Effects of hydraulic shear on calcium carbonate fouling in drip irrigation systems near the wall of water flow

This study aims to determine the influence of hydrodynamic conditions on the formation of calcium carbonate fouling and the appropriate control threshold. A systematic investigation was implemented to explore the influence of hydraulic shear on the growth mechanism of calcium carbonate attached to the fouling process, together with the effect of changes in the total amount of fouling. The control threshold of hydraulic shear was also clarified on the growth of calcium carbonate fouling. This experiment was carried out to evaluate the 12 near-wall hydraulic shear forces (0-0.70 Pa) on the dry weight, crystalline phase composition, crystal size, and apparent morphology of calcium carbonate fouling in the irrigators of drip irrigation systems using a shear simulation device, the Couette-Taylor reactor, and with the aid of scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The results showed that the dry weight of calcium carbonate fouling and different shear forces were significantly (P<0.05) dominated the formation of calcium carbonate fouling. Total amount of calcium carbonate fouling showed a trend of first increasing and then decreasing with the increase of shear force. The trend of the total amount of calcium carbonate fouling with the shear force was consistent with the Gaussian growth model. The most suitable shear force was at (0.42 ± 0.02) Pa for the formation of calcium carbonate fouling. The total amount of calcium carbonate fouling was 6.2-8.5 and 1.8-3 times of 0 and 0.7 Pa, respectively. The surface roughness of calcium carbonate crystals gradually increased with the increase of shear force, while the number of fragmented crystals also increased in the crystal surface morphology. The crystal size of calcium carbonate first increased and then decreased with the increase of shear force, reaching the maximum at about 0.25 Pa. In terms of crystal composition, the proportion of calcite was tended to decrease and then increase with the increasing shear, while the aragonite showed the opposite pattern. The content of both types of materials showed a linear increase with the increasing shear at the low shear (0-0.3 Pa), whereas, a linear decrease with the increasing shear at the high shear (>0.5 Pa), with the maximum content of calcite and aragonite between 0.35-0.5 Pa. In the shear force on calcium carbonate fouling growth, there was the thicker boundary layer of mass transfer between the material wall and water flow at the low shear force. The dispersion rate of Ca2+ and CO32- ions was the dominant factor of calcium carbonate fouling deposition. The increase of shear force was accelerated the ion dispersion rate, leading to the faster fouling growth of calcium carbonate. However, the exfoliation effect of water flow on the calcium carbonate played a dominant role in the fouling growth of calcium carbonate at the high shear force. Furthermore, the stripping effect of water flow on calcium carbonate was occupied a dominant role in the growth of calcium carbonate attached fouling, as the shear force increased. The stronger stripping was found in the calcium carbonate crystal structure, as the mass transfer layer thinned, where more broken grains appeared. In turn, there was the decrease in the total amount of calcium carbonate attached fouling. Therefore, the hydraulic shear at the near-wall surface of the irrigator flow channel was controlled at 0-0.24 and 0.65-0.7 Pa, in order to reduce the formation of calcium carbonate fouling. The finding can provide a strong support to the application and promotion of highly clog-resistant irrigators for the better quality in the water drip irrigation.