Chlorine has been used in municipal wastewater treatment since the early 20th century but it wasn’t until the Federal Water Pollution Control Act of 1972 that regulations were put in place to establish standards for wastewater treatment and the quality of effluent discharged into the environment. Residual chlorine can make its way into the environment via treated wastewater and can be toxic to both plant and animal life. In response, some facilities have installed dechlorination equipment to reduce residual chlorine levels.
What is Dechlorination?
The dechlorination process typically involves the use of a reducing chemical such as sodium bi-sulfite (NaHSO³) or sulfur dioxide (SO²). These chemicals require special provisions for handling, storage and emergency response training. In the last step of wastewater treatment by a plant using chlorine treatment, sodium bi-sulfate is often dosed into the wastewater in a dechlorination basin. To properly manage residual chlorine, effluent is treated with sodium bi-sulfate which must be thoroughly mixed and have the correct amount of contact time to be effective. Precise dosing and control of the reducing chemical is crucial to achieve the desired result.
The History of Municipal UV Water Treatment
Regulations around residual chlorine accelerated the development and adoption of UV (ultraviolet light) for treating municipal wastewater. In 1986, the USEPA Design Manual: Municipal Wastewater Disinfection was published. It included comprehensive pilot data on UV systems and design guidelines for the application of this rapidly growing technology.
UV light is an effective physical treatment alternative that will not create by-products or discharge a chemical residual which can impact the environment.
The Cost Benefits of UV Wastewater Treatment
The initial capital expense and the operating expenses over a finite length of time (typically 20 years) can be added together to determine the total cost of implementation.
Chlorine | Sodium Hypochlorite | UV Treatment | |
|---|---|---|---|
Chemical Supplies | $ $ $ Purchase and delivery of chlorine gas cylinders | $ $ $ $ $ $ Purchase and delivery or on-site generate of hypochlorite | N/A |
Electricity | $ Low electricity requirements | $ Running chemical feed pumps and aeration equipment if applicable | $ $ $ $ Electricity is required to power the UV lamps |
Replacement Parts | $ Minimal replacement parts required | $ Minimal replacement parts | $ $ $ Replacement parts for UV systems are primarily comprised of replacement lamps |
Operators | $ $ $ $ $ $ Labor required for changing chlorine cylinders, maintaining lead detection and emergency equipment, maintaining on-site chemical distribution and storage equipment. | $ $ $ Labor required to maintain pumps, generators, storage tanks, water conditioning equipment, de-scaling equipment, on-site chemical distribution piping. | $ Labor includes replacing UV lamps periodically and ensuring that quartz sleeves that house the UV lamps are kept clean. |
Leak Response | $ $ $ $ $ $ Costs of responding to and repairing leaks are high | $ $ $ Costs of responding to and repairing leaks are moderate | N/A UV lamps contain a small amount of mercury. Leak response and emergency preparedness plans are not required; however, local guidelines must be adhered to. |
Ensuring Compliance | $ $ $ $ $ $ Time-intensive administration for compliance with regulated risk management plans, emergency response plans and community right-to-know programs. | $ $ $ Sodium Hypochlorite is corrosive. Procedures must be in place to ensure proper transportation, handling, storage and spill protection | $ No special preparedness programs or risk mitigation are required. |
Training | $ $ $ $ $ $ Employees must be trained on process management, risk management plans, and evacuation procedures in addition to routine operation of the system. | $ $ $ Training programs must be in place to ensure chemicals are properly transported, stored and handled. | $ UV equipment is straightforward to operate. It requires no certifications for operators. |
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UV Savings Calculator
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How Does UV Treatment Work?
UV treatment uses a fundamentally different process than chemical-based systems. Clarified wastewater, typically secondary effluent, is directed through open channels where it flows past a series of ultraviolet lights that are submerged in the effluent. As microorganisms in the water flow past the array of UV lamps, they are exposed to ultraviolet light at a wavelength of typically 254 nm (inactivation zone). These powerful photons in the UV-C range alter the genomic structure of microbes rendering the microbe unable to reproduce and therefore becomes non-infectious.
Benefits of Choosing UV for Municipal Water Treatment
UV provides effective inactivation of microorganisms through physical treatment of water. The retention time required to achieve inactivation is usually in the range of a few seconds which reduces the physical footprint and capital costs of installation for UV systems.
Chlorine Treatment | UV Treatment | |
|---|---|---|
No Treatment By-products (DBP’s) | ✘ | ✔ |
No Chemical Residue | ✗ | ✓ |
No Chemical Spill Risk | ✘ | ✔ |
Effective Against Cryptosporidium and Giardia | ✗ | ✓ |
Well-Suited for Changing Regulations | ✘ | ✔ |
The top things that you don’t have to worry about with UV
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Comparing Different Types of Municipal Wastewater Treatment
Chlorine | Sodium Hypochlorite | UV Treatment | |
|---|---|---|---|
Transportation | Chlorine gas is delivered to the treatment plant by truck or train. | Has a limited shelf life, requiring frequent transportation and deliveries. | Lamps are typically delivered once or twice a year. |
Operators | Chlorine must be handled with caution. Training programs and certifications must be in place and routinely practiced and updated. | Corrosive and agitating if ingested or inhaled. It must be handled and stored with caution. Containment and leak protection equipment must be in place. | Employees handle the UV Lamps. |
Public | By-products from treatment with chlorine remain in the water when it is discharged into the environment. | By-products from treatment with Sodium Hypochlorite remain in the water when it is discharged into the environment. | UV does not produce by-products from treatment. |
Performance | Cryptosporidium & Giardia are resistant to chlorine. | Cryptosporidium & Giardia are resistant to sodium hypochlorite. | UV inactivates chlorine-resistant microorganisms like Cryptosporidium & Giardia. |
Environment | Chlorine is toxic to aquatic life. Dechlorinating (removing residual chlorine) requires the addition of a second chemical, which subsequently depletes dissolved oxygen that is essential for the survival of aquatic life. | Dechlorination is required when using sodium hypochlorite for treatment. Dechlorinating (removing residual chlorine) requires the addition of a second chemical, which subsequently depletes dissolved oxygen that is essential for the survival of aquatic life. | UV treatment is a physical process and does not chemically alter the water conditions for aquatic life and wetlands. |
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