Get Your Trenches in Order

Sequential distribution design allows the installer to work around drainfield layout challenges posed by obstacles or an unusually shaped lot

Question:

I’ve been pumping septic tanks, but I’m thinking about getting into the installing business. In April you wrote about ponding and the biomat, then you mentioned sequential distribution. What does this mean?

Answer:

First, I want to stress that if you get involved with installing systems, be sure and get adequate training. Most states have educational programs for onsite sewage treatment systems. Your local health department or zoning office can give you information on available training programs.

I have written about the advantages of sequential distribution in the past, but now may be a good time for a refresher.

When trenches are used for soil absorption in an onsite sewage treatment system, septic tank effluent must be distributed in the most efficient way. For gravity distribution of effluent, distribution boxes were used for many years. There was an inlet pipe from the septic tank into the distribution box and an outlet pipe for each trench. The outlet pipes were supposed to be at the same elevation so an equal amount of effluent would flow to each trench in the system.

There were two basic faults with this system. First, the trenches would likely not have the same capacity to treat sewage. The soil would have different properties along the length and depth of each trench. So each trench would be able to treat different amounts of effluent. Also, it was difficult to install trenches exactly the same length because of lot size or other features.

Secondly, it was virtually impossible to install a distribution box so all the outlets would be at the same elevation when the box was operating. To do this effectively, when installing the distribution box, water would need to be poured into the box and the outlet pipes adjusted. Most installers weren't doing this because it was too time-consuming. And even if the box were perfectly installed, settling of the soil during backfilling would often change the level of the box.

STUDYING FAILED SYSTEMS

After World War II, a lot of houses were constructed for veterans and financed through the Federal Housing Authority. Septic systems were used for some of these houses. Many of the septic systems failed because one of the trenches would have sewage surfacing. This was called “side hill seepage” at the time.

The FHA authorized research to determine why the systems were failing. The study showed distribution boxes were the problem, not the side hill slope. Too much effluent was discharged by the distribution box into one trench and that trench failed. Other trenches in the system often received little, if any, effluent.

The FHA study recommended use of serial distribution rather than distribution boxes. Serial distribution connects the end of one trench to the beginning of the next trench. For effluent to get to the end of the last trench it would need to flow through all the trenches in the system. In that way the entire trench system would be used. If effluent surfaced, it would be from the last trench. This meant the septic system would be hydraulically overloaded. The family was just using too much water.

However, the problem with true serial distribution is that if plugging occurred in a trench, the remainder of the trenches in the system would not receive effluent.

Sequential distribution is a modified serial distribution system. Each trench has a drop box installed at the head end. Effluent from the septic tank flows to the first drop box. This drop box directs all the effluent flow into the first trench.

DESIGN ADVANTAGES

When the biomat forms and the liquid level rises to the top of the first trench, effluent begins to flow to the second trench in the system. The first trench continues to treat septic tank effluent at its capacity, but excess effluent flows to the second trench. When the second trench is treating effluent at its capacity, effluent will flow to the third trench in the system. Each trench receives only the amount of effluent it is capable of treating. The trenches are supplied with their effluent in sequence by the drop box system. This is the reason for the name “sequential distribution.”

Septic tank effluent is supplied to the head end of each trench in sequence. The drop boxes are connected with 4-inch plastic pipe. The accompanying sketch shows the pipe connections and trench layout of a sequential distribution system.

An advantage of the sequential distribution system is that trenches do not need to be the same length, so trees or other obstacles can be avoided. Sometimes the shape of the lot makes it difficult to have all trenches the same length. All that is required with sequential distribution is the total length of the installed trenches meet the design for the soil absorption system.



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