basic question re. sewer pipe sizing

[entp]

Hey everyone,

I'm trying to determine the rule of thumb for sewer pipe sizing. I know that for water pipes, sizing is usually based on Max Day Demand (gal/day converted to gpm), which is double Average Day Demand (the average day over the year).

Are sewer pipes also sized for Max Day flow, or in other words, a peaking factor of 2 over the average day flow? Or are they sized based on Peak Hour flow, which is double Max Day flow, => max day x 2, => average day x 4?

I hope my question makes sense to someone reading it.

Thanks,
entp

 

[civilperson]

Use eight inch minimum for mains and check with the local agency for lateral sizes,(6" or 4" are normal). The maximum predicted flow also comes from the regulating agency. Usually all the watershed area upstream of your connecting point must be included in your sizing. A specified housing density times the undeveloped area must be added to your specific development flow. Keep minimum velocities at 3 fps and maximums at 10 fps. Parallel the average slope of the grade of the surface along your sewer alignment. ANSWER: THERE IS NO RULE OF THUMB, just experience giving good estimates.

 

[jartgo]

In my experience the sewer pipes (collection system) is sized based on the local (usually state) regulations. They will tell you what to allow for flow from various typical users. That would be your "average daily flow" then you need to convert that to an hourly flow allowing for the "time of use for that facility" On top of that, you need to add your peak factor ranging anywhere from 2.5 up to 4.0, usually based on population.

In summary...check with your local regulations.

 

[rconner]

There are many references for sizing of sewer pipelines. I think you received good advice e.g. to check with local authorities on this subject. When you are looking to see what all can go into engineering examinations of this subject, I think pretty good references have traditionally been the “Ten States” Recommended Standards for Wastewater Facilities (I think previously mentioned and available from online at relatively low price) and also ASCE’s “Gravity Sanitary Sewer Design and Construction” (ASCE MOP No. 60/WPCF MOP No. FD-5). From these references, I think the intended function of a gravity sewer is sort of summed up with a statement, “It is essential, therefore, that sanitary sewers have adequate capacity for the peak flow and that they function at minimum flows without operational; problems (16)”.
The devil is of course in the details. In this regard, there may be a sort of rule of thumb I’ve seen presented in the USA that might be helpful in some cases i.e. that roughly 70%-80% of a community’s domestic and industrial water supply returns as wastewater; however, it is obvious that much more water than this has gotten into many working sewer systems in the past (as a result of original “combined” storm and sanitary designs, all manner of infiltrations for various reasons and from whatever source/path including poor quality manhole and pipes/joints etc. into various parts of the systems, and even illegal connections etc.) that in general along with rainfall/flood events and other factors are probably some reasons for higher apparent per capita usage and/or high multiplicative “peaking factors” being used in some areas. The ASCE MOP previously mentioned also devotes quite a bit of copy to designing for “future” (e.g. 50 year or more) population and industry flow trends. I suspect, as also may be the case to some extent with hydrology, to do this right requires quite prescient judgment (or an accurate crystal ball!) if there are not to be future overflows/floods (in the case e.g. of near exponential development in many areas)!

 

[PELS]

There is no rule of thumb per se`. The common response above is to check with the design standards of the jurisdictional agency. The design standards normally spell out what typical (average) flows to expect for different types of land use, plus peaking factors and I/I. (Residential flows are different from industrial or commercial.)

The design process we go through in this region:
1. Determine the type of contributing flows (residential commercial, industrial, recreational, etc.) Sometimes there is a combination of any or all of the above.

2. Determine the average daily flow from these uses(should be given in design standards of the agency).

3. Apply a Peaking Factor (Qave x factor = peak flow) add this to Qave.

4. Determine I/I (Infitration & Inflow). Agency should have a factor for this too.

5. The sum of 2-4 above = Q design

6. Based on the design Q, use Manning's formula to determine pipe size and slope.

Generally speaking, pipes with a minimum velocity of 2 feet per second carry these flows:

6" pipe @ 0.0050 = 0.26 MGD
8" pipe @ 0.0033 = 0.45 MGD
10" pipe @ 0.0025 = 1.2 MGD

 

[bimr]

Sewers are sized for peak hour flow. The ratio of peak hourly flow to daily average flow is given in empirical charts as per the following:

http://www.ilga.gov/commission/jcar/admincode/035/03500370ZZ9996dR.html

 

[onyeuko]

If you can send me your e-mail address, I will send you a report of a project that I designed. The problem is that the report is in Italian language to which I do not have time to translate for you right now. However, you can use any soft ware for language translation like that from google to translate.
Teddy

 

[onyeuko]

The chart on the site

http://www.ilga.gov/commission/jcar/admincode/035/03500370ZZ9996dR.html

(for determination of the ratio of peak hourly flow to daily average flow)is not legible. Can some-one post a copy that is legible or give a link to a site where it could be downloaded in clearity.
Teddy

 

[bimr]

At population of 1,000, the ratio is 3.8

At population of 10,000, the ratio is 3.0

At population of 100,000, the ratio is 2.0

The chart is published in Water Supply and Wastewater Disposal by Fair and Geyer.

 

[onyeuko]

Please give me some thing with which I can carry out any design as such, that will not limit me to the figures listed above.
Teddy

 

[entp]

Hey everyone,

Thank you all for the good information.

I do realize we have to check with the local agency before the design gets too detailed, was just looking for some planning ballpark estimates. Thanks also for the population based peak factors, and the references, those are very useful.

I have another question about wet well sizing, so I'll start another thread for that.

Thanks,
entp