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

Bad smells in my rainwater tank , what should I do?, smelly rainwater... This is a question frequently asked of our Foudebassin experts.

How to combat bad smells?

• 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 regular inoculation with specific bacteria. This can be a very delicate problem, especially if this water is used for laundry.

Odors often appear after a hot period during which rainwater consumption has been reduced (often during the holidays).

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

Certain specific situations can explain these odor problems in the rainwater tank: excess pollen in the water, organic waste from birds, little pumping in the tank (stagnant water), decomposition of leaves and many trees nearby. In some 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 delivers rainwater to the tank is essential. However, if you did 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 my rainwater tank water give off bad odors?

There are three factors that cause the release of bad odors:

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

You must beware of the deposits that accumulate at the bottom. This organic waste will form a layer at the bottom of the tank. When we smell the odors, it is often because of the anaerobic bacteria that cause fermentation at the bottom (release of a sulfur or sewer odor).

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

How to treat the bad odors of my rainwater tank?

In the past, and at the beginning of rainwater tanks, bleach was recommended. This treats the problem in the short term, to improve water quality:

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

Bleach works in the 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 preventive solutions for odors?

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

Your tank must also be minimally protected from temperature rises. We recommend, if you are only at the idea stage, an underground tank. For an excellent result, reducing odors and improving water quality, you can use a specific air pump (it looks like what we find in aquariums but bigger).

Is rainwater harmful?

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

This is why rainwater can only be used for non-sanitary applications and never in the kitchen.

Diagnosis: where does the odor of your tank 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 answer depends on the amount of accumulated sludge and the equipment already in place.

• Open the manhole and smell: an egg or sewer 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 a dozen centimeters of sludge, an emptying is generally to be planned.
• Check the inlet filter and the gutters: if they are clogged, leaves and debris end up in the tank.
• Note when the odor appears: often after a hot period where we have little drawn from the tank.

To go further on the risks related to smelly water, also consult our article Smelly rainwater, should I worry?. If the odor manifests rather in the house, at the level of the drains, the problem may come from the pipes: see how to avoid sewer odors at home.

Aerating and inoculating the tank, step by step

The most durable combination combines two actions: continuous aeration, which brings oxygen to the bottom, and an input of bacteria, which gradually break down the organic matter of 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 tank aeration kits 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 moisture and the elements, preferably above the water level.
3. Place the diffuser at the bottom. Lower the weighted hose to the bottom of the tank so that the bubbles cross the entire water column, where the sludge is located.
4. Add the bacteria. Spread a biological treatment like BactoRain 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 normally and note the evolution of the odor and the color of the water.

Rainwater harvester that smells bad: what to do?

Whether it is an underground tank or a simple harvester (barrel, above-ground tank) placed under a gutter, the origin of the odor is the same: water that stagnates, poorly oxygenated and loaded with organic matter (leaves, roof dust, sludge at the bottom). Small harvesters, exposed to the sun, are particularly affected in summer.

1. Observe first: open the lid and look if there are leaves, deposits or sludge. An egg odor often signals water deprived of oxygen.
2. Clean the inlets: clear the gutter, the downpipe filter and the screen to limit what enters the tank.
3. Empty and rinse with clear water if the deposit is significant, without aggressive products that would disrupt the bacterial balance.
4. Limit light and heat: opaque and well-fitted lid, shaded location if possible.
5. Circulate the water: an tank aeration kit brings oxygen continuously, and the bacterial treatment Aquipond BactoRain accompanies the operation.

For a larger volume tank, go back to the section "Aerating and inoculating the tank, step by step". Regular maintenance and moving water avoid the return of odors most of the time.

Maintaining the tank over the years

Once odors are mastered, the goal is to prevent them from coming back. Three areas require a little attention: the input 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 adapt according to odors.
• Air pump: let it run continuously and plan the replacement of membranes and valves every 5 years approximately, 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 really retains organic waste.
• Drawing: in case of prolonged absence, know that water that stagnates in the heat is more subject to odors.

Find all the treatments for rainwater tanks in our store.

Common mistakes

Some reflexes aggravate the problem or only solve it for a few days. Here are the most common.

To complete your installation: our prefiltration foams and filters for tanks, the pumps for rainwater tank and the tank accessories.

Frequently asked questions