Minimum slope for effluent pipe

[Surcharged]

I am designing a STEG (Septic Tank Effluent Gravity) collection system for a 10 house development. The house will discharge to a septic tank, solids will then settle out. Effluent will then gravity flow to a pump station. A couple of questions: First, how should I size the pipe? Second what is the minimum slope of the pipe. Should be different than traditional sewer since solids are not present. Thanks.

 

[PEDARRIN2]

Size the piping based on the expected flow so the pipe is only 1/2 to 2/3 full (to allow for gravity flow).

The minimum slope is of course no slope. Slope on the pipe, again, will be based on the flow you want to achieve. There are charts that indicate different size pipes at different slopes at different partial flow ratios - all of which give a flow rate.

Even though there will not be solids, you are still dealing with sewage and will have non solid buildup on the piping over time (think what your lavatory drain looks like after awhile). Therefore, it is probably still best to size, determine the slope for a velocity of at least 2 ft/sec. This is the "scouring velocity" that most drainage pipes should have. This will keep the buildup to a minimum.

 

[semo]

Part of the idea of STEG systems is that the pipe size can be reduced and slopes can vary (even become negative) in areas as there are no solids present.

You need to make sure that your effluent line does not hold water at an elevation that would backup into your homes. Otherwise, there is no minimum slope.

Another thought is to put effluent filters in your septic tank to keep the solids from leaving them if the O&M is poor. Someone needs to check the tanks annually for sludge buildup and then have them pumped out or you will get solids negating the purpose of STEG.

You can look at some of the EPA handbooks for some sizing information. Typically it is based upon peak flow and pipe half full. EPA has a peak flow equation of 0.5n+20 gpm where n is the number of upstream connections.

With this and the slope you can use mannings eq. to size the pipe if it all has a positive grade.

 

[bimr]

I assume that you are talking about the gravity pipe. There is no need to size such a pipe, since it would most likely be considered to be a service sewer. Check your local plumbing code.

The sewers between the house and public collection system are called “service sewers”. As such, service sewers have a minimum slope of 1% and a minimum size of 4” diameter. Some towns are now requiring the service sewers to be 6" diameter for cleaning purposes. 4" diameter would probably be adequate for your application.

Here is an example of a plumbing code:


i) Service Connections

Sewer service connections shall meet the same criteria as public sanitary sewers described elsewhere in this Subpart C except as noted in this subsection (i). Roof and foundation drain connections to the sewer service connection are prohibited except as provided for in 35 Ill. Adm. Code 306.302. The service connection tap into the public sewer shall be watertight and shall not protrude into the public sewer. If a saddle type connection is used, it shall be a commercially available device designed to join with the types of pipe that are to be connected. All materials used to make service connections shall be compatible with one another and with the pipe materials to be joined, and shall be corrosion-proof.

1) Size

Service sewers and fittings shall be a minimum of 4 inches in diameter, but shall not be less than the diameter of the plumbing pipe from the building.

2) Slope

Service sewers shall have a minimum slope of 1%.

3) Alignment

When straight line alignment is not maintained on service connections, cleanouts or manholes shall be provided at points of changes in alignment.

http://www.ilga.gov/commission/jcar/admincode/035/035003700C03200R.html

 

[BADMANWINS]

Am I the only one that has a major problem with STEG / STEP? You are purposely created anaerobic wastewater with the associated odor/corrosion and treatment issues????

 

[jartgo]

BAD, can you provide more information as to what your concerns are with these type of systems? Thanks.

 

[dicksewerrat]

That system is a nightmare waiting to happen. Who pumps the solids? Who pays? When is it pumped? How are the costs shared? Why not ask for a city main to the site of this Holding tank and each house hooks up with a gravity line? Or build the line and turn it over to the sewer authority?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Surcharged]

Dick Sewer Rat

The site is 15 miles from nearest city sewer manhole. STEP systems are used extensively around Mobile Alabama. We went on a tour there last year (sponsored by Orenco for full disclosure). If a private utility owns the system then they will pump the tanks, replace pumps etc. STEG systems like the one we are proposing should be even less maintenance.

 

[Watthour]

it seems that Richard the Rat has seen the kind of thing many of us have. The constant maintenance, repair, pretreatment, and hauling fees will nickel and dime the developers/owners to death. The potential problems are significant.

Ever consider a portable/packaged treatment plant for the development? They can be had for under $300K and meet even the strictest effluent standards. The latest designs are serving as few as 15 homes or a single convenience store, and discharging directly to streams or in drip irrigation systems with no holding for freezing weather. Several stricter states, such as PA, NJ, and MA have all accepted NPDES applications for these plants for surface or stream discharge. I'm guessing AL is similar.