Dead legs in building water systems can cause severe problems, including persistent positive Legionella test results and an increased incidence of Legionnaires’ disease. A basic understanding of dead legs, how to recognize them, and how to manage the problems they cause can help building owners reduce the risks associated with Legionella and other pathogens in their water systems.
Dead legs are sections of water system piping with low or no flow due to system design and/or operation. Common examples include capped piping, closed cross connections, low point drains, cooling tower equalization lines, bypass lines, and out of service rooms or equipment. Intermittently operated faucets, showers, chillers, heat exchangers, and pumps can also become dead legs depending on how long they are out of service.
Biofilms are the link between dead legs and Legionnaires disease. Biofilms are communities of surface attached bacteria surrounded by a sticky, gel-like secretion often called slime. Although biofilms start out microscopic in size, they can grow into visible biofouling deposits in just a matter of days. A wide range of problem causing microorganisms including Legionella bacteria can grow to high levels protected from disinfectant or biocide additions within biofilms. The end result of biofilm includes corrosion, reduced heat transfer, clogged piping, and difficulty remediating positive Legionella counts.
The stagnant water in dead legs provides ideal conditions for biofilms to form. Without flow, the water in a dead leg does not receive disinfectant or biocide treatment. This allows bacteria to attach to system surfaces to start the biofilm formation process. Any sediment accumulations also foster biofilm growth. Once established, biofilms and associated microorganisms provide a protective environment for any Legionella entering a building with the source water to multiply to high levels. Eventually, Legionella may be released into the bulk water to cause disease.
Established biofilms are tenacious and difficult to remove, even when subjected to high levels of chlorine or other disinfectants. The lack of flow and access for cleaning makes biofilms growing in dead legs especially difficult to remove. The best control strategy is to eliminate the dead leg. Where this is not practical, a combination of design, operational, and maintenance strategies is required to help manage biofilm problems resulting from dead legs.
- Design: Design strategies involve mechanical alterations to provide continuous or intermittent flow. These strategies include removing dead legs or unused equipment, installing jumper piping across dead legs, and installing drain lines to allow flushing. As a general rule, terminated piping should be less than 2X the diameter to avoid creating a dead leg.
- Operational: Dead legs can also be managed by continuously or periodically establishing flow through off-line piping and equipment in coordination with disinfectant or biocide additions. A common control strategy for cooling tower systems involves programming the building management system to establish flow through off-line system components (chillers, heat exchangers, pumps, piping, etc.) at least twice per week in conjunction with biocide feed. Likewise, tower basin equalization lines or low point drains can be automatically flushed using motorized valves and timers.
- Maintenance: Effective maintenance procedures are also required to help manage biofilms and dead legs. Systems or components that have been off-line for more than a defined period of time (~five days) should be disinfected upon start up. Disinfection and physical cleaning may also be required in response to out-of-range microbiological test results. It’s important to address any accumulated debris in equalization lines, drains, side stream filters, and remote sumps, and to ensure equipment and piping receives circulation during disinfection procedures. The addition of a newly developed biofilm conditioning treatment can significantly improve removal.
The stagnant water associated with pressure testing new building water systems, or additions to existing systems, makes them particularly susceptible to biofilm-related problems. Even if the system is drained, trapped water can result in severe biofouling. New piping systems and additions should always be cleaned and disinfected prior to start up.
Dead legs in water systems can cause serious and costly problems. Design, operational, and maintenance strategies must be employed to help manage biofilm problems where dead legs cannot be eliminated. Contact Chem-Aqua for more information about how we can help you manage your water treatment problems, including biofilms.