Septic system design - stream crossing

[CEmonkee]

Hi,

I'm doing some preliminary layout for a septic system that will be placed on an extremely tight site. The septic tank will be located on the north side of a stream, the drainfield on the south side, necessitating a stream crossing (under the stream bed).

When the transport line exits the tank it will be approximately 3' below grade. At the point where the pipe is at the stream bank, the pipe will need to be 11.5' below grade in order to be 3' under the stream bed. This gives a total drop from the tank to the stream bed of 8.5'. The length of pipe from the septic tank to the stream bed is 35', giving an approximate pipe slope of 24%.

The drainfield is upslope from the stream, so after the stream is crossed the pipe will slope upward to the drainfield (approximately 19%).

Needless to say, this will be a pressurized system, probably 1" PVC pipe (which will need to be encased in ductile iron or steel where it crosses the stream).

Has anyone ever had to design something like this? Are the slopes too extreme?

Any general advice on the overall design would be appreciated (things to look out for or consider, etc.).

Thanks!

 

[WL123]

So, I gather your going to use a pump most likely. Here in Massachusetts, the pipe would not be less than 2-inches in diameter if pumped and not less than 4-inches in diameter if gravity, and you could incase it in thicker bigger diameter PVC. The slopes are ok for a pumped system. You would have a septic tank flowing gravity into a pump chamber, pumping to a dosed field, possibly.

I don't know where your from, but you have to check the local or state regulations for what you can do.

 

[CEmonkee]

Hi WL123,

It will be a pressurized system, most likely with timed dosing. The flow will be 240 gpd.

I'll check if there is a minimum diameter for the transport line.

Thanks for your input, I appreciate it.

 

[chicopee]

How is the pressurized system operating if you have a power outage?

 

[CEmonkee]

Hi chicopee,

I'll size the pump chamber for extra storage to allow for power outages.

 

[diarmud]

Call me paranoid but adding extra storage to allow for power outages is very subjective.

You will have power outages.

Check with the local municipalities, county or state and find out their minimum requirements and or guidelines.

That way when the power fails, you will have some protection from liability. Once there is a significant problem, you wil have a cranky homeowner. . .

Good luck,
Dermott

 

[tkall]

CE:
The slopes are not too extreme. Size the forcemain based on a hydraulic analysis and select a pump based on the analysis. 1"...2" pipe means nothing unless you compute the system H-Q curve and determine an operating point for a pump. Is the residence on a well? If so power outages are not a problem. If not, a battery powered high level float will provide residents with the necessary warning to stop using water. Most people don't race to the faucets and tub to start using water as soon as the lights go out any way. It sounds like the line will not drain. Freezing may be a concern. I would try to locate the tank and drainfield so that the line will drain when the pump stops. If the end of the pipe is lower in elevation than the liquid level in the pump chamber and the pipe flows full you may get a siphon so you will need a siphon breaker unless you are ok with that. Also, I can see no good reason for timed dosing other than it seems like a fad right now. Demand dosing will give you and your customers fewer headaches and, if you size everything correctly, demand dosing is timed dosing...isn't it?
I hope this helps
Tom

 

[CEmonkee]

Hi Tom & Dermott - thanks for your replies.

The residence will be on a well.

I don't think siphoning will be a problem... the pipe will exit the pump chamber, then slope down to go under the stream bed, then slope upward to the drainfield.

I may be able to avoid timed dosing... not sure yet.

Thanks again!

 

[dicksewerrat]

If this is a remote site, look at a backup generator for Reefer, heat system and pump.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com