Reverse Osmosis Systems (RO)

September 1, 2022

What is it?

Reverse osmosis is a technology that removes impurities and dissolved solids by passing them through a semipermeable membrane which filters out all the unnecessary impurities.  The membrane effectively eliminates dissolved salts, minerals, chloramines, chlorine, nitrates, dirt, and heavy metals present in water. Reverse Osmosis can effectively remove more than 90-99.99% of all the contaminants including minerals from the drinking water supply.

Cut away view of a reverse osmosis membrane element inside a pressure vessel.

Wasted water

A traditional reverse osmosis filtration system uses more water than it produces of RO water. To make and gallon of clean RO water it takes between 1 gallon and multiple gallons of unfiltered water.

During the reverse osmosis process, there is cross-flow filtration through a membrane. The clean RO water (permeate or product water) goes to the water storage tank, and the waste stream ( RO reject water, or brine) that has all the contaminants and dissolved inorganics, goes down the drain. 

The common RO system takes 4 gallons of water to make one gallon of clean RO water. The amount of water used is based on the water supply and the RO system itself. Factors include contaminants present, amount of dissolved solids in the water, water temperature, membrane recovery ratio, condition of filters/membrane, size of the system, the pressure at which it operates, age of RO system, and type of membrane that is used. If the incoming water is loaded with of contaminants, the system has to work harder (and use more water) than if the water supply had less contaminants. Using a whole home filter to help buffer some of the contaminants before going to the RO helps the membrane to live longer and work more efficiently.  

Membrane Recovery Ratio

Membrane Recovery Ratio is how much water is being “recovered” or saved as clean RO drinkable RO water (product water). The higher the recovery ratio the less waste water goes down the drain. 

  • Some RO systems can’t handle higher than 4:1 recovery rate because it leads to membrane failure. Most manufactures report higher then actual recovery rates due to the fact that they test in perfect conditions. In your home you will experience changing contaminants, and water temps. Most residential RO systems have a recovery rate between 10 and 25%. Many RO manufacturers do not disclose their ratios. If a manufacturer does not state the waste water ratio ask about it before purchasing a system. A good RO manufacture or a reputable water treatment dealer will be able to walk you through the various RO systems out there.

Mineral Loss

There are two types of minerals in water, organic and inorganic.  Most organic minerals for our body functions come from dietary plant foods. The World Health Organization (WHO) says that majority of healthy minerals needed for human body is from food or dietary supplementary sources and not from drinking tap water. Some minerals found in water can be harmful to human health.  Calcium and magnesium are essential elements for human body (WQA, 2011). There is an argument to suggest that we should make up this deficiency through water consumption (WQA, 2011). Tap water presents a variety of inorganic minerals which human body has difficulty absorbing.  Such as chlorine, ammonia, alum, and sodium fluoride. All these minerals are inert (which means they can’t be absorbed into a living tissue of a body). Inorganic minerals can be detrimental to human health. There presence in the body is suspect to a wide array of degenerative diseases, such as hardening of the arteries, arthritis, kidney stones, gall stones, glaucoma, cataracts, hearing loss, emphysema, diabetes, and obesity. What few minerals that are in “hard” (well) tap water, are poorly absorbed, or rejected by cellular tissue sites, and, if not evacuated, their presence may cause arterial obstruction, and internal damage as well.

pH change in water

RO will have little effect on the waters pH levels. The pH levels will automatically change when it is ingested and comes into contact with the food in your stomach. Even on an empty stomach, your stomach acid (pH 1.3-3.5) alone is already several times more acidic than RO water (pH 6-8).  The human body regulates pH levels constantly to find its balance. The normal human body will always maintain a neutral 7.4 pH balance. The healthy human body will restore pH  quickly and easily. Soft drinks and sports drinks typically have a pH level of 2.5, orange juice has a 3 pH and coffee has a 4 pH level and we drink these beverages all the time without problems 

The pH may drop for several reasons:

  • The carbon in the post filter is an acid washed carbon which may cause the pH to drop temporarily. Time frame is dependent on initial flushing and then use.
  • Dissolved gasses will pass through the membrane and are then stored under pressure in the tank. O2 and CO2 may cause the pH to drop slightly. Depends on raw water chemistry.
  • Incomplete rinsing of RO membranes. Membrane preservatives put in from the manufacture are acidic in nature.
  • Reduction in the mineral content especially the alkalinity. This reduces the buffering capacity of the water and may  increase the dissolved gasses effect.

Types of Reverse Osmosis Systems or ROs

3 stage 

Is mainly used for large scale purification of water. 

A standard RO system is equipped with 3 separate filter stages. Sediment, carbon, and reverse osmosis. Each filter has a specific role to make the RO function. 

Sediment filtration removes dust, dirt, particles, and rust in the water. Sediment filters are  placed as the first stage of filtration to help remove bigger particles so later stages can work more efficiently.

Carbon filtration is the second step. Carbon takes care of  tastes, odors, cloudiness, colors, and various chemicals such as VOCs (volatile organic compounds), chlorine and chloramines. Carbon filtration is the most widely used water filtration on the market as they are very effective in removing chlorine in the water and give the final water a very crisp and clean taste. Carbon can’t remove contaminants of smaller molecule size or harmful contaminants such as heavy metals and arsenic.

Reverse Osmosis is the final stage of the filtration process and it is also the main part the entire RO system. In this stage, the water is pushed through a  membrane. Up to 99% of all impurities such as lead, arsenic, and nitrates are removed from the water.

5 stage 

Is mainly used in home drinking water. They are used when high yield amounts of clean RO water are not needed. It has the same 3 filtration steps as the 3 stage but the additional 2 are.

Carbon Block. This is another carbon filter to help break down and remove any contaminates that made it through the first carbon. 

Alkaline or polishing filter. 

Polishing Filter is used to clear up any leftover impurities from the RO membrane. This filter leaves the water crystal clear

Alkaline Filter is used to raise the alkalinity of the water and “soften” the water by removing any excess unwanted minerals. Some experts say that alkaline water may be beneficial to your health by helping to balance acidic body chemistry.

Zero Waste Ro systems

Zero waste RO systems use the same technology as a standard RO or even a high efficiency RO system. The difference is in a Zero waste system it puts the waste water back into your homes water supply. Its recycling the water making it 100% efficient. 

The drawback of this water recycling is that if you do not send the recycled water back to a filter or softener, you could be using water filled with contaminates for washing your hands or bathing. Theres a few ways to make sure this doesn’t happen.

   1. send the reject water back to the front of your system where your whole home filters are or your softener.

  1. Install a filter after the RO reject tie in point so that the larger Contaminets are being cleaned out. The downfall to that is there is another piece of equipment that needs to be maintained.
  2. Tie the water back into the water heater so it can be dissipated by the water tank. The downfall to this is your sending contaminates to sit in your water heater. This could cause bacteria to grow or cause premature wear on the heater itself.