Feasible process development and techno-economic evaluation of paper sludge to bioethanol conversion: South African paper mills scenario


Ash content of the sludge effected ethanol concentration and process economics.

De-ashing of the sludge from recycle mills is necessary for ethanol production.

Solids loading of 21% (w/w) and 14.5 FPU/gds of enzyme are the generic conditions.

Higher IRR values were obtained with paper sludge feed rates in excess of 30 t/d.

Monte Carlo analysis predicted a probability of 95% for IRR values above 25%.


Paper sludge samples collected from recycling mills exhibited high ash content in the range of 54.59%–65.50% and glucose concentrations between 21.97% and 31.11%. Washing the sludge reduced the total ash content to between 10.7% and 19.31% and increased the concentration of glucose, xylose and lignin. Samples were screened for ethanol production and fed-batch simultaneous saccharification and fermentation (SSF) was optimised for the washed samples that resulted in highest and lowest ethanol concentrations. Maximum ethanol concentrations of 57.31 g/L and 47.72 g/L (94.07% and 85.34% of the maximum theoretical yield, respectively) was predicted for high and low fermentative potential samples, respectively, and was experimentally achieved with 1% deviation. A generic set of process conditions were established for the conversion of high ash-containing paper sludge to ethanol. Techno-economic analysis based on three different revenue scenarios, together with Monte Carlo analysis revealed 95% probability of achieving IRR values in excess of 25% at a paper sludge feed rate of 15 t/d. Feed rates of 30 t/d and 50 t/d exhibited a cumulative probability of 100%. This study presents the technical feasibility and economic viability of paper mills expansion towards bioethanol production from paper sludge.


  • Paper sludge;
  • Bioethanol;
  • Simultaneous saccharification and fermentation (SSF);
  • Fed-batch;
  • Techno-economic analysis;
  • Monte carlo analysis

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