I saw a question in an online group for onsite installers: “Bed or trench — is one better?”
The responses to the query all seemed to be: Bed if you have the room; I’ve always installed beds; I would do a bed; etc.
I realize it’s the internet and anyone can answer anything they want. But the responses showed me there are disconnects to important information that answers that question in detail, and should be part of any installer’s basic training.
Yes, one is better and the answer has been around for decades.
The answer, as far as I can find, came from E. Jerry Tyler and Jennifer Erickson. Tyler was professor of soil science and director of the Small Scale Waste Management Project at University of Wisconsin - Madison, and Erickson was a research assistant at the SSWMP.
Erickson and Tyler have at least two articles available at the SSWMP website that discusses this in much detail.¹ These free publications are available online at soilsfacstaff.cals.wisc.edu/sswmp/online_publications.htm.
(Please note that any mention of Tyler and Erickson is referring to these two publications.)
Their findings indicate that a system that maintains an aerobic environment will last longer and increase its loading rate more than one that doesn't get the same amount of oxygen.
Because oxygen enters the distribution cell from the sidewalls, the longer and narrower the better for the system. The more sidewall available, the more potential for oxygen. They state that systems “must be long and narrow, and close to the oxygen source," i.e. shallow.
In the early 1990s when I designed systems that were 10 feet wide, they would get redlined by a state plan reviewer that said (paraphrasing here) "systems wider than 8 feet have been shown to prematurely fail” starting in the middle. Oxygen enters shallow systems through the sidewalls. That oxygen has to make it to the middle of the system from the sidewalls — the wider the system the less oxygen will make it to the middle. Trenches have way more sidewall for more oxygen transport. And the narrower design makes it more likely the oxygen will get to the middle from each side.
In Wisconsin our in-ground distribution cells have a maximum width of 6 feet. In most systems there will be more than one cell and a minimum of 3 feet between each cell is required. This maximizes the amount of sidewall in our systems. Tyler and Erickson mention having “separated infiltration areas to maximize oxygen supply.” This would refer to multiple trenches.
Tyler and Erickson continue: “If the system design can be manipulated to supply oxygen into the soil exceeding the rate needed, the life and loading rate of the onsite wastewater system can be greatly increased.” If the life and amount of effluent the system can handle could both be “greatly” increased, trenches seem like the best option of the two for sure.
When I saw the question about bed vs. trench, most responses voted for beds, and nobody mentioned the information discussed in this article — and the answer should be a slam dunk. This is information we’ve had since at least the 1990s.
Our industry needs to do a better job of education and training. Basic concepts such as this should not be lost over time. It should be remembered, taught and improved upon.
Another topic I noticed in the online installer forum that appears to need addressing is methods of determining soil sizing and vertical separation from limiting factors. In Wisconsin limiting factors include high groundwater, slowly permeable soil, bedrock or redox features indicating seasonal (fluctuating) high groundwater.
In the online posts I’ve seen such comments from around the country such as, “What is the perc rate?” and “Does it meet a 1 foot separation from groundwater?”
Just using Wisconsin examples, we stopped using perc rates in about 1990 and changed to morphological evaluations. If you’re just using perc rates, you’ll know a speed or rate that the water will go through the ground. But how fast the water is moving does not take into account how much soil treatment the effluent will have before coming into contact with zones of saturation, groundwater or creviced bedrock. And 1 foot separation between a system and groundwater? Wisconsin’s vertical separation between a system elevation and a limiting factor is 3 feet in most instances, or 2 feet in some instances where the effluent has been highly pretreated.
I have heard some other state codes that have emulated some of these Wisconsin restrictions and design criteria of soil evaluation. E. Jerry Tyler, who was instrumental in the previous discussion of narrow systems, was a major force behind Wisconsin’s and other state’s methods of using the morphological method of evaluating soil and helping to set the 3 foot separation distance.
In a paper in which Tyler was an author², it mentions “Some bacteria and viruses added to the wastewaters are pathogens. Their movement during unsaturated flow is expected to be limited to within a meter.” There’s your 3 feet. Again, an article full of insights about how these systems work is available on the SSWMP site.
It would be great to see the soil testing methods and vertical separations be more uniform for the purpose of protecting public health, which is what we all strive to do.
If you have a choice and want to do right by your customer, you install trenches instead of a wide bed. And while you are at it, read the publications at SSWMP (these documents are important foundational texts for our industry), join your state onsite association (or start one) and attend or host as many training opportunities as possible. None of us know it all, and continuing to learn will only make you a better installer.
About the author
Todd Stair is vice president of Herr Construction, Inc., with 34 years’ experience designing, installing, repairing, replacing and evaluating septic and mound systems in southeast Wisconsin. He is the author of The Book on Septics and Mounds and a former president of the Wisconsin Onsite Water Recycling Association.
¹Soil Oxygen Delivery to Wastewater Infiltration Surfaces, and A Model for Soil Oxygen Delivery to Wastewater Infiltration Surfaces. Both by Tyler and Erickson
²The Soil as a Treatment System SSWMP Publication 6.9 by E.J. Tyler, R. Laak, E.McCoy, and S.S. Sandhu
