**2.3. Stillage from corn-to-ethanol process**

remove up to 90% of biochemical oxygen demand (BOD) but the chemical oxygen demand (COD) removal efficiency is just in the range of 20–50% [16–19]. Secondly, the United States Environmental Protection Agency (US EPA) have strict and specific policies about these industrial wastes, such as the US EPA CMOM (capacity, management, operation, and maintenance program, including grease control program), the US EPA final pulp and paper cluster rule and amendments, the US EPA CWA (Clean Water Act), the FOG ordinance/FOG management policy, and so on. That could be considered as the driving force to push industries to treat these wastes before discarding. Finally, these materials from industrial waste contain high organic content, which means they have the potential to be treated anaerobically as the energy feedstock.

Pulp and paper industry produces a large quantity of wastewater of high organic strength

every ton of paper produced [22]. In the paper manufacturing processes, pulping and bleaching processes creates most of the wastewater streams [23, 24]. These wastewaters typically have high organic content (COD 800–4400 mg−1) [24–26], high biological content (BOD 300– 2800 mg−1) [24–26], and high dye content (1200–6500 color unit) [24–26]. Several steps of treatment process were generally involved, including a primary clarification process to remove the suspended solids, a secondary treatment process to remove most of the air lagoons, and a final biological treatment process (aerobic) to remove the biological content (BOD5) [25, 26]. However, due to the recalcitrant chemical properties, the final effluent always still contains

Anaerobic treatment technique has not been widely used in the pulp and paper industry yet [27, 28]. One major advantage of anaerobic treatment is that the process is capable of treating high-organic strength streams that are not suitable for aerobic processes [30, 31]. Furthermore, it has the added benefit of lower treatment cost because the produced biogas can be diverted to energy generation [32]. Traditional treatment technique of energy-rich wastes should be avoided as far as possible mainly because of their low energy recovery efficiency [33], but the recovered biogas from anaerobic digestion process has a high methane content (60–80%) and can be directly used as fuel [34]. One of the current research issues is most of the evaluations for pulp and paper wastewater are only focused on synthetic waste stream in the lab-scale environment (reactor size 5–50 L) [29, 35–40], which makes the results less representative to large-scale industrial fabrications, a research utilizing pilot-scale system, and practical waste

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

of wastewater is generated for

[20, 21]. Even with the most modern operations, about 60 m<sup>3</sup>

large amount of high molecular weight organic compounds [25].

streams directly from the paper mill would be more helpful and relevant.

**2. Substrates**

**2.2. Brown grease**

**2.1. Paper mill effluents**

254 Advances in Biofuels and Bioenergy

Based on the increased demand of renewable energy, bio-ethanol as an alternative energy source was considered and has enormous economic and strategic advantages. In the past decade, the national total annual fuel-grade ethanol production has increased from 1.77 billion gallons (in 2001) to 13.95 billion gallons (in 2011). In 2005, 67% of this ethanol was produced from dry mill corn [63], and this percentage has kept on increasing because of the low cost of this technology [64].

In a typical bio-ethanol production process, corn mash has been fermented and distilled to produce high purity ethanol, and the fermentation residue is called whole stillage, which is centrifuged to produce wet cake (precipitate) and thin stillage (supernatant). About 50% of the thin stillage is recycled as backset. The remainder is further concentrated by evaporation to produce syrup and blended with dried wet cake to create a feed product known as distiller's dry grain with soluble (DDGS). The effluent of evaporation process was purified and recycled as water reuse.

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].

as an upflow flooded-bed reactor, the HRT is about 1.7–2.4 days. The feeding and operational

Biogas Recovery from Anaerobic Digestion of Selected Industrial Wastes

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

257

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%.

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

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.

Substrate FC + SW EOP + SW EOP + SW EOP + DO + SW EOP + DO EOP + DO + SPL

d) 2.96 ± 0.70<sup>1</sup> 3.02 ± 0.38<sup>1</sup> 2.25 ± 0.81<sup>2</sup> 2.75 ± 0.70<sup>2</sup> 1.59 ± 0.48<sup>2</sup> 1.44 ± 0.48<sup>2</sup>

32.7 ± 2.6 34.3 ± 1.6 31.5 ± 3.1 34.5 ± 1.6 32.2 ± 1.8 33.3 ± 0.9

Flow scheme Downflow Downflow Upflow Upflow Upflow Upflow

HRT (d) — — 2.44 ± 0.83 1.72 ± 0.51 2.12 ± 0.91 1.82 ± 0.55 pH in digester 6.92 ± 0.39 7.23 ± 0.11 7.42 ± 0.10 7.60 ± 0.48 7.25 ± 0.02 7.26 ± 0.09

production and the cumulative COD

yield was calculated as the ratio of

production rate; these six


kg-COD-1

characteristics are summarized in **Table 1**.

**Figure 1** shows the plots between cumulative CH4

periods were considered as steady state periods. The CH4

for the substrates evaluated.

OLR (kg-COD/m<sup>3</sup>

(°C)

1

2

Temperature of effluent

Based on the volume of packing media.

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.

VI, see **Figure 1**) that the system has a stable and consistent CH4

the slopes of the two curves in **Figure 1**. The values range from 0.22 to 0.34 m<sup>3</sup>

**Operating periods 1 2 3 4 5 6** Duration (days) 1–36 45–81 82–135 136–142 143–148 149–156

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 be improved and the total cost will be reduced.
