Water treatment facilities have been shifting from using chlorination to chloramination as a primary disinfectant since 2004, when the EPA enacted the Disinfectants/Disinfection By-Products (D/DBP) Rule mandating the decrease of DBPs. After the switch to chloramination, an unexpected lead concentration increase was detected in the Washington, D. C., and Greenville, NC water systems. These increases may be associated with the switch from chlorination to chloramination. Decomposition of chloramines results in higher ammonia loading to drinking water distribution systems, which may increase nitrification. Nitrifying bacteria may facilitate lead corrosion via two mechanisms: use of nitrite or nitrate as an alternative electron acceptor and destruction of alkalinity leading to a reduction of pH.
This project explored the roles that nitrifying bacteria play in lead corrosion in drinking water distribution systems. Hypothesized lead corrosion factors provided by nitrification (the presence of nitrate, nitrite, and an acidic environment) were imposed under abiotic conditions. The effect of nitrifying bacteria on lead corrosion was also examined. The effectiveness of several lead corrosion inhibitors (orthophosphate, zinc orthophosphate, alkalinity dosing, and pH control) was examined in the presence of nitrifying bacteria and under abiotic conditions. Nitrifying bacteria were also tested for tolerance to different concentrations of chloramines.
The presence of 2 mM nitrate or nitrite significantly increased lead corrosion. Nitrate served as an electron acceptor in the corrosion process. Lead corrosion occurred concurrently with the disappearance of nitrate and formation of nitrite. Reduction of nitrite was not quantified despite increased lead corrosion. Lead corrosion, arising from abiotic denitrification, was greater for aged coupons than for freshly cleaned coupons in the presence of nitrate. The presence of an acidic environment also significantly increased lead corrosion. When nitrifying bacteria were allowed to grow, lead corrosion factors (the presence of nitrite and an acidic environment) developed. Increased lead corrosion occurred in the presence of ammonia bio-oxidation to nitrite. Lead corrosion was higher for aged coupons than freshly cleaned coupons in biotic treatments. This suggested that the primary cause of lead corrosion for biotic treatments with a freshly cleaned coupon was the development of an acidic environment while biotic treatments with an aged coupon were susceptible to development of an acidic environment and abiotic denitrification of nitrite.
Under biotic conditions, total lead concentrations were significantly reduced for orthophosphate, pH control, and zinc orthophosphate treatments. pH control showed the greatest reduction in lead corrosion (86.9%). Zinc orthophosphate inhibited the growth of nitrifying bacteria and reduced total lead concentrations by 56.2%. Orthophosphate reduced total lead concentrations by 30.1%. Orthophosphate and alkalinity treatments also reduced total lead concentrations under abiotic conditions.
Chloramine doses as low as 0.10 mg/L Cl2 effectively inhibited ammonia bio-oxidation to nitrite when added to an AOB culture growing in a defined medium. Chloramine doses of 0.10 or 0.25 mg/L Cl2 were not inhibitory when added to an AOB culture following four days of growth in a defined mineral medium, in the absence of chloramine. Chloramine doses as low as 0.10 mg/L Cl2 effectively inhibited ammonia bio-oxidation to nitrite when added to an AOB culture growing in tap water, when the chloramine was added immediately or following eight days of growth in tap water in the absence of chloramine.