Destabilization and Aggregation in Turbulent Pipe Flow

Abstract

The application of hydrolyzing aluminium and iron salts in chemical water and wastewater treatment requires a rapid and uniform distribution of the chemicals in the raw water. This can be attributed to the short reaction times involved in the formation of hydroxocomplex species and their adsorption onto the particle surface, which are in the range of 10^-4-1 s. However, coagulation experiments in stirred continuous flow reactors do not allow defined reaction times on this order of magnitude to be adjusted. Therefore, a pipe flow reactor for particle destabilization and aggregation investigations has been constructed, permitting a minimum defined reaction time of 0.05 s. This plug flow type reactor has been used to investigate the destabilization kinetics of silica particles with aluminium and prepolymerized aluminium under controlled flow conditions during the very first reaction phase.

The results show that aluminium hydroxocomplex species with a maximum destabilization potential axe formed within an interval of less than 0.1 s. Increased reaction times result in the formation of hydrolysis species, characterized by a decreasing destabilization efficiency. After 6 s, the hydrolysis reaction is terminated and sweep coagulation is the dominating process. There are indications that pre-polymerized aluminium products are more stable after dosing and dilution than polymeric species of Al³⁺, formed during the hydrolysis reaction after dosing. The importance of high-intensity mixing during the destabilization phase is emphasized by these results. A comparison with data from technical scale plants leads to the conclusion that coagulation process efficiency can be improved with a proper design of the mixing device for the chemicals.