After the treatment process is started in the septic tank the effluent enters the soil treatment phase of the process (baffles prevent the larger floating solids from entering the drainfield). The soil treatment system, more commonly called the drainfield, is comprised of distribution pipe laid in a trench partially filled with gravel and covered with topsoil.

As the effluent enters the drainfield it percolates through the gravel bed where a large portion of the pathogens are destroyed. Pockets of oxygen created by the uneven shape of the gravel allow the more efficient aerobic bacteria to exist. As the effluent exits the drainfield the natural soil completes the treatment process. By the time the effluent has traveled 2-3 feet through the soil all the remaining pathogens have been destroyed and the water is drinking quality. The cleaning process continues as the water migrates through the soil

Phosphorus and nitrogen are utilized by the vegetative life covering the drainfield and chemically changed in the soil. A large portion of the moisture is returned to the atmosphere though evaporation (evapotranspiration).

Obviously not all sites are going to be perfect.  When dealing with high water tables, a nearby body of water, little or no soil, extremely slow soils, small lots, etc. and a standard system will not adequately perform the treatment process alternative systems can be designed to ensure the treatment process is performed before the effluent is discharged to the environment.  Today the lowly on-site septic system has evolved to the point where they can actually treat wastewater better than a multimillion dollar sewage treatment facility.  And although these alternative type systems will cost more they will in most cases still be far cheaper than a treatment facility.    

Tip for Buying a Home: If you are buying a home with a septic system, do not overlook the status of the septic system!!!  The codes have changed dramatically over the last decade rendering more than half of all systems technically illegal.  Up to this point most regulatory agencies have been content to ignore these old systems.  However as the environmental damage from these old noncompliant systems becomes evident, a few states/counties are starting to require the systems be brought up to current codes before the property can be sold.  However this is not a perfect process and what is starting to happen more frequently is people are told by the health depts. after they buy the home and move in that the system needs to be replaced and they as the new owners will be held responsible...an unforeseen cost that few can afford. 

When you find a house you are serious about, hire your own independent septic contractor to perform a full compliance inspection, don’t depend on the Realtor to give you the straight story (they get paid after the sale is complete and a septic inspection will only slow down that process) and home inspectors inspect homes, not septic systems (a good inspector will tell you this).  Even many county inspectors will only flush the toilet 3 times and look in the yard for surface discharge.  In many cases once you buy the house the problem is yours and there have been cases where the people have been kicked out of their home weeks after moving in because the failed system is an immanent health risk to the public.

A full inspection should include:

• Tank pumped and inspected for cracks/crumbling concrete, missing/inadequate baffles, excessive drain back from the drainfield (indicating a failing system).

• Size of tank.

• Size and depth of drainfield.

• Depth of seasonal water tables should be determined and the amount of separation from the trench/bed bottom to the water table.

• Any non-complying parts of the system should be noted. Any future code changes that would put the system in non-compliance should be reported. Example: Even if the system meets code requirements today, the system should be assessed if it will meet any proposed changes in the septic code requirements in the near future (and a good septic contractor will be aware of pending changes).

• Dollar estimates for any suggested up-grades should be provided.

• This inspection assessment should be provided to the sellers and buyers in writing and these costs should be incorporated into the selling price.

This inspection will cost $200-$500 but it is worth every penny if you find any problems. Any up-grades can/should be negotiated into the selling price of the home. If no problems are found then you know you are starting with a good system and it is up to you to take care [of the system] from the beginning.