Technical Guidance: Stormwater Treatment Credit for Rainwater Harvesting Systems

Revised September 22, 2008

Overview:

It is the policy of the Division of Water Quality to enable and encourage the use of rooftop rainwater harvesting systems (cisterns) to reduce stormwater runoff pollution from an individual site. Collecting and storing rooftop runoff, and providing a consistent, dedicated, and reliable end use, will reduce the volume of runoff and enable the reduction in size of other required stormwater treatment systems on the site. In watersheds requiring nutrient removal from stormwater, dedicated uses of the collected rainwater or proper treatment/infiltration can reduce stormwater nutrient removal requirements. This policy establishes the credit that will be allowed in DWQ permitting programs that consider impervious built upon areas (BUA) and that rely on calculations of runoff volume and peak flow for sizing stormwater Best Management Practices (BMPs).

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What a great way to control stormwater.