Let's talk that through before you go there
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There are two major concerns here: 1) Equipment failure causes the ATO to "Fail On" and kills your take and destroys your house and 2) the intermittent on/off cycling due to micro-dosing of RO water degrades the membrane performance AND quickly exhausts your DI due to a phenomena known as "TDS Creep".
The best way to solve problem one is to "air gap" the RODI from the tank so a Fail On does not flood the sump. This requires a storage container and a dedicated ATO pump. If the ATO pump fails on, you can only pump what is in your bucket. If you really don't want to do this (or can't), then you must include some kind of redundancy that is exterior to your ATO controller. For example, if you use a standard ATO system (pick a brand, any brand), a lot of them will use some kind of float or optical sensor that is connected to a powered unit and the powered unit controls a top off pump and/or RO/DI solenoid valve. If you have your RODI directly connected, I would plug the ATO into a controlled outlet that switches off if a mechanical float in your sump goes off. So if the ATO fails, you have another level of failure mitigation. Some ATO units have redundancy built in and some are higher quality than others. But at the end of the day, you are trusting thousands of dollars in investment to a single chip controller that costs a few tens of dollars at most. If you direct connect, have a backup.
The TDS creep is a little harder to understand and mitigate. Every time you shut the RO unit off, the TDS on the supply (intake) water and permeate (clean water) side will equalize. This is counter-intuitive, but it is because RO membranes are not "Filters", thus the salt/TDS can flow both ways across the membrane. It takes a pressure and flow to get clean water through the membrane without the salt. Remove either one and salt can easily go through the membrane. The only mitigation to this is either 1) never turn the system off
, 2) waste the first gallon of RO permeate each time the unit turns on, or 3) batch your RO/DI water is large enough quantities that the first gallon of water having high TDS just is not significant. Sure, the DI resit will remove this TDS, but at the cost of exhausting your DI resin. In an automated setup where you cannot flush the water, you end up just spending a lot more money on DI beads. There is no consequence to your tank....just the wallet.
BTW, no RODI systems for sale in the hobby have any means to deal with TDS creep, you just have to replace the DI cartridges more often if you perform many smaller batches instead of fewer larger batches. I learned through experimenting that wasting that first gallon of permeate makes a huge difference in how long your DI lasts. I was working on automating that before Hurricane Ida forced me out of the hobby.
Yes and no. The equipment has come a long way, but the basics are exactly the same. Unfortunately, what I see is over time the equipment has gotten far more expensive despite more mass production and lower quality despite becoming more expensive. The fundamentals remain the same.
