A Case Study on Settling Process in Inclined-Tube Gravity Sedimentation Tank for Drip Irrigation with the Yellow River Water

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Article A Case Study on Settling Process in Inclined-Tube Gravity Sedimentation Tank for Drip Irrigation with the Yellow River Water

Keyuan Wang , Yunkai Li, Shumei Ren and Peiling Yang *

College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, China; [email protected] (K.W.); [email protected] (Y.L.); [email protected] (S.R.) * Correspondence: [email protected]; Tel.: +86-138-1177-8078

Received: 22 April 2020; Accepted: 8 June 2020; Published: 12 June 2020

Abstract: A sedimentation tank which can remove fine sediment with low cost and high efficiency is of great significance for the wide application of drip irrigation techniques with the Yellow River water. In this study, the settling process of an inclined-tube gravity sedimentation tank which has high removal efficiency for fine particles in practice was thoroughly investigated. The sediment concentration distribution in the tank was measured by an optical back-scattering turbidimeter. The sediment thickness at the tank bottom was also measured. In addition, the size grading of sediment deposited at different positions on the tank bottom and at different heights in the inclined tubes was also measured by a laser particle size analyzer. It was found that the removal efficiency of fine sediment was 64.7–69.7% in the inclined-tube gravity sedimentation tank, which was higher than that of the sedimentation tank without inclined tubes (with a sediment removal rate of 20.7–32%). The sediment was mainly deposited in the flow adjustment area and settlement area with inclined tubes. A suitable height for the inclined tubes was 70–90 cm. In addition, the water inlet, baffle, and overflow weir in the tank negatively affected the fine sediment settling in two experiment cases. The experimental results enhance our understanding of the sedimentation characteristics in the tank, and indicate the direction for the subsequent structural optimization of the tank.

Keywords: ﬂow adjustment board; sediment concentration; sediment size grading; sediment thickness

1. Introduction The YellowRiver is the main irrigation water source of the YellowRiver irrigation area, which covers an area of up to 5.18 106 hm2. However, the high sediment concentration and the fine sediment of the × Yellow River can easily block drip irrigation emitters [1–3]. A sedimentation tank is the most traditional and common equipment used to separate water and sediment in an environmentally friendly way and it plays a very important role in ensuring the safe operation of the drip irrigation system in the Yellow River irrigation area. Many studies have been carried out to improve the sediment removal efficiency in sedimentation tanks with conventional structure. Bajcar et al. [4,5] observed the effect of different water inlet and outlet positions in a tank on sediment deposition. Kreb [6] and Goula et al. [7] found that the movement speed of solid particles toward the tank bottom was accelerated and the amount of deposited sediment was increased when a flow control baffle was set at the inlet of the tank. Tamayol et al. [8] found that setting baffles at the swirl zone of a tank had the best effect on improving the sediment removal efficiency of the tank. Shahrokhi et al. [9] improved the settling efficiency of a tank by arranging baffles at suitable positions at the bottom of the tank. They also carried out research to obtain the proper arrangement angle and quantity of baffles [10–12]. Csepai et al. [13] and Zong et al. [14] tested the improvement effect of an overflow weir on the settling efficiency of a tank.

Water 2020, 12, 1685; doi:10.3390/w12061685 www.mdpi.com/journal/water Water 2020, 12, 1685 2 of 17

Hua et al. [15], Zhang et al. [16] and Wu et al. [17] proved that the flow kinetic energy dissipation and sediment settling was increased by setting a flow adjustment board in a tank. However, the sediment size in the Yellow River is extremely fine, and the volume of sediment with a particle size of less than 100 µm accounts for more than 90% of the total sediment volume. It is difficult for a tank with a traditional structure to effectively settle fine particles in the Yellow River. Based on that, an inclined-tube gravity sedimentation tank which was mainly inspired by the ideal sedimentation theory was designed. Its most important feature was the installation of inclined tubes in the tank, which improved settling efficiency by reducing sedimentation distance and expanding the sedimentation area. At present, this kind of sedimentation tank has begun to be used in the Hetao irrigation district in Inner Mongolia, and it has worked very well in practice. A lot of experimental works have been conducted to demonstrate that tube settler had a better efficiency of sediment removal than traditional settler [18–24]. These studies used the influent and effluent water turbidity from the tubes as the main measurement for determining tube settler efficiency, but these studies all neglected the influence of the tank’s structure on the tube settler efficiency. Scholars were also concerned about the appropriate layout parameters of inclined tubes. Sarkar et al. [25] used a dimensional analysis method to study the influence of inclined-plate layout parameters (spacing, length, roughness of the plate, etc.) on the settling efficiency. Salem et al. [26] proposed that the most important requirements for achieving effective separation in a tank with inclined plates were its hydraulic performance and the equal distribution of suspensions between settler channels. Kshitija Balwan et al. [27] conducted a pilot scale model installed at Ichalkaranji municipal water treatment plant, and found that the optimum removal was observed for the length 60 cm and inclination 45◦ of inclined tubes. Subramani and Thomas [28] conducted a laboratory scale tube settler model, and the results showed that the optimum inclination angle was 55◦ for settling the flocculent particles. Demir [29] proposed an empirical equation for the inclined-plate layout angle and the settling efficiency, while Abdolzadeh et al. [30] found that the angle of inclined tubes had no effect on settling velocity in high-speed or high-Reynolds-number flow. The above literature summary shows that there is no consensus on the appropriate layout parameters of inclined tubes. Therefore, it is necessary to study the layout parameters of inclined tubes for the Yellow River water treatment. Contrast with a large number of studies focusing on the part of inclined tubes in the inclined-tube sedimentation tank, there are a few research works about the observation of the settling process in the tank. Bandrowski et al. [31] tested the removal efficiency of different kinds of suspended solids in the tank. Xie et al. [32] and Ignjatovic [33] proposed the calculation formula of sediment removal efficiency in the tank by theoretical analysis. Based on this, research was carried out to observe the settling process of the whole inclined-tube gravity sedimentation tank in this paper; this will be beneficial to the improvement of the structure and the settling efficiency of the tank. This study mainly focused on the following two aspects:

(1) To prove that the inclined-tube gravity sedimentation tank is more eﬀective than the sedimentation tank with a conventional structure on the ﬁne sediment removal. (2) To investigate the settling process of the inclined-tube gravity sedimentation tank in detail, and provide a direction for subsequent structural optimization of the tanks.

2. Experimental Setup and Procedure

2.1. Configuration of Inclined-Tube Gravity Sedimentation Tank The main structure of an inclined-tube gravity sedimentation tank includes the forebay, intake sluice, sedimentation tank, overflow weir, clear water tank and drainage hole. In the sedimentation tank, the water inlet, tank, flow adjustment board, baffle, inclined tubes and overflow weir are included (see Figures1 and2a). The working process of this tank is as follows: First, by opening the intake sluice, the water flows from the forebay into the tank. Then, the water flows through the flow adjustment board from the flow adjustment area (FAA, located between the water inlet and the flow adjustment board) to the stable settlement area (SSA, located between the flow adjustment Water 2020, 12, 1685 3 of 17 board and the baffle), making large particles deposit at the tank bottom firstly. Next, through the gap between the baffle and the tank bottom, the water flows down into the settlement area with inclined tubes (ISA, located between the baffle and the back wall of the tank, see Figure2b). By shortening the sedimentation distance and increasing its area, fine particles can be deposited on the tube wall and slide down to the tank bottom. Finally, clean water overflows from the overflow weir and flows into the clean water tank through the filter screen. The sediment on the tank bottom can be washed away by opening the drainage hole. The most significant feature of the inclined-tube gravity sedimentation tank was that the inclined tubes were installed in the latter part of the tank, so the latter part of the tank was divided into two parts which were named SSA and ISA in this study. The main function of the SSA was to make the flow velocity distributionWater 2020, 12, x uniform FOR PEER REVIEW and settle large particles, and the ISA mainly depended on3 of the 18 function that the inclinedthe baffle), tubes making could large reduce particles thedeposit settling at the tan distancek bottom and firstly. increase Next, through the settling the gap between area to improve the settlingthe e ffibaffleciency and ofthe finetank sediment.bottom, the water In contrast flows down tothe intoinclined-tube the settlement area gravity with inclined sedimentation tubes tank, the latter part(ISA, of located the conventionally between the baffle structured and the back sedimentation wall of the tank, tank see Figure was a 2b whole). By shortening part which the was just sedimentation distance and increasing its area, fine particles can be deposited on the tube wall and like the SSA with increased length, so the water flowed through this part straightly and overflows from slide down to the tank bottom. Finally, clean water overflows from the overflow weir and flows into the overflowthe weir.clean water The tank sedimentation through the filter tank screen. with The conventional sediment on the structure tank bottom may can be washed effective away for settling large sediment,by opening but thethe drainage effect for hole. settling fine sediment was very limited.

(a)

(b)

(c)

Figure 1. SchematicFigure 1. Schematic diagram diagram of the of inclined-tubethe inclined-tube gravity gravity sedimentation sedimentation tank: tank: (a) flow (a direction) ﬂow direction in the in the inclined-tube gravity sedimentation tank, (b) vertical view of the inclined-tube gravity sedimentation inclined-tube gravity sedimentation tank, (b) vertical view of the inclined-tube gravity sedimentation tank, and (c) cross-sectional view of A-A of the inclined-tube gravity sedimentation tank (all the tank, anddimensions (c) cross-sectional are in centimeters). view of A-A of the inclined-tube gravity sedimentation tank (all the dimensions are in centimeters).

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overflow weir

(a) (b) Figure 2. PhotographsFigure 2. Photographs of the inclined-tubeof the inclined-tube gravity gravity sedimentation sedimentation tank: tank: (a) ( afull) full view view of the of inclined the inclined-tube- gravity sedimentationtube gravity sedimentation tank, (b) detailed tank, (b) detail viewed of view inclined of inclined tubes. tubes.

2.2. ExperimentThe Establishment most significant feature of the inclined-tube gravity sedimentation tank was that the inclined tubes were installed in the latter part of the tank, so the latter part of the tank was divided into two In thisparts part, which two were experimental named SSA and cases ISA are in this introduced. study. The main function of the SSA was to make the flow velocity distribution uniform and settle large particles, and the ISA mainly depended on the 2.2.1. Casefunction 1 that the inclined tubes could reduce the settling distance and increase the settling area to improve the settling efficiency of fine sediment. In contrast to the inclined-tube gravity sedimentation The experimenttank, the latter was part conductedof the conventionally at the experimentstructured sedimentation station of tank China wasAgricultural a whole part which University was in the just like the SSA with increased length, so the water flowed through this part straightly and overflows Hetao irrigation district, Inner Mongolia. A counterpart sedimentation tank with a design flow of from the overflow weir. The sedimentation tank with conventional structure may be effective for 3 60 m /h, asettling length large of 10 sediment, m, height but the of 2.2effect m, for and settling anoperating fine sediment water was very depth limited. of 2 m, was established in the station. A flow adjustment board was installed 1.2 m away from the front wall of the tank in order to make the flow2.2. Experiment smooth Establishment and steady. The hole of the board was designed to be gradually larger from the bottom toIn the this top part, and two from experimental the middle cases are to theintroduced. sides, and the holes on two sides were symmetrical (see Figure3). Inclined tubes were installed at the latter part of the tank in order to ensure better 2.2.1. Case 1 sedimentation performance for the fine particles. The diameter of the hexagonal-prism inclined tubes The experiment was conducted at the experiment station of China Agricultural University in the was 50 mmHetao and irrigation the vertical district, height Inner Mongolia. of the tube A counterpart was 1 m. sedimentation There was atank 60 ◦withangle a design between flow of the 60 tube and 2 the horizontalm3/h, plane.a length Theof 10 layingm, height area of 2.2 of m, the and tubes an operating was 10 water m .depth The of height 2 m, was between established the in bottom the of the tubes andstation. that of A theflow tank adjustment was 0.8board m. was A installed baffle was 1.2 m designed away from inthe frontfront wall of theof the tubes tank in to order ensure to that the water flowedmake into the flow the bottomsmooth and of thesteady tubes. The throughhole of the the board gap was (with designed a height to be gradually of 0.4 m) larger between from the baffle the bottom to the top and from the middle to the sides, and the holes on two sides were symmetrical and the bottom(see Figure of the3). I tank.nclined Thetubes overflow were installed weir at withthe latter a length part of ofthe 2tank m wasin order set to at ensure the rear better of the tank. In this study,sedimentation the wall performance at the right for side the fine along particles the. flow The diameter was called of the thehexagonal right-prism wall andinclined the tubes opposite one Water 2020, 12, x FOR PEER REVIEW 5 of 18 was calledwas the 50 left mm wall. and the vertical height of the tube was 1 m. There was a 60° angle between the tube and the horizontal plane. The laying area of the tubes was 10 m2. The height between the bottom of the tubes and that of the tank was 0.8 m. A baffle was designed in front of the tubes to ensure that the water flowed into the bottom of the tubes through the gap (with a height of 0.4 m) between the baffle and the bottom of the tank. The overflow weir with a length of 2 m was set at the rear of the tank. In this study, the wall at the right side along the flow was called the right wall and the opposite one was called the left wall.

FigureFigure 3.3. SchematicSchematic diagramdiagram ofof thethe ﬂowflow adjustmentadjustment boardboard (all(all thethe dimensionsdimensions areare inin centimeters).centimeters).

2.2.2. Case 2 In order to compare the settling efficiency of an inclined-tube gravity sedimentation tank with that of a conventionally structured sedimentation tank, an additional experiment case was conducted on the inclined-tube gravity sedimentation tank while the inclined tubes were not installed in the tank. The experimental water came from the Yellow River trunk canal next to the experimental station. Due to the fluctuation of sediment concentration in the trunk canal during the experiment, an appropriate amount of riverbed sediment was added to the forebay to ensure a constant sediment concentration of 3–3.5 kg/m3 during the experiment. A 3D coordinate axis was used to clearly express the positional information in the tank, where the x, y, and z axes denoted the flow direction (longitudinal), direction from the right wall to the left wall (transverse), and that from the bottom to the top of the tank (vertical). The origin of coordinates was represented by O (see Figure 1b,c).

2.3. Measurement The sediment concentration at different locations in the sedimentation tank was measured in two experiment cases, while the sediment thickness, size grading of sediment deposited at the tank bottom and that at the inclined-tube walls were only measured in case 1. In order to verify the settling efficiency of the inclined-tube gravity sedimentation tank, the sediment concentration and size grading in the test water source were measured once a week during the experiment.

2.3.1. Measurement Locations Sediment concentration measurement was carried out at 19 longitudinal sections (named sections 1–19) from upstream to downstream over the range x = 20–980 cm. The longitudinal spacing between adjacent sections was 60 cm. An additional section was set up at a distance of 20 cm in front of the flow adjustment board and the baffle, separately (see Figure 4a). The details of the measurement locations are illustrated in Figure 4b, where 36 sediment concentration samples were collected at each section. The measurement points are represented by the symbol “+”. Height in the range of 0.8–1.8 m was occupied by inclined tubes in the ISA in case 1, so there were only two top- layer measurement points of sediment concentration below the free surface measured in the ISA. In case 2, the sediment concentration of all measurement locations in the ISA was measured. Sediment thickness measurement at the tank bottom was also carried out at all sections. The measurement locations are presented in Figure 4b and are represented by the symbol “○”. The sediment size grading measurement locations are as follows. The measurement locations at the tank bottom are illustrated in Figure 4b, represented by red “×” symbols. The locations were measured at all sections. The measurement locations of inclined tubes are shown in Figure 4c. There

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2.2.2. Case 2 In order to compare the settling efficiency of an inclined-tube gravity sedimentation tank with that of a conventionally structured sedimentation tank, an additional experiment case was conducted on the inclined-tube gravity sedimentation tank while the inclined tubes were not installed in the tank. The experimental water came from the Yellow River trunk canal next to the experimental station. Due to the fluctuation of sediment concentration in the trunk canal during the experiment, an appropriate amount of riverbed sediment was added to the forebay to ensure a constant sediment concentration of 3–3.5 kg/m3 during the experiment. A 3D coordinate axis was used to clearly express the positional information in the tank, where the x, y, and z axes denoted the flow direction (longitudinal), direction from the right wall to the left wall (transverse), and that from the bottom to the top of the tank (vertical). The origin of coordinates was represented by O (see Figure1b,c).

2.3. Measurement The sediment concentration at diﬀerent locations in the sedimentation tank was measured in two experiment cases, while the sediment thickness, size grading of sediment deposited at the tank bottom and that at the inclined-tube walls were only measured in case 1. In order to verify the settling eﬃciency of the inclined-tube gravity sedimentation tank, the sediment concentration and size grading in the test water source were measured once a week during the experiment.

2.3.1. Measurement Locations Sediment concentration measurement was carried out at 19 longitudinal sections (named sections 1–19) from upstream to downstream over the range x = 20–980 cm. The longitudinal spacing between adjacent sections was 60 cm. An additional section was set up at a distance of 20 cm in front of the flow adjustment board and the baffle, separately (see Figure4a). The details of the measurement locations are illustrated in Figure4b, where 36 sediment concentration samples were collected at each section. The measurement points are represented by the symbol “+”. Height in the range of 0.8–1.8 m was occupied by inclined tubes in the ISA in case 1, so there were only two top-layer measurement points of sediment concentration below the free surface measured in the ISA. In case 2, the sediment concentration of all measurement locations in the ISA was measured. Sediment thickness measurement at the tank bottom was also carried out at all sections. The measurement locations are presented in Figure4b and are represented by the symbol “ ”. The sediment size grading measurement locations are as follows. The measurement locations# at the tank bottom are illustrated in Figure4b, represented by red “ ” symbols. The locations were measured × at all sections. The measurement locations of inclined tubes are shown in Figure4c. There were eight inclined tubes measured in total, and they were divided into two rows along the x direction. Two rows of inclined tubes were each 40 cm away from the side walls. The four inclined tubes close to the left wall of the tank were named L1–L4 along the x direction. The other four inclined tubes close to the right wall were named R1–R4 along the x direction. In every measured tube, five locations of sediment were measured, which were marked as I–V from the bottom to the top of the inclined tubes.

2.3.2. Measurement Methods An optical back-scattering turbidimeter (OBS3+) was used for the sediment concentration measurement. It was ﬁxed on a platform which could freely move along the tank longitudinally, transversely, and vertically, so it could reach every measurement location in the tank. The quantity of suspended sediment was acquired by receiving the scattered light of infrared radiation. Measurement was repeated three times at every measurement location, and the average value was taken as the measurement result to ensure measurement accuracy. The correlativity between the turbidity and Water 2020, 12, 1685 6 of 17 concentrationWater 2020, 1 of2, xthe FOR suspended PEER REVIEWsediment was established as follows. The measurement accuracy6 of 18 was 4% were10 mg eight/L: inclined tubes measured in total, and they were divided into two rows along the x ± ± 5 2 β = 3.5344 10− α + 0.6975α 27.107 (1) direction. Two rows of inclined tubes were× each 40 cm away from− the side walls. The four inclined tubes close to the left wall of the tank were named7 2 L1–L4 along the x direction. The other four inclined γ = 3 10− β + 0.0011β + 0.1228 (2) tubes close to the right wall were named× R1–R4 along the x direction. In every measured tube, five wherelocationsα refers of the sediment value measured were measured, by the which OBS3 +were, β refers marked the as turbidity I–V from value, the bottom and γ torefers the top the of sediment the concentrationinclined tubes. value.

(a)

(b)

(c) Figure 4. Measurement locations: (a) locations of measurement sections, (b) measurement locations arrangement in each section, and (c) measurement locations of inclined tubes (all the dimensions are in centimeters). Water 2020, 12, 1685 7 of 17

A laser particle size analyzer (Mastersizer 3000) was used to measure the size grading of deposited sediment. The grading data were acquired by receiving and measuring the energy distribution of the scattered light. The particle size measurement range of the Mastersizer 3000 was 0.01–3500 µm, and the measurement error was less than 1%. In order to ensure measurement accuracy, each sample was measured five times repeatedly, and the average value was taken as the measurement result. The sampling method of sediment samples was as follows: when the experiment case ended and the water in the tank was drained, the inclined tubes in the tank were taken out firstly, then deposited sediment on the measured tubes wall (shown in Figure4c) was sampled separately according to different heights. Then, sediment samples at different measurement locations on the tank bottom were collected, and sediment samples were finally taken back to the laboratory for measurement. The sediment at different measurement locations was dried and then screened for 2 mm to remove impurities such as grass roots in sediment samples. Screened soil samples (0.2 g) at each position would be tested by following the operation steps of the instrument. In fact, actual particles (especially solid particles) are not usually spherical. In order to describe the size of irregularly shaped particles by particle size, the concept of equivalent diameter was introduced in the particle size measurement. In this experiment, particle size measured by the laser method was called the equivalent volume diameter, that is, the diameter of a sphere, the volume of which was equal to the volume of the particle. The particle diameter was automatically calculated by the software associated with the particle size analyzer. The sediment thickness at the tank bottom was directly measured by a ruler when the experiment case ended and the water in the tank was drained.

2.3.3. Calculation Method The surface measurement points in the ISA of section y = 180 cm is the closest to the overflow weir, so the sediment concentration in this area is close to that in the overflow. In this study, the average sediment concentration of these measurement points was regarded as the sediment concentration of overflow. Therefore, the sediment removal rate could be calculated by the following formula.

Ctw Cmp Rs = − (3) Ctw where Rs refers to the sediment removal rate, Ctw refers to the sediment concentration of the test water source, and Cmp refers to the average sediment concentration of the surface measurement points in the ISA of section y = 180 cm.

3. Result and Analysis

3.1. Sediment Concentration Distribution

3.1.1. Case 1 The distribution of sediment concentration at different locations in the sedimentation tank is shown in Figure5. Six y sections of y = 20, 40, 80, 120, 160 and 180 cm were analyzed in the plots. The sediment concentration of each section showed the same trend along the x direction. The concentration at the same height of the same y section gradually decreased along the x direction and finally reached about 1 kg/m3 when flowing through the overflow weir. In the FAA, the sediment concentration at the lower part of the tank (z = 20 and 60 cm) was large (8–10 kg/m3), in the middle (z = 100 and 140 cm) it was lower than at the bottom (4–7 kg/m3), and the concentration on the surface (z = 180 and 190 cm) was similar to that of the water source (3–4 kg/m3). The sediment concentration in the middle and lower parts of the FAA was larger than that of the water source. This was caused by a large amount of sediment settling down at the tank bottom of the FAA in a long running time. With a large flow velocity, the flow from the water inlet washed away the Water 2020, 12, 1685 8 of 17 sediment deposited on the bottom, and the sediment was turbulently diffused upward with the flow in the FAA. In the SSA, the distribution of sediment concentration along the x direction was different at different heights. The concentration at the lower part (z = 20 and 60 cm) rapidly decreased in the front part (x = 120–300 cm) of the SSA, and in the latter part it continued to decrease but at a slower Water 2020, 12, x FOR PEER REVIEW 8 of 18 rate. Because the flow velocity decreased after passing through the flow adjustment board, large-sized sediment3. particles Result and among Analysis the re-suspended sediment from the FAA bottom quickly settled to the tank bottom again. However, the settling velocity of fine sediment was small, leading to a slowdown 3.1. Sediment Concentration Distribution decreased rate of the sediment concentration in the latter part of the SSA. The sediment concentration in the middle3.1.1. Case flow 1 (z = 100 and 140 cm) slowly decreased in the SSA as a result of the sediment particles in theThe middle distribution flow beingof sediment smaller concentration than those at different at the lower locations part. in Thethe sedimentation variation curve tank ofis sediment concentrationshown in in the Figure upper 5. Six flow y sections (z = 180of y = and 20, 40, 190 80, cm) 120, was160 and smooth 180 cm in were the analyzed SSA, which in the plots. was causedThe by the fact that thesediment sediment concentration particles of suspended each section in showed the upper the sameflow were trend very along small; the x direction. therefore, The the effect of concentration at the same height of the same y section gradually decreased along the x direction and flow turbulence on sediment settling was obvious. finally reached about 1 kg/m3 when flowing through the overflow weir.

(a) (b)

(e) (f)

Figure 5. DistributionFigure 5. Distribution of sediment of sediment concentration concentration at di atﬀ differenterent measurement measurement locations (case (case 1): 1): (a) (a) section of y = 20 cm,section (b )of section y = 20 cm, of y(b=) section40 cm, of (yc )= section40 cm, (c of) sectiony = 80 of cm,y = 80 (d cm,) section (d) section of yof= y 120= 120 cm, cm, ((ee)) section of section of y = 160 cm, and (f) section of y = 180 cm. y = 160 cm, and (f) section of y = 180 cm. Water 2020, 12, 1685 9 of 17

In the ISA, only the sediment concentration in the upper part was measured. Compared with the concentration of the upper part of the end of the SSA, that of the upper part of the ISA was significantly decreased, from 2.5 down to 1 kg/m3. The sediment concentration curve of the upper layer of the ISA was smoothly distributed along the x direction, indicating that the settling efficiency of different locations in the ISA was similar. The sediment concentration at a height of z = 180 cm was slightly larger than z = 190 cm. Among different y sections, the concentration of y = 180 cm was greater than other sections because that section was the closest to the overflow weir, and the overflow suction affected the flow pattern in the area, resulting in an increase in sediment concentration.

3.1.2. Case 2 Similar to the case 1, the distribution of sediment concentration at diﬀerent locations in the tank of six y sections of y = 20, 40, 80, 120, 160 and 180 cm was also analyzed (see Figure6). The sediment concentrationWater 20 of20 each, 12, x FOR section PEER REVIEW showed the same trend along the x direction. 10 of 18

(a) (b)

(e) (f)

Figure 6. DistributionFigure 6. Distribution of sediment of sediment concentration concentration at di atﬀ differenterent measurement measurement locations (case (case 2): 2): (a) (a) section section of y = 20 cm, (b) section of y = 40 cm, (c) section of y = 80 cm, (d) section of y = 120 cm, (e) of y = 20 cm, (b) section of y = 40 cm, (c) section of y = 80 cm, (d) section of y = 120 cm, (e) section of section of y = 160 cm, and (f) section of y = 180 cm. y = 160 cm, and (f) section of y = 180 cm. The sediment removal rates of the two cases were obtained (see Table 1) according to the calculation of the test water source’s sediment concentration and that of the surface measurement point in the ISA of section y = 180 cm.

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In the FAA and the SSA, the sediment concentration distribution of case 2 is similar to that of case 1. In case 2, inclined tubes were not installed in the tank, so the sediment concentration distribution in the middle and lower part of the ISA was obtained. O1 In the lower part of the ISA: The flow velocity became large when the water in the SSA flowed to the ISA from the gap between the baffle and the bottom of the tank, washing away the sediment that settled in the front section of the ISA. The sediment concentration of lower flow increased significantly in the front end of the ISA, and then decreased along the x direction, while only slightly increased at the end of the ISA, which was caused by the swirling flow in the triangle area formed by the tank bottom and the back wall. O2 In the middle part of the ISA: Sediment concentration decreased slowly along the x direction. O3 In the upper part of the ISA: Sediment concentration almost did not decrease along the x direction, which indicated that it was difficult to remove the fine sediment in the Yellow River water only by prolonging the length of the sedimentation tank. The sediment removal rates of the two cases were obtained (see Table1) according to the calculation of the test water source’s sediment concentration and that of the surface measurement point in the ISA of section y = 180 cm.

Water 2020, 12, x FOR PEER REVIEW 11 of 18 Table 1. The sediment removal rate of two cases. Table 1. The sediment removal rate of two cases. Case Ctw Cmp Rs Case Case 1 3–3.5 kg/m3 1.06 kg/m3 64.7–69.7% Case 1 3–3.5 kg/m3 1.06 kg/m3 64.7%-69.7% Case 2 3–3.5 kg/m3 2.38 kg/m3 20.7–32% Case 2 3–3.5 kg/m3 2.38 kg/m3 20.7%-32%

The sedimentThe sediment removal removal rate rate of of case case 1 1 was was 64.7–69.7%, 64.7%-69.7% which, whichwas was significantlysignificantly higher higher than than that that of case 2.of Sedimentcase 2. Sediment concentration concentration almost almost did not did decrease not decrease along along the x thedirection x direction in the in ISA the ISAwhen when inclined tubesinclined were not tubes installed were not in installed the tank. in the This tank. showed This showed that the that inclined the inclined tubes tubes had had a great a great eff ecteffect on the removalon theof fine removal sediment, of fine and sediment, the inclined-tube and the inclined gravity-tube sedimentation gravity sedimentation tank had great tank advantages had great over advantages over the sedimentation tank with a conventional structure in terms of the removal of fine the sedimentation tank with a conventional structure in terms of the removal of fine sediment. sediment. 3.2. Sediment Thickness at the Bottom of the Tank 3.2. Sediment Thickness at the Bottom of the Tank The sedimentThe sediment thickness thickness of of six sixy ysections sections ofof yy = 20,20, 40, 40, 80, 80, 120, 120, 160 160 and and 180 180 cm cmand and the average the average valuevalue of the of sixthe sections six sections of sedimentof sediment deposition deposition thickness thickness alongalong the the xx directiondirection are are shown shown in inFigure Figure 7. The sediment7. The sediment thickness thickness at the at bottom the bottom of the of the tank tank was was large large at at both both ends ends andand small in in the the middle middle..

FigureFigure 7. Sediment 7. Sediment thickness thickness ofof didifferentﬀerent sections sections at at the the bottom bottom of the of thetank. tank.

In theIn FAA,the FAA, the the thickness thickness was was large, large, up up to to 4–84–8 cm.cm. During During the the experiment, experiment, sediment sediment was wasadded added into theinto test the watertest water source source to to ensure ensure that that the the sediment sediment concentration concentration was was maintained maintained at 3– at3.5 3–3.5 kg/m kg3. /m3. The addedThe added sediment sediment was was easily easily deposited deposited at atthe the bottombottom of of the the FAA FAA and and the the large large particles particles in the in flow the ﬂow also settled in the FAA. Therefore, the sediment thickness in this area was significantly larger than also settled in the FAA. Therefore, the sediment thickness in this area was signiﬁcantly larger than that in other areas. In y = 20 and 40 cm sections, the sediment thickness was small at the location of x = 20 cm, which was caused by the high rate of flow from the water inlet washing away the sediment. In the SSA, the thickness in the front part (x = 120–320 cm) was 2 cm and that in the latter part (x = 320–480 cm) was 1 cm, which was identical to the variation of sediment concentration in the tank’s lower layer of case 1. This was caused by the large-sized particles settling in the front part of the SSA, while the small particles settled slowly in the latter part. As for the ISA, the sediment thickness at the bottom of the tank was small just in the front part (x = 480–560 cm), while it was large in other parts (x = 560–980 cm). This was also due to the increase in the flow velocity near the baffle, washing away the sediment that settled in the front section of the ISA. Then, the flow with sediment moved up into the inclined tubes. The fine sediment in the flow easily settled on the tube walls, and then slid down to the bottom of the tank, so the sediment thickness became large in the latter part of the ISA. At the same time, the variation of the sediment thickness in different y sections along the x direction was analyzed. It was found that the thickness of the y = 20 and 180 cm sections in the SSA and the ISA was smaller than that of other sections, indicating that a number of swirls were formed

Water 2020, 12, 1685 11 of 17 that in other areas. In y = 20 and 40 cm sections, the sediment thickness was small at the location of x = 20 cm, which was caused by the high rate of flow from the water inlet washing away the sediment. In the SSA, the thickness in the front part (x = 120–320 cm) was 2 cm and that in the latter part (x = 320–480 cm) was 1 cm, which was identical to the variation of sediment concentration in the tank’s lower layer of case 1. This was caused by the large-sized particles settling in the front part of the SSA, while the small particles settled slowly in the latter part. AsWater for 2020 the, 12, ISA, x FOR the PEER sediment REVIEW thickness at the bottom of the tank was small just in the12 front of 18 part (x = 480–560 cm), while it was large in other parts (x = 560–980 cm). This was also due to the increase in theat flow the corner velocity between near thethe basideffl walle, washing and the awaybottom the of sedimentthe tank. As that a result, settled the in sediment the front in sectionthis area of the ISA. Then,was washed the flow away with by the sediment flow. moved up into the inclined tubes. The fine sediment in the flow easily3.3. settled Sediment on the Size tube Grading walls, at Different and then Lo slidcations down in the to Tank the bottom of the tank, so the sediment thickness became large in the latter part of the ISA. At theDuring same the time, experiment, the variation the sediment of the size sediment grading in thickness the test water in di wasfferent measuredy sections once a along week. the x Water 20M20ost, 1 of2, xthe FOR sediment PEER REVIEW particle size was less than 100 μm (see Figure 8). The sediment size grading of12 of 18 direction was analyzed. It was found that the thickness of the y = 20 and 180 cm sections in the SSA test water was similar to that of the Yellow River. andat the the corner ISA was between smaller the than side that wall of and other the sections, bottom indicatingof the tank. that As a a number result, the of swirlssediment were in formed this area at thewas corner washed between away by the the side flow. wall and the bottom of the tank. As a result, the sediment in this area was washed away by the flow. 3.3. Sediment Size Grading at Different Locations in the Tank 3.3. Sediment Size Grading at Different Locations in the Tank During the experiment, the sediment size grading in the test water was measured once a week. MostDuring of the sediment the experiment, particle the size sediment was less sizethan grading 100 μm in (see the Figure test water 8). The was sediment measured size once grading a week. of µ Mosttest water of the was sediment similar particle to that of size the was Yellow less thanRiver. 100 m (see Figure8). The sediment size grading of test water was similar to that of the Yellow River.

Figure 8. Percentage graph of sediment particle size accumulation of test water.

3.3.1. Sediment Size Grading at Different Locations on the Tank Bottom The distribution curve of the sediment size grading along the x direction when y = 20, 100 and 180 cm is shown in Figure 9. Nine characteristic particle sizes of d10, d20, d30, d40, d50, d60, d70, d80 and d90 were analyzed. The variation trends of the sediment size grading in the three y sections were similar. In the FAA, except for the particle size of d90 being different when y = 100 cm, all characteristic

particle sizes changes were distributed as a “crest” in all three y sections; i.e., the particle size was Figure 8. Percentage graph of sediment particle size accumulation of test water. large at x =Figure 80 cm, 8.while Percentage it was small graph at of x sediment= 20 and 100 particle cm. Meanwhile, size accumulation the characteristic of test water. particle sizes 3.3.1.in Sediment this area were Size Gradingsignificantly at Di largerﬀerent than Locations that of the on test the water Tank source. Bottom It also verified the inference 3.3.1.above, Sediment namely, Size that Grading the large at- sizedDifferent sediment Locations with poor on mobilitythe Tank of Bottom the test water source settled at the Thebottom distribution in the FAA. curve The sediment of the sediment size grading size at grading x = 100 alongcm was the largerx direction than that when at x =y 20= cm20, as 100 a and The distribution curve of the sediment size grading along the x direction when y = 20, 100 and 180 cmresult is shownof the flow in Figurevelocity9 at. Ninethe water characteristic inlet being large particle. The flow sizes stroke of d10 on ,thed20 flow, d30 adjustment, d40, d50 board,, d60, d70, d80180 andcmthen isd90 formedshownwere a in analyzed.vortex Figure in the9. The Nine corner variation characteristic between trends the board particle of the and sediment sizes the bottom of d10 size of, d20 gradingthe, tank,d30, ind40and the, washedd50 three, d60 awayy, sectionsd70 , d80 wereand d90the similar. sedimentwere analyzed. at the bottom, The variation in which trends the small of -thesized sediment particles sizewere grading re-suspended in the in three the flow. y sections were similar. In the FAA, except for the particle size of d90 being different when y = 100 cm, all characteristic particle sizes changes were distributed as a “crest” in all three y sections; i.e., the particle size was large at x = 80 cm, while it was small at x = 20 and 100 cm. Meanwhile, the characteristic particle sizes in this area were significantly larger than that of the test water source. It also verified the inference above, namely, that the large-sized sediment with poor mobility of the test water source settled at the bottom in the FAA. The sediment size grading at x = 100 cm was larger than that at x = 20 cm as a result of the flow velocity at the water inlet being large. The flow stroke on the flow adjustment board, then formed a vortex in the corner between the board and the bottom of the tank, and washed away the sediment at the bottom, in which the small-sized particles were re-suspended in the flow. (a) (b) Figure 9. Cont.

(a) (b)

Water 20202020,, 1212,, 1685x FOR PEER REVIEW 1312 of of 1718

(c)

Figure 9. SedimentSediment size size grading grading along along the the x xdirectiondirection at atthe the tank tank bottom: bottom: (a) (sectiona) section of y of = y20= cm,20 cm,(b) (sectionb) section of y of = y100= 100cm, cm,and and (c) section (c) section of yof = 180y = 180cm. cm.

In the SSA, FAA, the except distribution for the particle of sediment size ofsized90 gradingbeing diinff eacherent y whensectiony =was100 completely cm, all characteristic consistent. particleEach characteristic sizes changes particle were distributedsize decreased as a along “crest” the in x all direction; three y sections; it decreased i.e., the fast particle in the front size was part large (x = at120x–=32080 cm) cm, but while slowly it was in small the latter at x =part20 and(x = 100320– cm.440 Meanwhile,cm), and it increased the characteristic significantly particle only sizes at the in thisend area(x = 440 were–460 significantly cm). Because larger when than the that water of the flowed test water into source.the SSA, It alsothe velocity verified distribution the inference tended above, namely,to be uniform. that the In large-sized the front part, sediment the large with sediment poor mobility quickly of thesettled test to water the bottom source settledof the tank, at the whil bottome in inthe the latter FAA. part, The the sediment sediment size were grading small, at leadingx = 100 to cm slow was settling. larger than At the that end, at x the= 20 characteristic cm as a result particle of the flowsizes velocityincreased at thesignificantly water inlet as being a result large. of Thethe increase flow stroke in onthe theflow flow velocity adjustment near the board, baffle then. The formed fine asediment vortex in at the the corner bottom between of the tank the board was wa andshed the bottomaway by of the the flow tank, and and re washed-suspended. away the sediment at the bottom,In the ISA, in which the characteristic the small-sized particle particles size weres increased re-suspended in the front in the part flow. (x = 500–680 cm), while it decreasedIn the in SSA, the thelatter distribution part (x = 680 of sediment–920 cm). size At the grading end ( inx = each 920–y980section cm), was it increased completely significantly. consistent. EachDue to characteristic the fact that particlethe fine sizesediment decreased at the along bottom the ofx direction;the tank in it the decreased end of the fast SSA in the was front washed part (awayx = 120–320 by the cm)high but-speed slowly flow in theand latter moved part to (x =the320–440 bottom cm), in the and front it increased of the significantlyISA, the characteristic only at the endparticle (x = size440–460s in this cm). area Because were whensmaller. the The water latter flowed part intowas thenot SSA, affected the velocityby the high distribution-speed flow tended at the to beend uniform. of the SSA. In the The front fine part, sediment the large in the sediment flow was quickly quickly settled deposited to the bottomon the inclined of the tank,-tube while walls in and the latterslid to part, the the tank sediment bottom. were The sedimentsmall, leading particle to slow size settling. in the flow At the gradua end,lly the decreased characteristic alon particleg the x sizesdirection increased, so thesignificantly characteristic as particle a result size ofs the in the increase latter inpart the of flow the ISA velocity gradually near thedecreased, baffle. Theand that fine sedimentat the end atincreased the bottom significantly, of the tank which was washed was caused away by by the the vortex flow andat the re-suspended. corner formed by the bottom and theIn theback ISA, wall the of characteristic the tank, washing particle away sizes the increased sediment in here the and front re part-suspending (x = 500–680 the fine cm), sediment while it decreasedinto the flow. in the latter part (x = 680–920 cm). At the end (x = 920–980 cm), it increased significantly. Due to the fact that the fine sediment at the bottom of the tank in the end of the SSA was washed away by3.3.2. the Sediment high-speed Size flow Grading and movedat Different to the Locations bottom inin thethe frontInclined of the Tubes ISA, the characteristic particle sizes in this area were smaller. The latter part was not affected by the high-speed flow at the end of Figure 10 shows the sediment size grading distribution of the inclined tubes at different locations the SSA. The fine sediment in the flow was quickly deposited on the inclined-tube walls and slid to and heights. According to the different locations of the inclined tubes in the sedimentation tank, eight the tank bottom. The sediment particle size in the flow gradually decreased along the x direction, parts of L1–L4 and R1–R4 were analyzed in plots. In all measured tubes, the characteristic particle so the characteristic particle sizes in the latter part of the ISA gradually decreased, and that at the sizes at heights I (10 cm), II (30 cm), and III (50 cm) were significantly different, while the differences end increased significantly, which was caused by the vortex at the corner formed by the bottom and of those at heights IV (70 cm) and V (90 cm) were not obvious. the back wall of the tank, washing away the sediment here and re-suspending the fine sediment into L1, L2, R1 and R2 were in the front part of the ISA. In each tube, the characteristic particle sizes the flow. at heights I (10 cm), II (30 cm) and III (50 cm) were significantly larger than those at heights IV (70 3.3.2.cm) and Sediment V (90 cm). Size On Grading the one at hand, Different the Locationssediment inat thethe Inclinedbottom in Tubes the end of the SSA was washed away by the high-speed water, re-suspended in the lower flow of the ISA, and entered with the flow Figure 10 shows the sediment size grading distribution of the inclined tubes at different locations into the inclined tubes in the front part of the ISA. On the other hand, the sediment particle size and heights. According to the different locations of the inclined tubes in the sedimentation tank, eight gradually decreased along the x direction, so most large particles entered into the tubes in the front parts of L1–L4 and R1–R4 were analyzed in plots. In all measured tubes, the characteristic particle part of the ISA. sizes at heights I (10 cm), II (30 cm), and III (50 cm) were significantly different, while the differences of L3, L4, R3 and R4 were in the latter part of the ISA. The sediment particle size in the lower part those at heights IV (70 cm) and V (90 cm) were not obvious. of the flow was small, so in each tube, the differences between the characteristic particle sizes at different heights were small. The characteristic particle sizes of L3 and L4 were slightly larger than

Water 2020, 12, x FOR PEER REVIEW 14 of 18

those at the same height of R3 and R4. This could be attributed to the overflow suction of the overflow weir having an effect on the steady flow in the inclined tubes. At the same time, the characteristic particle sizes were not significantly different between the heights IV (70 cm) and V (90 cm) in all measured tubes, indicating that the effective vertical working height of the inclined tubes should be 70–90 cm. The sediment particle size of d90 at the outlet of the measured tubes was about 40 μm, which was significantly smaller than 95 μm, the particle size of Water 2020d90 ,in12 the, 1685 test water source, indicating that the inclined-tube gravity sedimentation tank performed13 of 17 well at removing fine sediment.

(a) (b)

Water 2020, 12, x FOR PEER REVIEW 15 of 18 (e) (f)

(g) (h)

FigureFigure 10. Sediment 10. Sediment size size grading grading distribution distribution atat different diﬀerent locations locations inin the the settlement settlement area area with with inclined inclined tubestubes (ISA): (ISA (a)): tube (a) tube L1, (L1,b) ( tubeb) tube R1, R1, (c) ( tubec) tube L2, L2, (d ()d tube) tube R2, R2, ( e(e)) tubetube L3,L3, (f) tube R3, R3, (g (g) )tube tube L4 L4 and and (h) tube R4.(h) tube (The R4. horizontal (The horizontal ordinate ordinate title dtitlex indicated dx indicated the the characteristic characteristic particle particle size.)size.)

L1,4. Discussion L2, R1 and R2 were in the front part of the ISA. In each tube, the characteristic particle sizes at heights I (10How cm), to remove II (30 cm)fine andsediment III (50 from cm) water were has significantly been a concern larger among than researchers. those at heights Many articles IV (70 cm) and V[34 (90–36 cm).] have On focused the one on hand, the removal the sediment rate of at different the bottom types in of the tanks, end and of the most SSA of thewas rates washed were away by thebetween high-speed 37% and water, 62%. re-suspended In this study, in the the fine lower sediment flow removal of the ISA, rate and of the entered inclined with-tube the gravity flow into the inclinedsedimentation tubes in tank the could front reach part of64.7 the–69.7% ISA.. OnThe the sediment other hand,particle the size sediment of d90 at particle the outlet size of gradually the inclined tubes was about 40 μm. This would greatly reduce the clogging possibility of emitters in drip irrigation systems, as a study has shown that irrigation emitter blockage would increase sharply when the particle size is larger than 50 μm [37]. In order to deposit the fine sediment in the water as much as possible, the size of sedimentation tanks with a conventional structure was often very large. For example, the diameter of a circular-ring sedimentation tank used in Xinjiang, China was 30 m [38]. A circular sedimentation tank used in Thessaloniki, Greece had a volume of 2960 m3 [7]. The water treatment efficiency per unit volume of the inclined-tube gravity sedimentation tank was found to be significantly higher than that of traditional tanks. It would significantly reduce the construction investment in tanks and the water treatment costs. The inclined tubes significantly increased the removal rate of fine particles. Many studies have found that during the settling process, the sediment in inclined tubes was delaminated [39–41]; that is, the large sediment settled in the inclined tubes first, while the fine sediment settled later. This phenomenon could also be explained by Stokes’ Law. The flow in the inclined tubes was laminar [32,42,43], so the terminal velocity of sediment particles was proportional to the square of the particle radius. Therefore, the larger the particle size was, the easier it would deposit on the inclined tubes wall. Due to the different settling speeds of sediment with different particle sizes in the flow of inclined tubes, the sediment particle size grading varied with the height of the tubes in this study. There have been few studies of the proper arrangement of inclined tubes height. Here, we found that the characteristic particle sizes of tube heights IV (70 cm) and V (90 cm) were not significantly different, indicating that the effective vertical working height of the inclined tubes should be 70–90 cm. The plate spacing and layout of the inclined plate had a great influence on the sediment settling velocity [25,29,30]. Similarly, the proper tube diameter and layout of the inclined tubes could greatly improve the sediment removal efficiency [32,33]. However, at present, there are few inclined tubes with a layout other than 60° in the market; more research works on arrangement parameters of inclined tubes need to be performed in the future by numerical simulation. The results of this paper can be used to validate the mathematical model of the numerical simulation. Some structures in the inclined-tube gravity sedimentation tank were not conducive to the stable settling of fine sediment. The flow velocity from the water inlet was high, which not only washed away the sediment that had been deposited at the bottom of the FAA and made it re-suspend, reducing the settling efficiency of the tank, but also affected the structural stability of the tank bottom, which is not conducive to the long-term safe operation of the tank. This disadvantage could be

Water 2020, 12, 1685 14 of 17 decreased along the x direction, so most large particles entered into the tubes in the front part of the ISA. L3, L4, R3 and R4 were in the latter part of the ISA. The sediment particle size in the lower part of the flow was small, so in each tube, the differences between the characteristic particle sizes at different heights were small. The characteristic particle sizes of L3 and L4 were slightly larger than those at the same height of R3 and R4. This could be attributed to the overflow suction of the overflow weir having an effect on the steady flow in the inclined tubes. At the same time, the characteristic particle sizes were not significantly different between the heights IV (70 cm) and V (90 cm) in all measured tubes, indicating that the effective vertical working height of the inclined tubes should be 70–90 cm. The sediment particle size of d90 at the outlet of the measured tubes was about 40 µm, which was significantly smaller than 95 µm, the particle size of d90 in the test water source, indicating that the inclined-tube gravity sedimentation tank performed well at removing fine sediment.

4. Discussion How to remove fine sediment from water has been a concern among researchers. Many articles [34–36] have focused on the removal rate of different types of tanks, and most of the rates were between 37% and 62%. In this study, the fine sediment removal rate of the inclined-tube gravity sedimentation tank could reach 64.7–69.7%. The sediment particle size of d90 at the outlet of the inclined tubes was about 40 µm. This would greatly reduce the clogging possibility of emitters in drip irrigation systems, as a study has shown that irrigation emitter blockage would increase sharply when the particle size is larger than 50 µm [37]. In order to deposit the fine sediment in the water as much as possible, the size of sedimentation tanks with a conventional structure was often very large. For example, the diameter of a circular-ring sedimentation tank used in Xinjiang, China was 30 m [38]. A circular sedimentation tank used in Thessaloniki, Greece had a volume of 2960 m3 [7]. The water treatment efficiency per unit volume of the inclined-tube gravity sedimentation tank was found to be significantly higher than that of traditional tanks. It would significantly reduce the construction investment in tanks and the water treatment costs. The inclined tubes significantly increased the removal rate of fine particles. Many studies have found that during the settling process, the sediment in inclined tubes was delaminated [39–41]; that is, the large sediment settled in the inclined tubes first, while the fine sediment settled later. This phenomenon could also be explained by Stokes’ Law. The flow in the inclined tubes was laminar [32,42,43], so the terminal velocity of sediment particles was proportional to the square of the particle radius. Therefore, the larger the particle size was, the easier it would deposit on the inclined tubes wall. Due to the different settling speeds of sediment with different particle sizes in the flow of inclined tubes, the sediment particle size grading varied with the height of the tubes in this study. There have been few studies of the proper arrangement of inclined tubes height. Here, we found that the characteristic particle sizes of tube heights IV (70 cm) and V (90 cm) were not significantly different, indicating that the effective vertical working height of the inclined tubes should be 70–90 cm. The plate spacing and layout of the inclined plate had a great influence on the sediment settling velocity [25,29,30]. Similarly, the proper tube diameter and layout of the inclined tubes could greatly improve the sediment removal efficiency [32,33]. However, at present, there are few inclined tubes with a layout other than 60◦ in the market; more research works on arrangement parameters of inclined tubes need to be performed in the future by numerical simulation. The results of this paper can be used to validate the mathematical model of the numerical simulation. Some structures in the inclined-tube gravity sedimentation tank were not conducive to the stable settling of fine sediment. The flow velocity from the water inlet was high, which not only washed away the sediment that had been deposited at the bottom of the FAA and made it re-suspend, reducing the settling efficiency of the tank, but also affected the structural stability of the tank bottom, which is not conducive to the long-term safe operation of the tank. This disadvantage could be improved by Water 2020, 12, 1685 15 of 17 reducing the flow velocity from the inlet and raising the inlet position. Moreover, the water flowing into the ISA from the gap between the baffle and the bottom of the tank resulted in the flow velocity increasing slightly and the sediment at the tank bottom being washed away; this result was also found in other research [44]. In addition, the suction effect of the overflow weir also negatively affected the stable settling of fine sediment. Different types of weirs proposed in many studies could inspire the optimization of the overflow weir structure used in this study [45–47].

5. Conclusions Compared with the sedimentation tank with a conventional structure currently used in the Hetao irrigation district, the inclined-tube gravity sedimentation tank was much more efficient at fine sediment removal. A better understanding of the sediment movement in the tank was of great significance for further optimizing the structural parameters of the tank and improving the settling efficiency. Therefore, instruments such as a turbidimeter and a laser particle size analyzer were used here to measure the sediment concentration distribution and sediment size grading at different locations in the tank, as well as the deposited thickness of sediment at the bottom of the tank. The main conclusions of the research are as follows.

(1) The sediment concentration decreased rapidly in the front part of the SSA and the ISA. Large sediment mainly deposited in the FAA and the front part of the SSA, and the ISA had a great effect on fine sediment removal. The sediment removal rate of the inclined-tube gravity sedimentation tank could reach 64.7–69.7%, significantly higher than that of the sedimentation tank with a conventional structure (with a sediment removal rate of 20.7–32%). (2) Except for some locations, the characteristic particle sizes at different locations on the tank bottom gradually decreased along the x direction. As for the particle size in the inclined tubes, it was larger in the lower part than in the upper part, and it was larger in the left tubes near the overflow weir than in the right tubes. The results showed that the effective vertical working height of the inclined tubes should be 70–90 cm, and the particle size of d90 at the outlet of the inclined tubes was significantly smaller than that in the test water source. (3) The water inlet, baffle, and overflow weir would reduce the fine sediment removal and require further optimization.

The creativity of this study lies in the systematic measurement of the sediment concentration and the size grading at diﬀerent locations in the tank, enhancing our understanding of the sedimentation characteristics in the inclined-tube gravity sedimentation tank. Further, the results of this paper can provide a technical basis for the structural parameter optimization of inclined-tube gravity sedimentation tanks and improvements of settling eﬃciency.

Author Contributions: Methodology, K.W. and P.Y.; investigation, K.W.; data curation, K.W. and S.R.; writing—original draft preparation, K.W.; writing—review and editing, K.W. and Y.L.; supervision, P.Y.; funding acquisition, P.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the National Natural Science Foundation of China (51621061) and the Major Science and Technology Projects of Inner Mongolia (2014[117]). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Acknowledgments: We are deeply grateful to the editor and reviewers for their insightful and careful review. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Han, S.; Li, Y.; Xu, F.; Sun, D.; Feng, J.; Liu, Z.; Wu, R.; Wang, Z. Effect of lateral flushing on emitter clogging under drip irrigation with Yellow River water and a suitable method. Irrig. Drain. 2018, 67, 199–209. [CrossRef] 2. Li, Q.; Song, P.; Zhou, B.; Xiao, Y.; Muhammad, T.; Liu, Z.; Zhou, H.; Li, Y. Mechanism of intermittent ﬂuctuated water pressure on emitter clogging substances formation in drip irrigation system utilizing high sediment wate. Agric. Water Manag. 2019, 215, 16–24. [CrossRef] Water 2020, 12, 1685 16 of 17

3. Zhou, H.; Li, Y.; Wang, Y.; Zhou, B.; Bhattarai, R. Composite fouling of drip emitters applying surface water with high sand concentration: Dynamic variation and formation mechanism. Agric. Water Manag. 2019, 215, 25–43. [CrossRef] 4. Bajcar, T.; Steinman, F.; Sirok, B.; Preseren, T. Sedimentation efficiency of two continuously operating circular settling tanks with different inlet- and outlet arrangements. Chem. Eng. J. 2011, 178, 217–224. [CrossRef] 5. Bajcar, T.; Gosar, L.; Sirok, B.; Steinman, F.; Rak, G. Influence of flow field on sedimentation efficiency in a circular settling tank with peripheral inflow and central effluent. Chem. Eng. Process. Process Intensif. 2010, 49, 514–522. [CrossRef] 6. Krebs, P. Success and shortcomings of clarifier modelling. Water Sci. Technol. 1995, 31, 181–191. [CrossRef] 7. Goula, A.M.; Kostoglou, M.; Karapantsios, T.D.; Zouboulis, A.I. A CFD methodology for the design of sedimentation tanks in potable water treatment: Case study: The influence of a feed flow control baffle. Chem. Eng. J. 2008, 140, 110–121. [CrossRef] 8. Tamayol, A.; Firoozabadi, B.; Ahmadi, G. Effects of inlet position and baffle configuration on hydraulic performance of primary settling tanks. J. Hydraul. Eng. 2008, 134, 1004–1009. [CrossRef] 9. Shahrokhi, M.; Rostami, F.; Said, M.A.M. Numerical modeling of baffle location effects on the flow pattern of primary sedimentation tanks. Appl. Math. Model. 2013, 37, 4486–4496. [CrossRef] 10. Shahrokhi, M.; Rostami, F.; Said, M.A.M.; Yazdi, S.R.S.; Syafalni, S. Computational investigations of baffle configuration effects on the performance of primary sedimentation tanks. Water Environ. J. 2013, 27, 484–494. [CrossRef] 11. Shahrokhi, M.; Rostami, F.; Said, M.A.M.; Yazdi, S.R.S.; Syafalni, S.; Abdullah, R. The effect of baffle angle on primary sedimentation tank efficiency. Can. J. Civil. Eng. 2012, 39, 293–303. [CrossRef] 12. Shahrokhi, M.; Rostami, F.; Said, M.A.M.; Yazdi, S.R.S.; Syafalni, S. The effect of number of baffles on the improvement efficiency of primary sedimentation tanks. Appl. Math. Model. 2012, 36, 3725–3735. [CrossRef] 13. Csépai, L.; Kabelka, H. Practical testing of automatically controlled overflow weirs. Water Res. 1996, 30, 749–752. [CrossRef] 14. Zong, Q.; Liu, H.; Tang, H.; Wu, J. Study of effect of spillway trough on spillway crest length of settling basin. J. Sediment Res. 2011, 3, 67–72. 15. Hua, G.; Liu, H.; Tang, H.; Zong, Q.; Gao, X. Experimental study of effect on sediment deposition in the new-type rectangular settling tanks. China Water Resour. 2010, 5, 8–10. 16. Zhang, Y.; Zhang, Y.; Chang, Z. An experimental study of hydraulic characteristics on automatic flushing desilting basin. J. Irrig. Drain. 2009, 28, 78–82. 17. Wu, J.; Zong, Q.; Liu, H.; Liu, C. Experiment study of flow’s adjusting with flow board in sediment basin. J. Water Resour. Water Eng. 2007, 18, 6–9. 18. Aldulaimi, S.M.S.; Racoviteanu, G. Performance of the tube settler clarification at different inclination angles and variable flow rate. Math. Model. Civ. Eng. 2018, 14, 13–25. [CrossRef] 19. Culp, G.; Hansen, S.P.; Richardson, G. High-rate sedimentation in water treatment works. J. Am. Water Works Assoc. 1968, 60, 681–698. [CrossRef] 20. Fouad, H.A.; Elhefny, R.M.; Marei, A. Evaluating the use of tube settlers and lamella plates in Increasing the efficiency of sedimentation tanks. J. Appl. Life Sci. Int. 2016, 7, 1–8. [CrossRef][PubMed] 21. Fujisaki, K. Improvement of settling tank performance using inclined tube settlers. Water Resour. Manag. III 2005, 80, 475–484. 22. Gurjar, A.; Bhorkar, M. Performance study of tube settlers module. Int. J. Eng. Res. Appl. 2017, 07, 52–55. [CrossRef] 23. Hassan, M.S.; Hassan, A.H. The efficiency of using tube settlers technique in comparison with the conventional treatment in Mosul city water treatment. Al Rafadain Eng. J. 2009, 19, 26–34. [CrossRef] 24. Yao, K.M. Design of high-rate settlers. J. Environ. Eng. Division 1973, 99, 621–637. 25. Sarkar, S.; Kamilya, D.; Mal, B. Effect of geometric and process variables on the performance of inclined plate settlers in treating aquacultural waste. Water Res. 2007, 41, 993–1000. [CrossRef] 26. Salem, A.; Okoth, G.; Thöming, J. An approach to improve the separation of solid–liquid suspensions in inclined plate settlers: CFD simulation and experimental validation. Water Res. 2011, 45, 3541–3549. [CrossRef] 27. Balwan, K.S.; Mujawar, A.; Karake, M. To Study the Effect of Tube Settler on Clarification; LAP LAMBERT Academic Publishing: Beau Bassin, Mauritius, 2018. Water 2020, 12, 1685 17 of 17

28. Subramani, T.; Thomas, S. Experiments on tube settler for the treatment of Fbw and optimization of plant operation for residual reduction. Int. J. Eng. Res. Appl. 2012, 2, 190–203. 29. Demir, A. Determination of settling efficiency and optimum plate angle for plated settling tanks. Water Res. 1995, 29, 611–616. [CrossRef] 30. Abdolzadeh, M.; Mehrabian, M.; Goharrizi, A.S. Numerical study to predict the particle deposition under the influence of operating forces on a tilted surface in the turbulent flow. Adv. Powder Technol. 2011, 22, 405–415. [CrossRef] 31. Bandrowski, J.; Hehlmann, J.; Merta, H.; Zioło, J. Studies of sedimentation in settlers with packing. Chem. Eng. Process. Process. Intensif. 1997, 36, 219–229. [CrossRef] 32. Xie, M.; Shi, Z.; Liu, X. The motion and removal efficiency of particles in circular tube settlers. In Proceedings of the 2009 International Conference on Energy and Environment Technology, Guilin, China, 16–18 October 2009; Volume 1, pp. 437–440. 33. Ignjatović,L. Efficient (tube) settling tanks. Water Sci. Technol. 1991, 24, 223–228. [CrossRef] 34. Casonato, M.; Gallerano, F. A finite difference self-adaptive mesh solution of flow in a sedimentation tank. Int. J. Numer. Methods Fluids 1990, 10, 697–711. [CrossRef] 35. Liu, X.; García, M.H. Computational fluid dynamics modeling for the design of large primary settling tanks. J. Hydraul. Eng. 2010, 137, 343–355. [CrossRef] 36. Maruejouls, T.; Vanrolleghem, P.A.; Pelletier, G.; Lessard, P. A phenomenological retention tank model using settling velocity distributions. Water Res. 2012, 46, 6857–6867. [CrossRef][PubMed] 37. Niu, W.; Liu, L.; Chen, X. Influence of fine particle size and concentration on the clogging of labyrinth emitters. Irrig. Sci. 2013, 31, 545–555. [CrossRef] 38. Zhang, J.; Shi, K.; Gao, Y.; Wang, J. Turbid water desilting characteristics of circular-ring desilting and sediment ejection basin. Trans. Chin. Soc. Agric Eng. 2014, 30, 86–93. 39. Doroodchi, E.; Fletcher, D.F.; Galvin, K.P.Influence of inclined plates on the expansion behaviour of particulate suspensions in a liquid fluidised bed. Chem. Eng. Sci. 2004, 59, 3559–3567. [CrossRef] 40. Laux, H.; Ytrehus, T. Computer simulation and experiments on two-phase flow in an inclined sedimentation vessel. Powder Technol. 1997, 94, 35–49. [CrossRef] 41. Schlieper, L.; Chatterjee, M.; Henschke, M.; Pfennig, A. Liquid–liquid phase separation in gravity settler with inclined plates. AIChE J. 2004, 50, 802–811. [CrossRef] 42. Clay Mcmichael, F. Sedimentation in inclined tubes And its application for the design of high rate sedimentation devices. J. Hydraul. Res. 1972, 10, 59–75. [CrossRef] 43. Yao, K. Theoretical study of high-rate sedimentation. J. (Water Pollut. Control Fed.) 1970, 42, 218–228. 44. Zhou, S.; McCorquodale, J.A. Modeling of rectangular settling tanks. J. Hydraul. Eng. 1992, 118, 1391–1405. [CrossRef] 45. Lv, X.; Zou, Q.; Reeve, D. Numerical simulation of overflow at vertical weirs using a hybrid level set/VOF method. Adv. Water Resour. 2011, 34, 1320–1334. [CrossRef] 46. Chen, Y.; Fu, Z.; Chen, Q.; Cui, Z. Discharge coefficient of rectangular short-crested weir with varying slope coefficients. Water 2018, 10, 204. [CrossRef] 47. Khode, B.; Tembhurkar, A.; Porey, P.; Ingle, R. Experimental studies on flow over labyrinth weir. J. Irrig. Drain. Eng. 2011, 138, 548–552. [CrossRef]

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