The Battle of Beds Vs. Trenches

A good design principle that results in better treatment is sometimes lost
The Battle of Beds Vs. Trenches

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Recently a colleague of mine pointed out that he is seeing lots of pictures in magazines of beds going in to serve as the final soil treatment and dispersal part of the system. These configurations are often associated with some of the rock alternatives as the distribution media.

While a lot of these configurations include pressure distribution as a part of the system, not all of them do. This led him to point out that from both a hydraulic and treatment perspective a longer and narrower system is better than a shorter and wider system. His concern and mine also is that if we move away from this guiding principle, our systems will struggle. This is the reason that systems have moved from bed configurations to trenches since the mid-1970s.

So what are the advantages? The obvious one is the direct increase in wastewater contact with the soil. Getting the wastewater in contact and then moving through the soil is where treatment will occur.

Here is an easy exercise that we used to teach in classes. Let’s take a soil treatment unit that is located in a loam soil and is designed to take a flow of 450 gallons per day. The long-term acceptance rate for this soil is 0.6 gallons per square foot. This means the soil-sizing factor is 1.67 square feet per gallon. So the soil area needed to accept this amount of septic tank effluent is approximately 750 square feet (1.67 square feet per gallon x 450 gallons = 750 square feet).

So let’s compare a bed that is 10 feet by 75 feet long with three 3-foot-wide trenches, which means there is 250 lineal feet of trench. Contact area for the bed is 2 x 10 = 20 plus 2 x 75 = 150 for a total of 170 lineal feet of contact. For 250 feet of trench the numbers are 2 x 3 x 3 = 18 plus 6 x 83.33 = 500 or a total of 518 feet of contact, or about three times as much!

Contact with the soil means that there will be more opportunity for treatment and for the effluent to infiltrate the soil in the trench configuration. This coupled with more opportunity for oxygen exchange through the soil, which is an important part of the treatment process, means that treatment will be more efficient when the oxygen has to travel less distance through the soil under the trenches as opposed to a bed.

About the Author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.



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