The key to a successful alum application is the formation of the floc and the precise placement of the floc on the lakebed. The floc is formed when the liquid alum mixes with lake water immediately after application. It is a whitish-green precipitant, is more dense than water and sinks through the water column at a rate of 1 foot every 2.5 minutes. Once it reaches the bottom, the floc’s active binding sites are in place to intercept phosphorus as it leaches from the lakebed.
HAB’s application technology maximizes the floc formation and settling by applying the alum through drop hoses that extend into the lake. There are jet nozzles at the end of each hose and the alum is applied under the pressure created from the on-board pumps. This setup effectively injects a stream of alum that flash mixes with lake water below the surface. Floc immediately forms at a depth of 2-3 feet. Forming the floc below the lake surface bypasses any interference in settling by avoiding the surface tension of the water and algal scums on the surface. It also forms the floc at a depth where it is less susceptible to wave action. All of this results in rapid settling in the target application zone, reduces drift and ensures that lake sediments receive the prescribed dose without gaps in coverage.
HAB’s application equipment is well-suited for the precision required for the Shield’s Lake restoration project. Any gaps in coverage would allow for phosphorus from untreated lakebed sediments to leach into the overlying water column and stimulate future algal blooms. An on-board computer system ensures that application is complete and uniform by integrating the location of the barge with GPS, the barge speed, water depth and alum flow rates. The exact dose is programmed into the software and inline flow sensors measure the flow of alum through the distribution lines in real time. Automated values, which are controlled by the computer, open and close as needed to adjust for barge speed and location to make certain the prescribed dose is applied. Tadd Barrow (HAB’s application specialist) follows a grid and each application path is recorded and displayed on the guidance monitor to prevent gaps in the application. At the end of the project, HAB produces a coverage map showing every application path and confirms the complete coverage of the lake.