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  • Lynda Smith

Failure of Septic Drainage Fields


Lets have a look at why Septic Tank Drainage Fields may fail and what the options are if they have already failed.

What is a septic drainage field? Septic drainage fields, sometimes referred to as septic drain fields, leach fields or foul water soakaways, are subsurface wastewater treatment facilities which are used to remove contaminants and impurities from the effluent that emerges from a septic tank.

If you are buying a house with a septic tank and a soakaway drainage field, then there are many problems that can arise after purchase. It is often a soakaway drainage field that is the problem, either incorrectly designed or blocked, so it is wise to get a septic tank survey done before purchase.

Firstly, a septic tank soakaway drainage field is NOT a 'hole filled with stones or crates'! These soakaways are for roof water only and are illegal and unacceptable for sewage effluent. A septic tank soakaway is a drainage field of solid, perforated pipes, laid in parallel trenches, on 300mm of washed drainage stone.






The key legislation around the design and build of a drainage field is the Government’s Building Regulations 2010, which states that a drainage field must be:

· At least 10m from any watercourse or permeable drain
· At least 50m from the point of abstraction of any groundwater supply and not in any Zone 1
groundwater protection zone
· At least 15m from any building
· Sufficiently far from any other discharge to ground to ensure the overall capacity of the ground
is not exceeded
· Down water/ slope of any groundwater source

In addition:
· No underground services or water pipes are allowed to be located within the dispersal area
· No access roads, driveways or paved areas should be located within the disposal area
· A percolation test must be carried out
· Drainage fields must ensure aerobic contact between liquid effluent and the subsoil. The minimum
depth of the pipes should be 500mm below the surface
· Drainage fields should be constructed using perforated pipe laid in trenches of uniform
gradient that is not steeper than 1:200
· The perforated pipes must be laid on a 300mm layer of clean shingle or broken stone graded
between 20mm and 50mm
· Trenches should be filled to a level 50mm above the pipe and covered with a layer of geotextile to
prevent the entry of silt
· Drainage trenches should be from 300mm to 900mm wide, with areas of undisturbed ground 2m
wide being maintained between parallel trenches
· An inspection chamber should be installed between the septic tank and drainage field

All septic tanks and their soakaway drainage fields have problems sooner or later.
These problems often occur after periods of wet weather, with the septic tank overflowing, septic tank effluent appearing at ground level, seeping into ditches or backing-up the pipes. These are common septic tank problems.

Most common septic tank problems and failures, with both brick and fibreglass ones, are due to the drainage field and not to the septic tank itself.

How quickly the drainage field problem becomes a complete failure depends on several factors, which include how often the septic tank has been emptied, the nature of the soil, the height of the winter water table, heavy rain periods and the deterioration of the septic tank itself.

Signs of a failed septic tank soakaway problem:

· Septic tank effluent surfacing on your land
· Strong or bad odours coming from the septic tank or drains
· Pollution of nearby ditches or streams with effluent
· Slow flushing of toilets
· Gurgling in the drains
· Ground movement (dips) near the septic tank or soakaway drainage field
· Toilet overflowing
· Overflow at washing machine hook-up
· Overflows into shower or bath when the sink empties
· Drain inspection chambers have standing effluent in them. These should be empty.

The reasons for septic tank and soakaway drainage field failure

1. Septic Tank Maintenance

All septic tanks require emptying ONCE A YEAR. They only hold 12 months sludge storage and if sludge starts to empty into the drainage field soil, it very quickly blocks the air spaces (porosity) in the soil, the effluent cannot soak away, you suffer a soakaway failure and the septic tank fills up, backing up the system.

2. Depth of the septic tank soakaway drainage field

All septic tank soakaway drainage fields must be constructed in the AEROBIC soil layer. This is soil which has oxygen in the air spaces between the particles and only occurs in the top 1 meter of the soil. This is because a foul water drainage field has two functions - the first is to soak the liquid away and the second is to treat the effluent and digest it via aerobic ( oxygen breathing ) soil bacteria. Aerobic bacteria cause no problems for soil porosity.
Unfortunately, if the drainage field is deeper than one metre below ground level; and this includes the 300mm. gravel bed underneath the pipe, then it is in the ANAEROBIC soil layer and the bacterial growth is anaerobic.
Anaerobic bacteria cause huge problems as they produce a black slime which blocks the soakaway and the porosity of the soil.
The fact is that most modern 'onion' shaped septic tanks have outlet levels deeper than one metre as standard! This almost certainly ensures that the soakaway drainage field is constructed in the anaerobic layer, making failure inevitable.

3. Nature of the soil and its effect on the soakaway drainage field performance

It is impossible to soak effluent away into a clay or clay-based soil in a normal soakaway drainage field. The porosity is so bad that they often fail within the first 5 years as their air spaces become filled with the high levels of suspended solids, often above 1200mg/litre, in the effluent. (This level of suspended solids can also be found in badly maintained sewage treatment units, and the same thing applies - you ruin your own soakaway drainage field!) In any case, even with a sandy soil, the porosity is eventually destroyed by the high level of suspended solids and by the black slime that results during the decomposition, although it may take 15 to 25 years. The air spaces fill with solids and the soil turns from a 'sponge' into a 'brick'. Adding a pump to the system to try to force the effluent into the drainage field only makes matters worse. The solids are 'blasted' into the soils air spaces, blocking the porosity even quicker and hastening your soakaway drainage field's demise!

4. Winter water table and its effect on the soakaway drainage field

If the winter water table becomes higher than the septic tank outlet level, then the outlet pipe to the drainage field starts to drain water from the drainage field back into the septic tank. There is one thing that is certain: water ALWAYS finds its own level. This usually causes the septic tank to fill and settlement chambers mix with clarified effluent with the result that, when the level subsides, the resulting effluent is full of solids which then block the soil porosity in the soakaway drainage field.

5. Deterioration of the septic tank

Many septic tanks' internal divisions/fins/rods and particularly the metal struts and bolts, (in the case of some 'Onion' septic tanks) and mortar joints (in the case of brick built ones), etc deteriorate, as raw sewage is a very corrosive environment. Concrete tanks are corroded by the hydrogen sulphide gas given off by the decomposing sewage. This turns into sulphuric acid on the lids and walls of the tank and disintegrates the concrete.

6. Sodium Binding in the soil

Excess sodium (salt) in soils with fine particles of silt or clay causes Sodium Binding. This results in the clay particles binding together, resulting in a waterproof layer being formed around the soakaway trench. Sodium is in washing powder, detergents, sweat, dishwasher tablets, water softeners and water from vegetable cooking, etc.

7. Another house joining the septic tank system

Septic tanks and soakaway drainage fields are designed to cope with a certain volume of liquid per day. You cannot increase this volume without increasing the size of the septic tank and drainage field to match.

8. Heavy Rain and its effect on the septic tank system

Periods of unusually heavy rain can also cause severe issues. If the soil cannot absorb the rainfall, it cannot cope with the extra liquid volume from a sewage system as well.

9. More people using the septic tank system

If you have bought a house that previously had only one or two people using the system and you are a family of four, this will have vastly increased the amount of water entering the soakaway drainage field. The previous owners may not have had any problems but that does not mean that you won't, as the soakaway drainage field may not be able to cope with the increased daily flow. This is an important point to bear in mind when buying a house with a septic tank treatment system.

How to solve the septic tank soakaway failure problem

You can install a complete new soakaway drainage field in a different part of your garden, if you are not in a Groundwater Source Protection Zone and have enough land (at least 100M², all of which is 15M from any building) of the correct soil type. You must also have a winter water table that never comes to within a metre of the bottom of the drainage field and no rock within 1M of the bottom of it. These are all new rules which you must comply with. Unfortunately, around 70% of all sites fail one or more of these conditions and if so, new below ground soakaway drainage fields are not an option.

You can scrap the entire system and install a full sewage treatment unit which CAN, if they comply with the General Building Rules, discharge to a ditch, stream, land drain etc. The effluent is gone from your property and with it, all the hassle of soakaways. This is now the cheapest option available if you have a ditch or other watercourse that is accessible.
If you have recently bought a house with a defective septic tank system, it is sometimes possible to use the Legal System to pay for the repair or replacement works, it you win. It is also often possible to have the total cost of the replacement system paid for by your Buildings Insurance company.

Finally, remember to maintain your septic systems. Owen Environmental Services can help provide you with a survey and we can arrange for septic tank emptying.

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