Inflow & Infiltration Numbers for Existing Sewer Drainage Area 1

[Persyn]

Hello -

I am working on a project in San Antonio, TX where we are trying to consolidate two existing (late 1960s installation) parallel sewer drainage lines (8" and 10") into one upsized line. I have sized the line based on a field survey of the drainage area and land use. Ideally, we would have flow meters installed, but this line was only recently added to the project and the deadline for submittal is coming like a freight train.

The drainage area is approximately 295 Acres and is about 2/3 commercial/light industrial/retail and about 1/3 residential. The existing sewer pipes are roughly 30-40 years old and 8" or 10" diameter for the entire drainage area, and the majority of them are concrete pipe.

With that background, my question is this: what Inflow/Infiltration numbers have some of you folks seen when doing projects in existing areas? The local water system uses a figure of 300 gallons/acre, but that is for new construction and does not seem accurate to me in this instance. It will not be possible to have the entire sewershed televised, or even portions of it, in time for the submittal. I am hoping there are some rough numbers that you folks may have experience with. Any comments are appreciated.

Mike

 

[LHA]

You stated late 60's installation. I pulled out my 1972 Metcalf & Eddy, blew off the dust and found that at that time I&I allowance was 3,500-5,000 gpd/mi for 8 inch and 4,500-6,000 gpd/mi for 12" pipe. I would use these (and cite M&E Wastewater Engineering, 1972 ed., page 43) for the old pipes.

If you are replacing them, cite the Ten States Standards, 1977 ed., Section 33.94). It is the guideling up north here (basically, the ten US states, plus Ontario which border the Great Lakes...I don't know if you follow TSS down there, but I'm sure you can use it as a reference). TSS allows 200 gpd/mi-inch of modern pipe. The amount should fall within 10% of Qavg or 5% Qpeak, both of which are used to quickly approximate I&I flows.