"Stagnant water" concerns are due to Microbes and Opportunistic Premise Plumbing Pathogens (OPPP).
OPPP are microorganisms that present an emerging infectious disease problem along with negative aesthetics such as tastes and odors.
A number of microbes could be present in finished water (even in safe city water):
- Heterotrophic bacteria
- Protozoans – amoeba, ciliates, slime molds
- Legionella
- Pseudomonas
- Mycobacterium
- Acinetobacter
- Sphignomonas
- Stenotrophomonas
- Aeromonas
- Methylobacterium
- Antibiotic resistant microbes (AMR)
Other Pathogens can be found in drinking water that is not properly treated, like Escherichia coli (E.coli), Salmonella, Shigella, Campylobacter, Cryptosporidium, and Giardia.
Premise Plumbing can be a great growth and proliferation environment for OPPPs
- Premise Plumbing may have low disinfection residuals.
- OPPPs can survive in low nutrient, low oxygen environments.
- OPPPs interact with protozoans, such as amoeba. This may enable them to resist treatment and persist in these environments.
- OPPPs are known to form biofilms, in which microorganisms aggregate together to form complex structures attached to pipe walls. Cell-to-cell communication, called quorum sensing, is involved in cell attachment and detachment in biofilms.
OPPP share these common characteristics:
- Chlorine/Chloramine resistance.
- Thermal tolerance.
- Biofilm formation.
Maintaining a chlorine residual is rarely enough to make much of a difference, and somewhat of a "false hope" in protecting the occupants/users of the building from the growth and proliferation of OPPP.
My general recommendations for minimizing and controlling OPPP in residential and commercial buildings are as follows:
- Do not presume that Chlorine/Chloramine will control bacterial growth without an additional Water Management Plan (WMP).
- Design the plumbing system to minimize dead legs.
- Maintain water heater temperature >120 F (49 C) and use POU tempering valves for downstream hot water delivery.
- Physically filter influent water below 1 Micron nominal to reduce turbidity and the ingress of microorganisms from the city supply.
- Scale control (or softening) to minimize hard water scale accumulation that will promote biofilm growth and proliferation.
- If salt-based softening is used, incorporate continuous onboard disinfection to prevent growth of biofilm and halophilic bacteria in the softener along with post-softening Ultraviolet (UV) disinfection.
- Insulate cold water piping to minimize ambient heat gain; water temperatures >65F (18C) encourage bacterial growth.
- Recirculate water through the building (separate hot and cold recirculation loops); movement minimizes growth.
- When space/budget allows, return recirculation should be further disinfected (Ultraviolet is excellent here) and/or physically filtered <1 micron.
- Flush all downstream fixtures at least once a week to minimize the potential for biofilm growth.
- Test downstream water periodically to confirm presence/absence of at least HPC bacteria (these are good general indicators of plumbing health). In more critical applications Legionella testing should also be performed.
The most common minimal beneficial measures that can be taken in residential and commercial buildings are as follows:
- Maintain continuous elevated water heater temperature >120F and use POU thermostatic mixing valves.
- Insulate hot water piping wherever possible and especially on all recirculation piping.
- Insulate cold water piping wherever possible, especially if proximal to radiant heating pipes.
Bacterial contamination should not be underestimated - As Water Quality Improvement professionals we owe it to our clients to do better.
