RO Systems – When to Change Filters?

Since Reverse Osmosis (RO) systems are so handy for brewing, many brewers use them to purify their water. But those systems typically have multiple filters that require occasional replacement.  When are you supposed to change them?

If you listen to the typical manufacturer or seller of these systems and filters, they recommend replacement at frequent intervals. That makes sense since the more you spend on filter replacements, the more money they make. You take it in the pocket.  A smarter way is presented below.

There are three filters that are likely used in typical RO systems: Sediment, Carbon, and Membrane. They each have differing replacement intervals. Most importantly, they don’t require replacement on a Calendar basis. Here’s what you should know.

Sediment filters are robust and they have a very long life. In fact, when used with typical municipal water supplies, you shouldn’t ever have to replace a sediment filter. Those filters actually filter BETTER as they age and the typical municipal water has very low sediment and won’t clog the filter. The best way to gauge if that filter actually needs replacement is to install pressure gauges on the filter’s inlet and outlet lines. Typically, the gauges will report a pressure difference of a few psi when the RO system is operating. But if that difference increases into the 10 to 20 psi range, then it’s time to replace the sediment filter.  Until then, don’t waste money on sediment filter replacement.

Carbon filters protect your RO membrane from chlorinated compounds. If you’re on a non-chlorinated well system, your system doesn’t need a carbon filter. Those on municipal water systems will need a carbon filter since the water is legally required to be disinfected. Keeping chlorine compounds away from the RO membrane extends the membrane life and keeps chlorophenols out of your beer.

To know when your carbon filter needs replacement, you’ll need to insert a SAMPLE TAP downstream of the carbon filter and test the carbon filtered water occasionally. A TOTAL chlorine (not FREE chlorine) swimming pool test kit is what is used to determine if your filter is removing ALL chlorine compounds. Inexpensive pool test kits that use an indicator chemical called OTO is what you’ll want to buy. They can measure chlorine and chloramines. The photo below shows a preferred liquid test kit. Don’t use kits with chlorine test strips since they aren’t very accurate at the low concentrations that you need to be able to discern.

Typical TOTAL Chlorine Swimming Pool Test Kit

Collect a water sample directly from your sink and perform a chlorine test on that water. You should see the water turn yellow, which indicates there is disinfectant in there (That’s a good thing). That test proves that your test kit is working. Then collect a sample from the sample tap downstream of the carbon filter. Collect the sample when the RO system is operating.  The chlorine test should show NO color. No color shows that no chlorine compounds are making it through the filter. If you see any color in the test, its time to change the carbon filter. Keep a spare carbon filter on hand so that you can immediately replace the filter.

Your RO membrane does require occasional replacement.  But, it is a relatively expensive unit and you’ll want to replace it as infrequently as possible. There are two reasons why RO membrane replacement may be needed: 1) the membrane is clogged and 2) the membrane is not taking enough of the dissolved salts out. 

The first condition takes some investigation, but no tools. If the RO system doesn’t seem to produce RO water as quickly as it used to, then it’s possible that the membrane is clogged and needs replacement. If your raw water is really hard, then it’s more likely that clogging can occur. If it’s cold out, don’t jump to the conclusion that your membrane is clogged when it doesn’t produce RO like it usually does. Cold water does reduce the production rate for RO systems.

Checking the second condition does require a tool, but its one that all RO system owners should have: a TDS meter. They’re inexpensive and durable (Get one!)  By regularly testing the Total Dissolved Solids (TDS) of your RO water, you can see if the TDS measurement is increasing beyond typical readings. If the TDS reading is climbing, it’s time to order a new membrane. There is one caution regarding TDS output from RO systems: TDS does increase slightly when the raw water temperature is higher. So, you should see lower TDS during the winter and higher during the summer.

As shown above, you don’t have to rely on a water filter seller telling you when to replace filters. Just a few enhancements to your system can put filter replacement decisions safely back into your hands.