Published: 17 October 2026
In short
- There is no single interval that suits everyone: tank volume, number of people and daily water use decide it.
- Sealed tanks fill up more often, while in leaking tanks the real risk is waste seeping into the soil and water.
- A slow-draining toilet, a smell around the tank cover and wet ground with lush green grass are the first signs that a tank is filling up.
- Noting the date of each emptying and how full the crew found the tank lets you predict the next one.
The way to find the right interval is to understand what affects your tank and to keep a record of a few emptyings. Once you do, you can plan the emptying on your own schedule, before the tank overflows.
What decides the emptying interval?
Each of these factors can change the interval on its own. When two of them increase at the same time, for example when guests arrive in summer and the washing machine also runs every day, the tank can fill up much sooner than you expect.
- Tank volume: a small tank fills up much earlier than a large one under the same use.
- Number of people living in the home: every extra person adds to the daily wastewater, and long stays by guests count too.
- Water use habits: a washing machine that runs often, long showers and a leaking cistern quietly fill the tank.
- Other water entering the tank: if rainwater from the roof or garden is connected to the tank, it fills faster in rainy months.
- Tank construction: a sealed concrete or prefabricated tank and an older tank with an open base behave very differently.
Why do sealed and leaking tanks need different schedules?
Every drop that enters a sealed tank stays there, so the time it takes to fill depends almost directly on how much water you use. These tanks are emptied more often, but no waste reaches the surroundings and it is easy to predict how full they are.
In older tanks with an open base or walls, part of the liquid soaks into the soil and the tank seems not to fill for a long time. Yet the sludge at the bottom keeps building up, the soil's ability to absorb gradually drops, and one day the tank starts to fill quickly. The seeping waste can reach the soil in your garden, a nearby water well and neighbouring plots, so emptying such a tank rarely is not a saving.
How do you set a schedule for summer and seasonal homes?
In homes used only in summer, the tank rests in winter and is loaded in a short time in summer. The calmest approach is to check the tank when you open the house for the season and, if needed, have it emptied when you close it. That way the waste does not sit in the tank all winter and cause odours, and the new season starts with an empty tank.
If you know the number of people in the house will suddenly rise over holidays and festivals, it also helps to have the tank emptied before the crowd arrives. An overflowing tank becomes a much bigger problem on a day when everyone is at home.
What signs tell you the tank is full?
Any one of these signs can have another cause, for example a drain line inside the house may be blocked. If two or more appear together, it is time to have the tank looked at.
- When you flush the toilet, the water goes down more slowly than usual or gurgles.
- There is a smell from the drains on the lowest floor of the house, especially the shower drain.
- The soil around the tank cover is constantly damp, and the grass is darker and thicker than elsewhere.
- There is a leak at the edge of the cover or at a spot in the garden.
- After rain, all of these signs become more noticeable at once.
Why does putting off emptying make the job bigger?
In a full tank, the liquid level rises to the inlet pipe and the wastewater from the house has nowhere to go. The first backflow usually comes out of the lowest drain, that is, the shower drain or toilet on the ground floor. At that point the job is no longer just an emptying and also includes cleaning the inside of the house.
In a tank that is not emptied for a long time, the sludge at the bottom hardens and forms layers. These layers both reduce the usable volume and make emptying harder, and sometimes the bottom has to be loosened with water. The pipe connecting the tank to the house can also fill with sludge in the meantime and may need to be cleared separately.
How do you keep your own emptying record?
The most reliable basis for predicting the next emptying is the history of your own tank. After a few emptyings it becomes clear how quickly the tank fills, and you can set the schedule accordingly.
You can keep the record in the notes on your phone or in a notebook that is easy to find at home. When the house is sold or rented out, these notes are also very valuable for the next occupant.
- The day and month of the emptying
- How full the crew found the tank and how much sludge they saw at the bottom
- How many people were living in the house at the time and whether any guests stayed for long
- Any crack or damage noticed on the cover, the walls or the inlet pipe
- Any odour, slow draining or overflow since the previous emptying
What can you do at home to lengthen the interval?
Everything that enters the tank either settles at the bottom or takes up volume. Changing a few habits at home can noticeably lengthen the time between emptyings.
- Do not put wet wipes, pads, cloth or cotton in the toilet: they do not dissolve and stay at the bottom.
- Do not pour frying oil or food scraps down the sink.
- Have a leaking cistern and dripping taps repaired.
- Spread the laundry and dishwasher loads across the week instead of piling them on one day.
- Make sure the rain gutters are not connected to the tank.
- Avoid pouring strong bleach and harsh cleaners into the tank: they weaken the bacteria that break down the waste.
