Drainage field size after a treatment tank. Are the building regulations misguided , or biased against treatment systems.

[saveasteading]

I'm hoping I am misunderstanding the regulations. 

Document H , Drainage and waste disposal.

 

Actually on reflection maybe Document H used to be silly but now is vague. The Scottish rules are still silly. So I will start with them.

 

Quote  The floor area of a sub-surface drainage trench required to disperse effluent from treatment plants or septic tanks may be calculated from the following formula: A= P x Vp x 0.25

 

so for a typical 6 person house with average percolation of 30s/mm. the result is 6 x 30 x 0.25.  = 45m2.

That is a fancy trench system measuring say 3m x 15m.

Perhaps that is sensible for a septic tank outlet where the stuff is still filthy.

6 persons would produce 6 x 130 litres in a day total.   Less than a m3, not all filthy, and it dribbles in.

 

Then it says to reduce it by 20% if it is a treatment tank.  (Not so long ago manufacturers  claimed this outflow was almost safe to drink.)

20% reduction seems ultra conservative.

 

In a discussion with a JCB driver in Scotland, as he lifted our treatment tank in,  he said they always just put in a rubble soakaway. Perhaps 20 times he had done this.

The building inspector was not interested. My submission showed a great big drainage field. With this in mind we left our drainage field at 'phase 1 of 3'.

 

Now to the English reg's.   https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf

 

For septic tanks the same formula applies.

However I don't see any reduction for treatment tanks.  I'm sure it used to but perhaps sense has prevailed.

At 1.54 it mentions package treatment works but is silent on what happens to the outflow.

 

Hey...  I have just found clause 1.11... It says "normally allows discharge direct to a watercourse".

 

 I am going to propose putting it into a soakaway and see what results from the bco. Does 'normally' cut it details-wise?

I must admit just pointing the pipe at a copse on one occasion and the bco didn't look. But back then the reg's said nothing on the subject.

 

Any feedback on what you have done is welcome.

I guess Scottish rivers are generally much cleaner, so they don't want anything going in. But I think it is silly, and perhaps ignored. 

Next time I would quote the English regs. even for a Scottish project. But I am working on an English one so it is now clear.

 

Thanks for being on this journey with me. It really has helped typing it down logically.

Perhaps it will simplify the process for some others too.

 

 

 

 

 

 

[ProDave]

Is this build you are doing now in England or Scotland?  you don't get to choose which set of regs you build to.

 

Our previous house was 5 bedroom so I believe it had to be sized for 8 people and with the Vp we had it worked out at 88 square metres.  It was indeed a big hole to dig, but we employed what turned out to be a very fast digger driver then who did it all with his 3CX.  He was so quick driving that thing around I swear he got a wheelie out of it.  He did it all in a day.

 

And building control were not in in the slightest interested.  They only wanted to inspect and pressure test the drain pipe runs.

 

I don't think some people realise how big they can be.  You often need more garden space for the soakaway than you do to actually build the house. And when you add the border requirements it becomes even more surprising how much space you need.  I was very glad not to have to do that this time (discharge to our burn)

[JohnMo]

Try failing a percolation test - we are on sand so water drained away to quickly. Ours is about the size of tennis court. F'ing huge.

[saveasteading]

9 minutes ago, JohnMo said:

we are on sand so water drained away to quickly.

On our Scottish project this was close to being an issue. The magic, fastest number seems to vary from 12 to 15. We had 17.

 

If it is faster then I think it can still be put before the EA or in Scotland SEPA.

The concern is of the sand washing away and sinkholes forming. Our sand was like rock to build on but porous with it (glacial deposits).

It might cost more to prove though, than building an alternative solution.

 

 

19 minutes ago, ProDave said:

England or Scotland?  you don't get to choose which set of regs you build to.

 

England. But I was looking for the logic behind it.  If based on proper science then that could be the makings of the proposal, as the regulations are generic of necessity.

Then I found the magic clause 1.11. Wouldn't you show that to the Scottish inspector if it suited?  I wonder why they are not concerned though. They know it is silly? Been told to keep out of it as policy?

[ProDave]

SEPA definitely do not allow discharge to a watercourse as a matter of course.  It was only after 2 different drainage proposals for land drainage had been rejected they said "why not discharge to the burn"  But even that I had to measure flow rates in the middle of summer to calculate dilution before they said yes.  Tense few weeks, no drainage = no building warrant.

[BotusBuild]

We put in what looked like a 3 rung ladder, in a square area measuring 6m (ish), for about 30m of pipe in 1m trenches, so 30m^2. All guided by local groundsman.

I submitted photos and the BC came round to check the coverage of the pipes with stone.

Site in England

[saveasteading]

15 minutes ago, ProDave said:

2 different drainage proposals for land drainage had been rejected

What did they reject? I might have proposed a soakaway or sand bed perhaps 10m from the burn, to filter the water a bit more.

I can see their point of view. Even a  badly installed 3 chamber tank will still do most of the work, even with the power off. But one of the other types might overflow fairly neat stuff if the power went off.

And we know that lots of people won't know or care about putting beach etc down the pan. 

I'm guessing from previous posts that your burn hurtles to the sea, and runs all year. But they can't go looking at every case.

 

Anyway I have had a good day. My English proposal just needs a soakaway or short French drain.  That is £1,000 not to spend.  I will put lots of numbers down so the BCO just skims to the end. and an obvious quote of clause 1.11.

 

I have more concern now about the roof water going to an existing trench  that was dug for twice as much roof, I know works, but can't really prove.

Again, it won't go to the EA so as long as the bco is happy, it is my shout.

 

BTW I am thinking of including a large water tank in-line, to use for summer watering. If I filter the incoming water and use a pump it doesn't even need to be deep or accessible.

What size would allow for keeping the garden alive during a hosepipe ban? 3m3??

£630. Will it ever repay itself? Probably not. £2/m3  x 15m3/year ???  x 20 years = £600.   But the garden not dying! priceless.

[ProDave]

50 minutes ago, saveasteading said:

What did they reject? I might have proposed a soakaway or sand bed perhaps 10m from the burn, to filter the water a bit more.

Initial proposal was a filter mound, the issue was although Vp was ok, in winter the water table was too high and a conventional soakaway would be flooded.  That was rejected because to get the space we needed it would have been closer than 10M to the burn.  Then a system using a Puraflow system (crates full of filter medium) which took up less space was proposed, but rejected for an unspecified reason.  So SEPA said why not discharge to the burn.

 

What always struck me as silly is they rejected a TP discharging into a filter mound less than 10 metres from the burn, so instead they allowed, even proposed the same TP discharging directly to the burn.  Logic surely says why not discharge to a filter mound that finishes say 2M from the burn?

 

Anyway happy with the result.

 

but if only we had known from the start.  The house was kept to one side of the plot to allow room for the filter mound. If we had known that discharge to the burn would be allowed, I would have moved the house over a couple of metrres but that would have meant a new planning application and all the delays and them probably a new building warrant, so we left it as it was.