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0 <br />i <br />' elevation) exist, additional renovation from the soil is curtailed. Unfor- <br />tunately, once a tank system is installed and buried, the above operating <br />' problems are not recognized until the entire system fails and raw <br />(untreated) sewage backs up into the house plumbing or seeps above ground. <br />' Thus, a septic tank system may on the surface appear to be functioning <br />properly when in reality it is providing very poor "treatment." The <br />following description of septic tank operation clarifies the capabilities <br />and limitations of this method of onsite disposal. <br />' 3.2.4 FUNCTIONS OF A SEPTIC TANK <br />' The most important function of a septic tank is to provide protection for <br />the absorptive ability of the subsoil. The tank conditions sewage so that <br />' it may be more readily percolated into the subsoil. Three functions take <br />place within the tank to provide this protection according to the Manual of <br />Septic Tank Practice: <br />' IRC.VB4 <br />6/4/87 <br />1. Removal of Solids: Clogging of soil with tank ef- <br />fluent varies directly with the amount of suspended solids in <br />the liquid. As sewage from a building sewer enters a septic <br />tank, its rate of flow is reduced so that larger solids sink to <br />the bottom or rise to the surface. These solids are retained in <br />' <br />the tank, and the clarified effluent is discharged. <br />2. Biological Treatment: Solids and liquid in the tank <br />' <br />are subjected to decomposition by bacterial and natural <br />processes. Bacteria present are anaerobic, which thrive in the <br />absence of free oxygen. The decomposition or treatment of <br />sewage under anaerobic conditions is termed "septic," hence the <br />' <br />name of the tank. Sewage which has been subjected to such <br />treatment causes less clogging than untreated sewage containing <br />the same amount of suspended solids. <br />3. Sludge and Scum Storage: Sludge is an accumulation of <br />solids at the bottom of the tank, while scum is a partially sub- <br />merged mat of floating solids that may form at the surface of <br />the fluid in the tank. Sludge, and scum to a lesser degree, <br />' <br />will be digested and compacted into a smaller volume. However, <br />no matter how efficient the process is, a residual of inert <br />solid material will remain. Space must be provided in the tank <br />' <br />to store this residue during the interval between cleanings; <br />otherwise, sludge and scum will eventually be scoured from the <br />tank and may clog the disposal field. <br />' IRC.VB4 <br />6/4/87 <br />