Question:
You seem to mention trench rock more than other products advertised for trenches. Is rock better to use?
Answer:
When I first started my educational programs at the University of Minnesota in 1972, rock was about the only material being used for distributing effluent by gravity in a drainfield trench. We discovered that the rock had to be at least 3/4-inch in size, but no larger than 2 1/2 inches. We also discovered the rock had to be clean and could not contain fine soil that would filter down and seal the bottom of a trench.
Onsite sewage treatment systems using rock-filled trenches were working well in Minnesota, and rock was generally used in our area. In an area in Texas where rock was not available, local onsite professionals developed a gravelless trench. This was a 10- or 12-inch corrugated drainage tubing with a geotextile fabric sleeve around the tubing.
Studied Gravelless Trench
We installed a gravelless trench system in Minnesota and studied its performance. We found it worked well to distribute septic tank effluent. A biomat developed on the geotextile fabric.
As a result of our research, we determined design values for sizing the absorption system. The performance was equivalent to a three-foot wide trench with six inches of rock under the distribution pipe. Part of our research result was learning that very fine sand had a slower acceptance rate than sandy loam soil. We incorporated that information into the Minnesota Code. The value is shown in the chart included here.
The chamber system had been used in other areas for many years. The first chambers were of masonry construction and very heavy. When plastic was developed with adequate strength, chambers began to be made of that product. In some areas, chamber systems are the most common type of effluent distribution system. Chambers are now being used to distribute effluent in drainfield trenches.
Today, other products on the market claim to be effective in distributing sewage tank effluent in the soil treatment system. I presume they work OK or they would not be on the market very long.
The reason I most often refer to trench rock is that I believe the values for sizing the soil absorption area have proven to be accurate. In the following table are the values I consider suitable for sizing the soil treatment area:
The advantage to using a trench no wider than three feet is that the bottom area can be reduced when there is more than six inches of rock under the distribution pipe. The reason for the reduction in bottom area is the increased sidewall area. The reductions are as follows:
• For 12 inches of rock below the distribution pipe, reduce bottom area by 20 percent
• For 18 inches of rock below the distribution pipe, reduce bottom area by 34 percent
• For 24 inches of rock below the distribution pipe, reduce bottom area by 40 percent
In addition, the sequential loading of a series of trenches using drop boxes allows the effluent level to reach the top of the rock layer. An additional six inches of sidewall on each side of the trench will then be exposed to effluent. The soil above the trench rock will get moisture by capillary attraction, which will be used by plants. (Erma Bombeck was wrong. The grass is always greener over the drainfield trench, not the septic tank.)
In my opinion, some basic research is still needed on the actual treatment capacity of various trench depths and trench widths using rock. However, lacking more accurate information, my opinion is that the above design values are conservative and will result in an adequately sized system.
Need More Research
Drainfield rock is heavy, and time and labor are required to install a system. There are other products on the market that advertise to distribute effluent in the soil. These are lighter than rock and require less labor. However, they are usually more expensive and many claim their soil absorption area can be smaller than for a rock system.
That claim may or may not be true. Again, I am not aware of accurate research showing the performance of other effluent distribution systems. The design information needed is: How many gallons of effluent will a foot of the system treat each day?
It seems to me that, in general, the onsite industry does consider the measure of sewage flows from individual homes to be necessary. However, the measurement of a home’s sewage flow can be made inexpensively with a water meter.
I have frequently suggested water meters be required in the Minnesota Onsite Code. One objection was that the inspector of the onsite system could not inspect the meter. My answer is to let the plumbing inspector inspect the water meter when the water system piping is inspected.
When the sewage flow is measured and recorded, a measurement of the amount of drainfield trench should be taken. Again, this is easily done if sequential distribution is used, and if there is an inspection well at the far end of each trench. The amount of sewage flow, together with the amount of trench being used for that flow, is essential information to determine the capacity of any type of effluent distribution system. Unfortun-ately, this type of important information is not being collected very often, if at all.
Therefore, my opinion is the sizing values for rock-filled trenches provide the best information the onsite industry has. This is not to say that there are not other suitable products available for effluent distribution. However, I am not aware of any information on their performance.
The Final Word
I will end with a comment by two onsite professionals who are also former colleagues, Jim Anderson and Dave Gustafson. The following comment appeared in an article in the December 2006 issue of Onsite Installer. What they wrote pretty well sums up my thoughts, too:
“Be very careful about downsizing systems with alternative media to bring costs in line with rock. At present, there is no definitive answer on soil-sizing factors in relation to rock. The best policy is to look at equivalent sizing and to make the decision based on all the factors. Also, know your state regulations on alternative products. Many states do not allow any reduction in drainfield size.”
