How to keep rainwater clean?

To keep rainwater clean, you need to address two things: prevent waste from entering the tank and maintain oxygen in the water. In the vast majority of cases, an inlet filter, an inlet that does not stir up the bottom, a closed tank protected from light, bottom aeration, and a pump that draws water near the surface are sufficient to keep the water clear and odor-free.

Rainwater is valuable in the garden: it is soft, chlorine-free, and costs nothing. But stored for several weeks in a tank, it can become cloudy, greenish, or smell like rotten eggs. Nothing is inevitable: in most cases, you simply need to adjust the installation. Experts in aquatic environments since 1998, we explain what happens in a tank and how to prevent it.

Why rainwater deteriorates in a tank

Rainwater is clean when it falls, but it is no longer completely clean when it reaches the tank. As it runs off the roof and through the gutters, it carries leaves, moss, pollen, dust, bird droppings, and sometimes conifer needles. All this organic matter ends up at the bottom of the tank.

At the bottom, microorganisms begin to decompose it. As long as there is oxygen in the water, this decomposition occurs without nuisance. The problem arises when the water remains still for weeks: the oxygen is depleted, and other microorganisms, which live without oxygen, take over. They produce foul-smelling gases, including hydrogen sulfide, which is responsible for the rotten egg smell, and the water becomes cloudy.

Two other factors aggravate the situation: light, which allows algae to develop if the tank is translucent or poorly closed, and heat, which accelerates decomposition and reduces the amount of oxygen the water can contain. An above-ground tank exposed to the full sun in summer often combines both.

Filter the water before it enters

The most effective action is also the simplest: stop waste before the tank. Every leaf that does not enter is a leaf that will not rot at the bottom.

• Clean the gutters at least in autumn, after the leaves have fallen, and in spring. A frogger (grate) at the entrance of each downspout prevents leaves from rushing in.
• Install a collector filter on the gutter downspout or a filter upstream of the underground tank. It retains coarse debris and lets the water through.
• Add a pre-filtration foam if your installation allows it: it retains finer particles. It is simply rinsed with clean water when it becomes clogged.

Remember to check these filters after heavy storms and in autumn: a clogged filter lets the water overflow, which then passes by or carries the debris into the tank.

An installation that does not stir up the bottom

Even with a good filter, a light deposit always forms at the bottom of the tank. The goal is to leave it alone. Three elements of the installation contribute to this:

• The calmed inlet: the inlet pipe goes down to the bottom and ends with an upward-facing elbow. The water enters gently, without creating a vortex that would put the mud back into suspension. It also brings fresh and oxygenated water to the bottom of the tank.
• The overflow: when the tank is full, the excess must leave through the surface. That is where pollen and fine dust float. A well-placed overflow "skims" the tank with every rain.
• Floating suction: the pump should not draw from the bottom, where the deposit is, but just below the surface, where the water is clearest. A float connected to the suction pipe follows the water level.

Keep the tank dark and cool

An opaque and well-closed tank is essential. Even weak light is enough for microscopic algae to develop and turn the water green. Check that the lid closes properly and that no opening lets in light, insects, or small animals.

For an above-ground tank, prefer a shaded location, or protect it from direct sunlight with a covering, a hedge, or a shelter. An underground tank has the advantage of keeping the water cooler all year round, which limits decomposition. A closed lid also prevents mosquitoes from laying eggs in it.

Oxygenate the water to prevent fermentation

This is often the missing piece. A small air pump, installed in a dry place, sends air to a diffuser placed at the bottom of the tank. The bubbles gently stir the water and maintain oxygen throughout the entire water column, including at the bottom, where bad odors form. The microorganisms that decompose the deposits then work under good conditions, without producing foul-smelling gases.

The sizing depends on the volume of the tank. We offer complete ready-to-install kits, with an air pump, diffuser, and hose, depending on whether your tank is from 1,000 to 5,000 liters or from 5,000 to 15,000 liters.

Maintenance throughout the seasons

A well-designed tank requires little maintenance, but it does require some. Here is a simple routine:

• In spring: clean the gutters and filters, check the operation of the air pump and the diffuser.
• In summer: monitor the odor and color of the water, especially during hot and dry periods.
• In autumn: clean the gutters after the leaves have fallen, rinse the pre-filtration foam.
• Every few years: drain and remove the deposit from the bottom if its thickness becomes significant.
• Air pump: replace membranes and valves according to the manufacturer's recommendations (about every 5 years for Secoh pumps, according to the product sheet).

To limit silting between drainings, a biological treatment such as Aquipond BactoRain can be added. It settles at the bottom and releases microorganisms that feed on organic matter. According to the product sheet, it is used at a rate of 200 g per m² of bottom, four times in the first year, then twice a year.

What uses for rainwater?

Clear and odor-free rainwater is perfectly suitable for watering the garden and vegetable patch, cleaning terraces and tools, or filling a pond, provided it is introduced gradually. Connected to a compliant installation, it can also supply toilets or the washing machine; check with your local municipality and an installer for the rules in force in your area.

However, stored rainwater is not considered potable: do not drink it and do not use it for cooking without specific treatment and appropriate testing.

If the water in your tank already smells bad, start by reading our article dedicated to bad odors in a rainwater tank: it details how to start over with healthy water.

Frequently Asked Questions

Why does my rainwater smell like rotten eggs?

Most often, the water lacks oxygen at the bottom of the tank. Organic deposits ferment there and produce hydrogen sulfide. Bottom aeration and better filtering at the inlet generally solve the problem.

Should I put bleach in a rainwater tank?

This is not a solution we recommend for water intended for the garden: it does not address the cause and can harm plants. It is better to limit the entry of organic matter and oxygenate the water.

How to prevent the water in the tank from turning green?

Green water means that light is entering the tank. Use an opaque tank, close the lid tightly, and place it in the shade if it is above ground.

How often should I clean a rainwater tank?

Filters and gutters are cleaned at least twice a year. The bottom of the tank is only drained when the deposit becomes significant, often every few years if the inlet is well filtered.

Where should I place the pump in a tank?

Avoid drawing from the bottom. Floating suction allows you to draw water just below the surface, where it is clearest.

Also read

• Bad odors in my rainwater tank, what to do?
• Rainwater that smells, should I worry?
• How to take care of the pond after rains and heavy storms?