Septic Tank & Drainage Field

[northnorfolk]

Hi. We bought our house last year and despite having a so called drainage expert take a look be fore we purchased, it soon became clear the drainage field was failing. Our expert, concluded that we just needed to dig out the old drainage field and replace with 8 tons of shingle. Hole was dug, old pipes removed, a new single open ended perforated pipe approx. 10m long was laid and just short of £3k was handed over. Not bad for a days work but being new to septics etc was hoping to get the right advice and outcome.

This was fine for four months and then we had to have another septic tank empty. A lot sooner than I expected. This was closely followed by us having the tank emptied on a 4 - 6 week basis. The field was clearly not working and every time we had the tank emptied, water would flood back into the tank from the field.

The tank itself is an old concrete ring style and holds approx. 5000 litres. this holds about 16 days worth of waste as we are using 300 litres a day on average. We seem to be getting water / waste into the tank ok, its getting rid of the stuff out the other end that seems to be the problem.

I have had at least four companies round to give me their opinion and frustratingly they all say something different, hence I am hoping for some ideas from the forum.

Having got fed up with the various options, I looked through Part H of the building regs and soon realised that some of the options being put forward don't even comply with b/regs. even more frustrating!

I don't have a huge area to play with and want to get it right. I have since dug four percolation test holes around the area I want to use and they came back with an average of 48 Vp. The area is approx. 40 square metres.

 

Is this area going to be sufficient?

Do I need to upgrade to a STP?

Can I use the herringbone layout for the pipework or do they have to be linked?

 

Please could you help, any advice would be welcome.

[nod]

It sounds like the one we replaced 

on our plot 

The seller had her old tank replaced with two lots of concrete rings 6 mtr deep for each 

So called experts 

Perforated pipe also which had turned her garden into a smelly swamp 

The guys that did this where also specialist 

 

A may be a bit late but for 3 k you could of had a treatment plant 

At least the discharge is clean 

Probably not much help now other than there seems to be loads of these so called experts out there 

[ProDave]

Firstly, a single 10 metre bit of perforated pipe is nowhere near enough.

 

40 square metres might be enough you need to look up the building regs calvulation that gives the required area based upn Vp and number of occupants.

 

The fact that the tank re fills as soon as emptied suggests your field is waterlogged and is not draining away.  How close to the existing drainage field were your new percolation test holes, and how deep did you dig them?

 

A treatment plant would be a lot better and the drainage field is much less likely to clog up.

 

If you are suffering from high water table, then you might need an above ground filter mound.

 

but your so called experts should know all this and have told you all of this.

 

Is there a watercourse anywhere?  That is a far better solution if available but would require an upgrade to a treatment plant.

[joe90]

18 minutes ago, ProDave said:

Is there a watercourse anywhere?  That is a far better solution if available but would require an upgrade to a treatment plant.

if not a watercourse a ditch that Is wet some of the year is what we did with a Vortex treatment plant and rumble drain.

[ProDave]

This is the forumla

 

The percolation test calculations for a  soakaway are as follows

 

Area (A) = V X P X 0.25 for septic tanks

V = the time is seconds for the water in the test hole to drop by 1mm.

P = the max. number of persons that the unit is designed to serve 

 

So with you percolation rate of 48 and at a guess 4 people, you would need 48 square metres.

 

With only 3 people that would be 36 square metres.

 

Better work on installing the largest infiltration field that you can fit in which sounds like it will be 40 square metres

 

You will need a matrix of lots of perforated pipe usually laid in a herringbone fashion to cover the whole area.

[northnorfolk]

Reading the b/regs they only show using a continuous loop pipe system. Is it ok to use the herringbone system?

[Miek]

19 minutes ago, northnorfolk said:

Reading the b/regs they only show using a continuous loop pipe system. Is it ok to use the herringbone system?

What's the problem with doing as recommended in part H? I would ask building control about deviations from part H. Might be ok as part H isn't actually the regs, just an approved guide.

 

I've just fitted a Vortex STP and when I did the calcs for the drainage field I realised it was going to cost more than the STP. Luckily I can discharge to a nearby water course, though you need approval from the environment agency to do this, or NRW in my case