Factors influencing copper corrosion and THM formation in low alkalinity drinking water

A pilot and bench scale study were conducted to determine the factors that influence THM formation at the Lake Major Water Treatment Plant in 2005. Lake Major Water Treatment Plant produces finished water that is low in alkalinity (∼10 mg/L as CaCO3), low pH (7.4), and TOC concentrations of 2.0 mg/L. Lake Major Water Treatment Plant uses free chlorine for secondary disinfection, and used a combination of poly-orthophosphates for iron, and copper corrosion control in the distribution system. This study considered THM formation at the plant, but also considered formation under stagnant condition in a constructed pipe rig located at the plant. Water parameters that were measured included pH, temperature, orthophosphate concentrations, TOC/DOC, THM, total/dissolved copper, copper corrosion rates and total/free chlorine concentrations.; The factors that influenced THM formation in the pilot study were temperature, free chlorine concentrations, stagnation time, initial THM concentrations, and pipe material. Additional experiments at a bench scale level determined that copper corrosion in new copper pipes is highly sensitive to pH. It was also concluded that finished water held stagnant in the presence of copper coupons increased THM concentrations as compared to blank samples without copper coupons. Other factors that influenced THMs at the bench scale level included stagnation time, and free chlorine dose.; Normally, water utilities collect THMs samples throughout the distribution including the plant and the far ends of the systems up to four times a year in order meet regulations established in the US or Canada. This study demonstrated that the chlorinated finished water held stagnant in plumbing in consumer's homes could have THM concentrations that are up to 70% higher than the levels leaving the plant. This suggests that consumers can be exposed to concentrations well above the regulation, especially during the warmer months of the year.; While this study only considered one water type, namely low alkalinity, low pH chlorinated finished drinking water, this study suggests that it is important for utilities to encourage consumers to flush drinking water through the plumbing in their home to ensure that THM concentrations reflect the levels in the distribution system rather than elevated levels in the home. This study is one of only a few that has considered the effects of copper corrosion on THM formation. It is important that research in this area continue in order to determine factors that influence THM formation under stagnant conditions.