What RO system should I buy?
My strong recommendation is to avoid proprietary water treatment systems that will force you to buy their replacement filters. RO systems are a 'mature' technology and they are simple. Virtually anyone can craft together the various system components and create a system that performs very well.
Assuming a brewer is looking at modest RO production of less than 100 gal/day, a system composed of standard 10-inch undersink filter housings for the particulate and activated carbon cartridges is a better option. DO NOT get a system with small filters that are typically called 'compact RO systems'. Those filters will be exhausted too quickly.
The most important component is the RO membrane. It is important to buy membrane cartridges that are 'name-brands'. Dow Filmtec or GE are pretty much the only name brands producing membranes for the residential market and Filmtec is prevalent. These membrane cartridges are a standard size for the residential market and they all fit a standard RO housing. Nothing special here, but know what you are buying.
Since the production rate for these residential RO machines is typically slow, you may not want to wait for the system to produce enough water for your current need. Having the system produce water prior to your need and storing it will improve the convenience of the system. Having a storage tank is an important component for delivering enough water on demand. The tank size should be large enough to supply the immediate demands. For brewers, the storage volume will typically be equal to the total of your mashing and sparging volumes.
There are two options for storing RO water, in a pressurized tank or in an open container. For best RO machine efficiency, discharging RO water to an open container is recommended. However, if you need to distribute the RO water, a pressurized tank might be best for you. Be aware that pressurized tanks cannot store and deliver the total volume they are rated for. There is a volume of air in those tanks that reduces the actual water storage capacity of the tank. The typical water storage volume available for pressurized tanks is about half the stated tank size. So a 4-gallon pressurized tank may only deliver about 2 gallons before it's exhausted. If your brewing day requires 10 gallons of RO water, then you may need to incorporate a 20-gallon pressure tank into your system. If you are using an open container, then its capacity would only need to be 10 gallons in this case.
A final consideration is to have a way to assess the performance of your RO machine. By appearances, most RO users will not know when their machine has started to fail and is no longer delivering the low-mineralization water they are expecting. The best protection for monitoring your machine's performance is to test the RO water for its Total Dissolved Solids (TDS) content. TDS meters are relatively inexpensive and can either be hand-held units or in-line units that can constantly monitor the water quality. In general, a properly operating RO system will deliver water with less than 25 ppm TDS. The actual reading will depend on the tap water quality feeding the RO system. By monitoring the TDS reading throughout the RO system's operation (often measured in years), the user can tell when the water's TDS is rising and membrane replacement is required.
The replacement of the particulate and activated carbon filters should be performed on a more regular basis. I feel that replacing particulate filters is a bit overstated. As long as the water still flows through the system, there is no strong need to replace the particulate filter(s). Particulate filters actually improve their filtering capability as they become clogged with fines. They only require replacement when they can't deliver enough water through them.
Activated carbon filters are another issue all together. Their most important purpose in a RO system is to remove all chlorine-based disinfectants from the water so that they won't destroy the RO membrane. 10-inch activated carbon cartridges are often quoted with a treatment capacity in the 1000 to 5000 gallon range. Fortunately, the capacity is dependent upon the contaminant being removed. In the case of chlorine compounds, the capacity of the filter can be quite high if the flow rate through the filter is very low. The flow through a RO system is quite low and therefore the capacity of the activated carbon filter is high. Since the purpose of this filter is to protect the membrane, regular replacement is appropriate. An annual replacement interval may be appropriate. If the RO membrane is failing in less than a couple of years, you may need to increase the activated carbon filter replacement frequency.
This should help out in purchasing and maintaining a RO system. Enjoy!
