Question:
Why is pressure distribution so necessary in sewage treatment mounds? In some cases the septic tank could flow by gravity to the mound, but we have to install a pump according to state code. I don’t think it’s right that we have to increase the price of the sewage system just because of a code provision.
Answer:
The reason for requiring pressure distribution in a mound is to get an even distribution of sewage tank effluent over the rock layer. The effluent then trickles down through the rock to the clean sand layer for final treatment.
There are three perforated laterals spaced 40 inches apart in the 10-foot-wide rock layer of a typical mound. They extend the length of the rock bed. Typically, 1/4-inch perforations are drilled every 36 inches along the bottom of each perforated lateral line.
The perforated lateral system connects to a header manifold. The manifold connects to the supply line from the pump. The pump station has controls set to deliver four doses each of about 25 percent of the average daily sewage flow to the rock layer of the mound. This amount of effluent will be spread over the entire rock layer.
The system I described does not have “perfect” distribution. The perforations are spaced 36 inches by 40 inches. This is 9.9 square feet of area. The effluent flowing down through the 9-inch-deep rock layer spreads out a little, but likely not more than about a couple of square feet. Therefore, some of the effluent will have to move laterally (sideways) in order to reach other parts of the sand layer.
Research has shown the rock bed and sand layer will remain aerobic with this perforation spacing and four doses each of 25 percent of the average daily sewage flow. The total daily application should not be greater than 1.2 gallons per square foot of the rock layer in contact with the clean sand layer. A heavier application will cause the mound to fail.
If effluent flowed into the mound by gravity, there would need to be 4-inch pipes with 1/2-inch perforations. Smaller perforations would plug with gravity flow.
In a 4-inch perforated pipe, the effluent would flow out of the first two or three perforations for a distance of less than one foot. There would be no way for effluent to get to the far end of the mound rock layer unless a biomat would form on the sand layer. And with proper mound performance there should be no biomat.
Experience has shown mounds using gravity distribution of effluent will fail in a fairly short period. That is why pressure distribution is required for long mound life.
The regulation to which you object is based on sound technical information and field experience. While it is true the cost of the onsite system will be increased when a pumping station is added, it is also true the life of the system will be much longer. And this is what we in the onsite sewage treatment industry must be interested in — long life systems.
In my opinion, 36 inches is the maximum spacing for perforations in a mound effluent distribution system with laterals spaced at 40 inches. Some codes allow a 5-foot perforation spacing. The rock layer of a mound should be no wider than 10 feet, so there would still need to be three perforated laterals. Or incorrect design might go to a 15-foot-wide rock layer, which further defeats the purpose of a mound.
With 5-foot perforation spacing and a 40-inch perforated lateral spacing in a 10-foot-wide rock layer, the area supplied by the perforation is 16.5 square feet. The 1-square-foot rock area in the clean sand under the perforation now has to try and supply effluent to 16.5 square feet. If the perforation spacing is 5 feet by 5 feet, the area is 25 square feet per perforation. I do not believe these spacings can result in suitable effluent distribution.
I realize drainfield rock is getting harder to procure and also requires equipment and labor to install properly. Synthetic drainfield media may function well in trenches where there is soil exposed along the sidewall of the trench. I am doubtful they will provide proper effluent distribution over the sand layer of a mound.
I’ve seen a photo recently showing tubes of synthetic media in a mound. A pressure pipe was shown leading into one of the four tubes of synthetic media. The perforated lateral will supply effluent only to that tube. Will the effluent be supplied to the other three synthetic media tubes and evenly over the sand layer? I don’t think so. Synthetic media cannot use the same specifications for distribution of effluent in a mound as those for drainfield rock.
Another photo showed synthetic media being placed in a mound system. There was no evidence of any pressure distribution system in that mound. I have heard some states do not require pressure distribution in a mound.
The question comes to mind, “How will the sewage tank effluent be evenly distributed over the sand layer of the mound with such synthetic media?” The answer is, “It won’t.” The problem for the onsite industry is when the mound fails, good technology gets blamed, rather than an incorrect design.
In answer to your question and comment, pressure distribution is absolutely necessary in a mound sewage treatment system. I would advise you to install mounds with rock layers no wider than 10 feet. The length of the rock layer of the mound determines the capacity of the mound. A 40-foot-long rock layer 10 feet wide is suitable for a three-bedroom house having an estimated sewage flow rate of 450 gallons per day.
A 50-foot-long rock layer 10 feet wide is suitable for a four-bedroom house having an estimated sewage flow rate of 600 gallons per day.
Good luck, and keep using drainfield rock in your mounds. Use pressure distribution and keep the width of the rock layer to no more than 10 feet.
