High-flow bioretention systems target the removal of suspended solids, total and dissolved nutrients and metals, oil and grease, trash, and debris. Components typically include plants, mulch and specially designed filter media mixtures often contained in a concrete container. The mulch layer and filter media capture and immobilize pollutants during storm events. Between storms, those pollutants decompose, volatilize, and are incorporated into the biomass of the system’s micro/macro fauna and flora. Stormwater runoff flows through the media and into an underdrain system at the bottom of the container, where the treated water is discharged. The high media flow rate reduces the footprint of these systems compared to traditional bioretention.
Inspection and maintenance of bioretention systems may take less than 30 minutes and is the least expensive to maintain of any stormwater MTD. Maintenance typically does not require a vacuum truck or confined-space entry and is typically recommended twice per year. Additional maintenance may be necessary depending on sediment and trash loading. Maintenance activities may include inspection of the vault and surrounding area; removal of tree grate and erosion control stones; removal of debris, trash and mulch; mulch replacement; and plant health evaluation and pruning or replacement as necessary.
The design of engineered bioretention systems differs considerably by manufacturer, with some including components such as screening devices, filter cartridges and sediment chambers. These differences can have a significant impact on the cost of maintenance. When evaluating engineered bioretention systems, engineers should inquire as to the following to ensure long-term maintenance costs are reduced:
- Is the first year of maintenance included?
- Some manufacturers include the first year of maintenance in the purchase price.
- How long is the recommended maintenance interval?
- Is there operational evidence that the system will continue to function as designed at the recommended maintenance interval?
- Does the system utilize a pretreatment chamber with filters?
- Pretreatment chambers with filters may need a vacuum truck to extract sediment and liquid, and the filters may need to be replaced, which will add significant cost.
- Does the system utilize filter cartridges?
- The replacement of cartridges may require confined-space entry, which will add significant costs.