Enhanced Removal of Natural Organic Matter During Lime-Soda Softening

In this research, a number of new approaches to increase the removal of NOM during the lime-soda softening process were investigated. Specifically, softening sludge recycling in the presence and absence of cationic polyelectrolytes, MgCl2 addition in the presence and absence of softening sludge, and the role of high magnesium lime were all evaluated. Recycling of softening sludge during lime softening was found to effectively improve NOM removal. A 554 mg/L sludge dose added with the stoichiometric lime-soda ash dose removed 28% of NOM present in a raw water as compared to only 10% NOM removal in the absence of sludge. In addition, Hydrophobic/Hydrophilic fractionation analyses, together with SUVA measurements indicated that hydrophobic fractions of NOM were preferentially removed. Further experiments showed that sludge recycling did not significantly affect pH, turbidity, hardness and level of inorganic elements in treated waters. The presence of polyelectrolyte CPAM was found to increase the sorption capacity of calcite particles for Aldrich humic acid by three times. Based on this, softening sludge was coated with CPAM and recycled during jar tests. Results showed that sludge coated with low doses of CPAM can improve NOM removal to level significantly higher than regular softening. However, the NOM removal level was comparable to that achieved by recycling bare softening sludge. The use of high magnesium hydrated lime and the addition of magnesium were found to be promising techniques to increase Mg(OH)2 precipitation and consequently NOM removal during the softening process. DOC removal increased as available magnesium in hydrated lime increased. A dolomitic quick lime sample, slaked at ambient pressure, however, hindered NOM removal due to lack of available magnesium and subsequent coprecipitation. As high as 43% NOM removal was achieved in the presence of 0.312 mM MgCl2 and 457 mg/L softening sludge dose as compared to only 13% for regular softening. DBP precursors were removed as indicated by THM concentration measurements. All the techniques discussed here can be implemented without any modifications in existing facilities or addition of new treatment step and/or equipments.