Sanitary flow rate and Fixture unit method

[gusongjun]

I am designing underground sanitary system in an industrial area. To calculate effluent rate, Fixture Unit method is used.

Effluent to MH1 = 5.8 l/s
MH2 =7.185 l/s
MH5=2.032 l/s
MH7=2.132 l/s

Question 1:

From Manhole 2 to Manhole 5, I add 5.8 l/s + 7.185 l/s = 12.995 l/s to select pipe and control the pipe slope. Is it correct to that total to design pipes from Manhole 2 to Manhole 5?

Question 2:

All discharged sanitary flows will be pumped. How to get the peak flow rate for the pump? I added all peak flow rates that calculated by Fixture Unit method. The total is 30.622 l/s.

The client estimated that there are only 500 people will there. All consumed water will be discharged to sanitary system. So population X water consumption per day X peak factor (5) = 3.1 l/s which is only 10% of the summary of peak flow rated by Fixture Unit method.

I am puzzled. Does anybody know how to decide the peak flow rate for the pump in this situation?

Many thanks for your kind concern.

 

[bimr]

The fixture method is primarily used by plumbing engineers for work inside buildings. The fixture method is not used by civil/environmental engineers for sanitary sewer design.

The minimum size of a collection sewer is 200 mm and that size will basically handle the flows that you are describing.

The collection sewer is sized to handle the peak flow rate. For example, if you have 20 people working in a factory 20 * 75 l/d = 1,500 l/day. Where 75 l/d is the assumed design flow for a factory worker. The average flow per minute is then 1.05 l/min. The peak factor is 4.0 so the design peak flow is 4.2 l/min. The sewer is sized for that peak flow.

Lift stations are sized to handle the design peak flow of the collection sewers that discharge into the lift station. You do not add an additional peak flow rate on to the lift station capacity.

 

[gusongjun]

To bimr:

Thanks for your reply.

Problem is I don't know how many people will work in each building. What I know is building layout. So I know the amount of closets, sinks, showers and so on. Then, I converted fixture units counted into water discharged (all consumped water will be discharged into sanitary system).

 

[bimr]

Here are some plumbing fixture standards from the plumbing code:

http://www.ilga.gov/commission/jcar/admincode/077/07700890sections.html

http://www.ilga.gov/commission/jcar/admincode/077/07700890ZZ9996amR.html

http://www.ilga.gov/commission/jcar/admincode/077/07700890ZZ9996anR.html

http://www.ilga.gov/commission/jcar/admincode/077/07700890ZZ9996aoR.html

http://www.ilga.gov/commission/jcar/admincode/077/07700890ZZ9996apR.html

http://www.ilga.gov/commission/jcar/admincode/077/07700890ZZ9996agR.html

 

[PEDARRIN2]

In general, you can assume 2 gpm for each water supply fixture unit.

It is assumed that whatever comes out the faucet or flush valve is generally what goes down the sanitary sewer.