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1. Introduction
Paper mills based on nonconventional agro residues are being encouraged due to increased demand of paper and acute shortage of forest-based raw materials. Small-scale pulp and paper mills are generally situated in rural areas due to availability of raw materials (wheat straw, rice straw, and baggase). The agro-based pulp and paper mills are highly water intensive, consuming 100–250 m3 fresh water/ton paper produced [1]. These units correspondingly generate large quantities of wastewater, approximately 150–200 m3 effluent/ton of paper produced. The environmental impact of wastewater emanated from small-scale pulp and paper mills is therefore of particular concern [2]. The paper manufacturing process involves three steps, pulping, bleaching, and finally paper making. Pulping can be done via chemical (Kraft pulping) or mechanical methods. Chemical pulping is the most commonly employed pulping technique in agro-based pulping mills [3]. The effluent emanating from the pulping process is called black liquor and it contains inorganic chemicals, chlorophenolic compounds, and fibre residues which have characteristically high levels of biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids, color, and organic compounds [4]. The discharge of black liquor from 30 ton per day agro-based pulp and paper mills without chemical recovery yields pollution load equivalent to a 100 tpd mill with chemical recovery [5]. The main component of this type of wastewater is chlorolignin. For ages, lignin has been well known for its resistance to microbial degradation because of its high molecular weight and presence of various biologically stable carbon-to-carbon and ether linkages [6]. But of late, few bacteria have been reported to be able to degrade lignin [7]. However, not much work has been undertaken towards lignin degradation by bacteria. In addition, the chlorophenolic compounds formed in chlorine bleaching are toxic, persist, bioaccumulate, and transform into other compounds which are more hazardous. Government agencies mark the standards for the discharge of wastewater into the environment; the BOD standard of 30 mg/L for discharge on inland surface water and 100 mg/L for disposal has been notified under the environment protection rule.
However, agro-based small-scale pulp and paper industries do not treat their wastewater to bring these parameters to the level of the standards laid down by the government bodies. They use sulfuric acid and polyelectrolyte for precipitation of lignin which also removes...