Can you explain reverse osmosis treatment and discuss its limitations and what it is effective at treating?

Question

Can you explain reverse osmosis treatment and discuss its limitations and what it is effective at treating?

Answer

Reverse osmosis is a water treatment process which primarily involves pushing water through a semipermeable membrane to separate pure water molecules from other ions, molecules, and larger particles. It is important to first understand the term “osmosis.” Osmosis is the movement of a solvent (like water) through a semipermeable membrane, from a less concentrated solution to a more concentrated solution. This natural process will continue until concentrations are equal. Pressure can be applied to the more concentrated side of the membrane to stop this flow, and this is called the osmotic pressure. If even more pressure is applied, in excess of the osmotic pressure, the flow is reversed. This technique/process is called reverse-osmosis. See figure 1 (EPA, 1):

Figure 1: osmosis and reverse osmosis visualized

While the membrane material and its interactions with the various components present in the solution is an important factor in determining removal efficiency, you can generally think of an RO membrane as a very fine filter, with the pore size being a primary guiding factor of what can be removed. Of the common membrane filtration technologies (microfiltration, ultrafiltration, nanofiltration, reverse osmosis), reverse-osmosis has the smallest general pore size, approximately 0.0001 micron (CDC, 2). This is small enough to remove most bacteria, viruses, and many common chemical contaminants.

A typical household reverse-osmosis system is shown in Figure 2. Most are point-of-use units meant to fit in small places, like under a sink. These systems typically have other filters (like a sediment filter and a carbon filter) in front of the RO membrane to help protect it and extend its life. They also will have a holding tank and a spout for dispensing the purified water. Some will have a carbon filter after the membrane for final polishing/cleanup. With units that are working properly, we typically expect to see a total dissolved solids removal rate of about 90% or more.

Figure 2: typical reverse-osmosis system (photo by Dan Webb)

Reverse osmosis systems are generally effective at removing (1, 2, 3): 

• Bacteria 
• Viruses 
• Protozoa 
• Total dissolved solids 
• Inorganic ions and salts, like sodium, chloride, copper, chromium, lead, fluoride, aluminum, barium, cadmium, hardness, iron, manganese, nitrate, radium, selenium, sulfate. 

Some may also remove arsenic, chlorine, and organic compounds, but often these can be difficult to remove, depending on various conditions. Most (probably all) systems will have a carbon filter within the unit, and this should help reduce the concentration of chlorine and organic components. It should also be noted that while RO system will remove microorganisms, it is best to use other methods to prevent these before they get to this treatment stage. If living organisms are present and get do get through the membrane, they can cause biological growth later, like in the final carbon filter, if one is present.

The nature of the membrane material, the chemical properties of the target contaminant, and the pH of the water can all affect removal efficiency. Charged species like ions tend to be more easily removed than neutral, uncharged components. For typical groundwater, with a pH of around 7 – 8, and typical reverse osmosis systems, boron can be difficult to remove. Similarly, arsenic can be challenging to remove sometimes, likely due to the nature of the arsenic species in groundwater sources. Arsenic typically occurs in water in two oxidation states: As(III), “arsenic three,” arsenite; and As(V), “arsenic five,” arsenate. Arsenic(III) tends to be present as an uncharged form H₃AsO₃, and is typically more difficult to remove than arsenic(V), which usually exists as H₂AsO₄^- or HAsO₄^2- (both charged species). Some consumers have had good luck improving arsenic removal by adding an oxidation step (like chlorine) ahead of the RO unit. 

To check specific analytes, it may be helpful to check NSF’s list of certified reverse osmosis drinking water treatment systems: http://info.nsf.org/Certified/DWTU/Listings.asp?TradeName=&Standard=058&ProductType=&PlantState=&PlantCountry=&PlantRegion=&submit3=Search&hdModlStd=ModlStd

Reverse osmosis systems produce about 1 gallon of purified water for every 4 gallons of water fed into the system, with the rest diverted to a waste drain. This can vary between units, though, so if this is a concern, it is important to research this ahead of time. 

The end water produced by reverse osmosis should have very low mineralization, with properties similar to distilled water. The pH will be low (can vary, but generally be slightly acidic… around 6 pH units), but because there is very little buffering capacity, this will generally change quickly as it mixes with other solutions or solids, and typically this is not a concern for consumption of RO water. 

As with other water treatment devices, it is important to remember to maintain reverse osmosis systems properly. As mentioned earlier, there will often be filters in place within the unit ahead of the membrane, and many consumers will have other/external treatments devices, first, like water softeners. These treatment steps and filters help to prevent fouling of the membrane, which helps to keep it at peak removal efficiency. Fouling can result from microbial growth/contamination, hardness/mineral precipitation, organic hydrocarbon coating, or particle accumulation on the surface of the membrane. Pre-membrane filters and post-membrane filters, in addition to the membrane itself, should be replaced on a regular basis to keep the unit working properly. Frequency will depend on the water conditions and household use.

References: 

1) US EPA, EPA 815-R-06-009: MEMBRANE FILTRATION GUIDANCE MANUAL 

2) Centers for Disease Control and Prevention: A Guide to Drinking Water Treatment Technologies for Household Use.

3) NSF: Consumer Fact Sheet, What is Reverse Osmosis

FAQ response provided by Daniel Webb, Illinois State Water Survey Chemist / Public Service Laboratory Coordinator, (217) 244-0625, danwebb@illinois.edu