Interactions of corrosion control and biofilm on lead and copper in premise plumbing

Abstract

Premise plumbing can contain copper and lead bearing fixtures, and although copper is considered primarily an aesthetic issue, the neurotoxic effects of lead present a significant public health concern. Utilities approach corrosion control in low alkalinity water by increasing the pH (>9) or adding a phosphate inhibitor at neutral pH. Phosphate inhibitors, pH and chlorine are known to affect lead and copper release through their direct action as corrosion inhibitors or oxidizing agents or through an indirect action via microbial growth. Biofilms are often an implied cause of premise plumbing corrosion, although little is known about their community structure or ability to store metals. The central hypothesis of this thesis is that biofilm contributes to lead and copper release in premise plumbing.

This thesis addresses a unique gap in corrosion literature by integrating traditional corrosion chemistry methods with microbiological and molecular biology techniques. The experiments used three distinct approaches: (i). electrochemical cell experiments to determine the key factors in decreasing lead and copper corrosion in galvanically coupled systems while maintaining microbial control (ii). galvanic macrocells using premise plumbing components to examine the unintended consequences of adding a phosphate based corrosion inhibitor and (iii). an annular reactor study to examine the impacts of two commonly applied corrosion control strategies. In the electrochemical cell experiments, the pH 9.2 with zinc orthophosphate and chlorine treatment achieved both goals: decreased lead and copper release and limited microbial growth. In galvanic macrocells experiments with premise plumbing components, zinc orthophosphate addition was shown to be positively correlated with increased bulk water bacteria, biofilm growth and biofilm community structure as measured by DGGE. Biofilm was also observed to sorb 3-29% of lead and 3-16% of copper from the bulk water. The comparison of the two corrosion control strategies showed that the majority of lead released was in the particulate form, and the results further alluded to the potentially significant role lead particles play in biofilm formation.