What Does Reverse Osmosis Do? The Process Explained

Reverse osmosis is the process of osmosis in reverse; it’s used in wastewater treatment to produce high-quality permeate free of harmful contaminants. 

Reverse osmosis is one of the best forms of wastewater treatment because of how effective it is and how simple the system is to use. And with two options for reverse osmosis, which we discuss in detail in this blog, you can choose how highly filtered your water is.
To fully understand how reverse osmosis works, it’s essential to understand the process of osmosis and how reversing that process produces high-quality water. 

The Process of Osmosis

To start, let’s review osmosis. Osmosis is a phenomenon that occurs naturally when a weaker saline solution migrates to a stronger saline solution. Plants are a great example of osmosis when the roots absorb water from the soil. 

If you were to separate salt water and fresh water in a container with a semi-permeable membrane, the fresh water would slowly migrate toward the salt water. 

A semi-permeable membrane is a membrane that lets some, but not all, atoms or molecules pass through. The semi-permeable nature of the membrane prevents unwanted particles from getting through. 

How Reverse Osmosis Works

Reverse osmosis is osmosis in reverse, but it doesn’t occur naturally. It requires energy – which osmosis doesn’t need – that is applied to the water. 

For example, in nature, fresh water would migrate towards salt water. But with the reverse osmosis process, the salt water would migrate toward the fresh water, with the semi-permeable membrane blocking out dissolved salts, organics, bacteria, and pyrogens. In this way, reverse osmosis acts as a water filtration system, providing clean water. 

Reverse osmosis uses a high-pressure pump to apply pressure to feedwater, pushing it toward the semi-permeable membrane. The water makes it through the membrane but leaves behind unwanted contaminants, and permeate comes out the other side of the membrane. 

During the reverse osmosis process, 95 to 99% of total dissolved solids are left behind in the reject stream behind the semi-permeable membrane. 

The total dissolved solids levels in the water determine how much pressure is needed for reverse osmosis to work. The pressure must be strong enough to overcome the naturally occurring osmotic pressure. 

With the way reverse osmosis works, cross-filtration is used instead of standard filtration. Standard filtration is where the contaminants are contained inside the filter media. 

Cross-filtration allows the solution to go through the filter with two different outlets. One outlet captures the filtered water, while the other outlet is filled with contaminated water. Cross-filtration is ideal for cleaning out contaminants and keeping the membrane surfaces clean. 

Single Pass RO vs. Double Pass RO

There are two types of reverse osmosis water systems: single pass and double pass. A pass is a stand-alone system; the single or double part refers to how many passes are contained in one system. A double pass reverse osmosis system has two membranes, while a single pass only has one membrane. 

A double pass reverse osmosis system will produce a high-quality permeate compared to a single pass system because the water has been filtered twice. 

A double pass system will also remove CO2 if you inject the system with caustic after the first and before the second pass. Adding caustic increases the pH of the water, thereby converting the CO2 to bicarbonate and carbonate.

Adding caustic can only happen in a double pass RO system since it would cause membrane scaling in a single pass system. Scaling occurs when inorganic compounds increase their concentration and exceed the solubility limits. The inorganic compounds then leak onto the membrane surface as scale. 

Scaling causes a high-pressure drop, decreases salt rejection, lowers the permeate flow, and, as a result, reduces the permeate water quality. Avoiding scaling will keep your reverse osmosis system running smoothly. 

What Contaminants Does Reverse Osmosis Remove?

A reverse osmosis system is capable of removing 95 to 99% of contaminants from water. Reverse osmosis eliminates ions, particles, colloids, organics, bacteria, and pyrogens. 

These contaminants are removed due to their size and charge. It won't get through if a contaminant weighs 200 in molecular weight. Likewise, a contaminant with a high ionic charge isn’t likely to get through the membrane. Contaminants with lower ionic charges, like calcium or sodium, will get through the semi-permeable membrane. 

So, while a reverse osmosis water system is highly reliable, it won’t remove all contaminants. Calcium and soda aren’t the only contaminants that can get through. 

If you need the guarantee that your water is 100% free of these contaminants, you’ll need to employ a secondary water filtration system. 

Reverse osmosis is ideal for treating brackish, surface water, and groundwater, regardless of the flow of water. Reverse osmosis water systems are commonly used in the pharmaceutical, boiler feed water, food and beverage, and metal finishing industries. 

Get Your Mobile Reverse Osmosis System from J.Mark Systems.

J.Mark Systems provides high-purity and water-softening reverse osmosis systems through WTRBOX, our brand of mobile wastewater treatment systems. We serve the aerospace, electronics, automotive, power generation, petrochemical, food and beverage, and pharmaceutical industries. 

Our reverse osmosis water treatment systems are skid-mounted, containerized, and mobile with wheels for transportation. Our treatment capacities range from 2 to 400+ GPM, with four options for you. We offer remote monitoring that you can connect to with WiFi, hotspots, or your existing internet connection. 

Best of all, we offer in-house financing and leasing, and we have a pilot program so you can ensure that WTRBOX from J.Mark Systems is right for your industrial wastewater treatment needs. 

Contact us today for your free water optimization audit, which will help us determine the right kind of wastewater treatment system for your industry/business. We have offices in Walker, Michigan, and Park City, Utah, and we serve all over the country from both locations. Fill out the form on our website to get started!