I have fitted a commercial " first flush diverter " to a small...

I have fitted a commercial " first flush diverter " to a small tank outside the kitchen.
We use the water for drinking, tea /coffee and cooking.
It is a similar set up to yours lucky.
But it does have a release valve on the bottom of the catchment pipe.
And a float which seals off the catchment pipe after it fills.

The tank is plumbed into a tap over the kitchen sink and we also have a filter between the
tank and the kitchen tap.
It works very well.

I bought it a couple of years ago from Bunnings

I note from a google search there are several different ones available.

The link below also sets out the size and length of pipe required to catch the appropriate volume
of water from the first run off with the sediment on the roof etc in it.
This is dependant on the catchment area ( roof size )


http://rainharvesting.com.au/product/downpipe-first-flush-diverters/