Movement and treatment of water containing Escherichia coli applied to soil by subsurface drip emitters

Abstract

Drip tubing as a means of applying domestic wastewater to soil is increasing in use, especially in clayey soils that are unsuitable for traditional disposal systems. Experiments were undertaken to evaluate operational constraints of drip tubing in regards to the reduction of Escherichia coli in wastewater that reaches the soil surface and the number of instances of wastewater surfacing in these clayey soils. Water colored with a dye and inoculated with E. coli was applied to soil through drip emitters at four dosing rates at two sites. Populations of E. coli in water that reached the soil surface were reduced by less than 20%, regardless of the dosing rate. Approximately 50% of emitters produced water on the soil surface at Site 1 and emitters at Site 2 produced few surfacings. Upon excavation, preferential flow was determined to be the mechanism for wastewater reaching the soil surface. Two depths of burial, 15 and 30 cm, were examined to determine depth effects on the frequency of wastewater reaching the soil surface using newly installed drip tubing. Deeper burial inhibited wastewater from reaching the soil surface, but did not eliminate preferential flow. E. coli populations were not reduced in surfacing wastewater when the depth of burial was increased. These same soils were found to remove greater than 99% of the cells when contaminated water was applied in laboratory column studies. However, this soil was free of preferential flow paths. When water was applied to a uniformly disturbed soil, the distribution of water was not uniformly dispersed spherically around the emitter. Instead, water flowed conically toward the microcosm surface when soil water (P[w]) was highest and dispersed into the bottom of the microcosm in drier samples. Some management practices showed potential to disperse domestic wastewater applied through drip emitters in a manner that will limit the instances of poorly treated wastewater reaching the soil surface.