UV Disinfection Systems
NSF International is a not-for-profit, non-governmental organization and a leader in standards development, product certification and education for food, water, indoor air and the environment. One of the many standards developed is the NSF/ANSI 55 Class A Standard. It has been established to set minimum requirements for the reduction of microorganisms using ultraviolet radiation (UV).
Global Gold Standard
All systems that are NSF/ANSI 55 Class A certified are designed to deliver a dose of 40 mJ/cm2. This minimum UV dose has been established as a failsafe set point to ensure 3-4 log reduction of viruses, bacteria and cysts found in typical drinking waters. The 40 mJ/cm2 dose must be achieved at “end of lamp life” and at the maximum specified flow rate of the UV system. To achieve this “global gold standard” of certification by NSF/ANSI, each system is tested with live organisms to establish a “kill rate” at a maximum flow. In simple terms, the calculation of dose required to deactivate pathogens is a function of UV intensity or power, UV transmittance of water, and the amount of UV exposure of the pathogens (flow rate).
At NSF, the calculation of the dose delivered by a UV system is determined by performing a series of bioassays of MS-2 Coliphage. This provides the log reduction of the organism achieved by the UV system. A dose response curve of the organism is also conducted at the same time of the bioassays. This data is then used to correlate log reduction of the organism to a UV dose. Hence, the log reduction of the UV system is converted to dose delivered by the UV system. The bioassays are performed at maximum flow rate and at “end of lamp life,” which is determined as the alarm set point of the UV system. The UV transmittance of the challenge water is decreased to simulate a decrease of UV intensity. The UV transmittance is lowered to at least 70% or to the alarm set point of the UV system, whichever is lower.
The other tests performed by the NSF/ANSI 55 Class A Standard are: the UV unit meets minimum structural integrity tests; extraction tests are conducted to ensure any parts in contact with the water shall not impart levels of extractable contaminants that exceed drinking water limits; and UV alarm performance tests are conducted to ensure alarm operates consistently.
The relationship between UV dose and flow rate is nearly linear, so that, for example, if both 10 and 20 gpm NSF/ANSI 55 Class A certified UV systems are operated at 10 gpm, the 10 gpm system would deliver about half of the dose of the 20 gpm system.
Maximum Flow Rate
Maximum flow rate is fixed in all NSF/ANSI 55 Class A certified systems. This is achieved by integrating a flow restrictor in all certified system configurations. Therefore, a certified system will not run at flow rates beyond the flow that it is certified for. For example, a 10 gpm system will not allow more than 10 gpm of water to flow through the system.
Flow restrictors are often called dynamic orifices as they vary the size of the opening that water is allowed to flow through. The flow element contracts as the pressure increases allowing less water to flow—the greater the pressure, the smaller the opening. This characteristic allows flow restrictors to have better pressure drop values at lower flows as compared to orifices. UV systems that do not have flow restrictors cannot be certified as NSF/ANSI 55 Class A systems, and conversely, removing the flow restrictor or installing a certified system without the restrictor invalidates the NSF certification.
There are always tolerances on the flow restrictors that are typically 10% (some as high as 15%). In the case of a 10 gpm flow restrictor, this implies the flow may range from 9 to 11 gpm. UV Pure has designed their NSF/ANSI 55 Class A systems to ensure that even with the tolerance of the flow restrictor, the flow will not exceed the certified flow rate. Thus, the nominal flow rate of the flow restrictor has been selected at 90% of the unit’s capacity.
While the flow restrictor is critical to ensuring purification, all flow restrictors create a significant pressure drop as they approach their maximum flow limit. This is an important factor to consider when configuring a UV system. In fact, it is not unusual for a flow restrictor to cause a pressure drop of up to 40 psi as it approaches its limit. This is not a linear relationship, and therefore, at only a few gpm below a flow restrictor’s limit, there may not be a significant drop at all.
Why is This Important?
In residential or recreational properties, as well as small public facilities, the maximum flow required may be 7 or 8 gpm. Installing an 8 gpm certified system, for example, will limit flow to 8 gpm, which is enough to service the maximum flow required, but can induce a pressure drop of up to 35 or 40 psi at flows near peak requirements. A pressure drop of this magnitude is significant and will noticeably affect users. In other words, in a home installation, it may mean that there is not enough pressure for a shower in the morning when other appliances and taps are in use. Adding to the pressure drop are also pretreatment devices such as filters. As filters load or become fouled, the pressure drop across them will increase.
It is important to size certified UV systems so that the maximum flow rate of the unit is greater than the maximum required by the application so that pressure drop is minimized. UV Pure Technologies’ smallest NSF/ANSI 55 Class A certified system is rated at a maximum flow rate of 13.3 gpm. This flow rate ensures that any application with maximum flow needs of approximately 10 gpm or less will not experience noticeable pressure drops at that level.
Similarly, a certified 6 gpm system would work well with a low pressure drop at a flow rate of 4 gpm; however, this may be too low for today’s larger homes and recreational properties. Of course, the same relative calculations apply to small commercial and public applications and are true for larger scale use as well.
The curves in Figure 1 show pressure drops for several units. The certified systems show the pressure drop increasing as the maximum flow rate of the unit is approached. Although the pressure may increase upstream of the unit, the flow restrictor limits the flow below the unit’s capacity.
Where the application calls for a flow that exceeds the flow capacity of one unit, multiple units can be used. If 45 gpm is required for a golf course, for example, two Hallett 30 units can be installed to operate at about 22.5 gpm each. Not only does the pressure drop of the system decrease, but there is also a redundant unit in place. Having multiple units in place can allow for greater flexibility in terms of maintenance since one unit can always be on-line while another is being serviced. In addition, the Hallett units operate independently of each other so in the event of a unit fault or loss of power to the unit, the valve will automatically close, preventing untreated water from passing through the unit. Many units can be “ganged” together to operate in parallel to achieve flows of hundreds of gpm.
Some regulations call for units to be installed in series in order to double the dose. Having this arrangement of NSF/ANSI 55 Class A certified UV systems implies having two flow restrictors in series as well. The pressure drop across two units in series is almost double that of just one unit alone. This situation may call for a secondary booster pump to increase the water pressure downstream of the units.
Installing a certified system without the approved flow restrictor will void its certification. There may exist installations where the flow restrictor is simply not installed in order to increase the flow or eliminate the pressure drop. Another argument that can be made is that the maximum pumping capacity of a specific well pump may only be 6 gpm and therefore below the capacity of a 10 gpm certified system. However, if the UV system is installed downstream of the pressure tank, there may be sufficient demand on the system to momentarily exceed 10 gpm. In any cases where flow restrictor are removed to reduce pressure drop, the flow may go higher than rated capacity of the unit, but the water will not be safe, and the system is not certified. wqp