Date of Award

2016

Degree Type

Thesis

Degree Name

Master of Science in Chemical Engineering

Department

Washkewicz College of Engineering

First Advisor

Belovich, Joanne

Abstract

Digestate (D), the remaining substance after anaerobic digestion of a biodegradable feedstock, is rich in inorganic contents, which makes it a good candidate for growing algae for biofuel production. Previous studies showed digestate at around 1.25% to 1.75% (v/v) dilution is suitable for algae growth. In this study, magnesium sulfate (MgSO4) and dipotassium hydrogen phosphate/potassium dihydrogen phosphate (K-P) were added to diluted digestate growth media. Two sets of experiments were conducted in batch reactor mode to identify the digestate (D), magnesium sulfate (MgSO4) and dipotassium hydrogen phosphate/potassium dihydrogen phosphate (K-P) concentrations that would optimize the algae growth. Algae growth parameters, such as maximum growth rate (r) and maximum algae concentration (Xmax) were estimated by using non-linear regression with a four-parameter logistic equation. Average biomass productivity (Pa), instantaneous biomass productivity (Pi), and specific growth rate (ug) were also calculated. This study used a central composite design. A surface response regression equation was generated for each of these algae growth parameters; the equation contained linear terms, quadratic terms, and the first order interaction terms of the three factors (D, MgSO4, and K-P). The resulting regression models showed both the maximum growth rate and the maximum algae concentrations were mainly dependent on digestate. The highest maximum growth rate was obtained at around 1% (v/v) digestate dilution. Within the tested digestate dilutions (0.184 to 1.817% (v/v)), maximum algae concentration increases with digestate concentration. In addition, the data and analysis showed that digestate concentration of 1.4% (v/v) dilution and low K-P and MgSO4 concentrations would be expected to result in high average biomass productivity and instantaneous biomass productivity. The digestate concentrations does not alter the effects of K-P and MgSO4 on algae growth, but an interaction was seen between K-P and MgSO4. At low concentrations of these two factors (MgSO4 < 0.61 mmol/L and K-P < 2.81 mmol/L), both experiments 1 and 2 showed that lower the K-P and MgSO4 concentrations would yield higher maximum growth rate. The cause might be that the additional K-P and MgSO4 might promote larger cell production rather than cell replication, or the replicated cells may stay attached, which would lead to slower perceived growth rate but higher maximum algae concentration at the end of the batch growth.

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