Development of Self-Sustaining Phototrophic Granular Biomass for Bioremediation Applications

Natural aquatic biofilms (e.g. periphyton) play a major role in the degradation of conventional pollutants as well as xenobiotics that enter our aquatic systems. The remarkable ability of biofilms to degrade pollutants has been harnessed for purposes such as wastewater treatment. Recent developments in aerobic microbial granulation technology have brought about substantial improvements in biofilm-based remediation processes, offering several advantages such as high biomass retention, rapid biomass settling, high tolerance to toxicity, ability to withstand shock loading and low excess sludge production. We hypothesized that the diverse metabolic machinery and mixed microbial (bacterial, cyanobacterial and microalgal) functions of lotic biofilms could be exploited, if they can be successfully reproduced in the laboratory in the form of granular biomass. Accordingly, a method was developed for the cultivation of phototrophic aerobic microbial granules using bubble column photobioreactors. Mixed inoculum consisting of activated sludge and mixed microalgal cultures was added to column-type bubbled photobioreactors, which were operated in sequential batch mode with 24 h cycle time and 30% volumetric retention. Granulation of biomass was achieved within five weeks. The significance of the work is that it combines the advantages of both aerobic granular sludge and phototrophic biofilms. The bioreactors can be operated without addition of any external organic carbon source, as carbon fixation by the phototrophic elements can support the mixed microbial biomass in the reactor. This granular phototrophic mixed microbial biomass consortium has tremendous applications in environmental biotechnology, which was demonstrated by degrading a toxic model pollutant (phenol).