A CASE STUDY ON CHLORINATION/DECHLORINATION CHALLENGES AT THE 160 MGD 91ST AVENUE WASTEWATER TREATMENT PLANT IN PHOENIX, ARIZONA

The 91ST Avenue WWTP includes three plants (1, 2 and 3) that discharge into an Effluent Channel via three dedicated Chlorine Contact Basins (CCBs). Upstream of each CCB, a junction box diverts the secondary effluent on an as needed basis to Arizona Nuclear Power Plant (ANPP). The plant disinfection system was expanded and improved under the Chlorination Project. The project consisted of adding a 60 MGD CCB3, lining the Effluent Channel to minimize bacterial regrowth and improve blending of the chlorinated effluent between all three plants. This is critical in order to provide a representative Environmental Protection Agency (EPA) compliance sample. In addition, dechlorination was relocated to the downstream end of the Effluent Channel, after all three Plant flows are combined at a location approximately 100 ft upstream of the compliance point. The project team had to account for three constraints: 1) apply the dechlorination chemical after T10 is satisfied, 2) mix the dechlorination chemical into the effluent, given time and space limits, and 3) provide adequate mixing of all three Plant flows. An innovative approach was used that resulted in the design of a unique “facing weir structure” at the downstream end of CCBs 1 and 2 and a Plant 3 diffuser system (5 to 120 mgd). The design of these systems was constrained by limited available pressure head, cost, and structural constraints.