There are several types of Alternative Septic Systems, but the most common are: (click any system name to jump to that section)
North Carolina Low Pressure Pipe System (LPP)
North Carolina Modified Low Pressure Pipe System (MLPP)
Typically, an alternative septic system will perform better than a conventional system. This is due to the fact that alternative technology tends to do a better job of distributing the wastewater once it leaves the septic tank.
The common fallacy with the Conventional Septic System is being an "on-demand" system (something comes in, something must go out). This does not allow the septic tank, or the disposal trenches, ample opportunity to "work and rest". Most Alternative Septic Systems (not all) have a big advantage over conventional systems, in that they have the additional tank with a pump inside that sends wastewater to the disposal trenches.
The advantages of a tank and a pump are two-fold. One, wastewater can be stored in the second tank until enough has accumulated to "dose" the disposal trenches. Two, all of the trenches can receive part of the wastewater during each dosing cycle. Meaning, instead of all of the wastewater going into one trench, it can be "spread-out" over 2, 3, or more trenches. This will help to reduce over saturation that is commonly experienced with conventional gravity septic systems.
Systems with a pump (pressurized systems) achieve the more preferred work and rest type of system. The system receives a dose (work), and then has time for the trenches to dry-out (rest).
North Carolina Low Pressure Pipe System (LPP)
North Carolina is credited with creating the Low Pressure Pipe system (LPP). The LPP is typically a two-tank system, with a mechanism (pump, siphon, etc.) to allow pressurized dosing of the system trenches, commonly called laterals. Take a look at the simplified drawing below to note the typical location of the components.
The biggest advantage of the LPP system is the uniform distribution of the wastewater in the disposal trenches. By using pressure, ALL of the trenches get a smaller amount of wastewater. And, since the system can store wastewater, the trenches are allowed to rest during doses. This type of uniform dosing and resting makes it more difficult for a Biomat to form. Thereby giving the system a far greater chance of having a longer life.
The disadvantage to this type of system is the initial cost. Many states require that LPP systems be designed by a registered Professional Engineer. Depending on the requirements of the state, or county within the state, the price can range from $300.00 to upwards of $2500.00 per design. Additionally, some states will not allow LPP systems to be designed from a "perc test". Although I personally would not want a perc test done for a home where I was considering a septic system, they are cheaper to have done. The alternative is to have a soil map done by a soil scientist.
Another additional cost over a conventional gravity septic system, is the second tank (pump tank), plus the pump and electronic controls. Still, I would prefer the LPP to a conventional gravity septic system.
North Carolina Modified Low Pressure Pipe System (MLPP)
The Modified LPP system is the same as the LPP system, with the exception of Modifying the area of soil where the trenches will be installed. This is done in areas where the soils for a systems are suitable, but the depth of the soil is just shy of being adequate. Soil is added on top of the ground to artificially raise the bottom of the disposal trench. Meaning, if a trench were normally to be dug to a depth of 18 inches into the original soil, adding 6" of topsoil would mean the trench is now only 12 inches into the original soil. See below.
