Your Tap Smelled Like a Pool Last Month. The City Did That On Purpose.

If your water had a stronger chlorine smell or taste sometime between mid September and early October, you weren't imagining it. The City of Tampa's water department switched its disinfectant from chloramine to plain chlorine for three weeks, from September 14 through October 5, 2026, a routine step the city says is also used by other utilities around the Tampa Bay region and across the country. The water stayed safe to drink the entire time. What changed was which disinfectant was doing the work, and understanding why utilities bother making that swap is worth it even now that this year's switch is over, because it will happen again next year.

Most municipal water in this area is treated with chloramine day to day, which is chlorine combined with a small amount of ammonia. Chloramine is more stable than chlorine alone, so it holds its disinfecting power longer as water travels through miles of pipe to reach your tap. That stability is also its weak point. If any of that ammonia isn't fully bound to the chlorine, bacteria living in the pipes can start feeding on it, in a process water engineers call nitrification. Those bacteria build up as biofilm, a thin slimy layer on the inside of pipes and storage tanks, and they're hardest to control in the spots where water sits the longest: dead end lines, large storage tanks, warm months when demand is low. As the bacteria consume the ammonia, they break down the chloramine itself, which drops the disinfectant level in the water and lets the bacteria grow even faster. Left unchecked, the EPA notes this cycle can show up as biofilm growth, taste and odor complaints, or occasionally a coliform bacteria detection somewhere in the distribution system.

Switching to plain chlorine for a few weeks breaks that cycle. Chlorine without ammonia gives the nitrifying bacteria nothing to eat, which stops the process and lets utilities knock the biofilm back before returning to chloramine for the rest of the year. It isn't free, though: running plain chlorine for an extended stretch can raise the level of certain disinfection byproducts, which is exactly why utilities keep the switch short and scheduled rather than running chlorine alone year round. It is maintenance for pipes you will never see, timed to do the most good for the least trade-off.

For most households, the only noticeable effect was taste or smell, and the fix is the same one that works for any chlorine taste: run the tap for a few minutes before using it, or fill a pitcher and let it sit on the counter for a few hours so the chlorine has time to dissipate. That trick works specifically because this was plain chlorine. It would not work during the rest of the year, when the disinfectant is chloramine, since chloramine does not dissipate by sitting out the way chlorine does. If chlorine taste bothers you year round, a standard activated carbon filter handles plain chlorine well, but chloramine needs a longer contact time or a catalytic carbon filter to strip out fully, which is worth knowing before you assume a pitcher filter is doing the same job every month of the year.

Two groups needed to pay closer attention during the switch, and should keep paying attention going forward. Dialysis centers and other medical facilities that treat incoming water before it touches a patient follow their own protocols regardless of which disinfectant the city is running that month. And anyone with fish, whether in an aquarium or a backyard pond, should already be treating tap water before a water change, since both chlorine and chloramine are toxic to fish at the concentrations utilities use. A dechlorinator rated for both, not just whichever one is currently in the pipes, is worth keeping on hand year round rather than switching products every September.

If you want to know which disinfectant your own water provider uses day to day, and whether they run a seasonal switch like Tampa's, your Consumer Confidence Report or a call to the utility will tell you. It is a five minute check that explains a smell most people just shrug off instead.

Primary references: City of Tampa, Tampa Water Department temporary disinfection change notice, September 2026, tampa.gov. US EPA, Distribution System Toolbox fact sheet on nitrification, epa.gov.