Hydraulically speaking, the answer to this question has to do with pressure differential. I am going to assume that the ball valves lead to the plumbing that returns water to the display tank. (Note, gate valves are different and would be a better choice for this application to regulate flow quantity.) So the pressure you would have to overcome is the head pressure. As long as the "separate pump" is capable of overcoming the head pressure it will work. But keep in mind, if the "separate pump" stops, then the flow will partially reverse back through the UV, but would be restricted by the gate valve. Another thing to keep in mind, what is the maximum flow per sized plumbing. Is the combined output of the return pump plus the "separate pump" going to exceed the capabilities of the plumbing size?
