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1 <br />1 <br />' <br />' <br />If adequately designed, constructed, maintained, and operated, septic tanks <br />are effective in accomplishing their purpose. However, bacteria removal is <br />' <br />not a process of a septic tank. Although the sewage undergoes treatment in <br />passing through the tank, this does not mean that infectious agents will be <br />' <br />removed. Hence, septic tank effluents can be considered hazardous. The <br />liquid that is discharged from the tank is, in some respects, more <br />' <br />objectionable than that which goes in; it is septic and malodorous. <br />Further treatment, including the removal of pathogens, is effected through <br />percolation through the soil. Disease -producing bacteria will in time die <br />in the <br />out unfavorable environment afforded by soil. In addition, bacteria <br />are also removed by certain physical forces during filtration. This <br />' <br />combination of factors results in the eventual purification of the sewage <br />effluent. <br />' <br />3.2.5 DESIGN AND OPERATION GUIDELINES <br />Since there are no operational standards for septic tank systems, their <br />design and operation is governed by guidelines involving several <br />The first in <br />parameters. step the design of subsurface sewage disposal <br />systems is to determine whether the soil is suitable for the absorption of <br />' <br />septic tank effluent, and if so, how much area is required. The soil must <br />have an acceptable percolation rate, without interference from groundwater <br />' <br />or impervious strata. In general, two conditions must be met: <br />I. Chapter 1OD-6 requires an effective soil depth throughout <br />the drainfield of 42 inches or more below the bottom surface <br />of the drainfield gravel. Also, the water table elevation <br />at the wettest season of the year must be at least 24 inches <br />below the drainfield gravel. <br />2. Table 3-2 presents some representative values for absorption <br />' area as related to percolation rate. <br />' IRC.VB4 <br />6/4/87 <br />