The basic functionality of a septic system follow three general steps: collection, treatment, and dispersal.
The waste and wastewater generated by a domestic or commercial building is initially collected by a buried septic tank through a pipe network. A septic tank is the most common pretreatment unit for onsite wastewater systems (Figure 1).
Treatment of wastewater occurs through various processes depending on the system. The septic tank is responsible for removing settleable solids, oils, greases, and floating debris, which are stored in sludge and scum layers and digested by acid-forming microorganisms. The scope of wastewater treatment is to create the proper environment and workspace for the microorganisms. We want to create an environment that is conducive to productive microbial growth, and provide media for the microbial colonies to populate and effectively digest organics. Generated gasses from digestion are vented back through the building plumbing stack, continuously sealed in water tight containers that eliminate any unwanted smells. From Figure 1, it is seen that effluent water is only taken from the clear water layer out of the outlet tee through specialized filters that further purify the wastewater.
After primary treatment, the water needs a place to go. Effluent water flows out of the septic tank by gravity or with a pump to a drainfield that disperses the water back into the ground. Figure 3 and Figure 4 show effluent water being dispersed by pipes and allowed to naturally percolate through the soil to restore sources of groundwater. In many cases, the drainfields continue to filter out forms of nitrogen and phosphorous that could be detrimental to the environment. This water has now been purified, and can be safely returned to ground water or surface water sources.