Maintenance Problems - Flat Sanitary 1

[dirtwurxII]

I am doing a road design serving about 65 acers zoned office/lab. The property falls away from the road it fronts on. I designed the sanitary as a 12" to take advantage of a flatter minimum slope (0.22% rather than 0.44% for an 8")to extend sanitary as far as possible into the acerage with gravity . This concept was checked with the reveiwing agency at the beginning, everybody said the idea was acceptable. After full design and a very lengthy reveiw process we were notified of approval and were asked to provide several sets of drawings for them to stamp approved. Apparaently someone else looked them over and has a problem with the 12" at 0.22%. They are concerned with maintenance problems. This property could potentially have employment for 250 to 300 people. The sanitary I am tying into is a 12" @ 0.22% and we are at the upstream end so I am not clear why this is an issue. Any idea how to address their concerns and allow this to proceed?

 

[civilperson]

Must maintain 3 feet per second velocity for suspension of solids and self scour. If the flow is less than designed, then settlement and maintenance headaches occur. A pair of pipes with lesser diameters used instead of the single pipe can keep the flow at greater than 50% in one pipe during low flows and have the capability for the greatest design flows. Check dams in the Manholes or two elevation of flowlines can produce this regime. (Draw back is a greater minimum slope for the smaller pipe.)

 

[bimr]

Review the standards of the reviewing agency. The slope that you have proposed will give you the 2 feet per second velocity that meets the requirements that most reviewing agency have in their standards.

If you meet the standards, point them out to the reviewing agency. The reviewing agency will not argue against their own standards.

 

[dirtwurxII]

Velocity is dependant on volume. Since volume is unknown velocity cannot be determined.

 

[LHA]

You can approximat the volume, which is infinitesimal. Although 12" at .0022 will give over 2 fps at full flow, you will never expect even a small fraction of that. Therefore, you will have to flush the pipes periodically, or something.

Per:

http://www.pacode.com/secure/data/025/chapter73/s73.17.html

Offices (per employee)10gpcd*300cap/day=3000gpd*4PF=0.0186cfs
Qfof 12"" @ .0022=1.67cfs
Q/Qf=2.13
d/D (approx):0.05
Vf of 12"" @ .0022=2.13fps
v 0.1065fps


Engineering is the practice of the art of science - Steve

 

[Maury]

The standard design for sanitary sewer mains is for minimum velocity of 2.0 fps, flowing full. (This will also provide a velocity of 2.0 fps when half full.) A minimum velocity of 3 fps is reequired for storm sewers. The fact that the expected flow will not provide a half-full volume is not relevant, if it were, most upstream sections of sewer couldn't be built, since they rarely provide a discharge of even half the pipe capacity. The peak to average ratio for one building will probably be higher than the 4:1 ratio commonly used for sewer mains.

As bimr said, since your design meets the minimum slope requirement, they have no justification for not approving the plan.

 

[dirtwurxII]

Thanks for all your input. I am trying to secure a copy of their designb standards.

 

[rconner]

I do not know what standards are applicable in your area; however, at one time I thought at least "10 State Standards" applicable to wastewater had an additional specific requirement reportedly to "Minimum Solids Deposition", that could be an arguable limitation to simply installing bigger pipes at the flatter slope you desire.
The passage undeer that heading at least once read, "The pipe diameter and slope shall be selected to minimize settling problems. Oversize sewers will not be approved to justify using flatter slopes. If the proposed slope is less than the smallest pipe which can accomodate the design peak hourly flow, the actual depths and velocities at minimum, average, and design maximum day and peak hourly flow for each design section of the sewer shall be calculated by the design engineer and be included with the plans." [Notice the second sentence in the quotation that is literally quite restrictive -- in the context of this standard, I think when they say "oversize" they are talking about going larger than the "Minimum Size" of mainline sewer that was "8 inch" stated prior to the quoted passage in that same standard.]

 

[Maury]

I located a current copy of 10-States Standards and rconner is absoluely correct. Article 33.43 of the 2004 edition is just as he described.

 

[dirtwurxII]

All true. However they were OK with the idea at the start and didn't raise an issue until full design was completed. I am in reviewer hell here with Army Corps, EPA, The City, and The County. I am in for about 18 months since inception for 1800 feet of road and I really don't want to start over. I just want some way to justify the existing design and get this done. I think my best bet is to point out the the maintenance on the sewer if preferable to the maintenance on a pump station.