Sizing an air pump for a rainwater tank

For a domestic tank of 1,000 to 5,000 litres, a small diaphragm air pump combined with a diffuser of around 10 litres per minute is generally sufficient; for 5,000 to 15,000 litres, you move up to a more powerful pump and a diffuser of around 20 litres per minute. Beyond that, the choice depends on the depth and shape of the tank. In all cases, the diffuser must be placed at the bottom, and the pump installed in a dry area.

Aerating a rainwater tank is the best way to prevent fermentation and bad odours. You must choose a suitable pump: if it is too weak, it will not circulate the bottom; if poorly installed, it will wear out quickly or take on water. Experts in aquatic environments since 1998, we provide you with the guidelines to properly size your aeration.

Why aerate a rainwater tank

In a tank, water can remain still for weeks. Leaves, pollen and dust that arrive with the rain settle at the bottom and decompose. This decomposition consumes the oxygen in the water. When there is none left, microorganisms that live without oxygen take over and produce foul-smelling gases: this is the well-known rotten egg smell of tank owners.

An air pump connected to a diffuser placed at the bottom creates a flow of fine bubbles. As they rise, they carry the water from the bottom to the surface, where it is recharged with oxygen in contact with the air. The entire water column thus remains oxygenated, and decomposition occurs without nuisance. This is the principle of a micro-wastewater treatment plant, applied to a clean water tank.

The criteria that matter for sizing

Three parameters guide the choice of the pump:

• The volume of the tank. The larger it is, the more air is needed to circulate the whole. This is the first criterion, the one used to choose a kit.
• The water depth above the diffuser. The air must overcome the water pressure to exit the diffuser. The deeper the tank, the more pressure the pump must provide. A diaphragm pump delivers less air flow at greater depth than at the surface: consult its flow curve according to pressure.
• The shape of the tank. A long and shallow tank is less easily circulated with a single central diffuser than a round and tall tank. In a very elongated tank, two distributed diffusers give a better result than one larger one.

The length of the hose between the pump and the diffuser also plays a role: a very long or small-diameter hose creates pressure losses. For a classic domestic installation, ten metres is not a problem.

Our kits by volume

To simplify the choice, we have assembled complete kits that correspond to the most common domestic tanks. They include the pump, the diffuser and the hose: all that remains is to install them.

For a tank of more than 15,000 litres, or of a particular shape, contact us with the dimensions: we will offer you a suitable pump from the Secoh range, possibly combined with several diffusers.

Properly installing the pump and diffuser

1. Place the pump in a dry area and above the water level. A garage, a dry basement or a small ventilated shed is suitable. The pump must be able to draw in clean air and never be submerged.
2. Add a non-return valve on the hose, near the pump. If it stops, the water will not rise back into the hose up to the diaphragm.
3. Lower the hose without kinks through the manhole or a provided opening, fixing it so that it does not rub against an edge.
4. Place the diffuser flat at the bottom, ideally in the centre or opposite the water inlet. If it tends to float, use a weighted hose or a support.
5. Turn it on and check: you should see or hear regular bubbling on the surface.

How many hours a day to run the pump?

The simplest thing is to leave it running continuously. Diaphragm pumps like those from the Secoh brand are designed for continuous operation and consume little. Continuous aeration guarantees that no area of the bottom becomes oxygen-poor again, even in summer when the water warms up.

If you prefer to program the pump with a timer, keep regular and long operating periods, and monitor the smell of the water: its return indicates that the aeration is insufficient.

Air pump maintenance

A diaphragm air pump requires little maintenance, but it does require some:

• Regularly clean the pump's air filter, especially in a dusty room.
• Check that the bubbling remains regular: a drop may indicate a clogged diffuser or a worn diaphragm.
• Replace diaphragms and valves according to the manufacturer's recommendations; for Secoh pumps, our product sheets recommend maintenance approximately every 5 years.
• Check the hose: no kinks, no cracks, no crushing.

To go further, our guide on the maintenance of an air pump, diaphragms and diffusers details each operation.

Signs of insufficient aeration

A properly sized aeration is forgotten. If one of the following signs appears, the installation deserves to be reviewed:

• The smell returns, especially in summer or after a long period without rain: the air flow is no longer sufficient to oxygenate the bottom.
• The bubbling is weak or localized: partially clogged diffuser, kinked hose, or pump whose diaphragms are wearing out.
• The drawn water remains cloudy while the surface appears clear: the diffuser may be placed too high, or the sediment at the bottom is too thick.
• The pump heats up or becomes noisy: it is working against too high a pressure, often due to a dirty diffuser.

In most cases, cleaning the diffuser and replacing the diaphragms is enough to restore the original flow.

Useful add-ons

Aeration solves the oxygen issue, but a tank remains healthier if you also limit what enters it and what accumulates at the bottom. A filter at the inlet and a pre-filtration foam hold debris. A biological treatment like Aquipond BactoRain helps break down organic matter at the bottom. Finally, a floating suction on the watering pump allows you to draw the clear water from the surface.

Frequently asked questions

What air pump for a 5,000-litre tank?

A 5,000-litre tank is at the limit of our two kits. The 1,000 to 5,000-litre kit is suitable for a compact tank; if it is deep or if the water already tends to smell, the 5,000 to 15,000-litre kit offers more margin.

Where to place the diffuser in the tank?

Flat, at the bottom of the tank, ideally in the centre or opposite the water inlet. It is at the bottom that oxygen is lacking and where odours form.

Can the air pump be placed in the tank?

No. It must remain dry, in a ventilated area, and preferably above the water level, with a non-return valve on the hose.

Does an air pump consume a lot of electricity?

Small diaphragm pumps intended for tanks have a modest consumption. Consult the power indicated on the sheet of the chosen model to estimate the annual cost.

Is aeration enough to eliminate bad odours?

It treats the main cause, the lack of oxygen. If the sediment at the bottom is very large, a drainage and cleaning may be necessary before restarting healthy water.

Also read

• Bad odours in my rainwater tank, what to do?
• How to protect my air pump?
• Where to place a non-return valve on a pond: pump, filter, suction and air pump