Formation of Aluminum Containing Solids in Drinking Water: Influence on Pb/Cu Corrosion, Al Solubility and Enhanced Softening

Abstract

Aluminum salts are used as the primary coagulants in the majority of United States drinking water treatment plants. Despite decades of practical experience, there are important knowledge gaps regarding the effects of residual Al on distribution system materials as well as specific types of solids formed. The first phase of this work examined the formation of aluminosilicate deposits in copper and lead pipes using water from Denver, Colorado. It was anticipated was that these deposits could form barrier films on the pipe, protecting it from corrosion. However, the deposits had slightly detrimental effects on leaching of metal to water, and higher levels of aluminosilicates could further worsen corrosion by-product release.

The second phase of work attempted to extend understanding of aluminum solubility controls by accounting for effects of sulfate and formation of solids other than Al(OH)₃ during water treatment. Sulfate was found to destabilize small Al(OH)₃ colloids resulting in agglomeration into larger flocs from pH 5.0-6.2 . At pH 9.0 and above, Al-Mg, Al-Mg-Si and Al-Si solids were discovered to control Al solubility, while also having significant impacts on the precipitation of calcite in the presence of silica and overall softening effectiveness. This could be of considerable importance to water treatment practice. These solids also had some potential for removal of arsenic, TOC and boron.