Hey Tagamet,
There is no feed pump at all. Actually, when I first saw the Reeftek design, I almost cried because it was absolutely perfect for what I wanted to do, mod it so I could use it to just run it with one pump. I have two friends that have different calcium reactors (one Geo and another homemade) that run their reactors with one pump (it circulates and "feeds") at the same time and they have great tanks. They did a lot of testing at first, but the levels were good and they stopped, but they have amazing tanks... anyways... The whole concept is pretty easy.
If you have a closed circuit with one pump in the loop (ala closed loop), what goes out one end of the pump must come in on the opposite of the pump. If you control effluent flow with the ball valve, all of the excess flow goes back into the reactor to recirculate. This will actually be better for your pump as you will restrict it less as it can divert flow back into the recirculation loop, whereas a feed pump encounters high pressure resistance due to the slow flow out of the reactor.
AfricanGrey,
I think that you will find the reeftek reactor a great buy. Jorge is sending out all the new reactors with this mod and you can drive it with the one pump, although he recommends a feed pump to pressurize the reactor, although like I stated above, no pressurization needed for good calcium and alkalinity levels.
I have not been able to run it as I ran out of funds for now. I've just water tested but here is something that you can do, run it at a high rate of effluent flow to flush out any debris and let it slowly drop down and reach a steady state (once you've got this steady state going, you can then supply the CO2). You might have to do this for a couple of days, because I've found that Jorge uses teflon grease which actually can clog up the valve as the hole is very small. You might want to ask for teflon tape instead of the grease.
Peace,
John H.