Reverse Osmosis TT18
Published on May 12, 2014

Reverse Osmosis - Tech Talk 18

There are two general categories of reverse osmosis (R/O): low pressure R/O for fresh water (less than 100 psi) and high pressure R/O for making drinking water from seawater (over 1,000 psi). This Tech Talk discusses only the low pressure category, which uses thin film composite (TFC®) membranes.

R/O removes dissolved ions by forcing water through a semipermeable membrane. As water flows over the membrane under pressure, about 33% passes through and exits the center as purified water, permeate or R/O water. The remaining 67% rinses the outside of the membrane and removes contaminants as concentrate. Rejection of dissolved ions is about 92% efficient if the system is in good condition. For additional filtering, the permeate can then be sent through a deionizing filter (DI) for another 6% or 7% polishing of the water. R/O is not recommended for removing viruses or bacteria since a cut in a seal or membrane could allow the organisms to pass. In addition, very small organic compounds such as trichloroethylene or trihalomethane will not be removed by this process.

Performance Checks

Source water should be at least 50ºF (10ºC) and 40 psi. Do not connect to hot water or pressure over 100 psi. If the pressure is below 40 psi, a booster pump is required.

A pressure gauge is essential to monitor the backpressure from your membrane and will indicate clogging. The gauge will also indicate prefilter loading, as the pressure will decrease as the prefilter becomes clogged.

Total dissolved solids (TDS) levels will affect performance of R/O units. R/O membranes require more pressure to overcome the osmotic pressure as the TDS increases. The higher the level of TDS, the shorter the membrane life.

The R/O unit is engineered to receive potable municipal water. In areas with very hard source water, a household water softener should be added to extend membrane life because the membrane rejects sodium better than either calcium or magnesium. Measure TDS to determine if the membrane has failed. Use a TDS or conductivity meter to monitor your permeate water. For example, if the tap water reading is 100 ppm and the permeate water is 8 ppm, you know all is working well.


Membranes will clog quickly and stop producing permeate water without proper protection; usually a 1- or 5-micron (µ) prefilter is used to remove solids and activated carbon is used to adsorb chlorine (specialty carbons must be used for chloramine removal). When the carbon filter is exhausted, chlorine will pass through and destroy the TFC® membrane. The sediment and carbon prefilters should be replaced every 3 months or after production of every 1,000 gallons of purified water. The TFC® membrane should be changed every 1–3 years or when TDS readings in the permeate water are above 30 ppm. After installing a new membrane, discard all permeate for the first two hours (a new membrane is coated with a solution to prevent microbiological growth and must be rinsed thoroughly). Until it is fully hydrated, a new membrane may take up to 10 days for full production. DO NOT allow it to dry out. If you are not going to use the filter for more than 4 weeks, remove the membrane, place it in a sealed bag with 2 tablespoons of filtered water and store it in the refrigerator. Membranes and prefilters are usually interchangeable between brands.

Use and Storage

Your R/O water should be stored in food-grade containers. R/O water is so pure that it acts as a solvent and will dissolve metals like copper or brass. If it is not stored in a food-grade container, the purified water will leach the chemicals out of the container within 24 hours. R/O water is too pure to be used by itself for fish and plants. Additives are needed to replace essential minerals. Most marine mixes contain these minerals. For freshwater systems, use an additive such as R/O Right. The average cost to produce R/O water is 5 to 10 cents per gallon. This includes the cost of the filter and maintenance.