Effects of aeration on the flocculation and sedimentation of cohesive sediment in irrigation water

Abstract: An integrated irrigation of water, fertilizer, and gas has been one of the most effective ways to alleviate the shortage of water resources in the reaches of the Yellow River. However, high content of cohesive sediment in the Yellow River has posed a great challenge on the water quality, as well as the flocculation and sedimentation of sediment resulting from the fertilizer solution and bubbles. This study aims to explore the influence of aeration on the flocculation and settlement of cohesive sediment during irrigation. A typical clay mineral of kaolin in the Yellow River was selected as the experimental sediment, and tap water was used as the experimental water. A water quality test proved that there was a fewer content of iron and other cations easy to form sediment flocculation. A Venturi circulation aeration device was used to fabricate the milky uniform mixture of water and air. The diameter of bubbles produced by the device was generally 15 μm, and the average gas holdup was 5.42%. The experiments were then carried out under various sediment concentrations (1, 3, 5, 7, and 10 g/L), the different types of fertilizers (potassium sulfate fertilizer, compound fertilizer, and urea), and the mass fraction of fertilizer (0, 0.2%, 0.5%, and 1.0%). Some parameters were measured, including the relative concentration of sediment, the median settling velocity of sediment, and the average particle size of the settling sediment before and after aeration. The results showed that the 5-minute aeration in the Venturi circulation device significantly promoted the flocculation and sedimentation of cohesive sediment with a concentration of 3-10 g/L (P<0.05). After the aeration, the median settling velocity of sediment increased by 48.67%-70.98% in winter, and 33.04%-57.52% in summer, while, the average particle size of settling sediment increased by 7.62%-13.95% in winter, and 6.83%-13.24% in summer, where the differences were significant (P<0.05). There was an enhanced effect of the aeration on the sediment flocculation and settlement, with the increase of initial sediment concentration. Subsequently, the promotion effect decreased with the increase of fertilizer concentration, closely relating to the type and concentration of fertilizer applied in muddy water. When the potassium sulfate fertilizer, compound fertilizer, and urea muddy water with the fertilizer mass fraction of 0.2%-1.0% were aerated, the median settling velocity of sediment increased by 20.00%-32.12%, 18.71%-130.40%, and 91.19%-170.21%, respectively, while the average particle size of settling sediment increased by 2.47%-11.06%, 6.00%-12.87%, and 13.42%-16.89%, respectively. The aeration was greatly contributed to promoting the sediment flocculation and settlement in the muddy water containing urea. The variance analysis indicated that the aeration, fertilizer concentration, fertilizer type, and the interaction presented significant effects on the median settling velocity of sediment (P<0.01). There was also an outstanding interaction among the aeration, fertilizer type, and fertilizer concentration on the sediment deposition. Prior to the aeration, there was the largest value in the median settling velocity of the sediment in the potassium sulfate fertilizer muddy water, whereas, the smallest in the compound fertilizer and urea muddy water. After aeration, there was the largest value of 0.399-0.450 mm/s in the median settling velocity of the sediment in the potassium sulfate fertilizer muddy water, whereas, the smallest of 0.288-0.330 mm/s in the compound fertilizer muddy water. Consequently, the compound fertilizer with the appropriate concentration can be expected to be preferentially selected in the integrated irrigation of water, fertilizer, and gas, further slowing down the sediment deposition in the irrigation pipe network system. The findings are of great significance to clarify the sediment deposition of the water, fertilizer, and gas integrated irrigation system. A sound theoretical basis can be also offered to take reasonable measures for the sediment deposition in the pipe networks during irrigation.