TL;DR
Wash a water filter pitcher's reservoir, lid, and jug in warm soapy water at least weekly, and dry them fully before reassembling. The slippery film people find inside is biofilm -- ordinary bacteria growing on a wet plastic surface -- and it forms in any container that stays damp, filtered or not. Never wash a carbon filter cartridge itself, and never put a pitcher in a dishwasher unless it is labeled dishwasher safe.
What is the slime in a water pitcher?
That slippery coating is biofilm: a thin layer of bacteria and organic matter that grows on any surface kept persistently wet. It is not a defect in the filter and it is not usually mold, though mold can grow alongside it. Washing with soap and a brush removes it, and drying the parts between refills is what keeps it from returning.
Filtered water often has less residual disinfectant than tap water straight from the faucet, which is one reason a filter pitcher can develop film faster than a plain jug does.
That is not a reason to stop filtering. It is a reason to wash the pitcher on a schedule instead of topping it up for months. The same biofilm shows up in a cold brew pitcher and its mesh filter basket for exactly the same reason.
How do you clean a water filter pitcher?
Take the pitcher apart into its jug, reservoir, lid, and filter. Set the filter aside dry. Wash the jug, reservoir, and lid in warm soapy water with a soft bottle brush or sponge, paying attention to the reservoir's underside where the filter seats and to the pour spout. Rinse very thoroughly, dry everything with a towel, and let the parts air dry before reassembling.
The reservoir underside and the spout are the two spots people skip, and they are where film is thickest.
Rinse until there is no trace of soap. Water tastes strongly of any residue left behind.
The lid, the spout, and the fill flap
Lids have a hinged fill flap, a seam where the flap pivots, and sometimes an electronic filter indicator. The seam collects film and mineral deposits.
Wash the lid by hand and work the flap seam with a soft toothbrush. Keep water away from any battery-powered indicator if it is not sealed -- wipe that part with a damp cloth instead.
The pour spout is a narrow channel where water sits between pours. A thin brush or a cotton swab reaches it. Neglecting the spout is a common source of an off taste in water that is otherwise fine.
The filter is never washed
Carbon filter cartridges are not cleanable. Washing one with soap contaminates the media, and scrubbing one damages it. Rinse a new cartridge with plain water only if its own instructions say to, and otherwise leave it alone.
Change filters on the schedule the manufacturer states, measured in gallons or days rather than in how the water looks. A saturated filter stops adsorbing contaminants long before anything visible changes.
Black specks in the water are usually loose carbon fines from a new cartridge, which typically clear after the first flush the instructions describe. Persistent specks or a filter that has been in place well past its rated life is a reason to replace it. Filter change intervals are a maintenance question, not a cleaning one, and they belong to the manufacturer's spec.
Source: U.S. Environmental Protection Agency guidance on home water treatment units
Mineral scale versus film
Two different deposits show up in pitchers, and they need different treatments.
Slippery film is biofilm and comes off with soap and a brush. Chalky white or grey crust is mineral scale from hard water, and soap will not touch it -- it dissolves in acid.
For scale, a soak in diluted white vinegar or a citric acid solution followed by a very thorough plain water rinse works. Keep the filter out of the pitcher during any acid soak, and never mix an acid with bleach or any chlorine product in one container, because that reaction releases chlorine gas. Mineral scale behaves identically in a sparkling water maker and its carbonating bottles.
Dishwasher, or not?
Some pitchers are top-rack dishwasher safe, and many are not. Heat can warp thin plastic reservoirs and cloud the jug, and a warped reservoir no longer seats properly on the jug.
Check the label molded into the plastic or the manual. When in doubt, wash by hand -- it is a five-minute job.
Never put a filter cartridge, an electronic indicator lid, or anything with a battery into a dishwasher.
Storage and habits that prevent the problem
Do not store a pitcher with old water sitting in the reservoir for days. Refill it and use it, or empty and dry it.
If you go away for a week, empty the pitcher, wash it, dry it, and store the filter according to its instructions. A pitcher left full for a week comes back with film.
Keep it in the fridge if you can, since cold slows biological growth. If it lives on the counter, wash it more often. The same rules apply to a countertop water cooler and its dispenser taps, which has more parts but the same underlying problem, and to every wet-surface appliance in the small appliance cleaning guide.
What cleaning does and does not do
Washing removes biofilm and residue so the water tastes right and the pitcher stays pleasant to use. It does not purify water, and it does not prevent or cure any illness.
If you have a specific concern about your water supply, that is a question for your local water utility's annual report or a certified water test, not for a cleaning routine.
If the pitcher is one item on a longer list around the kitchen, house cleaning services can take the whole room instead.
FAQ
Why is my water filter pitcher slimy inside?
That film is biofilm, a layer of bacteria and organic matter that grows on any persistently wet surface. It is normal in containers that are refilled without being washed and dried. Take the pitcher apart, wash every piece in warm soapy water with a brush, rinse well, and dry it fully.
Can you put a water filter pitcher in the dishwasher?
Only if the pitcher is labeled dishwasher safe, and even then usually only on the top rack. Heat warps thin reservoirs and clouds the jug, and a warped reservoir stops seating properly. Filter cartridges and electronic indicator lids never go in a dishwasher under any circumstances.
How often should you clean a water filter pitcher?
Wash it weekly with regular use, and more often if it lives on the counter rather than in the fridge. Empty and dry it any time it will sit unused for more than a day or two. Regular washing is more effective than any product at keeping film from forming.
Can you wash a water filter cartridge?
No. Carbon filter media cannot be cleaned, and soap contaminates it while scrubbing damages it. Rinse a new cartridge with plain water only if its instructions call for it. Replace cartridges on the manufacturer's stated schedule in gallons or days, not based on how the water looks.
What are the black specks coming out of my water filter?
Loose carbon fines from a new cartridge are the usual cause, and they normally clear after the initial flush the instructions describe. If specks continue well past that, or the filter is past its rated life, replace it. Persistent black particles are a reason to change the cartridge, not to scrub it.
How do you remove hard water scale from a pitcher?
Remove the filter, then soak the jug and reservoir in diluted white vinegar or a citric acid solution, and rinse very thoroughly with plain water afterward. Scale is mineral and dissolves in acid rather than coming off with soap. Never mix acid with bleach or any chlorine product.
A pitcher is easy to fix in ten minutes, which is more than can be said for the rest of the kitchen on most weeks. Every home is different, so the real number depends on size, condition, and what you want covered. Get a quote and we will walk you through it.