Recycled pulp and paper sludge, potential source of cellulose: feasibility assessment and characterization

The pulp and paper industry stands out as an example of a technology based on a renewable resource, cellulose. The sludge, however, poses major environmental and public health problems. To effectively manage the sludge wastes, it is critical to fully evaluate its composition, possible environmental impacts, and the total amount of exploitable renewable resources. The study established the pH of the sludge to be 7.32 ± 0.98, an electrical conductivity (1.84 mS/cm), nitrogen concentration (2.65 ± 0.21%), and total organic matter (41.23 ± 3.11%). The cellulosic content was established to be 74.07 ± 2.71% which contributes to 53.07 ± 1.23% water holding capacity (WHC). The most abundant elements were C and O, followed by Cl, Si, Al, and Mg, with lower concentrations of S, Si, K, and iron. The polycyclic aromatic compounds (PAHs) levels ranged from 0.29 to 322.56 ng.g-1 with 1-methyl pyrene posting the highest concentration (322.56 ng.g-1. XRD peaks at 17.10°, 23.86°, 30.14°, and 36.57°, which imply the existence of CaCO3. SEM indicated that the sludge was majorly comprised of fibers materials with average particle sizes of 280 micrometers. TGA/DTG analysis showed that the sludge had the greatest cellulose and hemicellulose (64.7 wt. %).

Insights into the economic viability of cellulases recycling on bioethanol production from recycled paper sludge

The economics of Recycled Paper Sludge conversion into ethanol was here assessed with emphasis on integrating a cellulase recycling system. Without cellulases recycling this process presented positive economic outputs (payback period of 7.85 years; 10.90 Million US$ of accumulated NPV) despite the modest ethanol titers. Recycling both free and solid-bound enzymes allowed considerable savings of enzyme but also an increase on annual costs (0.88%), resulting on a superior economic output: payback period decreased to 7.25 years; accumulated NPV increased to 14.44 Million US$. Recycling exclusively the liquid fraction enabled a clear costs reduction, however, also total ethanol decreased, attenuating the abovementioned benefits. Targeting higher ethanol concentrations, superior solids consistencies were also evaluated. Despite a costs reduction, total ethanol decreased due to a higher ethanol retention on the solid. A sensitivity analysis further revealed that the cost of enzymes and ultrafiltration membrane may be critical on enzyme recycling economic feasibility.

Determinants on an efficient cellulase recycling process for the production of bioethanol from recycled paper sludge under high solid loadings

BACKGROUND

In spite of the continuous efforts and investments in the last decades, lignocellulosic ethanol is still not economically competitive with fossil fuels. Optimization is still required in different parts of the process. Namely, the cost effective usage of enzymes has been pursued by different strategies, one of them being recycling.

RESULTS

Cellulase recycling was analyzed on recycled paper sludge (RPS) conversion into bioethanol under intensified conditions. Different cocktails were studied regarding thermostability, hydrolysis efficiency, distribution in the multiphasic system and recovery from solid. Celluclast showed inferior stability at higher temperatures (45-55 °C), nevertheless its performance at moderate temperatures (40 °C) was slightly superior to other cocktails (ACCELLERASE^®1500 and Cellic^®CTec2). Celluclast distribution in the solid-liquid medium was also more favorable, enabling to recover 88% of final activity at the end of the process. A central composite design studied the influence of solid concentration and enzyme dosage on RPS conversion by Celluclast. Solids concentration showed a significant positive effect on glucose production, no major limitations being found from utilizing high amounts of solids under the studied conditions. Increasing enzyme loading from 20 to 30 FPU/g_cellulose had no significant effect on sugars production, suggesting that 22% solids and 20 FPU/g_cellulose are the best operational conditions towards an intensified process. Applying these, a system of multiple rounds of hydrolysis with enzyme recycling was implemented, allowing to maintain the steady levels of enzyme activity with only 50% of enzyme on each recycling stage. Additionally, interesting levels of solid conversion (70-81%) were also achieved, leading to considerable improvements on glucose and ethanol production comparatively with the reports available so far (3.4- and 3.8-fold, respectively).

CONCLUSIONS

Enzyme recycling viability depends on enzyme distribution between the solid and liquid phases at the end of hydrolysis, as well as enzymes thermostability. Both are critical features to be observed for a judicious choice of enzyme cocktail. This work demonstrates that enzyme recycling in intensified biomass degradation can be achieved through simple means. The process is possibly much more effective at larger scale, hence novel enzyme formulations favoring this possibility should be developed for industrial usage.

Valorizing recycled paper sludge by a bioethanol production process with cellulase recycling

The feasibility of cellulase recycling in the scope of bioethanol production from recycled paper sludge (RPS), an inexpensive byproduct with around 39% of carbohydrates, is analyzed. RPS was easily converted and fermented by enzymes and cells, respectively. Final enzyme partition between solid and liquid phases was investigated, the solid-bound enzymes being efficiently recovered by alkaline washing. RPS hydrolysis and fermentation was conducted over four rounds, recycling the cellulases present in both fractions. A great overall enzyme stability was observed: 71, 64 and 100% of the initial Cel7A, Cel7B and β-glucosidase activities, respectively, were recovered. Even with only 30% of fresh enzymes added on the subsequent rounds, solid conversions of 92, 83 and 71% were achieved for the round 2, 3 and 4, respectively. This strategy enabled an enzyme saving around 53-60%, while can equally contribute to a 40% reduction in RPS disposal costs.