Septic System Owner's Guide

Septic System Features

The design and installation of a septic system is controlled by local and state rules through a permit process. The design takes into consideration all specific site characteristics including the type of soil, size of house, and sewage-generating fixtures and appliances. If you are connected to a cluster system with other houses, you may have pipes, a lift station or a septic tank on your property with a shared soil treatment area in another location. All systems must be designed and installed by licensed professionals and inspected by qualified officials to ensure proper installation. Operation and maintenance of the system is nearly always the owner’s responsibility. Contact the local appropriate agency (planning and zoning, environmental services, etc.) with questions about local requirements.

The complete septic system is made up of three primary components:

• Plumbing: sewage collection
• Septic tank: primary treatment
• Soil treatment system: final treatment/dispersal

Note: In recent years some septic systems may also have an additional "pretreatment" step between the septic tank and the soil treatment system.

Because systems are individually designed and have been installed over many years, there are many variations in the features and descriptions that follow.

Plumbing: Sewage Collection

All sewage containing human wastes, nutrients, dirt, and other contaminants must be collected and delivered to the septic tank and soil treatment unit for treatment and dispersal. All water used in bathing, toilets, laundry, and dishwashing must be treated by the system. A sewage pump in the basement may be needed to move the sewage from basement fixtures up to the septic tank. Drains allowing sewage to enter the system should be equipped with strainers and other filtration devices to reduce the amount of food particles, hair, and lint entering the system.

Some older homes may have been plumbed to allow grey water to bypass the septic tank, but this causes damage by putting too many solids into the soil treatment unit. Most states, including Minnesota, require all sewage to be treated. However, water from roof drains, basement drainage sump pumps, hot tubs, swimming pools, and other sources of clean or chemically treated water should not be put into the septic system. These large volumes of water will overload the system.

Original and remodeled plumbing systems must be correctly designed and installed to allow trouble-free operation. Before remodeling, consider the impact of changes on the septic system. The addition of a bedroom may trigger a need to upgrade the septic system. Some homes are equipped with composting or incinerating toilets, which handle these wastes through a separate process.

Septic Tank: Primary Treatment

How the Tank Works

The contents of the septic tank separate into three layers:

• Floating scum layer - soaps, greases, toilet paper, etc., on the top of the tank.
• Liquid layer - water, liquid, and suspended solids in the center of the tank.
• Sludge - heavy organic and inorganic materials at the bottom of the tank.

Solids separate in the tank by gravity – lightweight materials float to the top and heavy materials sink to the bottom. Naturally occurring bacteria in the sewage begin to break down the organic materials. This is often referred to as primary treatment. Pathogens in the waste are NOT destroyed in the septic tank. Anaerobic bacteria that live with very limited oxygen in the septic tank prepare the sewage for final treatment in the soil treatment unit. Liquid leaving the septic tank is referred to as effluent.

Components of the Tank

The septic tank is the first step of the sewage treatment process. The septic tank is a solid watertight tank. Septic tanks are most commonly constructed of concrete, but occasionally, other materials designed specifically to accept sewage (polyethylene, fiberglass, or coated metal) are used. Some installations may have two tanks in a row or one large tank with two compartments. Increased tank capacity is necessary if a garbage disposal is installed. A few homes on small lots or in poor soil treatment situations may have a large holding tank to store sewage until the entire contents are hauled away for treatment at another location. For shallow installations in cold climates insulating material designed to be buried in the soil can be placed over and/or around tanks.

Several tank designs are available but all tanks must have inlet and outlet baffles, inspection pipes, and a manhole for cleaning Figure 1.

The inlet baffle forces sewage entering the tank to be mixed with the liquid contents to begin bacterial breakdown of organic materials and separation of solids. The inlet baffle also prevents the floating scum layer from clogging the inlet pipe.

The outlet baffle prevents scum from leaving the tank. If the scum layer reaches the outlet pipe, the pipe will become plugged. Scum in the soil treatment unit will clog soil pores and destroy its ability to treat sewage. Filtering devices, known as, "effluent screens," can be installed at the outlet of new or existing tanks to prevent solids from reaching the soil treatment unit. Regular maintenance is required to keep the filters from clogging and causing backups. Filters are a very good idea, required in some states, but are not a substitute for proper operating or maintenance practices!

Inspection pipes of 4-6-inch PVC (plastic) material should be located above the inlet and outlet baffles to allow for inspection of pipes and baffles. Clogs in the inlet or outlet pipes can be unplugged through the inspection pipes. When operating properly, the septic tank is always “full” to the level of the bottom of the outlet pipe.

Inspection pipes must always be capped. They may be cut off flush with the ground to ease lawn care; however, the pipes should be left “long” until the final grade on a new site is determined. Metal covers can help in locating the inspection pipes if the ground is covered with snow. In cold climates they can be insulated to prevent heat loss.

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Do NOT use inspection pipes to clean or pump a tank!

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The manhole in the cover of the septic tank is the large entrance (20"-24") through which the tank should be cleaned. The manhole may be buried below ground level but should be close to the ground surface for easy access. It may be raised from the cover of the tank with concrete or plastic risers for easier access. It is usually located in the center of the tank; however, some manufacturers locate it closer to the inlet end of the tank. There may be more than one manhole, in which case they are usually located at the ends of the tank. The manufacturer, installer or records may be able to tell you where it is. Covers may be concrete or plastic. Insulation may be added in cold climates.

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Be careful when removing the manhole cover! It is heavy and creates a large, dangerous opening!!

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A variety of manhole configurations exist.

The manhole allows proper cleaning and inspection of the tank (see Maintenance and Care). The manhole cover must be kept securely in place. If the septic tank cover does not have a manhole or inspection openings, sometimes a new cover with these features can be installed on an existing tank.

Soil Treatment System: Final Treatment/Dispersal

How the Soil Treatment System Works

All septic systems include the same basic plumbing and septic tank components. Final treatment of sewage occurs in the soil. Uncompacted, unsaturated (not full of water), undisturbed soil must surround the soil treatment system. This system may be a series of trenches or a mound (Figures 2 and 3). Soil treatment kills disease-causing organisms in the sewage and removes nutrients. There are millions of naturally occurring beneficial microscopic organisms in every tablespoon of soil that complete the sewage treatment process.

The beneficial bacteria in the soil need air to live. Therefore, a zone of unsaturated soil must be present below the effluent's point of entry into the soil for complete treatment. At least three feet of unsaturated soil above bedrock is the recognized standard. Some local units of government have established different requirements based on local conditions.

Each site has a unique shape and slope. The soil type, water percolation rate, sewage volume to be treated, and other factors determine how large an area is needed to properly treat the sewage. To provide the necessary area, the design may be a series of many pipes, or “stepped” down a slope.

The biomat is a thin layer of fine solids, dead bacteria, and soil bacteria that forms where the sewage meets the soil. This biomat layer regulates how fast liquid passes out of the trench or bed into the soil so the soil beneath the trench remains unsaturated. Once the sewage moves through the biomat layer and three feet of unsaturated soil, harmful pathogens have been destroyed and phosphorus removed.

Saturated soil is determined by its color and mottling. Mottling is the change in color of the soil that has been saturated for extended periods. Mottling and depth to bedrock are detected by soil borings or pits dug by professionals when the system is designed.

Site conditions and local requirements determine the type of soil treatment system required at each site. If there is three feet of unsaturated soil from the bottom of the trench to saturated soil or bedrock, the least expensive distribution and soil treatment system is gravity flow to a trench system, as illustrated in Figure 2. lf there is not the required soil depth for a trench, an aboveground mound system is required to achieve the three feet, as illustrated in Figure 3. A mound system is an elevated drainfield built with clean sand. There are many small variations in design, but all trenches and mounds accomplish the same treatment function. Mound systems require pumps and/or lift stations and a pressurized effluent distribution system.

Systems that discharge directly from a septic tank to a ditch, stream, lake, or field tile do not provide adequate treatment and represent a danger to humans and the environment.

Soil Treatment System (Drainfield)

Common terms for the soil treatment system are: drainfield, mound, seepage bed, leach bed, and soil absorption field. The soil treatment unit is where the final treatment and dispersal of the septic tank effluent takes place. A properly designed and installed soil treatment system will destroy all disease-causing pathogens and filter out the fine solids contained in the septic tank effluent. Phosphorus will be absorbed by (i.e., attached to) soil particles, and nitrate-nitrogen may move through the soil with the water.

Through the year water moves through the soil. In the summer, water in a shallow drainfield trench evaporates and supplies water and nutrients to grass and trees. The nitrates that remain in the downward percolating water will be either changed to gas by soil bacteria or diluted.

Nitrates in drinking water are rarely a problem with a soil treatment system. In sensitive areas, however, septic systems may contribute to elevated levels of nitrates in groundwater. This would be an issue in sandy soils, or shallow bedrock and karst areas where shallow wells are used for drinking water. In these cases the solution may include pretreatment of effluent or different wells.

The two most commonly used types of soil treatment (drainfield) units are trenches and mounds.

Trench: Drainfield trenches (Figure 2) effectively treat liquid flowing from the septic tank. They are preferred when possible because they are the most economical to install. A drainfield trench is a level excavation 18-36 inches wide. Trenches 36-60 inches wide are referred to as a "seepage bed." The trench contains a perforated pipe in a bed of ¾ inch to 2 ½ inch diameter rock covered by natural or synthetic permeable fibers. Some soil treatment systems use large 8- to 10-inch diameter plastic tube wrapped with fabric or a plastic chamber in the trench in place of rock. A minimum 6-inch deep layer of topsoil covers the trench. Sewage flows through the holes in the distribution pipe, to the rock (tube or chamber), through the biomat, and into the soil. Bacteria and fine sewage solids are removed or destroyed in this process. Dissolved organics provide food for the soil bacteria.

The distribution of effluent into a gravity soil treatment system is accomplished using drop boxes and distribution boxes. The covers of either kind of box can be removed for inspection and cleaning. All pipes from the house and septic tank to the soil treatment area are solid with sealed connections. In cold climates, insulated piping should be used for shallow installations in traffic areas such as driveways and paths.

The trench system may be laid out in one of many configurations to fit the property and allow for the necessary square feet of treatment area. There are often inspection pipes used to evaluate the system on one or both ends of the pipes. These can be cut off at ground level and capped for easier lawn maintenance. The ground surface level of the soil treatment area should always be level with, or slightly raised above the surrounding ground to avoid excess rainfall flooding the system.

Mound (Elevated Seepage Bed): A sewage treatment mound is a pressurized seepage bed built with clean sand to provide adequate soil depth between the sewage released in the mound and the saturated soil or bedrock underneath. It is equally as effective in treating sewage as a trench system as long as it is properly designed, constructed, and operated, and the septic tank is maintained correctly. In fact, mounds will frequently remove more nitrogen than a trench. The mound system (Figure 3), has a pressurized distribution system of 1½ or 2-inch perforated pipe in a layer of ¾ to 2 ½-inch rock covered by a synthetic permeable fiber. The mound is covered with topsoil and planted to grass.

At-Grade: When the soil conditions are between the soil depth for a trench or mound, a system known as an "at-grade" can be used. An at-grade is a bed of rocks containing pressurized pipe placed on the ground surface covered by a synthetic permeable fiber and topsoil.

Pumps and Lift Stations

If the final soil treatment unit is a mound, at-grade (see page 10), or drip system, is preceded by a pretreatment unit, or the liquid flowing out of the septic tank cannot flow to the trench by gravity, a lift station is needed. A lift station is a separate small concrete tank or second compartment of a septic tank containing a pump designed to move the liquid (Figure 5). A pump may also be necessary to move liquid from the septic tank to a pretreatment unit. The pump operates on a float-controlled switch. When the storage area in the tank is full it will pump the contents to the soil treatment unit. Timers can also be used in place of floats to spread effluent application evenly throughout the day. The pump has an emergency alarm indicator circuit to warn the homeowner when the water level is too high. If this happens, the problem needs immediate attention! Be sure to know where this alarm is, what it means, and what to do when it is activated. The manhole cover on lift stations should be at the surface for easy year round access.

The effectiveness of a septic system in treating sewage depends on how the homeowner uses and operates the system. Water-use habits, fixtures and appliances, product selection, and septic additives and cleaners all affect how well a septic system works. The septic system operates every time wastewater enters the system.

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