Microalgae cultivation has received much research attention in recent decades due to its high photosynthetic productivity and ability to produce biofuel feedstocks as well as high value compounds for the health food, cosmetics, and agriculture markets. Microalgae are conventionally grown in open pond raceways or closed photobioreactors. Due to the high water contents of these cultivation systems, they require large energy inputs for pumping and mixing the dilute culture, as well as concentrating and dewatering the resultant biomass. The energy required to operate these systems is generally greater than the energy contained in the resultant biomass, which precludes their use in sustainable biofuel production. To address this challenge, we designed a novel photobioreactor inspired by higher plants. In this synthetic leaf system, a modified transpiration mechanism is used which delivers water and nutrients to photosynthetic cells that grow as a biofilm on a porous, wicking substrate. Nutrient medium flow through the reactor is driven by evaporation, thereby eliminating the need for a pump. This dissertation outlines the design, construction, operation, and modeling of such a synthetic leaf system for energy positive biofuel production. First, a scaled down synthetic leaf reactor was operated alongside a conventional stirred tank photobioreactor. It was demonstrated that the synthetic leaf system required only 4% the working water volume as the conventional reactor, and showed growth rates as high as four times that of the conventional reactor. However, inefficiencies in the synthetic leaf system were identified and attributed to light and nutrient limitation of growth in the biofilm. To address these issues, a modeling study was performed with the aim of balancing the fluxes of photons and nutrients in the synthetic leaf environment. The vascular nutrient medium transport system was also modeled, enabling calculation of nutrient delivery rates as a function of environmental parameters and material properties of the porous membrane. These models were validated using an experimental setup in which the nutrient delivery rate, growth rate, and photosynthetic yield were measured for single synthetic leaves. The synthetic leaf system was shown to be competitive with existing technologies in terms of biomass productivity, while requiring zero energy for nutrient and gas delivery to the microorganisms. Future studies should focus on utilizing the synthetic leaf system for passive harvesting of secreted products in addition to passive nutrient delivery. / text

Mathematical models of a nitritation: anaerobic ammonia oxidation (anammox)-coupled biofilm in a counter-diffusion hollow fiber membrane bioreactor (HFMBR) and a nitritation: anammox-coupled biofilm in a co-diffusion rotating biological contactor (RBC) were developed and implemented using AQUASIM. Four different start-up scenarios on the nitritation: anammox-coupled biofilm in an HFMBR were investigated. The supply of oxygen was simulated with the flow through the lumen of the hollow fiber membrane. For the four scenarios, two scenarios investigated the start-up when nitrite was supplied in the feed while the other two scenarios investigated when the source of nitrite was through nitritation only. The results showed that the presence of nitrite in the feed facilitated the start-up of the reactor. In addition, the results also showed that increasing oxygen flux through the membrane up to a certain ratio of ammonia flux with oxygen flux affected reactor performance by improving nitrogen removal and reducing start up time. For the nitritation: anammox-coupled biofilm in an RBC, four different process options were investigated: the number of reactors, the initial anammox (AnAOB) biomass fraction, the bulk oxygen concentration and the maximum biofilm thickness. Modeling results revealed that the steady state total nitrogen removal in RBC reactors in series occurred primarily in the first and second reactors. It is concluded that the number of reactors in series dictates the effluent performance and, therefore, this number can be selected depending upon the desired total nitrogen removal. Simulation results also revealed that increasing the initial AnAOB biomass fraction from 0.01% to 1.0% had no effect in the steady state nitrogen removal but had an effect in the required time to reach the steady state total nitrogen removal and the maximum biofilm thickness. Modeling results of the third process option showed that increasing the bulk oxygen concentration in the reactor from 0.2 g/m3 to 5 g/m3 linearly increased the steady state total nitrogen removal and reduced the time to reach the maximum biofilm thickness. Beyond 5 g/m3, steady state total nitrogen removal decreased. In addition, simulation results revealed that the thicker biofilm clearly showed a more linear correlation between the increase in bulk oxygen concentration and the increase in the steady state total nitrogen removal within a range of bulk oxygen concentrations. The results showed that RBC performance could be controlled by several process options: the number of reactors in series, initial biomass fraction, the bulk oxygen concentration and the maximum biofilm thickness. The mathematical modeling results for the HFMBR and RBC have shown that both have potential as carriers for nitritation: anammox-coupled biofilms targeted at the removal of nitrogen in the wastewater. / Master of Science