From stock to floc: An investigation into the physical/chemical processes controlling aluminum sulfate and polyaluminum chloride behavior in a gravity powered drinking water treatment plant

Coagulation and flocculation processes have been utilized in municipal scale drinking water treatment plants for many decades. Coagulation mechanisms have been defined and flocculation parameters have been tested in many studies. However, this knowledge has not been successfully aggregated into an overarching theory of coagulation and flocculation that can be applied to the design, construction, and operation of drinking water treatment plants. To this end, a prototype chemical dose controller was designed and tested to be used in conjunction with a linear flow meter for the accurate and consistent dosing for coagulant chemicals into the influent water in a plant. Scalable, physically based algorithms for the automated design of the flow meter and dose controller are available online at aguaclara.cornell.edu.;A bench-scale water treatment plant apparatus with a hydraulic flocculator was created and validated to quantify flocculation performance for a wide array of conditions. Extensive data processing was also carried out to characterize the sedimentation of the flocculated suspensions. This powerful combination of bench-scale reactors and data analysis was used to study the rate of formation of PACl aggregates upon mixing with influent water and the effect of aggregate size on the subsequent formation of flocs that can be readily removed by sedimentation. Results show that, under the experimental conditions tested in this research, PACl self-aggregation consistently lowers attachment efficiency of the colloidal suspension, reduces the effectiveness of the flocculator, and reduces turbidity removal. Optimal performance is best accomplished by immediate, rapid, and efficient mixing of PACl with the influent water.;Mechanistically-based scalable algorithms for design and performance of hydraulic flocculators were developed in another study based on observations of residual turbidity for a range of influent turbidities (5--500 NTU) and coagulant doses (0:01 -- 0:15mM), for two hydraulic residence times (800 s and 1200 s) and for two coagulant types (polyaluminum chloride and aluminum sulfate). Seemingly disparate results were unified through creation of a composite design parameter that considers collision potential in the flocculator and coagulant surface coverage of colloids. When calibrated properly to the coagulant and source water to be treated, the predictive model is expected to be a powerful tool in the design and operation of hydraulic flocculators.