Ultraviolet (UV) light is used to disinfect many types of water, including low-quality water like primary treated wastewater and combined sewer overflows (CSOs). CSO treatment is becoming a regulatory requirement in many regions.

Chlorine is traditionally used to disinfect CSOs because of its low cost, but due to the adverse environmental impacts of chlorination byproducts, chlorine residual limits are becoming stricter. UV light is a proven alternative. It is a cost-effective technology for disinfecting low-quality wastewater as long as the system is properly designed.

Because stormwater treatment involves high flows with high suspended solids content, variable temperature, and disinfectant-resistant pathogens, the disinfection system must have rapid oxidation and powerful pathogen-killing capability. In these applications, chlorine systems can require long contact times and thus large equipment footprints and high construction costs.

As in all disinfection technologies, UV disinfection design is a function of the water quality being treated. Because UV disinfection is a physical (not chemical) treatment, the relationships between disinfection and water quality are more easily defined and quantified. Once the relevant water-quality parameters are defined for a stormwater event, it is possible to design a UV reactor to cost-effectively meet the disinfection requirements for future events.

 

Proper design

The objective in UV disinfection is to transfer UV energy into the water. Low-quality wastewater has low UV transmittance, and so the reactor design challenge is greater. The key to proper UV reactor design is to optimize the effective water layer between the UV lamps for the transmittance of the water.

In low-transmittance water, the effective water layers need to be shorter. This can be accomplished with more powerful lamps, narrower spacing, or hydraulic devices that induce streamlines and direct flow toward the lamps.

Each option must be evaluated against its trade-offs. For instance, higher-power lamps can use more energy but allow wider lamp spacing, leading to a lower-headloss reactor design. Alternately, with lower-powered lamps, the required narrow lamp spacing and hydraulic devices can increase headloss.

Higher headloss can result in water level increases in an open-channel UV reactor, leading to a large water layer above the lamps (short-circuiting), or leaving a large section of downstream lamps exposed to air. These zones with large water layers or exposed lamps provide little or no disinfection, and reactors with these hydraulic flaws will fail when challenged in full-scale operation.

Any fraction of the flow that receives less than optimal UV doses will limit reactor performance. These issues can be overcome by using sophisticated computational fluid dynamics (CFD) modeling with accurate irradiation models to design UV reactors for stormwater applications.

 

CSO challenges

Low-quality wastewater is typically high in suspended solids, and those particles can harbor microorganisms that resist disinfectants. UV dose-response curves are generated in a laboratory using calibrated collimated beam devices to quantify the relationship between applied UV dose and microorganism survival.

UV dose-response curves for microorganisms in low-quality wastewater typically have two slopes, characterizing easy-to-disinfect free-floating microorganisms and the more challenging particle-associated microorganisms (Figure 1).

A typical disinfection objective for low-quality wastewater ranges from one to three log reductions of the target or indicator organism. In response, design UV doses to meet these requirements do not need to be excessively high because the limits are typically reached by disinfecting the free-floating microbes.

UV disinfection technology testing is typically done at flow rates lower than those common in CSO applications. Where sampling data is limited, a UV system can be designed using information from a UV dose-response database. Trojan Technologies, for example, has an in-house microbiology laboratory that has analyzed more than 25,000 data points related to wastewater UV dose-responses and has a database that covers more than 20 years. It allows prediction of design UV doses for different indicator organism concentrations and associated statistical limitations.

For stormwater high in suspended solids, the effectiveness of any UV design dose depends on the disinfectability of the water and on the properties of the solids. The relationships between TSS and UV transmittance can be derived from the database and used for disinfection system sizing.

As one example from the database, typical water quality during a stormwater event can be 200 mg/l TSS at first flush and 90 mg/l TSS during the extended storm. UV transmittance varies from less than 20 percent at first flush to greater than 65 percent near the end of the storm.

This data, combined with water-quality data from sampling, provides a high level of confidence that the UV system design will consistently and cost-effectively meet the discharge requirements.

 

Essential features

Long experience shows that key design features will realize cost-effective UV disinfection for challenging waters. The features described below provide an example of a UV that has been tested, installed and now operates successfully in a number of low-UV-transmittance applications. The design effectively incorporates the features that overcome the challenges related to disinfecting low-quality wastewater.

UV energy source

Effluent flows by gravity through a fully submerged, tubular reactor, where it is exposed to high concentrations of UV light generated by medium-pressure (MP) high-intensity lamps. Contoured reactor walls tightly control the water layer around the lamps for consistent disinfection regardless of flow rate or water level.

UV modules house the lamps, quartz sleeves and cleaning system and pivot into the reactor opening at the upstream and downstream ends. Lamps are placed in a staggered array, spaced evenly, and optimized to balance the trade-offs between headloss generated and mixing induced. Vortex mixers optimize performance at lower UV transmittance values (Figure 2). The mixers are mounted on the quartz sleeves and increase flow turbulence and mixing around the lamps (Figures 3 and 4).

Monitoring

An effective system must respond to varying water quality during a storm to ensure full treatment and optimize power and lamp use. Key monitoring equipment includes UV intensity sensors to measure lamp output, flowmeters, and online UV transmission monitoring to track water quality (Figure 5). As operating conditions and water quality fluctuate, the UV system controller automatically and continuously calculates the power settings required to achieve the necessary UV lamp output and ensure adequate disinfection.

Maintenance

The design must consider the quartz sleeve fouling rate and extent and provide for fouling removal. The rate and degree of fouling on the sleeves can be accelerated in stormwater applications. Options for fouling removal include manual cleaning and semi-automatic or automatic cleaning mechanisms. Some UV systems today offer automatic mechanical and chemical cleaning to remove fouling completely, optimize UV light delivery, and reduce operator maintenance.

 

Full-scale application

An estimated one billion gallons per day of stormwater or very low-quality wastewater is being treated today with UV disinfection in North America, Europe, Australia and Asia. Among facilities using UV is the Cog Moors Wastewater Treatment Works near Cardiff in southwestern UK.

To protect public health, notably on local beaches, the Environment Agency Wales limits stormwater spills to three per bathing season. The plant uses activated sludge secondary treatment and has storm tanks to capture and store storm flows.

The plant team evaluated intermittent UV disinfection (during storm events only) against adding stormwater storage capacity. An analysis found that UV disinfection would cost significantly less and produce one-tenth the greenhouse gas emissions.

A pilot study was conducted to confirm the effectiveness of UV disinfection for the Cog Moors CSO effluent, since it was the first UV plant in the UK designed to treat stormwater flows. Since commissioning in 2009, the UV plant has operated successfully. Effluent quality monitoring has shown the plant meets the required bacteriological reductions, ensuring safe beaches.

 

Knowledge is power

To ensure reliable stormwater disinfection, the equipment manufacturer must know and understand the quality of the water being treated, as well as proper UV reactor design, including the science of delivering UV energy to the pathogens of concern.

The equipment selected should be from a manufacturer with good scientific understanding and a demonstrated history of applying UV for low-quality wastewater. The design trade-offs in headloss and power consumption, which are a function of UV lamp intensity, lamp spacing and mixing, must be evaluated. Finally, the UV reactor configuration must be verified through field testing and independent bioassay validation to guarantee performance at full scale.

 

About the Authors

Jennifer Muller is municipal UV market director and Wayne Lem is market manager with Trojan Technologies, a manufacturer of UV disinfection systems based in London, Ont. They can be reached at jmuller@trojanuv.com and wlem@trojanuv.com.