Yeah, I think because of your backround in water purification you know mostly the kitchen type set ups. The Hydro Store here has a r\o that I believe could fill a 100 gallon resi in certainly less than the 20-50 gallon per day you are describing. I think they are made especially for growing herbs, so naturally the membrane would be larger as you are saying. I will let ya know what I decide. Are you in CA....where abouts?