Black coiled corrugated land drain pipe - why can't it be used for septic tank drainage fields?

[DIYNotIan]

Evening, all - hope you're all having a good weekend

Think the title covers the main question. I know it is against building regs, but does anyone know why? What are the practical reasons why this type of pipe shouldn't be used?

I spoke to a few suppliers about it and their answers varied as to why black corrugated cannot be used. For example:

- Because if there is a problem with the tank and more solids were getting through to the drainage field, the pipe would clog more easily.
- Smooth pipe is better for even distribution across the whole drainage field. It ensures a smooth flow.
- You CAN use black corrugated land drainage pipe. But you should also follow building reg (!).

Can anyone shed any light on it?

Cheers

 

[scbk]

Are you making a new soakaway/repairing an old one? Will it be inspected - does it have to meet regulations? Is it a herringbone style field or a large deep pit?

 

[Ian H]

I though you just had to get a certain distance from the tank (maybe 5mtrs?) in solid pipe before you switch to land drain?

 

[frutbunn]

The coiled stuff should not be used the holes are quite small and they get a build up of film that eventually reduces or closes the holes.

 

[lostinthelight]

I've installed complete septic systems and deep soak away pits weren't allowed because of the lack of aerobic action below a metre
Drain field pipes had to be a foot under that or part untreated effluent could get into the water table.
Weren't allowed to use coiled pipe but those were the regs or it wouldn't pass but don't know the reason .
IanH, Went from tank to inspection chamber in solid which had rockers either side then into perforated

 

[frutbunn]

I though you just had to get a certain distance from the tank (maybe 5mtrs?) in solid pipe before you switch to land drain?

Nothing in the approved document H, and I've never asked for it and worked in a very rural area for 20 years so I've inspected hundreds of installations.

I didn't allow the coiled stuff for the reasons in my last post.

I've just tried to download the latest BS for small treatment works, the one referred to in the approved doc has been withdrawn, but it won't load for some reason! I'm a little out of touch as I've been working in urban areas for the last 18 years so I just want to check the latest guidance.

 

[Ian H]

I dug one up a few months back that had a meter of solid pipe from the tank then about 15mtrs of the flexi to a soakaway. That was as you say, even with the flexi blocked and exposed it didn’t leak until we sliced it, the holes were all silted up.

 

[DIYNotIan]

Crikey, how the weeks pass quickly!

Well first, many thanks for the responses.

@scbk - nothing is decided yet. But the plan if we did do it would be to replace an old drainage field. Without inspection But like I say, it's all ideas for now and I'm just curious RE the corrugated pipe and why BRegs say not to use it.

@frutbunn - are the slits in the corrugated land drainage pipes smaller than those in the rigid pipes? I've not seen them up close but on the internet they don't seem all that different. Also, if the baffle pipes in the tank are all correct, the tank is working properly, and a filter is added to the outlet T piece in the tank, would this help? Or is the biofilm more of a microscopic thing?

@Ian H - that's a useful anecdote. But do you think the same would have happened with solid perforated (slits) pipe? Or was it something to do with the corrugations. Seeing as it was going to a soakaway originally it obviously wasn't correct anyhow, so were any other corners cut? Was the pipe in a trench of gravel/stone? And would you say it was silted from the sewage or from whatever was around the pipe? Guess it would also be useful to know the condition of the tank and whether the effluent coming out the other side was as clean and solid-free as it should be. There's a lot of variables! But very helpful to hear of your experience.

Any further thoughts mist welcomed...

Btw, having spoken to a couple more suppliers it seems some of them aren't even aware that it shouldn't be used. I note also that it seems to be used (legally) in the US.

Cheers