The alum treatment on Upper Prior is progressing well and may be completed by as soon as Wednesday. The HAB crew has had some curious onlookers asking about the application technology they use on the barge and why we are so careful with the placement of alum. These are great questions and we have some excerpts of answers from the Upper Prior alum treatment website below.
Precise Alum Application
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.
The alum is injected a little ways under the surface of the lake. 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.
The application of alum from the barge is synced with an onboard computer and GPS system which ensures that there are no gaps in alum application. For more details check out the “Precise Alum Application” article on the Upper Prior Lake Alum Treatment website.
Lake Testing Ongoing During Treatment
District staff and HAB (our alum applicators) continue to test the lake every day during treatment to ensure the application is going to plan and nothing has gone awry. Testing ensures that the lake maintains proper pH levels and so far everything has been going smoothly and no problems have been detected.