Terminology

• Screen / debris excluder: removes leaves, sticks, pine needles, and trash
• First-flush diverter: sends the first volume of runoff to waste before the rest flows to storage
• Roof washer: device that traps the first flow and routes it away; often combined with a first-flush diverter
• First-flush volume: the volume diverted per storm
• Floating ball diverter: chamber that fills, then a floating ball seals the outlet so later water goes to storage
• Borehole drain: small hole that slowly empties the diverter after the storm
• Pre-filtration: screening upstream of storage

Standards

• First-flush volume rules of thumb: about 1 gal per 100 sq ft of roof for easily cleaned roofs in clean settings, and about 1 to 2 gal per 100 sq ft in Texas guidance; a TWDB training approximation is about 10 gal per 1,000 sq ft
• Contamination: roughly halves for each mm (0.04 in.) of rain flushed away
• Turbidity design method: needed volume from the roof's turbidity decay rate; a target of 20 NTU entering the tank is usually sufficient, 5 NTU leaving; potable use needs filtration to under 1 NTU
• Effectiveness: a 2025 study found diverters do not ensure E. coli removal but may reduce the risk of contamination
• Order: a state code requires the first-flush device to remove debris before water reaches storage and to follow pre-filtration
• Plumbing code: screen, diverter, and testing provisions are collected in the plumbing code reference

Components

• Downspout screen or leaf guard: at each downspout
• Debris separator or vortex pre-filter: removes large debris before storage
• Diverter chamber: standpipe or tank sized to the first-flush volume
• Drain: borehole or partially cocked valve that slowly drains the chamber
• Cleanout cap: removable cap for cleaning clogs and debris

Detailing

• Reset between storms: the chamber must drain slowly so it is empty for the next rain; a very fast drain wastes water, and closely spaced rains are then over-diverted
• Prevent full closure: keep the drain valve slightly open and add a stop or sign so users do not shut it fully
• Clog resistance: place the drain hole above the settled solids; provide a cleaning tool
• Discharge: send diverted water to a landscape area or drain; it can water ornamental plants or clean equipment
• Each downspout: its own screen and first-flush device where multiple leaders feed one tank
• Access: reach for cleaning without ladders or confined entry
• Test: fill the chamber with water to confirm operation; pour water upstream to confirm diversion

Uses

Specific uses

• Residential cisterns
• Commercial rainwater systems feeding flushing and irrigation
• Potable systems, where followed by filtration and disinfection

Pros and cons

Pros

• Removes a large share of roof pollutants
• Reduces sediment in the tank
• Simple designs have few moving parts

Cons

• Wastes some captured water
• Needs regular cleaning
• Users may defeat the drain by closing the valve

Manufacturing and cost

Construction

• Plastic or metal chambers and pre-filters that fit on downspouts or in the underground collection line; floating-ball designs are more durable than tipping designs

Main manufacturers and suppliers

• Rainwater Management Solutions
• Graf USA

Trade association: ARCSA, American Rainwater Catchment Systems Association.

Sources

• Grafman, L.: To Catch the Rain, section 2.4 first-flush, https://eng.libretexts.org/Bookshelves/Environmental_Engineering_(Sustainability_and_Conservation)/Book%3A_To_Catch_the_Rain_(Grafman)/02%3A_Components/2.04%3A_First-Flush
• Grafman, L.: To Catch the Rain, section 2.3 screens, https://eng.libretexts.org/Bookshelves/Environmental_Engineering_(Sustainability_and_Conservation)/Book%3A_To_Catch_the_Rain_(Grafman)/02%3A_Components/2.03%3A_Screens
• Grafman, L.: To Catch the Rain, section 4.4 first-flush sizing, https://eng.libretexts.org/Bookshelves/Environmental_Engineering_(Sustainability_and_Conservation)/Book%3A_To_Catch_the_Rain_(Grafman)/04%3A_Calculations/4.04%3A_First-Flush_Sizing
• Virginia Administrative Code: 12VAC5-635-240, https://law.lis.virginia.gov/admincode/title12/agency5/chapter635/section240/
• University of Florida IFAS: cisterns for nonpotable use, https://www.ctahr.hawaii.edu/hawaiirain/Library/Guides&Manuals/FL_Cisterns_NonPotable_Use.pdf