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  When you flush a toilet, wash your clothes, take a shower, or engage in any other activity that that uses and discards water you are making use of a sewage disposal system. There are two major types of systems, municipal sewer service, and the type that will be described here; on-site sewage disposal systems. The sewage leaves the house, building, or facility through what is referred to as the building sewer. The building sewer is a piece of pipe that is usually constructed of either cast iron or P.V.C.. The standard diameter of this pipe is 4", although 3" diameter piping has been used in rare cases. The sewage is carried to the first stage of treatment, in some systems this could be a cesspool while in most cases this would be a septic tank.

  First a brief description of a cesspool; a cesspool is a pit with open jointed linings or holes in the sides and or bottom into which raw, untreated sewage is introduced. The solids that were present in the wastewater undergo some treatment by way of digestion as they are contained in the pit. The effluent leaches into the surrounding soil. Cesspools can be constructed of fieldstones, brick, concrete, steel, or just about anything that would hold up in the soil for a few years.

  The septic tank tank can be described as a watertight structure into which untreated sewage is introduced. The function of the tank is to allow for the settling of solids, the separation of greases, the digestion of solids, and the discharge of effluent to the leaching facility. These tanks are primarily pre-cast concrete in construction but can be made of polyethylene or fiberglass. In the past steel was used to construct these tanks. Steel tanks have not been used for this application in over twenty years in Massachusetts, and because of their tendency to fail both structurally and functionally they are not considered a compliant component of a septic system in Massachusetts. In order for a septic tank to carry out it's function effectively they must meet certain size, shape, and configuration requirements. 

  Title V, which is a Massachusetts sanitary code has guidelines for the construction of septic tanks to maximize the effectiveness of this component. In accordance with these guidelines a septic tank must have a minimum effective liquid capacity of 200% of the design flow or a minimum hydraulic detention flow of 48 hours whichever is greater, but in no case can the effective liquid capacity as measured below the outlet invert be less than 1,500 gallons. This guideline provides for enough time for the solids to settle and the greases to separate to help prevent solids passing through to the leaching facility. To further protect the leach field gas baffles are used in septic tanks. Gas baffles deflect particles that are trapped in rising gas bubbles (a by-product of the digestion that occurs in the sludge at the bottom of the tank) away from the outlet tee, thereby blocking their exit to the leach field. The most common shape for a septic tank is rectangular in cross section. Massachusetts guidelines call for a length to width ratio of 1.5 to 1, with an effective liquid depth of 4 feet, an effective inside width of three feet, a minimum distance of six feet from inlet tee to outlet tee (helps prevent short circuiting of sewage from the inlet tee to the leach field) , and also call for 3 manholes with removable access covers. The placement of the inlet and outlet tees follow these guidelines: The minimum separation between the tees shall be no less than the liquid depth and shall be the longest direction (not diagonal) across the tank in plan view. The inlet pipe elevation shall be no more than three inches above the invert elevation of the outlet pipe, the inlet tee will extend a minimum of ten inches below the flow line, while the outlet tee will extend 14 inches below the flow line if the liquid level of the tank is four feet (19 inches for 5 feet etc.)

  The distribution box is a component of a septic system that is responsible for evenly loading the leach field. The D-box as it is commonly referred  to, is placed between the septic tank and the leach field, it is usually made of concrete but other materials, such as plastic can be used. The D-box outlets must all share the same elevation and be a minimum of 2 inches below the inlet elevation. The most important function of the D-box is to evenly   distribute effluent to the leach field, This is accomplished by installing the D-box completely level, or using an internal leveling device such as a dial-a-flow or a speed leveler, along with proper installation of the distribution lines leaving the D-box. The D-box must also have 6" of sump space below the outlet invert to trap any solids that escape the septic tank.

  The leaching facility or  "soil absorption system"  as it is referred to is where the effluent undergoes the next stage of treatment. Soil absorption systems come in many configurations but all are intended achieve the same results, further cleaning of the effluent that has left the septic tank. This is accomplished through digestion, mineralization, and the transformation (into gases and inert materials) of the effluent. Theoretically after the sewage has undergone all these processes it will be rendered harmless to health, safety, and the environment before it is introduced back into the water cycle.