A spent-lime filtration system was built at a culvert where stormwater flows into the lake. This system uses recycled lime to filter out phosphorus, a nutrient that can cause algae blooms and poor water quality. It is designed to remove about 45 pounds of phosphorus annually from stormwater entering the lake.
Spent lime consists of calcium and carbonate and is a byproduct of the drinking water treatment process. Since this material is fresh (i.e., recently precipitated), it has properties that allow it to bind with phosphorus. When water with dissolved phosphorus contacts the lime material, calcium from the lime binds with phosphorus and forms calcium phosphate, a solid material that remains within the treatment system.
Spent-lime treatment is a cost-effective BMP, using a waste byproduct of the drinking water treatment system typically disposed of via agricultural land application. Because only a short contact time (5–10 minutes) is required for the chemical reaction to bind phosphorus to the calcium in the lime, a fairly small BMP footprint can be used to treat a significant volume of water. Additionally, the spent-lime material has a significant phosphorus binding capacity and an estimated lifespan of 100+ years (unconfirmed in the field). Routine maintenance is required, including inspection of inlet and outlet structures, annual mixing of the lime material to maintain its porosity and hydraulic conductivity, and occasional addition of spentlime material to maintain the design depth (contact time) of the material.
In December 2021, Water Resources Coordinator Josh Maxwell gave a talk about how Lake Susan's spent lime system has enhanced phosphorus reductions.
Partners: City of Chanhassen