(421f) Seawater Floating Photobioreactor for Microalgae Growth In Wastewater

            Cultivating microalgae in photobioreactors (PBRs) which are located in the ocean instead of on land offers a promising potential for future supply of oil feedstocks for biodiesel without impacting food supplies. Using wastewater to grow the microalgae vastly reduces the amount of freshwater used in the process. Also, the ocean provides a large area which can be used to grow the microalgae.

Biodiesel:

            Biodiesel, a renewable liquid fuel, has the potential to satisfy our energy needs. It is clean-burning and more environmentally friendly than petrodiesel. Biodiesel is made from the transesterification of oils. Oils from soybean and rapeseed food-crops are common feedstock used to produce biodiesel in the US and Europe, respectively. Microalgae oil is an alternative non-food feedstock for biodiesel, and can generate 15 times more oil per acre than other plants.

Biodiesel from Microalgae Feedstock:

            Both plants and microalgae need water, chemical nutrients (fertilizers), light, and carbon dioxide to grow and produce oil. Chemical nutrients, especially nitrates, are one of the most expensive parts of the cultivation process. Unlike food crops, algae can grow in fresh or impaired water and do not require fertile soil. The purpose of this research is to grow microalgae in municipal wastewater contained in PBRs which float on the surface of the ocean. Municipal wastewater is readily available, provides economic benefits by supplying nutrients such as nitrate and phosphate, and reduces the water footprint. The algae will treat the wastewater in the PBRs and clean water will be released into the ocean. The microalgae grown in the PBRs are freshwater algae. Therefore, if one of the PBRs gets damaged, and the algae are released into the ocean, the algae will quickly die because of the uninhabitable environment.

Project Goal:

            The goal of this project is to design a system of PBRs which float on the surface of the ocean, contain microalgae and wastewater and added nutrients inside the PBRs, treat the wastewater, and produce microalgae oils which can be used as a biodiesel feedstock.

Approach:

            The first step in the project was to determine the baseline case by growing the microalgae in a land-based PBR. The PBR contained wastewater with added nutrients and was exposed to light energy. Then, the microalgae were grown in wastewater contained in PBRs which were designed to float on the surface of the “ocean”.  The metrics of both cases were compared.

Measurements/Metrics:

The measurement techniques were divided into several types:

- Algae Growth: While the algae were growing, their growth was monitored through turbidity readings and cell counts. Turbidity measurements indicate the darkness of the solution. A dark solution has a lot of algae whereas a light one has very little algae. When performing cell counts, the number of algae cells was counted in a known volume of solution using a microscope and counting chamber.

- Water Quality: The water quality of the algae solution was checked by monitoring the nutrient concentration and pH during the algae growing phase to ensure optimum growing conditions. It is important to track how the algae are growing so they are harvested when the maximum amount of algae are present.     

- Algae Biomass Production: After harvesting the algae were dried and the algae production per volume of solution was determined gravimetrically.

- Algae Oil Production: The next step was to extract the algae oil by solvent extraction. The oil production per unit volume of solution was determined gravimetrically. The extraction step is very important because the oil is the biodiesel feedstock.

- Process Efficiency: The heat of combustion of the algae was measured. The purpose of this step is to compare the solar energy input and algae oil energy output, or efficiency of the process.

Outcome:

The outcome of this project is the design of a PBR system which treats wastewater and grows oil-rich microalgae at the same time. The algae oil will then be used to produce biodiesel, a clean burning alternative energy source.