NOM coprecipitation with solids formed during softening

Abstract

The effect of precipitate characteristics on natural organic matter (NOM) removal during softening was evaluated by conducting jar tests with synthetic organic water under varying conditions—calcium only, magnesium only, and calcium plus magnesium—using humic substances extracted from Lake Austin, Texas. Results from these experiments showed that both calcium carbonate (CaCO3) and magnesium hydroxide [Mg(OH)2] adsorbed NOM during precipitation. Each solid was capable of achieving the same maximum percent removal under the conditions evaluated; however, a greater mass of CaCO3 was required to achieve the same degree of NOM removal. These research results have important implications for softening plants. First, such plants may be able to increase NOM removal by operating at a slightly higher lime dose, maintaining a minimum calcium concentration but avoiding Mg(OH)2 precipitation. Second, an increase in NOM removal with increasing initial magnesium concentration was attributed to the formation of Mg‐CaCO3 and suggests that some softening plants may be able to enhance NOM removal by adding magnesium. Third, results supported findings by other researchers that soda ash is detrimental to NOM removal and should be added to the second stage of a two‐stage softening process if NOM removal is to be maximized.