Overcoming UV Obstacles

The water industry has a reputation for being too conservative, too attached to legacy technologies in an age of innovation. ‘Risk averse’ is another descriptive term that gets bandied about, somewhat pejoratively. But is avoiding risk really a bad thing, especially when it comes to public safety?

To be sure, there is a fine line between avoiding risk and avoiding change. Some treatment plant managers and operators simply want to stick with what they’re used to, and that is less defendable if more efficient, and proven, technologies are available.

UV disinfection is certainly proven, but there still may be resistance due to unfamiliarity. Operators need to know the nuance of a technology — regular O&M requirements and how to prevent and control system upsets — in order to feel comfortable using it.

To that end, what follows are four operation challenges and solutions for UV, as reported by June Leng of HDR Engineering and Neil Frankenberg of the Metropolitan St. Louis Sewer District (MSD). Leng and Frankenburg presented their findings at a technical session entitled “A Large UV System in Action for Secondary Effluent Disinfection” during WEFTEC 2015. The session and its accompanying paper (co-authored by Leng, Frankenburg, and MSD’s Becca Coyle) reflects the “abundant operation knowledge [that] was collected” under varying flow and water quality conditions for two full disinfection seasons at MSD’s Lemay Wastewater Treatment Plant (WWTP).

Challenge 1: Controlling Flow to the UV System

The Lemay WWTP has a system design that splits the incoming flow into two paths — one system disinfects secondary effluent via UV disinfection, while the other system using an existing conduit (the wet weather system, commissioned in August of 2010) disinfects via chlorination/dechlorination. A junction chamber equipped with gates splits wet weather flow to the two disinfection systems.  A nine-foot gate connected to a distributed control system (DCS) is used by operators to manage incoming flow, but such a large gate can be a crude means of flow control — at least without supporting instrumentation — and mismanaged flow will ultimately overburden the UV system.

The lesson for Lemay was that more precise flow control was needed. New gate actuators with reduced deadbands were installed, and the plant used a combination of flow control and level control instruments to help ensure that the UV system was never (again) hit with more than its designed flow capacity of 240 MGD.

Challenge 2: Battling Regrowth for Compliance

At the Lemay facility, the UV-treated effluent is combined with the effluent treated in the wet weather system (by sodium hypochlorite) before discharge. Because E. coli counts for regulatory compliance are taken from the combined effluent, wet weather system issues become whole-plant issues. In this case, stagnant wet weather effluent led to regrowth in the conduit. The DCS system was not set to pump out the effluent until 24 hours an event. After discovering a high E. coli count from a grab sample, the plant changed the DCS setting for pumping out remaining effluent to one hour after an event.

That specific cause and resolution aside, the experience raised the specter of not being able to pinpoint the cause of high E. coli when effluent from multiple treatment schemes is combined before outfall. Since then, MSD added sampling points throughout the plant to more capably troubleshoot problems as they arise, with samples now taken at exit of each UV channel to monitor its true performance.

Challenge 3: Keeping the UV System Clean

As with all UV systems, the pathogens in the water must be sufficiently exposed to the UV light in order to be inactivated. Dirty equipment will inhibit this process, so it’s imperative to perform regular, effective cleanings and maintain/fix equipment that could foul the UV system — particularly the quartz sleeves. At Lemay, rags and debris would often bind to the sleeves’ mechanical wipers, resulting in frequent maintenance. More chemical cleans were needed as well, further burdening the staff.

There were other issues as well. The citric acid dip tanks had uneven bottoms, causing the modules to shift when placed for cleaning and making it difficult to align the lifting bars and extract the UV modules upon completion. Finally, there was an abundance of snails getting into the UV channels, with concern that these snails could foul the sleeves.

To help keep the system clean and fully effective, the Lemay WWTP is upgrading to mechanical bar screens at the head of the plant, replacing its 46-year-old comminutors. Additionally, the frequency setting for mechanical wiper cleanings was changed from every 12 hours to every three hours; fiberglass grating was installed to level the cleaning tanks; and a trace amount of paracetic acid (PAA) was added to the effluent in front of the UV system to control the snails.

Challenge 4: Owning the UV System

The last challenge noted from the observations at Lemay may be most obvious to some, and yet easily overlooked by many. It is that the UV system needs a ‘disinfection champion’ — an instrumentation/electrical technician to perform daily checks for failed lamps, bad ballasts, intensity levels indicating lamp-sleeve fouling, and to verify proper operation of the transmittance meter. Assigning someone ‘ownership’ of the UV system reinforces the importance of these tasks and provides the system the attention it needs to deliver on the promise of consistent water quality and regulatory compliance.