Question:

One of your columns in Pumper related to serial distribution. Why is serial distribution better than parallel distribution? My understanding of serial distribution is that the first trench must fill before the second trench is used or gets any effluent. Then the third trench will get liquid after the second trench is full, etc. If this is correct and all of the trenches are full, is this described as serial failure? Did serial distribution come about because parallel D-boxes would not distribute correctly?

Answer:

If all of the trenches are full of effluent with serial distribution, they will also be full of effluent with parallel distribution. In either case, all of the trench system will be used to treat the supply of effluent.

Serial distribution is achieved by connecting the end of the first trench to the beginning of the second trench. The end of the second trench is connected to the beginning of the third trench, and so on.

Parallel distribution is defined as supplying effluent to the head end of each trench in the system. Theoretically the same amount of effluent is supplied to each trench.

Both of these effluent distribution systems have serious faults. The advantage of the serial distribution system is that the first trench receives as much effluent as it can treat. When the first trench is full, the effluent flows on to the rest of the trench system. The major objection to serial distribution is the possible blockage of a trench. If this happens, the remainder of the trenches in the system will not receive effluent.

Parallel distribution using a distribution box must assume each trench is exactly the same. Each trench must be able to treat exactly the same amount of effluent. Each trench must be the same length. Each trench must have exactly the same soil texture. Each trench must have the same amount of distribution medium in contact with the soil. It must be further assumed exactly the same amount of effluent can be supplied to each trench.

However, the variation in soil texture and the variation in percolation rates is the reason each trench will not have exactly the same capacity. It is rarely true that soil texture is exactly the same over the entire area of the drainfield trenches. When soil textures vary, percolation rates vary. Construction procedures may also cause variations between trenches.

REAL WORLD PERFORMANCE

It is one thing to observe the flow from a distribution box sitting on a display table during a product demonstration. It is different to have a distribution box placed into an excavation area that isn’t entirely level. The bottom of the excavation may contain some fill used to replace excavated firm soil. This fill will likely settle unevenly, causing the final pipe outlets of the box to be at different elevations and have different flows.

A few questions come to mind regarding each installed system: Will water be added to the distribution box during installation to be sure all the outlets are at the same elevation? Will the distribution box be opened and checked a year later to be sure there has been no soil settling? Will the outlets be checked to see if they are all at the same elevation?

In my opinion, both the assumptions of equal trench capacity and equal distribution are not logical. Even if equal distribution of effluent could be achieved, it would be of doubtful value because the trenches likely will have unequal capacities.

A trench overloaded by effluent from an uneven distribution box will have sewage coming to the surface. The Federal Housing Authority studied “side hill seepage” in failed existing onsite systems. The study determined the cause of “side hill seepage” was the distribution boxes. The study condemned further use of distribution boxes in onsite sewage treatment systems.

So where do we go from here? The answer is to use a modified serial distribution system, which I call sequential distribution. The sewage tank effluent is directed into the head end of each trench. A distribution device known as a drop box is located at the head end of each trench. Four-inch supply pipes connect each of the drop boxes.

The sewage tank effluent flows into the first drop box, then into the first trench. As this trench begins to treat effluent, the liquid level rises in the trench. When the first trench is treating sewage at its capacity, excess effluent flows to the drop box of the second trench. When the second trench reaches its capacity, effluent flows to the drop box of the third trench, and so on.

IRREGULAR TRENCHES

Each trench will be supplied with the amount of effluent it can treat. Different trench lengths or characteristics are not important. Because a trench is full of effluent does not mean it has failed. Treated water is still percolating into the surrounding soil. Whenever effluent is not flowing into a trench, as overnight, or when the family is away, the liquid level in each trench will be dropping. When effluent begins to flow again, each trench takes as much effluent as it can treat.

Occasionally trenches need to be different lengths because of the irregular shape of the lot. Occasionally a lot owner wants to keep a valuable tree from being cut down. In situations like these, parallel distribution cannot be used. But these situations are adaptable to sequential distribution.