Assessing graphite and stainless-steel for electrochemical sensing of biofilm growth in chlorinated drinking water systems

Infectious diseases caused by pathogens (e.g. bacteria, viruses, protozoa) are the most common and widespread health risk associated with drinking water. Biofilms are known to harbour pathogens in drinking water distribution systems (DWDSs). Disinfectant dosing (e.g. chlorination) is an effective method to control biofilm growth, but overdosing can lead to formation of harmful disinfection by-products. One approach to prevent overdosing is the development of a dynamic biofilm-sensing system that can guide disinfectant dosing on a real-time basis. This study explored the use of electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurements for the development of a biofilm sensor. Two materials (graphite and stainless steel) were compared for their sensitivity as an electrode material for biofilm detection, using incubation experiments in which freshwater from a drinking water supply dam was the source of microbes for biofilm formation. EIS and OCP measurements were correlated with biofilm growth-related parameters (flow cytometer cell counts). Chlorination (4.4 mg Cl-2/L) was included as a treatment for biofilm destruction. Among a range of electrochemical parameters determined, double-layer capacitance derived from the equivalent circuit model of EIS showed the strongest positive linear relationship with cell density for each electrode type (R-2 > 0.9). Stainless steel was 10-fold more sensitive than graphite (2 x 10(-5) vs. 2 x 10(-6) % capacitance change/cell cm(-2) ratio), suggesting that stainless steel is a more effective material for biofilm sensing. The observable changes in capacitance were exclusively triggered by biofilm formation, and chlorine residuals did not affect the capacitance of either electrode type. The results indicate the potential of using stainless steel for development of a practical biofilm sensor for water utilities.