Influence of Polymeric Aluminum Oxyhydroxide Precipitate-Aggregation on Flocculation Performance

Abstract

Polyaluminum chloride (PACl) is a commonly used coagulant for water treatment. One mode of action of PACl [(AlO 4 Al 12 (OH) 24 (H 2 O) 12 ) 7þ ] is reported to be through the formation of polymeric Al-oxyhydroxide precipitate aggregates that bridge between colloids. Although many studies have considered the effectiveness of PACl under a spectrum of conditions for influent suspensions, little is known about the effect of precipitate aggregate size on the subsequent formation of flocs that can be readily removed by sedimentation. Self-aggregation of polymeric Al-oxyhydroxide precipitate and the rate at which this occurs may be used to guide the designs used for the mixing of coagulant with the raw water. In this research, a novel kinetic aggregation model was developed and validated to give the precipitate aggregate size before mixing with a colloidal suspension and entering a flocculator. Granular PACl, a form commonly marketed and used in the developing world, was used for experiments. Efficacy of polymeric Al-oxyhydroxide precipitates of varying aggregate size in flocculation and sedimentation of a kaolin suspension was experimentally observed. These data, in combination with modeling results and geometric analysis, show that under the experimental conditions tested in this research, polymeric Al-oxyhydroxide self-aggregation consistently lowers attachment efficiency of the colloidal suspension, reduces the effectiveness of the flocculator, and reduces turbidity removal. Generally, minimization of aggregate size and maximum turbidity removal is best accomplished by immediate, rapid, and efficient mixing of PACl with the influent water. The relationship between residual turbidity and PACl dose is consistent with results from a geometric adhesive model of coagulation in which surface coverage of colloids by polymeric Al-oxyhydroxide aggregates governs the effectiveness of collisions.

A Hydrodynamic and Surface Coverage Model Capable of Predicting Settled Effluent Turbidity Subsequent to Hydraulic Flocculation

“…It has been found, however, that the components of PACl self-aggregate and go on to form larger clusters (Swetland et al, 2013). For these experiments, the value of d C was chosen based on sizing experiments performed by Garland (personal communication) (21) to data from Re&4,000 experiments (Fig.…”

A bench‐scale study of potable reuse impacts on surface water treatment

“…This reduces the likelihood of collisions among the suspended particles and between the suspended particles and the added coagulant (Weber‐Shirk & Lion, 2010). Instead, at a given pH and coagulant dose, the coagulant particles are more likely to collide with each other and self‐aggregate when the concentration of suspended particles is reduced (Swetland, Weber‐Shirk, & Lion, 2013). Both fewer collisions between suspended particles and the possibility of coagulant self‐aggregation could affect overall floc formation in potable reuse blends.…”