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The Septic Tank The septic tank serves three functions: storage, settling and digestion. The tank must be able to store waste from high flow periods (e.g., the morning round of showers) until it can pass through the absorption field. Hence, the size of the septic tank is determined by the size of the household it is intended to serve. The formula accounts for the number of bedrooms in the house. The minimum size and the location of a septic tank and its absorption field must comply with state regulations. Sewage flow coming from the house is separated into three fractions after it enters the tank. Grease, oils and other light materials accumulate at the top of the tank in a layer of scum. The heavier solids settle into a sludge layer at the bottom of the tank. The intermediate layer is wastewater. Wastewater passes on to the absorption field. The scum and sludge are held in the tank by a set of baffles. Since the settling process requires time, tanks are designed so that liquids are retained for about 24 hours, under normal flow conditions, before discharging to the absorption field. Even though it has no moving parts, the septic tank requires regular maintenance. Annual inspections of the baffles are recommended to ensure that scum is not leaving the tank and entering the absorption field. Similarly, accumulated sludge must be removed on a regular basis. Otherwise, the tank capacity will be reduced so that solids are not able to settle out before the wastewater effluent leaves the tank. The minimum frequency of sludge removal (“pumping”) varies with the amount of use your system receives and may be regulated, in Pennsylvania, by local sewage management program ordinances. For the average family of four, a septic tank in normal use needs to be pumped out every three years. If you place heavier demands on a system, such as a large family or a garbage disposal, the tank may need to be pumped every year. Due to several considerations, including personal safety measure requirements to protect the person making a thorough inspection of the tank, a professional contractor should do septic tank pumping and inspections. Warning: Never enter the tank unless equipped with self-contained breathing equipment or actively operating high volume forced ventilation to force outside air to ALL areas in the tank. Do not breathe the air inside an unventilated septic tank. It may contain LITTLE or NO OXYGEN. The lack of oxygen can cause unconsciousness without warning. Death can occur in this type of situation! The Soil Filter When the wastewater leaves the septic tank it flows to the absorption field. The absorption field consists of a network of perforated pipes (often plastic) laid out in a bed of trenches lined with gravel. The pipes are connected to the septic tank through a small chamber known as a distribution box. The distribution box is designed to distribute liquids equally among the absorption field pipes. Wastewater then flows through the gravel and into the surrounding soil. Microorganisms in the soil decompose many of the remaining contaminants. However, the soil cannot remove dissolved solvents, automotive and lubricating oils, drain cleaners, and other household chemicals that can easily percolate into groundwater. If the absorption field is properly located, designed and installed it will accept septic tank effluent for a very long time. Design life is normally a minimum of 20–25 years. The design presumes that in operation: 1) the field does not become overloaded with liquid, and 2) the septic tank is properly maintained and does not allow sludge or scum to escape and clog the field. Minimizing household water usage and waste output will extend the life of the absorption field and help keep it in good operating condition. The size and placement of the absorption field is determined by the type and depth of soils on the site, as well as the sewage load it is expected to handle. Some soils are better than others for placement of an absorption field. In sandy soils, for example, water may pass through too quickly to be treated efficiently, while heavy clay soils may not allow enough flow. In addition, there must be sufficient soil above the water table or bedrock for complete treatment (generally about six feet). Some lake homes have septic systems that do not have sufficient capacity for the type of use they receive, are located too close to the water table, and/or are in poor soils. Your county conservation district can help you estimate the type, depth, and location of the various soils on your property. A final determination of their suitability for septic systems will be made by your local sewage enforcement officer. The absorption field ceases to function when the soils surrounding it become saturated. This can happen when the system is overloaded or when the water table rises to the level of the absorption field. When the absorption field becomes saturated, septic effluent can rise to the field surface, enter groundwater, or travel underground and resurface at another water source—quite possibly your lake or a stream tributary to your lake! As you can see, whenever the absorption field becomes saturated, the sewage you flush flows essentially untreated into the surrounding environment. |