Optimization of dissolved air flotation for algal harvesting at the Logan, Utah wastewater treatment plant

his 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 benchscale 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...