For the past two decades, and increasingly today, ultraviolet (UV) radiation has been successfully used around the world for municipal applications including wastewater and drinking water disinfection. UV is a cost-effective and reliable technology that protects the public against pathogenic microorganisms including protozoa, bacteria and viruses. As a growing alternative and in many cases, a direct replacement technology to chemical (chlorine) disinfection, UV does not produce harmful by-products and is non-toxic to the environment. Furthermore, UV technology is recognized as the “green” disinfection solution with a low environmental impact.
Globally, protection of precious water resources is now an ecological and economic necessity. Rapid urbanization, industrialization and population growth leads to more people on the planet but with a finite amount of water. Less than 1% of the world’s fresh water is accessible for direct human uses. This is the water found in lakes, rivers, reservoirs and underground sources (groundwater, aquifers etc.) that are shallow enough to be tapped at an affordable cost. Only this small amount is regularly renewed naturally by rain and snowfall, and is therefore available on a sustainable basis. Because water sources are finite, water quality and quantity have become serious issues. In terms of water quality for human use, the presence of pathogenic bacteria, viruses and protozoa in wastewater and drinking water represents a potential risk to the public. To prevent the transmission of waterborne diseases, regulations specify water treatment processes, nutrient removal, final effluent quality and disinfection criteria based upon the specific requirements of the receiving stream or reuse application.
As municipalities in developing countries face the challenge of treating water for the public in the most cost-effective manner, UV disinfection should be a prime consideration in the technology portfolio. UV technology has matured and is widely accepted as an alternative to or replacement for chemical disinfection in municipalities in the western hemisphere. UV disinfection systems have been successfully designed, tested and installed in thousands of municipal treatment plants around the world. While individual applications (e.g. wastewater, stormwater, drinking water) and treatment objectives differ, they have one important goal in common – to cost-effectively treat water for the public while minimizing the environmental impact of the treatment process.
UV Disinfection of wastewater is a physical process whereby ultraviolet lamps, producing energy in the UVC range (200-400 nm), are housed within a specifically designed treatment reactor(s). The UV lamps produce light photons that attack the microorganisms in wastewater as it flows through the reactor. Within only a few seconds of exposure, the DNA of the microorganisms is permanently altered and the bacteria can no longer reproduce or infect those coming in contact with the water. Important considerations in the design, sizing and proper performance of a UV System include, but is not limited to, the effluent flow rates, water quality, solids concentration, log reduction of bacteria required and type, quantity and arrangement of UV lamps. Each of these parameters is quantified and tested during reactor design and application sizing.
Lake Chapala is the largest natural freshwater lake in Mexico and is the main source of drinking water for the state capital of Guadalajara (with a population in excess of 4.5 million people). Providing a habitat to a variety of fish and a number of North American migratory birds, Lake Chapala has evolved into a growing oasis for nature, irrigation, tourism, recreation and fisheries. Average annual evaporation (1,910 mm) greatly exceeds annual precipitation (781 mm in average) making the runoff and streams feeding the lake a critical important component of the water supply.
There have been many stresses placed on this dynamic water source and looking to future environmental and economic prosperity, great effort has been placed on increasing the quality of municipal wastewater being discharged to the lake. Meeting pressing water quality concerns with diminishing municipal budgets has placed further pressures on infrastructure and equipment maintenance.
The State Water Commission of Jalisco (CEA Jalisco) is responsible for several municipal conventional activated sludge treatment plants discharging to Lake Chapala. Prior to 2011, the majority of plants were disinfecting with chlorine gas. However, with growing concerns over toxic chemical residuals and disinfection by-products negatively impacting the aquatic life, CEA Jalisco needed to add a dechlorination process.
Although adding dechlorination to the treatment process significantly lowers the risk of discharging toxic chlorine to Lake Chapala, CEA Jalisco would still be dealing with hazardous and volatile chlorine gas, posing a threat to plant operators as well as the neighboring public community. After reviewing these safety concerns, CEA Jalisco had to evaluate disinfection using sodium hypochlorite or consider moving away from chemical disinfection entirely to UV disinfection. UV is a physical process and does not generate any carcinogenic disinfection by-products nor does it impart a chemical residual.
Evaluating Disinfection Alternatives
Traditionally, disinfection using chlorine gas has been the most common method of wastewater disinfection. Chlorine gas itself is relatively inexpensive but is a highly toxic chemical that must be transported and handled with extreme caution. It is stored under pressure in large tanks and is released into the wastewater as a gas. It is a strong oxidizing agent that can be extremely dangerous to humans if exposed. As in the case study presented here, the use of chlorine gas required an additional dechlorination step to remove the chlorine residual in the wastewater to protect aquatic life.
Sodium hypochlorite is a diluted liquid form of chlorine. It is a clear, yellow liquid that is corrosive. Because it is diluted, it is not as volatile or toxic as chlorine gas – nor does its unintended release have the same disastrous potential. It is typically purchased and delivered to the treatment plant in large volumes. As with chlorine gas, sodium hypochlorite can create disinfection by-products would require a dechlorination step.
In the interest of balancing public safety and environmental protection with the need for effective disinfection, over 25% of wastewater treatment plants in North America have adopted UV as a more responsible treatment option than chemical approaches. UV disinfection is a physical process that instantaneously disinfects microorganisms as they pass by ultraviolet lamps submerged in the wastewater effluent. As well, UV is effective against chlorine-resistant Cryptosporidium and Giardia which are pathogens in surface water sources that can easily find their way into drinking water supplies. The UV disinfection process adds nothing to the water but UV light, and therefore, has no impact on the chemical composition of the water.
CEA Jalisco had three options to consider:
- Add a dechlorination step after their existing chlorination process.
- Replace chlorine with sodium hypochlorite disinfection and add a dechlorination step.
- Implement UV disinfection and eliminate chlorination and dechlorination step.
- Implement UV disinfection and eliminate chlorination and dechlorination chemicals all together.
Both economic and non-economic factors were considered by CEA Jalisco is making their final decision.
For any UV system retrofit into an existing chlorine contact tank, the majority of the cost would be from procurement and installation of the UV equipment. There is also the added benefit of additional space reclaimed because of the small footprint of UV relative to chlorination. UV disinfection occurs in seconds whereas chlorination requires several minutes of retention time.
The annual cost of operating and maintaining a disinfection system can have a significant impact on the economic evaluation of each option. The O&M costs include the cost of chemicals, electricity, replacement parts, and labor required to maintain each system. The hazards of chlorine gas results in a significant amount of investment into training staff, emergency preparedness planning and maintaining the chlorine system. Chlorine gas prices are relatively low, but this is often outweighed by the intensive maintenance and safety precautions needed for the system. Due to the corrosive nature of chlorine, piping and pumps are prone to leaks and scaling and subsequent replacement. Scaling build-up in piping and pumps require regular acid cleanings to remove. Therefore, these ongoing maintenance costs associated with chlorination systems must be addressed when comparing disinfection alternatives. The O&M costs associated with UV consists primarily of lamp replacement costs and the electrical cost of operating the UV system.
Any disinfection alternative evaluation should also take into account the noneconomic factors that can heavily weigh into the decision-making process. These factors typically include, but are not limited to operator & community safety, ease of operation, process reliability, constructability & space requirements and sustainability/environmental impact.
Considering the environmental impact of disinfection methods, CEA Jalisco soon realized the key benefits of UV are what it does not do. When disinfecting with UV, no toxic residuals are introduced and no disinfection by-products are created. When UV disinfection is utilized, UV light rapidly penetrates the cell walls of bacteria, virus and protozoa, permanently altering their DNA, rendering these microorganisms inactive, unable to reproduce and therefore infect. Wastewater disinfected with UV can be discharged directly into environmentally sensitive areas.
Chlorination with either chlorine gas or sodium hypochlorite gradually destroys microorganisms, but also imparts a residual that even at low concentrations is toxic for aquatic life. Although adding dechlorination post disinfection would essentially eliminate toxic residuals, the long-term effect of discharging dechlorinated compounds into the environment are still unknown. Also, dechlorination does not eliminate disinfection byproducts (i.e. trihalomethanes [THMs]) which are caused by chlorine oxidizing with certain types of organic matter. The dechlorination process, which consists adding chemicals such as sulphur dioxide, tends to deplete oxygen from the effluent which also affects downstream aquatic environments. A re-aeration process – an addition expense – is often included to increase dissolved oxygen levels in the final effluent before discharge.
Considering community and employee safety, converting from chlorine gas would have tremendous safety advantages. Although a chlorine dosing system is relatively easy to operate, it does require a high degree of safety training and maintenance checks. Sodium hypochlorite is less volatile, but is still very corrosive and also requires a high level of safety training and maintenance checks.
UV completely eliminates the need to transport and store volatile and corrosive chemicals. Requiring no special certification or training, UV is safe and simple for staff to operate.
The Solution – UV Disinfection
CEA Jalisco chose UV disinfection technology to cost-effectively treat municipal wastewater plant effluent while also minimizing the environmental impact of the disinfection treatment process.
The criteria for selecting a UV system were:
- Proven local installations
- Automatic lamp cleaning to reduce maintenance for sleeve cleaning
- Recognized local dealer offering technical support
System Design Parameters
- Peak design flow: 40 L/s (0.9 MGD)
- Ultraviolet transmittance: 60% UVT
- Disinfection limit: 1000 FC/100 mL
- Secondary Treatment: Conventional Activated Sludge
Following evaluation of UV equipment options, CEA Jalisco selected the TrojanUV3000Plus™, an open channel system with horizontal low pressure high output lamps, to disinfect secondary treated plant effluent. The TrojanUV3000Plus system is the most-installed UV system of its kind in the world, giving CEA Jalisco confidence in its performance and proven experience. Features that reduce labor requirements were also important to CEA Jalisco in equipment evaluation. The UV system selected has an automatic cleaning system with chemical and mechanical cleaning (ActiClean™) so the staff would not need to perform manual cleaning – an important consideration as it would free up operator’s time so they could do pro-active preventative maintenance and work on other equipment within the treatment plant.
Because UV is a nearly instantaneous physical process where effluent requires a contact time with of only seconds, the required treatment footprint is a fraction of that of chemical disinfection methods – where contact times can be in excess of 30 minutes and large concrete holding tanks are required. For plants converting from chlorine to UV disinfection the existing chlorine contact tanks are most often used for a retrofit. This approach not only reduces construction costs but enables surplus portions of the old chlorine contact tank to be used for water storage, by-pass or other purposes. After retrofitting the existing chlorine contact basin, the first TrojanUV3000PLUS installation was installed and operating in 2011. CEA Jalisco will have a total of six wastewater treatment plants discharging to Lake Chapala using the TrojanUV3000PLUS disinfection system.
The wide-spread adoption of UV wastewater disinfection is the result of the technology’s many benefits, including ensuring community and operator safety, providing public health protection, highly-effective treatment performance, minimal environmental impact, and ability to consistently meet increasingly stringent regulations. For these reasons, UV has been successfully applied over a wide range of applications from low quality wastewaters (primary effluent, blended effluent), typical wastewater (secondary effluent discharging to streams and rivers) as well as advanced wastewater (filtered effluents being reused or discharging to environmentally sensitive areas).
Around the world, municipal UV systems have been installed to safely and reliably disinfect wastewater. These systems are used to safeguard public health through pathogen removal, but they also minimize the operating costs and environmental impact of the treatment process. Drawing from global experience and track records with proven installations, Mexico is seeing an increasing number of wastewater plants disinfect utilizing ultraviolet disinfection. Each UV installation will be successful if the stakeholders proceed through planning and design phases with diligence. The UV Manufacturer can contribute valuable experience to the process by understanding the science of UV disinfection, understanding the impact of effluent quality on UV disinfection and by properly designing and validating their rectors over a range of operating conditions.