Futurology

Could be, but isn't, which is where some regulations probably need to come in. I'm familiar with the systems Plenty uses, and it's all automated.

Prime > start > feed > dump once dead

I've not seen another of these large scale startups doing anything different as of yet, which does make sense cost-wise. Any crop you grow won't ever use an exact amount of nutrients at cycle end with a completely neutral byproduct, and trying to reuse what is left would require a lot of expensive lab efforts which they don't care to invest in.

Example: say you start with a 9N-12P-34K solution, and after a month it degrades to 0-0-12. You can't just refill the nutrients with that same mixture you started with, or you'll damage or risk killing the crop with too much Potassium. You'd need to analyze the loop nutrients to know what level you're at for each nutrient, and adjust to get the mixture right to recharge properly. Currently all these systems just dump and recharge because it's cheap (for now) and easy, but these high concentrations of the various components just end up saturating an area the same as farm runoff. Even if you filter, that filtered medium needs to go somewhere.

There are fancier methods of nutrient filtration extraction and recapture just starting to become more feasible, and we should be looking at making sure these are being used for these large operations.