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The best thing to clean most rainwater tanks with is clean water and physical removal of dirt, leaves, biofilm, and bottom sludge. A hose, soft brush, wet vacuum, or tank-cleaning equipment may be enough, depending on the tank.
Avoid pouring strong household cleaners into a water tank. Detergents, scented bleach, solvents, and other chemicals can leave residues that are difficult to rinse away.
If a tank needs to be sanitized after cleaning, an appropriate chlorine treatment may be used when the tank material and intended water use allow it. Cleaning and sanitizing are two different jobs.
Cleaning vs. Sanitizing a Water Tank
It helps to separate these two steps.
Cleaning removes:
- Dirt
- Leaves
- Insects
- Sediment
- Algae
- Sludge
- Biofilm on tank surfaces
Sanitizing or disinfecting reduces microorganisms that remain after cleaning.
You normally get better results by removing physical contamination first. Chlorine is not a substitute for removing a layer of sludge from the bottom of a tank.
For large rainwater tanks, public-health guidance recommends focusing cleaning on accumulated sediment and debris and then rinsing the tank with clean water. Cleaning chemicals that could leave residues or release harmful fumes should be avoided.
What Should You Use to Clean a Rainwater Tank?
For routine cleaning, start with the simplest method that will do the job.
Clean water
A strong rinse with clean water is often the best starting point.
Use it to loosen and wash away:
- Loose sediment
- Dust
- Small leaves
- Insects
- Light surface deposits
Direct the dirty rinse water to a suitable disposal area rather than letting it collect around the tank foundation.
A soft brush
If deposits remain on accessible surfaces, a soft or medium-stiff brush can help.
Avoid aggressive scraping unless the tank manufacturer says it is safe. Some tanks have internal coatings or surfaces that can be damaged by harsh cleaning.
This is particularly important with some steel tanks. Removing a protective internal layer can increase the chance of corrosion.
A wet vacuum or tank-cleaning system
You do not always have to completely empty a large tank.
Where the tank design allows it, sediment may be removed from the bottom with equipment such as:
- A wet vacuum
- A siphon
- A sludge pump
- Professional tank-cleaning equipment
Removing bottom sediment without climbing inside the tank is usually preferable.
Should You Use Soap or Detergent?
For a large rainwater tank, detergent usually should not be your first choice.
Soap can help clean small removable water containers because they can be thoroughly rinsed afterward. The CDC, for example, recommends washing small water-storage containers with soap and water, rinsing them completely, and then sanitizing them when appropriate.
A large tank is different.
Detergent trapped in seams, plumbing, outlets, or a textured tank surface can be difficult to remove. It may cause:
- Foaming
- Taste or odor problems
- Residue in stored water
- Problems with downstream treatment equipment
For a rain barrel, IBC tote, cistern, or other large storage tank, follow the tank manufacturer's cleaning instructions before adding detergent.
Can You Clean a Water Tank With Bleach?
Bleach can be useful for sanitizing, but it should not be treated as a general-purpose tank cleaner.
First remove the dirt and sludge. Then decide whether sanitizing is actually required.
When chlorine bleach is appropriate, use a product intended for disinfection. CDC guidance specifies regular unscented household bleach containing 5% to 9% sodium hypochlorite for its water-storage-container sanitizing procedure. Scented, color-safe, splashless, and products containing added cleaners may not be suitable.
Never mix bleach with ammonia, acids, or other cleaners. Dangerous gases can result.
Do not use one bleach dose for every tank
The right chlorine treatment depends on factors such as:
- Tank volume
- Amount of water remaining
- Chlorine concentration of the product
- Amount of organic contamination
- Tank material
- Intended water use
- Required contact time
- Whether plumbing also needs treatment
For example, public-health agencies publish specific procedures for disinfecting contaminated cisterns after floods or other emergencies. Those concentrations and contact times are not automatically the right procedure for routine rainwater-tank maintenance.
If the tank supplies drinking water, follow current local health guidance and the instructions for your particular system rather than estimating a chlorine dose.
What Should You Not Use in a Water Tank?
Avoid putting a cleaner into a tank simply because it works well elsewhere in the house.
Unless the tank manufacturer specifically approves them, avoid:
- Scented bleach
- Laundry products with additives
- Strong household detergents
- Solvents
- Degreasers
- Fragranced cleaners
- Toilet or bathroom cleaners
- Abrasive chemicals
- Unapproved algaecides
- Unapproved mold treatments
Also avoid mixing cleaning products.
A chemical may clean a surface well but still be unsuitable for a tank that stores irrigation, household, animal, or drinking water.
How to Clean a Rainwater Tank Safely
The exact method depends on the tank, but a basic cleaning job usually follows this order.
1. Stop dirty water from entering
Before cleaning the tank, check where the contamination came from.
Look at:
- Roof surfaces
- Gutters
- Gutter screens
- Downpipes
- Leaf filters
- Tank inlet screens
- First-flush devices
- Overflow screens
A first-flush device diverts the first portion of runoff from the roof so that some dust, droppings, and loose debris do not immediately enter the tank.
Cleaning the tank without correcting a dirty collection system means the tank can become contaminated again quickly.
2. Isolate pumps and treatment equipment
Turn off equipment that should not run while the tank is drained.
That may include:
- Pumps
- UV units
- Automatic controllers
- Irrigation equipment
- Filters
Do not let a pump run dry unless it is specifically designed to do so.
3. Remove as much usable water as practical
Decide where the water will go before draining.
Do not discharge large amounts where they could:
- Erode soil
- Flood a building
- Undermine the tank base
- Enter an unsuitable drain
- Damage nearby structures
Evaluating cleaning a rainwater tank safely helps assess reserve capacity for likely demand and collection rates.
Some tanks also have special requirements when empty. For example, an empty lightweight plastic tank may need protection from wind, while some concrete tanks should not simply be left empty for long periods. Check the tank manufacturer's instructions.
4. Remove sediment and debris
Remove leaves, sludge, and other deposits without entering the tank whenever possible.
A layer of sediment often collects around the lowest points and outlet area.
5. Rinse accessible surfaces
Use clean water to rinse loose material toward the drain or removal point.
A suitable brush can be used where needed and where the tank surface allows it.
6. Sanitize only when needed
Routine physical cleaning does not always require chemical disinfection.
Sanitizing becomes more important after events such as:
- Known contamination
- Floodwater entering the tank
- Animal entry
- Sewage contamination
- Long periods of poor storage
- Instructions from a health authority
- Certain drinking-water system maintenance procedures
If contamination involves fuel, pesticides, sewage, floodwater, or other hazardous substances, do not assume chlorine will make the water safe.
Chlorine can control many microorganisms, but it does not remove many chemical contaminants or heavy metals.
7. Flush the plumbing when necessary
Contamination may not stop at the tank.
Water can remain inside:
- Pipes
- Pump housings
- Pressure tanks
- Filters
- Faucets
- Irrigation lines
After serious contamination, the entire connected system may need attention rather than only the storage tank.
Do You Need to Enter the Tank?
Avoid entering a large water tank to clean it.
A cistern or rainwater tank can be a confined space. Hazards can include poor air quality, lack of oxygen, toxic fumes, heat, difficult access, and inability to escape quickly.
Public-health guidance recommends professional tank cleaning when cleaning requires entering the tank.
Never climb into a tank simply because the hatch looks large enough.
For large deposits that cannot be removed from outside, use a qualified tank-cleaning service with suitable confined-space procedures and equipment.
What About Algae Inside the Tank?
If algae keeps returning, cleaning the algae is only part of the fix.
Algae usually needs light and nutrients. Check for places where sunlight can enter through:
- A translucent tank wall
- An uncovered opening
- A damaged lid
- Clear pipework
- Poorly sealed fittings
Also check whether large amounts of organic debris are entering from the roof.
Repeatedly adding chemicals without fixing the light or debris problem is unlikely to be a good long-term solution.
What About Sludge on the Bottom?
A small amount of sediment can accumulate even in a well-designed rainwater system.
Over time, roof dust, pollen, fine organic material, and particles too small for inlet screens can settle on the bottom.
How often it needs removal depends on the system. A tank receiving poorly screened roof runoff may accumulate sediment much faster than one with good gutter maintenance, inlet screening, and first-flush management.
Check the tank instead of cleaning according to an arbitrary calendar.
Cleaning a Tank Used for Drinking Water
A tank that supplies drinking water needs more care than a tank supplying garden irrigation.
Potable means water intended to be safe for drinking. Non-potable water is water used for purposes such as irrigation or some household uses where drinking-water quality is not required.
Cleaning a potable-water tank does not by itself make collected rainwater safe to drink.
Drinking-water safety depends on the whole system, including:
- Collection surface
- Gutters and downpipes
- Animal and debris exclusion
- First-flush management
- Tank condition
- Plumbing
- Prefiltration
- Appropriate treatment
- Ongoing maintenance
- Current laboratory testing
- Local drinking-water requirements
A filter or chlorine treatment should not be used as proof that unknown rainwater is safe to drink.
If rainwater will be used as drinking water, follow current guidance from your local public-health authority and use qualified water-treatment help where needed.
The Simple Answer
For most rainwater tanks, the best cleaner is clean water combined with physical removal of sediment and debris.
Use a suitable soft brush or sludge-removal equipment if needed. Avoid harsh household cleaners and unnecessary detergents.
Bleach has a different job. It can be used for sanitizing under the right conditions, but the correct product, concentration, contact time, and procedure depend on the tank and its intended use.
And if proper cleaning requires you to climb inside a large tank, that is the point where a simple DIY cleaning job should usually become a professional one.
Frequently Asked Questions
Can I use dish soap to clean a water tank?
Dish soap may be suitable for some small removable water containers that can be rinsed thoroughly. It is usually not the first choice for a large rainwater tank because soap residue can be difficult to remove. Check the tank manufacturer's instructions.
Is bleach safe for cleaning a water tank?
Regular unscented chlorine bleach can be used in certain water-container and cistern sanitizing procedures. It should be used at an appropriate concentration and only when compatible with the tank and intended water use. Never mix bleach with other cleaners.
Can I pressure wash the inside of a water tank?
Only if the tank manufacturer allows it and you can do the work without unsafe tank entry. High-pressure cleaning may damage some coatings, seals, liners, or tank surfaces. A hose and suitable brush are often safer.
How often should a rainwater tank be cleaned?
There is no useful cleaning interval that fits every system. Inspect the tank for sediment, debris, odors, algae, contamination, and changes in water quality. Good roof, gutter, screen, and first-flush maintenance can reduce how quickly debris builds up.
Do I need to disinfect my tank every time I clean it?
Not necessarily. Routine removal of sediment does not always require disinfection. Disinfection may be appropriate after known microbial contamination, flooding, animal entry, or as part of a drinking-water system procedure.
Can I use vinegar to clean a water tank?
Vinegar is not a universal water-tank cleaner or disinfectant. It can also react with some materials and must never be mixed with chlorine bleach. Use the cleaning procedure recommended for your tank instead.
Can I drink rainwater after cleaning the tank?
Cleaning the tank alone does not establish that rainwater is safe to drink. Potable use requires suitable collection, treatment, maintenance, current water-quality testing, and compliance with applicable local health requirements.
Should I climb inside my water tank to scrub it?
Large tanks can be confined spaces with serious ventilation and access hazards. If cleaning cannot be completed safely from outside the tank, use a qualified professional rather than entering it yourself.
