Treating stormwater for heavy metals can be complicated and expensive. Catch Basin Inserts (CBIs) are a low-cost alternative that may be appropriate as a primary treatment method for some applications, particularly total suspended solids (TSS). They can work well for facilities that are close to meeting benchmarks for metals, where the pollutants are composed of large particles and a low amount of dissolved metals. How do you know if a CBI will work for you?
Catch Basin Inserts function as mechanical filters. CBIs can function for small particles down to 50 microns (50 millionths of a meter) up to hundreds of microns. In general, the finer the particle the CBI can retain, the faster the CBI will clog with particles, or “blind.” CBIs are an ideal solution when a facility can filter out enough particles to meet benchmarks. They are also effective pre-filters for more sophisticated treatment systems. CBIs do not function well in environments with high levels of TSS or debris in the stormwater.
Catch basin inserts can provide a nominal amount of treatment for oils and suspended solids, but cannot treat chemical oxygen demand (COD), phosphate, nitrate or other regulated substances like dissolved metals.
There are several products that claim to treat dissolved metals in stormwater using chemicals or media integrated into the filtration media of a CBI. In general, CBIs CANNOT reliably treat significant amounts of dissolved metals. This is true for two reasons: 1) The residence time in a CBI in not adequate to affect chemical reactions, adsorption or ion exchange, and 2) the volume of chemicals or media in enhanced CBIs is too short to function economically.
Keep in mind that regulated metals represent a small fraction of the total number of ions in a typical stormwater stream. In areas like the Pacific Northwest one acre receives about one million gallons of precipitation per year. The combination of high volume of water with a large number of ions means that a “reactor” as small as a catch basin can only support a chemical process for a very brief period of time.
Choosing an insert is a simple process. First – collect a typical sample of the stormwater to be treated. Second – have the sample analyzed for particle size distribution. (Contact us if you need a referral to a laboratory). Third – have the sample analyzed for both total and “dissolved” metals. Dissolved metals are generally those smaller than 40 microns.
If dissolved metals are significantly higher than the benchmarks, then CBIs will probably not work as a sole treatment method. However, If you are close to meeting benchmarks and the particle size distribution has a significant amount of large particles (>100 micron) then a CBI might be all you need. Also remember that a well-chosen CBI media can greatly extend the life of down-stream treatment systems, and may still be a good business decision even though it cannot meet benchmarks by itself.
Armed with data and a good plan, you can then make informed decisions about how you can use Catch Basin Inserts alone or in conjunction with other treatment technologies.