Reverse Osmosis for Well Water


Like so many of the best inventions, reverse osmosis water treatment is based on a naturally occurring process  – osmosis.  This is the movement of a solute (water) through a semi-permeable membrane, like a cell wall, as occurs in the uptake of water by plants. In nature, the water will move through the membrane from a low concentration to a high one in order to achieve a balance across the membrane.  However, in water treatment, we are looking to move water through the membrane from a high concentration to a low one to create a complete imbalance – drinking water with little or no dissolved particles on one side and reject water with all the contaminants on the other.  Hence the term reverse osmosis.  This movement is achieved by applying pressure.

Process of Reverse Osmosis

Process of Reverse Osmosis

Generally speaking, your household water pressure or that produced by your well pump will be sufficient to produce 2-12 gallons of reverse osmosis-purified water in a 24 hour timeframe.  If more water is needed or your water pressure is insufficient, it is always possible to add a booster pump. Other typical components of a reverse osmosis (RO) system include a holding tank (usually under the sink) and pre-treatment. Although the efficacy of a reverse osmosis is strongly dependent on the quality of the membrane itself, the feed water quality is also critical. In fact, the feed water needs to be relatively free of hardness minerals (which can clog the filter) and iron/manganese, which may oxidize on the filter.

What does all that mean?  Depending on the hardness of your well water, you may require a water softener.  And while that may address low levels of iron or manganese, if these are present in higher amounts, then an oxidizing filter may also be in order. Otherwise, the rejection ability of the RO membrane will be compromised, requiring more frequent replacement (and therefore increased cost).  Even if these are not challenges with your well water, a complete RO system requires a sediment pre-filter to remove sand, silt, and dirt, and an activated carbon post-filter. Following this, it is not uncommon to incorporate a low-flow ultraviolet water treatment system to sterilize any bacteria, like coliforms, or viruses not blocked by the membrane (where membrane integrity is compromised).  In industry terms, each one of these filters is called a “stage”.  Hence the terms 3-stage or 4-stage RO system.

Now that you have an understanding of the basic functioning of an RO system, what can it remove?  In short, any particle larger than a water molecule.  Typical contaminants found in well water would include nitrates, arsenic and chloride and microorganisms (see note above).  On the other hand, an RO membrane will not remove dissolved gases like hydrogen sulfide (that’s the rotten egg smell contaminant), radon, certain pesticides and volatile organic chemicals (VOCs).  So as always, a water quality test is necessary to understand if an RO system can work for you and what, if any, pre-treatment will be necessary.

In short, reverse osmosis water treatment systems are a clever manipulation of a natural process.  Is reverse osmosis right for you?  Consider the pros and cons listed below:

PROS CONS
Effective removal of many common well water contaminants. Requires a storage tank and possibly a booster pump.
Chemical-free treatment Requires pre-treatment.
  Taste of water frequently described as “bland”
  Produces significant volumes of reject water (additional load on septic systems)