Part II of this two-part series explores how to manage risks in system design
Part one of this article introduced current trends affecting premise plumbing safety and the continued rise of Legionnaires’ disease cases in the U.S. According to the U.S. Centers for Disease Control and Prevention (CDC), nine out of 10 Legionella outbreaks could be prevented with more effective water management. This article will attempt to outline how plumbing engineers can implement fundamental design principles and execute comprehensive strategies that support and facilitate effective water management.
Managing Legionella Risks Starts at Conceptual Planning
Managing risks must be a priority of the building owner and discussed at the beginning of project planning. Engineers should include recommended control measures in early conceptual and schematic design documents. This ensures control measures are included in initial cost estimates and creates an opportunity to discuss the owner’s priorities concerning managing Legionella.
As the project moves through design development, engineers should focus on supporting key water management principles:
- Maintain water temperatures outside of Legionella growth range;
- Prevent water stagnation;
- Ensure adequate disinfection; and
- Prevent scale, corrosion and biofilm growth.
During construction, it is critical for engineers to review construction submittals through the lenses of Legionella control and critically assess all cost-saving proposals. The continuous increase in construction costs exposes effective design strategies and innovative products to elimination from the project in order to save money. This is when an owner’s commitment to managing risks is vital to the successful implementation of proven control measures.
Engineers must balance public health and safety, good engineering practice, sustainability, operational efficiency, and owner requirements, while also being cost conscious.
Focus on the Fundamentals
Good engineering practice should start by focusing on fundamental Legionella control measures. These control measures often have little to no cost impact. Two of the most common control measures are maintaining water temperatures (hot and cold) outside of Legionella growth range and preventing water stagnation. Unfavorable temperatures and water stagnation increase the likelihood of bacteria and biofilm growth.
Balanced Hot Water Recirculation is Critical
One of the biggest challenges to maintaining water temperatures is the domestic hot water recirculation system. Without proper balancing of the recirculation system, hot water circuits further away from the heat source (i.e. water heater) will struggle to meet or maintain design temperature while the hot water circuits closest to the heat source consistently maintain design temperature.
The reason is due to water taking the path of least resistance. In order to balance a system, valves are installed on each hot water return circuit to create resistance. This resistance encourages water to choose an alternate path while still allowing enough flow through the circuit to overcome heat loss. A properly balanced system will maintain and deliver hot water quickly and consistently throughout the building.
Multiple issues can affect the proper balancing of a system, including system design, valve selection, commissioning, scale, renovations, repairs or neglected maintenance. The bottom line is most recirculation systems do not work correctly. As such, there are multiple balancing valve technologies on the market designed to improve system balancing. Automated balancing valves are the best option to ensure a building consistently maintains hot water temperatures throughout the system. They provide a perfectly balanced system based on custom parameters, automate weekly maintenance to prevent scale build-up, monitor temperatures in real-time at each return circuit, and continuously log data for verification, risk assessment and compliance.
Address Stagnation Throughout System Design
Eliminating the potential for water stagnation using system design alone is not possible. However, engineers can reduce the potential for stagnation and support regular water exchange by using good engineering practices and manufactured solutions:
- Consider pipe routing, fixtures, and fittings to minimize dead legs and eliminate dead ends.
- Provide means to facilitate remedial or routine flushing (Routine flushing is a preventative action performed during periods of low water use or at seldom-used bypasses, mains, branches and outlets.).
An overlooked potential for stagnation and bacteria growth is the period between substantial completion and building occupancy. Engineers can address stagnation during this time by including an automated system designed to periodically flush mains and branches according to time, temperature, or usage. This ensures regular water exchange while the building is still unoccupied, thus minimizing bacteria and biofilm growth while maintaining residual disinfectant levels.
Minimize Aerosolization, Corrosion, Scale & Biofilm
Legionella is transmitted through aerosolization. Contaminated droplets of water enter the lungs where the bacteria can grow potentially leading to severe pneumonia. In building water systems, aerosolization can take place at showers, faucets, decorative fountains, cooling towers, humidifiers, etc. Engineers should consider selecting plumbing fixtures that minimize the potential of producing aerosols.
Sediment, scale, and corrosion provide a habitat and nutrients for Legionella. It is important to address these items through the thoughtful selection of equipment and piping materials:
- Water filtration removes sediment and provides nutrient control.
- Water conditioning reduces scale in facilities with hard water.
- Supplemental disinfection allows facilities to control residual disinfectant levels.
- Select piping material that is corrosion-free and immune to degradation due to chlorine, monochloramine or chlorine dioxide disinfectants.
Provide Control, Monitoring & Data-Logging Capabilities
Plumbing systems often lack the control and monitoring capabilities necessary for effective water management — even in the newest and most critical facilities. If controls and monitoring are installed, they almost exclusively require manual oversight and operation. Manual verification relies heavily on the operator’s time, diligence and expertise.
Engineers must consider designing smarter, more intelligent systems that take advantage of market innovations to simplify water management. These smarter systems automate fundamental control measures and allow operators to quickly respond to potentially hazardous system changes, perform risk assessments, and verify compliance with an established water management program.
What Does Comprehensive Plumbing Design Look Like?
Two long-term care facilities in the Southwest U.S. wanted to manage the risk of Legionella without the use of continuous supplemental disinfectant treatment. Engineers provided a comprehensive plumbing design by implementing a multi-barrier approach. They:
- Applied control measures at the point of source such as water conditioning to reduce scale; ultrafiltration for nutrient control; and UV disinfection to destroy harmful microorganisms.
- Selected equipment to reduce potential risk such as a booster pump with no accumulator tank and flow-through expansion tank at the water heater.
- Designed hot water storage and distribution temperatures outside of Legionella growth range.
- Selected CPVC piping for hot and cold water distribution.
- Utilized “flow splitters” in the cold water system to minimize dead legs.
- Recirculated hot water close to fixtures (This minimizes opportunity for bacteria growth/survival).
- Provided an intelligent system capable of automatically flushing valves to ensure regular water exchange, monitoring of control limits, and continuous data logging.
- Provided ports for temporary supplemental disinfection (remedial or routine).
Plumbing engineers play an essential role in creating safe and healthy buildings. By making a conscious decision to incorporate fundamental principles and comprehensive strategies in the design process, engineers can minimize the impact of emerging trends and give building operators the systems they need to manage risks successfully.