Question:
I have been installing some of the chamber systems, but the manufacturer says that they don’t need any distribution pipe. I’m wondering how the septic tank effluent can get down to the other end without a distribution pipe. In my rock systems I have to put in a distribution pipe. That’s what the code calls for.
Answer:
Your question is interesting to me since there continues to be a lot of discussion and misunderstanding about the distribution of sewage tank effluent in drainfield trenches and leach fields.
First of all, to briefly answer your question, the biomat forms on the bottom of the trench at the head end and gradually forms along the bottom as the effluent goes to the far end of the trench. When there is a biomat over the entire bottom of the trench, the effluent level in the trench begins to get deeper.
The only way to get sewage tank effluent to spread over the entire length of a trench immediately is to use pressure distribution. Gravity distribution will not accomplish this. When there is adequate elevation to allow the sewage tank effluent to flow into the soil treatment system by gravity, it is an unnecessary expense to install a pump system with controls.
The so-called distribution pipe that you mentioned is normally a four-inch plastic pipe with half-inch holes or perforations every five or six inches. There are usually two rows of perforations. The pipe is laid in a drainfield trench with one row of holes pointing down. The purpose is to “distribute” the effluent along the length of the pipe.
THE TEST
As an engineer familiar with hydraulics and fluid flow, I had some serious reservations about the distribution of effluent in such pipe. A colleague and I conducted some research on the four-inch plastic pipe described above. We supported a 50-foot length of the four-inch pipe in “saddles” so the slope could be changed for the experiment. We used an orifice system to accurately control the flow into the perforated plastic pipe.
At that time I was fortunate to have a septic tank system for our home. We measured the maximum outflow from the 1,200-gallon septic tank to be about two gallons per minute from the discharge of the automatic clothes washer. Normally the outflow from the septic tank would be far less than this, such as from a toilet flush.
Discharging two gallons per minute into the “distribution” pipe we found the majority of the flow went out the first two perforations and 100 percent of the flow always went out of the first three perforations. Changing the slope of the plastic pipe had no effect on the distribution. We have photos and data to verify these results.
I want to emphasize that with gravity flow, the pipe you have been installing in trenches will not distribute effluent down to the end of the trench.
Then how does the effluent get to the end of the trench? Going back to my initial comments, the effluent flows down through the rock layer and a biomat forms on the soil in the bottom of the trench. As the biomat thickens, effluent flows along the bottom of the trench toward the far end.
This fact has been known for many years, but does not seem to be accepted by all of the onsite sewage treatment industry. The illustration included here is from my files and shows “progressive crusting” under “trickle flow.” I believe it was originally published in the “Manual of Septic Tank Practice” by the U.S. Department of Health. It is now out of print. It shows what I have been describing about the gravity distribution of effluent in a trench system.
I believe use of the word “crusting” was unfortunate as this implies the bottom is sealing off. Such is not the case, as the biomat develops its Long Time Acceptance Rate for the soil in which the trench is located.
To answer your question, the manufacturer of those chamber systems is correct in stating another pipe is not needed for gravity distribution. The effluent will flow to the far end of the chambers as the biomat builds up along the bottom of the trench.
SIDEWALL DISTRIBUTION
While I’m on the subject I want to comment about the use of the trench sidewalls. There seems to be a misconception by many that once a trench is full of effluent it has “failed” and won’t take any more sewage.
A full trench will continue to treat effluent at its Long Term Acceptance Rate. The bottom of a trench does not have as much area as the trench sidewalls. The soil in the trench bottom serves to develop the biomat and spread the effluent along the length of the trench. But there is more soil area available to treat effluent along the side walls of the trench.
As effluent rises in the trench, more sidewall soil surface becomes available to treat the effluent. The soil near the top of the trench contains more oxygen. This reduces the thickness of the biomat and increases the flow of treated liquid out into the soil.
The beneficial effect of trench sidewall area was recognized many years ago by the Manual of Septic Tank Practice. The following quote was taken from the manual. The values have also been incorporated in the Onsite Sewage Treatment Code of the State of Minnesota.
“For trenches only, the bottom areas may be reduced if more than 6 inches of rock is placed below the distribution pipe: for 12 inches of rock below the distribution pipe the bottom area may be reduced by 20 percent; a 34 percent reduction for 18 inches; and a 40 percent reduction for 24 inches.”
Treat the soil gently as you install your onsite systems and they will do a good job for your customers.
