A municipal Boil Water Advisory (BWA) raises questions for homeowners who rely on a Reverse Osmosis (RO) purification system. RO systems use a multi-stage process to improve water quality by removing a wide range of contaminants. The central confusion is whether this superior filtration technology can bypass the necessity of boiling water. This article examines the specific threat a BWA addresses and the practical limitations of an RO system during a public health event.
The Purpose of a Boil Water Advisory
A Boil Water Advisory (BWA) is a public health directive issued when the water supply is contaminated with microbiological organisms. These advisories focus entirely on biological threats, including bacteria (E. coli), viruses, and protozoa (Giardia and Cryptosporidium). Boiling the water uses heat to neutralize these pathogens, making the water safe for consumption.
Advisories are typically issued when events compromise the integrity of the public water distribution system. This occurs following incidents like a water main break, a significant loss of pressure, or a failure at a treatment plant. These incidents allow untreated groundwater or sewage to enter the clean water supply lines, introducing the biological contaminants the advisory is designed to mitigate.
How Reverse Osmosis Systems Handle Pathogens
Reverse osmosis functions by forcing water through a semi-permeable membrane using external pressure. This membrane is the core of the system and relies on size exclusion, possessing an extremely fine pore size, generally ranging from 0.0001 to 0.001 microns.
For comparison, most waterborne bacteria are relatively large, measuring between 0.5 and 10 microns. Viruses are smaller, typically falling between 0.01 and 0.4 microns. A pristine, fully functional RO membrane is therefore capable of physically blocking virtually all known bacteria, protozoa, and viruses from passing into the final purified water stream.
Why the RO Storage Tank Poses a Risk
Despite the theoretical effectiveness of the membrane, the entire RO system setup introduces several practical vulnerabilities that make it unreliable during a BWA. The system’s pre-filters, which typically include sediment and carbon blocks, are designed for particulate and chemical removal, not for eliminating pathogens. When the source water is compromised, these pre-filters can harbor high concentrations of live bacteria and viruses, potentially contaminating the whole system.
The primary risk lies in the pressurized storage tank, which holds previously filtered water ready for use. While water filled before the BWA was issued is technically safe, the plumbing itself can become colonized by pathogens from the highly contaminated incoming source water. A significant danger is the potential for back-siphonage or cross-contamination within the complex network of tubing, post-filters, and the dispensing faucet. Because of these systemic vulnerabilities, relying solely on an RO system during an advisory is unsafe, and the water must still be boiled.
Required Actions During a Water Advisory
For owners of an RO system, the first action during a BWA is to immediately stop the system from producing more water. This is accomplished by turning off the water supply valve that feeds the RO unit, preventing the storage tank from refilling with contaminated water. Any water dispensed from the RO tap or stored in the tank must be treated exactly like tap water and brought to a rolling boil for at least one full minute before consumption.
Once the municipal advisory is lifted, the RO system must be thoroughly sanitized to ensure no pathogens remain.
Post-Advisory Sanitation
The necessary steps include:
- Draining the storage tank completely.
- Replacing all filters and the RO membrane, as these elements may have been exposed to high levels of contamination.
- Chemically disinfecting the entire system, including the tank and lines, often using a chlorine solution.
- Flushing the system extensively before the water is considered safe to drink again.