Phosphorus is the second-most-regulated pollutant in stormwater runoff after total suspended solids. However, engineers and regulators are still learning how to remove phosphorus from stormwater to promote healthy waterways and meet agency requirements.

One reason phosphorus is difficult to remove from stormwater is that it can be found in different chemical forms: organic, inorganic, particulate-bound, dissolved, etc. This speciation of phosphorus makes it challenging to remove from urban runoff.

In stormwater runoff, phosphorus is typically measured by Total Phosphorus (TP), Total Dissolved Phosphorus (TDP) and Orthophosphate (OP). TP includes both particulate-bound and dissolved phosphorus. Particulate-bound phosphorus is often attached to small silt and clay particles due to their high positive charge and large surface area. However, if the particles are small enough to pass through a 0.45-micron filter, they are classified as dissolved and measured as part of TDP. OP is the simplest form of inorganic phosphorus and is immediately available for uptake by algae and aquatic plants.

Dissolved phosphorus is much more difficult to remove than particulate-bound phosphorus because it relies on different treatment mechanisms. Gravity separation and basic filtration can physically remove particulate-bound phosphorus; however, dissolved forms require active removal processes such as adsorption or precipitation.

Further, while a portion of the phosphorus load may be bound to particulates, it may not stay that way, as it can dissociate into the dissolved phase. For example, phosphorus attached to organic compounds in leaves can transform into dissolved phosphorus as the leaves decompose. This phenomenon can be observed in the spring and fall when flowers and other plants die and decompose. It can explain why seasonality affects the ratio of TP to OP and why algae blooms within water bodies are more frequent during certain seasons.

Meeting TP removal goals often means treating both the particulate-bound and dissolved phosphorus phases and retaining any captured particulate phosphorus that may dissociate into the dissolved phase. Standard stormwater treatment practices designed for TSS treatment can adequately remove particulate-bound phosphorus from urban runoff. However, removing and retaining dissolved phosphorus is more complex and requires specialized treatment such as sorptive filtration and bioretention.