ADDITION OF BAF AND DAF IN THUNDER BAY: A CASE STUDY OF THE IMPACTS ON THE SOLIDS STREAM

The Atlantic Avenue WPCP in Thunder Bay, Ontario serves a population of approximately 105,000. In 2005 biological aerated filters (BAF) were installed at the plant for BOD, TSS and ammonia removal from municipal wastewater. The plant uses a 14 cell two-stage BAF system, consisting of an eight-cell BOD removal train followed by a six-cell nitrifying train. Each BAF cell is regularly backwashed and the waste secondary solids are directed to a dissolved air flotation (DAF) system, one of the first uses of DAF in this application. Dissolved air flotation was selected because of its ability to thicken high volumes of dilute solids.The waste secondary solids are thickened from 500 mg/L TSS to approximately 40,000 mg/L. Polymer is dosed into the DAF system to facilitate the thickening process. The DAF is designed to run at a maximum solids loading rate of 5 kg/m2/hr (1 lb/ft2/hr) and a maximum hydraulic loading rate of 7.5 m3/m2/hr (3 gpm/ft2). The DAF system comprises four rectangular tanks that were installed in a redundant primary clarifier. As with a conventional DAF system, the float is removed from the tank opposite the feed. However, in this application, a novel process was selected, whereby the subnatant is removed from the same end of the tank as the feed. The tanks are covered and the air exhausted to an activated carbon filter. The paper will present performance data for the DAF plant including thickened solids concentrations, polymer dose rates, and subnatant quality.Prior to start-up of the BAF and DAF system, Atlantic Avenue WPCP operated as a chemically enhanced primary treatment (CEPT) plant, with alum coagulant and polymer flocculant. The solids from the plant are stabilized in mesophilic anaerobic digesters. Following start-up of the BAF and DAF system, thickened secondary solids were directed to the anaerobic digesters, and the alum and polymer doses reduced. The biogas generated by the digesters was monitored; the flow remained relatively unchanged despite the incorporation of secondary treatment.Adding secondary treatment affected the performance of the centrifuges used to dewater the biosolids. Parameters such as cake dryness, polymer dose rate, and centrate quality will be presented in the paper.