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
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
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
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.
contact tank will be taken out of service and cleaned.
first pass of the chlorine contact tank will be isolated by constructing a new
wall at the beginning of the second pass.
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.
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.
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.
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.
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
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
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
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 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.