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You may have heard the term reverse osmosis thrown around when it comes to water filtration, but the precise process is typically less understood.
Reverse osmosis refers to the filtration process whereby feed water is forced through a semi-permeable membrane to separate filtered water from contaminant-filled waste water.
But this is probably best explained with a diagram…
First of all, here is our reverse osmosis diagram:
The diagram above shows the process of reverse osmosis in action. Put simply, the high-concentrate solution on the right (this is our unfiltered feed water) is being forced under pressure through the membrane and onto the low-concentrate solution on the left (purified water). As this is the opposite of what would naturally occur, water pressure is necessary to push the water through the process.
This is essentially the osmosis process in reverse. To better understand how reverse osmosis works, it’s important to get a solid understanding of the scientific term osmosis.
Read on for an explanation of osmosis including another diagram.
Osmosis refers to the natural phenomenon where water from a weaker saline solution will migrate towards a stronger saline solution through a membrane. This migration will continue until both solutions reach an equal concentration level.
This can be a little tricky to comprehend, so it may be helpful to offer a few real-world examples so you can better understand the process:
The important point to remember about osmosis is that it’s naturally occurring. This means no outside force is needed to make it happen. Contrast that with reverse osmosis – which needs an external force in the form of water pressure for it to work.
Here is how the osmosis process looks like in a diagram:
As mentioned previously, the working principle behind reverse osmosis is the use of pressure to force water through the semi-permeable membrane.
The RO membrane is used to separate the high-concentrate solution from the low-concentrate solution. This membrane is ultra-fine and will prevent anything larger than to 0.0001 microns from passing through it.
Another important point to note is unlike many other water filters, the RO membrane doesn’t trap any contaminants inside of it. Rather, it simply separates the feed water into two streams: one with clean filtered water sent on to the rest of the system, and the other with contaminant-filled wastewater sent down the drain.
The reverse osmosis process is highly effective at removing a wide array of contaminants from water, but it’s not perfect. Let’s take a look at some of its benefits and drawbacks.
Firstly, by using an ultra-fine membrane, reverse osmosis filtration is capable of removing an incredibly broad array of contaminants from water. It will remove nearly all contaminant particles larger than the membrane’s pore size – which is typically 0.0001 microns.
This results in incredibly pure water that is nearly completely contaminant free.
Removing nearly all contaminants from water results in incredibly pure, clean-tasting water. This includes sodium and other minerals that can impart an ‘off’ taste to your water.
Reverse osmosis will also remove turbidity and dissolved solids that can give water a cloudy appearance.
Reverse osmosis water is highly pure. Not only does it taste better, but as virtually all problematic contaminants are removed, any health concerns should be alleviated.
Reverse osmosis water is inexpensive when compared with other filtration methods – especially when compared with bottled water. Without getting into a breakdown of the cost of RO systems, once you have one installed, the ongoing maintenance costs are minimal.
There is a wide range of applications for reverse osmosis filtration including whole house units, under sink units, and countertop units. In addition, reverse osmosis is regularly used in commercial and industrial applications as well as for large-scale desalination.
Lastly, switching from bottled water to reverse osmosis water will result in significant cost savings. Even in the first year of purchase, the cost of a typical RO system will be less than purchasing bottled water.
One of the unavoidable downsides to RO water is the creation of waste water. This is a necessary part of the filtration process – as contaminants separated by the membrane must be flushed away.
Generally, RO membranes for home usage will generate waste water at a ratio of 4 parts waste to 1 part permeate (filtered water). This ratio can be improved significantly by installing a pump to increase the system’s efficiency. This can lower the ratio down to 1:1 or even 1:4 in some cases.
Another option to reduce waste is collecting and reusing the waste water. While waste water is filled with contaminants, that doesn’t mean it can’t be used for household tasks like mopping floors, washing your car, gardening, or cleaning patios and driveways.
Reverse osmosis will strip away beneficial minerals alongside harmful contaminants. This includes calcium, magnesium, potassium, and several others.
Fortunately, drinking water is not a major source of essential minerals for people. Usually less than 5% of mineral intake in the human diet comes from water.
If you’re still concerned about this, there are remineralization stages that can be added to RO systems which will add back minerals after they’ve been stripped away by the membrane.
Reverse osmosis filtration is a slow process, which is why most systems utilize a storage tank to keep a reserve of filtered water ready to use. This means you’ll need to wait an hour or more the first time you fill the tank, or after cleaning the system.
Good news, there are also tankless RO systems that use powerful pumps to force water through the system rapidly.
Some people find RO water to be ‘flat’ tasting – especially at first. Most people get used to this quickly though, so it’s not a major cause for concern.
Home-based RO units are not exactly small, so you’ll need a set amount of space under your sink, on a countertop, or where your main water line enters your home – depending on the system type.
Reverse osmosis is used in large-scale desalination operations, in commercial and industrial applications for purifying water, as well as in homes for small-scale water purification.
Reverse osmosis is commonly used in the home environment for water purification. The most common types are under sink (point-of-use) systems, which are installed underneath a kitchen sink and provide filtered water out of a separate faucet.
The diagram below outlines the filtration process of a typical under sink RO system.
Let’s take a look at the typical system components and filter stages found in an RO system.
The first pre-filter stage is a sediment filter designed to remove sand, silt, rust, and other large particles so they can’t clog the membrane or other filters.
A carbon pre-filter is commonly used to remove chlorine and/or chloramine from feed water before it can reach the membrane. Chlorine in particular can damage the RO membrane, so it must be removed before water moves to the next stage. Carbon filters will also reduce foul tastes/odors, as well as VOCs, pesticides, and several other organic contaminants.
The heart of any RO system is the ultra-fine RO membrane. This stage does the majority of the heavy lifting when it comes to contaminant removal. It uses water pressure to force water through the membrane – at which point it is separated into clean water and waste water.
A carbon post filter is often used as a final filtration stage to ‘polish’ the water and remove any remaining contaminants that have made it through the rest of the system.
The under sink module connects the filters and membrane that make up the RO system. It allows for easy attachment of the various system components and has a single input and output port.
The storage tank keeps a reserve of filtered water on hand. After water has passed through the RO membrane it flows to the tank for storage before flowing up to the RO faucet. RO storage tanks use internal pressure to trigger the system to start and stop filtering water with the help of the auto shut-off valve.
The RO faucet dispenses filtered water out of a separate faucet from your main kitchen faucet. This allows you to use unfiltered water for washing dishes and other cleaning tasks.
The drain saddle is where the drain line running from the RO membrane outlet connects to the drain pipe under your sink.
The feed valve connects the RO system to the cold-water line underneath your sink. Unfiltered water enters the system through this valve before any filtration begins.
The automatic shut-off valve (ASO) stops the system from constantly filtering water. When the storage tank is full, it triggers the ASO to shut off the filtration process. When you open the RO faucet, the storage tank’s internal pressure drops, deactivating the ASO and starting the filtration process again.
The check valve stops the flow of water back toward the RO membrane. Backward flow into the RO membrane can damage or rupture it, so it must be prevented using the check valve.
The flow restrictor – as the name suggests – restricts the flow of waste water out of the RO membrane. It’s needed to maintain pressure inside the RO membrane so it can work optimally.
If you have any questions about our simple reverse osmosis diagram please don’t hesitate to leave a comment below!
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