Sewer Sizing 1

[khllmrk]

I would like a second opion on the size of a proposed sewer line. The sewer line in question is to be a 10 inch dia. on a 3/10 % slope. Two high rize condominiums will be utilizing this line for there sewer requirements. One will be a 150 unit tower and the other will be a 100 unit tower.
There will be approximately 15,000 sq. ft. retail, commercial space in the lower level of the towers.Thanks for your help.

 

[TVE]

In order to answer your question the following information is required:

Population per residential unit
Average daily flow per capita
Average daily flow rate for retail/commercial space

That said, using our local standards or 2.5 cap/unit, 341 l/cap/day (75 igal/cap/d) for residential and 0.975 L/ha/s (7500 igal/acre/d) for commercial the total daily:

625 people = 2.47 l/s
Commercial = 0.14 l/s
Total = 2.61 l/s
Peaking Factor = 3.92
Peak Flow = 10.23 l/s

For 10" pipe, need a slope of .03% but velocity would only be about 0.69fps. Normally, you need around 2 to 2.5 fps to ensure the pipe cleans itself and deposits don't build up.

So, is the pipe oversized based on required capacity? YES. Pipe will only flow at about 31% of its capacity.

Is the pipe properly sized based on minimum velocity requirements to keep the pipe clean? YES.

Dave

 

[johnd43]

I have a rule of thumb, that works in metric I'm afraid. But applying it to your case, I would say you have your gradient too low.

I never design gravity sewerage with a gradient of less than the reciprocal of the diameter. Thus, if I were designing your pipe, it would not be flatter than 1/250, or 4 per 1000, where you are proposing 3 per 1000.

After that, you need to make sure it achieves self cleansing flow conditions at least twice per day. That is usually taken as a flow velocity of 600 mm (2 ft) per second, but it needs to be real - i.e with sufficient depth and duration to pick up and transport the deposited material. This led to the adoption of flushing tanks and the like in years gone by but they seem to have dropped out of consideration in recent times in UK. Perhaps others have a view on that.

Hope that helps.

 

[TVE]

Reading johnd43's response, it looks like I wasn't as clear as I could have been in my concluding statements...

Required Pipe & Slope to handle flow = 10" @ 0.03%
Required Pipe & Slope to ensure self cleaning = 10" @ 0.30%

So, yes you were correct in thinking the pipe was oversized based on the required flow BUT it is properly sized (@ 0.30%) to ensure it cleans itself out. If you want to reduce the slope, you can always investigate whether they will accept a 6"mm pipe.

 

[darugar]

my experience tell me that velocity of less than .76m/sec does not self clean the pipe.

 

[CBRoberts]

Hi, Chris Roberts, Port Elizabeth South Africa,

I would say your sewer is oversized. I agree with the rule of thumb regarding slope, and try never to lay a 150mm dia pipe at a grade of flatter than 1 in 150.

The Peak wet weather flow from your situation will probably be in the order of 10 l/sec at the most. For this I would use a 150 (6 inch)pipe at a minimum grade of 1 in 150.

In South Africa, we do not use anything less than a 150 mm dia pipe for sewers, except for individual house connections when we use 100 mm dia.

Regards
Chris