Question:

Of course, the flow numbers given in your article (“Depth Charge,’’ October 2007) are for trenches ... reflecting a common attitude (bias) found in many areas that all normal systems are trenches. A drainfield is, in this view, a network of trenches. But, in New Hampshire and Maine, with thin, rocky soils often with shallow water tables, trench systems are rare. Many contractors have never installed one. We have installed leach beds as a mainstay system since the beginning of design rule, with other types as a less common alternative, until the advent of the fabric-based systems in the ’90s. The proportion of bottom to sidewall is very different for beds versus trenches. Yet, as the bed ages, it becomes, in essence, a “fat trench.”

I see your numbers reflect an important factor: regarding the soil unit as a whole, rather than as, say, bottom area only. This is from the notes on reducing length based on depth of stone (gravel or rock).

Answer:

I appreciate you having taken time to write to me with your opinion of trench systems and describing the leach bed systems being installed in thin, rocky soils with shallow water tables in your area. Frankly, this doesn’t seem like a good treatment practice to me and I’ll explain why.

Any soil absorption system must have a large enough area of a suitable soil exposed for adequate treatment of the average daily sewage flow. Some of the key words here are “suitable soil” and “adequate treatment.” Suitable soils have percolation rates of from 1 to 60 minutes per inch and no seasonally saturated conditions closer than three feet to the bottom of the absorption area.

Do the math

The soil absorption area can be available in leach beds, but more absorption area is available in drainfield trenches. It is a common practice to size the absorption area based on the bottom area. For example, if 600 square feet of absorption area is called for in the design, this could be a leach bed 10-feet-wide by 60-feet-long. It could be 200 lineal feet of 3-foot-wide trench with a minimum of six inches of rock below the 4-inch pipe and two inches above the pipe for a total rock depth of 12 inches. It could also be 300 lineal feet of 2-foot-wide trench.

The total amount of soil exposed to sewage tank effluent is quite different in each of these three systems when the amount of sidewall area is added. The following table shows the amount of total absorption area of the three examples above:

It is for this reason that the bottom area of a leach bed should be sized 50 percent greater than a trench system. If 600 square feet is the required bottom area, a leach bed should be sized as 600 plus 50 percent of 600 for a total of 900 square feet. Then the soil exposed in a leach bed will begin to approach the amount of soil exposed in a trench system.

You referred to my column on increasing the depth of rock. Trench bottom areas can be reduced when there is more than six inches of rock under the distribution pipe. The reason for the reduction in bottom area is the increased sidewall area. The reductions are as follows:

• For 12 inches of rock below distribution pipe, reduce bottom area by 20 percent

• For 18 inches of rock, reduce bottom area by 34 percent

• For 24 inches of rock, reduce bottom area by 40 percent

The bottom area of a leach bed cannot be reduced with additional rock depth, simply because there is very little sidewall in a leach bed.

You state that “as the bed ages, it becomes, in essence, a ‘fat trench.’” As the biomat develops in the bottom of a leach bed, the effluent level rises. But I would not refer to it as a “trench.” As one can see from the table, there is very little sidewall in the leach bed as compared to either the 3-foot-wide or the 2-foot-wide trenches.

I do not understand how leach beds can be effective to properly treat sewage tank effluent where the soil is thin and rocky with shallow water tables. One could certainly keep the bottom of a trench as far above the shallow water table as the bottom of a leach bed. If there is any slope on the ground, a leach bed would require some excavation on the upslope side to keep the bottom level. This would not be necessary with shallow trenches.

The soils you describe as not being suitable for trenches do not seem to be suitable for leach beds either. From your description, it seems that sewage treatment mounds would be the most effective sewage treatment system.

I have written about mounds before, so this is just a little refresher course. Installation of a mound uses a 10-foot-wide rock bed with pressure distribution. There is no excavation of the original soil, which, as you state, is “thin.”

The soil should be slightly roughened, but handled with care to preserve the soil structure. A layer of clean sand is placed on the original soil after vegetation and other surface trash is removed. The top surface of the sand layer is level. The sand layer tapers down on each side of the 10-foot-wide rock bed to expose more of the original soil to the effluent.

Perforated lateral

Heavy construction equipment with wheels must not be used in mound construction. Track-type equipment with a low ground pressure must be used. There is a definite art and science to properly constructing a sewage treatment mound.

The pressure distribution system is perforated plastic pipe, usually 1-1/2 inch in diameter, placed on 40-inch centers in the 10-foot-wide rock layer. Perforations of 1/4 inch are drilled every 36 inches in the bottom of the perforated laterals. A pumping station is required in addition to the sewage tank. The pump delivers about 25 percent of the average daily sewage flow to the perforated lateral system with each cycle.

My opinion is that leach beds are not the best system to use for proper sewage treatment in the soils you describe. If those leach beds do in fact treat the sewage, then trenches can be used. If leach beds do not treat the sewage because of poor soil conditions, then sewage treatment mounds must be used if proper treatment is the goal, as it should be.