Situation Normal During a UV Disinfection Retrofit
The answer is yes at the Patuxent Water Reclamation Facility in Anne Arundel County, Md. A carefully considered construction sequence will keep the facility in operation during its retrofit from chlorine to UV disinfection. Construction started in June 2002 and is expected to be complete by summer 2003.
Moving Towards UV
The Patuxent Water Reclamation Facility was designed to treat 6 million gallons per day (mgd) domestic wastewater for discharge to the Little Patuxent River. PBS&J, an environmental science and engineering consulting firm, evaluated plant processes and, in Phase One of the plant upgrade project, proposed minor modifications to increase plant capacity to 7.5 mgd. The construction cost for these changes was approximately $1 million.
Next, Anne Arundel County, located south of Baltimore, conducted a study to evaluate the planned expansion of the plant to 9 mgd. Because PBS&J had helped the County rerate the plant to 7.5 mgd, the County was able to delay the overall expansion. A study comparing disinfection alternatives to treat the future 9 mgd capacity was included in Phase Two of the plant upgrade.
PBS&J found that chlorine and UV disinfection had comparable present worth costs. While the initial construction cost for a UV disinfection system was higher than the chlorine system, operation and maintenance costs for the UV system were significantly lower over a 20-year period. A new UV disinfection system outscored chlorine disinfection in non-economic factors such as ease of operation, safety, constructibility and maintenance.
Ultimately, the UV disinfection system was selected. The key deciding factor was the opportunity to remove most of the stored chlorine from the site. This decision greatly improved site safety and eliminated the need for a Risk Management Plan (RMP) along with mandated training and evacuation planning.
The UV disinfection system was designed for 9 mgd average flow with space for additional UV modules for future expansion.
Most of Maryland's wastewater treatment facilities are evaluating alternative disinfection technologies. Many choose sodium hypochlorite disinfection to eliminate onsite chlorine gas storage. UV was selected over sodium hypochlorite at the Patuxent plant to eliminate a reliance on chemicals. The only chemical used in UV disinfection is a dilute acid for cleaning tubes. Sodium hypochlorite does not carry the same risk as chlorine gas, but it still can be a dangerous chemical to have on site. Operational costs of a sodium hypochlorite disinfection system also are higher than costs for a UV system.
Choosing the Right System
A number of UV disinfection products from reliable vendors are available in today's market. Several manufacturers were reviewed, and two were short-listed for the Patuxent project.
Both of the short-listed manufacturers offered low pressure/high output lamps. Just a few years ago, the equipment selection would have been limited to either medium pressure/high intensity lamps that could not be installed in an open channel or low pressure/low output lamps. The low-pressure lamps can be installed in open channels, and lamp costs are 25 to 50 percent lower than medium pressure lamp systems. Low pressure/high output lamp systems use about half the lamps that low pressure/low intensity systems require.
One system provided a horizontal lamp configuration that is parallel to wastewater flow. The other system utilized a vertical lamp configuration perpendicular to wastewater flow. Operators from the Patuxent plant observed both systems installed.
Each system performs well, but the low pressure/high output vertical lamp configuration was selected for ease of maintenance. All electrical connections are located above the water surface and operators can simply open a hatch and replace a lamp in minutes.
The UV disinfection system will be installed in two channels to allow for isolation and redundancy. Four modules, each containing a total of 40 lamps arranged in five rows, will be placed in series in each channel.
The 320 lamps will provide a UV dosage of over 30,000 µWsecs/cm2 at a peak flow rate of 20 mgd (hydraulic peak flow at average daily flow of 20 mgd). This dosage has been shown to be adequate in studies and actual installations to meet a 200 MPN/100 ml fecal coliform discharge limit with a filtered effluent (e.g., UV transmissivity greater than 65 percent). The system will be able to treat 75 percent of the maximum rated flow with one module in each channel out of service. A bypass system or fifth UV module per channel could be added to accommodate future peak flows above 20 mgd.
The UV disinfection itself comprises just less than half the $1.65 million total Phase Two project cost. Cost savings were realized by reusing existing contact tanks and channels rather than constructing a new facility.
The water depth in the existing contact tank is 10¢, and the water depth with the new UV system will be approximately 5¢. Therefore, the floor of the first pass of the existing chlorine tank will be raised to accommodate the UV system. A 10≤ sluice gate provided at the end of the raised portion will allow for complete drainage of the UV channel with channel flow sloped towards the sluice gate. Existing sumps in chlorine contact tanks will be used for complete drainage of the system.
Influent channels will be modified to gradually reduce the channel width to approximately 2¢. This will allow for uniform laminar flow at the UV modules and minimum head loss. Grating will be placed over open channels for access.
New walls will direct effluent to a cascade aeration system. The second and third passes of the chlorine contact tank will be abandoned and covered with a sealed concrete system to drain rainwater away from the basins. Covering the abandoned portions of the tanks integrates the unused areas with the new system making the completed project look less like a retrofit.
The project also includes installation of an effluent flowmeter (required for permit purposes) and an uninterruptible power source to serve the influent pump station and the UV disinfection system.
Phasing is Key
A local contractor experienced in wastewater treatment facilities is constructing the facility. The contractor and the consulting engineer will work together to keep treatment plant staff informed of progress and scheduled modifications to operations.
Construction will take place in two chlorine contact tanks currently in operation. The following planned construction sequence is designed to maintain disinfection of the effluent at all times.
- One contact tank will be taken out of service and cleaned.
- The first pass of the chlorine contact tank will be isolated by constructing a new wall at the beginning of the second pass.
- The existing influent box to the contact tank will be modified to allow flow to bypass the first pass and enter the second pass directly. The second and third pass of the contact tank will provide chlorine contact time in a parallel configuration. This will allow for construction of the UV system in the first pass without interrupting chlorine disinfection.
- The same construction sequence will be used on the second contact tank. During the modification of the second tank, only two of the six passes of the original contact tank will be in service. However, at existing plant flow rates, this still will provide greater than 15 minutes of contact time at maximum plant flow rate in accordance with Ten States Standards. In addition, testing was conducted on the existing system to confirm that this amount of contact time would result in the necessary fecal coliform kill. To minimize time that the system is operating with only two passes; the Contractor is limited to 30 days to complete the modifications to each contact tank.
- Once the modifications of the second tank are completed, the Contractor will complete construction of the UV system in the first pass of both contact tanks.
- When the UV system construction is complete and the system has been tested, the first contact tank will be taken out of service again so final modifications can be made. Once these modifications are complete, one UV channel will be placed into operation.
- Once the UV system in one channel is proven effective, the second contact tank will be taken out of service and the second UV channel will be placed into operation.
When both systems are operational, additional accessory items will be installed, including a scum system, covers over the contact tanks and a bridge crane for removing UV modules.
Managing the Hydraulics
The UV disinfection system will have more head loss than the existing chlorine system. Modifications noted above in the construction sequence accommodate this change. The existing weir on the chlorine contact tank influent will be submerged. At a maximum 20 mgd flow, the influent distribution structure will have a total freeboard of 13≤.
The water surface elevation in the UV channel must remain nearly constant in all flow conditions to keep the lamps submerged. Effluent launders will maintain submergence of UV bulbs within a two-inch range between maximum and minimum flows. Two steps in the channel will maintain submergence of the last module to accommodate head loss at higher flows in the UV modules.
Operations and Maintenance
A new microprocessor controller will operate the UV system. Wastewater operators will be able to change set points through a main computer control system. Controls tied to the effluent flowmeter will operate lamps to maximize lamp life. The system will be powered from existing power supplies. A new 80kW diesel-driven emergency generator also will be provided.
Lamps will have an in-situ air scour cleaning system that includes a blower with sound enclosure. Two dip tanks will be used to periodically clean modules with a mild acid solution of citric acid, Lime Away or muriatic acid purchased in solid form and added to the dip tank. The dip tank will connect to an air blower system for agitation and mixing during cleaning.
A one-half-ton overhead bridge constructed of corrosion-resistant aluminum will remove modules for service and cleaning. Modules do not need to be removed to change lamps.
Chlorine still will be required for filter cleaning and filamentous control. Sodium hypochlorite will be used instead of chlorine gas for these needs so that chlorine gas can be eliminated completely from the site.
The Future of Disinfection
The only risk with UV disinfection is exposure to UV light. Precautions including eye shields are built in place, and there is no risk to the public.
The easiest option for any treatment plant is to continue using its existing chlorine disinfection facilities. But many facilities in the State of Maryland are taking a hard look at the use of chlorine gas. With new security concerns, these facilities are evaluating UV disinfection to avoid the risk associated with chlorine gas storage on site.