Size of drain line to septic tank

[Keoke]

Hi folks, I am trying to decide what size ABS drain line to a new septic tank. The tank will serve 23 fixture units and there will be about 100 feet between tank and house pitched at 1/4" per foot. Read some stuff about sizing this pipe that said I would be better off with the 3 inch pipe. The reason given for the 3' pipe was that a pipe should be half full of water during a toilet flush to move the solids along in suspension with water. Article says this can be done better with a 3 inch pipe then with a 4 inch pipe. Article also said that a 4 inch pipe was more likely to plug up due to the fact the solids are more likely to fall out water stream and dry cake in the pipe. In short the 4 inch pipe can not meet the half full rule on a toilet flush. This is especially true on a low flush toilet.
I very much like to get the boards opinion (3 or 4 inch pipe) on the above comments.

thanks again folks,
george

 

[inveresk]

George, I'd be very worried about the waste from 23 fixtures going down to a septic system via a 3" pipe. I think you're asking for blockages. Anything in the ground should be 4" minimum. 3" is ok for above ground if you have access handholes for rodding but not underground. 4" drains should always be used for wc's, nothing smaller, even if you only have one wc. The exception is when you have a wc fitted with a macerator. Then you can get by with a smaller bore pipe.

If you have 23 appliances, what type are they and are you counting dishwasher, washing machine, kitchen sink, laundry room sink as separate appliances? If you are, 4" would be fine but if you have five or six bathrooms you could consider 6". At least this would allow for future expansion of the house, or for an outhouse with wet accommodation to discharge into the same drain.

On gradient, you choose this once you size your pipes. Rule of thumb is 1 40 fall for 4", 1 : 60 for 6".1/4" per foot works out at 1 : 48 which should be ok. Too shallow and you risk blockages. Too steep and the water runs away too quickly leaving solids behind.

You don't need to use abs (which is normally only for high temperature discharges) for underground. By the time the water has hit the u/g drain, it will be cool enough for upvc. Underground, I never use anything else.

As a point of information, half of the Greek islands have small bore drains and I can tell you that in some of the islands you can't even put toilet paper down the pan because it causes blockages!!!

 

[MarlandS]

First thing would be what your state or local authority require, some areas are strict on this.

Second, wouldn't you have the shower/tub or washing machine also using the same line? That would eliminate dry caking, wouldn't it?

I help my neighbor ( a licensed septic installer) occasionally and I believe all he uses is 4 inch.

 

[DKinWA]

One time an inspector said he likes to see 4" ABS from the house to the tank, so I run 4" to keep the inspector happy. A happy inspector makes my job easier /forums/images/graemlins/grin.gif

 

[_RaT_]

Your points are valid. Even running all fixtures, the 3" will provide adequate drainage to the tank. That of course will never happen. My understanding is that a 3" line demands 1/4" per foot fall. A 4" can have a little less. Don't quote me on it. I have found our 3" main line to our septic tank to work flawlessly. You don't want more the 1/4" per foot fall or the liquids will leave the solids behind. If you have to make a radical drop, it is only done once almost to the tank or main sewer line. I've forgotten where in Northern California your located, but I have found our local building agency to be superb answering these questions. I'm in Placer county

 

[EddieWalker]

Drainage is one of those things you can way overthink. I have an engineer charging me $150 an hour to tell me what I already know, but need his stamp on some paper to show it's been engineered. He's wasting time and my money figureing out the amount of flow versus distance and the resistance of the inside walls of various sized pipes as they relate to the angle of the slope. It's enough to drive me crazy since we're going to install it the way we want anyway, but his official stamp still has to be there.

Anyway, for what your doing, three inch will handle a house easy. You don't have to worry about size of pipe for the solids as much as the slope. Too steep and the liquids will outrun the solids. Too shallow and nothing is moving. Rule of thumb is to keep it between 3 and 7 percent. I like to work with 5 percent since the math is easier for me and my margin of error goes both ways.

I've never heard of a drain line being too large, but you hear plenty of stories of one being too small.

I run three inch lines for my toilets, then up them to 4 inches when everything comes together. The price from three inch to 4 inch over such a short run is insignificant, but if you have to clear an obstruction, the four inch line is better.

Be sure to include as many clean-outs as you can. I put them at the beginning of every line, either at the foundation or in the side of the wall. Relying of the vents just makes things more difficult when you can put them in during construction for very little.

I also put a cleanout on the line from the house to the tank and/or where there's a turn. I have a long sweeping 90 on my drain as it leaves my house on it's way to the tank. I put a "T" there with a clean-out.

Eddie

 

[Highbeam]

"half full rule" Never heard of that rule.

I have heard of rules like a minimum of 2 fps velocity to scour the solids out.

In my area, we require 4" or 6" at slopes of 2% or 1% respectively. I would rather a 4" line for a single home as a 6" can be a bit large. Never would consider 3" for any mainline work, maybe for a sink or something upstream. Heck, a 2" line could probably handle most things but we all know better.

 

[450EXC]

I own a small drain cleaning business, we do mostly large commercial work now but in the old days I did lots of residential.

I would recommend 4" but I think even more important is how well the pipe is installed.

You can have the best pipe in the world and a poor installation will plug it up constantly. Bed the pipe good and make sure there will be no settleling to create low spots or dips. These will hold water and start buildups. No rocks around the pipe, etc. Use thick pipe-schedule 35 or 40. No stone inside the pipe during installation, or tools.

Plastic pipe dosnt really plug much if installed properly.

Hope this helps, Larry

 

[JJT]

Go with 4" pipe. If you have issues, you can always change out the flapper in your low flow to increase the volume of water. It's amazing what a little extra water can do....

 

[Keoke]

JJT, what is the scoop on changing the flapper valve in a low flow toilet to increase flush water volume?
This looks like a good idea. Fill in the details, please.
By the way guys, your advice is clear, I'm going with 4 inch schd 40 on a 120 foot run, 1/4 per ft pitch, to septic tank.

thanks for the help,