The First Flush Diverter is the unsung hero of a rainwater harvesting setup. It sacrifices itself to catch the bird droppings, dust, pollen, and roof grit that wash off during the first few minutes of rain. However, because it collects the dirtiest water imaginable, it requires regular maintenance. A neglected diverter can become a smelly, sludgy mess that blocks water flow and attracts mosquitoes. Here is how to clean it properly.
How Often Should You Clean It?
The frequency of cleaning depends entirely on your surrounding environment. If your roof is under heavy tree canopy, you may need to clean it every month. For unshaded roofs in drier climates, every 3 to 6 months might suffice. As a general rule, we recommend checking the diverter immediately after the first major rain following a long dry spell, and then establishing a routine from there.
Step-by-Step Cleaning Guide
Step 1: Drain the Chamber
Most commercial first flush diverters feature a slow-release drain valve (drip valve) at the very bottom. Over the course of a few hours after a rain event, this valve slowly empties the chamber, resetting it for the next storm. To begin cleaning, fully unscrew this valve to ensure all standing water has drained out into your garden or dedicated soakaway.
Step 2: Remove the End Cap
Most standard diverters are constructed from 3-inch or 4-inch PVC pipe with a threaded end cap at the bottom. Use a strap wrench if it's tight, and unscrew this large cap. Be prepared: a slug of dark, thick sludge will likely drop out as soon as the cap is opened. Have a bucket ready beneath it.
Step 3: Access the Internal Filter and Float
Depending on your model, there will usually be a plastic float ball and a coarse strainer immediately inside the housing or attached to the end cap. Carefully remove these components.
Step 4: Wash the Components
Take the end cap, the drain valve, the float ball, and the strainer to a garden hose. Wash them thoroughly. Pay special attention to the tiny hole in the slow-release drain valve. Use a piece of wire or a paperclip to ensure this hole is completely clear of debris. If this hole gets blocked, the diverter will remain full of water and will not catch the dirty first flush during the next storm.
Step 5: Flush the Main Pipe
Take your garden hose, turn it on full blast, and spray water up into the empty main PVC chamber of the diverter. This will dislodge any sludge stuck to the walls of the pipe. Let the water run until the water falling out of the bottom is completely clear.
Step 6: Reassemble
Once everything is clean, reinsert the strainer and the float ball. Screw the large end cap firmly back into place (hand-tight is usually sufficient; do not over-tighten, or it will be impossible to remove next time). Finally, screw the slow-release drain valve back into the bottom of the cap.
Conclusion
Cleaning a first flush diverter is a dirty job, but it only takes about 10 minutes. By maintaining this vital piece of equipment, you ensure that only the cleanest possible water ever reaches your storage tank, saving your downstream filters and drastically improving water quality.