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Processes
Disinfection
Disinfection
System Improvements
Disinfection of
WWMA's wastewater effluent is required to reduce the amount of harmful pathogens
being discharged into Brush Creek to acceptable levels. Since the Plant
went into operation, WWMA has been achieving its requirements for disinfection
through the use of chlorine contact. The Authority's NPDES Permit sets a
limit on the amount of pathogens allowable in wastewater effluent, as well as
the amount of chlorine residual remaining in the effluent after disinfection.
WWMA has been in compliance with current NPDES discharge parameters since the
permit has been in effect.
Existing System
Requirements
The
chlorine contact system is located near the microstrainer building and treats
effluent from the final clarifiers before discharging it into Brush Creek.
The table below lists the characteristics of the existing disinfection system.
|
Number
of Tanks |
2 |
|
Length
of Each Tank |
62 feet
(including outfall channel) |
|
Width |
13.75
feet each, 27.5 feet total |
|
Depth |
10.5
feet |
|
Maximum
Water Level |
9 feet |
|
Total
Volume of Each Tank |
11,820
cubic feet |
|
Contact
Time (@4.4 mgd) |
29
minutes |
Prior to August 1998, at
least two, 2,000 pound chlorine cylinders were kept on-site with one connected
on-line and the other on standby. Recent modifications to the Federal
Clean Air Act required changes to the chemical handling and storage practices at
the Plant. As a result, the Authority currently maintains one, 2,000 pound
cylinder on-site and only orders a new cylinder when the on-line cylinder
reaches a capacity of 500 pounds to ensure that the total amount of chlorine
maintained on-site does not exceed 2,500 pounds.
Annual chemical costs for
chlorine are approximately $6,500 per year. Chlorine, however, is
extremely toxic, and care must be taken when handling tanks or working in the
chlorine room in the control building.
Reasons for
Exploring Alternatives
Recently, many
municipalities in the tri-state area have been required to meet more stringent
levels of chlorine residual in the treated effluent. Some areas require a
zero discharge of chlorine residual in the effluent. In addition, new air
quality regulations require municipalities to prepare a detailed and lengthy
risk management plan if certain chemicals, such as chlorine, are being used and
stored on site. With these requirements come more costs and liabilities
for the Authority, as well as an increased need for safety.
WWMA's NPDES permit
currently requires that the chlorine residual in the plant effluent be
maintained at 1 mg/L or less. This requirement can be expected to change
within the next five years, mainly for reasons previously mentioned. As a
result, the Authority may be required by future permits to implement a system to
remove the chlorine from the effluent to reduced residual levels. HDR
Engineering, Inc. reviewed two disinfection alternatives for the Plant.
Alternative
Disinfection Techniques
Alternative I - Chlorination/Dechlorination
The first alternative for
the plant is to keep its current system of chlorine contact and implement a
dechlorination system for removing chlorine residual. This alternative
would involve the construction of an additional tank and mixing
system. The additional tank would be similar to that of the present
chlorine tanks and would most likely be located next to the microstrainer
building. Dechlorination involves subjecting effluent with chlorine
residual to a dechlorinizing chemical, usually sulfur dioxide (SO2).
The amount of sulfur dioxide required is typically on a one-to-one ratio.
In other words, 1 mg/L of sulfur dioxide would be needed to effectively remove 1
mg/L of chlorine residual. The sulfur dioxide works by combining with the
chlorine residual to form chlorides and sulfates already found in water.
Pros
and Cons
PRO:
Chlorination system already in place.
PRO: Initial costs are low
compared to UV disinfection.
PRO:
Operators familiar with chlorination system; dechlorination system is similar.
PRO:
Method has been proven to work for WWMA.
CON: Chlorine and Sulfur Dioxide
are extremely toxic and can be lethal.
CON: Additional chemical handling
would be required of the employees.
CON: More stringent air
regulations will require a costly and detailed risk management plan.
CON: Complete removal of chlorine
residual is impossible.
CON: Treatment not effective
during overflow situations.
CON: Extra storage space is
required for additional piping, monitoring equipment, chemicals, etc.
CON: Tank cleaning and
maintenance is difficult and hazardous.
Alternative II - UV Disinfection
The second alternative to
chlorination/dechlorination is disinfection using ultraviolet (UV) light.
UV disinfection has become a popular alternative to dechlorination due to its
effectiveness, ease of maintenance, and high degree of safety. The system
works in the following way:
- Ultraviolet bulbs are
placed in quartz crystals and mounted onto mechanical arms.
- The arms, in turn, are
mounted to a frame, which is retrofitted in the effluent channel of the
chlorine contact tank.
- The effluent water runs
through a weir in the UV frame, which controls the level of water passing
through the UV system.
- The UV bulbs are
lowered by the mechanical arms into the water passing through the frame and
subjected to a certain dose of UV light. The dosage depends upon the
quality and clarity of the water and the flow rate.
UV disinfection eliminates
the need for chlorination and leaves no chlorine residual. The UV system
is also much more effective at killing pathogens, especially in an overflow
situation. UV systems increase safety by eliminating the need for
chemicals such as chlorine and sulfur dioxide. UV is also low maintenance,
with bulb replacement simple and infrequently required. For the bulbs to
work effectively, they need to be clean. Most UV systems on the market
have mechanical self-cleaning devices that automatically brush away any residue
that accumulates on the bulbs. The system also includes a mechanism for
removing UV modules, consisting of a quartz sleeve, bulb, and cleaning
mechanism, from the reaction chamber. This removal mechanism and cleaning
system reduces the amount of maintenance required. The UV control system
monitors the lamp current, module status, and flow pacing control. The
flow pacing can be adjusted by the operator accordingly depending on variables
such as flow and clarity of the effluent. Another advantage of the UV
system is that it requires no additional space for storage or installation.
The UV system for the WWMA
plant would be sized for a peak flow of 13 mgd. As one might expect, the
initial cost of this technology is high; however, operation and maintenance
costs, which include power for the system and all bulb replacements, are
comparatively low.
Pros
and Cons
PRO: Proven technology that is
extremely efficient at removing harmful pathogens.
PRO: Requires much less manpower
and maintenance.
PRO: Maintenance and operation
costs are low.
PRO: More effective at
disinfection in an overflow situation.
PRO: Requires no chemical storage
and no additional space for installation.
PRO: Extremely safe when compared
to chemical handling and storage.
PRO: Absolutely no chlorine
residual; therefore, will meet future permit requirements.
CON: Initial capital cost of
equipment and installation is high.
CON: Plant personnel not familiar
with the technology.
CON: Bulbs are extremely hot when
in operation and emit UV radiation which can be harmful with prolonged exposure
(note that bulbs are not harmful when submerged).
Recommendations
Based on the overall comparisons of dechlorination versus UV disinfection in
terms of cost, maintenance, safety, and future plant considerations, HDR
Engineering, Inc. recommends that WWMA consider installation of a UV disinfection system.
Among the factors influencing this decision are: cost, future plant
requirements, and worker safety. While the initial costs of UV
disinfection seem high, the system will pay for itself in the future.
Savings in costs associated with risk plans, wiring, building construction,
chemical storage, plant liability, etc. are likely to shorten the time it would
take to recoup installation costs. UV systems have been proven to work
effectively, and costs are straightforward. The UV system eliminates the
need for chlorine, hence no chlorine residual will be present in the
effluent. In addition, because overflows can be a serious problem, UV
disinfection of effluent in an overflow situation provides increased pathogen
removal as compared to chlorination, reducing the impact of overflows on Brush
Creek. A UV system requires relatively low maintenance and would make
disinfection at the plant safer and easier.
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