In Greensboro, the wet detention pond is the most commonly used structural SCM for stormwater quality enhancement. This SCM improves stormwater quality by detaining runoff for an extended period of time to allow pollutants suspended in the runoff to settle out. As runoff enters the pond, its velocity is dramatically reduced, allowing suspended pollutants to settle.

Many pollutant particles found in stormwater runoff are very small and, because smaller particles settle slower than larger particles, the pond is designed to provide adequate detention time so smaller particles can settle out.

Wet detention pond components that help increase pollutant removal efficiency include the permanent pool, temporary pool, and forebay. The permanent pool prevents settled particles from re-suspending when runoff flows into the pond. The temporary pool is storage above the permanent pool used to control runoff during a storm event. To increase the detention time of the runoff, the temporary pool is slowly released. A separate smaller pond, called a forebay, is placed upstream of the main pond to trap a majority of the suspended solids in the runoff before it enters the main pond. Learn how wet detention ponds are inspected.

The bioretention system is a relatively new concept that mimics forest ecosystems to enhance stormwater quality. A bioretention system consists of a depression in the ground filled with soil media mixture, mulch, and plantings. It is designed to resemble a landscaped area, which gives the SCM a very natural and appealing image. The way runoff flows through the bioretention system is very similar, on a smaller scale, to watering potted plants.

Stormwater runoff enters the bioretention area and is temporarily stored in a shallow pond on top of the mulch layer. The ponded water then slowly filters downward through the soil media mixture and is absorbed through the plantings. As the excess water filters through the system, it is collected by an under drain pipe and discharged to a storm conveyance system.

Underground detention facilities are structural SCMs designed to provide temporary storage of stormwater runoff for quantity control purposes. The systems are typically installed beneath parking lots, streets, and parks to maximize property usage and lower development costs. Underground detention system design measures must be taken to trap and store sediments in locations where clean-out and maintenance can be easily performed. Dry extended detention ponds are designed for the water to exit the pond through the principal and emergency spillway.

Filtration systems used for stormwater treatment work similarly to those used in the drinking water purification process. Stormwater filtration systems consist mainly of a pretreatment, or sedimentation area, and the filter area. Runoff first enters the sedimentation area where the velocity is reduced, which allows larger pollutant particles to drop out. When the stormwater leaves the sedimentation area, it is spread evenly over the filter bed, where it flows downward through the filter media. As the stormwater flows through the filter, the filtration media traps and absorbs pollutants.

Different filtration media can be used, such as sand, peat, and compost. Filtration systems are beneficial when land space is scarce or expensive because they can be designed for underground or to border the perimeter of a parking lot or other impervious surface.

By building wetlands to treat stormwater, we can try to reproduce the superior pollutant removal capability of natural wetlands. Wetlands remove pollutants primarily through physical filtration and settling and by biological processes of wetland plants. This SCM is somewhat similar to wet detention ponds in that they both have a permanent pool and a temporary pool. Generally, stormwater wetlands have a shallower permanent pool than wet detention ponds so that wetland plant species can thrive in the basin. Runoff captured by the wetland area first enters a micropool or forebay, which is a relatively deep pool that promotes initial settling of larger pollutant particles. The stormwater slowly flows through the wetland's shallow areas where plants filter suspended pollutants and reduce nutrient pollution through uptake.