In the early 1900s, it was discovered that ultraviolet (UV) light had the ability to inactivate microorganisms. Testing showed that waterborne pathogens—disease-causing microorganisms—were inactivated when exposed to UV radiation. While this discovery remained unused due to the availability of lower cost chemicals, UV has been effectively applied in wastewater plants for the last 30 years.
Health concerns related to the use of chemicals in water are leading more engineers and system designers to investigate how UV can be used in place of or combined with traditional chemicals.
Today, UV light is being used for municipal wastewater, drinking water, industrial process waters and a host of other applications.
A recent industry trend has been the use of UV technology to treat residential wastewater.
How the Technology Works
For disinfection purposes, inactivation of microorganisms is carried out by a UV lamp’s UV-C light output, which targets the nucleic acids (DNA and RNA) of microorganisms. Exposure to UV-C light prevents the DNA and RNA from replicating; therefore, the microorganism is prevented from reproducing. Cells that cannot reproduce cannot infect and are therefore harmless.
The actual lamps are housed in quartz sleeves, which are in turn housed in a disinfection chamber or vessel. These sleeves not only help maintain maximum operating temperatures but also prevent the lamps from coming in contact with the wastewater. A quartz sleeve looks like glass but unlike glass, it lets the UV-C rays out.
While in the vessel, the wastewater is exposed to doses of UV energy. Simply put, UV dose is equal to lamp intensity multiplied by residence time. It is usually represented in microwatt seconds per square centimeter (mWs/cm2). Time is calculated as the hydraulic residence time in the UV system.
The intensity is a function of the lamp type, the arrangement of lamps and the energy-absorbing elements in the water that absorb or interfere with light before it reaches the targeted microorganism. The measurement of absorbing material is referred to as UV transmission (UVT). This is expressed as a percentage from 0 to 100—most wastewater plants average 65%.
There are many advantages to this type of disinfection, such as no need for toxic and expensive chemicals; fast treatment; low maintenance and simple and extremely low-cost operation. Because the UV disinfection process does not add chemicals or change the physical or chemical properties of the effluent, the wastewater is ready for discharge when it leaves the system.
Residential UV Systems
Most residential wastewater treatment systems require components that are both rugged and easily maintained. For UV systems in this environment, PVC construction fits the bill. PVC chambers are good for the harsh wastewater and the physical environment and provide a good alternative to traditional stainless steel systems. Because most residential wastewater treatment equipment already uses PVC piping, the UV systems are easy to install.
As with any UV disinfection system, maintenance is a prime concern. The system must be accessible for maintenance. This includes a yearly lamp change but more importantly, periodic quartz sleeve maintenance.
Due to the heat of the lamp and impurities in the wastewater, the protective quartz sleeves can become fouled. In order to prevent this fouling or build up, a quartz cleaning system must be implemented.
This task can be accomplished with a simple manual cleaning mechanism. Using the plunger mechanism, the end user or maintenance operator can wipe the sleeves. The mechanism is comprised of a rod with a handle and a number of bushings housing wiper rings.
By plunging the wiper mechanism back and forth over the sleeves, the operator is able to remove debris and other build up. Once cleaned, more UV light will get through the sleeves and into the wastewater. In order for the system to be effective, this maintenance must be done on a regularly scheduled basis.
System Design
In order to meet regulations or discharge permits, the system needs to be properly sized. To size a system, the following must be understood:
- Physical location for installation (environment);
- Flow rates (peak instant, average and length of periods of no flow);
- Physical size of the piping in the rest of the system;
- Understanding of how and who will maintain the system;
- Determining the discharge permit, which is expressed as a certain number of microorganisms per 100 mL sample; and
- Transmission of the waste- water (55% is low, 65% is average, 70% is found with filtration and 85% can be attained through membranes).
Once the numbers are obtained, they can be put into a calculation. The calculation used is called the EPA Point Source Summation Method, as outlined in the 1986 U.S. Environmental Protection Agency design manual.
These numbers are generally supported through biological testing, which is referred to as a bioassay. Bioassays can be performed in the lab, in the field and at testing facilities around the world.
End users now have the option to integrate a green technology for disinfecting their residential wastewater. While very effective, a proper maintenance plan is the key to success.
