Selecting Sites for on-Site Sewage Treatment Systems
David Gustafson, PE
University of Minnesota, Civil Engineering Dept.
St. Paul, Minnesota
Where to locate an on-site system, and what type of system to use, are two intertwined decisions, and the solution of one leads to the other.
The On-Site System in the Environment
The first issue in system siting is identifying the goals of the system. This can be both a political decision and a technical decision; our goal is to choose sites that are appropriate in technical terms,. As an industry, we are getting much better at identifying “touchy” conditions, such as shallow groundwater, shallow bedrock, and nitrogen-sensitive areas, at designing systems to deal with those issues, and at working out management strategies necessary for these systems to be most effective. Major environmental issues include public health and water quality, and system should be located so as to treat wastewater without causing environmental problems.
Siting the System
the key to proper system performance and system longevity is the proper siting of the system. This also will help to determine the type and size of the system necessary for proper operation of the system. Proper site selection can make a hug difference in the longevity and performance of the system.
The site chosen determines the type of system to be used.
The site also governs the system’s performance.
1. The Soil
The first factor to consider is the soil. Soil is a natural element of the site that has developed over many years and must be protected. In evaluating soil, consider three characteristics.
- ColorThe color of the soil gives clues to two major soil characteristics. The first is the depth of topsoil in the soil profile. Topsoil determination is based on the amount of organic matter (black material) in the soil. The depth of topsoil gives clues about water movement and development of the soil.
The second characteristic revealed by color is the depth of saturated soil at the site. Saturation of the soil changes the color of iron in the soil, so the soil color indicates the level of seasonal saturation of the soil. Saturation affects the operation of the system in two ways. First, the ability of the soil to treat waste in saturated soils is minimal. The flow pattern in saturated soil is not conducive for removal of problems, and the speed at which water can flow in saturated conditions will not allow for treatment. The second problem is that without separation of the system from saturated soil, the flow of oxygen in the soil is limited. this will reduce the soils’ ability to accept the water (the long-term acceptance rate, or LTAR), and will affect both the short-term and long-term performance of the system.
- TextureThe texture is the physical make-up of the soil, the mix of sand, silt and clay particles. Texture is an indicator of how water will move through the soil, and this mix of soil separates is the soil property that is typically used for sizing of the system. Properly matching of the soil texture to the sizing will go a long way in the system working the way it should, and enough research has been done that we can feel confident about sizing systems given soil texture at the site.
- Structurethe texture is not the whole story. The structure is how the particles of sand, silt and clay are held together in larger grains. If the soil forms these larger pieces, there is room between the pieces for air and water. if the soil is not made up of these grains, water will have problems moving through the soil. the structure is also the easiest property to damage. Compaction or smearing of the soil will greatly reduce a system’s ability to accept water, and will cause the system to fail prematurely.
2. The Slope
The slope is the next major site characteristic that directly affects the system, primarily due to limitations of construction techniques and equipment.
- MaximumWhen trying to identify a maximum slope, the technology for construction and distribution will have a great impact on the availability of choices. The construction equipment will have limitations for safe operation and these limitations must be respected.
The other slope issue is the method if distribution. If pressure distribution is used, the design must take into account the slope, or the distribution of the wastewater will not be even. If gravity distribution is used, drop boxes will allow for the largest flexibility of slope choices. The use of the distribution box is extremely limiting, and if the slope is much over 3% the potential for problems is greatly increased.
- ContoursThe placement of the system on the slope is more critical than the slope itself. The system must follow the contours for it to work at peak efficiency. Following the contours reduces the potential for the system to overload and allows for the system to stay level while being as shallow as possible.
- DrainageDrainage of the site should be away from all portions of the system. They key to a short system life would be the draining of rainwater into the septic tank, and then into the soil treatment system. Unfortunately, this situation is all too often the case. Run-off over the soil treatment system can also be a major problem.
When designing a system for a particular site, its long-term management must also be reviewed. Too many times, a system is installed and the management is so difficult it is not kept up. Easy access to clean the tank, service the pump, and evaluate the system will go along way towards making the system last.
- All Systems Need MaintenanceThis holds true for all systems. None of the system manage themselves, and the better the management, the better the systems will operate.
- More Technical=More MaintenanceAs the site gets tougher, the system has the tendency to become more complicated, and this complication increases the need for maintenance. So the less ideal the site, the more important it is to have easy maintenance access.
The final location on the site is decorated by local ordinance. It is critical that these regulations, the “setbacks”, do not interfere with the best choice for the system location.
- StructuresThe distance from the different structures is the least critical of the setback distances. these setbacks are in the rule to deal with three issues:
Strcutural integrity-the system should be far enough not to affect the stability of the building. In Minnesota, the assumption has been that the system would fail and therefore the design needed to take into consideration sewage running across the ground. With proper design and siting, this can be avoided.
Construction of the system-the location of the system should be buildable. A system located directly next to a structure is far more difficult to build than one a reasonable distance away.
Maintenance of the system-if a system is located to close to a structure the maintenance may be neglected.
- Surface WaterThe separation to surface water is again a function of concerns about failing systems and structural locations. The most important setback, in terms of protecting surface water, is the separation from saturated soil that gives the soil the ability to treat the wastewater. The large distances associated with keeping the system away from surface water bodies can sometimes be self-fulfilling prophecies of poor system performance, due to siting constraints.
- WellsThe well is the most important setback on the site. Again, the first distance must be the separation from saturated soil or bedrock, to take full advantage of the treatment capacity of the soil. The second issue is travel time. Given the soil type and figuring the amount of time wastewater would take to reach the well water, the distance that the system must be from the well can be predicted with some accuracy.
The System Choice
Given site characteristics and available management tools, the system choice becomes clear. Missing of any of the pieces, though, will lead to missing elements in the decision tree, and potential problems with the system choice.