Date of Award

5-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biosystems Engineering

Committee Member

Dr. Caye M. Drapcho, Committee Chair

Committee Member

Dr. William Bridges

Committee Member

Dr. Nhuan Nghiem

Abstract

As an increasing amount of carbon dioxide (CO2) and other greenhouse gases pollute the lower atmosphere, and as various chemicals and elements pollute the water and soil, many search for potential solutions. One possibility is bio-mitigation using algae. Algae use CO2 and common polluting nutrients like nitrogen and phosphorus to grow. The growth, in turn, produces biomass and other byproducts that are converted to commodities such as biofuels, pharmaceuticals, cosmetics, and food supplements. Cosmetics and gelatin products are commonly sold, while other chemical products have bottlenecks in the process that require further research to make it marketable. The potential bioproducts from algae vary depending on the algal cell composition. The concentration of nitrogen, carbon, phosphorus, and light affect the growth and biomass composition, although less is known about the impact of phosphorus. Scenedesmus quadricauda, the algal species selected for this study, has potential for high carbohydrate and high protein content. However, gaps in previous studies require further compositional analysis before accurate conclusions are drawn. Batch growth of algae in closed reactors and limited diffusion reactors was analyzed. Trials were completed at four different levels of initial phosphorus concentration with an adjusted initial pH. Results show that the biomass concentration was impacted by the initial phosphorus concentration and by the amount of light received. One study yielded a specific growth rate of 0.0098 hr-1 (R2 = 0.996) in the reactor with 1 mg/L of phosphorus. The calculated biomass yield was 245.4 mg biomass/mg P. The phosphorus content of the cells linearly increased as the initial phosphorus concentration increased (R2 = 0.99977), while the carbon and nitrogen content increased in a nonlinear trend. A molar C:N:P ratio of 1266:85.6:1 was estimated for the lowest initial concentration of 0.05 mg/L P and 33.9:4.03:1 for the highest (7 mg/L P). These results suggest a connection between the initial phosphorus concentration and the composition of the algal biomass. Being able to model the growth of algae and the change in concentration of various nutrients could aid in design and optimization of algal bioreactors for waste treatment or production. This study modified a previously created model to include phosphorus and updated it to have more flexible code for user input. The model for phosphorus utilization allowed MATLAB to predict changes in the biomass and phosphorus concentrations depending on the initial phosphorus concentration. The initial values for the model were adjusted to account for phosphates in the alkalinity measurement. This adjustment improved the accuracy of pH predictions, but requires further adjustments to the predictions of the biomass, alkalinity, TIC and carbonate values. The sensitivity analysis showed that within a certain range the half-saturation constant does not affect the output of the model. An increase of the half-saturation constant by more than 200% is required to notice a visible difference. The model still needs adjustments to improve how it models limitation by all nutrient species, especially the carbonate and phosphorus predictions.

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