In 2009, Clear Water had the opportunity to work with Bill Nye on a video about Advanced Stormwater Treatment utilizing Chitosan-Enhanced Sand Filtration (CESF). Watch the video below for a strong introduction to this great technology!
Chitosan is a derivative of the biopolymer chitin, which is derived from the shells of crustaceans such as crabs and shrimp. It has a wide variety of uses in different fields, from agriculture to medicine. In water filtration, it acts as a flocculent, causing suspended particles to rapidly coalesce into larger groups called "flocs" when added to sediment- and/or contaminant-laden water. The flocs either settle via gravity or are easily filtered out of the water. Chitosan is known to be "eco-friendly" because only very small amounts of it are needed to effectively treat water, and it does not persist in the environment; typically, it breaks down rapidly through microbial activity. The original patent for water treatment using chitosan was filed in 1967.
CESF is a safe and effective method for removing suspended sediment, metals, and other contaminates from stormwater runoff. It has been thoroughly tested over the course of many years on construction sites and has been granted a General Use Level Designation (GULD) by the Washington State Department of Ecology (Ecology) Chemical Technical Review Committee (CTRC) for discharge to surface waters of the state in flow-through, batch and infiltration modes. The GULD is the primary guiding document by the regulatory authority specifying the conditions of use. It specifies the approval process, allowable dose rates, monitoring, training, and design criteria.
A typical CESF system includes several key components:
Chitosan is usually added to the raw stormwater or wastewater in the "pretreatment" phase, before the water is pumped to settling ponds or containers. After being allowed to settle for an appropriate amount of time, the pretreated water is sent through a series of sand/ mixed media filters. The water going in and coming out of the filters is constantly monitored for pH, turbidity, and other parameters. If the filter effluent doesn't meet the requirements for discharge, it is sent back to the pretreatment phase to go through dosing and settling again. Water which meets permitted water quality benchmarks gets sent to a clean water holding area/ container or may go directly to the discharge point. At some sites, the water may be re-used onsite. Check out the diagram below for an overview of the process.
Before beginning a project, Clear Water always conducts treatability testing to determine the most efficient water treatment chemistry for the water at the site. Sometimes, other coagulents or flocculents which have been approved by the Washington State Department of Ecology for use in CESF may perform better than chitosan acetate. In those cases, Clear Water adjusts the CESF system to include the most effective pretreatment chemistries and dose rates. You can find out more about approved chemicals on the DOE's page for water treatment technologies.
Learn more about stormwater reuglations and permits on our resources page.