Bad smells in my rainwater tank, what should I do? Rainwater that stinks...

Bad odors in my rainwater tank, what to do? Stinky rainwater... This is a question frequently asked to our Foudebassin experts.

How to fight bad odors?

• Act at the source (directly in the tank)
• Install rainwater filters near the booster pump.

After a few years of operation, rainwater can start to smell bad if the tank does not have aeration or specific bacteria added on a regular basis. This can be a very delicate problem, especially if the water is used for laundry.

The appearance of odors often occurs after a hot period during which rainwater consumption has been reduced (often the case during vacations).

These bacteria are found in natural environments. However, in too large a number and with heat, significant sediment on the bottom, and stagnant water, these bacteria will produce an unpleasant odor (eutrophication of the tank).

Certain specific situations can explain these olfactory problems in a rainwater tank: excess pollen in the water, organic waste due to birds, little pumping in the tank (stagnant water), decomposition of leaves, and many trees nearby. In certain situations (layers of more than 10cm of dirt at the bottom of the tank) and a tank that has never been maintained, a complete maintenance and emptying may be necessary. Our experts can empty your rainwater tank.

In any case, it is important to limit the amount of organic waste in the rainwater tank. The presence of a suitable and well-maintained filter on the network that routes rainwater to the tank is essential. However, if you do not have a pre-filter, consider providing an air pump and long-term treatment. You can also add filters between your tank and your house.

Why does the water in my rainwater tank emit bad odors?

There are three factors that lead to the emission of bad odors:

• light
• heat
• organic waste (roofs, gutters, ...)

Be wary of deposits that accumulate on the bottom. This organic waste will form a layer on the bottom of the tank. When odors are smelled, it is often because of anaerobic bacteria causing fermentation on the bottom (release of a sulfur or sewage odor).

There is also the area where the water is drawn; we advise you to provide a float with a non-return valve fixed to the suction hose. We offer kits adapted to the diameter of your hose to draw water from the surface. This water is often of better quality for your toilets and showers.

How to treat bad odors in my rainwater tank?

Back in the day, and at the inception of rainwater tanks, bleach was advised. It treats the problem on the short term to improve water quality:

• an air pump for rainwater tanks
• a chlorine-free treatment based on bacteria (bactorain type)

Bleach works for a very, very short term and will even kill the natural bacterial environment. One of the solutions recommended by rainwater tank experts is the installation of an aeration system. No more odors and yellowish color of the water. These odors are emitted by anaerobic bacteria, and they hate oxygen in the water.

What are the preventive solutions for odors?

The first precaution, if you have never thought about cleaning your rainwater tank, is to clean the gutters and roofs.

Your tank must also be at least minimally protected from temperature rises. We recommend, if you are only at the idea stage, a buried tank. For an excellent result, to reduce odors and improve water quality, you can use a specific air pump (it looks like what you find in aquaristics, but larger).

Is rainwater harmful?

The bacteriological quality of rainwater can sometimes be very poor and can, under no circumstances, be compared to that of drinking water. It is water that is not treated as in a catchment station; furthermore, it contains very few minerals, unlike tap water that comes from springs.

It is for this reason that rainwater can only be used for non-sanitary applications and never in the kitchen.

Diagnosis: where does your tank's odor come from?

Before buying anything, take five minutes to understand what is happening in your tank. The odor is almost always a sign of fermentation at the bottom, but the right response depends on the amount of accumulated sludge and the equipment already in place.

• Open the inspection hatch and smell: a rotten egg or sewage odor indicates oxygen-free fermentation.
• Look at the color of the water: yellowish or cloudy water often accompanies these odors.
• Probe the bottom with a handle: beyond about ten centimeters of sludge, an emptying should generally be planned.
• Check the inlet filter and gutters: if they are clogged, leaves and debris end up in the tank.
• Note when the odor appears: often after a hot period where little water has been drawn from the tank.

To go further on the risks associated with smelly water, also consult our article Smelly rainwater, should I be worried?. If the odor is more apparent in the house, at the level of the drains, the problem may come from the pipes: see how to avoid sewage odors at home.

Aerating and seeding the tank, step by step

The most sustainable combination involves two actions: continuous aeration, which brings oxygen to the bottom, and a supply of bacteria, which gradually break down the organic matter in the sludge. One does not replace the other: air prevents fermentation, bacteria reduce the source of the problem.

1. Choose the kit according to the volume. Our aeration kits for tanks exist for tanks from 1,000 to 5,000 liters and from 5,000 to 15,000 liters. Each kit includes the Secoh air pump, a diffuser, and the air hose.
2. Install the pump in a sheltered place. The pump remains outside the tank, protected from humidity and bad weather, preferably above the water level.
3. Place the diffuser at the bottom. Lower the weighted hose to the bottom of the tank so that bubbles travel through the entire water column, where the sludge is located.
4. Add the bacteria. Spread a biological treatment like BactoRain while respecting the dosage on the product sheet. Its non-soluble formulation settles and acts directly on the bottom.
5. Let it run and observe. The improvement is gradual. Continue to draw water normally and note the evolution of the odor and color of the water.

Maintaining the tank over the years

Once odors are under control, the goal is to prevent them from coming back. Three areas require some attention: the supply of bacteria, the air pump, and the water inlet (gutters and filter).

• Bacteria: follow the rhythm of the BactoRain sheet (200 g/m², four times the first year then twice a year), to be adapted according to odors.
• Air pump: let it run continuously and plan to replace the membranes and valves every 5 years or so, as indicated on the kit sheet.
• Gutters and roof: remove leaves, moss, and debris, especially in autumn.
• Inlet filter: clean it regularly so that it truly retains organic waste.
• Drawing water: in case of prolonged absence, be aware that water that stagnates in the heat is more prone to odors.

Find all treatments for rainwater tanks in our store.

Frequently asked questions