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For most fresh water tanks, the safest general approach is to wash away dirt first, then sanitize with properly diluted, plain unscented chlorine bleach. Do not simply pour bleach into a dirty tank and assume the job is done.
The right amount and method depend on the tank. A small portable water container, an RV fresh water system, and a large cistern may use very different chlorine concentrations and contact times. Always follow the tank or vehicle manufacturer's instructions when they are available.
What Can You Put in a Fresh Water Tank to Clean It?
For a tank made to hold drinking or household water, suitable cleaning materials usually include:
- Clean water for flushing
- Mild soap or a cleaner approved by the tank manufacturer for removing visible dirt
- Plain, unscented household chlorine bleach for sanitizing
- A soft or stiff brush that will not damage the tank, if you can safely reach the inside
For bleach, check the label. CDC instructions for water-storage containers specify unscented household bleach containing 5% to 9% sodium hypochlorite.
Avoid scented bleach, splashless products unless specifically approved for the job, color-safe bleach, and products that contain added cleaners.
Also avoid mixing cleaning chemicals. In particular, never mix bleach with ammonia, acids, vinegar, or other cleaners. Dangerous gases can form.
Cleaning and Sanitizing Are Different Jobs
These two steps are easy to confuse.
Cleaning removes sediment, slime, dirt, and other material from the tank.
Sanitizing uses a disinfectant, usually chlorine, to reduce germs on cleaned surfaces.
A layer of sediment or organic material can interfere with disinfection. That is why a dirty tank should normally be drained and cleaned before it is sanitized.
For a tank with visible buildup, the basic order is:
- Drain the old water.
- Remove sediment and debris.
- Wash or scrub accessible surfaces if the tank design allows it.
- Rinse out the loosened material.
- Sanitize the tank and any plumbing that also needs treatment.
- Drain the sanitizing solution.
- Flush or refill as directed for that system.
Do not climb into a cistern or large tank to scrub it. A tank can be a confined space with serious risks from poor air, slippery surfaces, chemicals, and difficult access. Hire a qualified tank-cleaning service when entering the tank would be required.
How Much Bleach Should You Use?
There is no single bleach amount that is correct for every fresh water tank.
Different instructions are designed for different jobs. Using a concentration from one procedure for another type of tank can result in too little disinfectant or far more chlorine than necessary.
Small Water Storage Containers
For small food-grade storage containers, CDC guidance says to first wash the container with soap and water and rinse it completely.
To sanitize the empty container, CDC recommends a solution of:
1 teaspoon of unscented 5% to 9% household bleach in about 1 quart or 1 liter of water.
The solution should contact all of the inside surfaces for at least 30 seconds. It is then poured out, and the container can be air-dried or rinsed with safe water before filling.
This is a surface-sanitizing solution for a container. It is not a formula for adding that same concentration to an entire full tank of drinking water.
RV Fresh Water Tanks
RV manufacturers commonly provide their own procedure because the tank is connected to pumps, faucets, water heaters, filters, and other plumbing.
For example, Winnebago instructions for some RV systems call for preparing a solution using 1/4 cup of household bleach for every 15 gallons of fresh water tank capacity, then filling the tank and allowing the chlorine solution to move through the plumbing. Specific contact times vary by model and procedure.
Jayco manuals have also used a 1/4-cup-per-15-gallon method to create about a 50 ppm chlorine concentration in certain RV systems.
Treat this as an example, not a universal recipe. Check your current owner's manual before sanitizing an RV. You may need to bypass a water heater or remove a filter cartridge before chlorine is circulated.
Large Cisterns
Large cisterns may require a different procedure, especially after flooding or known contamination.
CDC's post-disaster cistern guidance, for example, calls for 3 cups of 5% to 9% unscented household bleach for every 100 gallons of water in the cistern, with the chlorinated water held in the cistern and connected plumbing for at least 12 hours.
That is a specific disinfection procedure for cisterns after contamination. It should not be substituted for routine RV or portable-container cleaning instructions.
A Simple Fresh Water Tank Cleaning Process
If your tank has no special manufacturer procedure, start by identifying what the tank is used for.
1. Drain the Tank
Remove the old water.
If the tank has a low-point drain, use it so sediment near the bottom can leave with the water.
Disconnect electrical power to pumps or other equipment when required before working on the system. Do not let a pump run dry.
2. Remove Visible Sediment
Look through the inspection opening if the tank has one.
Leaves, sand, roof debris, biofilm, and settled material should be removed before sanitizing.
For an accessible tank, a long-handled brush may help loosen material. Use tools that will not scratch plastic tank walls or damage liners.
A heavily contaminated cistern may need professional cleaning rather than a DIY rinse.
3. Wash If Necessary
For a small removable container, soap and water can be used before sanitizing, as recommended by CDC. Rinse thoroughly afterward.
For a built-in tank, use only a cleaning product or detergent that the tank manufacturer considers suitable. You do not want soap trapped in inaccessible plumbing or absorbed into unsuitable materials.
4. Sanitize
Use the chlorine concentration and contact time specified for your tank or system.
Measure the bleach rather than guessing.
Factor in simple water-tank cleaning methods to assess airflow and overflow provisions around the storage vessel.
If the system has filters, UV equipment, a water heater, treatment cartridges, or sensitive components, check their instructions before exposing them to concentrated chlorine. Some components need to be bypassed or removed.
5. Drain and Flush
Drain the sanitizing solution according to the applicable instructions.
Be careful where heavily chlorinated water goes. Large quantities can harm plants, septic systems, aquatic life, and waterways.
Refill with clean water and flush the lines until the system meets the manufacturer's instructions for returning it to service.
Should You Use Vinegar?
Vinegar can help with some mineral deposits and odors, but it should not be treated as a replacement for an established tank-disinfection procedure.
More important, never add vinegar to bleach. Acids can react with chlorine bleach and release dangerous chlorine gas.
If mineral scale is the main problem, check whether the tank or equipment manufacturer recommends a specific descaling method.
What About Baking Soda?
Baking soda may help with some odors and light cleaning, but it is not a reliable substitute for sanitizing a fresh water tank.
It also does not solve problems caused by sediment, algae, biofilm, contaminated source water, or dirty plumbing.
Cleaning the source of the problem is more useful than simply adding something intended to mask an odor.
Can You Use Hydrogen Peroxide?
Some water-system products use hydrogen peroxide or peroxide-based chemistry. However, concentration, contact time, tank compatibility, and effectiveness vary.
Do not substitute household hydrogen peroxide for chlorine using a guessed amount.
If your tank manufacturer specifies a peroxide-based sanitizer, follow those directions. Otherwise, plain chlorine bleach is supported by established public-health guidance for many water-storage sanitation jobs.
What Not to Put in a Fresh Water Tank
Avoid improvising with household chemicals.
Do not put these into a drinking-water tank unless the tank or treatment-system manufacturer specifically calls for them:
- Scented bleach
- Color-safe bleach
- Household disinfectant sprays
- Laundry detergent
- Strong degreasers
- Drain cleaner
- Ammonia cleaners
- Pool chemicals at guessed concentrations
- Fragrances or odor-masking products
- Pesticides or algaecides not approved for the intended water use
Never reuse a tank that previously held pesticides, fuel, petroleum products, or other toxic chemicals as a drinking-water storage tank. CDC specifically advises against using water containers that previously held toxic substances.
Cleaning a Rainwater Tank Requires More Than Adding Bleach
If the fresh water tank stores collected rainwater, tank cleaning is only one part of maintaining the system.
Sediment may keep returning if the collection system has problems such as:
- Dirty gutters
- Leaves entering the tank
- Missing inlet screening
- Poorly maintained first-flush equipment
- An uncovered overflow
- Animal access
- Sediment accumulating at the tank bottom
A first flush is a device that diverts some of the first roof runoff from a rain event. This water often carries more dust, droppings, pollen, and roof debris than the water that follows.
Correcting these problems can reduce how quickly the tank becomes dirty again.
Cleaning a Tank Does Not Make Rainwater Safe to Drink
Sanitizing the tank does not prove that the water inside is potable.
Potable means suitable for drinking.
Roof runoff may contain microorganisms, chemicals, metals, or other contaminants. Chlorine can control many microorganisms when properly applied, but it does not remove every possible chemical contaminant. EPA notes that disinfection does not remove contaminants such as heavy metals, salts, and many other chemicals.
If rainwater will be used for drinking, cooking, brushing teeth, or similar household uses, treat it as a whole-system water-quality issue. The system may need suitable collection surfaces, debris control, prefiltration, treatment, proper storage, ongoing maintenance, and current laboratory testing based on the intended use and local requirements.
Do not use appearance, smell, a simple home test strip, or one filter as proof that rainwater is safe to drink.
When Should a Fresh Water Tank Be Cleaned?
There is no maintenance interval that fits every tank.
Inspect and clean sooner if you notice:
- Sediment collecting on the bottom
- Slime on accessible surfaces
- Unusual odors
- Discolored water
- Long periods of storage
- Contamination entering through the inlet or lid
- Work performed inside the plumbing system
- Floodwater entering or surrounding the tank
- Animals or insects getting into the system
RV manufacturers may also call for sanitizing at the start of a season, after storage, or at least annually. Follow your vehicle's manual because schedules vary.
A rainwater tank that receives a lot of roof debris may require sediment management more often than a sealed tank filled only with treated municipal water.
Frequently Asked Questions
What is the best thing to put in a fresh water tank to clean it?
For many potable-water tanks, start with water and an appropriate cleaner to remove dirt, then use properly diluted plain, unscented chlorine bleach to sanitize. The exact bleach concentration and contact time depend on the tank and plumbing system.
Can I pour straight bleach into a fresh water tank?
Do not guess or pour in an arbitrary amount. Measure the bleach according to an established procedure for your specific tank. Manufacturers often recommend diluting bleach in water before adding it so concentrated bleach does not sit against one area of the tank or plumbing.
How long should bleach sit in a fresh water tank?
It depends on the concentration and system. Small-container sanitation may require only brief surface contact, while RV procedures may use several hours or longer and certain cistern disinfection procedures require at least 12 hours. Follow the procedure that applies to your system rather than choosing a time by itself.
Do I need to rinse a fresh water tank after using bleach?
For most tank-sanitizing procedures, the sanitizing solution is drained and the tank or plumbing is flushed or refilled with clean water afterward. Follow the manufacturer's instructions because the flushing process varies by system.
Can I use vinegar instead of bleach?
Vinegar can be useful for some mineral deposits, but it is not a direct replacement for a proven chlorine sanitation procedure. Never mix vinegar with bleach.
Will bleach remove algae and sediment from a tank?
Bleach can help sanitize cleaned surfaces, but it does not physically remove layers of sediment or heavy buildup. Drain and remove dirt first. Recurring algae or debris also means you should check light entry, screens, lids, gutters, and other parts of the collection system.
Is water safe to drink after the tank has been sanitized?
Not necessarily. Sanitizing a tank reduces contamination on the tank and plumbing surfaces, but it does not prove that the water source is safe. Drinking-water use requires suitable source water or a properly designed treatment system, maintenance, and appropriate testing.
