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Reverse Osmosis

Reverse Osmosis (RO) units are usually installed as point-of-use (POU) systems. They are generally placed at the kitchen faucet and used to purify water intended for drinking and cooking purposes only.

RO IS BASED ON the process of osmosis. Osmosis is the selective movement of water from one side of a membrane to the other. Pressure is applied to the contaminated water, forcing water through the membrane. Because contaminants can not move with the water across the membrane, only purer water passes through the membrane. The process splits the liquid flow into a purified stream of liquid which has passed through the membrane (permeate) and a concentrated stream of liquid which has passed over the surface of the membrane (concentrate).

BASIC COMPONENTS of an RO system should include a pre-filter to remove fouling agents such as rust and lime; an RO module containing the membrane; an activated carbon post-filter to remove residual taste, odor and some compounds from the purified water; a storage tank; and various valves, including a shut-off valve that stops the water.  There are enhanced RO systems available with pressure pumps and up to 10 filter stages.

RO is effective at filtering or reducing:

  • arsenic, chloride, fluoride, nitrate, sulfate
  • lead, mercury, other heavy metals
  • some organics
  • bacteria and viruses


Membrane selection affects performance and is based on various water characteristics such as acidity, hardness, total dissolved solids and chlorine content.  Another characteristic influencing an RO system's performance is water pressure. The higher the water pressure, the better the rejection of pollutants and the more purified the water.  Water pressures vary significantly changing the efficiency of the unit. Water production and rejection rates in most units decrease as the storage tank fills, since this increases pressure on the purified water side of the membrane. For maximum efficiency the storage tank should be emptied daily.

The amount of water passing through the membrane is referred to as flux and is generally expressed in gallons per square foot of membrane per day (GFD). Membranes  have a maximum recommended flux, the higher the flux the more water is produced. Contaminant rejection by the membrane remains constant and therefore the more water that passes through the membrane the higher the quality of permeate water.  Typical water pressure in homes may not be adequate for efficient RO treatment, however, booster pumps can be added. Doubling the pressure across a membrane more than doubles the output flow rate of purified water. 

The production rate of purified water is also influenced by temperature. The higher the water temperature, the better the production rate. A drop in temperature from 75 degrees to 45 degrees Fahrenheit virtually cuts the production of purified water in half.


RO systems require regular maintenance and replacement of various components. Pre-filters and post-filters need to be replaced on a regular basis. The length of time between changing pre-filters will depend on the water quality, especially the concentration of solids, The contaminant concentration, membrane rejection percentages, and efficiency of activated carbon. RO membranes typically last for one to three years, depending on operating conditions, membrane type and pre-filter performance. Consider maintenance.

It is difficult to know when to replace the various components of an RO system.  RO membranes may become clogged by rust particles, Fe/Mn bacteria, silt, etc., if pre-filters are insufficient water conditions, if this occurs the membrane cannot be regenerated. Verification of a system's performance can only be determined by chemical testing. However, if we have access to your water analysis and have installed your RO system we can usually accurately determine when filters and membranes need to be replaced, presuming that contaminant concentrations have not increased significantly over time.

A major disadvantage of RO is the large amount contaminated wastewater generated. This can be from as little as 50 percent to higher than 90 percent of the incoming water. This amount depends largely on water temperature and the pressure difference across the membrane. The warmer the temperature and larger the pressure difference, the smaller the wastage rate. Consider water consumption and disposal.

Efficiency is described using the term "rejection percentage," which is the percent of a particular contaminant that doesn't cross the membrane, i.e., is rejected by the membrane. However, rejection percentages do not tell the whole story. If an incoming concentration of a chemical is high, a rejection rate of 80 or 90 percent may not be sufficient to reduce contaminant concentration to safe levels. Consider testing your water before purchasing a solution.

Be sure to size your system to your needs.  RO water generation is relatively slow process, RO units are rated at optimal conditions (70 degree water at 60 PSI, into an empty storage tank) in US gallons (US) per day.  Expect half of the rated capacity if you have city or well water at 45 degrees and water pressure of 45 PSI. On the other side of the equation in optimal conditions you do not want to store RO water longer than a day.  RO like distilled water is relatively pure.  Pure water is frequently termed as the universal solvent because it "wants" to pick up or dissolve many of the things it comes in contact with. Consider your daily requirements.

Both Distilled Water RO water have been implicated in the leaching of nutrients from the body.  Our investigation of this while definitely not as thorough as the paper done by the WQA on this topic has not found scientific proof of this. 

Like Distilled Water RO can create rather "flat" tasting water, much of this can be alleviated by aeration and chilling the water.

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Last modified: January 12, 2023