Summary of Sulfate-Reducing Bacteria (from KnowYourH2O Website)

Sulfate-reducing bacteria (SRB), also linked to sulfur/hydrogen sulfide issues, are non-pathogenic nuisance microorganisms that thrive in oxygen-poor (anaerobic or “reducing”) environments. They obtain energy by converting natural sulfates in water into hydrogen sulfide (H₂S) gas.

What They Are & How They Become a Problem

These bacteria typically establish themselves only after dissolved oxygen, nitrate, iron, and manganese have already been depleted. They commonly live in:

• Deep wells or shallow wells connected to organic-rich environments that are low in oxygen like wetlands and lake sediments
• Stagnant plumbing or distribution systems (dead-ends)
• Biofilms in water softeners and water heaters (especially the hot-water side)
• Contaminated sources linked to sewage, petrochemicals, or brine

Their activity produces the classic “rotten egg” odor, metallic sheens, slimy coatings (yellow, brown, red, or black), and brown foam. Biofilms can promote microbiologically induced corrosion and elevate levels of iron, manganese, arsenic, and corrosion metals (aluminum, copper, lead, zinc). Their presence often signals a longer-term microbiological problem rather than a recent one.

Health Risks & Standards

SRB themselves are not considered a direct health threat. However:

• Hydrogen sulfide is detectable by most people at just 0.5 ppm.
• Levels under 1 ppm produce a musty/swampy odor; 1–2 ppm produce the strong rotten-egg smell and make water corrosive.
• Continuous exposure quickly dulls the sense of smell, so the gas may still be present even if you no longer notice it.
• H₂S can sometimes indicate possible wastewater influence that might also carry pathogens (e.g., E. coli, Salmonella, Pseudomonas, etc.).

There are no specific EPA numerical standards for SRB or H₂S. They are indirectly controlled through total coliform (absent) and heterotrophic plate-count (< 500 colonies/mL) rules, plus state secondary standards that address odor and corrosivity nuisances.

Warning Signs & Testing

Level 1 – Observational

• Sulfur/rotten-egg odor, especially from hot water
• Black discoloration or slimy coatings inside toilet tanks or water heaters
• Gassy, discolored, or off-tasting first-flush water after periods of non-use
• Higher standard plate counts or musty smells after returning from trips

Level 2 – DIY / Lab Testing

• Hydrogen sulfide test strips
• BART kits specifically for sulfate-reducing bacteria (often combined with iron-related and slime-forming bacteria tests)
• Laboratory packages that check for SRB, hydrogen sulfide, and broader water quality

Shock disinfection is sometimes recommended before further testing if bacteria are confirmed.

Suspect sulfur bacteria in your water? The Sulfur Bacteria Problem Suspected Kit tests for sulfate-reducing bacteria and related water quality issues — a smart first step before choosing a treatment system.

Treatment Recommendations

Short-term

• Inspect the wellhead.
• Perform shock disinfection of the well and entire distribution system (recommended twice), bypassing all treatment devices and removing in-line filters/aerators.
• Thoroughly flush the lines and disinfect the water heater.
• For municipal (city) water: flush the main if possible and install a clear filter housing right after the entry point and report the problem to your water supplier.
• You may want to check the disinfection residual of the water and the oxidation reduction potential (ORP).

Long-term

• Private wells/springs: chemical oxidation/disinfection + reaction tank + backwashable particle filtration (sometimes followed by other treatments). Professional design and ongoing maintenance are usually required. Periodic well cleaning may be needed if bioslimes and scale are present.
• Municipal systems: primarily the utility’s responsibility (higher disinfectant residual, pipe flushing, dead-end elimination). Homeowners can add a whole-house particle filter + UV system as a barrier.
• Note: Simple carbon filters can temporarily reduce low levels of H₂S but will not solve a bacterial problem long-term.
• Whole house water treatment systems — US Water Systems / Filtered Water.

The page lists specific recommended products (H₂S strips, BART kits, Well-Safe shock kits, sanitary well caps, chemical injection systems, whole-house filters, and related books) and emphasizes working with local professionals for lasting solutions.

Bottom line: Sulfate-reducing bacteria are a common cause of rotten-egg odor and black slime, especially in wells and hot-water systems. They are an aesthetic and corrosion nuisance rather than a direct health hazard, but proper testing and shock chlorination followed by ongoing control are the standard approach.

Want to learn more about your water? Our Drinking Water Guide for Well Water and City Water is an educational booklet covering water quality issues, testing, and treatment options for both private wells and municipal systems.

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