This research evaluated dissolved air flotation (DAF) as a separation method for algae and phosphorus from municipal wastewater at the City of Logan, Utah Wastewater Reclamation Facility. DAF uses the supersaturation of air to raise suspended algae and other particles to the surface, where they can be easily removed. DAF, in conjunction with chemical coagulants and flocculants, can approach 95% algae and phosphorus removal. The algae removed using the DAF process will be used in the production of biofuels and bioplastics.
A pilot DAF unit was used to determine the optimum alum dose for total phosphorus (TP) and algae removal. In addition, a bench-scale jar test unit was used to study the effects of various alum and polymer doses on removal efficiencies at different times of the day. An optimal alum dose was found to be 30 mg/L based on results from both the pilot and bench-scale units. No advantage to adding polymer was found.
Algae removal efficiencies on the pilot DAF ranged from 68-70%, and the effluent algae concentration was reduced to 10 mg/L. Approximately 65% of the total phosphorus was removed, from 1.1 to 0.4 mg/L, which is low enough to meet regulations anticipated to be promulgated by the state of Utah. Using the assumption that the molar weight of algae is 3,550 g/mole, the molar ratio of Al/TSS was found to be 30.1 and the molar ratio of Al/TP was found to be 7.5.
Extracellular polymeric substances (EPS) excreted by algal cells act as a natural flocculant and may allow for chemical usage to be minimized. Autoflocculation and bioflocculation, natural processes caused by EPS production and an increased pH level, were not observed to be a significant factor.
The chemical dosing rates provide the City of Logan with basic operational parameters for a full-scale (15 million gallons per day) DAF plant, providing an effluent phosphorus level below 0.5 mg/L. The alum will cost $1,118 per day, with a daily electrical cost of approximately $149. This full-scale DAF plant would harvest 1,563 kg of algal biomass per day, with a cost per kilogram of algae at $0.81

Elder, Andrew R.

English

Research being completed at the Logan, UT wastewater treatment plant could provide a locally-sourced gasoline replacement by collecting algae that grows naturally. This process would also provide the City of Logan with a method to meet certain standards set by the state of Utah.
Problem and proposed solution:
The water leaving Logan’s wastewater treatment plant has high levels of phosphorus and nitrogen, two elements that are found in all wastewater treatment plants. These two substances need to be controlled, because they kill fish and cause pond scum to grow in recreational reservoirs downstream. A unique method has been found to remove these elements.
The wastewater treatment plant is a lagoons design, consisting of a series of large, open-air ponds. Algae naturally grow in these ponds, and consume the phosphorus and nitrogen found inside. If the algae can be harvested from the ponds, the phosphorus and nitrogen can be removed as well. As an added benefit, the algae can be used to create a gasoline replacement, known as biodiesel.
Current research:
One research project completed at the Logan wastewater treatment plant was the optimization of a method to harvest this algae. Andrew Elder worked with a device called a Dissolved Air Flotation, or DAF, unit. This machine uses microscopic bubbles to lift the algae to the surface of the water, where it can be easily removed. (Removing the algae removes much of the phosphorus). The phosphorus not in the algae cells is removed by the small amount of chemical additives required for the DAF process to work.
The goal of this research project was to determine the optimum amount of chemicals required for proper algae harvesting. Additionally, there is a possibility that chemicals naturally secreted by the algae could help eliminate some additional chemical addition.
Two different methods were used to determine the proper chemical addition. First, a large DAF unit was used to determine the proper chemical dose. Next, a miniature DAF unit was used to study different chemical doses at different times of day. There is evidence that different amounts of sunlight could increase the quantity of these natural chemicals being produced.
Results:
The optimum amount of chemicals needed was determined to be 30 mg of aluminum sulfate per liter. The naturally-produced chemicals were found to have no significant effect on algae harvesting, at any time of day.
Conclusion:
This project demonstrated the ability for a DAF system to successfully harvest algae from the Logan wastewater treatment plant, with only minimal chemical addition. Along with meeting state of Utah requirements, this process could allow the Logan wastewater treatment plant to collect enough algae to create a significant amount of a locally-based, sustainable automobile fuel. Enough algae could be grown to fuel all of the City trucks with biodiesel. This process would save the City money and be beneficial to the environment at the same time

DigitalCommons@USU

Optimization of Dissolved Air Flotation for Algal Harvesting at the Logan, Utah Wastewater Treatment Plant

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2050&context=etd

Optimization of Dissolved Air Flotation for Algal Harvesting at the Logan, Utah Wastewater Treatment Plant

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