Percolation tests for Septic Tank and Sewage Treatment Plant systems
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What is a percolation test?
A percolation test for a sewage treatment plant or septic tank is a test to see how well the area where you intend to discharge your effluent drains. This is carried out at the depth you intend to discharge.
Why do you need to do one?
You need to carry out a percolation test to see if an installation is feasible and to inform its design. If you do not carry out a percolation test and discharge to the ground then it is likely your system will not work.
The percolation test shows how quickly if at all, the liquid can drain away at the depth and the area where you will construct your drainage field. So if the effluent cannot drain away as quickly as you are putting it in then the system will back up and fail.
How to set up a percolation test?
There is literally so much rubbish on the internet about the test should be carried out. We carry out our tests in line with Building regulations 2010 Part H2, the instruction is very clear, I personally cannot understand why there is so much confusion.
Firstly you need to know the depth at which your effluent will discharge. In systems fed by gravity, this will be the depth at which the pipe is before your connection to the proposed drainage field. If you have a pumped outlet it will be the depth you intend to pump to.
Again the depth piece causes much confusion. If you have a septic tank you need to create a secondary treatment area. Building regulations 2010 H2 section 1.39 stipulate that Drainage fields or mounds should be constructed to ensure aerobic contact between the liquid effluent and the subsoil. My view is that this should be in the top 750mm; speak to your building control advisor. Therefore your discharge cannot be deeper than 750mm; if the outlet from your tank is lower you will need to pump it up to the right height. If you do not ensure aerobic contact then your soakaway will block and fail quickly.
If you have a Sewage Treatment Plant then you do not need a secondary treatment area and the drainage field can be constructed at the natural depth of the outlet. Please see our guide on Septic tanks vs Sewage Treatment Plants if you are confused.
Okay so dig a square 1m x 1m and then dig down until you reach the required depth. If you need to go down further than 1m get a professional to do it. Once at that depth dig another hole 300mm x 300mm x 300mm, this is your test zone. Be careful and take steps so you do not bury yourself if there is a cave-in. You need two test holes so repeat.
How to undertake a percolation test?
Okay so fill up your 300mm test hole and go to bed, it has been a hard day you deserve it! Also, section 1.35 on the building regulations requires you to do this.
Okay, so you bounce out of bed the next day with the excitement of undertaking the test. Put a marker in the test hole 75mm from the top and another 75mm from the bottom. Fill the test hole again and wait. Once the water hits the first marker start your stopwatch when it hits the second stop the timer. Repeat this 3 times in both holes.
Just to note section 1.37 of the Building regulations say that the test should not be carried out during abnormal weather conditions.
What to do with the results of the percolation test?
Okay, so you will have 6 results, two holes with three tests in each. Add up all of the timings and then divide by 6 to give you the average. Then divide your average by 150, this gives you the average time in seconds for the water to drop 1mm the Vp. A Vp of between 12 and 100 is seen as favorable.
Now you have your Vp you can firstly see if a drainage field is viable and if so how big it needs to be. We will cover this in another article.
Like I said there is so much conflicting information on all things home septic. The regulation is confusing and contradictory but carrying out a percolation test is very clear. Please read it for yourself here.
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Based in the New Forest, Hampshire on the border with Dorset and Wiltshire we operate an expert team of off-mains drainage specialists. This encompasses Septic Tank, Soakaway and drainage system problems, installations, blockages, and servicing. In addition, any issues relating to cesspits and sewage treatment plants.
Homeseptic offers high quality, professional advice, repairs and installation for all aspects of effluent cleansing and disposal across the south of England. Whether you’re looking for septic tank installation, sewerage treatment plants or systems, piping or soakaway solutions, Homeseptic employs only the most competent engineers in the drainage and treatment industry and knows exactly which solutions suit each property.
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