For years, water treatment plant operators have struggled with the difficulties of trying to settle out very light floc in vast sedimentation basins, often resorting to expensive chemicals and polymers to get an adequate water quality. In these times of declining capital availability, shortages of land in crowded cities, with the public demanding high drinking water quality, I am pleased to see new technologies being tried. One that springs to mind is dissolved air flotation (DAF).
The idea behind this concept is that bubbles are good (something that the Friars of the Champagne region of France have claimed for centuries). If gravity is not enough to convince a floc particle to descend, then maybe a bubble or two will float the floc to the surface. The concept is simple and so is the practice. One of my group of companies, F.B. Leopold, has been using a mobile water treatment plant to demonstrate the process to consulting engineers and municipal operators all over the United States. The beauty of the system is the application to almost all water quality types: colored water, high-TOC water, manganese and iron-bearing waters, algae blooms and more.
The process operates like this. Take a percentage of the clarified water output and pump it into a pressure vessel at 80 psig. Into the pressure vessel goes a measured amount of air, which at this pressure dissolves into the water. When this is passed through special nozzles, a cloud of micro-bubbles forms, turning the water a milky white color. These bubbles get caught in the floc, or floc forms on them, reducing its density. The floc simply floats to the surface. A skimming device removes it as a high-solids sludge, and the clarified water goes to the filters for polishing.
The quality of the DAF can be exceptional, reaching 0.08 NTU on one occasion, leading the operator to say, “So what do we need filters for?” As Leopold has a 75-year history of filtration knowledge to draw on, they are well-placed to say that filters are the primary treatment process and that the DAF is there to help prevent variations in raw water quality.
Getting the lowest turbidity possible out of the clarifier is often not the best way of optimizing the water treatment process. Leopold makes sure the system is optimized by using a mobile pilot plant with on-board coagulation, flocculation, DAF, filtration and computer-controlled data management.
Typically, a pilot study generates 2 million results on water quality. That should be enough to prove to most people that these new technologies not only work but provide a lot of the answers to the growing needs of the municipal water industry.
