Algal-Bacterial Photobioreactors for the Degradation of Toxic Organic Pollutants
Abstract: The aerobic biodegradation of toxic organic volatile contaminants is always limited by the low aqueous solubility of O2. Therefore, intensive bubbling or surface aeration is required to supply the bacterial community with sufficient O2 to carry out pollutant mineralization. This can cause the hazardous volatilization of toxic volatile contaminants. Photosynthetic oxygenation overcomes these limitations by in-situ O2 generation via photosynthesis. More precisely, microalgae produce the O2 required by the aerobic bacteria to mineralize organic pollutants, which in turn use the CO2 released by the bacteria. This oxygenation mode is both safer because there is no risk of volatilization due to air bubbling, and less expensive, because sunlight is used as the main energy source. The potential of photosynthetic oxygenation to support the biodegradation of toxic organic pollutants was investigated. Photosynthetic oxygenation was able to support the complete removal of salicylic acid, phenol, phenanthrene and acetonitrile under continuous artificial illumination in enclosed photobioreactors without any external O2 supply. Microalgae activity often limited the biodegradation process due to their low growth rate and their low tolerance towards toxic pollutants. Special attention was therefore given to the selection of microalgae. Chlorella sorokiniana, Chlorella vulgaris, Scenedesmus obliquus, Selenastrum capricornutum and Scotiellopsis oocystiformis were compared in combination with pollutant-specific degrading bacteria for their ability to support the biodegradation of salicylate and acetonitrile. In both cases, C. sorokiniana exhibited the highest O2 production rates and highest tolerance and therefore supported the fastest removal. The inhibition of microalgae was especially problematic when treating highly concentrated toxic effluents as process inhibition occurred, even with pollutant resistant microalgae. In this regard, two liquid phase bioreactors were shown to bring process stability, by allowing a self-regulated delivery of the toxic organic pollutant to the microbial community. Thus, the addition of silicone oil or tetradecane into the culture medium permitted the biodegradation of phenanthrene at concentrations of up to 500 mg.l-1. Maximum phenanthrene degradation rates up to 24 mg.l-1.h-1 were achieved in test tubes under continuous illumination at 17.000 lux with silicone oil as organic phase. In addition, the presence of the microalgae enhanced phenanthrene mass transfer to the cells by releasing biosurfactants that increased the interfacial area between the organic and the aqueous phase. The inoculation strategy was very important to favor the start-up in batch photobioreactors. Thus, at the early stages of biodegradation, microalgal density limited the process up to a certain algal density where the process became limited by light supply to the photosynthetic cells. Therefore, a sufficient initial microalgal density must be present in order to avoid excessively long lag phases. The continuous photosynthetically oxygenated biodegradation of salicylate was efficient and stable for over 1 year of operation. Under optimised conditions sodium salicylate was removed at a maximum constant rate of 87 mg.l-1.h-1, corresponding to an estimated oxygenation capacity of 77 mgO2.l-1.h-1. Process efficiency was significantly influenced by temperature, light intensity, pollutant concentration, and wastewater flow rate. The control of microalgal concentration by biomass settling and subsequent recirculation into the photobioreactor significantly enhanced pollutant biodegradation. When increasing biomass concentration from 0.4 to 0.6 g.l-1 salicylate removal efficiency increased from 56 to 100 %. One of the main advantages of algal-bacterial systems was the possibility to combine the removal of several pollutants. Indeed, the use of microalgae was specially advantageous for the degradation of organonitriles for two reasons; first it reduced their hazardous volatilisations and secondly a significant amount of the NH4+ generated during their biodegradation was assimilated by microalgae. During continuous acetonitrile biodegradation at 3.5 days of HRT, up to 83 % of the NH4+ produced was removed using a C. sorokiniana in combination with an acetonitrile degrading mixed culture. In addition, the residual biomass produced during pollutant mineralisation exhibited good properties with regards of Cu2+ removal, with specific adsorptions of up to 8.5 mgCu.g-1 at 20 mg.l-1 of initial Cu concentration. This study showed that the implementation of a process configuration based on an adsorption column packed with the residual biomass and placed before the photobioreactor enhanced the stability of a photosynthetically oxygenated salicylate biodegradation process. This study clearly demonstrated the broad potential of photosynthetic microorganisms in environmental biotechnology.
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