Conventional onsite systems have been in use for decades and they do a very good job at what they were designed to do: reduce human contact with disease causing organisms. Conventional onsite systems include standard, pressure distribution and sand filter systems and they do a tremendous job at reducing bacteria as sewage is treated in the drainfield or sand filter. These systems also do a good job at reducing the quantity of solids (total suspended solids or TSS) and reducing a water quality measure called biochemical oxygen demand (BOD).
What septic tanks discharge:
TSS and BOD are commonly reported indicators of water quality that should be as low as possible. The quality of the wastewater that is discharged from septic tanks is available here (PDF document). You can see that the levels of BOD, TSS and bacteria (fecal coliform and E. coli) are very high.
Nitrogen in septic tanks comes in the form of ammonium (NH4) or Total Kjeldahl Nitrogen (TKN). In the table showing septic tank effluent quality, the amount of nitrogen in the wastewater is reported as Total Nitrogen (TN) in order to be able to compare the performance of a septic tank with the performance of, for example, a sand filter or a nitrogen reducing system where the nitrogen is typically in a different form (nitrate or NO3).
What sand filters do:
Sand filters were designed to replace a part of the treatment provided by soil by creating an engineered environment in which wastewater treatment can occur. In Oregon, sand filters contain two feet of a high quality clean sand. This two feet of sand, in combination with a minimum of two feet of unsaturated soil provide a total of four feet of unsaturated treatment before the wastewater reaches a limiting condition (for example, groundwater). The sand filter is designed to create an oxygen rich environment in which to treat wastewater. Because of this oxygen rich environment the NH4 and TKN found in the septic tank is transformed to nitrate (NO3). Chart 1, provided here, (Chart 1 is a PDF document) shows how levels of the different kinds of nitrogen in the groundwater below a sand filter changed over time after a new sand filter started operation.
What standard systems do:
Standard systems are designed to provide wastewater treatment in the soil in the drainfield. As with sand filters, the goal is to create an oxygen rich environment in the drainfield, which results in the transformation of TKN and NH4 to NO3. Chart 2 provides a look at the groundwater quality below a standard system (Chart 2 is a PDF document).
How nitrogen can be reduced:
The job that conventional onsite systems do to transform NH4 and TKN from septic tanks to nitrate is actually the first step needed to reduce total nitrogen in wastewater. The next step is to send the nitrate-rich water into a low oxygen environment. Once there, microscopic organisms use the oxygen in the nitrate molecule NO3), which frees the nitrogen to dissipate as a gas.
It is difficult to add this step to conventional systems because the transformation to nitrate occurs in the soil or the sand filter where it can be difficult to collect the wastewater to send it through an oxygen-poor process. Onsite systems can be designed to do this, which is the basic concept behind certain recirculating gravel filters and Alternative Treatment Technologies (ATTs) that are becoming more common in Oregon.