fall on sewage pipe system

st1979

Hi am putting in a sewage system and the client has been told to put in a fall of between 1:100 and 1:200.
The reason being it's a long system. About 200m from one end to the other using 110mm pipes. The ground has a natural fall towards the sewage system of 1:40 and 1:60.
I can easily put in drops at each junction/manhole but I am afraid of such a small fall.
Any comments appreciated

sydthebeat

"Client has been told".... By who?

st1979

He was told by architect who has made many mistakes already and can change his mind. The architect said he was told by a friend of his who is a builder. So both me and the client are not mad keen on his idea. I have put in pipes at 1:100 before but never 1:200 especially when there is no need with the site slope already there. It also doesn't leave much of a margin of error.

sydthebeat

Is it a traditional septic tank that the system is out falling to? Or a waste water system?

BryanF

Ask the arch for it in writing..

Vandy West

Irish water publish a guideline for foul sewers on their website, probably worth getting advice from a civil engineer on that long of a run. 1:200 will likely produce velocity less than required for the pipe to be self cleansing.

Metric Tensor

I don't want to be giving direct numbers here in case people might take it as gospel but the steepest numbers you're talking about are borderline for a 110mm pipe and the shallowest is definitely inappropriate for 110mm pipe.

You need to get a civil engineer to check it. Dropped manholes won't help make a fall shallower - they are mainly of help when the ground slopes too steeply.

In general though wider pipes can be laid at shallower falls so start prepping to buy a 225mm or 300mm pipe and then get an engineer to spec it!

st1979

Spoke to civil engineer and he recommends a fall of 1:60
I am not sure what difference the type of treatment plant makes. As I am talking about the pipe going into the treatment plant not coming out of it.

sydthebeat

st1979 wrote: »

Spoke to civil engineer and he recommends a fall of 1:60
I am not sure what difference the type of treatment plant makes. As I am talking about the pipe going into the treatment plant not coming out of it.

It does make a difference

a septic tank works on anaerobic bacterial action where a crust is formed over the water to keep out the oxygen. therefore you want as shallow as possible outfall into it to negate against splashes and or risk of breaking the crust.

a waster water treatment system system works on an aeration basis, with aerobic bacteria... where splashes and movement are very much wanted. therefore falls to a waste water treatment system dont have to be as shallow.

Czhornet

1 in 60 for a single house, 1 in 100 for 2 or more houses! (from CIF House Building Manual) doesn't matter on the length of pipe run.
Anything flatter than that, then be prepared to have a good set of sewer rods and getting you hands dirty every month!!!!

Czhornet

sydthebeat wrote: »

a septic tank works on anaerobic bacterial action where a crust is formed over the water to keep out the oxygen. therefore you want as shallow as possible outfall into it to negate against splashes and or risk of breaking the crust.

Use a Tee branch and a short piece of pipe inside the tank to get it below the crust level

dzer2

The thing about fall on the pipe is to wash the solids to the tank too much and the water flows away too little and no flow. Normally 100 mm in 6 myrtle pipe

Metric Tensor

Czhornet wrote: »

Use a Tee branch and a short piece of pipe inside the tank to get it below the crust level

This is essential. There should be one at the outlet from the second chamber too in order to protect the percolation area.

sydthebeat

Czhornet wrote: »

Use a Tee branch and a short piece of pipe inside the tank to get it below the crust level

100%

but dont be surprised how often this isnt included !