Question:

I’ve been in the septic business in southcentral Kentucky for 22 years, and for the most part, the changes I’ve seen have been positive. We went through an era where serial/drop-box systems were scorned and preference given to equal flow/D-box systems. (I prefer to call them unequal flow) Serial distribution is gaining momentum, thank goodness. Some think D-boxes are OK, but I still always recommend serial distribution.

And now some state officials have made a ruling to ban serial distribution on Group 4 soils, which is pretty bad stuff but is provisionally suitable. And it is virtually impossible to keep a D-box level on Group 4 soils.

When I questioned this Kentucky state edict, I was told that good installers should be able to set D-boxes for equal flow, and equal flow is just better on Group 4 soils. And I was also told, “In the event of failure with a serial distribution design, the effluent will surface on the lowest line.”

Do you have any idea why “equal” flow distribution is superior to serial distribution in Group 4 soils?

 

Answer:

There are very few times when I don’t quite know what to say or write. This comes close to one of them. I cannot understand the thinking of the officials in your state of Kentucky in regard to effluent distribution on fine-textured soils.

First of all, with a gravity flow system, I will state: “Sequential, or serial distribution as you called it, is far superior to “equal” flow distribution on all soils that are suitable for the installation of a trench system.

I am not familiar with your Group 4 soils, but I fail to understand why they should be treated differently than other soils. I am assuming that sequential distribution can still be used on other soils in Kentucky.

In an Internet search, I was able to find the Kentucky rules for onsite sewage treatment. It is a very detailed document and has some excellent provisions. I was not able to locate any reference to requiring the use of “equal” distribution on Group 4 soils. Apparently this has been a recent action by the state.

The Kentucky rules had some good provisions for Group 4 soils, such as requiring sewage treatment tanks to provide a better quality effluent.

Your Group 4 soils have a lot of clay particles and the required application rate is very low in the Kentucky rules, as it should be. And those soils must have the proper moisture content and be handled with extreme care when trenches are being installed.

The finer textured soil in most soil profiles usually begins at least a foot below the surface. The top 12 inches of the soil profile is where the plant root system is located and has a better tilth and a better structure. The top portion of the soil profile also has a better porosity so liquid infiltrates more easily into the soil than at the deeper depths.

The major reason that trenches up to three feet wide are much more effective than beds for the treatment of sewage tank effluent is the amount of sidewall area available in the trenches as compared to beds.

And the sidewall effect is the reason that the trench should contain effluent to the very top of the distribution medium for maximum flow and efficient treatment. An effluent level near the top of the trench provides moisture for growing plants and evaporation. This increases the capacity and efficiency of the entire soil treatment system.

If it were possible to load all trenches equally with effluent, and if all trenches had exactly the same soil structure and capacity to treat sewage tank effluent, then the effluent level in the trenches would gradually rise at the same rate. When the effluent level reached the top of all the trenches, the system would be at maximum efficiency, and at maximum capacity. But the first two conditions are not possible.

I cannot think of any reason why so-called “equal” distribution should be considered better for fine textured soils. The flow conditions described in the previous paragraph are never realized in a D-box system.

Consider what happens in a trench system with sequential distribution. The first trench receives all the effluent until it reaches capacity. But when it does reach capacity, the entire sidewall on both sides of the trench is treating effluent. The top portion of the trench is contributing water to the atmosphere through plants and evaporation.

When effluent flows to the second trench, the first trench continues to treat effluent at its maximum capacity. As the second trench fills, it begins to reach maximum capacity. The treatment process and infiltration into the surrounding soil continues until all the trenches using sequential distribution are full of effluent.

I would agree with the Kentucky officials that if a soil treatment system using sequential distribution is overloaded, surfacing will take place over the soil in the last trench. But this is not the fault of sequential distribution; it is the fault of the soil treatment system being too small for the amount of sewage flow.

And this is why I continue to advise that each onsite sewage treatment system should have a water meter installed to measure the amount of sewage flowing into the sewage tank and on into the soil treatment system.

In the real world of onsite sewage systems, each of the trenches in an “equal” distribution system is not going to receive the same amount of effluent. In the real world of onsite sewage systems, each of the trenches in a system will not have the same capacity to treat sewage. In the real world of onsite sewage systems, when effluent surfaces from one of the trenches in an “equal” distribution system, there is still treatment capacity available in some of the other trenches.

In the real world of onsite sewage systems, when effluent surfaces from the last trench in a system using sequential distribution, all of the trenches have been used to full capacity. The entire system is at full capacity, and is being overloaded.

To answer your question, in my professional opinion, “equal” distribution in a gravity flow system is never superior to sequential distribution. Sequential distribution is far superior in many ways.