# Paper-Mill Wastes for Bioethanol Production in Relation to Circular Economy Concepts: A Review
## 5 RPS for Energy Applications
### Paper Wastes as a Lignocellulosic Material  
Paper wastes are considered a good lignocellulosic biomass source for bioethanol production because of their high cellulose content (Table 6). Paper is usually recycled 3-4 times [135], and after that, most fibers become too short and incompatible for papermaking. Using such fibers may negatively affect paper product properties; therefore, they are rejected as waste. Since the recovery of recycled paper cannot solely be in the papermaking market, RPS may also be used for ethanol production, as the process waste is beneficial [136].  
Both RPS and other PMS may have a negative cost impact from a circular economy standpoint, as they are an environmental burden and typically landfilled by default. PMS and RPS were evaluated and demonstrated as bioethanol feedstock, presenting successful conversion (Table 7). The integral utilization of lignocellulosic biomass, namely paper sludge, has been considered within the bio-refinery concept and can be evaluated by circular economy monitors for environmental, economic, and even social aspects. The common evaluation methodology is the life cycle assessment (LCA), standardized in the ISO 14040 series [142].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Source** & **Cellulose** & **Hemicellulose** & **L Lignin** & **References** \\ \hline Newspaper & 40–69 & 25–40 & 18–30 & [137; 138; 139] \\ Kraft paper & 57.3 & 9.9 & 20.8 & [134] \\ Corrugated cardboard & 53–57 & 10–13 & 21–22 & [137] \\ Office and copy paper & 57–63 & 16–21 & 0–6.5 & [137; 139] \\ Paper towel & 53 & 13 & 18 & [137] \\ Paperboard SBB a  & 64 & 20 & 5 & [137] \\ Paperboard SUB b  & 43 & 12.5 & 24.3 & [137] \\ Bleached softword kraft pulp & 82 & 16 & 0.6 & [140; 141] \\ Bleached hardwood kraft pulp & 70–81 & 17–27 & 0.8–3 & [140; 141] \\ Unbleached kraft pulp & 72 & 20 & 3–7 & [142; 143] \\ Chemi-thermomechanical pulp (CTM) & 54 & 25 & 19–21 & [140; 141] \\ Municipal solid waste (MSW) & 15 & 10 & 8.5–15 & [144; 145] \\ Waste paper & 40–80 & 5–20 & 1–10 & [138; 146; 147] \\ Paperboard mill sludge (PMS) & 23.4–37 & 8.6–33 & 16.5–24.5 & [148; 149] \\ \hline \hline \end{tabular}
\end{table}
Table 6: Paper sources’ composition.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Lignocellulosic Source** & **Pre-Treatment** & \
\begin{tabular}{c} **Ethanol Yield** \\ **(L/Dry Ton Feedstock)** \\ \end{tabular} & **References** \\ \hline Recycled paper sludge (RPS) &
\begin{tabular}{c} Ozonation \\ \end{tabular} & 120 & [5] \\ \hline Recycled paper sludge (RPS) & Sterilization (autoclaving with Sodium Azide) & 93–112 & [63] \\ \hline Copy paper & None & 287 & [147] \\ \hline Copier paper & None & 198 & [150] \\ \hline Virgin pulp PS & None & 241 & [151] \\ \hline Corrugated recycle PS & None & 214 & [151] \\ \hline Newspaper & Oxidative line: 1.875\% \(w\)/\(w\) Ca(OH), 7.1 bar (absolute) & 290 & [152] \\  & O\({}_{2}\), 140 \({}^{\circ}\)C, 3 h & 220 \({}^{\circ}\)C & 419 & [153] \\ \hline Office paper & Diluted acid: 0.5\% \(w\)/\(w\) H\({}_{2}\)SO\({}_{4}\), 220 \({}^{\circ}\)C & 419 & [154] \\ \hline Recycled paper sludge (RPS) & Ash removal (TAPPI, 1995) & 289–332 & [6; 154] \\ \hline Waste office paper & Diluted acid: 1\% \(v\)/\(v\) H\({}_{2}\)SO\({}_{4}\), 50 \({}^{\circ}\)C, 3 h & 206 & [155] \\ \hline \hline \end{tabular}
\end{table}
Table 7: Paper products and paper waste conversion to ethanol.