Reducing Emitter Clogging in Drip Fertigation Systems by Magnetization Technology

Abstract

The issue of emitter clogging is a common phenomenon in drip fertigation systems, which causes uneven irrigation and fertilization. However, efficient and environmentally friendly methods are scarcely available for alleviating clogging. In the present study, we investigated the anti-fouling efficacy of three magnetic field strength levels (0, 0.4 T and 0.6 T) on emitter clogging in drip fertigation systems. Our results show that magnetized water treatment could effectively relieve emitter clogging and delay the occurrence time of clogging, which increased the average discharge variation rate (Dra) by 37.00–61.64% and decreased the dry weight (DW) by 53.00–69.29% compared with non-magnetized water treatments. X-rays were used to estimate the compositions of the main clogging substances, and the results show that phosphates were the dominant substances that induced emitter clogging. In addition, magnetized water treatment effectively reduced the contents of chemical and particulate fouling, as exhibited by a decrease in phosphates, silicate and quartz by 53.17–69.58%, 47.22–61.95% and 43.18–74.80%, respectively. In comparison, the higher strength of magnetized water treatment (0.6 T) was better in clogging control, which increased Dra and the Christiansen of uniformity (CU) by 24.64% and 43.96%, respectively, and the DW was reduced by 34.67% compared with that of 0.4 T. This study proves that magnetized water treatment is an effective, chemical-free treatment method with great potential for fouling control technology, and it is helpful for the further promotion of drip fertigation technology.

1. Introduction

Drip fertigation systems are a new technology that is recognized to improve the efficiency of water and fertilizer resource utilization, save water and fertilizer resources and increase crop yield [1,2,3,4]. However, fertilizers contain a large number of nutrient ions that tend to recombine and precipitate with ions in irrigation water, inducing emitter clogging [5]. The problem of emitter clogging not only directly determines the service life and efficiency of drip irrigation systems, but also causes a significant reduction in irrigation uniformity, which further affects crop growth and yield formation [6,7,8]. Emitter clogging has become one of the main obstacle factors limiting the development and application of drip fertigation technology.

Irrigation water’s quality characteristics are the most direct cause of emitter clogging [9], and lowering the pH of irrigation water by injecting acidic chemicals can significantly prevent the generation and accumulation of chemical precipitation, which is a common method for clogging control. However, it has the risk of acidifying the soil, acid-etching the drip irrigation system, causing damage to the crop and also has high requirements for the operator [10]. Therefore, finding a more effective, safe and environmentally friendly technology for emitter clogging control is urgently needed. Magnetization technology first emerged for industrial descaling. Studies have shown that when water passes through the magnetic field, the larger cluster structure of water molecules becomes smaller, which changes the physical and chemical properties of water, such as conductivity, pH and solubility [11], increasing the solubility of the solution to scale substances and achieving physical descaling [12]. At present, magnetization technology has been gradually extended to many fields, and it is mainly applied in agricultural production to promote seed germination [13], increase yield [14,15] and maintain soil moisture [16] and saline land improvement [17]. In recent years, some scholars have innovatively introduced it into drip irrigation systems in an attempt to solve the problem of emitter clogging. Aali [18] and Sahin [12] found that magnetized water treatment can alleviate the problem of clogging in drip irrigation systems with brackish water. In recent years, studies have also shown that magnetized water treatments can reduce emitter clogging in drip irrigation systems with brackish water [19], biogas slurry water [20], reclaimed water [21] and sand-containing water [22]. In addition, Wang [23] found that the efficiency of magnetization in easing emitter clogging is related to the type of fertilizers, and different fertilizer types were suitable for different magnetization intensities. At present, there are few studies on the mechanism of magnetization in alleviating the clogging of emitters in drip fertigation systems. Most available studies focused on irrigation water sources; therefore, the effects of magnetization on clogging control in drip fertigation systems and its mechanism are not clear.

Accordingly, the objectives of this study were to (1) clarify the effects of magnetized water treatment to effectively control clogging in drip fertigation systems; (2) clarify the effects of magnetized water treatment on the mineral composition of clogging substances; and (3) provide a theoretical basis for the prevention and control of emitter clogging in drip fertigation systems.

2. Materials and Methods

2.1. Experimental Setup

The experiment was conducted in the greenhouse of the National Precision Agriculture Demonstration Base in Xiaotangshan Town, Changping District, Beijing, China. A non-pressure compensating flat emitter was selected as the object. The drip lateral was 3 m long with 30 cm spacing in between, and altogether 10 emitters were installed in 1 drip.

Table 1. Parameters of drip irrigation emitters.

Rated Discharge (L h−1)	Flow Path Dimensions (mm)			Discharge Coefficient	Flow Index
	Length	Width	Depth
2.45	18.5	0.65	0.51	0.86	0.44

displays the primary emitter parameters. Magnetized water was generated using a magnetizer (manufacturer: Baotou Xinda, China; type: CHO) installed at the head of the drip irrigation system and connected to the main pipe.

The experimental irrigation water source was local groundwater, which is a common irrigation water source in the area.

Table 2. Water quality parameters of irrigation water.

Water Quality Parameters	pH Value	EC (ms cm−1)	TDS (mg L−1)	Ca2+ (mg L−1)	Mg2+ (mg L−1)	CO32− (mg L−1)	HCO3− (mg L−1)	SO42− (mg L−1)
Values	7.32–7.90	0.60–0.62	0.33–0.59	62.90–63.90	21.80–23.40	18.02–33.23	317.04–362.03	16.81–22.00

shows the water’s average water quality characteristics. The test fertilizer was water soluble and exhibited 64% of the total microelement nutrients (N-P₂O₅-K₂O: 15%-39%-10%, Zn + B ≥ 0.2%) (manufacturer: Yunnan Yuntianhua Co., Ltd. Yunnan, China).

The testing system’s platform was a home-designed emitter-clogging experimental platform (Figure 1). To accelerate the test process, the mass concentration of fertilizer was set at 1%. The magnetic field intensity was set at 0.4 T and 0.6 T, with the unmagnetized treatment as the control. Each treatment was an independent drip irrigation system, each system had 8 drip irrigation laterals, each lateral was 3 m long with 10 emitters attached, and all drip irrigation laterals were arranged on aluminum shelves with a length, width and height of 3 m, 1 m and 1.3 m, respectively. A PVC water collection tank was placed 30 cm below the laterals and set to a certain slope so that the water and fertilizer solution from the emitter above flows back to the tank, forming a circulating system. The fertilizer solution was replaced every 6 days. Irrigation water temperature measurements were conducted every week at 10:00 a.m. using a thermometer, and the range was 12.3–30.5 °C. The operating pressure was 0.1 MPa. The system ran for 4 h every day (am 7:00–9:00 and pm 16:00–18:00) accumulatively for 384 h.

2.2. Evaluation Parameters of Emitter Performance

The emitter’s outflow was measured at 48, 96, 144, 192, 240, 288, 336 and 384 h of the system’s operation with 3 replicates per treatment. A measuring cup was placed under the emitter along the drip irrigation laterals to measure the outflow of all emitters along the entire drip irrigation lateral for 3 min. Based on the weight of collected water, the emitters’ average discharge variation rate (Dra) was calculated as follows [24].

$$\mathrm{Dra}=\frac{{{\displaystyle \sum }}_{i=1}^n{q}_i}{nq}\times 100\%$$

In the formula, q_i is the flow rate of emitter i in L h⁻¹; q is the initial flow rate of the emitter in L h⁻¹; n is the total number of emitters installed along the lateral.

The outflow uniformity of the drip irrigation emitters can be represented by the Christiansen of uniformity (CU) and was calculated as follows [25].

$$\mathrm{CU}=100\left(1-\frac{{{\displaystyle \sum }}_{i=1}^n\left|q_i-\overline{q}\right|}{n\overline{q}}\right)$$

In the formula, q_i is the flow rate of emitter i in L h⁻¹; $\overline{q }$ is the average flow of each emitter along the lateral in L h⁻¹; n is the total number of emitters installed along the lateral.

2.3. Extraction and Testing of Clogging Substances in Emitters

2.3.1. Dry Weight of Clogging Substances

A sampling of the emitter was carried out at 192, 336 and 384 h of the system’s operation. Three emitters were intercepted from the head, middle and tail of a drip irrigation lateral. The total mass of emitters and clogging substances (DW1) was measured using a high-precision electronic balance (0.0001 g); then, the emitters were cleaned using a brush with deionized water, and the clogging substances were removed using an ultrasonic cleaner (manufacturer: Xinzhi, China; type: SB-25-12DT; frequency: 40 Hz). The cleaned emitters were dried at 50 °C and weighed to obtain the total mass (DW2). The difference between the masses of DW1 and DW2 was the dry weight (DW) of the clogged material in the emitters. The internal clogging of the emitter before and after sampling is shown in Figure 2.

2.3.2. Mineral Composition of Clogging Substances

The emitter’s dry clogging substances were then analyzed by an X-ray diffractometer (Bruker, D8-Advance, Karlsruhe) to obtain polycrystalline diffraction images. Then, the resulting patterns were analyzed by the TOPAS (Bruker Corp) software to obtain the mineral contents and components of the clogging substances.

2.4. Statistical Analysis

Data were processed and plotted using Microsoft Excel 2019 and Sigmaplot software 12.5, and they were statistically analyzed using SPSS 22.0. Analysis of variance (ANOVA) was applied to determine significant differences among treatments. Pearson’s correlation analysis was used to study the correlation of Dra, CU and DW between the magnetized water treatment and the control.

3. Results

3.1. Effect of Magnetized Water Treatment on the Hydraulic Performance of Emitters

The average discharge variation rate (Dra) and Christiansen of uniformity (CU) are widely used to evaluate the hydraulic performance of an emitter. Generally, according to the recommendations given by the International Organization for Standardization, clogging is determined (as a standard) when the actual flow rate of the emitter is less than 75% of the design [26]. As shown in Figure 3, the Dra and CU of the drip irrigation systems under different treatments show a slow decline in the early stage (0–192 h) and then a rapid decline in the late stage (240–384 h). At the end of the system’s operation, the Dra of CK, 0.4 T and 0.6 T treatments were 32.67%, 69.67% and 94.31%, respectively; the CU was 2.23%, 52.89% and 96.85%, respectively (due to the complete clogging of the CK treatment after 384 h of system operation; CU here is the data after 336 h of system operation). Magnetization significantly slowed the decline of Dra and CU (p < 0.01) and delayed the time of emitter clogging, which occurred at 240 h and 384 h of system operation for the CK and 0.4 T treatments, respectively, while the 0.6 T treatment emitter performed better at the end of system operation. Compared with the CK, at the end of system operation, the Dra of 0.4 T and 0.6 T treatments increased by 37.00% and 61.64%, respectively. With the increase in magnetic field intensity, Dra and CU significantly increased (p < 0.05). Compared with 0.4 T, the Dra and CU of 0.6 T increased by 24.64% and 43.96%, respectively, which indicated that the higher magnetic field intensity was better than that of lower magnetic field intensities in alleviating emitter clogging.

3.2. Effect of Magnetized Water Treatment on the Dry Weight of Fouling Material

The clogging substance’s dry weight (DW) is shown in Figure 4. The results show that the clogging substance’s DW gradually increased with the operation of the system. At the end of the system’s operation, the DW of CK, 0.4 T and 0.6 T treatments was 26.38, 12.40 and 8.10 g m⁻², respectively. Magnetization significantly reduced the DW (p < 0.05). Compared with CK, the DW of 0.4 T and 0.6 T decreased by 53.00% and 69.29%, respectively. Although there was no significant difference in the DW between 0.4 T and 0.6 T (p > 0.05), with the increase in magnetic field intensity, there was a tendency to decrease gradually. The DW of the 0.6 T treatment was reduced by 34.67% compared with the 0.4 T treatment.

3.3. Proportion of Mineral Components of Clogging Substances

The mineral component’s information on the fouling found inside emitters is shown in Figure 5. The results show that the clogging substances and their proportions mainly contained calcium hydrogen phosphate dihydrate (43.93–90.56%), ammonium zinc phosphate (2.06–27.64%), calcium zinc phosphate dihydrate (3.87–12.49%), calcium hydrophosphate (1.15–9.40%), alkaline feldspar (0.77–7.66%) and quartz (0.72–3.56%). While calcium hydrogen phosphate dihydrate and ammonium zinc phosphate were the main substances that induced emitter clogging (Figure 5), particle fouling (alkaline feldspar and quartz) and chemical precipitation (calcium hydrogen phosphate dihydrate, ammonium zinc phosphate, calcium zinc phosphate and calcium hydrophosphate) accounted for 2.05–11.21% and 88.79–97.95%, respectively. The effects of magnetization on the proportions of different components varied. With the increase in magnetic field intensity, calcium hydrogen phosphate dihydrate significantly decreased at 48 d and 84 d, while zinc calcium phosphate dihydrate showed an increasing trend, and the other components had no obvious change.

3.4. Mineral Components Content of Emitter

The dynamic changes in the content of mineral components of the emitter fouling are shown in Figure 6. With the operation of the system, calcium hydrogen phosphate dihydrate, calcium zinc phosphate dihydrate and calcium hydrogen phosphate content of the CK gradually increased, while zinc ammonium phosphate content decreased, and alkaline feldspar and quartz showed no significant change. The magnetized water treatment significantly affected the contents of calcium hydrogen phosphate dihydrate, zinc ammonium phosphate, calcium zinc phosphate dihydrate and calcium hydrogen phosphate (p < 0.01). As a whole, the contents show a significant decrease with the increase in magnetic field strength. Compared with CK, the magnetized water treatment showed a decrease in calcium hydrogen phosphate dihydrate, zinc ammonium phosphate and zinc calcium phosphate dihydrate contents of 4.42–15.56, 0.24–0.85 and 0.13–1.17 g m⁻², respectively. At 192 h and 336 h of the system’s operation, there was no significant difference in the content of calcium hydrogen phosphate among the treatments (p > 0.05), while the magnetized treatment decreased by 0.31–0.36 g m⁻² compared with CK at the end of the system’s operation. Similarly, alkaline feldspar and quartz contents were not significantly different among treatments at 192 h and 336 h of the system’s operation, but there was a gradual decrease with the increase in magnetic field strength at the end of the system’s operation, and the magnetized treatments were 0.15–0.20 g m⁻² and 0.09–0.16 g m⁻² lower than that of CK, respectively. On the whole, there was a tendency for the content of each mineral component to decrease with the increase in magnetic field strength between 0.4 T and 0.6 T, but there was no significant difference (p > 0.05).

The mineral components can be divided into three categories according to chemical elements: phosphate (calcium hydrogen phosphate dihydrate, ammonium zinc phosphate, calcium zinc phosphate dihydrate and calcium hydrophosphate), silicate (alkaline feldspar) and quartz. Magnetization was effective in reducing the content of all three categories; phosphate, silicate and quartz contents were reduced by 53.17–69.58%, 47.22–61.95% and 43.18–74.80% for magnetized water treatments, respectively, compared to the CK at the end of the system’s operation. Moreover, this effect was better with the increase in magnetic field strength.

4. Discussion

The emitter clogging issue of the drip irrigation system directly affects the uniformity of irrigation and fertilization, system operation efficiency and service life and in serious cases directly leads to crop yield reduction, which is one of the main obstacles limiting the popularization and application of drip fertigation technology. In this paper, we studied the effect of magnetization on the clogging of emitters by building a drip irrigation platform indoors. Our results show that the irrigation solution treated by a magnetizer significantly improved the Dra and CU of the emitter (Figure 3) and reduced the total amount of fouling in the emitter (Figure 4). Xiao [20,21] observed that in both biogas slurry drip irrigation systems and reclaimed water distribution systems, electromagnetic fields can significantly ease bio-fouling, which inhibits the growth of microorganisms, improves the Dra of the emitter and reduces the scale weight and extracellular polymer. In drip irrigation systems with brackish water, magnetizing irrigation water with electromagnetic fields and permanent magnetic fields could significantly inhibit calcium–silicon scaling in the emitter [19]. In addition, Wang [23] found that in the drip irrigation system with sandy water, magnetic fields can relieve physical clogging. This suggests that magnetized water treatments can be applied when controlling a single or compounded clogging of emitters, such as biochemical–physical clogging under multiple irrigation water sources. The possible reasons can be explained with the following: On the one hand, magnetic fields can induce voltage differences across bacterial membranes [27], and transmembrane pores can be opened under appropriate voltage differences [28], leading to an ionic imbalance and metabolic stress in microorganisms that deactivate cells [29], thus inhibiting the biological clogging of emitters. On the other hand, when irrigation water is magnetized, the original structure of the water molecular groups will be destroyed, making the original large associated water molecular groups into smaller ones, enhancing the activity of water, improving the solubility and permeability of minerals in the aqueous solution and affecting the growth and morphology of scale crystals [11], further making them less adherent to the pipe wall and easy to be washed away by the water flow [30], which is macroscopically manifested by a reduction in fouling in the emitter and an increase in Dra and CU.

Identifying the main substances that induce clogging in emitters is an important way to further understand the mechanism of clogging occurrences. In this experiment, it was found that the fouling components inside the emitters were consistent among different magnetized water treatments, mainly particulate (silicate and quartz) and chemical precipitation (phosphate) (Figure 5). The results of previous studies show that the minerals that induce the clogging of emitters are mainly particulate fouling (silicate and quartz) and chemical precipitation (carbonate and phosphate) [31,32,33]. However, no carbonate substances were found in our results, and it was mainly phosphates that induced the emitter’s clogging, with their content up to 88.79–97.95% of the total amount of mineral components. The reason may be that the water-soluble fertilizer used in the experiment is high in phosphorus (the content of P₂O₅ is 39%). After being dissolved in irrigation water, the content of phosphoric acid ions is higher and increases the probability of binding with a cation (Ca²⁺) in irrigation water. Therefore, cations in irrigation water preferentially combine with phosphate ions to form insoluble chemical precipitation. Meanwhile, it also indicates that the fertilizer type and irrigation water quality both affect the main components that induce the clogging of emitters. The results of our experiment also found that the magnetized water treatment significantly reduced the contents of phosphates, which is similar to the results of Xiao [20] in biogas slurry drip irrigation systems. The reason may be that, firstly, water molecules are subjected to the energy of the magnetic field after being magnetized, and motion is intensified, which is conducive to the movement of its chemical balance towards the direction of decomposition, resulting in smaller water molecular clusters. Secondly, when water molecules are subjected to the magnetic force of Lorenn, the hydrogen bond angle of water molecules decreases, which enhances the activity of water and increases the amount of minerals dissolved in aqueous solutions and its permeability [34], thus reducing the formation of chemical fouling.

In addition, it was also found that magnetization has no significant effect on particulate fouling (alkaline feldspar and quartz), but on the whole, particulate fouling has a tendency to reduce after magnetization (Figure 6). The results of previous studies also show that magnetization significantly reduced the content of silicate and quartz in the emitter [19,20], which may be due to the fact that a lower pH of water promotes silicate deposition [35], while magnetization can increase the pH of water (

Table 3. pH value of water solution before and after magnetization.

Parameter	CK	0.4 T	0.6 T
pH	7.50 ± 0.01 b	7.59 ± 0.01 a	7.63 ± 0.03 a

Different letters indicate significant differences (p < 0.05).

), further inhibiting silicate formation. Additionally, magnetization reduces the surface tension of water and reduces charge repulsion between solid particles (silicates and silica), thus accelerating the flocculation of particles. This flocculation, which is easily intercepted by screen filters, will avoid the deposition of silicates and quartz in drip emitters [20].

It was also found that with an increase in magnetic field intensity, the Dra and CU significantly increased (p < 0.05) (Figure 3), and the DW (Figure 4) and mineral components (Figure 6) tended to decrease, indicating that within the range of the magnetic field intensity set in this experiment, the greater the magnetic field intensity, the better the effect of reducing clogging. Liu [19] also observed that when using the same magnetic field generator (permanent magnetic field), a high magnetic intensity was better at inhibiting the clogging of the drip irrigation with brackish water than that of low magnetic intensities. However, the increasing anti-clogging effects observed in higher magnetic field strengths were not supported by all previous studies. For instance, some scholars have found that a stronger magnetic field (400 mT–1500 mT) application of PMF promotes the precipitation of aragonite polymorph [36]. Wang’s [23] results show that the best clogging control effect was achieved under 0.4 T treatment when potassium fertilizer was applied, while it was turned to 0.2 T when urea was applied. The results of Niu [22] also show that there was a peak in the effect of reducing emitter clogging as the magnetization intensity increased. This may be because the surface tension coefficient of the solution does not change linearly with an increase in magnetization [37]. This indicates that magnetization performs better on the clogging control of emitters in drip irrigation systems within a certain range. Meanwhile, it is also indicated that the clogging control effect of magnetization technology on emitters is complicated. The question of how a reasonable magnetic field intensity for different water sources can be set and the effect of magnetization on physical and biological clogging of the drip fertigation systems remains to be further studied. In general, in view of the effectiveness of magnetized water treatments in alleviating the clogging of drip fertigation emitters, as well as their ecological environmental protection and simple and safe operation, our study concluded that magnetized water treatments can be used as a green, safe and efficient clogging control method for drip fertigation systems.

5. Conclusions

The results show that emitter performance improved under magnetized water treatments. Compared with the control, the magnetized water treatment produced a higher Dra and CU and lowered the amount of accumulated clogging substances and their primary mineral components (phosphate, silicate and quartz). In addition, higher magnetic field intensities had better efficiency for reducing emitter clogging. This study revealed that magnetized water treatments are an effective and environmentally friendly method for reducing emitter clogging in drip fertigation systems.