applied sciences

Article Effects of Mixing Conditions on Floc Properties in Magnesium Hydroxide Continuous Coagulation Process

Yanmei Ding, Jianhai Zhao * , Lei Wei, Wenpu Li and Yongzhi Chi Tianjin Key Laboratory of Aquatic Science and Technology, School of Environmental and Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China; [email protected] (Y.D.); [email protected] (L.W.); [email protected] (W.L.); [email protected] (Y.C.) * Correspondence: [email protected]



Received: 29 December 2018; Accepted: 27 February 2019; Published: 7 March 2019


Abstract: Magnesium hydroxide continuous coagulation process was used for treating simulated reactive orange wastewater in this study. Effects of mixing conditions and retention time on the coagulation performance and floc properties of magnesium hydroxide were based on the floc size distribution (FSD), zeta potential, and floc morphology analysis. Floc formation and growth in different reactors were also discussed. The results showed that increasing rapid mixing speed led to a decrease in the final floc size. The floc formation process was mainly carried out in a rapid mixer; a rapid mixing speed of 300 rpm was chosen according to zeta potential and removal efficiency. Reducing retention time caused a relatively small floc size in all reactors. When influent flow was 30 L/h (retention time of 2 min in rapid mixer), the average floc size reached 8.06 µm in a rapid mixer; through breakage and re-growth, the floc size remained stable in the flocculation basin. After growth, the final floc size reached 11.21 µm in a sedimentation tank. The removal efficiency of reactive orange is 89% in the magnesium hydroxide coagulation process.

Keywords: magnesium hydroxide; reactive orange; mixing; coagulation; floc size

1. Introduction Magnesium hydroxide was used as a potential coagulant for reactive dyes wastewater treatment for many years [1–3]. Typical characteristics of this kind of coagulant include nontoxicity, an environmentally-friendly nature, recoverability, and rapid reaction [4]. Magnesium hydroxide precipitate has a positive superficial charge and it can adsorb negative dyes through charge neutralization or precipitate enmeshment [5,6]. Floc size settling properties are the main parameters influencing reactive dyes removal efficiency in real industrial scale unit operations [7–9]. A photometric dispersion technique and laser technique are useful in monitoring floc physical characteristics [10,11]. As previously found [12–14], magnesium hydroxide nucleation and precipitation processes are very fast; floc size can reach to 15 µm. Although floc size is not very large compared with other coagulants, the sedimentation process can meet the requirement of pollutant removal. During the magnesium hydroxide coagulation process, mixing conditions influenced the removal efficiency and floc properties. Rapid mixing brings the reactants together and homogenizes the solution. In the rapid stirring process, it will cause nucleation and precipitation of magnesium hydroxide. The magnesium hydroxide coagulation process had two stages including fast floc formation and growth of flocs. As for slow mixing, flocs remained stable and the re-growth process was not happened apparently in our previously found [13]. Several authors have found that operational conditions influenced the coagulation process and flocs properties especially for mixing conditions [11,15]. Effects of shearing on floc formation and growth are also related to coagulation mechanisms [16,17].

Appl. Sci. 2019, 9, 973; doi:10.3390/app9050973 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, x 2 of 8

Appl. Sci. 2019, 9, 973 2 of 8 conditions [11,15]. Effects of shearing on floc formation and growth are also related to coagulation mechanisms [16-17]. AlthoughAlthough there wasthere a largewas a amount large amount of data of on data the on characteristics the characteristics of floc of in floc the batchin the coagulationbatch coagulation process,process, there havethere been have limited been limited studies studies on the on relationship the relationship between between floc characteristics floc characteristics and mixing and mixing conditionsconditions using magnesiumusing magnesium hydroxide hydroxide as a coagulant as a coagulant in the continuousin the continuous process. process. The main The objectivesmain objectives of thisof laboratory this laboratory study werestudy to were evaluate to evaluate the role the of role rapid of mixingrapid mixing speed speed on coagulation on coagulation performance performance and flocand properties. floc properties. Furthermore, Furthermore, the effects the effects of retention of retent timeion on time floc on characteristics floc characteristics are also are assessed. also assessed.

2. Materials2. Materials and Methods and methods

2.1. Synthetic2.1. Synthetic Test Water test water and Coagulant and coagulant ReactiveReactive orange orange (K-GN) (K-GN) was purchased was purchased from Jinan from Xinxing Jinan Textile Xinxing Dyeing Textile Mill, Dyeing Shandong, Mill, China. Shandong, ReactiveChina. dye Reactive solutions dye with solutions pH 12 with were pH prepared 12 were with prepared K-GN with and K-GN deionized and deionized water to water provide to provide a concentrationa concentration of 0.25 g/L. of NaOH0.25 g/L. solution NaOH was solution used to was control used the to solution control pH the value solution (PHS-25 pH Shanghaivalue (PHS-25 · Jinke IndustrialShanghai Co.,Jinke Shanghai, Industrial China). Co., Shanghai, MgCl2 6H 2China).O (CP. TianjinMgCl2 Chemical·6H2O (CP. Reagent Tianjin Co. Chemical Tianjin, China)Reagent Co. was usedTianjin, to prepare China) the was coagulant. used to Magnesiumprepare the ioncoagulan was analyzedt. Magnesium with an ion ICS-1500 was analyzed (Dionex, with Sunnyvale, an ICS-1500 California,(Dionex, USA) Sunnyvale, ion chromatography California, system.USA) ion The chromatograp concentrationhy ofsystem. reactive The orange concentration in the solution of reactive was analyzedorange byin athe UV-VISIBLE solution was spectrophotometer analyzed by a (UV2550UV-VISIBLE Shimadzu, spectrophotometer Suzhou, China). (UV2550 The reactive Shimadzu, orangeSuzhou, characteristics China). are The shown reactive in Tableorange1. characteristics are shown in Table 1.

Table 1.TableReactive 1. Reactive orange orange characteristics. characteristics. λ NameName MolecularMolecular Structure Structure max (nm)λmax (nm)

Reactive orange Reactive orange (K-GN) 476 476 (K-GN)

2.2. Apparatus2.2. Apparatus and Procedures and procedures ContinuousContinuous experiments experiments were carriedwere carri outed at out temperature at temperature of 20 ±of 120±1◦C,℃ and, and in in order order to to obtain obtain a a real Appl.real Sci. continuous 2019continuous, 9, x steady steady experiment, experiment, K-GN K-GN removal removal efficiency efficiency remained remained stable stable at 2 h. at For 2 h. this For process this3 of process8 to to occur,occur, the 1 L the rapid 1 L mixerrapid hadmixer to had have to a have stirring a stirring speed atspeed 250 toat 250 350 to rpm. 350 The rpm. flocculation The flocculation basin wasbasin was everydivided 10divided minutes. into three into Floc parts three size (each parts distribution of (each 4 L) andof 4 aL)(FSD sedimentation and) awas sedime measured tankntation (30 tank L)by was Mastersizer(30 designed L) was designed for2000 the (Malvern removal for the removal of Panalytical,flocs (Figureof flocs Malvern,1). (Figure The UK) slow 1). and stirringThe each slow speed sample stirring was was speed maintained measured was main at three 80tained rpm times inat theto80 obtain rpm flocculation in the the average flocculation basin. results. 0.25 g/Lbasin. 0.25 DuringReactive theg/L orangeslowReactive mixing and orange 250 period mg/Land 250in magnesiumthe mg/L third magnesium flo ioncculation were ion pumped basin,were pumped zeta into potential the into rapid the was rapi mixer, measuredd mixer, respectively. respectively. by zetasizerThe totalThe Nano influent total ZS influent (Malvern flow was flow chosenPanalytical, was chosen at 30 L/hMalvern, at 30 and L/h 60 UK). and L/h, 60The in L/h, whichimages in which retention of flocs retention timefrom was timethe 2 rapid minwas and2 mixer,min 1 minand 1 min thirdin the flocculation rapidin the mixer,rapid basin, mixer, respectively. and respectively. sedimentation The operational The tankoperational were conditions captured conditions of the by continuousof IX71 the continuousdigital coagulation photomicrography coagulation process process are are (Olympus,shownshown in Tokyo, Table in2 Table.Japan). 2.

Table 2. Operational conditions of coagulation process.

Flux Rapid Mixer Flocculation Basin Sedimentation Tank speed time speed time time 250rpm 30L/h 300rpm 2min 80rpm 24min 60min 350rpm 60 L/h 300rpm 1min 80rpm 12min 30min

2.3. Floc size distribution and properties analysis

DuringFigureFigure the 1. Experimentalcontinuous 1. Experimental coagulation apparatus apparatus for process, forcoagulation coagulation sample of magnesium ofs magnesiumof flocs were hydroxide. hydroxide. taken from a rapid mixer, with the third flocculation basin and sedimentation tank using a tube with an inner diameter of 5 mm 3. Results and Discussion

3.1. Coagulation Behaviors Under Different Rapid Mixing Speed

3.1.1. Floc Size Distribution in Three Processes Continuous experiments were performed under 250 mg/L magnesium ion with 30 L/h to investigate the effects of rapid mixing conditions on coagulation performance and floc size distribution. According to FSD, average floc size decreased when rapid mixing speed increased in the rapid mixer and sedimentation tank. In the flocculation basin, floc size tended to be stable or slightly broken with the increase of rapid mixing speed to 300rpm. This is consistent with the findings that repulsive forces tend to stabilize the suspension and prevent particle agglomeration [13,18]. As shown in Table 3, the average floc sizes 8.39, 8.06 and 8.04 μm were obtained with rapid mixing speeds of 250, 300 and 350 rpm in a rapid mixer, respectively. Small floc had the same trend to aggregate relatively large flocs. Average floc size was 8.06 and 7.89 μm in the rapid mixer and flocculation basin when the mixing speed was 300 rpm. In general, the flocs will grow in the flocculation basin, but it seems that floc size decreased in the flocculation process. In fact, during the flocculation process, particles larger than 11.25 μm accounted for 4.54% and 4.9% in the rapid mixer and flocculation basin, respectively. Particles smaller than 1 μm also decreased in the slow mixing period. As can be seen also in Figure 2 (sedimentation tank), particles smaller than 1 μm account for 6.2%, 10.0% and 12.6% with mixing speeds of 250, 300 and 350 rpm respectively. It was observed that the percentage of smaller particles increased with the increase of mixing speed. This indicates that the high mixing speed will break the flocs, and only part of the flocs will aggregate again after the flocs are broken. The general shape of the curves was broadly similar in different units. The average floc size decreased to a steady state; there was a dynamic balance between floc growth and breakage. When the stirring speed was increased from 250 rpm to 350 rpm, small average floc size could be observed in these three units. Following the breakage and aggregation period, there was only a partial flocs re-growth, showing that the broken flocs can only re-grow to a very limited extent. Appl. Sci. 2019, 9, 973 3 of 8

Table 2. Operational conditions of coagulation process.

Flux Rapid Mixer Flocculation Basin Sedimentation Tank speed time speed time time 250 rpm 30 L/h 300 rpm 2 min 80 rpm 24 min 60 min 350 rpm 60 L/h 300 rpm 1 min 80 rpm 12 min 30 min

2.3. Floc Size Distribution and Properties Analysis During the continuous coagulation process, samples of flocs were taken from a rapid mixer, with the third flocculation basin and sedimentation tank using a tube with an inner diameter of 5 mm every 10 minutes. Floc size distribution (FSD) was measured by Mastersizer 2000 (Malvern Panalytical, Malvern, UK) and each sample was measured three times to obtain the average results. During the slow mixing period in the third flocculation basin, zeta potential was measured by zetasizer Nano ZS (Malvern Panalytical, Malvern, UK). The images of flocs from the rapid mixer, third flocculation basin, and sedimentation tank were captured by IX71 digital photomicrography (Olympus, Tokyo, Japan).

3. Results and Discussion

3.1. Coagulation Behaviors under Different Rapid Mixing Speed

3.1.1. Floc Size Distribution in Three Processes Continuous experiments were performed under 250 mg/L magnesium ion with 30 L/h to investigate the effects of rapid mixing conditions on coagulation performance and floc size distribution. According to FSD, average floc size decreased when rapid mixing speed increased in the rapid mixer and sedimentation tank. In the flocculation basin, floc size tended to be stable or slightly broken with the increase of rapid mixing speed to 300rpm. This is consistent with the findings that repulsive forces tend to stabilize the suspension and prevent particle agglomeration [13,18]. As shown in Table3, the average floc sizes 8.39, 8.06 and 8.04 µm were obtained with rapid mixing speeds of 250, 300 and 350 rpm in a rapid mixer, respectively. Small floc had the same trend to aggregate relatively large flocs. Average floc size was 8.06 and 7.89 µm in the rapid mixer and flocculation basin when the mixing speed was 300 rpm. In general, the flocs will grow in the flocculation basin, but it seems that floc size decreased in the flocculation process. In fact, during the flocculation process, particles larger than 11.25 µm accounted for 4.54% and 4.9% in the rapid mixer and flocculation basin, respectively. Particles smaller than 1 µm also decreased in the slow mixing period. As can be seen also in Figure2 (sedimentation tank), particles smaller than 1 µm account for 6.2%, 10.0% and 12.6% with mixing speeds of 250, 300 and 350 rpm respectively. It was observed that the percentage of smaller particles increased with the increase of mixing speed. This indicates that the high mixing speed will break the flocs, and only part of the flocs will aggregate again after the flocs are broken. The general shape of the curves was broadly similar in different units. The average floc size decreased to a steady state; there was a dynamic balance between floc growth and breakage. When the stirring speed was increased from 250 rpm to 350 rpm, small average floc size could be observed in these three units. Following the breakage and aggregation period, there was only a partial flocs re-growth, showing that the broken flocs can only re-grow to a very limited extent. Appl. Sci. 2019, 9, 973 4 of 8

Table 3. Average floc size in different process units.

Average Floc Size (µm) Rapid Mixing (rpm) Rapid Mixer Flocculation Basin Sedimentation Tank 250 8.39 7.96 14.41 Appl. Sci. 2019, 9, x 300 8.06 7.89 11.21 4 of 8 350 8.04 7.95 10.58

6 5 5 250rpm 250rpm 300rpm 300rpm 4 4 350rpm 350rpm Flocculation basin Rapid mixer 3 3 2 2 Volume(%)

Volume(%) 1 1 0 0 0.01 0.1 1 10 100 1000 0.01 0.1 1 10 100 1000 Size(μm) Size(μm) (a) (b)

6 250rpm 5 300rpm 350rpm 4 Sedimentation tank 3

Volume(%) 2 1 0 0.01 0.1 1 10 100 1000 Size(μm)

(c)

FigureFigure 2. 2.Floc Floc sizesize distribution with with different different rapid rapid mixing mixing for forthree three stages. stages. (a) rapid (a) mixer; rapid mixer;(b) (b) flocculation basin; (c) sedimentationflocculation tank. basin; (c) sedimentation tank.

3.1.2. Removal Efficiency andTable Zeta 3. Average Potential floc size in different process units. As shown in Figure3, the K-GN removal efficiency after coagulation remained stable at about 89% Rapid Mixing Average Floc Size (µm) in three different rapid mixing conditions. Although Removal efficiency is not significantly influenced (rpm) Rapid Mixer Flocculation Basin Sedimentation Tank by rapid mixing speed, zeta potential increased with increasing rapid mixing speed. Changes in floc 250 8.39 7.96 14.41 characteristics also lead to changes in zeta potential. Rapid stirring speed may change the zeta potential 300 8.06 7.89 11.21 of the colloid, because the rapid mixing could change the floc size and their surface properties. Positive 350 8.04 7.95 10.58 magnesium hydroxide acts as a charge neutralization species. Similar results were also found in the magnesium hydroxide coagulation process for the removal of reactive dyes [18,19]. Zeta potential is 3.1.2. Removal Efficiency and Zeta Potential important in terms of the impact on steady state floc size and response to increased levels of shear. UnderAs differentshown in shear Figure conditions, 3, the K-GN the removal break-up efficiency flocs may after have coagulation different physicalremained properties. stable at about Floc 89%properties in three impacted different significantly rapid mixing on theconditions. overall process Although efficiency Removal [20 ].efficiency Electrical is charge not significantly or colloidal influencedproperties ofby the rapid magnesium mixing hydroxide-reactivespeed, zeta potential orange increased flocs would with beincreasing greatlyaffected. rapid mixing Based onspeed. this Changesobservation, in floc it can characteristics be reasoned also that lead charge-neutralization to changes in zeta ispotential. one of the Rapid mechanisms stirring speed for destabilization may change theand zeta removal potential of reactive of the colloid, orange [because21]. Normally, the rapid slow mixing mixing could conditions change alsothe floc affects size the and flocs their break-up surface properties.and growth. Positive However, magn inesium the magnesium hydroxide hydroxideacts as a charge coagulation neutralization process, species. magnesium Similar hydroxide results wereformed also and found flocs in grew the fast,magnesium then the largerhydroxide flocs coagulation broke into relatively process smallfor the particles removal in of the reactive slow mixing dyes [18,19].period. Zeta As previously potential foundis important [13], floc in growthterms of is the not impact significantly on steady influenced state floc by asize slow and mixing response period. to increased levels of shear. Under different shear conditions, the break-up flocs may have different physical properties. Floc properties impacted significantly on the overall process efficiency [20]. Electrical charge or colloidal properties of the magnesium hydroxide-reactive orange flocs would be greatly affected. Based on this observation, it can be reasoned that charge-neutralization is one of the mechanisms for destabilization and removal of reactive orange [21]. Normally, slow mixing conditions also affects the flocs break-up and growth. However, in the magnesium hydroxide coagulation process, magnesium hydroxide formed and flocs grew fast, then the larger flocs broke into relatively small particles in the slow mixing period. As previously found [13], floc growth is not significantly influenced by a slow mixing period. In this research, the effects of slow mixing Appl. Sci. 2019, 9, 973 5 of 8 Appl. Sci. 2019, 9, x 5 of 8

Inconditions this research, on floc the properties effects of we slowre mixingnot considered. conditions As onhas floc been properties mentioned, were large not considered.flocs breakage As and has beenre-growth mentioned, will happen large simultaneously flocs breakage and and, re-growth conseque willntly, happenthere is simultaneouslyless opportunity and, for the consequently, break flocs thereto re-grow. is less opportunity for the break flocs to re-grow.

90 0

-4 88 -8

-12 86 Removal efficiency Zeta potential -16

-20 potential(mV) Zeta Removal efficiency(%) Removal 84 -24 250 300 350 Rapid mixing (rpm) Figure 3. Effects of rapid mixing on removal efficiency efficiency and zeta potential.

3.2. Effect of Flow o onn Coagulation Performance For continuous coagulation reaction, the flow flow determinesdetermines the retention time of reactants in each reactor. The increase of flowflow indicates the decrease of retention time. In order to investigate the effect of flow flow on coagulation performance, continuous experiments were performedperformed under magnesium ion 250 mg/L, mg/L, rapid rapid mixing mixing speed speed 300rpm 300rpm with with 30 L/h 30 L/h and and60 L/h. 60 The L/h. retention The retention time in time the rapid in the mixing rapid mixingtank was tank 2min was and 2min 1min and for 1minflow of for 30 flow L/h ofand 30 60 L/h L/h, and respectively. 60 L/h, respectively. As shown in As Table shown 4, the in average Table4, thefloc averagesize decreased floc size with decreased increasing with influent increasing flow influent for each flow operation for each unit. operation As for unit. the rapid As for mixer, the rapid the mixer,average the floc average size was floc 8.06 size and was 7.25 8.06 μm and for 7.2530L/hµm and for 60 30L/h L/h, respectively. and 60 L/h, According respectively. to earlier According results to earlier[14], in results the stage [14 ],of in rapid the stage mixing, of rapid magnesium mixing, magnesiumhydroxide was hydroxide formed was and formed particles and grew particles in a grewvery inshort a very time. short A suitable time. A period suitable of period rapid ofmixing rapid was mixing necessary was necessary for good for coagulation. good coagulation. Reactive Reactive orange orangeremoval removal efficiency efficiency reached reached 89% and 89% 83% and for 83% 30 L/h for 30and L/h 60 L/h, and 60respectively. L/h, respectively. The retention The retention time in timeeach inunit each for unit60L/h for was 60L/h shorter was than shorter that than of 30 that L/h, of especially 30 L/h, especially in the rapi ind themixer rapid of 1 mixer min retention of 1 min retentiontime. This time. is too This short is to too form short magnes to formium magnesium hydroxide-reactive hydroxide-reactive orange flocs. orange flocs.

TableTable 4. Average size in differentdifferent processprocess units.units. Average Size (µm) FlowFlow (L/h) Average Size (µm) (L/h) RapidRapid Mixer Mixer Flocculation Flocculation Basin Basin Sedimentation Sedimentation TankTank 30 308.06 8.06 7.89 7.89 11.21 60 607.25 7.25 7.52 7.52 10.47

Figure 4 clearly indicates floc formation and growth in the magnesium hydroxide-reactive orangeFigure continuous4 clearly coagulation indicates floc system. formation In the and rapid growth mixer in the and magnesium flocculation hydroxide-reactive basin, average floc orange size continuousremained stable coagulation and flocs system. aggregated In the rapidto relatively mixer andlarge flocculation flocs in the basin, sedimentation average floc tank. size These remained two stablefigures and showed flocs aggregated the same trends to relatively that the large magnes flocs inium the hydroxide sedimentation coagulation tank. These process two figureswas similar showed to the sameprecipitation trends that process. the magnesium When magnesium hydroxide ion coagulation is added to process the alkaline was similar solution to thein rapid precipitation mixing process.period, the When reaction magnesium crystallization ion is added process to thewill alkaline happen solutionrapidly. inMagnesiu rapid mixingm hydroxide period, thecoagulation reaction crystallizationincludes magnesium process hydroxide will happen nucleation rapidly. and Magnesium a combination hydroxide of reactive coagulation orange includes into flocs. magnesium hydroxide nucleation and a combination of reactive orange into flocs. Appl. Sci. 2019, 9, x 6 of 8 Appl. Sci. 2019, 9, x 6 of 8 Appl. Sci. 2019, 9, 973 6 of 8

6 7 6 a Flow: 30L/h 7 b Flow: 60L/h 5 a Flow: Rapid 30L/h mixer 6 b Flow: Rapid 60L/h mixer 5 FlocculationRapid mixer basin 6 FlocculationRapid mixer basin 4 SedimentationFlocculation basin tank 5 SedimentationFlocculation basin tank Sedimentation tank 5 Sedimentation tank 4 4 3 4 3 3 2 3

2 Volume(%) 2 Volume(%) Volume(%) 2 Volume(%) 1 1 1 1 0 0 0 0 0.01 0.1 1 10 100 1000 0.01 0.1 1 10 100 1000 0.01 0.1Size(μm) 1 10 100 1000 0.01 0.1 1 10 100 1000 Size(μm) Size(μm) Size(μm) (a) (b) (a) (b) Figure 4. Floc size distribution with different flows for three stages. (a) 30 L/h; (b) 60 L/h. FigureFigure 4.4.Floc Floc sizesize distributiondistribution withwith differentdifferent flowsflows for for three three stages. stages.( a(a)) 30 30 L/h; L/h; ((b)b) 60 L/h. 3.3. Images analysis Analysis 3.3. Images analysis The flocfloc size distribution can tell us the percentages of different floc floc sizes (shown in in Figure 44),), The floc size distribution can tell us the percentages of different floc sizes (shown in Figure 4), and the removal efficiencyefficiency is commonlycommonly used toto estimateestimate thethe coagulationcoagulation performance.performance. In order to and the removal efficiency is commonly used to estimate the coagulation performance. In order to gain further insight into the flocfloc and sediment properties,properties, image analysis was used to predict flocfloc gain further insight into the floc and sediment properties, image analysis was used to predict floc characteristics by IX71 digital photomicrography. In In Figure Figure 55,, thethe averageaverage sizesize ofof flocsflocs inin differentdifferent characteristics by IX71 digital photomicrography. In Figure 5, the average size of flocs in different reactors is clearly indicated. Th Thee floc floc formation and growth processes were carried out in the rapid reactors is clearly indicated. The floc formation and growth processes were carried out in the rapid mixer and no more aggregation occurred due to thethe strong repulsionrepulsion betweenbetween positively charged mixer and no more aggregation occurred due to the strong repulsion between positively charged particles ofof magnesium magnesium hydroxide hydroxide in the in flocculationthe flocculation basin. Inbasin. the sedimentationIn the sedimentation tank, flocs aggregatedtank, flocs particles of magnesium hydroxide in the flocculation basin. In the sedimentation tank, flocs toaggregated form relatively to form large relatively particles. large This particles. is consistent This is with consistent the findings with ofthe FSD findings value of which FSD arevalue shown which in aggregated to form relatively large particles. This is consistent with the findings of FSD value which Figureare shown4. As in for Figure Figure 4.5a,d, As theyfor Figure clearly 5a show and that5d, increasingthey clearly flow show caused that aincreasing decrease offlow the averagecaused a are shown in Figure 4. As for Figure 5a and 5d, they clearly show that increasing flow caused a sizedecrease in the of rapid the average mixer. Thesize averagein the rapid floc size mixer. of magnesium The average hydroxide-reactive floc size of magnesium orange hydroxide- is 8.06 and decrease of the average size in the rapid mixer. The average floc size of magnesium hydroxide- 7.25reactiveµm. orange This is is also 8.06 consistent and 7.25 withμm. This the findings is also consiste of a previousnt with study the findings [14]. of a previous study [14]. reactive orange is 8.06 and 7.25 μm. This is also consistent with the findings of a previous study [14].

Figure 5.5. FlocFloc image image analysis analysis ( (aa)) rapid rapid mixer for 3030 L/h;L/h; ( (bb)) flocculation flocculation basin for 3030 L/h;L/h; Figure 5. Floc image analysis (a) rapid mixer for 30 L/h; (b) flocculation basin for 30 L/h; (c))sedimentation sedimentation tank tank for for 30 30 L/h; L/h; (d (d) )rapid rapid mixer mixer for for 60 60 L/h. L/h. (c)sedimentation tank for 30 L/h; (d) rapid mixer for 60 L/h. 4. Conclusions 4. Conclusions 4. ConclusionsIn this research, the effects of mixing conditions on magnesium hydroxide continuous coagulation performance and floc properties were investigated. Rapid mixing speed plays a significant role in floc Appl. Sci. 2019, 9, 973 7 of 8 formation and growth. The function of the rapid mixer is to provide a place for the rapid nucleation and precipitation of magnesium hydroxide. In this stage, coagulant and reactive orange formed flocs. In the flocculation basin, small flocs will form more dense flocs, but average floc size remains stable. After the sedimentation process, flocs aggregate to form relatively large flocs. Therefore, appropriate rapid stirring is conducive to the formation of flocculation and the removal of pollutants. Increasing the flow will cause a short retention time; the average floc size decreases from 11.21 to 10.47µm when flow increases from 30 to 60 L/h. The coagulation behavior indicates that magnesium hydroxide is an effective coagulant for reactive orange removal with an efficiency of 89%.

Author Contributions: Investigation, W.L.; data curation, L.W.; writing—original draft preparation, Y.D.; writing—review and editing, J.Z.; funding acquisition, Y.C. Funding: This research was funded by the Technology Research and Development Program of Tianjin, China, grant number 16YFXTSF00390. Acknowledgments: The authors would like to acknowledge the support from Tianjin Key Laboratory of Aquatic Science and Technology. Conflicts of Interest: The authors declare no conflict of interest.

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