Dairy production has grown more efficient as the number of cows on each facility increases; however, this expansion has also increased the concentration of localized manure that can lead to environmental concerns such as greenhouse gas emissions or eutrophication. This dissertation investigates a sustainable alternative to conventional dairy manure treatment methods that incorporates hydrothermal carbonization (HTC), algae cultivation, and membrane distillation (MD). HTC converts dairy manure into a low-grade coal that can be used as an energy source, while expelling nutrients into the HTC aqueous product (HAP). The algae can be cultivated on the HAP, where they consume the nutrients, reduce eutrophication risk, and when harvested, can be used as a protein supplement for the cattle on-site. The algal supernatant can be treated with MD to produce clean water and replace conventional reverse osmosis systems. The parts of the project investigated in this work include the potential to remediate the HAP, cultivate the algae, and treat the supernatant using MD.Arthrospira maxima, Chlamydomonas reinhardtii, Chlorella vulgaris, and Scenedesmus obliquus were microalgae strains selected for this work due to their high protein content or potential to be cultivated on wastewater. All four species are capable of heterotrophic growth. The screening process revealed dilution to 5% HAP was required for successful cultivation, with little growth observed at higher concentrations. A. maxima remediated the most COD, TN, TP, and NH3 in the HAP, followed by C. reinhardtii; both strains averaged 43% protein content. The other strains removed fewer nutrients and had lower protein content when grown on HAP. Of the four species, A. maxima had the highest growth rate but required a bicarbonate pH buffer that introduced other environmental complications. Both A. maxima and C. reinhardtii were moved forward as the preferred candidates for nutrient utilization and dietary supplements as buffered and non-buffered species.
The supernatants of the two strains were treated using MD and compared to their respective HAP controls to assess the effects of algae cultivation and the buffer on membrane operation and distillate quality. The water flux of both supernatants resembled the flux of the unbuffered HAP control. The flux of the buffered HAP control was greatly reduced compared to unbuffered HAP, but was restored after cultivation with A. maxima. The distillate produced from the A. maxima supernatant had reduced COD, TN, TP, and NH3 concentrations while the distillate produced from the C. reinhardtii supernatant had increased concentrations of COD and NH3 compared to their respective controls. Fluorescence was used to characterize the types of organic species removed during algae cultivation or MD treatment revealing the detected distillate species shared properties with simple aromatics or biological byproducts. A simplified A. maxima regrowth experiment revealed the buffer can be recycled back into cultivation if extra nutrients were provided, but a second growth cycle on the supernatant is not possible.
A. maxima was shown to successfully reduced eutrophication or greenhouse gas risks compared to traditional dairy manure management methods. Future work should focus on further improving the system by reducing water usage and alleviating complications associated with the bicarbonate buffer
