# Paper-Mill Wastes for Bioethanol Production in Relation to Circular Economy Concepts: A Review
## 3 Paper Mill Wastes
### Waste Cycle in Pulping and Bleaching Processes  
Paper production also results in waste stream generation, mainly during the pulping and bleaching stages. Kraft pulping results in a stream of weak black liquor containing \(\sim\)15% total solids, including spent chemicals and solubilized biomass. This weak black liquor is concentrated in evaporators, resulting in 70-80% total solids [36; 41]. The produced heavy black liquor is then recovered and burned by spraying it into recovery boilers. Salt smelt, consisting mainly of Na\({}_{2}\)CO\({}_{3}\) and Na\({}_{2}\)S, is generated and leached in water, generating green liquor. The green liquor is causticized to form white liquor, which is recovered and reused in the pulping process, as well as in washing water [89]. The thickened pulp moves on to the bleaching process [41]. The bleaching stage is the major source of effluent in a pulp mill (up to 85%), and the chlorinated effluents are more complicated to treat for reutilization within the mill [32]. Most compounds identified in bleaching effluents are derived from lignin or other wood components [12; 56]. There is also a large amount of colored pollutant compounds (200 to 300 kg ton\({}^{-1}\) of pulp), mainly chloro-lignin, which are resistant to conventional biological degradation. One of the most effective treatments for bleaching effluents is a combination of ozone with fixed bed biofilm reactors, which provides maximum elimination of COD substances with a minimal dosage of ozone. This technology transfers the oxidative residue into a biological excess sludge. It was found that ozone addition upstream from bio-filtration not only enhances the biodegradability of \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Country**} & \multirow{2}{*}{\begin{tabular}{c} **Paper** \\ **Production** \\ **(MTons)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Paper** \\ **Consumption** \\ **(Million Metric** \\ **Tons)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **MSW** \\ **(kg Capita\({}^{-1}\)** \\ **Day\({}^{-1}\))** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **\% Paper within** \\ **MSW (\% wt.)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Paper** \\ **Recycling Rate** \\ **(\%)** \\ \end{tabular} } & \multirow{2}{*}{**References**} \\  & & & **Tons** & & & \\ \hline Argentina & 1.3 & 1.9 & 1.2 & N/A & N/A & [66] \\ Australia & 3.4 & 3.5 & 1.8 & 6.7 & 53 & [3; 66] \\ Belgium & 2.1 & 2.8 & 1.3 & N/A & 60 & [66] \\ Brazil & 14 & 9 & 1.1 & 13.1 & 63.4 & [67] \\ China & 125 & 109 & 1 & 8 & 35 & [66; 68] \\ Egypt & 0.7 & 1.7 & 1.4 & 10 & N/A & [66; 69] \\ France & 8 & 8.7 & 1.5 & 16.2 & 54 & [66; 70] \\ Germany & 22.6 & 20.5 & 1.7 & 8.1 & 83 & [66; 71] \\ Greece & 0.4 & 1 & 1.4 & 20 & 37 & [66; 72] \\ India & 17 & 17 & 0.4 & 6 & 29 & [66; 73] \\ Iran & 0.8 & 1.8 & 0.2 & 7.8 * & N/A & [66; 74] \\ Israel & 0.4 & 0.9 & 1.7 & 24 & 39 & [75] \\ Italy & 8.9 & 10.5 & 1.4 & 19 & 46 & [66; 76] \\ Japan & 26.1 & 26 & 0.9 & N/A & 72 & [66] \\ Korea & 11.6 & 9.5 & 1 & 35 & 85 & [66] \\ Malaysia & 1.8 & 3.1 & 0.9 & 17.8 & N/A & [64; 66] \\ Mexico & 5.7 & 8.9 & 1 & 5.89 * & 7 & [66; 77] \\ Pakistan & 0.9 & 1.4 & 0.7 & 2.4 & N/A & [64; 66; 78] \\ Portugal & 2.2 & 1.2 & 1.2 & 27 & 50 & [66; 79] \\ Poland & 4.6 & 6.4 & 0.9 & 19.1 * & 34 & [66; 78] \\ Russia & 8.5 & 6.9 & 0.9 & N/A & N/A & [66] \\ Saudi Arabia & 1.2 & 2.1 & 1.3 & 28.5 & N/A & [66; 80] \\ Singapore & 0.1 & 0.5 & 0.9 & 21.2 & 40.3 & [64; 66; 81; 82] \\ Spain & 6.2 & 6.7 & 1.3 & 27 & 59 & [66; 83] \\ South Africa & 2.2 & 2.3 & 2 & 18.2 & 59 & [66; 84] \\ Turkey & 3 & 4.9 & 1.1 & 15 & 38 & [66; 85] \\ United Arab & 0.2 & 0.9 & 1.7 & 22 & N/A & [66; 86] \\ Emirates & UK & 3.7 & 8.9 & 1.3 & 10.5 & 56 & [66; 87] \\ USA & 73 & 70 & 2 & 26 & 50 & [88] \\ \hline \end{tabular}  
* Data from the capital/biggest city; N/A not available.  
\end{table} 
Table 4: Waste and paper recycling.