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Traditionally, the septic tank-soil absorption system has been used to provide on-site treatment and disposal of liquid wastes. It is an very good system for disposal of liquid wastes since it is simple, inexpensive and relatively maintenance free. When properly designed, installed and maintained, it is a very satisfactory system. (See Figure 1)
Figure 1
The septic tank is used to provide partial treatment of the raw wastewater. Its primary purpose is to protect the soil absorption system from becoming clogged by solids suspended in the raw wastewater. The wastewater is discharged from the home directly into the tank where it is retained for a day or more. During this time, the large solids settle to the bottom where a sludge blanket develops while the greases, oils, and other floating particles rise to the top to form a scum layer. (See Figure 2)
Figure 2
In addition to acting as a settling chamber and providing storage for the sludge and scum, the bacteria in the septic tank digest or break down the waste solids that have been removed. Anaerobic bacteria, organisms that live without oxygen, feed on the sludge, reducing its volume. In the process, soluble organic matter is released from the sludge into the effluent. Methane and carbon dioxide gases are also produced which are vented from the tank through the building vent. Only about 40 percent of the sludge volume can be reduced in this manner, however, and about once every two or three years it is necessary to pump the tank to remove the accumulated solids. If this is not done, the tank will fill with sludge to a point where the settled solids will be re-suspended and washed out into absorption fields where they can quickly clog the soil pores.
The clarified liquid flows from the septic tank to the soil absorption system for final treatment and disposal. It is an odorous liquid high in partially degraded waste constituents, suspended solids, organic material and ammonia nitrogen. Disease-causing bacteria are present in high numbers and viruses are sometimes present.
A properly designed and constructed soil absorption system is a very effective biological and physical filter which is able to break down the organic and other chemical substances and remove pathogenic organisms including viruses.
This is true, however, only where the soils meet rather specific criteria as outlined in chapter COMM 83 of the Wisconsin Administrative Code. Basically this means that accept in certain cases, suitable soil must exist to a depth of at least three feet below the soil absorption system. Quite often these conditions are not met and failing systems result, causing public health hazards and nuisances.
Many soils in the state do not meet the necessary criteria and thus are unsuitable for on-site liquid waste disposal. For example, red clay soils in the northern and eastern part of the state are very slowly permeable, as are soils formed in tight glacial till in the central and southeastern portions of the state. Construction of conventional subsurface absorption systems in such soils often results in surface discharge or seepage of effluent causing health hazards and/or surface-water contamination. Shallow loamy soils over creviced bedrock, on the contrary, are too permeable. Here effluent may contaminate groundwater with disease causing bacteria and viruses due to insufficient purification by soil percolation. Soils of this type occur in Door County and Southwestern Wisconsin. Soils with constant or periodic high groundwater occur locally all over the state. The total land area occupied by these problem soils is estimated to exceed 50 percent of the state.
Where soil conditions meet minimum COMM standards, a conventional system may be installed.
Conventional System
Seepage trenches are constructed from one to six feet in width and contain one distribution pipe each. Trenches are at least three feet apart. The trench system can take advantage of shallow construction, has more sidewall absorption area for filtration, and is less likely to have the bottom compacted by equipment during construction. These factors provide for longer system life. Trench systems may or may not need more total land area, depending on soil conditions. Also, trenches can be designed to fit into restricted spaces such as between trees or on sloping sites. A conventional system may or may not use a pump, depending on site conditions. Since a pumped conventional system receives effluent that is pumped more evenly across the entire system, a "pumped conventional system" can be expected to have a longer life than a "gravity conventional system". (See Figure 3)
Figure 3 (trench of conventional system)
Alternative Soil Absorption Systems
There are two several alternatives to a conventional soil absorption system. The most common are in-ground pressure distribution systems, at-grade systems, and mound system.
In-ground Pressure Distribution System
This system has some similarities to the conventional system previously discussed. It does have absorption trenches and utilizes partially treated effluent from a septic tank, as do all of the conventional systems. But here the similarity ends. The pressure distribution system uses small diameter laterals in the drain field with single holes spaced a specified distance apart. An effluent pump is selected to provide a minimum pressure to the distribution laterals. The pump, which is located in a separate tank or chamber, will discharge a calculated volume of effluent to the distribution network. Due to the pressure applied, the effluent will be evenly distributed throughout the absorption field. (See Figure 4)
Figure 4
At-Grade Systems
Under normal conditions an at-grade system may be installed on a site that has at least three feet (36 inches) of suitable soil above any limiting factor, such as high groundwater or bedrock. An at-grade system may be installed on slopes up to 25 percent.) An at-grade system is a logical step between the in-ground pressure system and the mound system. The design and construction of an at-grade system is similar to the mound system, except that there is no sand fill under the bed, and the gravel-soil interface will not be level when it is on a sloping sites. (See Figure 5)
Figure 5
Mound Systems
The mound design was developed to provide safe disposal and treatment of effluent at sites that have one of three restrictions. The restrictions are slowly permeable soils, shallow soils over bedrock and seasonal or periodic saturation. To overcome these restrictions, an alternative is to build a mound of medium sand, raising the absorption system above the natural soil surface.
The objective of a mound designed for slow permeability is to spread the effluent out sufficiently to compensate for the slow downward movement of effluent once it reaches the slowly permeable layer. A mound over shallow permeable soil over creviced bedrock is designed to provide additional sand fill in which purification can take place prior to effluent reaching the bedrock. For soils with seasonal or periodic saturation the objective is to purify the effluent before it reaches the zone of saturation. (See Figure 6)
Figure 6
The depths of suitable soil required over the three limiting factors of bedrock, slow permeability, and seasonal or periodic saturation are provided in chapter COMM 83, Wisconsin Administrative Code.
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