Poking Perforations in a Theory

More than 5 feet of perforated pipe at the head of a drainfield is a waste of money

Q: The method I and many other installers use to install leach lines is to suspend perforated pipe prior to installing rock, which requires walking in the trench, both to shoot the grade to maintain level in the bottom and installing the pipe level on supports. What do you think of this practice?

A: I’ve devoted several columns to preaching about the importance of avoiding foot traffic in trenches during drainfield construction. So I’m not going to get in my pumper’s pulpit about that topic. But your mention of using perforated pipe points out that a little story of soil absorption system history may be in order.

This is how I think drainfield trenches and beds developed:

Many years ago a drainage expert likely figured out if 1-foot lengths of clay or concrete tile could drain water out of wet soil, they could also let septic tank effluent drain into dry soil. So drain tile was laid in the soil with 1/4-inch spacing. Some of the septic tank effluent did drain into the soil. But it didn’t take long for the biomat to form in the soil around the 1/4-inch crack and slow down the flow to practically nothing since there was so little soil area.

So the next step was to put pea gravel under and around the 1-foot lengths of tile to spread the effluent out over the soil. The pea gravel also clogged up because it was so fine, so larger rock with a 1/2-inch minimum size was used.

TILE BECOMES OBSOLETE

This worked well and the technique was used for many years. The designers and installers didn’t recognize the effluent was draining out of the first crack or two of the tile and down into the rock where the biomat formed in the soil under the rock layer.

Then plastic pipe came into the picture. Plastic pipe began to be used for agricultural drainage and the clay and concrete tile makers were pretty much out of business. The 4-inch clay or concrete tile became 4-inch perforated pipe. For sewage systems the perforated plastic pipe was placed on the rock layer instead of clay or concrete tile.

But the operation of the trench was the same. The effluent flowed out of the first two or three perforations and down through the rock layer to the soil below. The biomat formed on the soil layer and the effluent gradually moved along the length of the trench. Beyond the first few perforations, the so-called distribution pipe remained dry. And it stayed dry until the effluent level rose up into the pipe from the bottom of the trench.

So why install perforated pipe that really isn’t doing anything useful? I needed some good information to prove this, so I set up an experiment. I obtained some 4-inch perforated pipe from a septic system installer and set 50 feet of pipe on supports so I could adjust the slope.

I used an orifice system to obtain an accurate and constant flow into the perforated pipe. I experimented with the pipe at various slopes and found the water never got past about three perforations, even at maximum flows out of a septic tank.

The perforations were 5 inches apart. This gave a total distribution distance of 10 inches (two 5-inch spaces between the perforations). I collected data, took pictures and presented a technical paper on my research to the Society of Agricultural Engineers.

TRY IT YOURSELF

If you don’t believe my research, set a 10-foot-length of perforated pipe, with the perforations pointing down, on a couple of saw horses. Place an elbow on one end of the pipe so you can pour water into the pipe. Now pour water into the elbow at a constant rate no faster than 2 gallons per minute. This is about the maximum flow out of a septic tank when an automatic washer discharges. The flow out of the septic tank is a lot slower for a toilet flush. You can try slower rates, too, but the flow must be steady.

In my opinion, a length of 10 inches is not very good distribution of gravity flow of effluent in a trench. I believe it is a waste of fossil fuel and money to lay 100 feet of 4-inch plastic perforated pipe in a trench. It is even more critical now as we face an energy crisis. In my opinion, only 5 feet of perforated pipe at the head end of the trench is needed in a gravity flow system.

Check out the drawing I’ve included below. It shows 5 feet of 4-inch perforated pipe with the perforations pointing down. The pipe is open on the end in case solids coming through from a full septic tank or a dropped baffle plugs the perforations.

In case the 5 feet of pipe gets plugged with trash, it can be accessed by the sanitary tee without any digging. One should dig to open the drop box so the 5 feet of pipe and the drop box can be cleaned out.

I do think an inspection pipe at the far end of every trench is a good idea and should be required.

I presented the idea of only 5 feet of perforated pipe at the Onsite Sewage Treatment workshops I taught for years in Minnesota. A number of times I had installers tell me they had used only a short section of pipe at the head end of the trench or bed. They said the systems worked fine.

IS IT A CODE ISSUE?

In regard to your comment, I would like to ask you a question, “Why are you setting pipe supports and why are you placing perforated pipe in what you call leach lines?” The answer likely is, “Because it’s in the code.”

However, the perforated pipe is essentially useless for distributing septic tank effluent and the grade it is set on doesn’t make any difference. I would hope more government officials, at both state and local levels, would understand how useless and wasteful it is to install plastic perforated pipe the entire length of a drainfield trench.



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