There really is no across the board answer to this question.
The treatment septic waste relies upon the action of both anaerobic and aerobic bacteria.
In a simple septic tank & leachfield system the primary digestion takes place by anaerobic bacteria in the septic tank which is primarily free of air. The anaerobic bacteria break the solid matter down and converts all the waste to a thin liquid sludge which then goes to the leachfield.
The leachfield lines are then run relatively close to the surface where some air can penetrate the soil by normal atmospheric pressure and get into the leachfield lines. The air in the leachfield lines then supports the growth of the aerobic bacteria which performs the final digestion of the waste and the excess liquid is then absorbed into the soil.
Years ago it was common to install septic tanks with leachbeds running three parallel runs of 60' to 80' or more, but as the cost of land rises the size of building lots has decreased. The problem is further hampered by our desire to build larger and large structures which then leaves even more limited space for the septic tank leachfield. As a result the current code standards are set at almost the bare minimum required to maintain a septic tank under the best of conditions which is evidenced by the recommendation to have the tanks pumped every 3 to 4 years.
Years ago pumping a septic tank was unheard of, in fact, the septic tank on my grandfathers farm was installed in 1958 and it was never pumped until last year simply because the people who bought the farm in 1979 did not know where the tank was and could not locate it until I happened to stop by to see the old place and I pointed out the location, which was about 300' from where they had been looking.
In some cases if you have one of the old style septic tank & leachfield systems where the leachfield has begun to slow down as a result of sludge saturation into the surrounding soil, the system can be dramatically improved by installing an above ground air vent into the leachfield lines. This will dramatically increase the amount of free oxygen in the leachfield lines which supports the aerobic bacteria and improves the leachfield efficiency. (You must check local codes carefully before installing a vent because there are a number of distance from existing structure requirements that must be met.)
In recent years they have begun making an improved type of septic tank that is baffled into multiple chambers. The primary chamber is still air tight and it supports the anerobic bacteria while the second chamber has an aeration system to support the formation of the aerobic bacteria.
The type and size of the aeration equipment is critically sized to the tank to insure that it will not produce excessive amounts of air that might agitate the sludge and force solid matter into the leachfield.
Recently they have introduced a third type of system which is being called a "Whitewater system." These type of septic tanks utilize a three chamber process. In the first chamber, which is called the "digester" receives the incoming effluent which is broken down by anaerobic bacteria in the conventional manner. The waste then flows into the "aereator chamber" where air is injected into the waste by either a selfcontained submersible aereator pump which is vented to the atmosphere or it may have a small air compressor mounted at a conventient remote location in the house, garage or utility shed with an air line to the tank. The third chamber of these systems is a sediment tank where any remaining solids will settle to the bottom of the tank and the resultant discharge is basically just plain water. In some areas where the soil will not pass a perk test for installation of a septic tank the whitewater systems are installed and the add a chlorinator to the final sediment tank. The homeowner must then keep the chlorinator filled with the large swimming pool chlorine pucks. (They usually require 5 to 6 chorine pucks per week.)
The singular advantage of the whitewater system with a chlorinator is that, while it does have a conventional leachbed, in a worst case scenario such as when the ground is to saturated to permit the leachbed to work properly the discharge of the system will meet or exceed the water quality standards that will permit the end of the leachbed to be piped to a nearby watershed or storm drain.