UNDERSTANDING PHOSPHATE FOR CONTROLLING METALS RELEASE AND BIOFILM FORMATION IN DRINKING WATER DISTRIBUTION SYSTEMS

Abstract

Corrosion is a common problem in drinking water distribution systems and leaching of contaminants such as lead is a concern for the health of all consumers. Populations who are more susceptible to lead exposure are infants, young children and pregnant women. Those who are exposed to lead for a period of time are at risk of developing negative impacts to the nervous system such as lower IQ, behavioral problems and intellectual delays.

Two experiments were conducted in order to evaluate the effect of orthophosphate and pH on metals release and biofilm formation in the distribution system: (1) bench scale simulated partial lead service line replacements were used in a controlled laboratory setting using two water sources (surface water and groundwater) in stagnant water conditions and (2) annular reactors were used in a controlled laboratory setting containing copper coupons with 50:50 lead tin solder to simulate pipe material and allow for biofilm accumulation. Additionally, a lead and copper monitoring program was completed in a non-residential building where the groundwater was the water source to determine the potential sources of lead and copper at the consumers tap.

The bench scale simulated partial lead service line replacement experiment resulted in the highest lead levels due to the presence of stagnant water conditions. Additionally, the groundwater had significantly higher lead levels than the surface water due to the presence of a high CSMR (7.7 to 8.3) which was 11 to 12 times higher than the recommended criteria of 0.7. The interaction between pH and orthophosphate was statistically significant in reducing lead levels for both water sources.

Since the groundwater source in the bench scale partial lead service line replacement resulted in significantly higher lead levels, this water source was chosen for the annular reactor experiment and at the tap monitoring. The effluent of the annular reactors had a CSMR between 7 and 8 which was 10 to 11 times the recommended criteria of 0.7. Orthophosphate significantly reduced bulk water total lead levels. Furthermore, chlorine significantly reduced both bulk water and biofilm cATP concentrations. The addition of orthophosphate significantly reduced bulk water cATP concentrations, but significantly increased biofilm cATP concentrations. Lastly, both pH and orthophosphate played a significant role in reducing biofilm total lead concentrations.

The monitoring program determined the at the tap CSMR to be between 7 and 8 which indicated a corrosive environment was present. First draw total lead samples resulted in two locations being above the action level: raw groundwater and bathroom. The sources of lead throughout the distribution system in this case were brass faucets and valves. Additionally, high copper concentrations were found at four out of the five locations throughout the monitoring program due to the presence of copper piping and components along with corrosive water with a high CSMR. A high percentage of samples were not only above Health Canada’s aesthetic objective, but also above the EPA action level and the WHO guideline.