**2.1. Paper mill effluents**

Pulp and paper industry produces a large quantity of wastewater of high organic strength [20, 21]. Even with the most modern operations, about 60 m<sup>3</sup> of wastewater is generated for 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 large amount of high molecular weight organic compounds [25].

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