As an affiliate, we may earn a commission from qualifying purchases. We get commissions for purchases made through links on this website from Amazon and other third parties.
A rainwater tank should be cleaned by removing accumulated sediment and debris, rinsing the tank, cleaning the inlet and screening parts, and fixing anything that is letting new dirt or pests enter. For most home systems, the safest approach is to clean the tank without climbing inside it.
You do not need to scrub a rainwater tank on a fixed schedule just because it contains water. The better approach is to inspect it and clean it when sludge, leaves, odor, algae, or contamination make cleaning necessary. EPA guidance also treats tank inspection and cleaning as part of an ongoing maintenance plan rather than a one-time job.
Before You Empty the Tank
Do not start by simply opening the drain.
An empty tank can behave differently from a full one. Above-ground plastic tanks may become easier for strong wind to move. Some concrete tanks should not remain empty for long. Buried tanks can have additional structural and groundwater concerns.
Before draining:
- Check the tank manufacturer's cleaning instructions.
- Turn off pumps and other electrical equipment connected to the system.
- Stop automatic top-up water if your system has it.
- Divert new rainwater away from the tank while you work.
- Check the tank for cracks, leaning, damaged supports, or loose plumbing.
- Make sure discharged water will not wash soil away from the tank foundation.
- Pick a time when you can refill the tank appropriately afterward.
If electrical equipment has been flooded, damaged, or exposed to water where it should not be, have it checked by a qualified electrician before using it again.
Do Not Enter a Large Rainwater Tank
A tank that is large enough to climb into can be a confined-space hazard.
A confined space is an area that is large enough to enter but has limited ways in or out and was not designed for people to occupy. Tanks are a common example. Hazards can include low oxygen, harmful gases, heat, slippery surfaces, difficult rescue, and electrical hazards.
For a homeowner, the practical rule is simple:
If cleaning requires your body to go inside the tank, use a qualified tank-cleaning professional.
If the job requires confined-space entry, compare the typical cost to have a water tank cleaned by a qualified service.
Do not assume an empty tank is safe to enter. OSHA guidance specifically warns that an empty tank can still contain serious atmospheric or physical hazards.
What You Will Usually Need
For a tank that can be cleaned entirely from outside, you may need:
- Garden hose with clean water
- Long-handled brush suitable for the tank material
- Siphon hose or suitable sludge-removal equipment
- Buckets
- Gloves and eye protection
- Replacement screens or seals if damaged
- Tools needed to open the tank's normal inspection hatch or drain
You may not need all of these.
Avoid using solvents, strongly scented cleaners, abrasive chemicals, or other products unless the tank manufacturer says they are suitable. Cleaning chemicals can damage tank materials or leave residues in the water.
How to Clean a Rainwater Tank
1. Clean the Roof, Gutters, and Inlet First
There is little value in cleaning the tank if the next rainfall immediately washes leaves and dirt back into it.
Remove accessible debris from:
- Roof collection areas
- Gutters
- Downspouts
- Leaf screens
- Rain heads
- Tank inlet screens
Use normal roof-access safety precautions. Do not work from an unsafe ladder or climb onto a steep, wet, or unstable roof.
Also clean the system's first-flush diverter, if it has one. A first-flush diverter sends the first portion of dirty roof runoff away from the tank before cleaner runoff enters storage.
Check that it drains and resets as designed.
2. Use or Drain Most of the Stored Water
If the existing water is suitable for irrigation or another non-potable use, it may make sense to use as much as practical before cleaning.
Non-potable means water that is not intended for drinking.
When draining the remaining water, send it somewhere that will not:
- Erode soil around the tank
- Flood a building
- Damage a foundation
- Create a muddy walking area
- Enter an unwanted drainage system
If you suspect sewage, floodwater, fuel, pesticides, chemicals, or another serious contaminant entered the tank, treat the water as contaminated rather than simply releasing it onto the yard.
3. Remove Sediment From the Bottom
The layer on the bottom of a rainwater tank is often called sludge. It can contain fine roof dust, pollen, decomposed plant matter, insects, and other material that settled out of the water.
You do not always have to empty the whole tank to remove it.
Depending on the tank design, sludge may be removed using:
- A bottom cleaning or scour outlet
- A siphon operated from outside the tank
- Suitable pumping equipment operated from outside
- Complete draining followed by rinsing
Some official rainwater guidance recommends checking tanks for sludge every two to three years, while cleaning sooner if sediment is clearly accumulating. Actual buildup varies greatly with the roof, trees, screens, first-flush system, weather, and tank design.
Never place a gasoline- or diesel-powered pump inside the tank or next to an opening where exhaust could collect. Combustion engines can create deadly carbon monoxide.
4. Rinse the Tank
Once most loose sediment is gone, rinse the accessible inside surfaces with clean water.
For many tanks, rinsing is more important than aggressive scrubbing.
A long-handled brush may help loosen dirt that you can safely reach through an access opening while remaining outside the tank. Choose a brush that will not damage the tank surface.
Avoid aggressive scraping.
This is especially important with some metal tanks because harsh cleaning can damage protective interior surfaces and encourage corrosion.
Rinse dirty water toward the drain or cleaning outlet and remove it.
5. Inspect the Tank While It Is Empty
An empty tank gives you a useful chance to inspect areas that are normally hidden.
Without entering the tank, look for:
- Cracks
- Corrosion
- Damaged lining
- Loose fittings
- Failed seals
- Root intrusion
- Light entering through holes or gaps
- Insect access points
- Damaged inlet screens
- Damaged overflow screens
- Heavy algae growth
- Excessive remaining sediment
If you see structural damage, stop and get advice from the tank manufacturer or a qualified contractor.
Do not try to make major structural repairs from inside a large tank yourself.
6. Clean Screens, Filters, and Connected Parts
Tank cleaning should include the parts that control what gets into the tank.
Inspect and clean the:
- Leaf screens
- Inlet basket
- Mosquito screens
- Overflow screen
- First-flush device
- Pump strainer
- Floating intake, if fitted
- Accessible prefilters
A blocked inlet can cause water to overflow where it should not. A damaged mosquito screen can allow insects into the tank.
NSW Health recommends closely fitting fine insect-proof screens on tank inlets and overflows and regular inspection for damage.
If you have cartridge filters downstream of the pump, service them according to their own maintenance requirements rather than assuming that cleaning the tank also cleans the filters.
7. Close the Tank and Refill It
When the sediment and rinse water are removed:
- Close the drain or cleaning outlet securely.
- Reinstall screens and baskets.
- Make sure the inspection hatch seals correctly.
- Restore the inlet connection.
- Refill the tank naturally or with an appropriate approved water source.
- Restore the pump only after there is enough water for safe pump operation.
- Check the system for leaks.
Some pumps need to be primed after the tank or suction line has been drained. Follow the pump manufacturer's instructions rather than running it dry.
Running a water pump without enough water can damage it.
Should You Disinfect the Tank After Cleaning?
Routine physical cleaning and disinfection are not the same thing.
Cleaning removes:
- Sludge
- Leaves
- Dirt
- Biofilm
- Other physical material
Review safe cleaning products for water tanks to verify tank material choices for the intended end use.
Disinfection is intended to reduce microorganisms.
A tank used only for suitable garden irrigation may not need routine chemical disinfection after ordinary sediment cleaning.
Disinfection becomes a more important question after events such as:
- Floodwater entering the tank
- Animal contamination
- Sewage contamination
- Suspected microbial contamination
- Work on a drinking-water system
CDC guidance for cisterns exposed to floods or heavy contamination includes physical cleaning followed by disinfection before the system returns to service.
Do not simply pour an estimated amount of bleach into a tank.
Chlorine products come in different concentrations, and the required process depends on tank volume, water condition, intended use, contact time, and the rest of the plumbing system. Some products also contain additives that are not suitable for drinking-water treatment.
For a tank supplying drinking water, use a procedure supplied by your current public health authority, tank-water professional, or other qualified water-treatment professional.
Cleaning a Tank Used for Drinking Water
Rainwater intended for drinking needs more care than water used for gardens or outdoor washing.
Cleaning the storage tank does not prove the water is safe to drink.
Roof runoff may pick up microorganisms, animal waste, metals, chemicals, roofing material residues, and airborne contaminants before it reaches the tank.
A drinking-water rainwater system therefore needs to be considered as a complete system, including:
- Suitable roof and collection surfaces
- Debris control
- First-flush management where appropriate
- Protected storage
- Appropriate filtration
- Appropriate disinfection or other treatment
- Plumbing designed for the intended use
- Routine maintenance
- Current laboratory testing
A filter alone does not establish that rainwater is potable.
Potable means suitable for drinking.
If the tank supplies drinking or cooking water, especially as a home's main water source, have the treatment plan and testing needs reviewed using current public health guidance for your location.
What About a Tank Contaminated by Floodwater?
Flood contamination is different from normal sediment buildup.
Floodwater can carry sewage, microorganisms, chemicals, fuel, and other contaminants. If floodwater has entered a rainwater tank, do not assume that draining visible dirt from the bottom makes the water usable again.
CDC advises treating a cistern exposed to floodwater as contaminated and cleaning and disinfecting it before returning it to drinking-water service.
Professional help is a good choice if:
- The contamination source is unknown.
- Sewage may be involved.
- Fuel or chemicals may be involved.
- The tank or plumbing has been damaged.
- Tank entry would be required.
- The system supplies drinking water.
Some chemical contamination cannot be corrected simply by chlorinating the water.
How Often Should a Rainwater Tank Be Cleaned?
There is no single cleaning interval that fits every tank.
Inspect the system regularly and pay attention to sediment buildup.
Government rainwater guidance commonly recommends checking for sludge about every two to three years.
A tank may need attention sooner if:
- Trees hang over the roof.
- Gutters collect heavy debris.
- The inlet has poor screening.
- There is no effective first-flush system.
- Dust levels are high.
- Sediment appears in the water.
- The water develops an unusual smell or appearance.
A well-designed collection system may go much longer between major cleanings because much of the dirt is stopped before it reaches storage.
Reduce the Need for Future Tank Cleaning
The easiest sludge to remove is the sludge that never reaches the tank.
Keep up with the parts before the storage tank:
Keep gutters clear
Leaves eventually break down into smaller material that can enter storage.
Maintain inlet screens
Clean screens allow water to flow while keeping larger debris out.
Maintain the first-flush system
A first-flush diverter can reduce the amount of early roof wash entering the tank, but only if it drains and resets correctly.
Keep access openings sealed
A poorly fitting lid can allow insects, small animals, dirt, and sunlight into the tank.
Screen the overflow
The overflow needs to pass water freely while preventing pest entry.
Check the tank periodically
Look for leaks, corrosion, movement, damaged plumbing, blocked screens, and unusual water appearance before they turn into larger problems.
When to Hire a Professional Tank Cleaner
Professional cleaning is the better choice when:
- A person would need to enter the tank.
- The tank is very large.
- The access opening is difficult to reach.
- Heavy sludge cannot be removed from outside.
- The tank is buried.
- The tank may have structural damage.
- Sewage or chemical contamination is possible.
- Electrical equipment has been damaged.
- The tank forms part of a drinking-water system and you are unsure how to disinfect or recommission it.
The most important dividing line is tank entry. Tanks can qualify as confined spaces and may contain hazards that cannot be detected by looking through the hatch.
Frequently Asked Questions
Can I clean a rainwater tank without emptying it?
Sometimes. If the main problem is bottom sediment, sludge may be siphoned or pumped from outside the tank while leaving much of the water in place. Whether this works depends on the tank shape, access, and amount of debris.
Do I need bleach to clean a rainwater tank?
Not for every routine cleaning. Removing sludge and rinsing the tank may be enough for many non-potable systems. Disinfection is a separate step that may be needed after contamination or as part of a drinking-water treatment plan.
Can I pressure-wash the inside of my rainwater tank?
Check the tank manufacturer's instructions first. Aggressive cleaning can damage some coatings, linings, or protective surfaces. A rinse and gentle brushing are often safer than trying to strip the interior surface completely clean.
Is sediment at the bottom of a rainwater tank normal?
Some sediment buildup is common because small particles can pass through roof screens and settle in storage. Heavy or rapidly increasing sludge can indicate that gutters, inlet screens, or the first-flush system need attention.
How often should I remove sludge from a rainwater tank?
Inspect rather than relying only on the calendar. Guidance from NSW Health recommends examining tanks for sludge at least every two to three years and removing it when present. Tanks exposed to heavy debris may need attention sooner.
Is rainwater safe to drink after I clean the tank?
Cleaning alone does not establish drinking-water safety. Drinking rainwater requires suitable collection, storage, treatment, maintenance, and water-quality testing appropriate to the system and local requirements.
Can I climb inside a water tank to scrub it?
Homeowners should avoid entering tanks. A tank large enough to enter may be a confined space with atmospheric, heat, fall, electrical, or rescue hazards. Use a qualified professional when cleaning requires tank entry.
What should I do if floodwater entered my rainwater tank?
Treat the tank as contaminated. Avoid using the water for drinking until the system has been properly cleaned, disinfected where appropriate, and assessed for the intended use. Floodwater can introduce microbial and chemical contaminants that ordinary sediment cleaning may not address.
