How to Clean 100 Gallon Water Container | Keep Your Water Safe

A 100-gallon water tank is only as good as its cleanliness. Whether you filled it for hurricane season or use it as a stationary cistern, sediment and biofilms build up fast. The cleaning process is not complicated, but skipping one step can leave you with stale-smelling water or a health risk. Here is the exact sequence to restore that container to potable condition, based on public-health guidance for water storage.

What You Need Before You Start

Gather these items before you open the tank: a stiff-bristle scrub brush or a power washer, a garden hose long enough to reach a drain area, unscented household bleach (5.25% sodium hypochlorite), a clean bucket, rubber gloves, and eye protection. If the container is indoors or has a small opening, ensure good ventilation and have someone nearby in case you need help—several public-health procedures explicitly warn against entering a tank alone.

The bleach must be unscented. Scents and additives coat the inside of the tank with chemicals that are hard to rinse out and may react with the water you plan to store. If your container has held anything other than potable water—fuels, chemicals, or unknown liquids—do not use these steps. The protocols below are for food-grade or potable-water-rated tanks only.

Step-by-Step Cleaning Process for a 100-Gallon Water Container

Step 1: Drain it completely. Open the valve or siphon out every drop. Standing water at the bottom dilutes the bleach later and leaves sediment in place. Tilt the container or use a pump if gravity alone cannot empty it fully.

Step 2: Scrub the interior walls and floor. This is the most skipped step and the one that matters most. Bleach alone cannot dissolve caked-on sediment or biofilm—you must physically remove that layer first. Use a stiff brush on a long handle or a power washer with a clean-water setting. Rinse the interior with potable water after scrubbing, and drain again.

Step 3: Disinfect with the correct bleach solution. Public-health authorities give different dose-time combinations depending on the tank’s use. For a 100-gallon potable container, the most practical and widely cited method comes from the Alaska Division of Public Health: add ½ cup of unscented household bleach per 100 gallons of tank volume, fill the container to the brim with clean water, and circulate the solution through any connected faucets until you smell bleach at every tap. Recirculate that same water back into the tank for 15–30 minutes, then let it sit for at least 6 hours—overnight is better.

Step 4: Flush and rinse until the chlorine is gone. Drain the bleach solution completely. Refill the tank partway with fresh water, slosh or circulate it, drain again, and repeat until you cannot smell chlorine. Some guidance uses a test strip to confirm a free chlorine residual of zero. If your container is connected to a plumbing system, flush every faucet and pipe run separately during this step.

Disposing of chlorinated water responsibly matters: draining a 100-gallon bleach solution onto a lawn can kill plants. Run the water into a gravel pit, a driveway, or a sanitary sewer cleanout rather than onto a garden.

Common Mistakes That Ruin the Cleaning

The biggest error is relying on bleach alone and skipping the scrub. Physical removal of debris comes before disinfection in every official protocol. The second mistake is cutting the contact time short. Draining early leaves a live biofilm behind that regrows in days.

Another frequent miss: failing to clean the spigot, hoses, or plumbing attached to the tank. If your container feeds water through a pipe, bacteria hiding in the line will reinfect the freshly cleaned tank as soon as you open the valve. Disconnect any removable components and run the bleach solution through them during the contact-time window.

For those ready to purchase a new or replacement container, your guide to the best 100-gallon water containers compares models on durability, portability, and food-grade safety.