When designing a commercial drinking water system using ultraviolet (UV) light for disinfection, you will need to know the local regulations, how to determine flow rates, what options may be required and whether you need pre-treatment. When speaking about commercial establishments, I am referring to restaurants, daycare centers, schools, hospitals, parks and other entities that serve the public.
Because commercial establishments using groundwater fall under the watchful eyes of the local department of health, it is important to check with them before installing your UV system.
Regulations vary from state to state and from county to county, so knowing the local rules will make your installations go a lot faster.
Flow Rate System Sizing
Most water professionals and engineers will use the maximum pump output to determine the flow rate. However, many state agencies require that sizing be based on fixture counts. The science behind fixture counts (sinks, toilets, showers, washing machines, drinking fountains and other water using devices) is based on well-established engineering charts (see Table 1). A numerical value is assigned to each fixture. After totaling the fixture count, use a cross-reference chart to determine the load rate in gpm (see Table 2). [download pdf for images]
UV System and Options
As noted, there are different regulations for UV systems based on the locality. Most agencies require system validation. System validation is based on a biological test that uses real microorganisms to determine the unit’s “killing” power. The validation test is called a bioassay and involves counting microorganisms before and after the UV system.
Depending on the state agency, this can be done by an independent agency or by meeting Standard 55, which is performed by entities like NSF and WQA. In either case, a UV dosage of 40,000 uWs/cm2 (Microwatts) is required. Because Standard 55 validated systems only go up to about 50 gpm, the larger flow rates need to be biologically validated by outside testing agencies.
In addition to validation, there are other options that may be required. The following is a list of items that may be required:
UV monitor. This is a device that provides the relative output of the lamp as expressed from 0–100%. With new lamps, this device reads 100%. If the output falls, it may be due to lamp aging or quartz sleeve fouling. It should be noted that many states require the output to be expressed numerically and not via a bar graph.
Quartz cleaning system (wiper). This is a device that cleans the protective quartz sleeve. Due to impurities in water, the quartz sleeve can become fouled. To combat this problem, many systems are offered with either manual or automatic cleaning systems. This allows the end user to periodically clean the sleeve.
Flow control valve. This is a device that regulates the flow to the unit and insures that the water is receiving the required amount of UV. Solenoid valve. This is a device that will shut off the water in the event of a lamp failure or low UV output.
Warm up delay. UV lamps can require some time to warm up. This is especially true for cold well water. This device will keep the solenoid closed for a minimum of two minutes, which will allow the lamp to come up to full power. Remote alarms. The remote alarms can be both visual and audible and will alert the owner to a UV problem. Because UV systems are often in remote places within the establishment, these alarms are generally installed in occupied areas like a kitchen.
Plumbing supplies. It is important to include shut-off valves before and after the UV system. This will allow for easier servicing. In addition, the use of unions before and after the system will allow for removal of the system in case of emergency.
As with the UV system design, many agencies have requirements for treatment prior to disinfection. These include water softeners, reverse osmosis systems, back-washing carbon/media filters and cartridge filtration. All of these treatment options will need to be installed ahead of the UV system.
The purpose of any pre-treatment is to remove impurities from the water. While the primary concern is public health, the removal of impurities will actually help the UV system’s efficiency. Any solids, including dissolved, will interfere with the proper transmission of UV light and cause a potential quartz fouling problem.
If we use cartridge filtration as an example, we need to select the proper cartridge size expressed in microns. For most applications, a 5-micron filter is required to remove the larger solids in the water. However, if the system needs to meet state requirements, then more investigation needs to be done.
Many states are requiring the use of 5-micron filters followed by 1 micron or 1 micron absolute filters. When sizing these systems, you must determine the pressure drop. Most manufactures will provide you with a flow rate per cartridge. Table 3 shows examples of flow per 20-in. cartridge at various micron ratings. As with the UV system, optional items are a good idea. These include shut-off valves as well as pressure gauges before and after the filters.
A New York state high school with a growing student base had to upgrade their water system to meet demands. The upgrade included adding new wells. Based on the fixture count at the school, which was 1,750, they had to design a system to treat 300 gpm.
Local regulations required that the system be designed with a 5 micron sediment filter, followed by a 1 micron absolute filter, followed by a validated UV system with wiper, UV monitor, solenoid valve, flow control valve, two-minute warm up delay, remote alarms and a complete set of back up lamps.
This installation is a great example of how water professionals can design a system that meets even the toughest state regulations. wqp