Offgrid living

When the rain is on the way from the roof drains to the storage tank, make it go through a tee with the side of the diverted to a first flush drum. When rain starts to flow, it will go into the first flush receiver first, but once that is full, water will back up in the pipe and then water can continue on its way to the storage tank. Every house is a little different so how that is arranged is a little pipe engineering puzzle. I used a closed-top drum, and provided a vent pipe that extended up above the roof drain pipe. So the drum filled completely to the top. Note that the PRESSURE of water depends on the height of the water column, not the amount of water, so if the vertical pipe down to that sealed drum were too great, the pressure could make the drum rupture (even if the pipe to the drum were only a small diameter).

I had a drain valve for the bottom of the drum. After it had stopped raining and a day or two passed and no more rain - I would drain it, then close the drain to be ready for the next rainy period.

I used HDPE plastic for my drum. I bought a galvanized pipe niple the same size as the PVC pipe I was going to use for my piping system. I cut four slots across the threads with a side-grinder, so I could bend the threaded part of the niple into a little more of a taper, and used that invention as a thread-cutting tool. I drilled a hole into the drum with a hole saw a little smaller than the PVC adapter, then screwed in the modified pipe niple to cut threads. After that, I could screw in the PVC adapter with some teflon tape. They make fittings called "bulkhead" fittings for connecting pipe to a tank, but they are expensive.