind of high strength wastewater, which exhibits a considerable pollution potential [67]. Up to 20 l of stillage will be produced for each l of corn ethanol [68, 69], and the pollution potential of generated stillage can reach to a chemical oxygen demand (COD) of over 100 g L−1 [69].

The problems described in the previous paragraph have a significant negative impact on the industrial cost of the corn to ethanol process. Thus, a gate-to-gate life cycle assessment for thin stillage treatment was needed to provide a synergistic effect for energy recovery and cost saving. AD technique could be used to remove COD from thin stillage and also to convert the organic fraction of thin stillage into methane, which is a readily in-plant-usable energy source for ethanol industries [69]. Once this AD process was linked as a gate-to-gate life cycle to the ethanol production chain, the efficiency of the complete cradle-to-gate evaluation will

As mentioned in Section 2.1, most of the current researches related to paper mill effluent treatment are focused on lab-scale experiments; therefore, an upscaling research is necessary to predict more comprehensive and representative results. In this study, a pilot scale sequential reactor system was introduced to evaluate the biotreatability of paper mill waste stream. Various waste streams from different paper making process were used, including liquid waste from bleaching process (DO), liquid discharge from alkaline extraction operating process (EOP), foul condensate from chemical pulping process (FC), and screw press liquor from dewatering operation process (SPL). For pH adjustment purpose, as well as improve the biodegradability, a small volume of wasted sugar water (SW) from a food processing plant

The entire pilot system was established on a property outside of a pulp and paper mill, waste streams were obtained from the paper on a daily basis. The whole system consists of an equal-

The AD column is a cylindrical column with 1.07 m in diameter and 2.60 m in height, 85% of the AD column was packed with commercial ceramic bio-packing media. The discharge of the

The evaluation lasted for 156 days and was divided into six periods according to different feeds and operating conditions. Initially, the packed-bed column was operated as a downflow digester. From the 80th day, the flow direction was changed, and the AD column is operating

to blend all substrates equalized. Before sending to the

continuous stirred tank reactor was used for predigestion.

aerobic tank for final aeration, and the sample was taken

be improved and the total cost will be reduced.

256 Advances in Biofuels and Bioenergy

**3. Biogas production for different substrates**

was also blended in, as a co-digestion substrate used in this study.

**3.1. Anaerobic treatability of paper mill effluents**

ization tank with a volume of 2.1 m<sup>3</sup>

packed-bed AD column, a 0.95 m<sup>3</sup>

AD column will be fed to a 0.95 m<sup>3</sup>

on each tank on a daily basis.

2 Based on total volume of the packed-bed digester.

Note: Day 37–44 was in maintenance and recovery mode.

**Table 1.** Pilot-scale feeding activities and conditions during six operating periods.


**3.2. High-rate anaerobic digester to treat brown grease**

adjustable volume of 0.2–1.0 m<sup>3</sup>

Date 4/13/11–7/26/11 7/27/11–8/7/11 8/8/11–

tion tank (1.5 m<sup>3</sup>

Days of operation

Days of intensive evaluationa

Sedimentation tank

Influent COD (mg L−1)

Influent VS (mg L−1)

a

b

Activity Seeding and

initiating

Data were collected in five different periods for analysis.

The high-rate anaerobic digestion system employed in this work comprises three CSTRs and a clarifier: a balance tank (BAL), a facultative tank (FAC), an anaerobic digester (AD), and a final sedimentation tank (ST). The BAL and FAC are rectangular shaped tanks having an

a Plexiglas window at the top for observing the mixed liquor in the digester. Various liquid waste streams from paper mill wastewater including foul condensate (FC) and screw press liquor (SPL) were blended as an effort to minimize the water use in the feed. The sedimenta-

) has a cylindrical shape with a conical bottom at 1:1 slope.

evaluation schedule and the corresponding operating parameters in each stage.

The evaluation period lasted for 343 days. Excluding the system start-up, maintenance, and feeding transition periods, process data were collected for 238 days. The evaluation was divided into five intensive evaluation periods (I–V). During each operating period, a steady stage (S1–S5) defined as a state with relatively consistent biogas production and organic removal) was selected for intensive measurement and data analysis. **Table 3** summarizes the

**System start-up I II III IV V**

/ 1–12 13–90 91–135 136–217 218–238

No No Yes Yes Yes Yes

Feeding BG BG BG BG + FC BG BG + SPL

OLR<sup>b</sup> / 2.0 ± 0.2 2.7 ± 0.3 0.8 ± 0.2 0.6 ± 0.2 0.9 ± 0.3 HRTb / 7.3 ± 0.6 8.9 ± 0.9 15.2 ± 1.1 15.8 ± 1.9 11.0 ± 0.1

OLR and HRT in S1 and S2 were calculated based on AD only, while in S3–S5 were calculated based on AD + ST.

Establish BG steady state

**Table 3.** Feeding characteristics and reactor configuration during the evaluation.

S1–S5 stands for five selected stages with intensive evaluation and stable data consistency.

/ 1–12 (S1) 34–45 (S2) 107–133 (S3) 184–217 (S4) 218–238 (S5)

/ 34,510 ± 2557 56,570 ± 3894 26,570 ± 6264 33,881 ± 9176 30,200 ± 1503

/ 13,965 ± 1262 23,937 ± 1625 10,139 ± 754 13,224 ± 3236 13,225 ± 1891

Add ST Establish BG + FC steady state

10/24/11

in diameter and 3.8 m in height) with adjustable reaction volumes of 4.3, 5.8, and 7.6 m<sup>3</sup>

. AD is a cylindrical tank with a total volume of 7.6 m<sup>3</sup>

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10/25/11–12/7/11 12/8/11–2/29/12 3/1/12–3/21/12

Back to BG steady state Establish BG + SPL steady state

(1.6 m

259

. It has

\*n stands for sample size, i.e. testing times for raw industrial waste streams. # dCOD stands for dissolved COD concentration.

**Table 2.** Initial characteristics of the evaluated substrates.

**Figure 1.** Cumulative CH4 production at STP and cumulative COD mass digested during the evaluation period. There are six linear stages (I–VI) during which the data were used for calculating the CH<sup>4</sup> yield.

#### **3.2. High-rate anaerobic digester to treat brown grease**

**Parameters Foul condensate** 

258 Advances in Biofuels and Bioenergy

Alkalinity (mg L−1 as

CaCO3 )

#

**(n = 11)\***

COD (mg L−1) 2973 ± 142 2886 ± 381 3901 ± 1940 4498 ± 2020 dCOD (mg L−1)# 2740 2445 ± 151 2890 609 ± 189 TS (mg L−1) 406 ± 104 4718 ± 522 4744 ± 532 8768 ± 7957 VS (mg L−1) 210 ± 14 2497 ± 346 1903 ± 136 3742 ± 1666 VS/TS ratio 0.53 ± 0.1 0.53 ± 0.02 0.4 ± 0.02 0.5 ± 0.1 TSS (mg L−1) 357 ± 577 868 ± 365 388 ± 127 4048 ± 1750 VSS (mg L−1) 339 ± 461 758 ± 339 296 ± 204 1997 ± 875 TSS/VSS ratio 0.83 ± 0.25 0.86 ± 0.04 0.79 ± 0.23 0.49 ± 0.06

pH 9.28 ± 0.18 5.19 ± 1.04 9.29 ± 0.29 8.44 ± 0.83 TN (mg L−1) 52.2 ± 4 4 ± 1.3 27 ± 43.3 2.3 ± 0.1 TP (mg L−1) 0.24 ± 0.09 6.33 ± 0.18 3.98 ± 5.22 0.41 ± 0.04

Conductivity (ms cm−1) 5 ± 5.7 — 4.6 ± 0.4 — Sulfide (mg L−1) 52.2 ± 18.1 <0.5 <0.5 — Sulfate (mg L−1) <40 — 106 ± 23 — Chloride (mg L−1) — — 335 ± 39 —

\*n stands for sample size, i.e. testing times for raw industrial waste streams.

are six linear stages (I–VI) during which the data were used for calculating the CH<sup>4</sup>

dCOD stands for dissolved COD concentration.

**Figure 1.** Cumulative CH4

**Table 2.** Initial characteristics of the evaluated substrates.

205 ± 50 — 915 ± 263 —

production at STP and cumulative COD mass digested during the evaluation period. There

yield.

**DO filtrate (n = 8) EOP filtrate (n = 4) Screw press liquor** 

**(n = 13)**

The high-rate anaerobic digestion system employed in this work comprises three CSTRs and a clarifier: a balance tank (BAL), a facultative tank (FAC), an anaerobic digester (AD), and a final sedimentation tank (ST). The BAL and FAC are rectangular shaped tanks having an adjustable volume of 0.2–1.0 m<sup>3</sup> . AD is a cylindrical tank with a total volume of 7.6 m<sup>3</sup> (1.6 m in diameter and 3.8 m in height) with adjustable reaction volumes of 4.3, 5.8, and 7.6 m<sup>3</sup> . It has a Plexiglas window at the top for observing the mixed liquor in the digester. Various liquid waste streams from paper mill wastewater including foul condensate (FC) and screw press liquor (SPL) were blended as an effort to minimize the water use in the feed. The sedimentation tank (1.5 m<sup>3</sup> ) has a cylindrical shape with a conical bottom at 1:1 slope.

The evaluation period lasted for 343 days. Excluding the system start-up, maintenance, and feeding transition periods, process data were collected for 238 days. The evaluation was divided into five intensive evaluation periods (I–V). During each operating period, a steady stage (S1–S5) defined as a state with relatively consistent biogas production and organic removal) was selected for intensive measurement and data analysis. **Table 3** summarizes the evaluation schedule and the corresponding operating parameters in each stage.


a S1–S5 stands for five selected stages with intensive evaluation and stable data consistency.

b OLR and HRT in S1 and S2 were calculated based on AD only, while in S3–S5 were calculated based on AD + ST. Data were collected in five different periods for analysis.

**Table 3.** Feeding characteristics and reactor configuration during the evaluation.

The characteristics of BG feedstock, FC, and SPL are shown in **Table 4**. Since the BG has an extremely high organic content (~1 kg-COD kg-BG-1, **Table 4**), the feeding stream was diluted to the range of 25,000–50,000 mg L−1 COD. FC and SPL have a relatively low COD concentration and solid content compared with BG (**Table 4**). In addition, their mild alkalinity (**Table 4**) effectively offset the mild acidity in BG. FC is a liquid substrate with relatively low solid content (TS = 400 mg L−1), its major organic content is in the dissolved phase (dCOD is >90% of total COD, **Table 4**). SPL has a TS content less than 1.0 wt%. Its dCOD concentration is <20% of total COD concentration (**Table 4**), which indicated the major organic content is in the solid phase.

The daily biogas production during the evaluation is summarized in **Figure 2**. In S1, the biogas production is 5–6 m<sup>3</sup> d−1. The biogas production (~7 m<sup>3</sup> d−1) was higher in S2 because of the higher organic removal. During S3–S5, the average daily biogas production was lower than S1 and S2 since the system OLR was reduced. In S3, the COD removal efficiency was higher than S4 and S5, leading to higher biogas production (~5.6 m<sup>3</sup> d−1) compared to that in S4 and S5 (~3.5 m<sup>3</sup> d−1) (**Table 5**). The easily digested dCOD in FC may account for this increase. Another reason for the lower biogas production was the lower OLR applied in S4 and the slightly lower organic removal in S5. Generally, the biogas production trend in S3–S5 was

consistent with the organic removal (**Table 5**), suggesting that the biogas production was not

content.

/CO<sup>2</sup>

to be anaerobically treated as a biofuel feedstock and there has been an ongoing commercial effort to build large-scale digesters using BG as the primary substrate. Using BG for biofuel

The integrated anaerobic-aerobic system employed in this work contains three CSTRs, two transfer tanks, two clarifiers, and one serious CSTR aeration basin. The receiving tank (REC)

recovery could serve as a profitable model for converting waste to renewable energy.

Facultative tank (FAC) is a cylindrical CSTR with total volume of 0.35 m<sup>3</sup>

placed inside to divide the whole basin into four equal-sized serious tanks (0.6 m<sup>3</sup>

Two clarifiers were set after AD (anaerobic clarifier) and after aerobic basin (aerobic clarifier),

Stillage feedstock was obtained daily from the ethanol plant. Generally, the raw stillage has a COD concentration of ~100 g L−1 [66]. In this study, to make the experimental results more comprehensive, the feeding concentration was adjusted to different ranges, which will be discussed later. Homogenized feeding substrate from REC was pumped to FAC for predigestion.

.

and 1.5 m height), the operating level is adjustable from 0.15 to 0.30 m<sup>3</sup>

) and between AD clarifier and aerobic basin (0.15 m<sup>3</sup>

content), with a CH4

(0.6 m diameter

for each).

. Anaerobic digester

yield. BG has the industrial potential

(1.2 m width × 2.5 m length ×1.5 m height).

(2.1 m diameter and 3 m height) and the

). The aerobic basin is rectangu-

. Two transfer tanks were respectively set between FAC and AD

(0.7 m width × 3 m length × 1.2 m height), three baffle plates were

kg-VS−1. The addition of paper mill waste streams (FC

Biogas Recovery from Anaerobic Digestion of Selected Industrial Wastes

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261

significantly affected by the addition of co-substrate.

**Figure 2.** Measured daily biogas production and CH4

and SPL) as co-substrate did not adversely affect the CH<sup>4</sup>

**3.3. AD addendum unit to improve corn-to-ethanol process**

is a rectangular CSTR with total volume of 4.5 m<sup>3</sup>

(AD) is a cylindrical CSTR whose volume is 10.4 m<sup>3</sup>

respectively, both of them have a volume of 0.7 m<sup>3</sup>

yield in the range of 0.40–0.77 m<sup>3</sup>

operating level is 7.2–9 m<sup>3</sup>

lar whose volume is 2.5 m<sup>3</sup>

(0.04 m<sup>3</sup>

The pilot-scale system produced biogas of excellent quality (75% CH<sup>4</sup>



a Here BG stands for pretreated brown grease in solid phase, thus the unit of COD, TS, and VS is mg kg−1.

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

Brown grease was used as the primary substrate and the other two liquid wastes were used as co-substrates in part of the evaluation.

**Table 4.** Substrate characteristics.

Biogas Recovery from Anaerobic Digestion of Selected Industrial Wastes http://dx.doi.org/10.5772/intechopen.72292 261

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

The characteristics of BG feedstock, FC, and SPL are shown in **Table 4**. Since the BG has an extremely high organic content (~1 kg-COD kg-BG-1, **Table 4**), the feeding stream was diluted to the range of 25,000–50,000 mg L−1 COD. FC and SPL have a relatively low COD concentration and solid content compared with BG (**Table 4**). In addition, their mild alkalinity (**Table 4**) effectively offset the mild acidity in BG. FC is a liquid substrate with relatively low solid content (TS = 400 mg L−1), its major organic content is in the dissolved phase (dCOD is >90% of total COD, **Table 4**). SPL has a TS content less than 1.0 wt%. Its dCOD concentration is <20% of total COD concentration (**Table 4**), which indicated the major organic content is in the solid phase. The daily biogas production during the evaluation is summarized in **Figure 2**. In S1, the bio-

d−1. The biogas production (~7 m<sup>3</sup>

than S4 and S5, leading to higher biogas production (~5.6 m<sup>3</sup>

**(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

pHb 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

Here BG stands for pretreated brown grease in solid phase, thus the unit of COD, TS, and VS is mg kg−1.

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

**Parameters Brown grease (BG)a**

higher organic removal. During S3–S5, the average daily biogas production was lower than S1 and S2 since the system OLR was reduced. In S3, the COD removal efficiency was higher

Another reason for the lower biogas production was the lower OLR applied in S4 and the slightly lower organic removal in S5. Generally, the biogas production trend in S3–S5 was

) / 205 ± 50 /

pH of brown grease was measured by suspending 100 g brown grease in 1 L tap water. Tap water has pH of 8.05 and

Brown grease was used as the primary substrate and the other two liquid wastes were used as co-substrates in part of

d−1) (**Table 5**). The easily digested dCOD in FC may account for this increase.

**Foul condensate (FC)**

**(μ ± σ, n = 11)**

d−1) was higher in S2 because of the

d−1) compared to that in S4 and

**Screw press liquor (SPL)**

**(μ ± σ, n = 13)**

gas production is 5–6 m<sup>3</sup>

260 Advances in Biofuels and Bioenergy

Alkalinity (mg L−1 as CaCO3

alkalinity of 55 mg L−1 as CaCO3.

**Table 4.** Substrate characteristics.

a

b

the evaluation.

S5 (~3.5 m<sup>3</sup>

consistent with the organic removal (**Table 5**), suggesting that the biogas production was not significantly affected by the addition of co-substrate.

The pilot-scale system produced biogas of excellent quality (75% CH<sup>4</sup> content), with a CH4 yield in the range of 0.40–0.77 m<sup>3</sup> -CH4 kg-VS−1. The addition of paper mill waste streams (FC and SPL) as co-substrate did not adversely affect the CH<sup>4</sup> yield. BG has the industrial potential to be anaerobically treated as a biofuel feedstock and there has been an ongoing commercial effort to build large-scale digesters using BG as the primary substrate. Using BG for biofuel recovery could serve as a profitable model for converting waste to renewable energy.

#### **3.3. AD addendum unit to improve corn-to-ethanol process**

The integrated anaerobic-aerobic system employed in this work contains three CSTRs, two transfer tanks, two clarifiers, and one serious CSTR aeration basin. The receiving tank (REC) is a rectangular CSTR with total volume of 4.5 m<sup>3</sup> (1.2 m width × 2.5 m length ×1.5 m height). Facultative tank (FAC) is a cylindrical CSTR with total volume of 0.35 m<sup>3</sup> (0.6 m diameter and 1.5 m height), the operating level is adjustable from 0.15 to 0.30 m<sup>3</sup> . Anaerobic digester (AD) is a cylindrical CSTR whose volume is 10.4 m<sup>3</sup> (2.1 m diameter and 3 m height) and the operating level is 7.2–9 m<sup>3</sup> . Two transfer tanks were respectively set between FAC and AD (0.04 m<sup>3</sup> ) and between AD clarifier and aerobic basin (0.15 m<sup>3</sup> ). The aerobic basin is rectangular whose volume is 2.5 m<sup>3</sup> (0.7 m width × 3 m length × 1.2 m height), three baffle plates were placed inside to divide the whole basin into four equal-sized serious tanks (0.6 m<sup>3</sup> for each). Two clarifiers were set after AD (anaerobic clarifier) and after aerobic basin (aerobic clarifier), respectively, both of them have a volume of 0.7 m<sup>3</sup> .

Stillage feedstock was obtained daily from the ethanol plant. Generally, the raw stillage has a COD concentration of ~100 g L−1 [66]. In this study, to make the experimental results more comprehensive, the feeding concentration was adjusted to different ranges, which will be discussed later. Homogenized feeding substrate from REC was pumped to FAC for predigestion.


a Typical value of operating parameters including pH, T, ORP, TN, TP, VFA and alkalinity were based on the description of typical anaerobic digestion systems. Typical values of CH4 yield were based on earlier literature. b For comparison purpose, VS removal efficiency in S1 and S2 has not been corrected by biomass calculation.

removal because the organic loading of the stillage was mainly in the solid phase (VS 67,224 mg

standard temperature and pressure, STP). All the gas volumes mentioned hereafter have been


kg-VS fed−1 in period I and 0.414 m<sup>3</sup>

AD process could be integrated to traditional corn-to-ethanol process as a treatment technique to the thin stillage product. The generated methane will partially replace the nonrenewable fuels and a large amount of energy could be saved from the removed evaporation

the total energy output in this anaerobic system is 16.8 MJ kg-VS fed−1 in period I and 13.7 MJ

The energy saving is calculated based on several areas. In traditional process, the stillage treatment process including evaporation and syrup flash drying will take 38 MJ for each gallon of 95% ethanol produced [65], and DDGS treatment process will take 8.4 MJ [70]. In periods I and II, the evaporation process was removed to save 38 MJ, and DDGS productivity was decreased by 45.2 and 39.8% (SCP was considered as the same quality animal feed with DDGS); thus, the saved energy from these two processes was calculated and listed in **Table 6**.

LHV. The consumed energy of applied anaerobic system was mainly focused on three mixing pumps in REC, FAC, and AD, respectively and the aeration activity in the aerobic system dur-

The power of the mixing pump was calculated based on the Camp-Stein equation for mixing

*P* = *G*<sup>2</sup> *V* (1)

sured VS concentration in thin stillage and the mean VS removal efficiency, the CH<sup>4</sup>

Energy recovered from produced methane was calculated based on the CH4

yield was 0.790 m<sup>3</sup>

production at STP and cumulative VS digested during two intensive evaluation periods (I and


kg-VS digested−1 in period II. Based on the mea-

(50.00 MJ kg−1) and our CH4


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263

kg-VS digested−1 (at

kg-VS fed−1 in period II.

yield could

yield,

yield and it is

L−1, COD 122,743 mg L−1). The calculated CH4


production yield was calculated based on the ratio of the two slopes.

process. Based on the lower heating value (LHV) of CH<sup>4</sup>

ing period II. The energy cost of transfer pumps is negligible.

normalized to STP) in period I, and 0.824 m<sup>3</sup>

be converted to 0.507 m<sup>3</sup>

**Figure 3.** Cumulative CH4

II). CH<sup>4</sup>

kg-VS fed−1 in period II.

with an impeller:

**Table 5.** Anaerobic digestion operating parameters and system performance in five selected stages.

This predigestion process will initially introduce a series of microbial strains, and to eliminate potential process inhibitors as reported earlier [55–57]. Afterwards, the predigested substrates were pumped continuously to AD for digestion. The effluent of AD was transferred via gravity to clarifier for sedimentation. The upper flow of clarifier was pumped to aerobic basin for aerobic treatment.

The evaluation period lasted for 171 days. Excluding the system set-up period, process data were obtained mainly from day 100 to day 171. Two intensive evaluation periods (day 100– 116, period I, and day 161–171, period II) were applied in the study. These two intensive periods were corresponding to two different scenarios of anaerobic treatment for stillage in this study. In period I (day 100–116), the system organic loading rate was 8.54 kg COD m−3 day−1, the raw thin stillage (~120,000 mg L−1 as COD) was directly fed into the REC without dilution, and no aeration process was added. The purpose of this period was to maximize the methane production by anaerobic treatment. In period II (day 161–171), the system organic loading rate was reduced to 40% of the period I, which is 3.50 kg COD m−3 day−1. The thin stillage was diluted to ~50,000 mg L−1 as COD before fed to the REC, and the aeration process was operated to further polish the AD effluent and to produce another economic product, single cell protein (SCP). These two scenarios will be discussed later.

The cumulative CH4 production and digested VS in stage I and II are shown in **Figure 3**. The CH4 yield was calculated as the ratio of the two slopes. The CH4 yield was reported based on VS

**Figure 3.** Cumulative CH4 production at STP and cumulative VS digested during two intensive evaluation periods (I and II). CH<sup>4</sup> production yield was calculated based on the ratio of the two slopes.

removal because the organic loading of the stillage was mainly in the solid phase (VS 67,224 mg L−1, COD 122,743 mg L−1). The calculated CH4 yield was 0.790 m<sup>3</sup> -CH4 kg-VS digested−1 (at standard temperature and pressure, STP). All the gas volumes mentioned hereafter have been normalized to STP) in period I, and 0.824 m<sup>3</sup> -CH4 kg-VS digested−1 in period II. Based on the measured VS concentration in thin stillage and the mean VS removal efficiency, the CH<sup>4</sup> yield could be converted to 0.507 m<sup>3</sup> -CH4 kg-VS fed−1 in period I and 0.414 m<sup>3</sup> -CH4 kg-VS fed−1 in period II.

AD process could be integrated to traditional corn-to-ethanol process as a treatment technique to the thin stillage product. The generated methane will partially replace the nonrenewable fuels and a large amount of energy could be saved from the removed evaporation process. Based on the lower heating value (LHV) of CH<sup>4</sup> (50.00 MJ kg−1) and our CH4 yield, the total energy output in this anaerobic system is 16.8 MJ kg-VS fed−1 in period I and 13.7 MJ kg-VS fed−1 in period II.

This predigestion process will initially introduce a series of microbial strains, and to eliminate potential process inhibitors as reported earlier [55–57]. Afterwards, the predigested substrates were pumped continuously to AD for digestion. The effluent of AD was transferred via gravity to clarifier for sedimentation. The upper flow of clarifier was pumped to aerobic basin for

Typical value of operating parameters including pH, T, ORP, TN, TP, VFA and alkalinity were based on the description

**Stages 1 2 3 4 5 Typical range a** pH 7.34 ± 0.05 / 7.12 ± 0.08 7.10 ± 0.07 7.01 ± 0.17 6.5–8.5 [33]b T (°C) 36.0 ± 0.7 36.3 ± 0.7 34.3 ± 1.8 34.3 ± 2.1 37.9 ± 1.0 35–40 [33]b

ORP (mV) −209 ± 14 −228 ± 24 −243 ± 40 −247 ± 37 −263 ± 23 −400–−150 [31] TN (mg L−1) 591 ± 83 409 ± 37 237 ± 74 314 ± 50 306 ± 46 60–1000 [32] 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 [32]

VFA (mg L−1 as HAc) 274 ± 97 / 199 ± 76 394 ± 84 469 ± 378 <1800 [31]

COD removal efficiency (%) 42.1 ± 6.7 50.6 ± 5.8 73.8 ± 11.0 61.7 ± 12.3 53.5 ± 8.7 / VS removal efficiency (%) 26.8 ± 7.9<sup>b</sup> 37.1 ± 4.3<sup>b</sup> 72.7 ± 7.4 57.9 ± 13.2 56.4 ± 9.9 /

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 /

For comparison purpose, VS removal efficiency in S1 and S2 has not been corrected by biomass calculation.

**Table 5.** Anaerobic digestion operating parameters and system performance in five selected stages.

) 3087 ± 282 / 1455 ± 457 2478 ± 291 2204 ± 222 1500–5000 [33]a

kg-VS−1) 0.40–0.49 0.58–0.77 0.49 0.48 0.45 0.11–0.42 [52]a

yield were based on earlier literature.

DO (mg L−1) 0.01 ± 0.00 / 0.06 ± 0.04 0.15 ± 0.05 0.10 ± 0.03 /

The evaluation period lasted for 171 days. Excluding the system set-up period, process data were obtained mainly from day 100 to day 171. Two intensive evaluation periods (day 100– 116, period I, and day 161–171, period II) were applied in the study. These two intensive periods were corresponding to two different scenarios of anaerobic treatment for stillage in this study. In period I (day 100–116), the system organic loading rate was 8.54 kg COD m−3 day−1, the raw thin stillage (~120,000 mg L−1 as COD) was directly fed into the REC without dilution, and no aeration process was added. The purpose of this period was to maximize the methane production by anaerobic treatment. In period II (day 161–171), the system organic loading rate was reduced to 40% of the period I, which is 3.50 kg COD m−3 day−1. The thin stillage was diluted to ~50,000 mg L−1 as COD before fed to the REC, and the aeration process was operated to further polish the AD effluent and to produce another economic product, single cell protein

production and digested VS in stage I and II are shown in **Figure 3**. The

yield was reported based on VS

,

[72]

aerobic treatment.

Alkalinity (mg L−1 as CaCO3

262 Advances in Biofuels and Bioenergy

CH4

CO2

H2

CH4

a

b

yield (m3


of typical anaerobic digestion systems. Typical values of CH4

The cumulative CH4

CH4

(SCP). These two scenarios will be discussed later.

yield was calculated as the ratio of the two slopes. The CH4

The energy saving is calculated based on several areas. In traditional process, the stillage treatment process including evaporation and syrup flash drying will take 38 MJ for each gallon of 95% ethanol produced [65], and DDGS treatment process will take 8.4 MJ [70]. In periods I and II, the evaporation process was removed to save 38 MJ, and DDGS productivity was decreased by 45.2 and 39.8% (SCP was considered as the same quality animal feed with DDGS); thus, the saved energy from these two processes was calculated and listed in **Table 6**. Energy recovered from produced methane was calculated based on the CH4 yield and it is LHV. The consumed energy of applied anaerobic system was mainly focused on three mixing pumps in REC, FAC, and AD, respectively and the aeration activity in the aerobic system during period II. The energy cost of transfer pumps is negligible.

The power of the mixing pump was calculated based on the Camp-Stein equation for mixing with an impeller:

$$P = G^2 \mu V \tag{1}$$


save the dilution water usage and the operation cost, generally the system will use less flow rate rather than dilution in real industries. Thus, the applied HRT in period II should be the calculated result, which is mentioned in kinetic analysis section (5.34 days). The calculated energy consumption in mixing pumps is listed in **Table 6**. For each pump, a 70% pump efficiency was assumed.

The power of the aerator (mostly an air diffuser) is the main operation cost of the aerobic section. The power requirement was estimated based on the reported typical energy requirement

sum up, compared with the traditional ethanol making process, the anaerobic integrated process could save 42.2 MJ (period I) or 52.2 MJ (period II) for each gallon of 95% ethanol produced.

The calculation of industrial cost saving was similar to the energy saving. The power cost saved by thin stillage treatment process was calculated based on electricity (price based on US EIA report 2013). The operation cost of anaerobic system was calculated based on the energy

from USDA livestock and grain market report (2013). The price of the SCP was assumed to be the same with DDGS. Since the anaerobic system cost and DDGS productivity reduction is the capital of the integrated system, in **Table 6**, they were shown in cash-negative format. After the calculation, the integrated system saved 12.2¢ (period I) or 39.6¢ (period II) for each gallon of 95% ethanol produced. Period II has a higher cost saving because the system applied in period II has just treated 40% of the generated stillage; thus, the energy consumption was lower. For a typical ethanol plant with 100 million gallon 95% ethanol yr−1 productivity, by applying this AD integrated system, the cost saving of the plant could reach \$ 12.2 million by completely treating thin stillage with AD, or \$ 15.8 million by partially (40%) treated. This amount is higher than the reported amount (\$ 7–17 million, most likely 10 million) in the study of Schaefer and Sung [71] because the gate-to-gate life cycle assessment was more comprehensive in this study.

By changing the influent condition, two different scenarios of anaerobic digestion were studied in this research. For each gallon of 95% ethanol produced, when thin stillage was fed

system energy saving was 42.2 MJ, and industrial saving will be 12.2 cents compared with traditional dry mill process, which means a typical ethanol plant could save 12.2 million dollars per year. When thin stillage was partially (40%) fed to a smaller sequential anaerobic-aerobic

industrial saving will be 39.6 cents, which means a typical ethanol plant could save 15.8 million dollars with this 40% of thin stillage. This study shows that thermophilic AD is a better

In this chapter, three kinds of waste streams from real industries were selected to investigate their anaerobic treatability, economic feasibility, and applicability to the practical plants. Generally, these selected waste streams were applied to a pilot-scale anaerobic-aerobic biological treatment system to convert their organic fraction into renewable energy in the form of CH4

directly to the anaerobic digester without dilution, the produced CH4

will be 0.464 m<sup>3</sup>

use of thin stillage and is applicable to practical dry mill ethanol plants.

**4. Benefit of anaerobic digestion process in biofuel recovery**

, the estimated result is shown in **Table 6**. To

http://dx.doi.org/10.5772/intechopen.72292

265

will be 0.568 m<sup>3</sup>

at STP, and system energy saving was 52.2 MJ. The

at STP,

.

comes from US EIA report 2013, and the price of DDGS comes

Biogas Recovery from Anaerobic Digestion of Selected Industrial Wastes

form Metcalf and Eddy [72], which is 30 kW/10<sup>3</sup> m3

consumption. The price of CH4

system, the produced CH4

Three scenarios (traditional, high thin stillage feeding, and low thin stillage feeding) were applied.

**Table 6.** Summary of energy and industrial cost saving in traditional ethanol making process and integrated processes for producing one gallon of 95% ethanol.

where *P* is the power requirement (W), *G* is the average velocity gradient (S−1), *μ* is the dynamic viscosity (N S m−2), and *V* is the reactor volume (m3 ). In this study, the applied *G* was a typical value in rapid mixing operations reported by Metcalf and Eddy [72], which is 1000 S−1. *μ* was water dynamic viscosity at 60°C, 4.66 × 10−4 N S m−2. *V* was calculated based on the flow rate (1.9 × 105 L h−1 thin stillage in period I and 7.6 × 10<sup>4</sup> L h−1 thin stillage in period II) and the applied HRT. HRT in the system is 5 days for REC, 1.5 days for FAC, and 12.6 days for AD in period I. In period II, the HRT for REC and FAC was kept the same, the HRT for aerobic basin is 1 day. To save the dilution water usage and the operation cost, generally the system will use less flow rate rather than dilution in real industries. Thus, the applied HRT in period II should be the calculated result, which is mentioned in kinetic analysis section (5.34 days). The calculated energy consumption in mixing pumps is listed in **Table 6**. For each pump, a 70% pump efficiency was assumed.

The power of the aerator (mostly an air diffuser) is the main operation cost of the aerobic section. The power requirement was estimated based on the reported typical energy requirement form Metcalf and Eddy [72], which is 30 kW/10<sup>3</sup> m3 , the estimated result is shown in **Table 6**. To sum up, compared with the traditional ethanol making process, the anaerobic integrated process could save 42.2 MJ (period I) or 52.2 MJ (period II) for each gallon of 95% ethanol produced.

The calculation of industrial cost saving was similar to the energy saving. The power cost saved by thin stillage treatment process was calculated based on electricity (price based on US EIA report 2013). The operation cost of anaerobic system was calculated based on the energy consumption. The price of CH4 comes from US EIA report 2013, and the price of DDGS comes from USDA livestock and grain market report (2013). The price of the SCP was assumed to be the same with DDGS. Since the anaerobic system cost and DDGS productivity reduction is the capital of the integrated system, in **Table 6**, they were shown in cash-negative format. After the calculation, the integrated system saved 12.2¢ (period I) or 39.6¢ (period II) for each gallon of 95% ethanol produced. Period II has a higher cost saving because the system applied in period II has just treated 40% of the generated stillage; thus, the energy consumption was lower. For a typical ethanol plant with 100 million gallon 95% ethanol yr−1 productivity, by applying this AD integrated system, the cost saving of the plant could reach \$ 12.2 million by completely treating thin stillage with AD, or \$ 15.8 million by partially (40%) treated. This amount is higher than the reported amount (\$ 7–17 million, most likely 10 million) in the study of Schaefer and Sung [71] because the gate-to-gate life cycle assessment was more comprehensive in this study.

By changing the influent condition, two different scenarios of anaerobic digestion were studied in this research. For each gallon of 95% ethanol produced, when thin stillage was fed directly to the anaerobic digester without dilution, the produced CH4 will be 0.568 m<sup>3</sup> at STP, system energy saving was 42.2 MJ, and industrial saving will be 12.2 cents compared with traditional dry mill process, which means a typical ethanol plant could save 12.2 million dollars per year. When thin stillage was partially (40%) fed to a smaller sequential anaerobic-aerobic system, the produced CH4 will be 0.464 m<sup>3</sup> at STP, and system energy saving was 52.2 MJ. The industrial saving will be 39.6 cents, which means a typical ethanol plant could save 15.8 million dollars with this 40% of thin stillage. This study shows that thermophilic AD is a better use of thin stillage and is applicable to practical dry mill ethanol plants.
