he liquid in most septic tanks stinks, but in some there is hardly a smell to be noticed. The ONSITE WASTEWATER DISPOSAL Table 2.1 Comparison of Typical Septic Tank and Aerobic Unit Effluents Parameter Aerobic Unit Septic Tank BOD (mg/L) Fecal coliforms (no./100 mL) 19,000 290,000 Suspended solids (mg/L) Total nitrogen (mg N/L) Source of data: National Environmental Health Association and * U.S. Environmental Protection Agency. difference may be partly due to the diet and habits of the individual family. This brings us to the other major, but less common, method of residential sewage treatment known as aerobic. The process is essen- tially the same as has been used in municipal treatment plants for more than 75 years. Air is vigorously bubbled through the sewage SO that aerobic bacteria can live in the sewage and digest the solid material. Other types of aerobic treatment units, such as aerobic lagoons and rotating biological contactors (bio-discs) exist but are used less frequently as home systems. The qualities of the liquid existing in these aerobic treatment units are significantly different than those of the liquid coming out of a septic tank. Table 2.1 indi- cates some of those differences. A typical aerobic treatment unit can be seen in Figure 2.2. BOD stands for Biochemical Oxygen Demand. This is a measure of how much oxygen is needed by the bacteria to digest the organic material in the sewage, which is their food. The higher the BOD, the more organic material is present, and the nastier the sewage. Fecal coliforms are bacteria which live in the guts of warm-blooded crea- tures such as people. While most fecal coliform bacteria are not dangerous, their presence indicates the possible presence of disease-causing bacteria. Suspended solids is a measure of the solid material which will reach (and may eventually clog) the drain field. And total nitrogen is a measure of the potential nitrate pollution as wastewater enters the ground water under the drain field. High nitrate concentrations have been correlated