Soil Dispersal

by Jim Anderson and Dave Gustafson

(Published April 10, 2000 in Focus 10,000)

This is a relatively new term that describes the ways wastewater is applied to soil for final treatment. The methods of effluent distribution or dispersal can be described under two broad categories, gravity and pressure.

Gravity systems include rock-filled trenches and beds, chambers, and gravelless pipe. Pressure systems also can incorporate the use of chambers and gravelless pipe in trenches and beds. Additional pressure systems include sewage treatment mounds, at-grades, low pressure pipe and drip pipe.

All of these dispersal methods have the same objectives, to apply the wastewater to soil in a manner that allows movement into or through the soil and to treat the wastewater.

Which method should be used and how effective it will be depends on a number of site factors including but not limited to soil texture and structure, depth to water table or bedrock, land slope, vegetation, and household water use patterns.
In gravity dispersal systems, effluent flows through a water-tight pipe from the septic tank or other pretreatment unit to either a drop box or a distribution box, where the effluent flows from an outlet pipe at the top of the box to the first trench in the series or the seepage bed. Drop boxes and trenches are superior to distribution boxes and beds from the perspective both of treatment and of efficiency of operation.

Inside the trench or bed, the effluent will flow out through the holes in the first length of distribution pipe and, in a rock-filled system, move through the rock layer and into the soil.

Absorption of effluent by the soil is dependent on a complex series of processes. The factors involved include the initial soil characteristics, quality of effluent, development of biomat and depth of ponding (this operation would require an article by itself to explain how these factors interact).

As mentioned above, trenches are preferable to beds when gravity distribution is used. Longer and narrower configurations allow more direct soil contact with the effluent as well as better opportunity for oxygen exchange through soil around the soil treatment unit.

The advantage to the geometry can be illustrated this way. If I have a sewage flow rate that requires 300 square feet of bottom absorption area, if I construct a 10-foot by 30-foot bed, I have the bottom area and 80 linear feet of sidewall in contact with the soil. If, instead, I have a 3-foot wide trench, it will be 100 feet long. Here there are 206 linear feet of sidewall in contact with the effluent. Obviously, there is a lot more area of soil in contact with the effluent which will improve both acceptance and treatment.

Until now we have focused on rock-filled trenches or beds. There are a number of products available that act as rock substitutes in trenches and beds. These include polyethylene chambers and gravelless pipe.

Gravelless pipe is corrugated tubing surrounded by geotextile fabric. There are two rows of half-inch holes spaced along the tubing. When effluent is introduced by gravity, it flows in the pipe and out of the holes. Generally, it is recommended that the pipe be set at an elevation so that before effluent flows from one trench to another using drop boxes, the entire pipe is filled with effluent. This means that the entire area around the pipe allows effluent contact with soil. Soil absorption and treatment processes occur as in rock-filled beds and trenches.

Chamber systems are also constructed of plastic, typically in a dome shape. Holes or slots are cut into the sides of the chamber. When sewage effluent is applied by gravity, the effluent is directly introduced to the soil surface at the bottom of the trench or bed. Since the top is generally solid and there is limited sidewall contact, the systems do not have all of the advantages connected with sidewall contact.

The future will bring other products substituting for rock in trenches and beds. These include used tires and expanded polystyrene. Each will have its own unique characteristics.

The other means of soil dispersal requires some type of effluent distribution under pressure. In general, this means that in addition to a septic tank for pretreatment, there will be some type of dosing chamber with a pump.

Spreading the effluent out over the soil area allows a degree of control over how much effluent is delivered that is not present in a gravity distribution system. This allows for management of the development of the biomat and more flexibility in terms of where the effluent is applied.

Typical pressure distribution systems include sewage treatment mounds, at-grades and drip systems. In mound and at-grades, the effluent is delivered to the soil treatment unit through small diameter (1-2-inch) pipe with 1/8- to 1/4-inch holes. As in gravity systems, the effluent is applied to rock or rock substitute (gravelless pipe or chambers) in either trenches or beds.

The advantage detailed above is that effluent is evenly distributed over the entire operating area of the system. This increases effluent contact with well aerated soil, improving treatment efficiency and reducing or eliminating development of a biomat. Mounds and at-grades are well proven systems that have been demonstrated to work very well in cold climate areas such as ours.

Drip dispersal systems are just beginning to be installed on a regular basis in Minnesota. Drip systems use technology basically similar to drip irrigation systems used in landscape and orchard settings. Here effluent is delivered under high pressure through a series of small-diameter tubing with very small holes. The concept is to drip water evenly through a number of holes across a wide area. This provides excellent effluent-to-soil contact. Septic tank effluent cannot be used directly because it will plug the holes. Therefore, additional pretreatment is necessary. Usually, this is done through a sand filter or another set of in-line filters. There are also some questions about how deep in the ground these products should be installed to avoid freezing, but it appears this may be a very effective way to apply sewage effluent to soil.

Any of the methods described above can be effective in treatment of septic tank effluent. However, the type of system should be selected on the basis of site, wastewater characteristics, and costs. Each has advantages and disadvantages