Can anyone help me to design our foul waste system?

[Omnibuswoman]

We are going to install a waste treatment tank as we are almost 100m from the main sewer connection point and told it will cost £££ to connect. I've read the Binding Regulations and have checked the rules re distance from house etc.

 

However, I have become stuck on the design of the trenching/pipework system from the house to the tank, and the drainage field. We had a quote from a local company for this, but they whacked it up by over £2k between July and January, without any persuasive explanation, so I am now considering the feasibility of getting the installation done ourselves with the help of a local chap with a digger. 

 

I'm struggling to work out where to start. Do I first need to work out where the pipe will exit from under the slab? As it is a passive house the drains will come out from under the slab, I guess meet up in a sensible location, and from there flow to the tank.

 

Attached is a copy of the site plan with my initial thoughts. Is anyone here able to help me to work this out please?? Or is this one of those things where I am better off just paying a professional to sort it out?

Many thanks,
M

Plan of sewerage system.pdf

[Wumpus]

Not sure I’m going be a lot of help, but as I understand it from our foul drainage design, the drainage field also needs to be a min of 15m from any building, so you might need to allow for that too. At least that’s what the drainage designer told me.

 

I was also told we need sufficiently slow percolation/infiltration rates. In our case the tests were too fast if we dug down, so we have a minimal dig and then 500mm of cover over the top to give the right rates.

Edited February 19, 2022 by Wumpus

[Russell griffiths]

You will probably have far more than 1 pipe exiting from your slab, remember you cannot tee into a pipe under your slab as you wouldn’t be able to show how you can clear a blockage. 

So you will probably have a few pipe exiting. 

The main thing is making sure you have the fall to the tank, you need a 1::40 fall to the tank from the house. 

You also don’t really want the tank in an area you will have vehicles, you can no problem but you will need extra concrete and stuff to protect the tank, so better in a grass area out of the way. 

Get your floor plan with every single waste water pipe marked on it, toilets showers, kitchen, bootroom, laundry. 

You can then have a bit more of an idea.

getting the soakaway to work is a different matter and you will need to look into a percolation test, you will need this for the rainwater anyway. 

[Mr Punter]

You're on quite a steep slope with the tank below the house, so fine there.  The drainage field size is done by calculation.  The bit between the house and the tank is straightforward but you need to post up a floor plan.  It would be best to have stack pipes on outside walls rather than the centre of the footprint.  These are often internal, so the drainage is at the edge of the slab.

 

Are you doing a separate system for rainwater?

[nod]

All of the above contain good advise 

But your building control will tell you how he or she wants it’s done I wouldn’t over complicate things 

Keep it really basic for planners 

 

I’m doing our drainage design at the moment 

We have a 70 Meter run from our tank to the dyke at the bottom of the field 

I’ve shown this as 100 mil pipe 

Which is fine with planners 

BC will probably insist on 150 mil 

Two and a half times the price 

i wit and see 

 

[ETC]

@Omnibuswomanpost floor plans showing the location of all sanitary appliances - including the location of any condensing boiler which needs a foul drain connection - and I’ll take a look.

 

However keep the layout simple, place ICs at all changes of direction and gradient, place a(S) VP at the highest IC/MH and you won’t go wrong.

 

Work out the inlet invert level of the treatment plans and work back from there. Falls should be around 1:40 and cover needs to be a minimum of 600mm for a driveway and 300mm for a garden.

 

Quite straight forward to do a plan - a bit more complicated root work out falls and invert levels. Also depth of cover will determine the diameter of any ICs or MHs.

[ProDave]

Siting the tank and the pipework to it is the easy bit.  That is just reading building regs for where you are and applying them.

 

The difficult bit, and often the "make or break" of a system is the drainage field.  THAT is what you have to work out FIRST.  It can end up taking up a LOT of space and that does not look to be a huge plot.  so the drainage field needs designing first and then you work out what space is left where you might be able to squeeze the treatment plant into.

 

The basics are you first dig a test pit or 2 and perform percolation tests, and then some calculations tell you what area of drainage field you need.  then looking up the BR clearance distances you work out will that area of drainage field actually fit, and if so where, and what space is left over for the treatment plant and pipework etc.

 

What, if any of that has been done?

 

P.S this should all have been done at planning stage.  I hope you don't get any nasty surprises in this process.

[Omnibuswoman]

Thanks all. Ok first thing, floor plans are attached. 

 

@ETC There won't be a condensing boiler - we will have an ASHP outside the utility room feeding the hot water tank inside the utility room cupboard. Both bathrooms are on top of each other. We will have a stack pipe inside the house in the same area. 

 

@Mr Punter The rainwater system will be separate - the roof will drain to a water garden, and from there down to a pond in the south westernmost corner of the garden.

 

@ProDave We haven't had a percolation test done yet, but we know that drainage is likely to be slow - the ground is Kate Brook Slate. My parents, across the valley, had a klargester installed about 10 years ago in the same ground, and my dad said that the percolation test was a nightmare. He filled up the hole, and 3 hours later it was still full! The local company from who we obtained a quote for this work offered a working assumption of 42m soakaway length, based on the size of the house/tank that we will need and their knowledge of the ground in this area, but said that this was subject to a percolation test being done to finalise the calculation.

 

We own the land to the East of the red boundary line (south of the crossed out garden), and will be able to extend the soakaway into that area.

 

I have also asked South West Water to contact me to quote for connecting to the main sewer. It goes straight down the road at the bottom of the drive, so it is possible we might be able to connect to it without enormous cost, but our SE previously said that she thought that we would be bound by the nearest manhole which is 20-30m down the road, i.e. £20k-30k cost for that. I took her word for that, but realised today that I should check with SWW to be certain that a WTP is the way to go.

 

 

 

[Omnibuswoman]

Mods/admins, please can someone remove the docs I have posted as I have not properly edited. I'll post some screenshots instead.

Thanks!
 

[Omnibuswoman]

[ProDave]

Okay, that is good news that you have extra land available should you need it.

 

No 1, get a firm quote for a mains sewer connection, don't ASSUME anything.

 

No 2 get a percolation test done if No 1 looks too expensive, it is virtually impossible to design a private treatment system without that, and (certainly up here BC would not let you proceed without.)

[Mr Punter]

You can often connect to the sewer pipe via a saddle on the top, subject to the pipe size.  You do not have to connect via a manhole.  So much better than a treatment plant.  You should be able to get an asset plan from the water co and it will probably include the pipe material and diameter.

[saveasteading]

On 19/02/2022 at 15:03, Russell griffiths said:

you need a 1::40 fall to the tank from the house. 

I have heard this said before, but a 100dia pipe can go at 1:80 and that makes a huge difference to the depths of the pipes and the digester tank.

 

I think 1:40 is perhaps deliberately conservative in case of imprecision.

[PeterW]

 UPVC is fine at 1:80 so is easy to the tank but I would also seriously consider a mains connection. If it’s over 225mm as a main run then you can saddle it as @Mr Punter said. 
 

 

[saveasteading]

I think you need a professional.

(Drainage is a particular skill....lots of people can work out pipe sizes, but getting the best solution for you is cost related too.)

I am, and drainage is a speciality, but I would not give more advice without seeing the site circumstances, and then the advice is likely to be as above. Percolation test,  (diy is fine for feasibility)  and also get a quote for mains connection and the drain to it.

It is sometimes cheaper to pump sewage, even downhill,  when the distance is long. This is because the pipe is much smaller diameter and can wind around the driveway etc and to the final connection, without manholes. 

 

I thought the small area of land was the main problem but now see you have more. Poor percolation is a challenge, and may not be feasible at all for soakaway, esp as you already have the rainwater to deal with.....but it may work. 

In theory the stuff coming out of the treatment tank can be nearly clean....think about combining with rainwater after treatment, but that needs advice too.

 

If your rainwater lagoon was to overflow, where would it go?

 

 

 

 

[saveasteading]

On 19/02/2022 at 19:20, nod said:

BC will probably insist on 150 mil 

They can't if you can show that 100 suffices. who does the calculations?