The Effect of Flocculants and Water Content on the Separation of Water from Dredged Sediment

Abstract

Dredged sediment has high water content, and its engineering characteristics are poor; therefore, the treatment and disposal of dredged sediment is difficult, and the utilization efficiency of these resources is low. Various methods can be used to dehydrate dredged sediment, among which flocculation dewatering is the most widely used. In this study, the basic properties of dredged sediment were examined, and the flocculation dehydration effect of dredged sediment was analyzed in relation to the polyaluminum ferric chloride (PAFC) dosage, water content, and time. There is an optimal flocculant dosage added during dredged sediment processing. Dredged sediment with high water content has obvious flocculation effect. Flocculant can speed up the flocculation dehydration of dredged sediment in a short time, but long-term dehydration has no advantages compared with natural dredged sediment; the dehydration effect was even weaker than that of natural dredged sediment dehydration in the later stages of sedimentation. Meanwhile, this paper divided the settling process into different stages; the settlement coefficients and compaction settlement indexes in different settling stages can well reflect the flocculating performance of flocculants at each settling stage.

1. Introduction

Large amounts of dredged sediment are produced in the dredging process; this sediment is characterized by high water content and poor mechanical properties, and using the resources in dredged sediment is difficult [1,2]. Dredged sediment pollutes the environment during transportation, and if not treated with concerted efforts, dredged sediment will pose a threat to human health and the environment. Various methods can be used to dehydrate dredged sediment, such as cement curing technology [3,4], electroosmotic technology [5,6], vacuum preloading technology [7,8], and flocculation dehydration treatment [9,10]. Due to its low solid content, rich organic matter, and strong compressibility, dredged sediment cannot be directly used in engineering applications, so dehydrating and reducing treatments have become necessary for the application of these resources [11]. Dehydrated sediment can be used as base pavement and construction materials [12,13], but to do so, the water content of sediment must be reduced.

The flocculation and dehydration of dredged sediment are feasible and effective methods. Many kinds of flocculants, such as organic and inorganic flocculants, can be used [10,14,15]. Inorganic flocculants are affordable and simple to use, so they are often preferred by water-treatment workers. Polymer inorganic flocculants such as PAFC and polyaluminum chloride can perform adsorption electric neutralization, adsorption bridging, and precipitation net trapping and produce a stronger coagulation effect [16]. Many results on inorganic flocculants have been published, and the flocculation effect of compound coagulants is stronger than that of single flocculants; when APAM: FeCl₃ was 1:5, the flocculation dehydration effect of dredged sediment was the best according to the results of a settlement column test [17,18]. After the FeCl₃ conditioning of sewage sludge, the consolidation performance of sludge is improved; as the added amount of FeCl₃ increases, the consolidation coefficient of sludge increases, which is crucial for reducing the volume of sludge. Additionally, the vacuum preloading dehydration efficiency is also good; this method substantially decreases the volume of dredged sediment, and the water content considerably decreases [19]. PAFC is a compound flocculant that can be used to remove color; given its effects, PAFC is often used in water or wastewater treatment [20,21]. The flocculating mechanism of flocculants is achieved through double-layer compression, charge neutralization, sweep flocculation, and bridge aggregation [22]. The addition of flocculants can destabilize the silt, causing the particles to sink, and the subsidence of particles is affected not only by gravity but also by resistance, where the larger the resistance, the weaker the downward force and the lower the particle subsidence rate. According to the different sedimentation velocities of particles, the velocities can be divided into different sedimentation stages [23]. Flocculants are added to accelerate the flocculation precipitation by affecting the sedimentation rate in different sedimentation stages. In most cases, the purpose of adding flocculants is to quickly separate particles and water; that is, during the initial stage of precipitation, the higher the precipitation rate, the better the application of the flocculant in wastewater treatment. The speed at which silt particles sink varies with the water content of silt; as such, our aim in this study on the flocculant dosage was to find the most appropriate flocculant amount when the silt flocculation precipitation effect is the best. Meanwhile, the effect of the water content on dredged sediment dehydration was also studied, and the aim was to achieve an economic and efficient way to dehydrate the dredged sediment.

Dredged sediment is a special kind of soil containing organic matter and is characterized by high compressibility and high water content. Due to the differences in the properties of wastewater, such as the solid content, the results of the flocculation dehydration effect and the mechanism differ to some extent between dredged sediment and wastewater treatment. Here, dredged sediment was taken as the study object, and PAFC was used to conduct flocculation tests on dredged sediment. We thus studied the process of flocculation dehydration and explored the optimal PAFC dosage to incorporate in the sediment; we also studied the influence of different water contents on the flocculation dehydration of dredged sediment, and then, we divided the settling process by two methods; we put forward the reference indexes of flocculation performance for dredged sediment. This study provides some references for the practical flocculation treatment application of dredged sediment under different conditions.

2. Materials and Methods

2.1. Materials

Dredged sediment was taken from a river in Huainan, China. Through XRF analysis, we found that the main oxides in the dredged sediment were SiO₂, Al₂O₃, and Fe₂O₃ with contents of 68.8%, 17.0%, and 6.7%, respectively. The mass fractions of the elements in the dredged sediment are shown in

Table 1. Chemical element mass fraction of dredged sediment.

Element	Si	Al	Fe	K	Ca	Ti	Mg
Content (%)	59.95	14.18	13.20	5.82	3.09	1.52	1.30

. The Si, Al, and Fe contents were 59.95%, 14.18%, and 13.20%, respectively. The main metal elements were Al, Fe, K, and Ca in Table 1; the particle grouping is shown in Figure 1. The dredged sediment was mainly composed of clay and silt particles, and clay particles more strongly impacted the engineering properties of the sediment.

The XRD test results of the sediment are shown in Figure 2. The mineral composition was mainly quartz, and the main clay minerals were kaolinite and illite. The details of PAFC are shown in

Table 2. Characteristics of PAFC.

Characteristics	PAFC
pH (1% water solution)	4.5
Al2O3 (%)	29.5
Fe2O3 (%)	3.8
Basicity (%)	75.5
Insoluble matter (%)	0.8
As (mg/L)	0.002
Mn (mg/L)	0.0068
Pb (mg/L)	0.0026
Hg (mg/L)	0.0002
SO2−4 (%)	8.5

; PAFC is an inorganic polymer flocculant formed by the coagulation hydrolysis of aluminum and iron salts and is a yellowish-brown powder that easily dissolves in water, benefitting from the advantages of both aluminum and iron salt. During flocculation, PAFC produces fast hydrolysis and effective flocculation.

2.2. Test Methods

(1)

We first removed the supernatant after settling the dredged sediment and then removed 1000 mL of the dredged sediment, placing 50 mL into each of 7 measuring cylinders (100 mL). Then, we added 50 mL of water to each measuring cylinder and evenly stirred the contents. PAFC (2, 3, 4, 5, 6, 7, or 8 g) was placed into the 7 beakers containing 100 mL river water; after mixing until even, 2 mL of mixed PAFC solution was added into the diluted dredged sediment measuring cylinder, 2 mL of clean water was added to the natural control group, and the flocculation effects were observed.

(2)

We added 10, 20, 30, 40, 50, 60, 70, 80, 90, or 100 mL of the bottom dredged sediment to the measuring cylinder, and then we injected water into the measuring cylinder. The total volume of the dredged sediment with injected water was 100 mL. We added 5 g of PAFC into 100 mL of river water to prepare the PAFC solution, and then we added 2 mL of the PAFC solution to each measuring cylinder to observe the flocculation precipitation effect. The flowchart for the study is shown in Figure 3.

3. Results

3.1. Effect of Flocculant Dosage

As shown in Figure 4, the flocculation effect was related to flocculation duration and flocculant dosage. When the PAFC dosage was 0.05 g/mL, the best flocculation effect was achieved in the first 20 min; after 40 min, the PAFC dosage of 0.04 g/mL produced the best flocculation; at 80 min, the PAFC dosage of 0.02 g/mL produced the best flocculation effect; at 140 min, the precipitation advantage without adding PAFC gradually became prominent until the end of the test. PAFC is a coagulant [24], having the properties of both of polyaluminum and polyferric coagulants [25]. In the early stages, PAFC broke the suspension balance system and formed flocs, which were larger in diameter and settled faster. Over time, the amount of dredged sediment particles used for flocculation in the upper part gradually decreased, whereas the density in the middle and bottom parts gradually increased. Due to the interaction between the particles and floc groups, the sinking resistance of the dredged sediment particles and floc groups in the middle and bottom increased. Due to the flocculant, the water in the floc groups was increasingly unable to discharge. When the dredged sediment was in the late stage of sedimentation, and the main actions were the sedimentation and compression of the dredged sediment, the flocculation and sedimentation were minimal. Because compression and consolidation are slow processes, the water separation from the dredged sediment was not obvious [16,23]. We selected a PAFC flocculation time of 40 min as an example to illustrate the optimum PAFC dosage. When the PAFC dosage was lower than 0.04 g/mL, the flocculation effect gradually strengthened; when the content was higher, the flocculation effect gradually weakened. Small amounts of flocculants can destabilize dredged sediment and improve the flocculation effect, but a large amount of flocculant may change the zeta potential and then restabilize the dredged sediment [21,26].

The initial sedimentation effect of the dredged sediment with the addition of flocculant was stronger than that of the natural dredged sediment. Over time, the volume with flocculant added to the supernatant was less than that in the natural because flocs impede the consolidation of dredged sediment; this may be because the different sinking flocs contact each other, and voids exist in the flocs; the flocculation sinking rate gradually decreases. Flocculants can increase the initial settling performance of dredged sediment; however, as long as the settlement duration is long enough, the final settlement value tends to be the same or even less than that of natural settlement. This is because the particles are relatively dense by compression; conversely, the dredged sediment with flocculant at the bottom due to the flocs, the volume of the flocs, and the internal pores are larger, and the water within and between flocs cannot be discharged.

3.2. Effect of Water Content on Flocculation Effect

As shown in Figure 5, the water content strongly impacted the flocculation effect of PAFC. For dredged sediment with 90 mL of added water, the PAFC flocculation effect was the strongest, and the supernatant volume was approximately 50 mL after 15 min. With the decrease in the amount of added water, the flocculation effect of PAFC gradually weakened. When the added water volume was less than 50 mL, the PAFC flocculation effect was very weak; this occurred because the dredged sediment had a higher solid content, the particles and flocs interfered with each other, and the sinking speed decreased. The sinking speed of the sediment is strongly related to the water content; the more water added, the faster the flocculation sinking speed. For natural dredged sediment, the subsidence speed was the slowest due to having the maximum solid content. PAFC made the solid particles form flocs, and flocs sunk first under the action of gravity; the particles with a high solid content contacted each other, so the resistance encountered in the sinking process was larger, the interference was stronger, and the settling effect was poor; this indicated that the solid content strongly influences the flocculation subsidence of dredged sediment [27]. Adding PAFC can shorten the free sedimentation stage, and the PAFC sedimentation effect is reflected in the initial sedimentation stage of dredged sediment. Dredged sediment with a high solid content requires dilution; if the solid content is low after dilution, a larger volume of dredged sediment needs to be treated; if the dilution degree is too low, the flocculation and dehydration effect of dredged sediment are weak. The results of the test showed that the dilution of sediment with a high solid content should be based on a ratio of water added to a sediment volume of 1:1~1.5:1, which ensures not only that the correct volume of the dredged sediment is treated but also a faster flocculation speed. However, in engineering applications, the duration of the construction period should be considered, and the water separation from dredged sediment should be reasonably optimized.

3.3. Settlement Stages of Dredged Sediment

By fitting the relation curve shown in Figure 5, we obtained Figure 6, which relates the change of the solid–liquid interface and time. The flocculation settling process can be divided into the free settling stage (S₁), interference settling stage (S₂), and compaction stage (S₃), as shown in Figure 7. In S₁, particles settle through gravity, buoyancy, and the resistance of the alternating medium. In S₂, at the end of free settlement, the dredged sediment concentration increases, and the dredged sediment particles and the walls interfere with each other to prevent the dredged sediment particles from settling. In S₃, particles settle to the bottom of the wall, and the particles produce gravity compression under the downward force of the resultant force. The water inside the dredged sediment is squeezed out, and the settlement at this stage is minimal and slow. The variation V of the dredged sediment solid–liquid interface is

V = V₁ + V₂ + V₃

(1)

where V₁ represents the free settlement volume, V₂ represents the interference settlement volume, and V₃ represents the volume change in compaction.

The time corresponding to the critical points of the free and interference sedimentation stages is t₁, and the time corresponding to the critical points of the interference and compaction stages is t₂, the end of the compaction stage time is t₃ Then, we concluded that

$${\mathrm{C}}_1{=\frac{{\mathrm{V}}_1}{{\mathrm{t}}_1},\mathrm{C}}_2=\frac{{\mathrm{V}}_2}{{\mathrm{t}}_2-{\mathrm{t}}_1},{\mathrm{C}}_3=\frac{{\mathrm{V}}_3}{{\mathrm{t}}_3-{\mathrm{t}}_2}$$

(2)

where C₁ is the free settlement coefficient, C₂ is the interference settlement coefficient, and C₃ is the compaction settlement coefficient.

By converting the ordinate in Figure 6 to the time logarithmic function (lgt), we obtained Figure 8. It divides the subsidence into the main settlement stage and compaction stage in Figure 9. The tangent intersection point before and after the reverse bending point in the curve was taken as the dividing point between the main settlement stage and compaction stage.

$${\mathrm{C}}^{*}=\frac{\mathrm{V}-{{\mathrm{V}}_1}^{*}}{\mathrm{l}\mathrm{g}\frac{{\mathrm{t}}_2^{\prime }}{{\mathrm{t}}_1^{\prime }}}$$

(3)

Here, ${{\mathrm{V}}_1}^{*}$ is the main settlement volume, and ${\mathrm{C}}^{*}$ is the compaction settlement index.

Therefore, C and C* both reflect the settling performance of dredged sediment. We analyzed the changes in C and C* for different amounts of added flocculant and water. We assumed that the maximum settlement rate was taken as the critical point between the free settlement stage and interference settlement stage in Figure 4. By fitting Figure 4, the maximum inflection point of the curve was taken as the critical point between the interference settlement stage and compaction stage, we estimate the C value, which is shown in Figure 10; the free settlement coefficient of the dredged sediment was high when the added PAFC content was 0.04 g/mL, but the interference settlement coefficient and compaction settlement coefficient were lower. This indicated that adding flocculant can accelerate the sedimentation rate in the free sedimentation stage; that is, the sedimentation rate will be high when the appropriate amount of flocculant is added, but over time, the flocculation process changes.

In Figure 5, we considered the tests that dredged sediment with 50, 60, 70, 80, and 90 mL of added water, respectively, and we did not consider other test groups with a high solid content. Similarly, Figure 11 shows that the amount of water added strongly influences the sedimentation of dredged sediment; the larger the amount of water added, the faster the sedimentation speed of the dredged sediment in the free and interference sedimentation stages; however, the compaction coefficient in the compaction stage was small, resulting in a poor compaction effect.

As shown in Figure 12, the compaction effect gradually weakened with increasing added water amount; that is, the more the water we added, the stronger the effect in the main settlement stage. The main stages influenced by the flocculation and dehydration of dredged sediment were the free and interference settlement stages.

3.4. Flocculation Dehydration Process and Flocculation Precipitation Mechanism

3.4.1. Flocculation Dehydration Process

The sediment–water separation process is relatively complicated, as the properties of silt affect its water-holding capacity. Flocculants generally affect the electrical properties of sediment and thus its stability. After adding flocculant, flocs are produced by sediment particles under the action of flocculation, and some water remains inside and between the flocs; flocs also increase the particle size of the sediment. Regardless of the free sinking of sediment particles or the sinking of flocculated flocs, resistance occurs during the sinking process, including the forces between flocs, between particles and flocs, and against the inner wall of the measurement cylinder. The lower the resistance, the faster the sinking speed, and the higher the resistance, the less easily the particles sink. Therefore, the flocculation dehydration process of silt is generally as follows: (1) The flocculant affects the zeta potential of the sediment and destabilizes the sediment. (2) The sediment particles form floc groups under the action of the flocculant. (3) The floc groups sink under the action of their own gravity, and sediment–water separation occurs. (4) The floc is subjected to resistance, and the sediment–water separation rate is affected. The dredged sediment flocculation precipitation process is shown in Figure 13.

3.4.2. Flocculation Precipitation Mechanism

PAFC not only has the coagulability of polymeric aluminum, the strong adsorption activity of polymeric iron, rapid precipitation, and a wide application range; it is also economical and practical [28]. The flocculation mechanism of PAFC is as follows [21]:

Fe³⁺ + H₂O→[Fe(OH)]²⁺ + H⁺

(4)

Al³⁺ + H₂O→(AlOH)²⁺ + H⁺

(5)

This is a simplified explanation of the PAFC flocculation mechanism: the real mechanism is more complex. The PAFC flocculation mechanism is affected by the hydrolysis and adsorption characteristics of general aluminum salt and iron salt, its own unique surface complexation, and the processes of surface hydrolysis and subsidence. We simply assume that colloidal particles adsorb dissolved positively charged monomers and multinucleates via base complexes, and then electrical neutralization occurs. The sediment is in an unstable state, resulting in “roll-up” flocculation, and polyaluminum and poly-iron are complementary and synergistic [28]. When the dosage is excessive, the flocculation effect weakens because the flocculation forms colloids again, which is a restabilization phenomenon, which negatively affects the dehydration of dredged sediment.

4. Discussion

Dredged sediment can be simply viewed as a two-phase, solid–liquid system. Due to the high water content in dredged sediment, coarse particles usually settle first via gravity, fine particles are often suspended, and these particles (~1 nm–10 μm) do not easily sink due to Brownian motion [29]. The settling rate of a single particle in the fluid is usually higher than that made up of many solid particles; the settling velocity of particles is related to the particle concentration [30,31]. For dredged sediment treatment, the settling rate is an important parameter; the concentration changes the stress state of the particles and the flow conditions near the particles during the sinking process; the concentration must be taken into account. The difference in concentration leads to the difference in properties for dredged sediment to affect the flocculation effect; it has been verified in the above tests.

The stages of flocculation precipitation can be divided into the free settling stage, interference settling stage, and compaction stage in Figure 7. The settlement is divided into three settlement stages, which is different from the literature; it can explain the flocculation action better [23]. The flocculant can change the settling rate in each stage, and C₁, C₂, and C₃ can indicate the settlement performance of each stage. Meanwhile, it also can be used as the parameter indexes of flocculation effect. In Figure 9, it is also divided into the interference settlement stage and compaction settlement stage; compared with the three-stage division, the two-stage division can explain the effect of flocculant on the dredged sediment dehydration better in the initial stage.

There will be the consolidation stage after long-term settlement for dredged sediment; the dredged sediment in the consolidation stage can be regarded as silty soil [32]. In Figure 10 and Figure 11, the consolidation effect is weak during the test time, which can be regarded as the settlement dehydration has been completed in the free settlement stage and interference settlement stage. The flocculant dosage and water content mainly have a great effect on the free settling stage and interference settling stage; they have little effect on the compaction stage, especially the water content. The sedimentation parameters given in this paper can reflect the sedimentation performance in different sedimentation stages during the sedimentation process; they differ from the performance of the flocculation evaluation parameters of overall flocculation [33]. In Figure 12, C* can be an evaluation index for the main sedimentation stage or the compression consolidation stage, but it is limited to dredged sediment with high water content; however, the compression consolidation exists in the sedimentation process, especially for the high solid content dredged sediment.

The flocculation settlement of dredged sediment has a great relationship with time. The flocculants can affect the settlement rate in different settlement stages, but they cannot promote the final consolidation settlement, and it may hinder the final consolidation settlement due to the pore structure of the floc. Although this paper divides the stages of the precipitation and puts forward parameters to evaluate the flocculation effect, the properties of dredged sediment are quite different, and the water content is positively correlated with the dredged sediment volume. In practice, the dilution degree of dredged sediment and flocculant dosage should be accorded to the laboratory test. Meanwhile, the treatment time and water content requirements after treatment also need to be considered.

5. Conclusions

In this study, the following conclusions were obtained through the laboratory testing of dredged sediment:

• The flocculation effect of dredged sediment is related to the flocculant dosage, water content, and flocculation time. The optimal dosage of PAFC changed with time, the optimal dosage of PAFC has the best flocculation effect, the flocculation effect decreased when the PAFC content exceeded the optimal dosage, and the dilution of the dredged sediment was conducive to flocculation precipitation. The flocculation precipitation of dredged sediment had an optimal flocculation precipitation time, after which the flocculation effect substantially decreased and was even lower than that of the free sedimentation dredged sediment.
• The higher the water content, the stronger the flocculation precipitation effect. In the initial stage, the flocculant can markedly increase the flocculation precipitation rate, but over time, flocculation hindered the sedimentation and compression processes; the higher the solid content, the stronger the effect.
• The settlement coefficient and compaction settlement index can reflect the settling performance of dredged sediment at different settling stages and can provide reference for selecting flocculant.
• In the PAFC flocculation process, the solid–liquid equilibrium system of the sediment was first broken, and then the particles precipitated through the synergistic flocculation of aluminum and iron salts. The appropriate amount of PAFC should be added: too little does not produce the desired effect, and too many lead to sediment particle restabilization.
• In engineering applications, the flocculant content should be adjusted according to the solid content. For dredged sediment with a high solid content, dilution is required before flocculation dehydration.