Reverse Osmosis (RO) Explained
What is osmosis?
Osmosis is a natural process that balances two bodies of water separated by a semipermeable membrane. Water flows through the semipermeable membrane from the less concentrated solute (fewer contaminants) to the highly concentrated (more contaminants) to restore equilibrium between the two sides.
A semipermeable membrane has small holes that trap contaminants but allow water to flow through. This flow of water may be reversed if pressure is applied to the solute with higher concentration.
What is reverse osmosis?
Reverse osmosis occurs when pressure is applied to solute with a high concentration of contaminants. For example, when pressure is applied to a volume of saltwater, the salt is left behind when the water passes to the other side of the membrane. Now, the saltwater is drinkable. The fresh water produced is called the permeate. The concentrate left over is the waste or brine.
How does a reverse osmosis system work?
Unfiltered water, or feed water, flows to the more concentrated side of the RO membrane, where pressure is applied. The pressure forces the water through the semipermeable membrane. Both the contaminants and the membrane contain ions with like charges. Since like charges repel, the contaminants cannot cross to the other side of the membrane.
Stages of RO systems
Reverse osmosis is only one stage in an RO system. RO systems are made up of 3, 4, or 5 stages of filtration. In an RO system, water is always treated with reverse osmosis in conjunction with prefilters and post-filters. Each type of system contains one or more of the following filters:
- Sediment filter: Reduces particles like dirt, dust, and rust
- Carbon filter: Reduces volatile organic compounds (VOCs), chlorine, and other contaminants that give water a bad taste or odor
- Semi-permeable membrane: Removes up to 98% of total dissolved solids (TDS)
When water first enters an RO system, it goes through prefiltration. Prefiltration gets rid of sediment, chlorine, and particles causing bad tastes and odors that could clog the RO membrane. Next, water goes through reverse osmosis. Particles dissolved in water, even too small to be seen with an electron microscope, are removed.
Afterward, the water flows to the storage tank, where it is held until needed. A reverse osmosis system makes water slowly. If you were to turn your faucet on to fill a glass of water at the actual membrane production rate, you would wait at least 5 minutes. 2-3 ounces of fresh water are produced every 60 seconds, which is why a storage tank is necessary. A reverse osmosis system continues to filter water until the storage tank is full, and then it shuts off. Once you turn on your faucet, water comes out of the storage tank through another postfilter to polish drinking water before it gets to the tap.
What happens to the filtered contaminants?
Rejected impurities are carried down the drain. Unlike traditional filters, the RO membrane is self-cleaning. As the source water flows through the system, it is divided into two streams. One stream carries the filtered water to your tap. The second stream carries the rejected salts, dissolved pollutants, and minerals to the drain.
Because the contaminants are carried away with the brine water, they are not able to clog the membrane. A charcoal or carbon filter, on the other hand, gradually loses its ability to trap the impurities and contaminants as water passes through.
Does a reverse osmosis system waste water?
A small percentage of water goes to the drain carrying the rejected contaminants, so this water is used for a purpose, not wasted. The brine water allows you to have better water, just like a dishwasher users water so you can have clean dishes.
Our job is to create efficiencies so that the amount of water going down the drain is minimalized. Adding a permeate pump gives us one way to do this. Permeate pumps reduce the rejected water by 75 to 80%. Also, every RO system we sell has an automatic shut off device. So once the tank is full, no more water does to the drain.
Many companies and homes with RO systems use the concentrate water for landscaping or artificial lakes. This drain water will have a higher TDS reading but will be safe for the lawn and garden.
Reverse osmosis vs. bottled water
Bottled water wastes more water than reverse osmosis. Think about all the water required to make a bottle of water. First of all, the water in the bottle was most likely made using reverse osmosis. Not to mention, the production of the bottle required water and petroleum use. Petroleum was also used to deliver the bottled water to the store. And afterward, the truck may even get washed.
Our theory on how RO water helps the environment
If you use water from the city supply, then the water entering your home has been treated by a municipal plant to make the water safe to drink. When water drains from your home, chemicals must be removed before it’s recycled or sent to the riverbeds for nature to filter through the hydrologic cycle. Water is often treated by dilution (clean well water is blended with not so clean water). The diluted water is easier to treat.
But if the water draining from your home has been through a reverse osmosis system first, it is free of chemicals. Chemicals have already been grabbed by the carbon filter. The brine water simply has a slightly higher concentration of inorganics. An RO system takes care of dilution to make the job of the waste treatment more efficient. RO systems speed up the recycling process because no new chemicals are introduced to the water supply when RO water drains from your house.
Does reverse osmosis demineralize water?
Yes. Is that necessarily bad? No. Some minerals in water are fine to drink, but the EPA recommended that the number of TDS not exceed 500 parts per million, which means a large portion of the U.S. should remove some. Stripping minerals from your water through reverse osmosis will not harm people who have enough food to eat. Food is the primary source of essential nutrients, not water.
If you’re drinking water without minerals, the water is filtered before entering your body. Reverse osmosis does the work of your kidney. Our bodies are 70-80% water, and that water primarily hydrates, lubricates joints, and aids organ function. You don’t need minerals to do those things. You would have to drink excessive amounts of water for your body to absorb enough to matter anyway. If you’re deficient in minerals, eat your fruits and veggies.
Will a reverse osmosis system soften water?
Yes. Water is softened through the reverse osmosis purification process. However, using the RO system to treat hard water (above 7 grains of hardness) can shorten the life of the membrane. This will lead to more frequent replacements for the RO membrane. If you have hard water, use a water softener to treat it instead.
A water softener, however, does not treat water for contaminants. It only removes calcium and magnesium to treat hard water and make washing clothes and showering easier. An RO system is safe to use with a water softener. It removes the sodium added by the softener, and soft water extends the life of the RO membrane.
A water softener can also remove iron from the water before it goes to your RO system. Iron can stain your toilet and shower, turn your hair and clothes orange, and clog the RO membrane.
What does a reverse osmosis system remove?
While the RO membrane only removes some types of contaminants, an RO system also includes sediment and carbon filtration for a broad spectrum of reduction.
Does a reverse osmosis system remove…
- Fluoride? Yes.
- Salt? Yes.
- Sediment? Yes.
- Chlorine? Yes.
- Arsenic? Yes.
- VOCs? Yes.
- Herbicides and pesticides? Yes.
- Many other contaminants? Yes. The contaminants listed are some the most popular ones treated with an RO system, but the system also removes a slew of other contaminants.
- Bacteria and Viruses? No. Water entering your RO system should already be microbiologically safe. Reverse osmosis may remove some bacteria, but bacteria could grow on the membrane and potentially enter your water supply. To remove viruses, we recommend UV disinfection.