Freeze Thaw Pond Design

Abstract

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.