Effect and Mechanism of Micro-Nano Aeration Treatment on a Drip Irrigation Emitter Based on Groundwater

Abstract

The problem of emitter clogging has become the main obstacle restricting the application and promotion of drip irrigation technology. Studying the process of emitter clogging helps improve irrigation efficiency and save water resources. A large number of researchers have tried to solve the problem of emitter clogging from many perspectives. However, the influence of micro-nano bubbles as well as generated blockage on the clogging process of drip irrigation systems is less studied. Here, the influence of aeration on emitter clogging was studied by adding micro-nano bubbles to groundwater. Four different emitters were selected. Two treatments, micro-nano aeration and non-aeration, were set up, with a total of eight sets of experiments, running for 1500 h. The degree of emitter clogging was quantitatively characterized using the discharge ratio variation (Dra). The Christiansen uniformity coefficient (Cu) and statistical uniformity coefficient (Us) were used to evaluate the influence of emitter clogging on the performance of the drip irrigation system. Compared with the non-aeration treatment group, the Dra of aerated E1–E4 decreased by 64.74%, 54.22%, 64.20%, and 94.69% in 800 h, respectively. At the same time, the Us of the aerated E1–E4 decreased by 100%, 60.05%, 92.32%, and 100%, while the Cu of aerated E1–E4 decreased by 76.64%, 53.79%, 74.11%, and 100% compared with the unaerated group. The Cu and Us of all emitters under the aeration treatment were smaller than those comparison group. As for the blockage, the main components were typical physical blockage SiO₂ and chemical blockage CaCO₃. Most of the blockages in the non-aeration treatment group are 5–10 μm in length, while those in the aerated treatment group were generally less than 5 μm. Aeration treatment made the blockage more broken and dense and more likely to accumulate in the flow channel, obstructing the flow of water and thus intensifying the clogging process. As a result, micro-nano aeration treatment increased the risk of emitter clogging, accelerated the development of blockage in the emitter, and disturbed the uniformity of the entire drip irrigation system. This study provides a reference idea for solving the problem of blockage in drip irrigation systems.

1. Introduction

As one of the most water-scarce countries in the world [1], China’s per capita available water resources amount to only one quarter of the world average [2,3] Water shortage has become an important factor restricting the sustainable development of China. Agriculture is the largest user of freshwater by far, consuming 70–86% of available water resources in the world [4,5]. China is the world’s largest irrigator, with an irrigated area of 74 million hectares in 2019, including 68 million hectares of arable land, accounting for 50.3% of the country’s total arable land [6]. The discrepancy between the supply and demand of agricultural water is prominent. Compared with traditional border irrigation, drip irrigation usually combines emitters with a closed and low-pressure pipe system, which effectively avoids the direct contact between irrigation water and crops and human bodies, and has long been considered one of the most suitable irrigation methods [7]. Since the 1960s, when the Israelis created and developed the world’s first drip irrigation system, this advanced irrigation and fertilization technology has gradually become increasingly popular [6]. As one of the most widely used high-efficiency water-saving irrigation technologies, drip irrigation can deliver water evenly and quantitatively to the soil according to the water consumption of the root soil. On the one hand, it can promote crop growth and increase yield, and on the other hand, it can also effectively save water and improve fertilizer utilization [8,9,10]. However, the generated blockage of the emitter during the operation process restricts the safety and lifespan of drip irrigation systems. Although the quality of irrigation water meets the standards of farmland irrigation, a large number of suspended solids, dissolved salts, microorganisms, and other impurities often accumulate, settle, and adhere in the drip irrigation system, which can easily cause the clogging of emitters [11,12]. It is very important to study the causes and mechanisms of clogging in drip irrigation.

Aerated drip irrigation is a technology that involves mixing water with air and transferring it to the root zone to improve soil oxygen content, which has obvious technical advantages and good application prospects [13,14]. Numerous studies have shown that aeration can reduce the production of plankton, accelerate the degradation of pollutants in water, and purify water quality [15,16,17]. Benlouali et al. [18] found that aeration can degrade organic matter and part of the inorganic salts in the sediment, and reduce the possibility of clogging by studying the sediment produced after aeration in irrigation systems. Zhu et al. [19] designed a series of pot experiments to assess the relevant changes in the antioxidant system of tomato roots. It was found that aerated drip irrigation improved the water and nitrogen absorption efficiency of tomatoes indirectly, though affecting the antioxidant system and the growth of plant roots. By studying the effect of aeration on dripper clogging in muddy water, Niu [20] indicated that the adhesion of fine particles in muddy water at the entrance of the flow channel was the main reason for dripper clogging by aeration. In summary, research on aerated drip irrigation has made some progress.

With the development of research on aerated drip irrigation, scholars have continuously innovated in aeration methods. In recent years, many outstanding studies have been carried out regarding micro-nano bubbles. Micro-nano bubbles are bubbles with diameters between micro-bubbles and nano-bubbles, sized between 200 nm and 50 mm [21]. Compared to bubbles produced via the traditional aeration method, micro-nano bubbles have the characteristics of a large specific surface area, long retention time, high adsorption performance, and negative surface charge [22,23]. Due to these properties, Micro-nano bubbles are commonly used in pollutant removal [24], soil quality improvement [25], and hypoxia remediation [26,27]. So far, research on micro-nano aeration drip irrigation has mainly focused on promoting crop seed germination [28], improving crop quality and yield [29,30], and reducing environmental damage in the process of fertilizer production [31]. In recent years, some scholars have also carried out research on the effect of micro-nano bubbles on the clogging process of drip irrigation systems. For example, Li et al. [32] found that adding micro-nano bubbles to drip irrigation systems could not only inhibit the emitter-clogging process, but also improve the uniformity and lifespan of drip irrigation systems effectively. Tan et al. [33] explored the alleviating effect of nanobubbles on clogging in biogas slurry irrigation systems, and concluded that nanobubbles could effectively decrease biofouling, chemical precipitation, and particulate fouling adhering to the emitter. However, in actual agricultural production, the blockage of the emitter seriously restricts the lifespan and application of drip irrigation systems. Therefore, it is important to solve the problem of emitter clogging and improve the efficiency of the drip irrigation system. The combination of drip irrigation technology and micro-nano bubble technology has brought new opportunities for the construction of eco-friendly agriculture, and is also conducive to the sustainable development of the environment [32,34].

The complexity of the irrigation water determines the diversity of the clogging. Generally, there are three kinds of clogging: physical clogging, chemical clogging, and biological clogging [35]. The most common physical clogging of sediment particles in the channel is caused by flocculation and sedimentation. The structure and size of the runner are important factors affecting the emitter-clogging process. In the meantime, water quality is also an important factor affecting the emitter. The sediment content, particle size, composition, and other factors can change the resistance of suspended particles to water flow, thereby changing the following ability of suspended particles and the transportation capacity of water flow. Ultimately, the anti-blocking ability of the emitter is affected [36]. Due to the complexity of the actual test, these three conditions rarely occur individually, and the appearance of blockages is often the result of several things.

At present, the influence of micro-nano aeration on drip irrigation systems and blockage components is less studied. The study of the blockage state and composition helps reveal the clogging mechanism of the emitter under micro-nano aeration conditions, and makes clear the clogging rule of the drip irrigation system. Therefore, in this article, groundwater acted as the irrigation water, and four kinds of emitters with the same rated flow rate and pressure were selected. Two treatments of micro-nano aeration and non-aeration were designed, and the experiment ran for 1500 h. The effect of micro-nano aeration on the clogging of different emitters was studied by monitoring the dynamic change in the discharge ratio variation (Dra). The uniformity of the drip irrigation system in different periods was evaluated using the Christiansen uniformity coefficient (Cu) and statistical uniformity coefficient (Us). The composition and morphology of blockage were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy depressive spectrum (EDS). The results show that adding micro-nano bubbles to the drip irrigation system increases the risk of clogging, accelerates the clogging process, and disturbs the uniformity of the whole drip irrigation system. This study provides a reference idea for solving the blockage problem of aerated drip irrigation systems, and helps to propose targeted solutions in the future.

2. Materials and Methods

2.1. Emitter Selection

To explore the clogging mechanism of aerated drip irrigation, four commonly used emitters in the market were selected as the research objects. Before the test, the coefficient of variation (C_v) and the relationship between the pressure and flow rate of the emitters were determined (ISO, 9261) [37]. Among them, the flow–pressure relationship and the coefficient of variation C_v were calculated as follows:

q_e = kp^m

(1)

$$C_{\mathrm{v}}=100 \times \frac{{\mathrm{S}}_{\mathrm{q}}}{ \overline{\mathrm{q}}}$$

(2)

where k is the flow coefficient; p is the working pressure, MPa; m is the emitting exponent; q_e is the flow rate of the emitter, L h⁻¹; S_q is the standard deviation of the flow rate for the emitter, L h⁻¹; $\mathrm{and} \overline{\mathrm{q}}$ is the average flow rate of the emitter.

2.2. Experimental Setup

The experiment was carried out in the Quality Testing Center of Water Conservation Irrigation Equipment of the Ministry of Water Resources, and the local groundwater was selected as the irrigation water source. To study the effect of micro-nano aeration on the clogging process of different emitters, four kinds of emitters commonly used in agricultural production were selected in the experiment: an inlaid patch emitter (with pressure compensation; Toro, Xiamen, China), inlaid patch emitter (without pressure compensation; Toro, Xiamen, China), inlaid cylindrical emitter (Huawei, Shanghai, China), and above-pipe dripper (Toro, Xiamen, China) marked E1, E2, E3, E4. All the emitters have the same rated flow rate and rated pressure, in which E1 and E4 have pressure compensation functions. The hydraulic performance of each emitter was tested before the experiment, and the structural parameters and performance indexes are shown in

Table 1. Performance indexes of different emitters.

No.	Rated Flow Rate (L h−1)	Rated Pressure (kPa)	Type of Emitters	Compensation Capabilities	Emitter Space (m)	Discharge Coefficient k	Flow Index m	Cv
E1	2.0	100	Inlaid patch	Pressure compensation	0.5	0.04	1.679	1.79
E2	2.0	100	Inlaid patch	Non-pressure compensation	0.5	0.47	0.236	1.08
E3	2.0	100	Cylindrical	Non-pressure compensation	0.5	0.51	0.191	1.40
E4	2.0	100	Above-pipe dripper	Pressure compensation	0.5	0.01	2.026	2.97

.

In the experiment, to explore the influence of micro-nano bubbles on the clogging process of drip irrigation system, it is necessary to design aeration and non-aerated treatments to explore the role of micro-nano bubbles. Two test groups were established: aeration treatment (A) and non-aeration treatment (UA). The two groups were conducted simultaneously on two identical test platforms. The length, width, and height of each test platform are 10.0 m, 1.0 m, and 1.4 m, respectively. The platform was composed of three layers of iron frames, as three groups of repeated experiments. All drip irrigation belts were installed on the platform, and E1–E4 were fixed on each layer with a spacing of 0.2 m. The test equipment is shown in Figure 1. The equipment consisted of the header and the test part. The header included two series of water (120 L and 200 L in volume); a frequency conversion pump (power 1.5 kW, head 50 m, rated flow rate 3 m³ h⁻¹; Taiqiang, Taizhou, China); a filter (120 mesh); three pressure gauges (0.4 lever; Ruyi, Shanghai, China); three valves, etc. In addition, the aeration treatment group included a micro/nanobubble generator (bubble content: 84–90%, bubble diameter: 0.2–4.0 μm; Zhongjing, Shanghai, China) at the header, compared to the unaerated group.

The experiment started in November 2021 and ended in March 2022, running for 10 h every day (8:00–18:00), with a total operation time of 1500 h. To compensate for the loss of water caused by evaporation and droplet splashing, groundwater was replenished to the scale line before the test every day. The filter was cleaned at the end of the daily operation. During the operation of the drip irrigation system, the working pressure of the system was monitored with a precision pressure gauge, and it was stabilized at 0.1 MPa by controlling the valve. To collect the blockage generated by the drip irrigation system at the end of the experiment, the capillary tubes were not washed during the test.

2.3. Emitter Clogging Monitoring

To monitor the clogging process of the emitter, the flow rate was measured every 4 days after the test started. The system was steadily operated for 30 min at rated pressure before the test. Then, the cylinder was placed directly below the measuring point every 5 s. Ten minutes later, it was removed following the placement sequence and time interval. The amount of water was then measured with a cylinder. To reduce the error of the test, each test was repeated three times.

The average discharge ratio variation Dra (%) was used to quantitatively characterize the clogging degree of the emitter, and the calculation formula is as follows:

$$\mathrm{Dra}(\%)=\frac{{{\displaystyle \sum }}_{i=1}^{\mathrm{n}}{q}_i}{\mathrm{n}{q}_{new}}$$

(3)

where q_i is the flow rate of the ith emitter during the test, L h⁻¹;

q_new is the average flow rate of the emitter before the test, L h⁻¹;

n is the number of emitters.

The Dra represents the degree of reduction of the average flow rate of the emitter. The smaller Dra is, the greater the decrease in the average flow rate, and the more severe the clogging is. Regarding the problem of emitter clogging, the draft of ISO/TC 23/SC18 suggests that when the actual flow rate of the emitter is less than 75% of the design, the emitter is considered to be blocked [38,39,40]. This article refers to this standard to evaluate the clogging degree of the emitter. Therefore, Dra ≤ 75% is used as the criterion of emitter blockage in the test.

Generally, different clogging degrees can be divided according to the Dra value. When the Dra ≥ 95%, it is considered that no clogging occurred. When 75% ≤ Dra < 95%, there is considered to be slight clogging, and when 50% ≤ Dra < 75%, it is defined as clogging. When 20% ≤ Dra < 50%, it is considered severely clogged, and when Dra < 20%, complete clogging has occurred.

2.4. System Performance Evaluation

The uniformity coefficients are important performance indicators used to measure the irrigation quality of drip irrigation systems, and can also be used to evaluate the influence of emitter clogging on drip irrigation systems. The Christiansen uniformity coefficient (Cu) was used to evaluate the uniformity of the drip irrigation system herein, and can evaluate the spatial uniformity and reflect the randomness of each emitter in the drip irrigation system [41]. The influence of manufacturing deviation, topographic difference, pipe head loss, and clogging on uniformity can be comprehensively evaluated using the statistical uniformity coefficient (Us) [42]. The formulas are as follows:

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

(4)

$$Us(\%)=\left(1-\frac{S}{\overline{q}}\right)$$

(5)

where q_i represents the observed water output value of the ith emitter, mL;

$\overline{q}$ represents the sample mean;

n represents the number of measurement points.

S represents the standard deviation of the sample observations.

According to the ASAE standard EP 458 [43], when the Us value is greater than 90%, the performance of the evaluation system is “excellent”. When the value stands at 80~90%, the evaluation system performance is “good”, and the evaluation system can be defined as “qualified” at 60~80%, When the Us stands at 60% and below, the evaluation system performance is “unqualified”. Due to the serious clogging of some emitters in the experiment, the flow rate between different emitters varies greatly. Therefore, part of the Cu and Us values were negative, and when negative values were calculated, in this article, we assigned them a score of 0 [36].

2.5. Water Quality Analysis

The quality of irrigation water is also linked to the development of blockages. Some metal ions and acid ions in groundwater are prone to forming chemical clogging under suitable conditions. When the content of indicators such as total nitrogen and total phosphorus is high, it is easy for biological clogging to form. The production of this kind of clogging has a great impact on the operation of drip irrigation systems. Therefore, the monitoring of water quality can help to analyze the formation process and type of blockage. The groundwater used in the test was replaced every 2 days and stored in the tank for reserve before the test. The water stored in the water tank was tested regularly. Several parameters of groundwater were determined in the laboratory, including temperature, pH, dissolved total solids (TDS), electrical conductivity (EC), chemical oxygen demand (COD), biological oxygen demand (BOD), total salt content, total nitrogen, total phosphorus, and the content of some elements: Ca²⁺, Mg²⁺, Fe²⁺, Mn²⁺, Na⁺, CO₃²⁻, HCO₃⁻, SO₄²⁻. Among them, the temperature, pH, TDS, and EC were tested using the HANNA multi-parameter test pen (HI98129); the COD was determined using the dichromate method. The BOD was determined using the 5-day BOD dilution and seeding method. The content of total salt was measured by gravimetry (HJ/T 51-1999); the concentration of total nitrogen was determined by ultraviolet spectrophotometry (Jingke, Shanghai, China); and the content of total phosphorus was determined using the ammonium molybdate spectrophotometric method. The contents of Ca²⁺, Mg²⁺, Fe²⁺, Mn²⁺, and Na⁺ were determined with an inductively coupled plasma optical emission spectrometer (Agilent 5800 VDV, Santa Clara, CA, USA). The concentrations of CO₃²⁻ and HCO₃⁻ were determined via titration. The concentration of SO₄²⁻ was determined with an ion chromatograph (925 Compact IC Flex, Metrohm, Switzerland). The total number of bacteria was determined by Zhengzhou Pini Testing Technology Co., Ltd. (Zhengzhou, China), using the plate-counting method. In addition to daily measurements of temperature, pH, EC, and TDS, other parameters of groundwater were measured three times during the test, on the 20th, 40th, and 100th day after the test started. The average values of each parameter obtained during the water quality testing are shown in

Table 2. Water quality parameters of the test water.

	pH	TDS	EC	COD	BOD	Total Salt Content		Total Nitrogen	Total Phosphorus
		ppm	μm·cm−1	mg·L−1	mg·L−1	mg·L−1		mg·L−1	mg·L−1
UA	8.70	2541	5081	15.65	4.65	432		39.2	0.385
A	8.70	3021	6052	14.58	1.90	380		38.95	0.335
	Bacteria Number	Ca2+	Mg2+	Fe2+	Mn2+	Na+	CO32−	HCO3−	SO42−
	CFU·mL−1	mg·L−1	mg·L−1	mg·L−1	mg·L−1	mg·L−1	mg·L−1	mg·L−1	mg·L−1
UA	4.15 × 103	4099	136	0.29	0	234	21.811	357.204	280.1
A	4.75 × 103	4025	136	0.20	0	231	22.76	369.256	276.3

.

2.6. Blockage Analysis

After operating the drip irrigation system for 1500 h, the experiment was completed. All treatments that presented clogged emitters were tested. In order to reduce the external influence, the drippers were cut with sterilized scissors, placed in a prepared aseptic bag for encapsulation, and then transferred to the −80 °C refrigerator for storage immediately. To reduce the error, three samples were taken for dissection and sampling. The test samples were classified according to the type of emitter and aeration or not, labeled A-E1, UA-E1, etc., respectively. SEM was used to investigate the microscopic structure of the flow channel blockage. XRD was employed to determine the composition and crystalline phase. To further quantitatively reveal the phase composition of blockage, the element types and relative weights were determined by EDS.

2.7. Data Analysis

Excel was used to sort out the experiment data; SPSS 22.0 was used for statistical analysis. A three-factor analysis of variance was used to test whether the type of emitter, aeration, and system operating time have a significant or extremely significant impact on Dra. XRD results were analyzed using Jade 6, and the crystal phase was determined through peak identification, mineral identification, and peak intensity. Solidworks 2020 was employed to draw the experimental equipment. Based on this platform, a model of single parts was established, and then the 3D model of the target object was obtained through virtual assembly. No specific statistical methods were used in this article.

3. Results

3.1. Influence of Aeration Treatment on the Average Flow Ratio Dra of the Emitter

Since the flow rate of the emitters in each emitter treatment group quickly decreased to 0 after 1000 h, it was not necessary to continue the comparison, so the statistics were calculated to 1000 h. Figure 2 shows the change in the Dra of each type of emitter with or without aeration treatment. As can be seen from Figure 2, the clogging process of the four types of emitter was similar regardless of the aeration treatment. In the beginning, the Dra curve of all emitters decreased steadily and no clogging occurred, which indicated that the emitter clogging was slow at the initial stage. As time went by, when slight clogging occurred, the process of the emitter clogging accelerated and turned into blocked or severely blocked. As can be seen, with the aeration treatment, the Dra reduction in the four kinds of emitters was significantly greater than the comparison group after 400 h, and the difference became more and more significant as time increased. When the system ran for 560 h, all the aerated emitters were blocked obviously with Dra < 75%, while the non-aeration treatment groups had Dra > 75%. When the system ran for 800 h, the Dra of the unaerated E1–E4 was 57.00%, 62.74%, 73.27%, and 70.38%, which were generally blocked. At the same time, the Dra of E1–E4 with aeration treatment was 20.10%, 28.72%, 26.23%, and 3.74%, which were already severely blocked or completely blocked. Until 1000 h, the Dra values of aerated E1–E4 were 5.86%, 10.72%, 13.21%, and 0.33% while those of the unaerated E1–E4 were 39.3%, 26.76%, 60.27%, and 47.92%. The Dra values of aerated E1–E4 were reduced by 85.09%, 59.94%, 78.08%, and 99.31%, compared with the unaerated group, respectively. It can be seen that the aeration accelerated the clogging of the drip irrigation system and reduced the anti-clogging performance. The clogging process of the different emitters was also different. The UA-E3 has the maximum Dra value at 1000 h, indicating that the E3 has better anti-clogging performance under the unaerated condition. The Dra of E4 changed the most under-aerated or unaerated treatment, demonstrating that the aeration treatment had a greater impact on E4, while E2 changed the least, indicating that aeration had less influence on it.

It can be seen that the micro-nano aeration treatment shortened the normal operation time of the system significantly. When the experiment ran to 1000 h, the Dra of the four types of emitters of the unaerated group ranged from 26.76% to 60.27%, while the aerated group ranged from 0 to 13.21%.

Table 3. Continuous operation time of drip irrigation systems with different emitters when Dra = 75%.

No.	Continuous Operation Time When Dra = 75%
	UA	A
E1	580	540
E2	740	440
E3	760	490
E4	750	510

shows the statistics of the continuous operation time of the drip irrigation system when Dra = 75%. The operation times of the aerated group were ranked from longest to shortest as follows: E1, E4, E3, E2. Aeration treatment sped up the process of E2 and E3 from slight clogging to severe clogging. According to Table 3, aeration treatment accelerated the clogging process of the emitter, and had a particularly significant impact on E2 and E3, while E2 and E3 had no pressure compensation function.

3.2. Regression Analysis of Factors Influencing Emitter Clogging

To quantitatively analyze the statistical relationship between the Dra and other factors such as aeration treatment (AI), emitter type (E), and system operation time (T), the data analysis software SPSS 24.0 was employed to conduct a multiple linear regression analysis of relevant factors. In the regression analysis, the non-quantitative independent variables were assigned, in which the AI of the aerated treatment group was set to 1, while the unaerated treatment group was 0. E was set to 1–4 according to E1–E4. The regression equation obtained was as follows:

Dra = 144.390 − 21.536AI + 0.400E − 0.083T

(6)

The regression analysis coefficients are shown in

Table 4. Regression equation coefficient table.

Parameters	Unstandardized Coefficients		Standardized Coefficients	t	Sig.
	B	Standard Error	Beta
Constant	144.390	3.924	—	36.800	<0.001
AI	−21.536	1.861	−0.356	−11.571	<0.001
T	−0.083	0.003	−0.825	−26.801	<0.001
E	0.400	0.832	0.015	0.480	0.631

. It could be concluded that AI and T had a significant effect on the clogging of the emitter, and the significance of their T-value was p < 0.001, indicating that the two factors both had a significant influence on the change in Dra. According to Table 4, the E did not reach a significant level, which meant that the influence of this factor on emitter clogging was not obvious in this experiment. In other words, emitter type was not the main factor causing the clogging. It cannot be considered that this factor did not affect the emitter clogging.

In the multiple linear regression analysis, the standardized regression coefficients were used to compare the effect of different independent variables on the dependent variable. The larger the absolute value of the standardized coefficient was, the greater its effect on the dependent variable. According to their degree of influence on emitter clogging, the three influencing factors were ranked from most to least: T, AI, and E. In summary, when the operation time and the emitter type were determined, aeration treatment had a significant effect on the clogging of the drip irrigation system.

3.3. Influence of Drip Irrigation Emitter Clogging on System Uniformity under Micro-Nano Aeration Treatment

The influence of emitter clogging on system performance was reflected in the decrease in system uniformity. Cu and Us were used to evaluate the degree of clogging. The variation of Cu and Us was shown in Figure 3 for different treatment and emitter types with operating time. It can be seen that the change process was similar to Dra; both Cu and Us decreased with time.

At the early stage of the experiment, the Cu and Us of E1–E4 were maintained at a high level, and there was no significant difference between aerated and unaerated groups. Aeration treatment had little influence at this stage. For the four types of emitter with the same flow rate, the uniformity coefficient of the different types varied greatly. In Figure 3C, the evaluation of unaerated E1–E4 was still “excellent” until 480 h. It was not until 800 h that the “unqualified” evaluation appeared. As for the aeration treatment, after 480 h, the Us changed rapidly from “excellent” to “qualified”. 560 h later, all emitters were assessed as “unqualified”, and no longer suitable for continued use. At 800 h, the Us of the aerated E1–E4 decreased by 100%, 60.05%, 92.32%, and 100% compared with the non-aeration treatment group. At the same time, the Cu of aerated E1–E4 decreased by 76.64%, 53.79%, 74.11%, and 100% compared with the unaerated group. Among the four types of emitter, the uniformity coefficient of E4 changed the most, while E2 changed the least, which was consistent with the results of Dra. Overall, the Cu and Us of the unaerated group were better. The results further indicated that the micro-nano aeration treatment significantly reduced the system uniformity and shortened the lifespan of the drip irrigation system.

3.4. Dynamic Change of Emitter Dra and Cu

The fitting results of Dra and Cu for each type of emitter with or without aeration treatment are shown in Figure 4. With the increased operation time, the Dra and Cu decreased synchronously. The fitted slope of the E1–E4 unaerated group is 0.374~1.469, which is significantly smaller than 1.246~1.513 of the aerated group. The larger the slope, the more likely the emitter was to block suddenly. According to Figure 4, the aeration treatment made the emitters more prone to blocking. The slope of emitter E4 under both aerated and unaerated conditions was larger and similar, indicating that this type of emitter was easily blocked. The slopes of E1, E2, and E3 under aeration treatment were 1.349, 1.246, and 1.404, respectively, which were significantly higher than those of 0.374, 0.590, and 0.814 without aerated treatment, indicating that aeration had a greater influence on the clogging process of E1–E3. Micro-nano aeration on the one hand increased the sensitivity of Cu and Us to Dra; on the other hand, it reduced the uniformity of the emitter in the drip irrigation system significantly.

3.5. Blockage Analysis

The only difference between the aerated and unaerated treatment groups was whether micro-nano bubbles were added to the system. The main component of micro-nano bubbles is air, and the aerated treatment did not bring other new substances. It could be considered that the element of aerated and unaerated groundwater in the test was the same. Therefore, this article selected the blockage generated by E1 as the sample to analyze. Six drippers were selected from the aeration and non-aeration treatment groups of E1 for dissection and sampling. It was observed that each dripper was blocked with yellow-white solids, and these blockages were attached to the surface of the filter grid and the wall of the flow channel tightly, which directly caused the emitter clogging. The blockage collected from E1 under different experimental conditions was selected for further study. Figure 5A,B are the SEM images of the blockage under the aerated and unaerated treatment of E1, respectively. As can be seen from the picture, the surface of the blockage was rough, with different shapes and sizes. Most of them appeared as broken columnar or needle-shaped, with a length of 2–10 μm. Comparing Figure 5A,B, it can be concluded that whether micro-nano bubbles were added or not, there was no significant effect on the morphology of the blockage. In the meantime, microbial communities and extracellular polymers could be found in blockages. Upon further comparison, the blockage with aeration treatment was observed to have a tighter structure, more broken, and smaller volume. This may be attributed to the flow of micro-nano bubbles affecting the growth of crystals in the drip irrigation system. This was related to the physical shearing of micro-nano bubbles [33].

As shown in

Table 5. Results of energy dispersion spectroscopy analysis of blockage in E1.

	C	O	Na	Mg	Al	Si	S	Cl	Ca	Fe
UA (%)	18.21	47.78	0.53	1.77	0.14	2.32	0.30	0.49	21.82	4.01
A (%)	19.21	36.21	1.97	1.46	0.59	2.09	1.70	3.42	20.74	12.28

, there is no significant difference in EDS results between the aerated treatment group and the comparison group. The blockage was mainly composed of C, O, Mg, Si, Ca, and Fe elements, of which the three elements accounting for the largest proportion were C, O, and Ca, indicating that the main substance presented in the blockage was CaCO₃, which was a typical representative of chemical clogging substances. In addition, the compounds SiO₂ and MgCO₃ might also exist in the blockage, which needs to be further demonstrated. The presence of Fe in the spectrum was due to the residue of the iron scissors used to peel the blockage out of the runner.

After the test, the blockage in the flow channel of E1 was collected for XRD analysis, which was employed to characterize the composition and crystalline phase of the substance. As shown in Figure 6, there was no obvious difference between the XRD spectrum of the aerated treatment group and the unaerated treatment group. The peaking positions of the two groups of blockages were the same, which further indicated that the aeration treatment did not affect the composition and crystalline phase of the blockage. The main peaks located at 26.2°, 27.2°, 33.1°, 37.9°, 45.9°, 48.3°, 50.2°, and 52.4°, can be ascribed, in order, to the (111), (021), (012), (112), (221), (041), (132) and (113) crystal planes of CaCO₃ (JCPDS No. 05-0586), respectively. For SiO₂, several apparent diffraction peaks could be discovered at 26.3°, 29.5°, and 31.2°, which were consistent with the (216), (412), and (420) crystal planes of SiO₂ (JCPDS No. 51-1379). It could be observed from the XRD spectrum that the intensity of the diffraction peaks was strong, while the peaks matched well with CaCO₃, indicating that CaCO₃ was still the main component with high crystallinity. On the contrary, the diffraction peaks intensity of SiO₂ were weak and not obvious. The characteristic peak of MgCO₃ was not found in the XRD spectrum, but it cannot be concluded that these substances did not exist, possibly due to the low content and high dispersion in the blockage making them difficult to detect via XRD [44].

The main blockages of each type of emitter were CaCO₃ and SiO₂. Since the water of the experiment was local groundwater, it appeared as a yellowish suspension. Although the filter could filter out large particles, some small-sized sediment still entered the drip irrigation system to cause physical blockage. As time went on, the deposited particles increased gradually, and these blockages began to agglomerate under the promotion of micro-nano bubbles. When blockage entered the flow channel with the flow, a part of the sediment was deposited due to the sudden decrease in the flow velocity at the entrance of the flow channel. Ca²⁺ and HCO₃⁻ in water make it easy forchemical precipitation to form under weak alkaline conditions, which aggravates the clogging of the emitter.

This was consistent with the results of EDS, showing that CaCO₃ and SiO₂ were the main blockages. It could be seen that micro-nano aeration had no essential effect on the main composition of the flow channel blockage, but it had a great impact on the adhesion, generation, and development process of the blockage.

4. Discussion

Because of the clogging of drip irrigation emitter, researchers have carried out extensive research, analyzed the mechanism of clogging, and summarized many blockage prevention and control methods. At present, the most commonly used control methods include periodic acid and chlorination treatment [45,46], optimal design of the emitting flow channel [7,47], reasonable configuration of the filtration equipment [48], etc. In recent years, attention has been focused on aeration treatment. The most direct effect of micro-nano aeration on the clogging of drip irrigation emitters is reflected in the change in the Dra of the emitter. At the end of the experiment, compared with the comparison group, the Dra of E1–E4 with aeration treatment decreased significantly, and the normal operation time of the system was greatly reduced, which was consistent with the research results of some scholars [20,49].

The flocculation and precipitation process of sediment in the drip irrigation system affected the efficiency and lifespan of the whole system. The micro-nano bubbles introduced in the aeration process, increased the turbulence degree of the water flow, and promoted the collision and combination of sediment particles, thereby forming flocs. Moreover, the flocs condensed with each other during the settlement process. The volume of sediments gradually increased [50]. In the process of aerated irrigation, when sediment moved to the inlet of the dripper with the flow of water, the sediment particles were easily settled at the entrance of the flow channel due to the sudden decrease in cross-sectional area and the change of flow direction [51]. Guo Qing et al. [49] found that the smaller particles in irrigation water were easy to adhere to the flow channel and the inner wall of the capillary tube due to their strong adhesion, which is manifested by aeration increasing the deposition of sediment in the emitter. Niu et al. [20] believed that aeration made it easier for small particles of sediment in the muddy water to adhere to the flow channel inlet and accelerated the clogging of the channel inlet, which is the main reason for the rapid blockage of the dripper caused by aeration treatment.

Studying the composition of the blockage can help to propose a method to solve clogging in drip irrigation systems. Al₂O₃ and SiO₂ particles are generally considered to be typical representatives of physical clogging substances [52,53], as they originate from irrigation water and do not participate in chemical reactions. These two substances have strong adsorption properties and are easy to adhere and deposit on the flow channel and the inner wall of the capillary tube [54]. MgCO₃ and CaCO₃ are the typical representatives of chemical clogging substances. The pH of the groundwater in the experiment was 8.7, and the drip irrigation system presented a weakly alkaline environment, which contributed to the formation of chemical clogging substances. These suspended solids easily recombined with other blockages on the emitter, causing more severe clogging.

It can be seen from SEM that the blockage density of the aerated group was higher, while the unaerated group was loose. This is because aeration increases the deposition of particulate matter in groundwater. The small particles in the irrigation water were more likely to form aggregates under the action of micro-nano bubbles and the adsorption water film between the particles [55,56]. At the same time, the turbulence of the aerated water flow increased the collision frequency between particles and the density, making it difficult for water to pass through, thus aggravating the clogging of the emitter [49].

Aeration of the soil has the advantages of increasing crop yield and promoting crop seed germination [28,29]. However, when farmers choose to add air to irrigation water in agricultural production, they should also consider that aeration treatment increases the risk of clogging in drip irrigation systems. Therefore, it is recommended to flush the drip irrigation system regularly to reduce the risk of clogging caused by aeration and increase the lifespan of the system. This also reminds experts that when developing and modifying the emitter, they should take into account the risk of aeration-induced clogging of the drip irrigation system. The designed emitter should be more suitable for use in actual production. At present, there are few studies on the effects of micro-nano aeration on drip irrigation systems and blockage components. In this article, by studying the hydraulic performance of drip irrigation systems under aeration treatment, it was found that aeration treatment not only exacerbates the clogging of drip irrigation system, but also reduces the uniformity and lifespan of the system. The blockage was characterized by XRD, SEM, and EDS, and the type of blockage was determined, which provides a reference for the study of the emitter clogging in aerated drip irrigation systems and will help to propose targeted solutions in the future.

5. Conclusions

In the experiment, micro-nano aeration had similar effects on different emitters. The changes in Dra were small and relatively stable in the initial stage, and clogging did not occur at this time. Once the clogging occurred, the Dra of drip irrigation system began to decline rapidly. The Dra, Cu, and Us values of the aerated group are all smaller than those of the unaerated group, and this difference became more and more significant in the middle and later stages of system operation. The changing trend of Cu and Us over time was the same as that of Dra, indicating that micro-nano aeration aggravates this phenomenon. Aeration treatment not only exacerbates the clogging of drip irrigation systems, but also reduces the anti-clogging performance. Experimental results have shown that the aeration treatment significantly accelerated the clogging process of E4, but had little impact on E2. Micro-nano aeration treatment reduced the uniformity of the groundwater drip irrigation system, making the clogging more violent. The sensitivity of the Cu and Us to the change in the Dra was significantly increased, and the uniformity and lifespan of the system were greatly reduced. The types of blockage generated by E1–E4 were mainly physical and chemical blockage, and the main components were CaCO₃ and SiO₂. Micro-nano aeration treatment significantly exacerbated the clogging risk of the emitter based on groundwater. The blockage of the aerated group was more broken and dense, and the smaller volume made it easy for the micro-nano bubbles and water to accumulate in the flow channel, preventing the water from passing through, thus aggravating the blocking process. It is recommended to flush the drip irrigation system regularly to reduce the risk of clogging at the inlet of the flow passage under aeration treatment. Experts should also design and transform the emitter according to the actual situation, develop a drip irrigation system suitable for aerated irrigation, and improve the anti-clogging ability of the emitter.