School addition adding onto existing school line 2

[cenc]

If someone can help, it would be greatly appreciated. I've read some threads and looked online, but pretty confused right now.

I've got an existing middle school that has a 8" and 4" tieing into an existing manhole, with an 8" outfall to an existing pump station. The County has stated the lift station is rated at 88 gpm @ 146' TDH. The existing school has 645 students. Not sure what the DFU count is.

The addition has 134 DFU, which the MEP provided a 4" out of the addition. The new 4" from the addition ties into the existing 8", prior to the pump station.

To determine if an 8" line will be sufficient with the additional flow, should I calculate the DFU's for the existing school or use a PE of 0.25/person?

In addition, how do I determine if the additional flow is satisfactory for the existing pump station?

This project is under construction, so I've somewhat got my back against the wall to figure this out quickly.

Thank you.

 

[jgailla]

clemsonfan,
I would not use the DFUs for analysis. Your state should have a standard for the amount of sewage generated for a particular use. If there is a local land development manual, that may also have guidance.
A school would typically be 15-20 gpd per student plus 15 gpd per worker. If I couldn't get the information, I've assumed workers are 15% of the student total.
This will get you your average daily flow. You then apply a peaking factor to get your peak design flow in gpm.
Generally, 10 States has a peaking factor based on population. For schools, I generally use between 5 and 6, although some on this forum would say that is too high.
You will then need the wetwell dimensions and especially the pump curve.
If your station is capable of pumping 88 gpm, you should be OK for a school up to about 1000 students.

 

[RWF7437]

Go measure it ?

 

[bimr]

You can calculate it either way, using DFU's or gal per person. Plumbing engineers use the DFU system. Wastewater engineers use the gallons per capita system.

The state where your project is located should provide some guidelines. Here is an example:

http://www.ilga.gov/commission/jcar/admincode/035/03500370ZZ9996bR.html

Note that these figures are conservative since most plumbing codes now require low water use fixtures.

Many schools also no longer have showers for students because of concern for possible situations.

So you are looking at 15 gallons per person per day.

645 students X 15 gallons per day / 1440 minutes per day X 6 (Peaking Factor) = 40 gpm.

An 8" diameter sewer flowing half full at 1/8-inch/ft slope will discharge 240 gpm.

As jgailla posted, you should have not problems with the existing capacity of the 8" sewer.

 

[cenc]

Thank you for the responses. Looking forward to the day I'm able to offer some assistance to someone.

 

[nackra]

Don't forget that the PEAK flow to the Pump Station may require upgrade of pump(s). A LOT of people wait for half-time break to use facilities.