Sanitary sewer design

[ngneer]

I'm currently working on a design for a sanitary sewer. The neighborhood currently utilizes septic tanks, drainage fields. I'm concerned with the lack of flow in the sewers since very few homes will be connected. I realize I need to increase slope but then my manholes/pipe will become excessively deep. For instance, I will have a 200' run of 8 inch pvc with only one home connection. Also, I noticed the standard tip of designing for minimum velocity of 2 ft/sec @ half-full. The pipe will never come close to half-full. I would appreciate any advice.

 

[dicksewerrat]

Why are you designing this? some systems are dying? The municipality may have to pass an ordinance requiring all to hook up. This would be a costlymaintenance headache for them if they only have one customer.

 

[ngneer]

I work in the public works department. It's politically driven as with most projects: residents (voters) are tired of pumping tanks and limiting their water consumption. It seems as though there is no way to avoid the need to periodically flush the proposed lines.

 

[BobPE]

ngneer:

You have the right answer, flushing the lines. Now the question is what is the easiest and cheapest way to do it. I think if it were my design, I would design flushing manholes at stratigic locations to acconplish main cleaning. The designs I have done in the past all worked well and as flows pick up via connections, they are easily decommissioned.

Good luck in that political world you are in there!!!

BobPE

 

[roakesh]

Well, you need a minimum of 2 ft/sec velocity for gravity pipe system to transport organic material and some municipality requires a minimum 8" dia pipe. Periodical flushing of manhole is an option but sometimes maintanence cost of periodical flushing over a 50 to 70 years of time is not worth comparing the cost of additional depth of cover it needs, unless there is a significant amount of differnce in depth to be incur to accomodate design standards.

 

[dicksewerrat]

When I worked for a living, in the Public sector, we had a benchmark of cleaning every sanitary main every three years. No sewer main is totally self cleaning no matter what the engineer says. With CMOM and GASB34 you ahve to do these things.
This also makes you look at each line more often and finding other types of problems before they become crises.

 

[cvg]

why not look at more modern technology? Instead of gravity system, try low pressure system. Cost of intallation may be considerably lower and much of the cost can be deferred until customers eventually connect to the mainline. Flushing of gravity lines is eliminated. Manholes are eliminated. Infiltration is eliminated. Look at the following link or many other references online. try a google search using the following key words:

small diameter pressure sewer mains

http://www.epa.gov/ORD/NRMRL/Pubs/1991/625191024front.pdf.pdf

 

[EricC]

Along the same lines as the previous post, there are some areas where a vacuum system is used as well. As the previous post, these are normally used in areas with undulating topography that precludes gravity sewers or in areas where there are fewer customers. Something to consider...I have no experiences good or bad, just wanted to throw out another idea.

 

[KRSServices]

Regarding the vacuum sewer system, I have had the pleasure of managing such a system, and I recommend staying as far from that idea as possible, particularly if there is winter and subsequent freezing.

It sounds as though you are trying to design the serives for acreages or similar. One consideration in your design is the connection (and cost) of the private system to the public line. I might suggest that a tank and pump out system might be better served, but I guarantee that the municipality will be in for some costly maintenance headaches if it intends on owning the systems on private lands. If gravity flow, the lines have to be deep enough to protect from freezing (if applicable) or at least enough to provide 1% or 2% grade for drainage. If the residence is set back far enough or it is an older home, additional costs to modify and connect to the system will be becessary.

If the manholes are going to be excessively deep, will the proposed system be able to tie into the existing system without need of a lift station?

As an aside, regarding the bylaws, or statutes, how is the cost of this collection system being distributed? Are the majority of owners in favour or a handfull? Is the municipality borrowing to put the system in, and if so, is the municipality intending on recovery via a frontage bylaw or specified area? I ask because in some cases, a project such as this can easily be scuttled if there are sufficient objections raised by the ratepayers.

KRS Services

http://www.krs-services.com

 

[ngneer]

Thanks to all for your input. Of the 65 homes with existing septic tanks, I'd say approx. 50 want to connect. Most of those desiring connection have existing problems, especially those homes with children wanting to take long showers. Also, the homes with tanks in the front yard will have an easier time. The difficult evaluation will be to limit a manhole depth, thus requiring some homeowners to pump to the proposed lines.

As far as funding, the homeowner will be responsible for tie-in to the main, and I believe we have received a grant from the state to install the mains. Being an staff engineer in the public works department, I'm not totally exposed to the financial workings of the municipality.

 

[dicksewerrat]

The property owners may also be eligible for grants or low cost loans to connect. The City should pass an order to have all connect at the time of construction. The City could bid out the connections and get a good price for the whole ballgame.