# Paper-Mill Wastes for Bioethanol Production in Relation to Circular Economy Concepts: A Review
## 3 Paper Mill Wastes
### Waste-Water Treatment and Paper Sludge  
The paper industry stands out as one of the largest consumers of industrial process water and energy (per product ton). In the United States, it leads in industrial water consumption and ranks third in energy consumption. Additionally, it holds the fourth position among industrial sectors in emissions of toxics release chemicals into both air and water [90]. Among recovered fibers products, the tissue paper manufacturing process generates the highest amount of waste (10 times more than packaging paper) [91]. During the recycling of 1 ton of wastepaper for papermaking, approximately 300 kg of RPS is generated. This constitutes most of the generated waste from paper production and recycling, contributing up to 28.4% of waste per unit of produced paper [92; 93]. A typical process for water treatment in a P&P mill involves primary treatment to eliminate suspended solids, followed by secondary treatment. The resulting solid residue obtained after thickening is called primary sludge (PS) and is generated in significant quantities. It is reported that one ton of paper yields approximately 30 kg of PS [94]. The composition of PS includes organic matter, nitrogen, phosphorus, paper fillers, low biodegradable organic matter (mainly cellulose), and it possesses a high C/N ratio (150 to 250) [95]. The primary sludge is further treated for the stabilization of organic matter by destructing pathogens and reducing volatile substances. This treatment is aimed at improving the quality of effluent for safer disposal. As preparation, the PS is sometimes neutralized to a pH of 6.5 to 8.5, suitable for biological treatments, by adding acid or alkali. Then, biological treatment is applied using oxygen-consuming (aerobic) microorganisms for the conversion of soluble organic matter into CO\({}_{2}\) and water for biochemical oxygen demand (BOD) reduction [96]. Most of the biological effluent treatment plants towards secondary sludge in the P&P industry employ activated sludge systems [97; 98], yet some variations of these systems include filters and sequence reactors such as moving bed biofilm reactors (MBBRs) and membrane bioreactors (MBRs). Sometimes, anaerobic treatment is used, followed by an aerobic biological one [36; 99; 100]. The treated waste then flows into a coagulative precipitation clarifier, with coagulation agents, to remove colloidal particles such as lignin and paper fillers. Depending on its purity level, the effluent water from this process can be reused within the mill or released into the environment. The secondary paper sludge (SPS) that is precipitated in the clarifier is extracted as solid content at a concentration of approximately 3%, directed to a dehydator, and de-watered to reach a solid content of 40-60% [92]. SPS is considerably more difficult to de-water compared to primary sludge. Therefore, most pulp and paper facilities opt to de-water a mixture of primary and secondary sludge. The primary organic components of the secondary paper sludge are microbial extracellular polymeric substances (EPSs), non-biodegradable materials, and microbial cell biomass [100]. Additionally, it contains a higher amount of nutrients than primary sludge and maintain a low C/N ratio (from 5 to 30) [95]. Due to the organic content, the landfilled paper mill sludge is subject to aerobic and anaerobic decay. Theoretically, when landfilling 1 ton of low-ash paper mill sludge (<30% ash), approximately 2.69 tons of CO\({}_{2}\) and 0.24 tons of CH\({}_{4}\) are released into the environment [101]. SPS volumes are lower than those corresponding to the PS, since most of the heavy, fibrous, or inorganic solids are removed in the primary clarifier. In addition, SPS has a higher content of lignin and nitrogen, as well as kaolin, calcium, and other fillers, vs. PS [100]. In most mills, PS and SPS are mixed to form a combined sludge, with a variety of SS:PS ratios [102].