Uniformity of wastewater dispersal using subsurface drip emitters

Abstract

An on-site wastewater treatment project site with two separate drip fields produced data on emitter flow rates and uniformity after 6 years of operation. The site served a two-bedroom residence in Weslaco, Texas, with treatment through a septic tank and subsurface flow constructed wetland. Filtration was accomplished with a small sand filter and screen filter. Results represent a worst-case scenario because the air relief valves were improperly installed and maintenance on the system was lacking. A pressure compensating (PC) emitter (Netafim Bioline 2.30 L/hr) and a pressure dependent (PD) emitter (Aqua-Drip 3.79 L/hr) were evaluated. When new, the PC emitters produced a mean discharge of 2.33 L/hr with a manufacturing coefficient of 0.043. The PD emitters, when new, produced a mean discharge of 4.30 L/hr and a manufacturing coefficient of variation of 0.016. The testing protocol was verified with the collection of data on new emitters. Two individual drip fields contained PC emitters (Netafim Bioline 3.50 L/hr) and PD emitters (Aqua-Drip 2.35 L/hr). The PC emitters were installed in a 200 m continuous length of tubing and the PD emitters were installed with ten individual lines of 15.24 m connected with a supply and return header. Wastewater with an average BOD₅ of 23 mg/L was applied to two drip fields for 6 years. Emitter flow rates for 313 PC emitters were reduced to a mean discharge of 0.95 L/hr with a coefficient of variation of 0.74 and the 251 PD emitters were reduced to a mean discharge of 1.52 L/hr with a coefficient of variation of 0.35. Two shock chlorination treatments with chlorine concentrations of 500 mg/L and 1000 mg/L were used to increase the emitter's flow rate. Sixty PC and 61 PD emitters were evaluated. The initial average flow rate of the PC emitters was 0.818 L/hr. Average flow rates for the PC emitters increased significantly to 0.859 L/hr and 0.954 L/hr following the 500 mg/L and 1000 mg/L shock chlorination treatments, respectively. The initial flow rate of the PD emitters was 1.54 L/hr. The field flushing cycle represented an increase in flow rate to 1.60 L/hr. The shock chlorination treatments increased the average flow rate to 1.71 L/hr and 1.77 L/hr following the 500 mg/L and 1000 mg/L treatments respectively. All increases in mean discharge were statistically significant. Uniformity and over-application of wastewater were evaluated by analyzing the soil profile on a 1.22 m grid over the entire drain field. Statistical uniformity was 48.1 percent and 71.4 percent for the PC and PD emitters, respectively. The uniformity coefficient resulted in similar results with 70.1 percent for the PC emitters and 85.6 percent for the PD emitters. PC and PD fields caused an over-application of 55.3 percent and 58.5 percent of the field area, respectively.