Vortex mixers mounted on quartz sleeves housing medium pressure UV lamps provide additional mixing and direct the low UVT water toward the high intensity light source.An important consideration in the operation of a low %T UV system is the ability to respond to varying water quality conditions. Over the course of a storm, water quality can vary significantly and the UV system must respond accordingly to ensure full treatment performance and to optimize power and lamp use.
Key monitoring equipment includes UV intensity sensors to measure lamp output, flow meters and on-line UV Transmission monitoring to track water quality throughout a storm event. As operating conditions and water quality fluctuates, the system controller automatically and continuously calculates operational power settings to achieve the required UV lamp output and ensure adequate disinfection.
Another design challenge equally critical to proper performance and long-term operation is quartz sleeve fouling rates and fouling removal. Due to low UVT, higher solids and in some cases the type of coagulants used, the rate and degree of fouling on the quartz sleeves can be accelerated in low UVT applications. Fouling must be removed to maximize the UV light transfer to the water.
The Trojan system uses an automatic cleaning mechanism that provides both mechanical (wipe-action) and chemical (dissolve-action) to remove fouling, optimize UV light delivery and reduce operator maintenance.
Case Study
It is estimated that over one billion gallons per day of stormwater and/or very low quality wastewater is currently being treated with UV disinfection at facilities in North America, Europe, Australia and Asia. Prior to selecting UV technology, several of these municipalities underwent a comprehensive bench scale and/or full scale field testing project to confirm UV’s effectiveness on their challenging effluent.
One particular municipality that evaluated, tested and installed UV for CSO treatment is the Cog Moors Wastewater Treatment Works (WwTW) facility located near Cardiff, UK, in the Barry catchment (southwestern UK region). The region in South Wales served by the Cog Moors WwTW includes three popular beaches. To protect public health, the Environment Agency Wales introduced stringent consents and stormwater spill limits of only three per bathing season, posing a compliance challenge for the Cog Moors WwTW.
The plant is equipped with an activated sludge process for secondary treatment and uses storm tanks to capture and store the storm flows. The facility evaluated several options that would enable compliance with Bathing Water Directives (including the limit on spill events per season). Options included: provision of additional 25,000 m3 stormwater storage capacity and UV disinfection of 2,380 liters/second (54 mgd) storm flow with UVT down to 30%.
