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

where *P* is the power requirement (W), *G* is the average velocity gradient (S−1), *μ* is the dynamic

**Table 6.** Summary of energy and industrial cost saving in traditional ethanol making process and integrated processes

Total N/A 12.2 39.6

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

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

). In this study, the applied *G* was a typical

viscosity (N S m−2), and *V* is the reactor volume (m3

**Scenario I II III**

Raw material Corn grain 0.357 Bushel (12.6 L) Main product 95% ethanol 1 gallon (3.785 L)

EtOH process

By-products Biosolids (pound) DDGS 6.43 DDGS 3.52 DDGS 3.52 and SCP 0.35

Total N/A 42.2 52.2

@STP) N/A 0.568 0.464

N/A −18.3 from three

N/A 38 38

N/A 3.8 3.4

N/A 18.7 15.3

N/A 68.7 68.7

N/A 6.9 6.1

N/A −33.1 −8.2

N/A 9.9 8.2

N/A −40.2 −35.2

mixing pumps

High thin stillage feeding to maximum methane production, all the generated thin stillage will be treated Low thin stillage feeding to produce methane and single cell protein, 40% of the generated thin stillage will be

−4.5 from three mixing pumps, and −0.02 from air diffuser

treated

Description Traditional

264 Advances in Biofuels and Bioenergy

Methane (m3

From stillage treatment process

From DDGS treatment process

From methane recovered

From power saved by stillage treatment

From power saved by DDGS treatment

From operation of AD system

From methane produced

From biosolids produced

for producing one gallon of 95% ethanol.

process

process

From operation of applied AD system

Energy saved (Megajoule)

Industrial cost saving (US cents)

> 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 .

For paper mill effluents, the improved COD removal efficiency (55–70%) and the substrate utilization rate (0.28–0.46 d−1) indicated that it is anaerobically treatable. The CH4 yield (0.22– 0.34 m<sup>3</sup> -CH4 kg-COD−1) showed that the application of anaerobic technique has the potential to improve the energy footprints of the pulp and paper industry. For brown grease, the COD removal efficiency had somewhat been sacrificed (58%) to the maximum methane yield (0.40–0.77 m<sup>3</sup> -CH4 kg-VS−1). The obtained high CH4 yield showed that using brown grease for biogas production could serve as a profitable model for converting waste to renewable energy. For thin stillage, based on the high CH4 yield (0.464–0.568 m<sup>3</sup> CH4 @ STP per gallon 95% ethanol produced) and reducing energy consumption, a typical ethanol plant (producing 100 million gallon 95% ethanol per year) could save 12.2–15.8 million dollars per year, which indicated anaerobic digestion is a better use of thin stillage and is applicable to practical dry mill ethanol plants.

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Biogas Recovery from Anaerobic Digestion of Selected Industrial Wastes

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