02/27/2023 | Environmental | 9 MINUTE READ
Storm Water Management and Disinfection With UV
When it rains, the water created from the storm can pick up many different contaminants that need to be filtered out before the water is used for any application. One technique that’s proven to be effective is disinfecting the water with ultraviolet rays.
UV is considered a potent disinfect because of its germicidal ability. The ionizing radiation that’s produced from this type of light is capable of breaking chemical bonds and eliminating microbes. It’s possible for this light to kill viruses and bacteria as well as protozoans.
Because of the high concentration of contaminants found in storm water, the best disinfectant is a powerful UV system that can get rid of more than 99% of all contaminants that are present in the water. In this article, the UV disinfection process will be outlined to better understand how it treats water.
What is Storm Water?
California gets around 23 inches of precipitation each year. Whenever it rains or snows, the water will quickly get into the ground in certain areas. For instance, forested areas accommodate around 18-20 inches per hour of infiltration. In comparison, an average lawn usually accommodates two inches per hour. When rain builds up on impervious surfaces like rooftops and pavement, there’s no infiltration.
The water that’s unable to infiltrate will flow over the roofs, ground, and gutters before reaching storm drains. Once storm water gets into a drain, it will be washed into the nearest waterway, which can be anything from a stream to a lake.
Let’s say that a home has around 1,000 square feet of impervious surfaces. This equals 623 gallons of storm water runoff that must be managed in a standard rainstorm. This water will have nowhere to go. Storm water can also be gathered from highways, parking lots, construction sites, and farm fields.
This water eventually flows into local water supplies. Issues with erosion, pollutants, property damage, and flooding can lead to uncontrolled runoff of storm water. Local regulations are in place that indicate how storm water runoff should be managed in a municipality. These regulations detail the best practices that businesses and residents can benefit from.
The latest improvements to UV technology as well as the safety problems attributed to the chlorination treatment have caused UV disinfection to become more widely used in household and municipal systems. At the moment, there are a small number of UV-based water treatment facilities in the U.S. However, there are over 2,000 facilities in Europe.
Many towns and cities throughout the U.S. still have older infrastructure in place. This infrastructure can involve combined sewer systems, which are systems that combine sewage and storm water in a single pipe. While these systems were mainly used in the late 1800s, they are still present in many locations.
Over the years, urbanization has increased, which has resulted in combined sewer systems being unable to handle to excess storm water and sewage. In this scenario, it’s common for combined sewer overflow (CSO) to occur, which results in the water entering nearby rivers and streams. Today, combined sewer overflows are a risk to human and aquatic health.
Municipalities have begun to transition to ultraviolet disinfection because of the ineffectiveness of chlorination. The CSO discharges can consist of viruses, chemical substances, and pathogenic microorganisms. Since the flow rate of CSO is highly variable and has an irregular composition, chlorine doesn’t produce the necessary results.
In comparison, UV systems provide strong filtration for suspended solids and can get rid of contaminants without changing the physical or chemical properties of the water. As mentioned previously, the waves of light produced by UV systems are highly efficient against every known virus and bacteria, which ensures that harmful pathogens don’t get into the environment.
Principles of UV Disinfection
Ultraviolet radiation has wavelength zones of UV-A, UV-B, and UV-C. The shortwave UV-C zone has the germicidal properties that are required for disinfection. Low-pressure mercury arc lamps are able to emit UV light at around 254 nanometers. These lamps consist of an inert gas and an elemental mercury that are contained in a quartz tube.
At 254 nanometers, nucleic acids are able to effectively absorb the ultraviolet energy, which ensures that the bonds in these acids break. The result of this process is that the microorganism is killed. Over the past few years, more powerful UV lamps have been made at power levels that extend from 200-320 nanometers. These lamps are available for businesses that are using ultraviolet systems for industrial applications. Keep in mind, however, that standard UV disinfection systems are powerful enough for household use.
While most ultraviolet radiation systems operate at or near 260 nanometers, some lamps produce radiation at a range of 185 nanometers. At this range, the total organic carbon in the water is significantly reduced. When using a standard UV system, nearly 95% of the radiation will pass through a small quartz sleeve before reaching the untreated water. This water then flows over the lamp, which is kept at a temperature of 104 degrees Fahrenheit.
UV Radiation (How it Works)
UV radiation alters DNA and impedes reproduction in microorganisms. This process allows the organisms to be killed without needing to be removed from the water. All UV disinfection systems have a minimum exposure time of 16,000 µWatt•sec/cm². However, the majority of lamps provide an intensity that reaches closer to 30,000-50,000 µWatt•sec/cm².
Coliform bacteria can be destroyed at an intensity of 7,000 µWatt•sec/cm². Make sure that you replace the lamp occasionally to ensure the UV lights still function correctly. UV systems should also be outfitted with a device that warns the owner when the intensity has fallow below the ideal germicidal range.
When used by itself, UV radiation doesn’t improve the odor, clarity, or taste of water. While this form of radiation is highly effective at destroying contaminants, the disinfecting process only occurs within the unit. There isn’t any residual disinfectant that remains in the water to kill bacteria once the water passes through the UV source. At the right intensity, all pathogens should be destroyed.
One issue with UV disinfection is that dissolved organic matter, inorganic contaminants, and suspended matter can absorb the radiation, which results in the UV doses that are delivered to the contaminants being lower than intended. This issue can be mitigated by using a powerful lamp that ranges from 30,000-50,000 µWatt•sec/cm².
Inactivation Doses for Giardia and Cryptosporidium
The ultraviolet dose is a combination of exposure time in seconds and UV light intensity, which is shown as mJ/cm2. Different microbes have different sensitivities to ultraviolet light. For instance, Giardia and Cryptosporidium are considerably more sensitive to UV than viruses and bacteria. It’s possible for these parasites to be killed with even low-pressure systems. The UV dose that’s needed to eliminate waterborne pathogens is:
- Cryptosporidium – Less than 10
- Giardia – Less than 10
- Salmonella – 8.2
- Vibria cholerae – 2.9
- Hepatitis A – 30
- Shigella sonnei – 8.2
- Poliovirus – 30
- Rotavirus – 36
UV Irradiation Pretreatment
Activated-carbon filtration or sediment filtration should occur before you send water through a UV unit. Turbidity and particulate matter need to be removed before the UV disinfection process takes place. UV disinfection often takes place as the final treatment after filtration, reverse osmosis, or water softening have occurred.
Types of UV Disinfection Devices
Most UV treatment units contain a cylindrical chamber that houses a UV bulb. A simple quartz sleeve encases this bulb. In order for water to flow parallel to the bulb, electrical power is needed.
All UV systems must be made with stainless steel to keep the electronic components from corroding. UV units can have different features. As mentioned earlier, some units include a UV emission detector that will warn you when the light intensity is dropping or when cleaning is required.
Maintenance of a UV System
The light source housing needs to be kept clean at all times to ensure that the UV radiation reaches the intended destination. The cleaning solution should contain 0.15% citric acid or sodium hydrosulfite. Keep in mind that these units are meant for continuous operation, which means that they should only be turned off if you don’t need to treat the water for a few days. When the system isn’t being used, it should be flushed to keep it clean.
Over time, UV lights become less effective. Make sure that the lamp is replaced at least once every year. Within the initial 100 hours of operation, the lamp could lose as much as 20% of its overall intensity. The new intensity level should be maintained for a few thousand hours.
Quick Facts about UV Water Treatment
Some quick facts about UV water treatment include:
- UV disinfection won’t add chemicals to your water
- UV disinfection is effective against viruses, bacteria, and many parasites
- UV disinfection doesn’t leave residual disinfection
- The lamp intensity should be at least 16,000 µWatt•sec/cm²
- UV is the final system used when treating storm water
- The device should contain a UV emission detector
- Regular lamp replacements and maintenance are required
Capacity of UV Disinfection Systems
These systems are point-of-entry systems that can treat all of the water in a home. The capacity of a UV system ranges from 0.5 gallons per minute to a few hundred gallons per minute.
Prefiltration is necessary to get rid of particles, color, and turbidity that could guard microorganisms from the ultraviolet source. Consider using a phosphate injection system or water softener to keep minerals from coating the lamp.
Before using a UV treatment system, make sure that you don’t use it as a substitute for a proper well design. If the source of water is a dug well, the entire well may need to be replaced. The recommended treatment process for storm water involves:
- Get information on your water source
- Test your water annually
- Identify any infrastructure issues that require repair
- Install the proper water treatment systems
Treating storm water is essential to make sure that all of the organisms, inorganic matter, and chemicals that accumulate are filtered out of the water before it’s used for another application. While other treatments are required to filter contaminants out of this water, UV light can be used to remove the viruses and bacteria that other treatments were unable to get rid of.
Posted by Dominic O'Donnell on February 27, 2023
Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.