Optimizing alum coagulation for turbidity, organics, and residual Al reductions

Increases in residual dissolved Al from alum coagulation associated with low water temperatures should be minimised to avoid problems in the distribution mains and as a precautionary approach to possible health effects of Al. Temperature-controlled jar-tests (0.1 to 17.0°C) were used to evaluate optimisation of a plant using alum coagulation at pH 6.0 followed by activated silicate addition. pH adjustment was assessed during coagulation and flocculation (i.e. before and after activated silicate) in order to control residual Al by precipitation without affecting turbidity and natural organic matter (NOM) reductions. The effects on NOM reduction were marginal until pH 6.4 in all conditions tested. Turbidity reductions by sedimentation considerably worsened when increasing pH, especially at the lowest temperature. Experimental conditions to eliminate this negative effect were found to be by increasing the pH after silicate addition (from the coagulation pH of 6.0 to between 6.1 and 6.3). By pH adjustments after silicate addition, up to 90% decrease in dissolved Al could be obtained at pH ∼6.8 (from 180 to ∼20 μg/L) at low temperatures. At this pH level, however, turbidity reduction reached minimal values (10–20%) while NOM reduction was clearly affected, indicating partial NOM re-dissolution. Slight pH adjustments of coagulation pH (up to 6.1) or flocculation pH (up to 6.3—after silicate addition) promised significant dissolved Al minimization (down to ∼50 μg/L) without compromising turbidity or NOM reductions.