Influence of the Mixing Energy Consumption Affecting Coagulation and Floc Aggregation

The operational significance of energy-intensive rapid mixing processes remains unaddressed in coagulation and flocculation of insoluble precipitates (flocs), which play an important role in the removal of impurities from drinking water supplies. In this study, the influence of rapid mixing and associated mixing energy on floc aggregation was examined for a surface water source characterized by a high fraction of aquatic humic matter. Infrared spectral analyses showed that the colloidal complexes resulting from ligand exchange between iron and dissolved natural organic matter (DOM) were not substantially influenced by the mixing energy input. This signified that DOM removal by coagulation can be achieved at lower mixing intensity, thereby reducing energy consumption. In contrast, macroscopic investigations showed the coagulation mixing energy affected floc size distributions during the slow mixing stage in flocculation and, to some extent, their settling characteristics. The results from analysis of floc properties clearly showed that more mixing energy was expended than necessary in coagulation, which is typically designed at a high mixing intensity range of 600-1000 s^-1 in treatment plants. The key findings from this study have practical implications to water utilities to strategically meet water quality goals while reducing energy demands.