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For most rainwater tanks, start with clean water, a soft or stiff nylon brush, and a small amount of mild dish soap if the tank maker allows it. Rinse the tank well so no soap is left behind.
If the tank needs to be sanitized, regular unscented household chlorine bleach may be suitable for some tanks. Use only the concentration and method recommended for your tank and intended water use. Do not mix bleach with other cleaners.
The right cleaner depends on what the tank is made from, what was stored in it, and how you plan to use the water afterward.
Cleaning and Disinfecting Are Different
Cleaning removes dirt, slime, leaves, sediment, and other material from the tank.
Disinfecting is a separate step used to reduce or kill germs after the tank has been cleaned. Dirt can make disinfectants less effective, so cleaning should normally come first.
A tank used only for garden irrigation may not need the same sanitation process as a tank supplying water for household use.
If rainwater may be used for drinking, cleaning the tank is only one part of water safety. A clean-looking tank does not mean the water is potable, which means safe to drink.
What Can You Use to Clean a Rainwater Tank?
Clean Water and a Nylon Brush
For routine cleaning, plain water and physical scrubbing are often enough.
A long-handled nylon brush can loosen:
- Slime on tank walls
- Algae
- Dirt
- Fine roof debris
- Sediment near the bottom
Avoid wire brushes unless the tank manufacturer specifically allows them. Metal brushes can scratch plastic and damage protective coatings inside metal tanks.
Mild Dish Soap
A small amount of mild dishwashing soap can help remove greasy dirt or stubborn buildup from some tanks.
Check the tank manufacturer's instructions first. Some tanks, liners, coatings, seals, and treatment systems may have special cleaning requirements.
If you use soap, rinse repeatedly with clean water. Soap left in the tank can cause foaming and may affect stored water.
For a basic household water-storage container, CDC guidance recommends washing with dishwashing soap and water and rinsing completely before sanitizing.
Unscented Household Chlorine Bleach
Bleach can be useful when a tank must be sanitized or disinfected, rather than simply scrubbed clean.
Use regular, unscented chlorine bleach suitable for sanitizing. CDC guidance for water-related applications specifies unscented household bleach containing about 5% to 9% sodium hypochlorite.
Do not use:
- Scented bleach
- Splashless bleach unless specifically approved for the purpose
- Color-safe bleach
- Bleach mixed with added cleaners
- Products when you do not know their active ingredient or strength
Never mix chlorine bleach with ammonia, acids, vinegar, toilet cleaners, or other cleaning chemicals. Dangerous gases can form.
The amount of bleach also matters. Cleaning a tank wall, sanitizing an empty container, disinfecting a cistern, and treating stored water are different jobs. They should not all use the same bleach concentration.
For example, CDC guidance for cleaning a cistern after certain contamination events uses a specific bleach-and-water scrub solution, followed by rinsing and draining. CDC provides a different procedure when the goal is disinfecting water inside a cistern.
Follow current health guidance and the tank manufacturer's directions rather than guessing at a bleach dose.
What Should You Not Clean a Tank With?
Strong chemicals are usually unnecessary for normal rainwater-tank cleaning.
Avoid using products such as:
- Gasoline or other petroleum solvents
- Paint thinner
- Pesticides
- Drain cleaner
- Strong acids
- Strong alkaline cleaners
- Scented household cleaners
- Chemicals that leave fragrances or coatings
- Unknown industrial cleaning products
These substances may damage plastic, coatings, seals, valves, or liners. Some can also leave residues that are difficult to remove.
Vinegar is sometimes suggested as a general household cleaner, but do not assume it is suitable for every water tank. More importantly, never combine vinegar or another acid with chlorine bleach.
How to Clean a Rainwater Tank
The exact process depends on tank size and design, but the general order is simple.
1. Stop New Rainwater From Entering
Temporarily divert the downpipe or inlet so another rain shower does not refill the tank while you work.
If the system has pumps, UV units, filters, treatment equipment, or automatic controls, isolate them according to their instructions.
2. Drain the Tank Safely
Drain the tank through its normal outlet where possible.
Think about where the water will go. A large cistern can release a great deal of water. Do not discharge it where it could flood a building, wash out soil, enter a septic system too quickly, or damage nearby property.
3. Remove Loose Sediment
Leaves, insects, roof grit, and fine sediment can settle on the tank floor.
Remove accessible debris before washing the walls. A wet vacuum or other suitable equipment may help with some small tanks, provided it can be used safely around water.
Do not climb into a tank to reach sediment.
Large tanks and cisterns can be confined spaces. They may have unsafe air, difficult exits, slippery surfaces, or other hazards. Internal tank entry should be left to people trained and equipped for confined-space work.
4. Scrub Accessible Surfaces
Use clean water and a nylon brush or other tank-safe cleaning tool.
If needed and allowed by the manufacturer, use a small amount of mild detergent.
Pay attention to accessible areas around:
- The inlet
- Overflow
- Tank lid
- Screens
- Outlet
- Float valves
- Fittings
You do not need to make an outdoor rainwater tank look brand new. The goal is to remove accumulated material that could affect water quality or clog the system.
5. Rinse Thoroughly
Rinse away loosened dirt and any cleaner.
Explore suitable cleaners for water tanks to evaluate tank material choices for the intended end use.
If detergent was used, continue rinsing until there is no visible foam or slippery residue.
6. Sanitize Only When Needed
Sanitizing may make sense after contamination, extended storage, certain repairs, flooding, or when preparing a tank for a more sensitive water use.
Follow guidance appropriate to your tank and water use rather than adding a random amount of bleach.
For example, CDC recommends cleaning before disinfecting because dirt and other material can reduce how well a disinfectant works.
7. Clean the Rest of the System
Tank cleaning will not help for long if dirty water immediately flows back into it.
Check the parts upstream and downstream, including:
- Gutters
- Downpipes
- Leaf screens
- Inlet screens
- First-flush diverter
- Overflow screen
- Pump intake
- Prefilters
A first-flush diverter sends the first portion of runoff away from the tank. This can help keep some roof dust, droppings, and loose debris from entering storage.
What About Algae?
If the tank has green growth inside, cleaning can remove existing algae, but you should also fix the reason it is growing.
Light is often part of the problem. Check for:
- A translucent tank
- An uncovered inspection opening
- Light entering around the lid
- Clear pipes exposed to sunlight
- Damaged covers
Blocking light is usually more useful than repeatedly adding chemicals.
Do not assume algae treatment products intended for pools, ponds, or aquariums are suitable for a rainwater tank. Their intended use and chemical formulation may be different.
What If the Tank Smells Bad?
Odor can come from several sources.
Common causes include:
- Decaying leaves
- Dead insects or small animals
- Stagnant water
- Organic sludge
- Contaminated roof runoff
- Dirty filters
- Poorly screened openings
Drain and inspect the system before trying to cover the smell with a chemical cleaner.
If fuel, pesticides, solvents, or other toxic chemicals have entered the tank, adding bleach does not make the water safe. EPA notes that boiling and disinfection do not remove contaminants such as many chemicals, salts, and heavy metals.
Serious chemical contamination may require draining the tank, professional assessment, and possibly replacement of affected materials.
Cleaning an IBC Tote
Be especially careful with used IBC totes.
An IBC tote should not be considered suitable for water storage merely because it has been washed out.
You need to know what it previously contained. Do not use a container that previously held toxic chemicals for drinking-water storage. CDC gives the same warning for water-storage containers.
For garden rainwater, confirm that the tote's previous contents are compatible with your intended use and that the tote itself is suitable for storing water.
Unknown tote history is a good reason to choose another container.
How Often Should You Clean a Rainwater Tank?
There is no useful one-size-fits-all schedule.
Inspect the tank and collection system periodically and clean when buildup makes it necessary. A well-screened system with clean gutters may collect sediment much more slowly than an unprotected tank.
Check sooner if you notice:
- Heavy sediment
- Bad odors
- Algae
- Insects
- Dirty water
- Blocked screens
- Reduced pump flow
- Debris after storms
- Evidence that animals entered the tank
Good pretreatment can reduce cleaning work. Gutter guards, leaf screens, suitable inlet screens, first-flush equipment, and screened overflows all help keep larger debris out.
Be More Careful With Drinking-Water Systems
Cleaning and disinfecting a tank does not by itself make collected rainwater safe to drink.
Roof runoff can pick up microorganisms, animal waste, chemicals, roofing material residues, dust, and other contaminants before it reaches storage.
A drinking-water system needs to be considered as a complete system. That may include suitable collection materials, debris removal, prefiltration, treatment designed for the contaminants of concern, correct operation, maintenance, and current laboratory testing.
A single sediment filter, carbon filter, UV unit, chlorine treatment, home test strip, or water-quality meter cannot by itself establish that roof-collected rainwater is safe to drink.
For potable use, follow current local health and plumbing requirements and use qualified water-treatment help when needed.
Frequently Asked Questions
Can I use dish soap to clean a rainwater tank?
A small amount of mild dish soap can be used with some tanks if the manufacturer allows it. Scrub accessible surfaces and rinse very thoroughly so no soap remains. Plain water and brushing may be enough for routine dirt and sediment.
Can I use bleach to clean my water tank?
Regular unscented household chlorine bleach can be used for some sanitizing and disinfection jobs. The correct concentration depends on the job, bleach strength, tank design, and intended water use. Follow the tank manufacturer's directions and current public-health guidance rather than guessing.
Can I use vinegar to clean a rainwater tank?
Vinegar may be compatible with some materials, but it is not a universal tank cleaner or a substitute for an appropriate disinfection process. Never mix vinegar or any other acid with chlorine bleach.
Do I need to disinfect my tank every time I clean it?
Not necessarily. Routine cleaning removes sediment, slime, and debris. Disinfection is a separate step and may be appropriate after contamination or for certain water-storage applications. The need depends on how the water will be used.
Can I pressure-wash the inside of my tank?
Only if the tank manufacturer permits it and the work can be done without entering the tank. High-pressure water can damage some liners, coatings, fittings, and plastic surfaces. Never climb into a cistern simply to pressure-wash it.
Can I clean a tank that previously held chemicals?
Do not assume cleaning makes a chemical container safe for water. Containers that previously held toxic chemicals should not be reused for drinking-water storage. If the previous contents are unknown, choose another tank for sensitive uses.
Will cleaning my rainwater tank make the water safe to drink?
No. Cleaning reduces dirt and buildup but does not establish drinking-water safety. Potable rainwater requires a suitable whole system, appropriate treatment, maintenance, current laboratory testing, and compliance with applicable local requirements.
