Posted by Jeff Wahl on 8/27/2022 to UV System Education
There are many common misconceptions about which water treatment equipment is best-suited to treat various contaminants in relation to residential water sources in Canada. This comparison of Ultraviolet Disinfection and Reverse Osmosis highlights two of the most common types of systems that are commonly installed in rural and municipal water source applications. Understanding what each system can treat and how it works is important when considering the installation of drinking water equipment.
Ultraviolet (UV) Disinfection systems transfer electromagnetic energy from a mercury arc lamp to an organism's genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it destroys the cell's ability to reproduce, rendering it unable to replicate or cause harm.
Treatment Method: Microbe Disinfection
Reverse Osmosis (RO) is a process which uses a membrane under pressure to separate relatively pure water (or other solvent) from a less pure solution. RO can remove pollutants from water including lead, pesticides, fluoride, pharmaceuticals, arsenic and many other contaminants.
Treatment Method: Contaminant Separation
The two systems differ in their operation and method of treating water
Point of Entry (POE) - UV
Devices typically treat all water entering a house or a building.
UV Systems disinfect water using UV Rays to ensure that there are no bacterial contaminants that could cause illness active in the water. This process physically removes nothing from the water and functions through the inactivation of the bacteria. It is suited for treating an entire home or just one tap in some cases. They are installed where the water enters a home, typically after a water meter or pressure tank in rural applications. Proper maintenance requires annual lamp changes, quartz sleeve cleaning and a plumbing disinfection.
A UV System is installed for the treatment of bacteria (e.Coli & Coliform) and are commonly sold as Drinking Water Systems by hardware stores, water treatment dealers and plumbing companies. Most UV Systems are installed to treat an entire home, however the systems should be sized to determine the flow rate of water in a home. It is important that the water entering the system contains no hardness, iron, sulfur or tannin. Critical to the proper operation is the ability for the UV Ray to "see" the water; this refers to the protective quartz sleeve that surrounds the UV lamp. That sleeve must remain clean at all times, or the full force of the UV light properties cannot be applied to the water.
Point of Use (POU) - RO
Devices treat water from a single outlet, faucet or fixture.
RO Systems use a series of filters and a membrane to reduce contaminants such as chlorine, sodium, dissolved solids, lead, pcb's, herbicides, pesticides and various chemicals. This process takes time to produce the purified water and is typically suited to a single drinking water faucet at the kitchen sink or refrigerator. They are installed under a kitchen sink or directly below the sink in a basement. Proper maintenance requires filter changes on 3, 6 or 12 months intervals and membrane changes every 2-3 years. Proper disinfection of the system should be completed with every maintenance interval.
Reverse Osmosis systems would be used to remove sodium, chemicals and dissolved solids and are commonly sold as drinking water systems by hardware stores, water treatment dealers and plumbing companies. Systems require minimum operating pressures to work effectively and should not be operated on water that is microbiologically unsafe.
Understanding the Impact of Total Dissolved Solids (TDS)
Water is commonly known as the universal solvent, so when it encounters soluble material, particles of that material are absorbed into the water. This process creates total dissolved solids.
TDS, which is completely dissolved in a water supply and generally invisible to the human eye, refers to the amount of minerals, metals, organic material and salts that are dissolved in a certain water volume that is expressed in mg/L or parts per million (ppm). The Canadian Drinking Water Quality recommended guideline for TDS is less than 500 ppm.
UV disinfection does not alter or reduce the level of total dissolved solids present in a water supply. Reverse osmosis will reduce total dissolved solids in direct correlation with the incoming TDS level present in the source water, the water temperature and pressure supplying the system.
An Example for Drinking Water Considerations
Drilled Well Water Source Containing 928 ppm TDS
- The installation of a UV system would not change TDS levels.
- The installation of a Reverse Osmosis system would reduce that level to within what is acceptable in the Canadian Drinking Water Quality Guidelines.
The Role of Water Testing
In order for both systems to operate correctly, the incoming water should be tested prior to the installation of a UV system or Reverse Osmosis drinking water system. Parameters such as hardness, iron, sulphur, tannin, total dissolved solids, pH, chlorides, sulfates and sodium can potentially have an impact on how well either system effectively treats the water, therefore it is critical that they are identified in the water before installation.
For example, consider that water hardness can foul the UV quartz sleeve. This prevents it from achieving proper disinfection rates and allows bacteria to pass through the UV system untreated and into a home's plumbing. Water hardness can also foul the reverse osmosis membrane, effectively preventing it from allowing water to pass through and reduces or stops drinking water production.
Without proper testing, both systems can operate ineffectively
In comparison, the two systems operate differently and are generally used for very different applications when treating water. Based on years of experience treating rural water, it has been observed that it is commonplace for people to perceive UV systems as an acceptable stand alone method of providing safe drinking water. An ultraviolet disinfection system is best suited for entire building applications (POE) and reverse osmosis is generally used for single tap drinking water treatment (POU). There are applications where reverse osmosis is used to treat the water for an entire building and, in some cases, UV is used at a single tap. When selecting either system, it is essential to know the overall quality of the incoming water, and to size the system correctly for the piping size and flow rate of incoming water. UV systems and Reverse Osmosis are often installed together in a home to ensure bacterial disinfection for all of the water, and for drinking water at a single faucet in the kitchen.
Best Practice Recommendation
If you own or are considering purchasing either system, consider how each operates and what each system treats in order to ensure safe drinking water. It is possible for a reverse osmosis system to be contaminated by bacteria and a UV system to allow contaminants to pass through untreated when used in the incorrect applications. If you are unsure about your water quality or have questions, consult a certified water treatment professional who can provide you with assistance in selecting a UV or Reverse Osmosis treatment system.
Volume 6 - Issue 2 Wahl H2O - Water Awareness
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