What would be involved ...
Why reinvent the wheel?
Simple inlet-outlet filter systems offer effective treatment without electricity and little or no wasted water. However, these systems do not incorporate an automatic back wash cycle which is designed to redistribute the media, dispose of media fines and minimize particulate accumulation on top of the media bed.
Down flow systems pass through a compacted media bed providing for increased contact time which improves contaminant reduction and overall capacity. However, the accumulation of particulates within the media tank will eventually lead to severe channeling. Water following the path of least resistance flows through only a small portion of the media resulting in diminished performance and flow restriction.
A thorough manual back wash upon installation is essential and periodic back washing (monthly is typically adequate) will greatly improve the long-term performance and capacity of a down flow inlet-outlet GAC system. Adding an automatic valve (Pentair 5810, 5812, 5600, or Clack CK10, WS155) eliminates the need to remember.
But is there a water check ( i have a test kit) I can do on the Rinse water tank and deem the membranes are bad/ carbon needs to be changed?
Your test kit is likely just testing water hardness.
You'll want a chlorine test kit...easily found at pool and spa dealers...to determine if the GAC is exhausted. And, a TDS meter (continuity tester) for measuring the Total Dissolved Solids in your spot free rinse tank. Cheap meters are sufficient and you can find them online. Heck, ZeroWater even includes a reliable TDS meter with every drinking water pitcher. TDS readings of less than 50ppm is good. TDS above 50ppm will spot.