Lactic acid has many industrial applications. It acts as a precursor of polylactic acid that is promptly broadening in the market as a suitable substitute of petrochemical-based plastics. Polylactic acid is a biodegradable and biocompatible polymer. During combination of homopolymers poly-l-lactic acid and poly-d-lactic acid, high optical purity is used by which regular structures are formed in the crystalline phase. The ratio of poly-l-lactic acid and poly-d-lactic revamps the properties and disintegratability of polylatctic acid.
However D-lactic acid is non-edible and can be toxic. Hence there arose challenges in producing optically pure lactic acid using economical renewable resources. Large amounts of waste is produced by European paper industry of which seventy percent of recycled paper production. So it is indispensable to identify economical and environmentally sustainable application to avoid harmful deposition in landfills. Portuguese researchers proposed an approach to improve the production of lactic acid using recycled paper sludge as an attractive alternative raw material.
The selective production of L(+)-lactic acid from recycled paper sludge by simultaneous saccharification and fermentation had already been implemented under a pulsed fed-batch mode using lactobacillus rhamnosus. Many studies have made clear that lactic acid promotes an important inhibitor effect both on cell growth and on lactic acid production. On preferring a fed-batch strategy, a high lactic acid concentration is achieved significantly limiting the conversion.
Various approaches had been suggested to avoid product inhibition on lactic acid fermentation, but product removal is the most effective approach. Susana Oliveira Marques and her colleagues proposed membrane separation processes, which have many advantages in terms of energy efficiency, separation capacity, etc. and also possesses increased capacity, yield by operating at high cell densities and avoiding product inhibition.
The authors’ objective is to intensify simultaneous saccharification and fermentation (SSF) implementation into a membrane-recycle bioreactor in which the substrate was fed to the fermentor and the reaction mixture is continuously recycled through an external filtration unit. Thereby, maintaining a stream with constant product concentration while extending operation the product is removed as soon as it was formed.
To get consistent product concentrations, high dry matter contents should be used for running simultaneous saccharification and fermentation processes. However, on handling high solids loadings, the operational feasibility of membrane bioreactors is doubtful as they disintegrated membranes functionality due to increased cake layer formation and membrane fouling. Simultaneous saccharification and fermentation processes not only grow microbial cells, but also deals with residual lignocellulosic solid material providing a very high content of suspended solids.
According to the authors, it was very important to adequately select the module configuration and process conditions, so as to decrease the concentration polarization phenomena. Instead of using high flow velocities and lower transmembrane pressures, dynamic membrane filtration configuration should be adopted to promote higher membrane shear rate.
Henceforth the team of author’s study proposed flat sheet filtration module will be allowed by promoting the feed stream pass along the surface at a high cross flow velocity, and thus operating as a dynamic cross-flow filtration system. Porous asymmetric polymeric membranes with an ultrafiltration process would be implemented. Polysulphone and polyether sulphone membranes, exhibiting very good chemical and thermal stability could be utilized for micro and ultrafiltration.
Thereby authors suggested the best approach to run the simultaneous saccharification and fermentation process for the lactic acid production from recycled paper sludge into the in house membrane recycle bioreactor (MRB) based on the product inhibition along with the limitations imposed by the high solids concentrations. It would improve other simultaneous saccharification and fermentation processes dealing with high-solids concentrations, also using cost-effective lignocellulosic feedstock biorefineries. For instance, the fermentation that produces butanol, an advanced biofuel platform, is also strongly affected by end-product inhibition, and thus this bioprocess might also be enhanced by applying the authors’ proposed strategy. In terms of alternative feedstocks for bio-production of lactic acid, the combination of recycled paper sludge with a lignocellulosic material exhibiting lower ash content, such as brewer’s spent grains, might be important by reducing the gathering of unreacted inert to elimination.
The attractiveness of the innovative system proposed by these authors thus lies in the application of membrane assisted technology, under a MRB configuration, to a SSF process dealing with high suspended solids loadings, which might improve several biorefinery processes.
S. Marques1, C.T. Matos1,4, F.M. Gírio1, J.C. Roseiro1, J.A.L. Santos2,3, Lactic acid production from recycled paper sludge: Process intensification by running fed-batch into a membrane-recycle bioreactor, Biochemical Engineering Journal, Volume 120, April 2017, Pages 63–72Show Affiliations
- Laboratório Nacional de Energia e Geologia, I.P. (LNEG), Unidade de Bioenergia, Estrada do Paço do Lumiar 22, 1649-038 Lisboa, Portugal.
- Departamento de Bioengenharia, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
- IBB, Institute for Bioengineering and Biosciences, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
- Current address: European Commission, Joint Research Centre (JRC), Institute for Environment and Sustainability (IES), Sustainability Assessment Unit, Via Fermi, 21021, Ispra, VA, Italy
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