Membrane bioreactor (MBR) assisted bacterial contamination removal from bioethanol fermentation system

Bioethanol fermentation is an anaerobic process in which the monomeric sugars are converted into ethanol and carbon dioxide by yeast fermentation. As a sustainable source of energy that can be used as an alternative for fossil-fuel consumption, the production volumes have been shown to increase over the years. A major issue hindering efficiency in1st generation industrial ethanol fermentation from sugar based feed stock is unwanted bacterial contamination. The presence of bacterial contamination in a fermentation process leads to the drop in final ethanol yields, process profitability, yeast metabolic activity and the quality of final product, and increasing downstream processing costs. In industrial scale fermentation, a contamination free process is hardly achieved as complete sterilization and maintaining sterility of the feedstock and instruments involved is very costly and laborious. As the contamination may occur from various pathways, the use of conventional anti-microbial compounds are not fully effective for long term bacterial control and system disinfection. In this study, applying a physical bacterial decontamination of the fermentation to replace acid or base treatments and antibiotic addition was investigated. A submerged membrane bioreactor (MBR) using Integrated Permeate Chanel (IPC) flat sheet membrane panels were used for this purpose. Ethanol producing microorganism was selected as Saccharomyces cerevisiae as it is the most widely used strain for bioethanol production. The contaminant - Enterobacter cloacae- had been previously isolated from a lab-scale bioethanol fermentor located in Biotechnology Department of University of Boras. Both strains were cultivated separately and concomitantly to determine growth characteristics, metabolites and cell morphology in synthetic medium and analyzedthroughspectrophometry, HPLC analysis, microscopy, plate inoculationand cell dry weight measurements. A selective separation of a co-culture of Saccharomyces cerevisiae and complex of Enterobacter cloacae were first investigated by placing the cultures within a phosphate buffered saline solution in MBR to determine the interaction of both strains with the membrane surface in the absence of nutrients and therefore metabolites. Dilution rates of 0.25 to 0.75 h-1were practiced using poly ethersulfune (PES) membrane with pore sizes of 1-2.4 µm. The effect of initial bacterial inoculum size (up to 0.25 g/l in the bioreactor) on its washout regime was studied and it was observed that in optimum conditions up to 93% could be effectively removed through the permeate line by this MBR set up.xxiv Upon establishing the optimum filtration conditions for bacteria strain in the absence of nutrients, the concomitant presence of both species in continuous fermentation using sugar based nutrient medium was studied. As expected in fermentation conditions the process was hindered by the growth of bacteria as well as yeast; never the less continuous bacteria washout was achieved with a smaller bacteria inoculum (0.001 g/l) and slower dilution rate (0.11 h-1).