Reverse osmosis: Which filters and when to change?

Standard 3 stages RO

The small 3-stages systems have only 2 pre-filters: sediment and active carbon, both inline, that means filter housing and filter is built together and it is "use and throw away".



Standard 5-stages RO system

The large 5-stages reverse osmosis units are produced in

Taiwan and China. Most systems are sold as drinking water filters because the water in many countries is quite dirty. First filter is a sediment filter, removes and and rust.

Filter No. 2 in the standard system from the Far East is an active carbon filter (Granulated Active Carbon), which consists of baked carbon particles that have large capacity to absorb chlorine and other chemicals. Europe does not have the same amount of chlorine in the water as in for example USA. Therefore the active carbon filter (GAC) is not as relevant in Europe - but use it anyway.

Filter no. 3 is a carbon Block filter that is better than a GAC carbon filter. It weighs more than the GAC filter.


It looks like this:

  1. Sediment filter 5µ
  2. Active carbon filter 5µ
  3. Active carbon block filter 5µ

You just use the RO system as it is the first 6-12 months. After this period of time it is recommended to replace the active carbon filter with a sediment filter 1µ.


System with 2 filter housings 10"

  1. 10" Active carbon block filter 5µ
  2. 10" sediment filter 5µ/1µ
  3. membrane
  4. A. Drinking water: 10" inline active carbon filter 5µ.
    DI filter


System with 3 filter housings 10"

  1. 10" carbon block filter 5µ
  2. 10" sediment filter 5µ
  3. 10" sediment filter 1µ
  4. membrane
  5. 10" inline aktivt kul efter-filter 5µ.

System with 3 filter housings  10" + optional harpiksfilter

  1. 10" sediment filter 5µ
  2. 10" carbon block filter 5µ
  3. 10" sediment filter 1µ
  4. membrane
  5. DI filter inline refill patron or another large 10" filter housing


System with 3 filter housings 10", where 3rd filter housing is used as DI filter

  1. 10" carbon block filter 5µ
  2. 10" sediment filter 5µ/1µ
  3. membran
  4. optional DI filter in 3rd filter housing 10"
    Advantage: no need to buy a 4th filter housing.

When to change filters and membrane?

The more often you change your filters, the better protected is the membrane and the longer the membrane lasts, and since the membrane is significantly more expensive than the filters, it makes good sense.

The 10" filters for the large filter housings have a capacity of 11.000 liters or 3.000 Gallons. The small inline filters for the small 3 stages systems has a capacity of 6.000 liters or 1.500 Gallons. Remember to calculate with your waste water factor.

If you make 100 liters of osmosis water a week with a membrane having a waste water factor of 3, you have 100 +300 = 400 liters of water a week through your filters. That makes 400 * 52 = 20,000 liters a year.

Otherwise you measure when what should be changed:

Immediately after installing a new system you measure:

  1. tap water, as it is. This value is normally constant.
  2. the value BEFORE the membrane, ie after the pre-filters (if this value increases with time, it's time to change filters). This requires that you have a meter as shown below DM-1.

  3. the value AFTER membrane (if this value increases and the values for the pre-filters have not changed AND the diaphragm worked for some years, it's time to change the membrane). Membranes CAN hold more than 3 years, but  most shops tell you to change every 3 years.

  4. the value after the resin filter. If this rises and ALL other values are unchanged, consider whether you want to change resin

The serious user monitors these values permanent with a TDS Monitor, that measures at two points simultaneously.

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