36/Plumbing Engineer June 2021
By Tim Keane and Gary Klein
et us be clear: Legionella bacteria live in our water systems and enter our buildings through the water supply. Typically, if Legionella is present in the municipal water system, the levels are too low to measure. However, given the right conditions, Legionella can mul- tiply to disease-causing levels in the premise plumbing system. The task we face is how to design premise plumbing sys- tems to minimize the factors contributing to the growth of Legionella while also minimizing construction and mainte- nance costs and maximizing water and energy conservation over the life of the system. The most significant problem today in addressing these tasks is the common misconception that achieving them all is impossible. They are absolutely achievable. More impor- tantly, if we are to reach our net-zero carbon goals, achiev- ing these tasks on new construction ASAP is an absolute requirement! Often buildings looking for LEED certification will forgo the water component because the Legionella risk is too high. This is a real issue. Trying to achieve low water use on buildings designed for much higher water use is a significant Legionella risk. In 2016 we, along with Marc Edwards, jointly pre- sented a program at the International Emerging Technology Symposium titled, "Water Quality, Water Savings and the Water-Energy Nexxus: Three Issues, One Solution." We discussed the numerous variables impacting Legionella growth in premise plumbing systems: tempera- ture, water age, disinfectant residual, total organic carbon, microbial population, biofilm, water velocity and plumb- ing materials. The impact of some variables on Legionella growth, such as piping material (CPVC, PEX, copper, etc.), is difficult to quantify. Two of these variables, temperature and water age, have a huge impact on the Legionella growth risk in building water systems. These two variables also directly impact water conservation and energy efficiency. You can describe water age in several ways, but the simplest is the volume of water in the premise plumbing system, including all tanks and piping divided by the daily water consumption. Temperature is well-documented as key to Legionella control. Temperature alone can be used for Legionella con- trol or temperature can be used in combination with other controls such as disinfectants. Several factors can impact the efficacy of water temperature in controlling Legionella : the presence and amount of biofilm, disinfectant residual, ambient temperature and contact time. The architect and the plumbing design engineer directly impact the stored water volume and, consequently, water age. The architect does it by determining the location of the wet rooms, the number of plumbing fixture appliances and the associated flow rates and flush volumes. The plumbing design engineer does it by selecting pipe sizes and piping layout. Both are guided by the applicable codes and stan- dards.
: Cost-Effective Control of this Serious Pathogen
Recognizing the contamination risk difference in hot versus cold premise plumbing systems is critically important.Previous Page