Impact of working pressure and surfactant concentration on air-water transmission in drip irrigation tape under cycle aeration

Abstract: Aerated subsurface drip irrigation (ASDI) refers to the irrigation with aerated water based on Venturi principle by the use of subsurface drip irrigation (SDI) system. ASDI provides a source of oxygen to root environment that may suffer from temporary hypoxia, and therefore unlocks yield potentials of crops. But the efficiency of ASDI is dependent on a number of factors. Among which, proper operating pressure and surfactant concentration are of great significance for aeration efficiency and the uniformity of air and water flow in the aerated irrigation water stream. Proper control of air-water transmission will offer the industry an opportunity to further optimize the use of ASDI system and improve water use efficiency and minimize the environmental impacts of irrigation. The primary objective of this study was, therefore, to explore optimum working pressure and a surfactant sodium dodecyl sulfate - SDS addition on the flow uniformity of water-air irrigation in the drip pipeline. Nine combination were tested (3 working pressure at 0.05, 0.10, 0.15 MPa and 3 concentration level of SDS of 0, 5, and 16 mg/L, respectively). Each combination has aeration treatment and non-aeration treatment. Results showed that the aerated concentration was increased with increasing operating pressure even without surfactant addition. However, aerated concentration decreased with increasing operating pressure under surfactant addition. Under the same operating pressure, the aerated concentration increased with increasing surfactant concentration. The addition of surfactant significantly increased the equilibrium concentration in aeration treatment. The average value of water flow uniformity under aerated drip irrigation was greater than 95%, suggesting that there was no negative impact of surfactant addition on the water flow during oxygation. And the air flow uniformity of each combination remained above 70%. The oxygen transmission efficiency was improved with the addition of SDS. The surfactant concentration had a positive effect on the oxygen mass transfer coefficient, while the operating pressure had a negative effect on the oxygen mass transfer coefficient. The surfactant addition has greatly shortened the aeration time. The deviation between the observed and calculated aerated concentration were less than 5%, indicating that the calculation method was reliable for the aerated irrigation water stream. The operating pressure at 0.1 MPa and surfactant concentration of 5 mg/L was optimal combination. This research demonstrates great importance of the improvement of aeration efficiency and the air-water transfer for oxygation that can significantly reduce the cost of operation.