Why Pretreat Your Stormwater Infiltration System?

Infiltration systems can be an effective way to manage stormwater, reduce runoff, and support groundwater recharge. But like any stormwater BMP, they need to be protected if they are expected to perform over the long term. That is where pretreatment comes in. Pretreatment removes trash, sediment, debris, and other pollutants before runoff enters an infiltration system. This can help extend the life of the system, simplify maintenance, reduce long-term liability, and support local stormwater compliance requirements.

Protecting the infiltration surface

Infiltration systems depend on open void space and the ability for water to move into the surrounding soil. When sediment enters the system, it can accumulate on the infiltration surface and reduce performance over time. Once an underground infiltration system begins to clog, restoring capacity can be difficult and expensive. In some cases, maintenance may require confined-space access, specialized equipment, or partial system reconstruction. Pretreatment helps reduce that risk by capturing sediment and debris before it reaches the infiltration system.

Making maintenance easier

A good pretreatment strategy can move routine maintenance to a location that is easier to access. It is usually much simpler to clean a pretreatment structure than to remove sediment from an underground chamber, pipe gallery, or rock bed. This can reduce maintenance liability and help lower lifetime costs for the owner. In many cases, the cost of the pretreatment BMP will more than pay for itself after one or two maintenance cycles. Also, for engineers, this is an important design consideration. The goal is not just to get the system approved. The goal is to design a system that can be maintained for the life of the project.

Supporting local requirements

Pretreatment may also help satisfy local municipal or Water Board expectations for infiltration systems. Many agencies are increasingly focused on protecting groundwater quality and ensuring that infiltration BMPs do not become long-term maintenance problems.

For example, the California State Water Board is developing a Statewide Urban Stormwater Infiltration Policy intended to provide a clearer, risk-based framework for urban infiltration projects. While the final requirements are still developing, the direction is clear: infiltration design will continue to receive close attention, especially where groundwater protection and long-term performance are concerned.

Pretreatment options

Hydrodynamic separators, such as the CDS, are commonly used for pretreatment upstream of infiltration systems. CDS has been used in stormwater applications for decades and is approved through the Washington Department of Ecology TAPE program for pretreatment applications.

For sites with higher flow rates or trash compliance needs, other pretreatment options may also be appropriate. A DSBB, for example, can provide a practical option where high flows, debris loading, or full trash capture requirements need to be considered.

The main takeaway

Pretreatment is not just an add-on. It is a key part of protecting an infiltration system. By removing sediment, trash, and debris before runoff reaches the infiltration area, pretreatment can help extend service life, simplify maintenance, reduce owner liability, and support compliance with local stormwater expectations. For many projects, the better question is not whether pretreatment is worth considering. It is how to provide pretreatment in a way that is simple, accessible, and maintainable for the life of the project.