What is the minimum recommended filter media depth for Level 2 Bioretention Filter and Basin?

• A. 12 inches
• B. 36 inches
• C. 6 inches
• D. 48 inches

Answer: (B)

Depth of filter media determines how much treatment and storage the bioretention system can provide. A deeper media layer gives more volume for water to pass through, increasing contact time with the media to remove sediments and pollutants, while also supporting a more robust root zone for vegetation that aids stabilization and pollutant uptake. For Level 2 designs, the minimum filter media depth is 36 inches, ensuring adequate treatment capacity and storage for typical storm events and reducing the risk of clogging over time. Shallower depths, like 12 inches or 6 inches, don’t provide enough treatment volume for the higher loads of Level 2, while a depth of 48 inches is not the minimum unless site conditions require it.