Yan Zhang, Marc Edwards, Ameet Pinto, Nancy Love, Anne Camper, Mohammad Rahman, and Helene Baribeau

OBJECTIVES:

The objective of this research was to comprehensively study nitrification in drinking water systems targeting two aspects: the impact of water quality and pipe corrosion on nitrification occurrence and the impact of nitrification on water quality and pipe corrosion.

BACKGROUND:

In the United States, utilities are increasingly using chloramine to comply with regulations for disinfection by-products. The ammonia formed via chloramine decay can support autotrophic microbial nitrification. Nitrification can create levels of nitrite that exceed the maximum contaminant level (MCL), stimulate growth of heterotrophic bacteria, contribute to loss of disinfectant, and also create problems with lead and copper contamination from corrosion of premise plumbing systems. Previous research indicates that nitrification was detected in about two thirds of medium and large U.S. systems using chloramines. Given the high costs of corrosion to utilities and consumers, and other possible health concerns related to nitrification, it is critical to better understand nitrification.

APPROACH:

The study used a systematic approach to study nitrification in drinking water systems, including a comprehensive literature review, bench and then large scale studies, followed by utility sampling. The specific research conducted in this study included the following:

* Investigated effects of significant trace micro and macro nutrients on nitrification activity, and studied the role of different pipe materials and pipe corrosion in modifying nutrient levels and nitrification activity

* Examined the interplay between nitrifiers and heterotrophic bacteria

* Verified anecdotal links established between nitrification and pipe corrosion in both well-controlled laboratory and simulated home plumbing systems, and conducted a mechanistic basis for these links

* Examined the role of nitrification in modifying disinfectant efficiency

* Conducted utility sampling to document nitrification effects on corrosion and the effects of corrosion on nitrification

RESULTS/CONCLUSIONS:

Plumbing materials had profound impacts on the incidence of nitrification in homes. Effects were due to toxicity (i.e., release of Cu⁺²), recycling of nitrate back to ammonia substrate by reaction (zerovalent iron, lead, or zinc materials), or release of nutrients that are essential to nitrification by leaching from concrete or other materials.

Phosphate plays a key role in determining where, when, and if problems with nitrification will occur in a given water distribution system. High levels of phosphate inhibitor can provide nutrients to nitrification, reduce the concentration of Cu⁺² ions, and make nitrification more likely, but phosphate can also sometimes lower corrosion rates and increase the stability of disinfectant and its efficacy on controlling nitrifiers.

Dependent on circumstances, nitrification had an increased, decreased, or no effect on aspects of materials corrosion. For example, nitrification markedly increased lead contamination of low alkalinity potable water by reducing the pH, but dramatically decreased leaching of zinc to potable water from galvanized iron. Nitrification did not affect copper solubility in low alkalinity water, but is expected to increase copper solubility in higher alkalinity waters.

Experiments also verified that nitrification could affect the relative efficacy of chlorine versus chloramines in controlling heterotrophic bacteria in premise plumbing.

APPLICATIONS/RECOMMENDATIONS:

Many of the key findings of the research are captured in a decision tree that can be used by utilities, regulators, and scientists. If there is nitrification in the main water distribution system or in the premise plumbing sampled for lead and copper under the USEPA Lead and Copper Rule (LCR), under certain circumstances significant effects on lead corrosion (i.e., low alkalinity water) and copper corrosion (i.e., high alkalinity) can be anticipated. It may be necessary to implement corrosion control or nitrification control to stop these problems.

Even when problems are not apparent through sampling in the pre-1986 homes in the LCR sampling pool, “hotspots” can occur in other “worst case” consumers' homes or in schools. Specifically, buildings with galvanized iron, iron, or plastic plumbing materials can sometimes develop rampant nitrification problems that can be self-perpetuating due to rapid loss of disinfectant. Other worst case situations can occur for homes near the end of the distribution system or which utilize GAC treatment devices. These hotspots could be sites where secondary disinfection is ineffective, or where lead, copper, and other corrosion problems are exacerbated. Depending on the seriousness of the problems, corrosion or nitrification control could be implemented to try and improve water quality in these situations.

RESEARCH PARTNER:

USEPA

PARTICIPANTS:

Thirteen utilities from the United States and Canada participated in this project.