Removing Dissolved Pollutants from Stormwater – The Difference Between Adsorption and Absorption

Stormwater runoff is a significant source of water pollution, carrying various pollutants from urban areas into natural water bodies. Most stormwater treatment efforts have focused on the capture of total suspended solids. Still, significant pollutant loads can be attributed to dissolved pollutants, such as heavy metals, nutrients, and organic compounds, which can have detrimental effects on aquatic ecosystems and public health.

Dissolved pollutants are more mobile, bioavailable, and are captured via different mechanisms than particles. While gravity separation and basic filtration can physically remove particulate-bound pollutants, dissolved forms require active removal processes such as adsorption and absorption. While there is only a one-letter difference between the two terms, how they remove pollutants differs greatly.

Adsorption Vs. Absorption
In stormwater treatment, adsorption is widely used to remove dissolved pollutants. Adsorption is a process in which molecules or ions adhere to the surface of a solid, forming a thin layer. In effect, pollutants bind or “stick” to the filtration media. 

Absorption is a process where a substance is taken up and dissolved within another substance. The most common examples of absorption are when oil is wicked into oleophilic media or when water is absorbed by organic matter, perlite, or other porous media.  

Some media and pollutant combinations blur the line between adsorption and absorption processes. For example, adsorbed metals or other contaminants that are initially retained on filter media surface may migrate deeper into the media particle structure via micropores or by diffusion as accumulated pollutants develop a concentration gradient between the surface of the media particles and their internal volume. Compared to adsorption, absorption is more durable to bind pollutants to media.

Both adsorption and absorption methods are environmentally friendly, as they do not produce harmful byproducts or chemicals that can further pollute water bodies. When the sorption capacity of a particular media is known, the amount of media needed can be easily scaled up or down to accommodate different stormwater volumes and pollutant loads. However, in most urban stormwater settings, the occlusion of filter media due to the accumulation of sediment usually triggers the need for maintenance before the sorption capacity of the media is exhausted. 

An example of a treatment system that uses both adsorption and absorption is the Stormwater Management StormFilter, an underground device that utilizes rechargeable, media-filled cartridges that can be customized using different filter media to target site-specific pollutants. In testing verified by the Washington State Department of Ecology TAPE program the StormFilter system has demonstrated the ability to remove common stormwater pollutants using a variety of media, including zeolite, perlite, activated carbon and PhsophsoSorb.