With Heron Highlights, Heron Innovators presents informative articles highlighting the remarkable features of Suspended Air® Flotation.
Heron Highlights for July 2011 features the remarkably rapid rise rate of flocculated solids with Suspended Air® bubbles attached. Each bubble is covered with a chemically active film which strongly attaches to the flocculated solids in the wastewater, achieving high rate and efficient flotation. By contrast, DAF (Dissolved Air Flotation) uses high pressure to dissolve a small amount air in water with no chemically active agent, and depends on physical entrapment and a weak surface bond to float the flocculated solids.
A recent journal article reports a comparison of SAF with DAF in flotation of algae from a wastewater treatment pond. Algae are notoriously difficult to harvest. DAF has been the method of choice in the past, preferred over various methods of filtration due to problems with clogging of the filter media. Experiments by Patrick Wiley at Humboldt State University conclusively demonstrated that flocculated algae with attached Suspended Air® bubbles floated over twice as fast as with plain air bubbles produced by DAF.
The common measure of hydraulic loading rate in flotation is a rate of flow per unit of surface area, typically expressed as gallons per minute per square foot (gpm/sq. ft.). For a given flow rate of water to be treated, a higher hydraulic capacity means that less flotation area is needed, and that means a smaller equipment footprint and lower installation cost.
With the conversion of gallons to cubic feet, the hydraulic loading rate can be expressed as cubic feet per minute per square foot. This reduces to feet per minute, and represents the average downward face velocity of the water towards the effluent underflow baffle. The solids must rise faster than this if they are not to be swept out with the underflow.
In the article by Wiley et al., thoroughly mixed samples of flocculated algae were floated with dissolved air and with Suspended Air® emulsion. The time for the solids to reach the top of the sample container was recorded, and the results were expressed in terms of rate of flow per surface area (liters per minute per square meter). Table 5 of the Wiley article summarizes the results for the DAF and SAF conditions producing the best flotation performance. Converting these results to rise rate, the SAF solids rose at an average of 2.03 feet per minute and the DAF solids rose at the rate of 0.88 feet per minute.
Similar conditions to the Wiley et al. SAF flotation experiments are shown in the video to the left. Here, a flocculated suspension of ferric hydroxide is being floated with Suspended Air® emulsion. It takes approximately 6.5 sec. for all the solids to reach the top of the sample container, for a rise rate of 3.1 feet per minute. This is in line with the Wiley et al. results, and translates to a maximum allowable hydraulic loading rate of 23 gpm/sq. ft.
The video to the right shows a pilot Suspended Air® flotation demonstration conducted at Red Bluff, California in 1995 on activated sludge mixed liquor. The pilot flotation cell is 18 square with approximately 4 ft. water depth. A typical flocculated solids clump with Suspended Air Emulsion bubbles attached has a rise rate of almost 25 feet per minute. This is typical of SAF operation at many installations, and translates to a maximum hydraulic loading rate of 186 gpm/sq. ft. before washout of solids would start to occur. Because of other factors in full-scale operation, the Heron Innovators ClearFloater flotation cell is designed for a nominal 15 gpm/sq. ft., giving it a generous factor of safety, and the ability to be operated at a higher hydraulic loading rate at peak flow conditions.
This photo compares the rise rate of DAF treated solids with SAF. The Standard DAF solids took 4 minutes to rise 1 foot, for a rise rate of 0.25 feet per minute, translating to a maximum hydraulic loading rate of 2 gpm/sq. ft. The SAF solids were already at the top of the graduated cylinder 30 seconds after beginning flotation.
Remember: SAF is re-writing the book on flotation technology. Dont be fooled by imitators. Always ask for Suspended Air® Flotation by Heron Innovators.