I love the water treatment idea but like any piece of equipment it needs maintenance and it will only work if run properly . RO units are simple but can still drive you nuts with little things like blown o-rings and scale build up . They need a good high pressure pump to force the water thru the membranes also there is waste water that needs to be disposed of . There are many more things that I could talk about but the point is nothing is as easy as fire and forget . I'm not yet familiar with the sand process and it sounds great . Getting a steady volume and upping that volume would be key
Getting the produced water as clean a possible before entering the sand to me is important maybe that water going through a wash tank first then another pump to push that water through the sand ( and the chemical treatment ) I'm just pointing out nothing comes easy and things can breakdown somewhere in process that is the challenge but this can work , keep it simple .
This why we also need oil and that's why I want to know when do we start drilling .