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

Abstract

BackgroundIn 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.ResultsCellulase 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/gcellulose had no significant effect on sugars production, suggesting that 22% solids and 20 FPU/gcellulose 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).ConclusionsEnzyme 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.

Co-production of biofuels and value-added compounds from industrial Eucalyptus globulus bark residues using hydrothermal treatment

“…A slight increase in solids loading to 17.5% (w/v) showed to be feasible despite the increased mechanical constraints from a higher viscosity. Interestingly, and opposing to what would be expected for a higher solids loading [56], the ethanol production yield increased from 60 to 64% in the case of an SSF approach (SSF2 and SSF3, Table 3). We hypothesize that a superior initial glucose production (enabled by a higher solids loading) could have represented a slight advantage for cells growth and posteriorly for ethanol production.…”

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

“…It is worth mentioning that although these results may provide some economic insights into general strategies of enzyme recycling, they are highly specific for RPS material. As recently shown by Gomes et al (2018), recycling viability is strongly dependent on the enzymes fractionation and recovery at the end of the process, which itself may strongly vary according to multiple factors, one of the most important being the employed substrate and its composition (Rodrigues et al, 2014.…”

“…1A). An extended description of this process has been reported by Gomes et al (2016Gomes et al ( , 2018.…”

“…Considering the results and specifications described by Gomes et al (2016Gomes et al ( , 2018, a cellulases recycling scenario was envisioned assuming the recovery of both liquid and solid fractions, with only 50% of fresh enzyme being utilized on each recycling stage (Rec2Frac). As expected, there was a clear reduction in the total cost of enzyme -38%and consequently the corresponding contribution to the total costs D.G.…”

“…Nevertheless, this material also contains a considerable amount of sugars that can be valorized through multiple products, namely bioethanol. In fact, several studies have already demonstrated the practical feasibility of RPS for the production of bioethanol (Marques et al, 2008a;Gomes et al, 2016Gomes et al, , 2018 or lactic acid (Marques et al, 2008b). Chen et al (2014) have recently demonstrated that from an economic point of view, the conversion of different paper manufacturing residues is in fact viable to implement on an industrial scale.…”