My logic is closely mirrored by the previous posts but I can't help but throw in a "I knew I was right all along" comment.
I built my own, using a pressure washer pump rated at 8 gpm and then sized the upstream and downstream plumbing
to handle 10 gpm. I use the engine driven option since I often run the engine while leaving and arriving and manuvering as necessary so it's easy to run it an hour or two a day.
It's plenty big enough for me at 160gpd capacity.
Although I've seen some really NICE home built untis, with electronics
to handle flushing
, automatic ppm sampling and divertor valves, mine is all manual. read the gauges, turn the valves, taste the outflow and when it's right, fill the tanks