Piloting DAF for Full-Scale Implementation to Mitigate Impacts of Changing Source Water Quality

Abstract

In the last few decades, water treatment facilities in Atlantic Canada have experienced changes in their source waters, specifically elevated natural organic matter (NOM) levels. This is widely referred to as brownification and has been reported in surfaces waters across the Northern Hemisphere. Lake recovery is noted as one factor behind brownification. Lake recovery is characterized by increasing acid neutralization capacity, alkalinity, and pH, which often correspond to increases in NOM concentrations. Climate change is also expected to promulgate the impacts posed by lake recovery with higher temperatures and increased frequency of runoff events. The conditions associated with lake recovery are also favourable for harmful algal blooms to occur which can produce toxins that are harmful to humans, and are difficult to remove through conventional water treatment processes. Treatment plants with source waters experiencing brownification are noticing significant historical increases in chemical dosing for coagulation processes, leading to increased operational costs and waste residuals. Dissolved air floatation (DAF) has been noted as being an effective clarification process for removing low density contaminants such as NOM and algae. DAF poses great advantages as it can reduce chemical dosing as well as reduce flocculation time and tank volumes. This work evaluates the operations of a DAF pilot plant in Nova Scotia for the intended purpose of informing design decisions for full scale implementation. Parameters that were investigated included coagulant dose, coagulant type, and flocculation time. Results suggest that these factors may influence removal mechanisms and that consideration must be taken to effectively target contaminants of concern. The project was operated throughout various seasons which provided data depicting seasonal effects on the process. Data has shown DAF clarification to provide promising results in terms of turbidity and natural organic matter removal. Longer flocculation times may prove to be more robust with DAF but shorter flocculation times provide adequate results and are far more economically friendly.