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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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Last modified: May 3, 2018