An Investigation For The Need Of Secondary Treatment Of Residential Wastewater When Applied With a Subsurface Drip Irrigation System.

Two subsurface drip irrigation (SDI) systems were installed and monitored at two sites in Tennessee.  These locations were residential developments served by a septic tank effluent pump (STEP) collection system, a recirculating media filter (fine gravel media), and SDI dispersal.  At both locations, SDI plots were established to receive primary treated (septic tank effluent) and secondary treated (recirculating media filter effluent) wastewater.  In close proximity to randomly selected SDI emitters, soil samples were extracted.  Soil cores were analyzed to determine hydraulic conductivity and pore water samples were analyzed for nitrate, total nitrogen, total carbon, and total phosphorus.  Results indicate that the primary-treated sites had lower hydraulic conductivity values, higher nitrate and higher total nitrogen levels than the secondary-treated side and the background soil. Interestingly, the primary treated side had less total carbon and the background phosphorus concentration was twice that of the primary and secondary treated sides.  The primary effluent application site showed a decrease in concentration for all constituents with increased depth.  Secondary treatment does result in a higher quality effluent but is not needed when applying effluent with a SDI.