Clusters of Concern
To the Editor:
I have been concerned for some time about very large and cluster systems that seem to be so popular today. Aside from some very real construction problems, of more concern is the dynamics of flow through the soil.
I refer especially to the numbers for the large systems described in the article “Serving a Community” in the August Onsite Installer. The values commonly used for designing onsite soil treatment systems were developed for small systems. The values reflect the rate of liquid movement through a developed biomat. This would be through the trench or seepage bed bottom and sidewalls.
These values, such as 0.345 gal./sq. ft./day, also assume that there will be sufficient oxygen in the soil for soil bacteria to continue to break down the biomat on the soil side of the trench or bed. These soil acceptance rates do not apply to an application over a broad area of soil. For example, in the August issue a system is being designed with drip irrigation using the rates of 0.18 gal./sq. ft./day and 0.345 gal./sq. ft./day.
The system capacities are to be 92,400 gpd and 320,000 gpd. This is a lot of liquid! The amount of 92,400 gallons applied at a rate of 0.18 gal./sq. ft./day would require 11.8 acres of area. The application rate of liquid would be 0.29 inches per day, or 106 inches per year. For 320,000 gallons, 41 acres of area would be required.
This is 106 inches of liquid being applied to that area each year. This amount is in addition to the rainfall that might be received. Does the soil profile have the capability to transmit this amount of liquid downward to the water table?
The water table is indicated to be at a depth of 6 feet. This will complicate the dynamics of flow in the soil, and much of the applied liquid will need to move laterally away from the application area. If the soil characteristics do not allow the liquid to move laterally, then the liquid will move laterally on the surface of the soil.
The point here is that design values used for soil acceptance rates in small underground trench and seepage bed systems simply do not apply to large systems. This is to say nothing about the nutrients, salts and other compounds that may be in the liquid flowing down to a shallow water table. If the water table is used as a water supply for nearby residents or municipalities, there may be serious problems.
Wisconsin found out many years ago at Westboro that large seepage beds failed. The beds were sized using design values for small systems. These values simply did not apply to seepage beds 100 feet or more wide. The failure of the large seepage beds was reported in a paper titled “Performance Characteristics of a Community Wastewater Absorption System” by R.L. Siegrist, D.L. Anderson, D.L. Hargett and R.J. Otis. This paper was presented at an Agricultural Engineering Symposium on Individual Sewage Treatment Systems.
Minnesota at present is hiring consultants to diagnose the failure of some large systems designed by engineers. The problem is that engineers do not understand that the design values for individual onsite treatment systems do not apply to large systems that apply large quantities of liquid over a relatively small area. The engineers who design these large systems also apparently do not understand the dynamics of flow through porous media, both laterally and in the downward pattern toward the water table.
I also believe your editor over-simplified matters in his August column in stating to installers that “If you know the basics of single-family systems, you also know most of the fundamentals of large-scale and cluster systems.” I also cannot accept the statement that “the soil does its absorptive work in large systems just as in those you now design and install.” There is a vast difference in the fluid dynamics of small and large systems.
I think that Ontario has the right idea with its Municipality Responsibility Agreement. The provisions are such that if a large onsite system fails, the nearby municipality agrees to take over the treatment responsibilities. An agreement like this is desperately needed where these large systems are being designed and installed in the United States.
I realize that there is a vast movement in the onsite industry today toward large cluster systems. For the sake of the onsite industry, I hope they are properly designed and I hope they work. I do have real serious doubts about the future of the industry moving in the direction of these large systems. There have already been some serious problems.
Roger E. Machmeier
Chisago City, Minn.
