I see that you don’t like the width of the rock layer in a mound to be more than 10 feet. What’s the reason for that? I see lots of beds that are a lot wider than 10 feet. Why can’t a rock layer in a mound be more than 10 feet, when beds are a lot wider than that?

Let me first explain how a sewage treatment mound works. The purpose of the mound is to provide treatment for sewage tank effluent where there is shallow soil depth over a restricting layer. The topsoil layer may be only a couple of feet thick above the saturated soil layer below. The soil is saturated when there is a discoloration called mottling.

The reason that the soil is mottled is that liquid moves through it very slowly. At times during the year, the soil layer is totally saturated. The saturated layer turns anaerobic, dissolves iron and becomes gray. When the soil dries out, some of the iron oxidizes and turns brown. The soil color is gray with brown spots or streaks. This is called mottling.

The saturated layer of soil may let water move through it at a rate of a half-inch per day or even less. The amount of effluent applied to the rock layer of the mound is 1.2 gallons per day per square foot. This is equivalent to a liquid application of almost two inches every day.

The liquid moving down through the rock layer and the clean sand layer of the mound must spread out over the saturated layer. There must be at least four times the amount of underlying soil as the width of the rock layer of the mound. This additional area allows the liquid to spread out and percolate downward through the underlying soil.

So the clean sand layer spread over the soil must be at least 40 feet wide for a 10-foot-wide rock layer in a mound. This is another 30 feet of width beyond the rock layer. For higher sewage flows, the length of the mound must be increased and not the width.

As an example, suppose that the rock layer of the mound was 20 feet wide. The clean sand layer would need to extend at least another 60 feet. This would be 80 feet of width, plus the dikes on each side. And the height of the rock layer would need to be increased to allow the liquid to move sideways away from the rock layer.

I hope this explanation gives you a better idea of why the rock layer width in a sewage treatment mound should be no wider than 10 feet.

I would like to comment on your statement that “beds are a lot wider than that.” It is true some very wide beds are being designed and installed. In my opinion, the very large beds now being designed and installed may have problems to continue to accept sewage tank effluent at the design rate. There also may be problems of adequate pathogen removal and concentration of nitrates in the groundwater. There are more factors to consider than just “getting rid” of the sewage tank effluent.

One problem will be groundwater mounding under the wide bed. If the underlying soil will not continue to accept a flow of 2.0 inches per day, a water table will rise under the bed. As this water table “mounds up” it will slow down the flow from the wide bed.

Another problem will be the lack of soil oxygen under the center of the bed. The oxygen must come in from the side of the bed. In a wide bed, the distance is too great for adequate oxygen movement. Oxygen is necessary to prevent anaerobic conditions in the soil under the bed. When the soil under the bed becomes anaerobic, the biomat gets thicker and the flow through it slows down. This is why individual seepage beds should not be wider than 10 feet.

Many years ago at Westboro, Wis., the lack of oxygen in the soil under 100-foot-wide beds slowed the design acceptance rate of 2.0 inches per day to a rate of 0.55 inches. This reduced acceptance rate resulted in the hydraulic failure of the beds.

Another problem is an adequate evaluation of the soils under the wide bed. Soil texture is often quite variable from place to place in an area. It would be unlikely that the soil texture would be exactly the same over the bottom of a wide bed. If the original area was not level, the excavation depth will vary and different layers of original soil will be exposed.

The percolation rate may be measured by the standard test using a 6- or 8-inch diameter hole. This test measures the horizontal permeability of the soil in the location of the test. For a very wide bed, the vertical permeability of the various soil layers under the bed is the information needed for design purposes.

Finally, construction practices must not allow any heavy equipment into the bottom of the wide bed. It may be difficult or not possible to follow this important rule on very wide beds. I have been told that there have been hydraulic failures of large beds recently installed. The design criteria used for individual onsite sewage treatment systems simply do not apply to large and wide seepage beds. Unless the designers recognize this, there will be more failures of wide beds.