The membranes used in the RO filter's can vary in capacity and rejection ratio. The better membranes are rated at 75 gpd or higher. One of the top brands is Dow Filmtec. An equivalent filter is an applied membranes filter (ie the Dow membrane manufacturer).
Be leery of anyone who says they obtain zero ppm water from a RO filter.
Even the best RO membranes have only a 96% to 98% rejection rate. That means if your water goes in at 400 ppm TDS, it will come out at bets with a TDS of between 8 and 16 ppm. A good DI cartridge could drop that to Zero ppm. Does that means everything but H2O has been removed. No. It just means conductive salts have been removed.
However in general the only major difference in most RO filters is the bells and whistles they add. For examples: 1) Pressure gages are sold on some so you can see how much the pressure drips going through the pre filters. If there is a significant drop you need to change the prefilters. Most people such as reef keepers and growers just replace the prefilters every 6 months whether they need it or not. 2) Prefilters are sold in different micro ratings. You do not need prefilters smaller than the typical 5 micron pore size of your carbon block filter. 3) Carbon filters can be GAC (granulated activated carbon), carbon block or catalyzed carbon blocks made to remove chloroamine. Unless You have chloroamine in your water a simple GAC filter is all that is needed, however carbon block filters are m now often sold as cheaply as the GAC filters. 4) Filter systems can also have DI (deionizing) cartridges. Very few growers will ever benefit by the extra TDS removal provided by the DI filters. In reality most of the removal capacity of a DI filters resins are quickly used up by CO2 from the carbonates left in the water after the Calcium and magnesium are removed by the RO filter. 5) Some filter systems come with TDS meters built in to show the TDS of the water before it enters the DI filters and after it leaves the DI filters. This will give you an indication of the RO membrane performance and the DI cartridge performance. 6) Some RO filter systems come with a pressure booster pump or a permeate pump to improve the total water volume produced if you have low tap water pressure. 7) Some systems are made principally to provide filtered water for drinking and therefore come with a small storage tank and faucet. They often have membranes with onlt a 85% rejection rate.
The best option for an RO filter is the membrane reverse flush set up. This allows the flushing of minerals and sediments out of the membrane and its housing. This increases the life span of the expensive membrane and keeps the membrane production rate from declining.
http://www.thefilterguys.biz/ro_di_systems.htm
99% of growers will obtain great results cheaply with a simple RO filter system from The Filter Guys. I would recommend the simple $124 model. Check with your water supplier to see if they use chloramine. If so order the optional carbon filter made to remove chloramine. The guys that put together and sell these filters are great guys. They will put together what ever you want. They answer emails quickly even on week ends and holidays. They will even give you therir home phone numbers. Great guys, greta equipment and great prices. I gurantee if you find some expensive system you think is the greatest send a link of the system to The Filter Guys and they will provide an equivalent or evem better system for a lower cost.
