Premise plumbing, the pipes and fixtures at the building level, present a unique challenge for maintaining drinking water quality. Of particular concern are opportunistic pathogens, including Legionella pneumophila which can regrow in premise plumbing and cause disease in immunocompromised populations. The goal of this work was to explore engineering methods for control of L. pneumophila and total regrowth. The first line of study involved a series of experiments with simulated glass water heaters (SGWHs) to investigate interactions between specific water chemistry factors and L. pneumophila regrowth, and the second used laboratory grade purified water to investigate the limits of a nutrient control approach for biological stability.

Several water chemistry factors including assimilable organic carbon (AOC) content, granular activated carbon (GAC) biofiltration, plumbing materials, copper concentrations and temperature were investigated using SGWHs. AOC is the carbon available for bacteria growth in drinking water. Results indicated that AOC reduction may be a promising method for controlling L. pneumophila and total bacteria regrowth, but there may be a point at which AOC reduction is no longer effective. Prior GAC biofiltration removed organic carbon and was effective in controlling total bacterial regrowth in SGWHs, but actually encouraged L. pneumophila regrowth.

A wide variety of materials typically encountered in premise plumbing was investigated and only had limited effect on proliferation of L. pneumophila and total bacteria. The effects were dynamic, even with long-term studies. Copper pipes held promise for control of L. pneumophila, as did copper concentration across a range of pHs. Aqueous copper concentration released from pipes was dependent on temperature, however, and thus this control method may not be applicable in all hot water lines.

The peak temperatures for L. pneumophila proliferation fell between 41 and 45 °C, temperatures which could be encountered in a hot water distribution system when the water heater is set to 48 °C, as is often recommended with scalding and energy concerns. A constant temperature of 53 °C seemed to provide control of L. pneumophila, but recolonization is possible even at these high temperatures.

Work with laboratory grade water indicated that extreme control of nutrients was not enough to completely control regrowth in premise plumbing. With stagnation in the cleanest conditions, a 2-log increase of a diverse group of bacteria was observed within 10 days. As drinking water can never achieve such nutrient removal, this study presents the limits of nutrient removal as a strategy for regrowth control.

This work explored both the potential and the limitations of several mechanisms for controlling regrowth in premise plumbing. Understanding how these water chemistry factors affect L. pneumophila and total bacterial regrowth is critical to identifying the most effective engineering controls.