A Novel Membrane Biofilm Reactor for Microalgae Cultivation and Harvest and Wastewater Treatment

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The algae industry is an important part of the global economy. In addition to bioproducts, algae are increasingly recognized for their use in the bioenergy and waste management sectors (nutrients removal from wastewater and abatement of greenhouse gas (CO2)). However, the cell concentration of suspended microalgae culture is very dilute and usually less than 0.5-2 g/L. Consequently, microalgae grown in open ponds or closed photobioreactors require large volumes of water and a significant amount of energy input for cultivation, harvest, and dewatering. It is estimated that 20-30% microalgae production cost and about 90% of the equipment cost are associated with the harvest and dewatering of algal biomass, respectively. Thus, novel technologies for microalgae cultivation, harvest, and dewatering are highly desirable to improve the economy of microalgae production. As an alternative to open ponds and closed photobioreactors, algal biofilm systems have drawn great attention in recent years, due to their unique features, such as high biomass density, easy biomass harvest, and increased biomass productivity. The key advantage of microalgal biofilm (MB) lies in the higher cell concentration (up to 100 times of suspended cell concentrations), which greatly reduces harvesting effort as the MB can simply be scraped or vacuumed off the surface and can be used without the need for further concentration. However, the high cell density in MB requires a larger amount and faster transfer rate of CO2 for biofilm growth. The traditional strategies of directly bubbling CO2 gas into photobioreactors or microalgae ponds have a low transfer and utilization efficiency with 50% to 90% CO2 existing back to atmosphere. Thus, novel CO2 delivery technologies are vital to increase CO2 transfer and utilization efficiencies and reduce energy cost, particularly for MB systems. The proposed research program focuses on the development of a novel membrane carbonated microalgal biofilm bioreactor (MCMBR) for microalgae cultivation, harvest, and wastewater treatment. This novel system utilizes hydrophobic gas permeable membranes to deliver molecular CO2 into MB grown on outside surfaces of the membrane, while nutrients (N and P) and photons are transported into the MB from an opposite direction. This novel bioreactor system combines the advantages of high cell density biofilm systems and a high CO2 transfer and utilization efficiency through membranes (via a synergistic effect) and has a low energy consumption and thus can achieve a much higher process efficiency that is not achievable with current microalgal technologies. The outcome of this research is the development of a novel MCMBR system for microalgae cultivation, harvest, wastewater treatment, and greenhouse gas (CO2) abatement. This research will benefit the Canadian environmental and energy industries and train highly qualified personnel (HQP) in areas that are critical to Canada.