Effect of polyaluminum chloride on enhanced softening for the typical organic-polluted high hardness North-China surface waters

The treatment of organic-polluted high hardness surface water by enhanced softening via precipitation using various coagulants, such as FeCl3, AlCl3 and polyaluminum chloride (PACl), was investigated in bench scale experiments. Higher removal of natural organic matter (NOM) (UV254 removal reaches about 50%) is achieved by enhanced softening with coagulant addition conditions as compared with conventional coagulation at natural pH or by softening alone without coagulation. It is found that PACl could enhance the formation of Mg(OH)2 precipitate to remove NOM efficiently at relative lower pH range (pH <10). Under this circumstance, the pH for enhanced softening can be decreased significantly into the practical pH range for treatment plant operation. The efficiency of enhanced softening is affected significantly not only by pH, coagulant type and dose, but also by raw water quality such as NOM characteristics, magnesium content and hardness. NOM with more hydrophobic content and higher molecular weight may be removed more easily in some situations. For water with higher content of Mg2+, the inflexion pH for favorable UV254 removal is lower. It is of practical value for organic-polluted water with high content of magnesium to be treated by enhanced softening with PACl.