Septic tanks and 2020

6th Sep 2018

There are quite a few websites and businesses claiming that any form of off-mains sewage treatment system that involves a septic tank, of which there are hundreds of thousands in England, must be replaced by a more effective package sewage treatment plant before the beginning of 2020. It’d be great for us as a business if it was true, but it simply isn’t.

What the Government says is:

“If you have a septic tank that discharges directly to a surface water you will need to replace or upgrade your treatment system by 1 January 2020.”


Note that this applies to England only. Scotland, Wales and Northern Ireland each has its own laws in this respect.

“A surface water” is a stream, beck, culvert, river, ditch, pond, lake, tarn, etc. – anywhere with flowing fresh water on the surface (and it must flow).

A septic tank is one part of a two part wastewater treatment system and it has two functions:

1.To hold raw sewage long enough to allow the reliable separation of solids from liquids
2.To store those separated solids until their storage starts to have a detrimental effect on the effectiveness of the separation processes

If you’re one of those people who is now shouting at the computer screen that “My septic tank has never been emptied and it’s working perfectly well, you’re only saying that so that you can charge me for emptying it – unnecessarily!” consider this: how do you know it’s working perfectly well? Because it doesn’t smell? That’s one of the reasons why you have soil stack or soil vent pipe. Because it’s not backing up? Your tank could be leaking into the ground, or the raw sewage could be flowing straight out of the other wide and down a hole in the ground. Because you can’t see any polluting effects on the ground surface? Do you know how a drainage field is supposed to work, and why?

The second part of the two part wastewater treatment system is the drainage field. In terms of how much work the two parts of the system do, the septic tank does the “heavy lifting” even though that amounts to only about 30-40% of the sewage treatment process. It’s the drainage field that is responsible for the remaining 60-70% of the treatment process: effective treatment of the liquid that flows out of the septic tank depends on the drainage field being correctly designed and constructed.

Percolation testing – timing how long it takes for water to drain away – tells us how permeable or “receptive” the ground is to the septic tank liquor. If it takes a long time for liquid to drain away from the test holes the ground is less permeable, i.e. less able to allow liquid to flow through it, probably because the soil structure is dense with small particles, like silt and clay, or even rock. If liquid drains away quickly then the ground is more permeable, and probably contains sand and gravel, both of which give an open structure to soil. The area over which the drainage field is built is a function of a) the soil’s permeability and b) the volume of septic tank liquor that will be applied to it, and that volume is governed by the number of people using or living in the property that the drainage field serves.

The drainage field doesn’t exist just to spread the septic tank liquor over a wide enough area to allow every toilet flush and bath of water to soak away before the next slug of wastewater comes rushing down the pipes and thus avoid the pipes “backing up”. No; the primary purpose of the drainage field is to facilitate the completion of the greater part of the wastewater treatment process that is only started by the septic tank. The biological activity and chemical reactions that take place in a drainage field and the soil beneath it to convert highly polluting septic tank liquor into far more simple and non-polluting chemical compounds such as water, carbon dioxide and nitrogen are involved and intriguing but they’re not the subject of this piece. What it’s important to know is that if your septic tank is not followed by a drainage field and drains directly to a surface water, or has a drainage field with an “overflow” to a surface water, then the requirement to do something applies to you because you’re discharging highly polluting septic tank liquor into that surface water with adequate treatment.

So, do you need to replace your septic tank with a package sewage treatment plant? The answer is only if you wish to carry on making that discharge to the surface water - the alternative is to design and build and new drainage field to the current British Standard BS 6297:2007+A1:2008 Code of practice for the design and installation of drainage fields for use in wastewater treatment. Note that a pit-type, rubble-, gravel- or crate-filled “soakaway” as shown in the Building Regulations for rainwater is not suitable or permitted for sewage treatment.

The smallest drainage field you have for a 3 bedroom house with the best percolation test results under the British Standard would be 19m2, which equates to 42m of trench at 450mm wide and would cost about £2,000 plus VAT to install. The worst percolation tests permissible would need 278m of 450mm wide trench and cost around £11,000 plus VAT. That’s towards the top end of what you’d pay for the supply and installation of a package sewage treatment plant, so if you have a 3-, 4- or 5 bedroom house and access to land that drains fairly well a package sewage treatment plant, with its ongoing running and maintenance costs, isn’t necessarily the right solution for you.

If you’d like a free-of-charge, no obligation visit to your property for us to cast an eye over your septic tank and drainage field to give an opinion on whether or not it complies with the 2020 requirement, get in touch.

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