Question:

Do you need the same amount of vertical separation between the bottom of a seepage trench and seasonal high water table for effluent treated with a septic tank and effluent treated with an aerobic treatment plant? If not, what do you recommend for each?

Answer:

First of all, the soil under the seepage area must contain soil air with oxygen. Oxygen is needed for the aerobic bacteria, which eat or break down the biomat on the soil side of the trench or bed. If the soil pores are saturated with water, they cannot contain soil air. No oxygen is present and no soil bacteria are present. The biomat continues to get thicker and thicker until little, if any, effluent can move through it. As a result, the trench or bed fails hydraulically.

So some distance is required between the bottom of the seepage area and the saturated soil to allow for the presence of oxygen in the soil pores under the seepage area. How much separation is needed? Scientists have performed research on effluent flow through the soil and the results indicate that a 2-foot separation is an absolute minimum. The recommendation is a 3-foot separation should be used to have a factor of safety.

Secondly, there needs to be a depth of soil with open pores for adequate treatment. The pathogens (bacteria and viruses) move through the soil with the effluent. The rate of flow through the biomat is much slower than the flow rate of the soil. So only the fine soil pores will have any liquid because of their greater capillary attraction.

This flow through only the fine soil pores is called partially saturated flow. The soil bacteria live in the open soil pores and destroy the pathogenic bacteria. An adequate depth of aerated soil is required for this pathogen destruction to take place. There needs to be a separation between the bottom of the seepage area and the saturated soil.

WIDE SEEPAGE BEDS

Are there bacteria and viruses in septic tank effluent? Are there bacteria and viruses in aerobic tank effluent? Yes on both counts.

Therefore, a separation distance between the bottom of the seepage area and the saturated soil is needed for two reasons: to allow adequate downward flow of the effluent, and for adequate pathogen removal.

I see no difference between septic tank effluent and aerobic tank effluent with respect to flow through the soil and removal of pathogens. In my opinion, 3 feet is a realistic separation distance.

I will add a note of caution in regard to installing wide seepage beds. The soil oxygen must move down through the soil along the side of the bed and then move sideways under the seepage bed. In a very fine soil, the movement of soil air will be very slow. The soil pores under the center of the seepage bed will have far less oxygen than the soil pores near the sides of the bed.

The wider the seepage bed becomes, the less oxygen will be present under the center of the seepage bed. Anaerobic conditions will take place and the biomat will thicken, resulting in slow or zero flow into the soil. The wide bed will fail hydraulically.

Question:

Why is an uneven distribution box different than a drop box? Won’t an uneven distribution box work the same as a drop box?

Answer:

I presume by an uneven distribution box you mean one where the outlets to the trenches are at different elevations. This likely happens more often than not when the excavation for the distribution box is backfilled and the box location is possibly run over by an equipment wheel.

So, let’s say most of the effluent flows to a single trench from an uneven distribution box. As you know, a single trench will not be able to treat all of the sewage tank effluent. This is also true with a drop box system.

What happens when the trench supplied by the uneven distribution box gets full of effluent and can handle no more? This depends on the elevations of the distribution box and the trench.

If the ground surface is nearly level, then effluent can back up in the pipe from the trench to the uneven distribution box. When this happens, the liquid level will rise in the uneven distribution box and effluent will flow out of the next lowest box outlet to another trench.

If all of the trenches are at the same elevation as the distribution box, when each of them becomes full, the effluent will back up to the uneven distribution box. Under these conditions all the trenches have received effluent, but in a random order.

A HINT AT FAILURE

All the pipes leading from the distribution box to the trenches will contain effluent. This may cause a freezing problem in a cold climate.

But, a more common installation is for the trenches to be installed on sloping ground below the distribution box. When the trench receiving most of the effluent from the uneven distribution box is lower than the distribution box, what will happen when it fills up? As the trench fills and liquid builds up in the supply pipe, the pressure on the effluent will cause it to seep to the ground surface. This is called surface seepage and the system will have failed, even though most of the trenches never received much, if any, sewage tank effluent.

Some of my other columns have explained drop boxes and sequential distribution in detail. Briefly, all of the effluent flows to the first drop box and trench in the system. When the first trench is full, the extra effluent flows down the slope through a watertight pipe to the drop box leading to trench number two.

There is never any effluent standing in any pipes with a drop box system. If sewage ever surfaces from this system it will be the last trench in the system. This will happen only if more wastewater is used than the system was designed for.

BOTTOM LINE

I strongly advise you to always use sequential distribution with drop boxes for your trench systems.