Warning for High Rainfall Periods

 22 October 2023

Those with Septic Tanks or Sewage Treatment Plant need to be careful.
If you empty any underground tank today then the recent rains could well float the tank out of the ground. It may not be dramatic as this Sewage Treatment Plant was the day after emptying.
STP broken after emptying from high rainfall
But the pressure is huge.
Imagine holding a football below the surface in a swimming pool. Let go and it shoots to the surface... Water in the ground has a similar effect.

Why does it happen?

For clay or sand, if after a heavy period of rain, the soil is almost fluid. The tank is fine while it is ballasted down with sewage. But, when it is emptied the unit can move upwards, dramatically as the photo shows, or less so but still can cause a problem if the tank moves upwards just an inch or two.
The pipes to and from the tank are broken, or lifted. As you add more water, say over a week, then the pipe and tank may settle down and the damage is unnoticed. However, a constant leak at the joints will simply add more water into the ground. But if a stone has slipped down the side, then the GRP tank could be resting on that stone, eventually causing a rupture at the bottom of the vessel. The tank in the photo may have been lifted without rupturing the GRP carcase, or the pipes may be able to be relaid. It will be re-installed.
Though, this time it should be done with the manufacturer's instructions at hand. Generally, this means concrete. Tons of it! Not just to support the tank but to hold it down when it is empty. Had the installer surrounded it with concrete, up to the inlet and outlet pipes, then it would not have moved.

The Problem With Heavy Rain

With rain as heavy as we are currently experiencing, the ground, already saturated, is being asked to take even more from roofs and hard standings. Before the house was built all the water that fell on each acre of land had that area to deal with the water to send by drains to ditches or by run-off. Or simply absorbing it. If water could not get away, then puddles would form. Indicating the saturated nature of the land.

We come along and cover some of the land with houses. Over time we add sheds greenhouses, house extensions and concrete/ tarmac driveways!  Everything we do covers up part of the earth.


So we have to ask how is the land able to cope?

One of the consequences is serious flooding. This hits the headlines but the consequences for those Sewage Treatment Plant are serious too. If the septic tank or stp is just relying on gravity to get the water into the soakaway (aka drainage field), then as the ground gets wetter and wetter, the water backs up into the unit or septic tank and then into all the manholes.


Now we have a real problem. If the manholes fill up the kitchen waste water overflows on to the path. Lavatory water fills up the pan.

Getting a desludger in to reduce the level will help for a couple of days providing it doesn’t empty it. The tanker will be taking ground water out as much as sewage, and far more than normal. The tank could be full again because of the amount of water in the ground.

If you use a desludger be sure not to empty it too far below the inlet level to prevent the tank coming out of the ground.


A Temporary Solution


Using a fish-pond pump could well solve the immediate problem but won’t do the pump any good. Sewage water is corrosive over time. 

Care has to be taken too with electricity and wet environments. It is a long term problem that should be resolved at a later date. Fixing a pump station in the garden down stream of the unit is probably the answer. Getting our engineer out to advise is a good step to take. Buying a small pump, lay flat hose and cable could be a good short term answer. But be careful with electrical stuff.


Placement of the pump is important. If you place it not far below the inlet pipe then there is no danger of over emptying it. The pond pump should be stood in a bucket of clean tap water and the water flushed though the pump to enhance its life!


Fish-pond pumps should be used only to reduce the tank water level down to the normal outlet level. No further!


If the tank does not have a proper concrete surround then it can move upwards. Just moving an inch will affect the pipes to the tank. Just dangle the pump at outlet level. Stand and watch  it, don’t leave it  or it will run dry and burn out. Or invest in a pump with its own float level. Only then can you leave it.

But it still needs to be checked. It’s a makeshift temporary solution to get you out of a mess.

  1. Keep children away from the open tank.
  2. Make sure the hose is long enough. Too close and you will find the pump is pumping the same water over and over again as it circulates through the soil. These heavy rainfalls- more than a normal month’s rain in a day on top of a period of saturation - are to be expected because of climate change.
  3. Investigate your See where the rain water soakaways /  drainage fields are in relation to the foul water one is. Does it need diverting away from the important foul soakaway? 
  4. Get your black smith or garage man to make a pricker (a metre long steel bar 20 mm diamete, one end pointed like a pencil. The other with a tee handle 30 mm dia 300 mm long). Search for pipes and listen to the noise it makes! Ping! Means old clay pipes. If it bounces it’s plastic. Practice first.


What To Do

Re: Flooded Soakaway/ Drainage Field

We will get to you when we can but with so many suffering the same problem we can’t do it all at once. Let us know if you have sorted the problem with a pump. We will still make an appointment when we can.

Re: Floating Septic tank or STP

Get it in writing from the installer that your Septic Tank or Sewage Treatment Plant or any other underground apparatus, has been installed according to the manufacturer's recommendations. If you have had this happened to you, This email address is being protected from spambots. You need JavaScript enabled to view it. and we will have a rogues gallery!