ULTRAVIOLET DISINFECTION IN THUNDER BAY: ONSITE COLLIMATED BEAM TESTS TO ESTABLISH A DESIGN BASIS FOR UV TREATMENT AFTER TWO STAGE BIOLOGICAL AERATED FILTRATION

The biological aerated filtration (BAF) installation at the Thunder Bay Atlantic Avenue Water Pollution Control Plant (WPCP) is the first municipal installation of its kind in Ontario and one of the largest operating BAF installations in North America. It is now intended to further extend the Atlantic Avenue WPCP to include ultraviolet (UV) disinfection of the treated effluent instead of the current chlorine gas disinfection.In early 2006 a series of collimated beam tests were undertaken with BAF effluent from the plant. The samples were shipped to external certified laboratories, where they were analyzed the next day. The data obtained was quite inconsistent and considered not adequate for use as a design basis for the future disinfection facility. It was therefore decided to undertake on-site collimated beam tests.The tests allowed the designer to:Confirm feasibility of UV disinfectionObtain a representative data set and a higher confidence factor in the dataObserve the influence of BAF operating conditions and variationsObserve the influence of rain eventsInvolve and train City staff in light of the future projectIt appears that the multiple barrier configuration of the Atlantic Avenue WPCP (Chemically enhanced Primary clarification → BAF Carbonaceous removal stage → BAF Nitrification stage) is capable of producing a high quality effluent, which can be disinfected to the disinfection limit with a relatively moderate dose:Average UV transmittance (UVT) of the plant effluent during the test period: 58%Average total suspended solids (TSS) concentration in the plant effluent during the test period: 6.7 mg/lAverage required UV dose for disinfection target: 14.5 mWs/cm2Maximum dose based upon 95 percentile data: 21.0 mWs/cm2