Finding a slimy green film in your Brita pitcher is unsettling, but it’s a common, well-understood problem — not a sign your water is unsafe or your filter is broken. Here’s what’s actually growing, why it happens, and how to keep it from coming back.
What’s Actually Growing in There
What people call “algae” in a filter pitcher is usually a mix of algae and bacterial biofilm — a thin, slimy layer that forms when microorganisms already present in trace amounts in your water find a place to take hold and multiply. It’s not mold (which tends to look fuzzy or spotted rather than slimy), though mold can occur separately under similar conditions.
Three things drive this growth: light, warmth, and time. Algae specifically needs light to grow, so a pitcher stored somewhere sunny is at meaningfully higher risk than one kept in a dark cabinet or the refrigerator. Bacteria don’t need light, but they thrive in the same warm, stagnant conditions.
Why It Happens
Your filter removes the chlorine that would otherwise suppress growth. Municipal tap water is disinfected with chlorine specifically to control bacterial and algal growth. Brita filters are designed to remove that chlorine taste and smell — which is exactly what makes the water taste better, but it also removes the residual protection that was keeping microbial growth in check once the water sits in the pitcher.
Well water lacks that protection from the start. If you’re on a private well, your water was never chlorinated in the first place, so a pitcher filled with well water has even less built-in resistance to growth. (If you’re on well water and haven’t tested it recently, see our guide on well water testing.)
Light and warm storage accelerate it. A pitcher left on a sunlit counter or near a window gives algae exactly what it needs. Refrigeration slows growth substantially — both by limiting light exposure and by keeping temperatures below the range where these organisms multiply quickly.
Infrequent cleaning lets it accumulate. Even in ideal conditions, a pitcher that’s rinsed but never actually washed will build up a biofilm layer over weeks, whether or not visible algae ever forms.
An overdue filter. Filters lose effectiveness at the end of their rated life, and a spout or lid that doesn’t seal tightly can let in dust, airborne spores, or contamination between fills.
Is It Harmful to Drink?
For most healthy adults, ingesting a small amount of the algae or biofilm from a pitcher is unlikely to cause serious harm, though it’s not something to make a habit of. Possible symptoms from prolonged exposure include mild digestive upset — nausea, stomachache, or diarrhea — that typically resolves on its own. If you have ongoing symptoms after drinking from a contaminated pitcher, or if you’re serving this water to an infant or someone with a compromised immune system, it’s worth being more cautious and consulting a doctor rather than waiting it out.
The more important point: if you’re seeing this regularly despite reasonable cleaning, it’s worth checking your water source directly rather than assuming the pitcher is the whole story — particularly if you’re on well water.
How to Clean It Out
- Empty and disassemble the pitcher, reservoir, and lid completely. Set the filter aside in a clean spot — don’t let it sit in the sink.
- Wash with mild dish soap and warm water, scrubbing any visible slimy buildup by hand rather than relying on a quick rinse.
- For stubborn buildup, a solution of white vinegar and water is effective at breaking down mineral and biofilm deposits — let it sit for a few minutes before scrubbing.
- Rinse thoroughly with clean water to remove all soap and vinegar residue.
- Air dry completely before reassembling — don’t put pieces back together while still wet.
- Reinstall the filter (or install a fresh one if it’s due) and run a partial pitcher of water through before drinking, discarding it if you notice loose carbon particles.
Preventing It Going Forward
- Store the pitcher out of direct sunlight — a cabinet, cupboard, or the refrigerator is ideal.
- Keep it refrigerated between uses when possible; the lower temperature meaningfully slows both algae and bacterial growth.
- Wash the pitcher on a real schedule (roughly every 2–4 weeks, more often if you’re on well water), not just when you notice a problem.
- Replace the filter on schedule — most standard filters are rated for about 40 gallons or 2 months, whichever comes first; longer-life filters are typically rated higher. Check your specific filter’s packaging for its rated capacity.
- Keep the lid and spout sealed properly between fills to limit airborne contamination.
- Avoid drinking directly from the spout, which can introduce bacteria from your mouth into the reservoir.
FAQ
Does this mean my tap water is unsafe? Not necessarily. This is a pitcher-specific issue related to storage conditions and filter chemistry, not usually a sign that your incoming water itself is contaminated — though if it happens repeatedly despite good cleaning habits, testing your water source is a reasonable next step.
Can I just wipe it out instead of a full wash? For routine maintenance, yes, but a full wash with soap (or vinegar for stubborn buildup) periodically is what actually prevents biofilm from establishing in the first place.
Should I throw out the filter if I see algae? If the growth has reached the filter cartridge itself rather than just the pitcher walls, replacing the filter is the safer choice — a contaminated cartridge can’t be effectively cleaned.
Have a specific question about your water? Reach out through the Contact page — I read everything.