Long-lasting biofouling formation on transparent fouling-release coatings for the construction of efficient closed photobioreactors.

In order to build an efficient closed-photobioreactor (PBR) in which biofouling formation is avoided, a non-toxic coating with high transparency is required, which can be applied to the interior surface of the PBR walls. Nowadays, amphiphilic copolymers are being used to inhibit microorganism adhesion, so poly(dimethylsiloxane)-based coatings mixed with poly(ethylene glycol)-based copolymers could be a good option. The 7 poly(dimethylsiloxane)-based coatings tested in this work contained 4% w/w of poly(ethylene glycol)-based copolymers. All were a good alternative to glass because they presented lower cell adhesion. However, the DBE-311 copolymer proved the best option due to its very low cell adhesion and high transmittance. Furthermore, XDLVO theory indicates that these coatings should have no cell adhesion at time 0 since they create a very high-energy barrier that microalgae cells cannot overcome. Nevertheless, this theory also shows that their surface properties change over time, making cell adhesion possible on all coatings after 8 months of immersion. The theory is useful in explaining the interaction forces between the surface and microalgae cells at any moment in time, but it should be complemented with models to predict the conditioning film formation and the contribution of the PBR's fluid dynamics over time.

Microalgal biofouling formation on tubular cellulose-ester membranes during dewatering by forward osmosis.

This work assesses the biofouling formation of a microalgal consortium, cultivated in wastewater, on dialysis tubular membranes with no supporting layer, in both batch and continuous FO dewatering modes. The biological adhesion strength was compared with the predictions from the Baier and Vogler biocompatibility theories, employing critical surface tension (γc) and water adhesion tension (τ0), respectively, as measurable parameters of surface wettability. The results indicate that most of the tested membranes presented amphiphilic surface characteristics (τ0=22 to 45 mJ.m-2, θW ≈ 65˚) with a minimal biological adhesion tendency, which is compatible with the Vogler criteria. However, the membrane exposed the longest time to the microalgal culture presented more hydrophobic characteristics and poor wettability. The existing thermodynamic models succeeded in predicting cell-cell and cell-surface interactions as a competitive phenomenon. Nevertheless, the XDLVO model was used to determine changes in the cell-to-surface attraction dynamics. This assessment of microalgal foulant-membrane interfacial interactions helps to enhance understanding of the fouling mechanisms present on a novel FO membrane surface.

Adsorption Analysis of Exopolymeric Substances as a Tool for the Materials Selection of Photobioreactors Manufacture.

An improved method that allows the robust characterization of surfaces is necessary to accurately predict the biofouling formation on construction materials of photobioreactors (PBR). Exopolymeric substances (EPS), such as proteins and polysaccharides, have been demonstrated to present a similar behavior to cells in terms of surface adhesion. In this work, these EPS were used to optimize parameters, such as EPS concentration or adsorption time, to evaluate accurately the adsorption capacity of surfaces and, with it, predict the biofouling formation in contact with microalgae cultures. Once the method was optimized, the characterization of seven commercial polymeric surfaces was submitted to different abrasive particles sizes, which modified the roughness of the samples, as well as protein and polysaccharide lawns, which were prepared and carried out in order to evaluate the characteristics of these substances. The characterization consisted of the determination of surface free energy, water adhesion tension, and critical tension determined from the measurement of the contact angle, roughness, surface zeta potential, and the EPS adhesion capacity of each material. This will be useful to understand the behavior of the surface in the function of its characteristics and the interaction with the solutions of EPS, concluding that the hydrophobic and smooth surfaces present good anti-biofouling characteristics.