| The water source in the Yellow River drip irrigation(DI)area contains a amount of sediment particles and nutrients that remains even after strict filtration treatment.This water quality condition greatly increases the clogging risk of drip emitters and induces a complicated clogging mechanism in the practical fertigation engineering.For the sake of controlling drip emitter blockage,it is of great significance to investigate the intrinsic relationship between the water quality,irrigation temperature,fertilizer characteristics,fertilizer concentration and the clogging behaviors of drip emitters in the fertigation systems.In this thesis,the inline cusp-shaped,saw-tooth,and labyrinth-channel emitters were used to perform a series of in-door anti-clogging tests with artificial muddy water(sediment concentration of 0.8 g/L,sediment particle size of<0.1 mm)as the irrigation water source.The discharge relative average(Dra)and irrigation uniformity(Cu)of the drip emitters were determined under different irrigation temperature(i.e.,an average value of 23 oC at summer and 5 oC at winter),fertilizer type(i.e.,urea:[CO(NH2)2],phosphate fertilizer:Ca(H2PO4)2·H2O,potassium fertilizer:K2SO4,compound fertilizer:[CO(NH2)2]+KH2PO4+KNO3)and fertilizer concentration(i.e.,0,0.25,0.5,0.75,1.0,5.0,10.0,20.0and 30.0 g/L).In addition,the adsorption trends of different fertilizers on sediment particles,the hydrostatic flocculation and sedimentation behaviors of sediment particles as well as the surface morphology,composition and microorganism counting of the clogging materials in the emitter channels were combined to reveal the emitter clogging mechanisms under the cross-coupling effects of physical-chemical-biological interaction and determine the reasonable threshold of fertilizer concentration.Finally,a comprehensive evaluation model was established to assess the clogging risk of drip emitters in the fertilization systems.The main conclusions were listed as follows:(1)Irrigation temperature influenced the effective irrigation frequency and the sensitivity of emitter clogging to sediment particle size and fertilizer concentration.The effective irrigation frequency in summer with an average temperature of 23 oC was 1.26?1.43 times as much as that in winter with an average temperature of 5 oC,indicating that the anti-clogging performance of drip emitters in summer was higher than that in winter.In winter,the more coarse particles with a diameter ranging from 0.067 to 0.1 mm in the irrigation water,the more sensitive of emitter clogging to fertilizer concentration was.While in summer,the more fine particles with a diameter ranging from 0 to 0.034 mm in the irrigation water,the more sensitive of emitter clogging to fertilizer concentration was.(2)Fertilizer characteristics were an important factor controlling the risks and types of drip emitter clogging in the fertigation systems.At a high fertilizer concentration of 30.0g/L,the risk of different fertilizer types for inducing emitter blockage followed the decreasing trend of phosphate fertilizer>potassium fertilizer>urea>compound fertilizer.The blockage mechanisms of drip emitters in the urea DI system were the combination of physical clogging caused by the agglomerates of molecular urea precipitates and sediment particles as well as the biological clogging induced by the growth of actinomycetes.The drip emitter clogging in the phosphate fertilizer DI system was ascribed to the apparent sedimentation phenomenon due to the high adsorption of fertilizer composition on sediment particles(i.e.,physical clogging).The blockage of drip emitters in the potassium fertilizer DI system was due to the coupling effect of chemical clogging(i.e.,the deposition of precipitates formed by the replacement of potassium ions with calcium and magnesium ions in water onto the flow channel walls of drip emitters)and biological clogging caused by the growth of bacteria.The drip emitter clogging in the compound fertilizer DI system was predominantly due to the microbial growth accelerated by the soluble nitrogen,phosphorus and potassium components(i.e.,biological clogging).(3)Fertilizer concentration had a significant influence on the emitter clogging risk and the dominant inducing factor in the fertigation systems.For the urea DI systems,when fertilizer concentration was 0.25?1.0 g/L,the risk of drip emitter clogging was low.The fine flocs formed by the flocculation of a few sediment particles could improve the following performance of sediment and thereby reduce the clogging risk of emitter.In contrast,when urea concentration was 5.0?20.0 g/L,the risk of drip emitter clogging was high.The small flocs further formed large aggregates.The apparent decrease of viscosity reduced the dragging ability and buoyancy of the irrigation water for the formed huge aggregates.Accordingly,the sediment particles were easy to precipitate in the flow channels.For the phosphorus fertilizer DI systems,the risk of drip emitter clogging was low at a fertilizer concentration of 0.25?0.75 g/L.The flocculation and settling speeds of sediment particles continuously decreased,while the dragging capacity and buoyancy of the irrigation water for the aggregates of sediment particles gradually increased within this range.Meanwhile,the total number of bacteria also decreased to the minimum.For the potassium fertilizer DI systems,a fertilizer concentration of 0.25?5.0 g/L was a low risk area for emitter clogging.The addition of potassium fertilizer inhibited the flocculation and settlement of sediment particles in the irrigation water.In addition,the sediment particles rapidly reached the flocculation equilibrium with a stable settling rate.For the compound fertilizer DI systems,the drip emitters exhibited favorable anti-clogging performance within a wide fertilizer concentration range of 0.25?20.0 g/L.The high conductivity and low viscosity of the irrigation water conjointly enhanced the fluidity and suspension of sediment particles.Reasonable concentration thresholds for different fertilizers were determined as 1.0?5.0 g/L for urea,0.75?1.0 g/L for phosphate fertilizer,5.0?10.0 g/L for potassium fertilizer and 1.0?5.0 g/L for compound fertilizer.(4)The analytic hierarchy process and fuzzy mathematics comprehensive evaluation method were used to determine the weight value and membership degree of each criterion layer and index layer.On this basis,a mathematical model was established to evaluate the clogging risk of drip emitters in the fertilization systems.By integrating the advantages of the two mathematical methods,the established model quantified the qualitative factors and made an objective assessment on the clogging complexity and risk degree of drip emitters.The results showed the weighted values of emitter channel structure,water quality,DI operation mode and irrigation environment on the clogging of drip emitters were 0.056,0.678,0.171 and 0.095,respectively.The water quality was the primary factor affecting emitter blockage.The risk of emitter clogging followed the incremental order of muddy water irrigation<compound fertigation with muddy water<urea fertigation with muddy water<potassium fertigation with muddy water<phosphate fertigation with muddy water.The evaluation results were consistent with the measured data,indicating a good applicability of the established evaluation method.The research findings of this thesis can provide important scientific basis for promoting efficient fertilization technology,reducing fertilizer cost and emitter clogging risk as well as prolonging the service life of DI equipment in the arid and semi-arid regions. |
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