rain water
Roofwater harvesting using slow sand filtration

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> First flush diverter 
2500 gallon cistern
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First posted 2010-09-02
page last updated 2012-02-10

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First Flush Diverter drawings and pictures


A first flush diverter "diverts" the most contaminated "first flow" of rainwater away from the storage system. Because water is an excellent solvent; after a dry spell, the first water runoff from a roof surface will contain the most contamination. The diverter is set up to divert between 10 and 25 gallons of water per 1000 square feet of roof surface into a seperate holding space; and then seal up and allow the rest of the runoff to flow into the storage/filtering system. When the rain stops, the diverter slowly empties the contaminated water from the holding space so it will be ready for the next rain event.
Diverter drawing The drain faucet is set to drip slowly enough to allow the holding space to empty between rain events. This diverter's drain is set at the same rate as the water flow from the downspout in a very light drizzle - taking about 3 days to empty. The correct setting will vary depending on the geographical location and amount of pollution in the air.

diverter




Detail of float device As the holding space (the inside of the barrel below the "barrel top" in the drawing) fills, the table tennis ball floats on the surface of the water until it gets to the 1 1/4 inch pipe with beveled edges, then it seals off the holding space and the clean water flows into the filter/storage system through the "output to filter and storage system" pipe. Note that the output for the clean water inside the 4 inch pvc is upside down. This further insures that all of the most pollluted water is kept out of the clean water pipe while the temporary storage is filling. When the rain stops, the holding space empties and the ball drops down to let contaminated water into the holding space at the start of the next rain event.

float seal




This is a picture of the diverter
diverter picture