Coupling algae biofilm cultivation with wastewater treatment for sustainable biomass production and nutrient recycling
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The main objective of this study was to model the transport phenomena through which algal biofilms are able to recover nutrients contained within secondarily treated wastewater and to gauge the potential of hydrothermal liquefaction process water as a viable nutrient source. The cultivation of algae in the form of biofilms has shown promise due to this technique's greater energy return on investment during cultivation. These biofilms are known to be nutrient transport limited, thus an understanding of the phenomena associated with nutrient transport into algal biofilms is necessary for the optimized design and effective implementation of fixed cultivation techniques. This thesis reports a numerical study that investigates the transport of dissolved nutrients from wastewater into biofilms and an experimental study of the suitability of hydrothermal liquefaction process water as a nutrient source for algal cultivation. For the numerical study, the biofilm is modeled as a flat surface whose composition and growth characteristics were obtained from values in the literature. The fluid over the biofilm is modeled as a one-dimensional flow over a flat surface. The transport of dissolved nutrients was assumed to be diffusion dominated into the biofilm and was described through Fick's law. A novel derivation of the concentration boundary layer was demonstrated for use in a fully developed, thin fluid layer. The areal algal productivity and the nutrient removal rate by the biofilm were determined using the generated model and were validated against the literature data. The results showed that biofilm productivity and nutrient removal rates are highly sensitive to the presence of boundary layers, with a nearly order of magnitude decrease in productivity within the first meter of the modeled reactor. Finally, a parametric study on the effects of fluid velocity and depth on nutrient removal rates was performed. The study indicated the nutrient uptake rates increased by 24% with a 50% reduction in fluid depth and increased by 118% with a ten-fold increase in fluid velocity In the second part of this thesis, nitrate and phosphate concentrations in the process water of hydrothermal liquefaction of municipal biosolids were determined by ion chromatography to gauge its potential as a nutrient medium for algal cultivation. A parametric study on the effect of process conditions with the intent of determining the best nutrient source for algal cultivation indicated that the aqueous nutrient content was largely unaffected by heating rate, but was highly sensitive to the type of biosolid.