How much per day?
You can always recycle it and hope that your tank is concentrating the heavy stuff.
Shaping the tank and creating a vortex in it may help.
You must also measure what you get so few more sensors are probably acceptable.
E,
If output is not enough you have two possibilities, more power or less recycling.
Industry usually has no space problems, so they use reservoirs, if possible.
You can also cascade the process, but since it's a forced operation you need more pumps.
Measuring apparatus can be just one that is selected between different membranes, if final quality is not absolute.
After process has started it is very stable, if recycling is only partial.
Measuring apparatus can of course adjust the flow constantly also, if needed.
The scale of expenses is probably few thousands, that's many qubes of raw water, but only "a drop" if need is constant.