Modeling of the residual chlorine distribution in drip irrigation system applying reclaimed water based on EPANET

Chlorination is an important practice to ensure the safety of drip irrigation system with reclaimed water application. However, the decaying process caused by chlorine reacting with irrigation water and drip irrigation pipeline will adversely affect the uniformity of residual chlorine distribution. The objective of this study was to evaluate the effect of different chlorination modes on the residual chlorine distribution in drip irrigation system using a model, which was constructed using the EPANET software package and could simulate the hydraulic performance and residual chlorine transport, attenuation and distribution in a drip irrigation system. The hydraulic and water quality parameters for the model were calibrated and validated based on the experimental data. The distribution of residual chlorine under 4 chlorination modes (chlorination concentration injected- chlorination duration of 1.3 mg/L-185 min (C1T4), 3 mg/L-80 min (C2T3), 6 mg/L-40 min (C3T2), and 8 mg/L-30 min (C4T1)) and 15 lateral lengths (10-150 m at 10-m intervals) were analyzed. The results showed that the hydraulic performance and residual chlorine distribution in the drip irrigation system were well simulated by the constructed model using the EPANET software. The index of agreement between the simulated and measured residual chlorine concentration of emitter discharge along lateral is above 0.75. The age of water discharged from emitters increased with the distance from the inlet of the lateral, while it ranged from 5.8-12.8, 1.2-16.9, and 1.1-27.3 min for the system with lateral length of 10, 80, and 150 m, respectively. The residual chlorination concentration decreased gradually from the inlet of the lateral due to the reaction of chlorine with water and pipe wall. The decay rate of the residual chlorine concentration decreased with the chlorine concentration injected. The relatively higher decay rate was attributed to the higher bulk reaction coefficient and wall reaction coefficient in the drip irrigation system with low chlorination concentration. Under a given injected chlorination concentration, the system with longer length of lateral produced higher variation of the residual chlorine concentration because of the relatively higher variation of water age. The average residual chlorine concentration of the drip irrigation system increased first and then decreased with increasing length of the lateral. A relatively lower decay rate of the average residual chlorine concentration and the residual chlorine concentration at the end of the lateral was observed for drip irrigation system with high injected chlorination concentration. For example, the ratio of average residual chlorine concentration to chlorination concentration injected decreased from 82% to 60% and from 91% to 82% when the lateral increased from 40 to 150 m for the system with 1.3 and 8.0 mg/L of chlorination concentration, respectively. Much higher deviation of the residual chlorine mass was observed than the water applied for all the chlorination modes. The deviations of the residual chlorine mass for the system with 10-m length of lateral were 22%, 24%, 36%, and 45% for the C1T4, C2T3, C3T2 and C4T1 modes, respectively. The C2T3 mode generally produced the lowest deviation and the highest Christiansen uniformity coefficient (CU) of residual chlorine mass while the length of lateral ranged from 10 to 150 m. A relatively shorter duration of chlorination (smaller than 40 min) and too low injected chlorination concentration (1.3 mg/L) probably produced higher deviation of the residual chlorine mass and thus decreased the coefficient of uniformity of the residual chlorine mass. The injected chlorination concentration of 3 mg/L with chlorination duration of 80 min was recommended, while the maximum lateral length could extend to 66 m with the coefficient of uniformity of residual chlorine mass exceeding 90%. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.