When to change prefilters

When to change prefilters:

An R/O for window cleaning only needs two prefilters; a sediment filter and a carbon block filter. The sediment filter is the first filter and the carbon block is the second filter. They are called prefilters as they are situated before the actual r/o membrane/s. They are needed to protect the membranes.

Sediment filter.

Simply, a sediment filter is there to filter out sediment from blocking up the carbon filter and the membrane/s. If your water is sediment free, then you may not need a sediment filter. I would always have one because you never know when a burst water main further down the street is going to dump lots of sediment into your water supply while you are out working. It’s happened to me.

Carbon filter or carbon block filter.

The purpose of the carbon block (C/B) filter is to remove chlorine from the water. This filter is very important as chlorine damages r/o membranes. The more damaged a membrane is, the less efficient it performs at removing dissolved solids from the water and will increase resin usage polishing that water off. The only cure for a damaged membrane is to replace it.

A C/B filter has a service life in the same way the engine oil in our vans have a service life; say 12500 miles or 1 year for example. The service life of a C/B filter is based on how much water that filter has to process. The service interval is based on the manufacturer’s specifications.

As an example we will consider the manufacturer’s specs of a Pentair Fiberdyne CFB-Plus C/B filter. The two most common sizes of prefilters used in window cleaning are a 10” and 20”.

If you have a 10” filter then a 10” Fiberdyne filter will process 37800 litres of water at 3.8 litres per minute. A 20” filter will process 75700 litres of water at 7.6LPM. These figures are the total amount your r/o uses to process pure, i.e., waste and pure combined.

Different manufacturers provide different service life requirements. A 10” Spectrum ECB C/B filter has a service life of 6000 and the 20” 12000 litres. Now, why would I purchase a set of Spectrum 20” prefilters from Daqua for £25.42 when I can buy a set of Fiberdyne C/B and Spectrum sediment filter for £35.42 from Daqua? (Prices as at 22/3/2022).

I still have a CCK 20” refillable coconut shell carbon filter GAC-20R-C under my desk I purchased from Osmotics 10 years ago. It has a service life of 2,500 US Gallons (10,000 litres.)

IMHO, the only way to know how much water your r/o is using is to fit a sub water meter to your r/o somewhere before the membrane/s.

I usually change my 20” prefilters every 3 to 4 months. Why do suppliers recommend we replace a C/B filter every 3 to 6 months? Manufacturers will specify a service life of a C/B filter based on a preset chlorine content in the water. Pentair say their estimated service timetable is based on “using 2 ppm free available chlorine at 0.5 ppm breakthrough.”

Doug from Daqua recently suggested on the forums that he tested some tap water in Scotland and found no chlorine in the water. Logically then, the C/B filter will last me much longer if my chlorine levels in my tap water are half of the chlorine levels specified by Pentair. But I don’t know what the chlorine content of my water is every day. It could be that my C/B filters are good for 150,000 litres; I just don’t know. So I have chosen to change my Fiberdyne filters to Pentair service timetable. It works for me in our area, as my Axeon HF5 4040 membrane is nearly 10 years old (purchased July 2012) and still producing at a rejection rate of 97%. If the chlorine content in another area is higher than their service life specifications, then one will need to change the C/B more regularly.

I hope this helps newbie window cleaners.

For those who say that they only replace their prefilters once a year and have no issues, then that’s fine. All I know is that the first r/o I purchased was 1 year old from a local cleaner. The first owner only changed prefilters when I purchased the unit from him. Those membranes were shot after a year. I had to replace them. When I started to replace prefilters every 3 months or so, then the replacement set of membranes lasted 6 years. So I know that I can’t go a year without changing prefilters.