Observations and a Geometric Explanation of Effects of Humic Acid on Flocculation

Dissolved organic matter (DOM) is ubiquitous in natural waters and has considerable influence on drinking water treatment, as the presence of DOM can create a need for increased coagulant doses in addition to being a precursor of disinfection byproducts. This work evaluated use of polyaluminum chloride as a coagulant for a synthetic water to determine the effect of DOM on the settled effluent turbidity. The research used a hydraulic flocculation performance model previously published by some of the authors and made additions to the model algorithm to incorporate the effects of humic acid (HA) on flocculation of inorganic particulate matter. Data were obtained using a laminar-flow tube flocculator and a lamellar tube settler. Two adjustable model parameters were used to fit data, one related to the capture velocity used for sedimentation and another that estimated the average size of dissolved HA molecules. The modified model that accounted for the presence of HA was able to independently predict the experimental results from 60 experiments at a different influent turbidity. The predictive model is expected to provide insights into the interactions between dissolved species and the coagulant nanoparticles and represents a new predictive approach for operating drinking water treatment plants.