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/m 2 /hr (1 lb/ft 2 /hr) and a maximum hydraulic loading rate of 7.5 m 3 /m 2 /hr (3 gpm/ft 2 ). 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.
The City of Winnipeg has been evaluating options for its new water treatment plant's (WTP's) residuals management. Several mechanical systems such as centrifugation and belt presses were reviewed. However, a cost effective and environmentally friendly solution was desired for final dewatering of the solids sludge. Due to the favourable climatic conditions, the feasibility of freeze thaw technology for the City of Winnipeg's residuals solids dewatering was evaluated. A desktop evaluation and pilot study were completed. The desktop study concluded that freeze thaw ponds have been effectively used throughout cold regions of Canada and the United States. Due to the large scale of the Winnipeg project and to obtain practical confidence in the technology, a pilot scale sludge thickening test device was constructed in order to more closely identify the anticipated sludge concentration and supernatant quality after thickening. From a ten-week pilot trial, it was noted that solids concentrations increased from approximately 0.6% by weight to as high as 8% by weight after freezing. The test results also showed that the sludge solids concentration as high as approximately 50% can be achieved with an adequate drying period. Conventional higher energy technologies do not provide such a high solids concentration. Upon review of the pilot work and desktop study, the application of the freeze thaw technology was accepted as the way forward for the City of Winnipeg solids dewatering project. Due to site constraints, operational and economic reasons, the design of the facility posed several challenges. This paper intends to highlight some of the design criteria and challenges.
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