**4. Benefits of anaerobic digestion process of BG**

In this chapter, the anaerobic digestion process of BG as well as its kinetic study was investigated. An average COD removal of 58% and VS removal of 55% was achieved. The organic removal efficiency was consistent and comparable with earlier studies. Kinetic analysis indicated that the pseudo-first-order degradation rate constant of BG was in the range of 0.10–0.19 d−1.

The organic removal rate and kinetic coefficient listed above indicate that anaerobic digestion process could be introduced as an effective pretreatment process of BG for initial COD removal and energy recovery. After anaerobic treatment process, the effluent had a consistent effluent organic strength (COD ~ 10,000 mg L−1) that can be treated aerobically. Based on the analysis, BG was found to be a readily digestible substrate. The recovered biogas could be considered as a readily in-plant-usable energy source; therefore, the energy utilization efficiency will be increased, and also the capital of organic treatment will be decreased, which is an industrial win-win situation. Once the AD process was linked as a gate-to-gate life cycle to the BG treatment chain, the efficiency of the complete cradle-to-gate evaluation will be improved and the total cost will be reduced.

The pilot-scale system produced excellent quality biogas (75% CH4 content), with a methane yield in the range of 0.40–0.77 m3 -CH4 Kg-VS−1. The addition of paper mill waste streams (FC and SPL) as cosubstrate did not adversely affect the methane yield. Currently, anaerobic digestion technique has not been widely used in the pulp and paper industry due to the recalcitrant nature of the paper process wastes. However, the combining treatment process of paper process and food process wastes does not show negative affections and could be considered as an alternative treatment method in the future.

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 as a profitable model for converting waste to renewable energy.
