AWWA WQTC60663

The methods that the City of Calgary Waterworks are using to investigate aluminumresiduals in their treated water are the focus of this paper. Several monitoring andmodeling approaches are introduced for consideration. The methods used by TheCity of Calgary to control dissolved aluminum can be easily adapted to any municipality'ssource 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 sourcewaters varied widely throughout the year (up to 200 mg/L as CaCO₃) with a temperaturerange of 1°C to 20°C, turbidity range of 1 to 500 NTU, and total organic carbon range of 1 to 6ppm. The high alkalinity of the source waters meant that coagulation reactions occurredat relatively high pH (7.8 to 8.5), which made it difficult to control aluminum in the treatedwater. Although the annual average aluminum residuals have generally been below 100µg/L (a Canadian operational guidance value), seasonal fluctuations in residualaluminum 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. Higheraluminum sulfate (alum) doses were found to reduce aluminum residuals due to theextra reduction in pH provided by the alum. The data also illustrated how a coagulantswitch from alum to a medium-basicity polyaluminum chloride (PACl) unexpectedlyexacerbated this problem.A three-phase project was conducted to investigate residual aluminum control. The firstphase included a review of literature and field data to determine a pH target necessaryto reduce aluminum residuals. The second phase developed a unique site-specific pHadjustment spreadsheet to predict operating costs and chemical dosages for sulfuric acidor carbon dioxide addition, as well as the predicted impact on water stability saturationindex by such addition. The third phase entailed the development of theoreticalaluminum solubility curves with Calgary's high alkalinity source water through benchscale experiments. Alum and PACl were tested to determine the required pHsuppression necessary to minimize aluminum residuals. Includes 9 references, table, figures.

Product Details

Edition:

Vol. - No.

Published:

11/15/2004

Number of Pages:

13

File Size:

1 file , 1.8 MB