Can a drain be *too* steep?

[Crofter]

I need to make a fall of about 6m over a distance of 34m, to connect my house to a new septic tank system.
I'm aware that there are regulations covering minimum gradient, 1:80 I believe, but I can't find anything about a maximum gradient. It has been mentioned to me that beyond a certain gradient you risk 'stranded solids' however I cannot find what that magic gradient might be, and any attempts to google it simply turn up people declaring the whole thing to be a myth anyway, suggesting you can go for whatever gradient you want.

I did email my BCO but he only said to keep the fall into the tank at 1:40, and use backdrop manholes. I can understand that the final flow into the tank needs to be at a low rate, but I don't see why it matters what I do upstream of that.

Part H has tables showing flow rates starting at 1:10 which whilst not stated as being any sort of limit at least implies that you can go that steep.
If I can go to 1:6 or so then it will save me a lot of hassle building backdrop manholes.

Cheers

[seanandruby]

Anything less than a 1:10 should go into a back drop.

[lutonlagerlout]

they really dont like anything too steep or separation will occur
as said we normaly keep between 1:30-1:50 and use roddable back drops
LLL

[Crofter]

Thanks, it seems there's no actual regulation on this, just good/best/accepted practise.

I got another reply from BCO which indicated 1:10 ought not to be a problem, he mentioned modern plastic pipes as being an advantage, I presume that is because they are smooth?

There will need to be some chambers at changes of direction, so I can incorporate some fall there, just don't want to do all 6m that way

[Captain Concrete]

You could keep the drain fairly shallow then use a Mascar bowl as a back drop, we've done this many times and Mascar bowls are fantastic for this and have never given us problems.

[GB_Groundworks]

i had to google mascar bowls

http://www.drainageonline.co.uk/Marscar....owl.htm