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Ultraviolet disinfection represents the most commonly used alternative to chlorine and chlorine-based chemicals.
By: Robyn Dack
Due to its effectiveness, low cost, and availability, chlorine has been the mainstay of the wastewater disinfection industry for more than 100 years. However, use of chlorine as a disinfectant has several disadvantages:
- Formation of regulated disinfection byproducts (DBPs)
- Trihalomethanes
- Haloacetic Acids
- Formation of emerging DBPs
- Nitrosodimethylamines
- Reaction with acetaminophen (a widely used painkiller) to form toxic intermediaries
- Toxicity to aquatic organisms requiring the use of a second chemical for dechlorination
- Inability to inactivate Cryptosporidium and safeguard public health where wastewater effluent is reused or discharged upstream of water supply sources
- May be hazardous to the operator and the public
Consequently, the use of alternative disinfection technologies has increased in the last two decades. Ultraviolet (UV) disinfection represents the most commonly used alternative to chlorine and chlorine-based chemicals.
Ultraviolet Disinfection
Ultraviolet light is emitted at a wavelength of 100 nanometers (nm) to 400 nm and is between visible light and X-ray in the electromagnetic spectrum. As shown in Figure 1, UV light can be further divided into UV-A (380 to 315 nm), UV-B (315 to 280 nm), and UV-C (280 to 200 nm). The wavelength most disruptive to microorganisms, known as the germicidal range (200 to 300 nm), is primarily in the UV-C portion.
Ultraviolet disinfection is a physical process that restructures the DNA of the target microorganism(s).
For UV light to be effective, it must be absorbed by the DNA and the UV dose must be adequate to restructure the DNA. When this happens, the DNA cannot replicate and the microorganism loses its ability to reproduce. Any organism that cannot replicate cannot infect. As illustrated in Figure 2, both the DNA and effluent UV absorbance are wavelength-dependent. The DNA absorbance peaks at a wavelength of 254 nm—the optimum wavelength. The effluent UV absorbance is relatively low [high UV transmittance (UVT)] in the germicidal range, allowing more UV light to reach the target organism. Because of this, Cryptosporidium and Giardia have been shown to be inactivated by relatively low UV doses. These two organisms are of primary concern in human health.
Ultraviolet Dose
The UV dose delivered is a function of several factors, such as non-ideal UV intensity distribution and non-ideal hydraulics. The theoretical UV dose is the product of the UV intensity and exposure time, and is expressed as follows:
UV Dose = I*t
I = UV intensity (mW/cm2)
t = Exposure time (s)
UV Dose = mW-sec/cm2 = mJ/cm2
The UV dose applied can be accurately determined in a laboratory with a collimated beam test, which is a batch test performed under controlled conditions. Such a direct determination of UV dose in a flow-through UV reactor or channel is not possible due to several complex factors, including water quality, hydraulics, lamp type and age, and sensor performance.
Ultraviolet transmittance is a gross measure of water quality. It is the amount of light that penetrates a given path length of the water sample. The UVT of the water is affected by the dissolved organics, total suspended solids, particle size, iron and manganese concentrations, hardness, and color. A high UVT value (low UV absorbance) correlates to good water quality and a reduction in the size of the UV system required to deliver a given UV dose.
Three types of UV lamp technologies are presently in use in wastewater disinfection. Table 1 shows a comparison of these lamps.
Depending on the manufacturer, lamp configurations include open-channel or in-pipe configuration installation and horizontal or vertical orientation.
Regulations
In Florida, wastewater UV disinfection systems are governed by the Florida Department of Environmental Protection (FDEP) Rule 62-600.440, “Disinfection—Design and Operational Criteria,” and Program Guidance Memo DOM-03-03/OWM-03-06, “Permitting Ultraviolet Disinfection Systems.”
The FDEP Rule 62-600.440 requires “reasonable assurance that public health is protected.” The same rule “acknowledges the potential harmful effects of chlorine” and “advocates alternative disinfection methods.” The rule, however, does not provide any guidance with respect to the design and operation of alternative disinfection systems.
The rule requires public access reuse systems to meet high-level disinfection. High-level disinfection is defined as total suspended solids (TSS) to 5.0 milligrams per liter or less prior to disinfectant application and fecal coliform values below detectable limits.
The FDEP Program Guidance Memo DOM-03-03/OWM-03-06 provides specific guidance for permitting UV systems. It states that the National Water Research Institute (NWRI) Ultraviolet Disinfection: Guidelines for Drinking Water and Water Reuse, Second Edition (2003) “should now be used when permitting UV disinfection systems to provide reasonable assurances that a UV system will be able to reliably meet Florida’s high-level disinfection criteria.” UV equipment validation per NWRI guidelines is a prerequisite for FDEP approval of the UV system.
The following is a summary of the NWRI criteria for UV system design following non-membrane filtration:
- UV dose of at least 100 mJ/cm2 under maximum day flow
- Filtered effluent UV transmittance of 55% or greater at 254 nm
- In Florida, the effluent quality must also meet turbidity or TSS requirements for reuse.
The NWRI guidelines allow a site-specific UVT value to be used in lieu of the above default value. The onsite design UVT is defined as the 10th percentile value of the six-month sampling data (three samples per day).
Valrico AWTP Expansion
The Valrico Advanced Wastewater Treatment Plant (AWTP) is a biological nutrient removal facility, owned and operated by Hillsborough County, FL. The original Valrico AWTP was constructed in 1989, with a design treatment capacity of 3.0 million gallons per day (mgd) annual average daily flow (AADF) and was re-rated to 4.0 mgd AADF in the early 1990s. The plant was expanded in 2000 to 6.0 mgd AADF and is currently undergoing an expansion to 12.0 mgd AADF.
Malcolm Pirnie Inc., a national environmental engineering and consulting firm, was hired to design the 4.0- to 6.0-mgd AADF plant expansion. The Hillsborough Water District was completely satisfied with the year 2000 expansion and therefore hired Malcolm Pirnie to undertake the current UV disinfection system and plant expansion effort. In the past five years alone, Malcolm Pirnie has provided wastewater engineering services to more than 260 clients, working on over 1,200 wastewater projects throughout the United States and abroad.
Based on the existing reclaimed water demand, the plant effluent is either pumped to the existing reuse customers, stored in the two existing 5.0-million-gallon ground storage tanks, sprayed into the 263-acre private spray field, or discharged into the river. The plant is permitted to discharge to three locations with the following restrictions:
- Public Access Reuse System—Permitted to discharge reclaimed water up to 6.0 mgd AADF
- Restricted Public Access Reuse System—Permitted to discharge 2.0 mgd AADF to the spray fields on Valrico AWTP site (263-acre area)
- Surface After Discharge—Permitted to discharge an annual average of 3.0 mgd AADF to an unnamed ditch that flows to Turkey Creek
Water from the Valrico AWTP is being used by reclaimed water customers for irrigation purposes. Table 2 outlines the existing reclaimed water customer demands for central Hillsborough County.
Currently 55% to 60% of the reclaimed water is reused, primarily for irrigation at residences, golf courses, and some commercial and industrial customers. The remaining 40% to 45% of the reclaimed water winds up being discharged to surface waters through permitted outfalls. Most of this discharge occurs during the rainy season when irrigation demand is low. The Valrico AWTP can actually run out of reclaimed water during the dry season and have to ration its use.
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Gaseous chlorine has been used for disinfection since the Valrico AWTP was constructed. Chuck Hammett of the Hillsborough Water District offered the following in deciding on UV disinfection: “In order to permit the expansion and still use chlorine, we would have had to either completely enclose the gaseous chlorine storage building and constructed chemical scrubbers, et cetera, due to FDEP rule requirements or change to bulk sodium hypochlorite (bleach) as a liquid. However, due to disinfection byproduct production caused by using chlorine and the limits of such on our discharge per FDEP permit to surface waters, we decided to go with the UV disinfection as part of the expansion, as the UV doesn’t promote creation of disinfection byproducts as does chlorine.”
Hammett goes on to say that “our 2005 Reclaimed Water Master Plan was approved and we are now updating it with the regulatory agencies as part of our treatment plant expansions. This plan was not the driver in our decision to utilize UV disinfection beyond the issue of disinfection byproduct formation and its effect on our discharge permits.”
The Valrico AWTP uses reclaimed water for certain internal uses, such as filter backwash, pump seal water, wash down stations, and irrigation. All facilities in the Hillsborough County Water District are designed this way in an effort to help limit the use of potable water for functions within the plant where reclaimed water will do the job just as well. This usage has nothing to do with the type of disinfection system utilized.
One component in the expansion to 12.0 mgd is the replacement of the existing chlorine gas disinfection system with a UV disinfection system. Presently chlorine is injected in the influent to the chlorine contact tank, which is located after the filters. The UV disinfection system equipment will be installed in the existing chlorine contact tank channels (Figure 3).
Prior to the design of the Valrico UV disinfection system, plant staff measured UVT of the filter effluent samples. As outlined in the NWRI guidelines, samples were collected three times a day for a minimum six-month period. The UVT data and corresponding flow collected during the study are shown in Figure 4 and summarized in Table 3.
In addition to the UVT measurements performed by Valrico plant staff, seven samples were submitted to an independent laboratory for verification. Figure 5 illustrates the correlation between the locally analyzed UVT data and the independently verified UVT data. The independent laboratory UVT results are shown in Table 4.
Design Criteria
Site-specific UVT monitoring resulted in a 10th percentile UVT value of 70% based on the analysis performed at the facility and 68.2% from the independent laboratory. Conservatively, a UVT of 65% was selected for final design. Using a UVT value higher than the NWRI guideline default value of 55% provided significant capital and operational and maintenance cost savings to the county. The design criteria for the Valrico UV system are presented in Table 5.
It should be noted that, although the use of UV disinfection at the Valrico AWTP will eliminate the need for continuous chlorine addition for disinfection, a sodium hypochlorite system is provided for maintaining chlorine residual in the reuse system to avoid regrowth. The hypochlorite would be added intermittently on an as-needed basis.
Project Schedule
The design for the Valrico AWTP expansion project was completed in September 2006. The remaining efforts and proposed dates of completion are detailed in Table 6.
Robyn Dack is a Forester Communications staff writer.
OW - May/June 2007 |
