Microbial EPS production from various substrates

Microorganisms that degrade organics in (waste)water also excrete biopolymers referred to as extracellular polymeric substances (EPS). New uses are increasingly found for such EPS in various industries. In this study, we showed how single substrates (glycerol, glucose, acetate, ethanol) and mixtures of two substrates non-equivalent in terms of bioenergetics, influenced mixed-cultured EPS recovery and characteristics. EPS were produced and harvested from membrane bioreactors treating nitrogen-limited (COD/N ratio 100) synthetic wastewater containing the aforementioned substrates. The reactors showed high COD removal and the ratios between the biomass-COD, EPS-COD, and oxidised COD fractions depended on the utilised substrate and the microbial community developed per substrate. Overall, the glycerol-based substrates resulted in the highest EPS recoveries (28 – 45% of influent-COD recovered as EPS-COD), followed by the glucose-based substrates (25 – 30%), while the reactors fed with sole ethanol (11%) and acetate/ethanol showed the lowest EPS yields (3 – 11%). With substrates of acetate and acetate/ethanol mixture, the formation of polyhydroxyalkanoates were observed, which likely decreased EPS production. Although the addition of ethanol as an auxiliary substrate to acetate increased EPS recovery from 3% (sole acetate) to 11% (acetate/ethanol ratio 1:1), reduced EPS recoveries were obtained when ethanol was added to glycerol and glucose substrates. Surprisingly, a higher EPS recovery (69%) was obtained when a mixture of acetate and ethanol was fed to the microbial community already producing high EPS yield from glycerol-based substrates. Microbial community analysis via Next Generation sequencing (NGS) of 16S rRNA gene revealed the growth of different and diverse communities of EPS-producing bacteria (mostly at the genus level) per reactor, as well as possible EPS-degraders. Furthermore, this study shows the possibility to employ different substrates to modulate basic EPS properties (namely monosaccharide composition, molecular weight and charge density), which is interesting for certain EPS applications such as for particle flocculation, heavy metal adsorption and as additives in the food industry.