Algal blooms in the reservoir of a direct filtration plant can cause a rapid reduction in filter run times. One technology that can help solve this problem is dissolved air flotation clarifiers installed in front of the filters.
Dissolved air flotation (DAF) is a clarification system that removes algae and other low-density particles or light floc from surface stored waters, and, in some cases, ground water. It works by creating a small floc using standard chemicals. No polymer is normally required.
Algae naturally want to float, so DAF attaches microscopic bubbles to the floc and floats them to the surface of the clarifier, creating a thick surface sludge blanket. Clarified water is almost totally free of algae, with many of the problematic species, such as Microcystis, Aphanizomenon, Anabaena, and Fragilaria, being removed along with their associated taste and odor problems in the finished water supplies.
The DAF process creates a sludge that actually lifts out of the water by the trapped air, so it free drains and can get very thick, removing the need to build thickeners. Loading rates on DAF clarifiers have been tested up to 10 gpm/sq. ft. on good quality reservoir waters. The DAF process can be very compact for retrofitting into existing areas on a direct filtration plant that is experiencing filter run time problems.
Flexible in operation, the process starts quickly and produces clarified water with low turbidity. It works well if the water treatment plant needs to be run in an enhanced coagulation mode to remove TOC. The solids loading on the DAF is increased in this mode of operation; however this means an adjustment on the desludging timer is required to pull the sludge off on a more regular basis. If the problem is seasonal, the system can be either left on or turned off.
The F.B. Leopold Company, Zelienople, Penn., will display its dissolved air flotation equipment at the AWWA Annual Conference and Exhibition.
