WHAT IS A SOIL TEST (PERC TEST)
The reason for the soil test is to determine what is the proper type of system and what size the system needs to be. During the soil test a backhoe is brought in to dig at least 3 holes/pits that vary in depth from 4’ to as deep as 11’. At this time the Soil Tester will examine each layer/horizon of the soil and record the depth, color, type or texture, structure, and consistency of each horizon. There are 12 types/textures of soil each being made up a varying percentage of sand, silt, and clay. Each soil texture can disperse only so much effluent. For example: course sand has a loading rate of 0.7 gallons per day/ square foot where clay has a rate of only 0.2 gpd/sq. ft. The drain field for sandy soils can be much smaller than those required for clay soils. The Soil Tester is also looking for signs of high groundwater (mottling) or bedrock that will limit the flow of effluent that is being dispersed back into the soil. There needs to be at least 36” of soil below the septic system for proper treatment of the effluent, if there isn’t groundwater contamination can occur. If there is determined to be a limiting factor in the soil a mound or at-grade type system needs to be installed.
WHAT IS A SEPTIC SYSTEM
A septic system starts with the plumbing as it leaves a sink, shower, or the toilet. The wastewater is carried away through a series of pipes that lead to the septic tank. In side the septic tank the dense, heavy solids (sludge) settles to the bottom where bacteria tries to decompose these solids. Materials that are lighter in weight (fats, oils, and greases) will rise to the top of the septic tank along with the gases released during decomposition to form the scum layer. The clear liquids then flow out through either a baffle or effluent filter that is mounted on the outlet side of the septic tank.
The clear liquid is then either pumped or flows by gravity to the drainfield. Drainfields may be either in the ground or above ground such as at-grade or mound systems. As this liquids moves through the ground it is further purified by the soil to remove bacteria, pathogens, and viruses. The main purpose of the drainfield is to distribute the treated liquid back to the ground water table where the process can be repeated.
Septic tanks can be made from steel, plastic, fiberglass, or concrete. The main purpose of the septic tank is to retain the solids so that bacteria can decompose it. You shouldn’t drive heavy equipment over the tank or cover the tank with a concrete slab. If your septic tank is made from steel, you will eventually need to replace it. I’ve seen steel tanks last from 25 to 40 years. If your tank is steel you need to watch for ground settling over your tank, this means that the cover is starting to cave in.
Since 2000 the State of Wisconsin has required that all new septic systems have effluent filters installed in the septic tanks. The effluent filter takes the place of the baffle and must filter out 1/8th inch particles and larger. Filters need to be cleaned periodically; this is why manhole covers must be left above ground level and padlocked shut. The frequency of cleaning depends on individual size and construction of the filter as well household usage. They vary in construction from a bristle brush, plastic disc or tube, or plastic box type.
Drain fields traditionally were constructed of crushed rock and perforated pipe. Trenches were dug with a backhoe or bulldozer that varied from 3’ to 12’ wide and then about 6” of crushed rock was spread into the bottom of the trench. 4” perforated pipe has then laid down and covered with 2” more of the crushed rock. A synthetic fabric was then laid on top of the rock to prevent soil from washing into the drainfield and plugging the air spaces between the rocks. Drainfields are now being installed that do not use crushed rock or gravel. These systems use either plastic chambers or polystyrene beads. Both systems work well and now are installed more often than rock because of the lightweight and ease of construction.
MOUND - AT GRADE SYSTEMS
Mound and at-grade systems are installed when there is less than 30” of suitable soils for a septic system. At-grade systems are installed directly on top of the existing topsoil of the property. Mound systems require that an approved washed sand is added above the topsoil and the drain field is installed on top of the sand. The more poorly drained the soil is the taller the mound gets to be.
Holding are a system of last resort in most counties in NW Wisconsin. The reasons a holding tank would be installed vary from no room for a mound/ at-grade system, or extremely high ground water. A holding tank has no drain field so all the wastewater from the home has to be hauled away by a licensed Septic Pumper. This can be very expensive because generally this wastewater must be hauled to a Municipal Waste treatment plant for disposal.