Long Sewer Pipes

[olmedo]

Our wastewater treatment is located more several meters away from the factory. If we maintain the usual slope of the sewer to comply with the flow velocity requirements, the sewer pipe would be around six meters deep approaching the treatment plant. How can we avoid this without using a lifting station?

 

[bimr]

What slope are you using? The minimum practicable slope for construction is about 0.0008 m/m.

That calculates out to 6,250 meters distance which is considerably farther than several meters.

For that distance, the cost of a lift station and force main may be attractive. You have to pump the wastewater at least once, either at the treatment plant or the lift station.

 

[Ussuri]

As its a gravity line the minimum gradient which achieves self cleansing velocity is dependent on the flow.

If your flow is very low you may need a pipe at a gradient of 1:50 or 1:75 to keep flow velocity up. I generally aimed for gradients of 1:100 or 1:150. You can go to around 1:300 or 1:350 as a last resort but I would expect solids to settle out.

Ultimately you can lay your sewer at any gradient you like but if its too shallow it will require frequent maintenance to prevent blockage.

You dont say exactly how far 'several meters' is or what your flow is, but I would expect a 6m drop at invert over around 1km say. How deep is the head of the pipe, because if several metres is only 100m say, then I would say your pipe is too deep to start with.

 

[bimr]

If you are using 110 mm sewer pipe, a minimum slope of 1.4% will provide you with a velocity of 0.75 m/s which should be adequate.

That minimum slope of 1.4% equates to 357 meters sewer length.

 

[DoctorSludge]

The definitive volume on the subject is the companion to Metcalf and Eddy 'wastewater engineering'. Look up Metcalf and Eddy and sewers on google and it should pop up. OK for the theory but here is the UK's engineering short cut to gravity sewer design.
for 10 inch line use fall of 1/100
for 8 inch line use fall of 1/80
for 6 inch line use fall of 1/60
for 4 inch line use fall of 1/40

It works in practice. Try to avoid the 4 inch line - I use 6 inch minimum for all sludge and sewage lines as they tend to self clean rather better than a 4 inch gravity line. Also do not forget the ventilation of the line to prevent 's' bend odour traps being dried out by suction in the line.

So to summarise the answer - use the largest diameter sewer necessary for the flow and the slope and vent it.

Hope this helps.

 

[stanier]

It is not unusual for the incoming carrier to need a lift station into the plant. Sydney's North Head actually comes in 60m below the sedimentation tanks.

The only way to escape this is to have low pressure sewerage schemes. Gravity is a wondeful thing but it only works downhill.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au