First Flush

September 29, 2015

Our only prefiltration system (except gutter guards) is the first flush diverter. It's a wonderfully simple system that allows the dirtiest water (the first to come off the roofs) to get diverted and disposed rather than entering the tanks. It is essentially a simple chamber to collect the water. It has a small nozzel on the outlet end so the water drains (much) slower than it fills. As the dirty water fills collection chamber, a ball floats up to the top and eventually blocks water from entering the chamber any more. At this point, water backs up and starts flowing to the tanks. The ball isn't essential, but it keeps the dirty water from mixing with the clean water once the chamber is full.

We built the chamber from two 6' lengths of PVC that Rob had laying around, plus a coupler he found on eBay. That's a bargain.

The tricky part of this system is getting the water to divert. Our first design and implementation was a complete failure. Originally, we just made the main line rise about 5" after a T. The idea was the flowing water would hit the rise, and rather than jumping the hump, it would divert in to the T until the chamber was full, and then it would have no where to go but over the rise. Nope. That water had momentum. It hopped that rise like it wasn't even there. Only a token amount of water made it into the chamber.

Back to the drawing board.

The second design is much awesomer. Rather than relying on static water to fight the momentum of the incoming water, it uses the incoming momentum to our advantage. The first place the water wants to go is in to the chamber. Only once the chamber is full will the water back up enough to take the diversion, go over a hump, and get to the tanks. Duh.

The last piece of the system is dealing with the output. For some reason we thought it had to go downhill until it reached daylight to drain. After awhile, the very helpful man from the manufacturer reminded us that we can let it drain underground to a leachfield. Duh #2.

One of our main concerns was wasting too much water. The chamber itself holds about 70 gallons, before water diverts to the tanks. Additionally, after the rain event, about 30-60 gallons more will be backed up and eventually wasted because of its inability to get over the diversion hump once it loses momentum and mass. It seems like a steep price to pay for each rain event, but at least it's a fixed cost and not a percentage loss as the basket filter would have given us.

Here is the first flush system.

Description	Cost
Assemble and connect (10.5)	$840
Attempt to glue ff in rain (2 hrs)	$80
Install new ff (5 hrs)	$400
First flush kit (x2)	$194
12" PVC coupler	$45
Glue	$21
Total	$1580