AWWA WQTC60663

The methods that the City of Calgary Waterworks are using to investigate aluminum residuals in their treated water are the focus of this paper. Several monitoring and modeling approaches are introduced for consideration. The methods used by The City of Calgary to control dissolved aluminum can be easily adapted to any municipality's source water. The City of Calgary Waterworks operates two large conventional water treatment plants, each with 450 ML/d (119 MGD) capacity. Historically, the alkalinity of the surface source waters varied widely throughout the year (up to 200 mg/L as CaCO₃) with a temperature range of 1°C to 20°C, turbidity range of 1 to 500 NTU, and total organic carbon range of 1 to 6 ppm. The high alkalinity of the source waters meant that coagulation reactions occurred at relatively high pH (7.8 to 8.5), which made it difficult to control aluminum in the treated water. Although the annual average aluminum residuals have generally been below 100 µg/L (a Canadian operational guidance value), seasonal fluctuations in residual aluminum levels ranged from 25 to 300 µg/L in the summer months. Treated water aluminum residuals were strongly correlated with raw water temperature, and frequently exceeded 100 µg/L when the raw water was warmer than 15°C. Higher aluminum sulfate (alum) doses were found to reduce aluminum residuals due to the extra reduction in pH provided by the alum. The data also illustrated how a coagulant switch from alum to a medium-basicity polyaluminum chloride (PACl) unexpectedly exacerbated this problem. A three-phase project was conducted to investigate residual aluminum control. The first phase included a review of literature and field data to determine a pH target necessary to reduce aluminum residuals. The second phase developed a unique site-specific pH adjustment spreadsheet to predict operating costs and chemical dosages for sulfuric acid or carbon dioxide addition, as well as the predicted impact on water stability saturation index by such addition. The third phase entailed the development of theoretical aluminum solubility curves with Calgary's high alkalinity source water through bench scale experiments. Alum and PACl were tested to determine the required pH suppression necessary to minimize aluminum residuals. Includes 9 references, table, figures.