Development of softening and ballasted flocculation as a pretreatment process for seawater desalination through a reverse osmosis membrane

doi:10.21203/rs.3.rs-2005992/v1

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Development of softening and ballasted flocculation as a pretreatment process for seawater desalination through a reverse osmosis membrane

https://doi.org/10.21203/rs.3.rs-2005992/v1

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published 11 Feb, 2023

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Efficient desalination through a reverse osmosis (RO) membrane requires the prior removal of blockade-causing substances from raw seawater. We achieve ultrahigh-speed processing by combining traditional softening with ballasted flocculation (SBF) for Ca²⁺ and Mg²⁺ removal. A mixture of Ca(OH)₂ and Na₂CO₃ alkaline agents was most suitable for removing t Ca²⁺ and Mg²⁺ by softening and reducing the amount of generated sludge. In addition, the softening treatment simultaneously removed the suspended solids and bacteria from actual seawater. The settling velocity of the suspended solids generated via seawater softening was extremely low. In case of SBF under optimum conditions for desalinating actual seawater using an anionic polymer flocculant and microsand, the settling velocity exceeded 3.5 cm/s, 833 times higher than the settling velocity of softening without ballasted flocculation. The silt density index of the treated seawater met the water-supply standard of RO membranes (i.e., < 3.0). Furthermore, the dewatering property of the SBF-generated sludge was considerably improved compared with that of the sludge obtained via conventional softening. SBF can efficiently and quickly remove the causative substances of RO membrane fouling from seawater, thereby improving the treatability of generated sludge. SBF provides a new pretreatment process for seawater desalination using RO membranes.

The rapid increase in the global population has raised the demand for freshwater resources for households, agricultural activities, and industries¹. One-fifth of the global population is currently facing water shortages². Therefore, technologies that produce freshwater from the vast amount of seawater are urgently required³. Reverse osmosis (RO) is an energy-saving, highly economical, energy-efficient desalination technology for seawater⁴, with a high freshwater-recovery rate⁵. Accordingly, the RO membrane process is commonly used in desalination facilities⁶ and the importance of seawater desalination technologies using RO membranes is well recognized. However, seawater desalination using RO membranes is prone to membrane fouling^7,8. The main mechanism of membrane fouling is blockage due to scale formation by Ca²⁺ and Mg²⁺ which exist in high concentrations in seawater, and inorganic substances such as silica and carbonates. Another problem is blockage by the organic matter⁹ contained in seawater and bacterial biofilms^10,11.

RO membrane fouling increases the operating costs¹² and time required for washing the membrane; decreases the membrane lifespan, flow rate¹³, and water permeation flux¹⁴; and deteriorates the permeated water quality. These problems seriously reduce freshwater production¹⁵. Therefore, prior to the RO membrane process, the causative substances of fouling must be removed clogging substances using a pretreatment process¹⁶. In modern conventional pretreatments, an acid and/or a scale inhibitor is added to seawater to lower its pH and prevent scale formation by inorganic substances^17,18,19. In addition, suspended solids and colloidal particles can be removed using a combined coagulation and ultrafiltration membrane process²⁰, coagulation and sand filtration²¹, and a dual-media filter with a sand filter and a cartridge filter²². However, these pretreatment processes cannot remove high concentrations of Ca²⁺ and Mg²⁺ from seawater, causing scale generation²³. Although chlorination can control biofouling²⁴, residual chlorine damages the RO membrane²⁵. Therefore, to improve the efficiency of seawater desalination, a new pretreatment process that can easily and quickly remove fouling substances from seawater without damaging the RO membrane is required.

Previously, Ayoub et al.²⁶ introduced seawater softening as a pretreatment process in RO membrane–based desalination. Softening has been long used to remove Ca²⁺ and Mg²⁺ from industrial wastewater and high-hardness water^{27,28,29,30,31,32}. In the softening method, the Ca²⁺ and Mg²⁺ ions in seawater are removed as insoluble salts with adding an alkaline agent, such as sodium hydroxide or calcium hydroxide. Ca²⁺ and Mg²⁺ can be effectively removed from the softened seawater, and the turbidity and bacterial amounts are decreased by the agglutination of the produced insoluble hydroxides and carbonates. Therefore, softening is an attractive option for the pretreatment of seawater before desalination via the RO membrane process. However, the method for seawater softening has not progressed because the sedimentation process is slow and a large amount of sludge is generated³³. Owing to these two disadvantages, the seawater softening method is hindered and its robustness to salinity fluctuations, the neutralization of strong alkali–treated seawater, and the characteristics of the generated sludge are unknown. If these two disadvantages could be resolved, seawater softening could be promoted as a new pretreatment process for RO membrane–based desalination.

Ballasted flocculation, trade-named ACTIFLO, was introduced as an innovative solid–liquid separation process in the 1990s³⁴. In this process, microsand and a polymer flocculent are used to considerably accelerate sedimentation by forming large flocs with a high specific density. At present, ballasted flocculation has been used in various solid–liquid separation processes for obtaining purified water from polluted wastewater ^{35,36,37,38,39}. Furthermore, as a method for reducing hardness of drinking water and industrial water, a process combining softening and ballasted flocculation has been developed by Veolia Water Technologies⁴⁰. Applying such processes, we propose that the fine precipitates of insoluble hydroxides and carbonates generated from seawater by softening can be settled at high speed via ballasted flocculation. In addition, the large flocs formed by the polymer flocculant should reduce the sludge volume and improve dehydration⁴¹.

In this study, we developed an ultrahigh-speed pretreatment technology for RO membrane–based desalination based on softening with ballasted flocculation (SBF). The study proceeds through the following steps: (1) selection of an alkaline agent and optimization of the seawater softening conditions, (2) selection of the polymer coagulant and optimization of the SBF conditions, (3) characterization of treated-water quality by the SBF treatment, (4) evaluation of sludge generation and dehydration using SBF, (5) neutralization of the strongly alkaline treated water with carbon dioxide gas, and (6) estimation of the silt density index (SDI) of the treated water. After following these steps, we obtained a novel pretreatment process for RO membrane–based desalination.

2.1. Removal of Mg²⁺ and Ca²⁺ using the softening agents

Figure 1 shows the pH dependence of the Mg²⁺ and Ca²⁺ concentrations when each alkaline agent was added to the water samples. Mg²⁺ was effectively removed by all alkaline agents and the concentration of Mg²⁺ fell below the lower detection limit (Mg²⁺ < 0.057 mg/L) above pH 11. During the softening process, hydroxide ions (OH⁻) were supplied to the seawater and Mg²⁺ was converted to insoluble magnesium hydroxide (Mg(OH)₂) and removed from the liquid phase. Meanwhile, Ca²⁺ was not removed by adding the alkaline agents NaOH and Ca(OH)₂, but the Ca(OH)₂ + Na₂CO₃ and NaOH + Na₂CO₃ agents effectively removed Ca²⁺. Above pH 11, the Ca²⁺ concentration of the samples softened using these agents was below the lower detection limit (Ca²⁺ < 0.065 mg/L). Ca²⁺ reacted with CO₃²⁻ to form insoluble CaCO₃ under alkaline conditions. The optimum addition volumes of the Ca(OH)₂ + Na₂CO₃ and NaOH + Na₂CO₃ agents for Ca²⁺ and Mg²⁺ removal were 40 and 72 mL per 1,000 mL of seawater, respectively.

2.2. Generation amounts of SS and sludge

Figure 2 shows the SS and sludge generation (SV_60min) amounts after adding each alkaline agent and adjusting the pH to 11. The SS content in the sample with the Na₂CO₃ additive was twice that in the sample without Na₂CO₃. After adding Na₂CO₃ in the forms of the Ca(OH)₂ + Na₂CO₃ and NaOH + Na₂CO₃ agents, the SS contents were observed to be similar and statistically equivalent (p > 0.05). Na₂CO₃ formed calcium carbonate and increased the SS content in seawater. Comparing the sludge amounts generated by the Ca(OH)₂ + Na₂CO₃ and NaOH + Na₂CO₃ agents, the SV_60min in the Ca(OH)₂ + Na₂CO₃ system was 25.2%, one-third of that in the NaOH + Na₂CO₃ system. The sludge produced using NaOH had more pore water than that produced with Ca(OH)₂, and the specific gravity of the sludge became lower. Ayoub and Merhebi³³ reported the high compressive properties of the Ca(OH)₂-treated sludge. To achieve the desired low amount of sludge generation, we selected Ca(OH)₂ + Na₂CO₃ as the most suitable alkaline agent for the softening treatment of seawater.

2.3. Relation between salinity and dosage of the alkaline agent

The alkaline-agent dosage in the softening treatment depends on the Mg²⁺ and Ca²⁺ concentrations in the seawater. Therefore, we investigated the optimum dosage of the alkaline agent at different salinities in the artificial seawater. At the optimum dosage, the Mg²⁺ and Ca²⁺ concentrations were reduced to below the lower detection limit. As shown in Fig. 4, the optimum dosage of the alkaline agent is linearly related to salinity. As the positive correlation was extremely high, the optimum dosage of the alkaline agent could be determined from the salinity of the seawater. The optimum dosage of the alkaline agent obtained from the correlation presented in Fig. 3 was then applied to actual seawater. Because the softening in seawater could be inhibited by turbidity and bacteria, we examined the optimum dosage of the alkaline-agent dosage for actual seawater. From the correlation between salinity and the alkaline-agent dosage, the optimum dosage rate of alkaline-agent in actual seawater was determined as 100%. The softening treatment was further examined at the alkaline-agent dosage rates of 110%, 120%, 130%, 140%, and 150%. Fig. S1 shows the Mg²⁺ and Ca²⁺ concentrations at the alkaline-agent dosage rates of 100–150% during the softening of three samples of actual seawater. At the 100% dosage rate, the Mg²⁺ and Ca²⁺ concentrations decreased considerably but the ions were not completely removed from the treated water. By contrast, at dosage rates above 120%, no Mg²⁺ or Ca²⁺ was detected in the treated water. Based on these results, the optimum dosage of the alkaline agent in actual seawater was multiply by 1.2 (120%) obtained with artificial seawater.

2.4. Treated-water quality of the actual seawater samples after softening

Table S1 shows the treated-water qualities after softening the seawater samples collected from different sampling points. Mg²⁺ and Ca²⁺ were almost completely removed from all actual seawater samples (removal rates ≈ 100%). The K⁺ removal rate was very low (3.8–6.8%), and the Na⁺ removal rate was zero (in fact, the Na⁺ concentration was higher in the treated seawater than in the raw water because the alkaline agent comprised Na⁺). Meanwhile, SS aggregated with the generation of Mg(OH)₂ and CaCO₃, affording high removal efficiency for turbidity. Furthermore, no E. coli and total coliforms were detected in the treated seawater and the number of heterotrophic bacteria was reduced by 95.9–100%. Ayoub et al.²⁶ similarly reported the complete removal of total and fecal coliforms above pH 10.5. Therefore, the softening treatment can simultaneously remove Mg²⁺ and Ca²⁺, SS, and bacteria from actual seawater. However, the settling velocity of the SS generated by the seawater softening treatment was extremely low. In addition, the SV_60min exceeded 29% and the amount of generated sludge was large.

2.5. Optimization of SBF

Figure 4 plots the floc-settling velocity versus the dosage of each polymer flocculant in the SBF process using artificial seawater. The optimum dosage of alkaline agent and 10 g/L of the microsand was added to artificial seawater. When added at a dosage of 15 mg/L, the four polymer flocculants AP335B, AP825B, AP335PWS, and AP120PWS, which have high- and medium degree of anion (Table S2), formed large flocs with considerably increased settling velocities (< 4.5 cm/s). At a dosage of 20 mg/L, the settling velocity was further increased more than 6 cm/s using AP825B and AP120PWS. For comparison, the settling velocity without the addition of a polymer flocculant was only 2.8 × 10^− 3 cm/s. Ballasted flocculation of the artificial seawater increased the settling velocity by a factor of 1.9 × 10³–2.3 × 10³. The highest floc-settling velocity (6.5 cm/s) was achieved by adding the polymer AP825B at a dosage of 20 mg/L. AP825B has the smallest molecular weight between the four high- and medium-anion polymer flocculants (Table S2). Therefore, the specific gravity of flocs is considered to be high because the water content of the formed flocs was low. By contrast, the anionic polymer flocculant AP119, which has extremely high anionic strength, and the nonionic NP800PWS exerted no ballasted flocculation effect. The anionic polymer flocculants with extremely high ionic strength and the nonionic polymer flocculants with no ionic strength are unsuitable for flocculation in the ballast treatment.

The volumes of sludge generation in the systems with various polymer flocculants after 3 min of standing (SV_3min) are compared in Fig. 5. In the absence of a polymer flocculant, the settling velocity of the floc was extremely low and settling was barely observed after 3 min (SV_3min = 99.0%). The polymer flocculants, which accelerated the floc sedimentation, markedly decreased the SV_3min with an increase in their dosage. Among the seven types of polymer flocculants, the four specified high- and medium-anion polymer flocculants, which accelerated floc-settling velocity, produced small amount of sludge (< 25%) at 20-mg/L dosage. Between the four polymer flocculants, AP335B and AP825B at a dosage of 20 mg/L achieved the lowest SV_3min, averaging 21.5% and 22.0%, respectively, with no significant difference between the two values (p > 0.05). Conversely, the nonionic polymer NP800PWS, which could not form flocs, generated a large amount of sludge. Based on the settling velocities and SV_3min values of the flocs, AP825B was determined as the most suitable polymer flocculant for the SBF treatment of seawater.

2.6. Relation between floc-settling velocity and sludge-generation volume at different microsand dosages

The settling velocity of the flocs was considered to depend on the microsand dosage. Therefore, the floc-settling velocity was investigated in SBF using the AP825B flocculant with different dosages of microsand (0, 3, 10, or 20 g/L). Figure 6 shows the relation of the settling velocity of the flocs to the flocculant dosage at each microsand dosage. Large flocs with considerably increased settling velocities were formed at the polymer dosages of > 15 mg/L, regardless of the microsand dosage. The microsand dosage influenced the settling velocity. In the SBF with 20 g/L of microsand, the settling velocity was maximized and reached 7.4 cm/s. When polymer was added in excess (> 25 mg/L), the settling velocity tended to decrease. The excessive addition of the anionic polymer caused the negatively charged particles to electrostatically repel each other, thereby hindering floc formation. Therefore, determining the appropriate amount of polymer flocculant for the given microsand dosage is an important part of the SBF treatment.

Figure S2 compares the volumes of sludge generation at each dosage of microsand. The SV_3min decreased considerably when the dosage of the polymer flocculant exceeded 10 mg/L. At polymer dosages of 15 and 20 mg/L, increasing the microsand dosage to 10 g/L decreased the amount of sludge to 20% because the increased specific density of the flocs caused compressive settling. However, when 20 mg/L of polymer was added, the SV_3min values were not significantly different at the microsand dosages of 10 and 20 g/L (p > 0.05). Based on the observed floc-settling velocities and SV_3min values, the optimum polymer flocculant and sand dosages of the SBF were determined as 20 and 10g /L, respectively.

2.7. Neutralization of the alkaline agent–treated water

The treated water generated in SBF is highly alkaline and must be neutralized. Herein, the treated water was neutralized under the optimum conditions. The pH changes during neutralization with H₂SO₄ and CO₂ are shown in Fig. S3. In the neutralization with H₂SO₄, the pH dropped sharply after neutralization because H₂SO₄ was added in excess, resulting in acidification. In addition, as sulfuric acid is a deleterious substance, restrictions are imposed on its use. Conversely, in neutralization by aeration using the CO₂ gas, the pH decreased to 5.8 at neutralization and remained 5.8 while further aeration with the gas. Neutralization of treated water is easily achieved using CO2, and CO₂ is harmless. Based on these results, the CO₂ gas was selected as the neutralizer of actual seawater after processing via SBF.

2.8. Application of SBF to actual seawater

After performing softening and ballasted flocculation under the optimum conditions, the settling velocity of the actual seawater exceeded 3.5 cm/s, 833 times higher than the settling velocity after softening without ballasted flocculation. The SV_3min value of each sample was < 32.5%. The SBF treatability results of each water-quality parameter in the actual seawater samples are given in Table 1. The treatability results matched those of the softening treatment (Table S1). Mg²⁺ and Ca²⁺ were almost completely removed from all actual seawater samples, with removal rates of ~ 100%. The average SiO₂ removal rate was 23.6%. The silicate molybdic acid method used in this study can determine the dissolved SiO₂, which exists in ionic, molecular, and colloidal chemical forms. The ionic fraction of SiO₂ would be removed via electrostatic adsorption in the aggregation process by Mg(OH)₂ used in the softening process. By contrast, the Na⁺ concentration was higher in the treated water than in the raw seawater because it was contributed by the alkaline softening agent. Meanwhile, the turbidity was removed at high rate because the SSs were aggregated with the insoluble Mg(OH)₂ and CaCO₃ products of softening. No E. coli and total coliforms were detected in the treated water, and the numbers of heterotrophic bacteria were reduced by 88.0–99.5%. Therefore, SBF can efficiently remove Mg²⁺ and Ca²⁺, SSs, and bacteria from seawater.

Table 1

The treatability of SBF for each water quality parameter for actual seawater
(a) Miyazaki Port
Parameter	Unit	Miyazaki Port
		Raw water	Treated water
		Mean ± SD (n = 3)	Mean ± SD (n = 3)
pH	-	8.2 ± 3.9 × 10^− 2	11.7 ± 4.1 × 10^− 2
EC	mS/cm	37.9 ± 9.4 × 10^− 2	39.2 ± 0.33
Turbidity	ppm	1.8 ± 0.70	2.1 ± 0.35
SiO₂	mg/L	1.3 ± 0.25	1.4 ± 0.85
Mg²⁺	mg/L	1,488 ± 51.1	BDL*
Ca²⁺	mg/L	464 ± 21.1	BDL
Na⁺	mg/L	9,985 ± 383.5	12,695 ± 245.3
K⁺	mg/L	376 ± 13.1	375 ± 13.0
E. coli	CFU/100 mL	5.3 ± 2.9	0
Coliforms	CFU/100 mL	57.3 ± 40.9	0
Heterotrophic bacteria	CFU/100 mL	5.8 × 10⁴ ± 2.5 × 10⁴	7.0 × 10³ ± 8.2 × 10²
(b) Aoshima Port
Parameter	Unit	Aoshima Port
		Raw water	Treated water
		Mean ± SD (n = 3)	Mean ± SD (n = 3)
pH	-	8.3 ± 3.0 × 10^− 3	12.5 ± 3.0 × 10^− 2
EC	mS/cm	40.6 ± 9.4 × 10^− 2	41.6 ± 0.49
Turbidity	ppm	2.6 ± 2.3 × 10^− 2	0.93 ± 0.14
SiO₂	mg/L	1.1 ± 9.4 × 10^− 2	0.6 ± 0.24
Mg²⁺	mg/L	1,595 ± 6.0	BDL
Ca²⁺	mg/L	474 ± 2.1	BDL
Na⁺	mg/L	11,105 ± 138.2	13,539 ± 163.4
K⁺	mg/L	386 ± 1.0	358 ± 3.7
E. coli	CFU/100 mL	0	0
Coliforms	CFU/100 mL	5.3 ± 2.1	0
Heterotrophic bacteria	CFU/100 mL	3.2 × 10⁴ ± 3.1 × 10⁴	6.7 × 10² ± 9.4 × 10²
(c)Tukunami River
Parameter	Unit	Tsukunami River
		Raw water	Treated water
		Mean ± SD (n = 3)	Mean ± SD (n = 3)
pH	-	8.4 ± 4.5 × 10^− 3	11.9 ± 3.0 × 10^− 2
EC	mS/cm	36.7 ± 0.62	37.0 ± 9.4 × 10^− 2
Turbidity	ppm	6.4 ± 0.84	1.5 ± 0.85
SiO₂	mg/L	1.3 ± 0.36	0.97 ± 0.52
Mg²⁺	mg/L	1,497 ± 5.7	BDL
Ca²⁺	mg/L	464 ± 10.7	BDL
Na⁺	mg/L	10,450 ± 110.0	12,415 ± 101.8
K⁺	mg/L	383 ± 8.1	354 ± 3.6
E. coli	CFU/100 mL	13.3 ± 1.7	0
Coliforms	CFU/100 mL	50.3 ± 9.0	0
Heterotrophic bacteria	CFU/100 mL	8.7 × 10⁵ ± 8.2 × 10⁵	4.0 × 10³ ± 8.2 × 10²
*BDL: Below detection limit

2.9. Evaluation of SDI of actual seawater

To assess the applicability of the seawater treated via SBF and neutralized using CO₂ in the RO membrane process, the SDIs of the raw and treated seawater samples collected form Miyazaki Port, Aoshima Port, and the mouth of Tsukunami River were computed and are presented in Table S3. The SDI values of all the raw seawater samples exceeded 5.7, over measuring range of the SDI measurement device. Actual seawater should not be directly passed through the RO membrane. Conversely, the SDI values of the treated seawater were below 2.9. The SDI of water applied to RO membranes should not exceed the standard value, which is generally set below 3.0^42,43. The SDI values determined in the present study confirmed that after the pretreatment using SBF, the seawater achieved the water-supply standard of RO membranes.

2.10. Sludge dehydration

Finally, we compared the specific resistances of the sludge generated via conventional softening and SBF. Figure 7 shows the results of the Buchner funnel test on the artificial seawater and actual seawater samples (Miyazaki Port and the mouth of Tsukunami River). After conventional softening, the sludges obtained from artificial seawater and both types of actual seawater samples yielded similar slopes in the plots of the reciprocal filtration rate versus the amount of passing water (with values of 4.40–4.80). The slope associated with the sludge obtained from the artificial seawater generated using SBF was less than 1/10 that of the conventionally softened sludge. The decreased slope of the reciprocal filtration rate implies the considerably improved water permeability of the sludge. The slope further decreased to 0.11–0.15 after the SBF of the actual seawater samples. Table S4 shows the Ruth constants⁴⁴ K and C obtained through the dehydration test of each sample. The K and C values indicate the filtration resistances of a sludge cake and the filter medium, respectively. In the case of artificial seawater, the K and C values were 9.9 and 2.1 times higher, respectively, in case of the sludge generated via SBF than those in case of the sludge formed using conventional softening. In the sludges obtained from actual seawater, the K and C values were 34.5–45.5 and 3.9–8.9 times higher, respectively, after the SBF treatment than those after conventional softening. The sludge generated via SBF exhibited an extremely high permeability, and its dehydration was considerably better than that of the sludge formed using conventional softening.

2.11. Advantage of SBF

As a pretreatment process for RO membrane–based seawater desalination, we investigated an SBF treatment process that combines the existing softening process with ballasted flocculation–sedimentation for ultrahigh-speed precipitation. Based on the removal efficiencies of Ca²⁺ and Mg²⁺ and the amount of generated sludge, the Ca(OH)₂ + Na₂CO₃ agent was determined as the best softening agent. The SBF process with the Ca(OH)₂ + Na₂CO₃ agent effectively removed Ca²⁺ and Mg²⁺ from actual seawater samples and reduced the turbidity and bacterial counts in the seawater. Furthermore, the seawater samples with different salinities could be treated by adjusting the dosage of the alkaline agent. The settling velocities of magnesium hydroxide and calcium carbonate produced via conventional softening were extremely low and could not meet the requirements of pretreatment technologies for practical RO desalination.

By contrast, SBF increased the settling velocity by 833 times compared with that obtained via conventional softening while achieving the same water quality. The optimum SBF conditions for treating seawater with a salinity of 3.5% were determined as follows: pH = 11, alkaline agent = Ca(OH)₂ + Na₂CO₃ at a dosage of 40 mL/L, anionic polymer flocculant = AP825B at a dosage of 20 mg/L, and microsand concentration = 10 g/L. The strongly alkaline treated seawater was easily neutralized to pH 5.8 via aeration using the carbon dioxide gas. The optimized SBF process reduced the SDI of seawater below the standard value (3.0) and considerably improved the dewatering property of the generated sludge compared with that of the sludge obtained via conventional softening.

Dissolved substances such as Ca²⁺ and Mg²⁺ cannot be removed by existing the pretreatment methods for RO membrane such as sand filtration and microfiltration (MF membrane) based on the principle of physical filtration. In addition, cleaning of sand and/or membrane is essential to maintain physical filtration. SBF can efficiently and rapidly remove the causative substances of RO membrane fouling from seawater and can greatly improve the treatability of the generated sludge. We therefore propose SBF as a new pretreatment process for RO membrane–based seawater desalination.

3.1. Seawater preparation

To perform basic studies and comparisons of SBF, we required raw water with a consistent water quality. Therefore, artificial seawater was prepared for the experiments. Table S1 shows the composition of the artificial seawater. The Ca²⁺ and Mg²⁺ concentrations were 400 and 1,289 mg/L, respectively. Assuming that the method developed based on artificial seawater will be used in practical applications, we collected actual seawater from five sites in the Miyazaki City, Japan (Fig. 8) : Aoshima Port (a small fishing port adjacent to a tourist beach), the mouth of the Tsukunami River (the brackish water area of a small river), Kizaki Beach (a sandy beach that serves as an international surfing venue), Shirahama Beach (a sandy beach), Miyazaki Port (the largest port in a berthing area for large vessels), and Sun-Marina Port (an artificial yacht harbor). The seawater samples were collected from the surface layer at each site in a polyethylene container and transported to the laboratory in the Miyazaki City for experimental study.

3.2. Alkaline agents

Softening can be performed with the lime aggregation method using calcium hydroxide (Ca(OH)₂) and sodium carbonate (Na₂CO₃)^45,46 or via the soda aggregation method using sodium hydroxide (NaOH) and Na₂CO₃²⁶. Therefore, four types of alkaline agents, i.e., only Ca(OH)₂, only NaOH, mixed Ca(OH)₂ and Na₂CO₃ (Ca(OH)₂ + Na₂CO₃), and mixed NaOH and Na₂CO₃ (NaOH + Na₂CO₃), were examined as softening agents in the present study. In the Ca(OH)₂ + Na₂CO₃ agent, 1.7 mol/L of Ca(OH)₂ and 2.0 mol/L of Na₂CO₃ were used based on the conventional softening protocol with respect to the Ca²⁺ and Mg²⁺ concentrations in artificial seawater⁴⁷. In the NaOH + Na₂CO₃ agent, 1.35 mol/L of NaOH and 0.675 mol/L of Na₂CO₃ were used as per the report by Ayoub et al.²⁶. The single Ca(OH)₂ and NaOH agents were prepared at the 1.7- and 5.0-mol/L concentrations, respectively.

3.3. Microsand and polymer flocculants used for ballasted flocculation

Silica sand (quartz content > 98%, specific gravity = 2.6, particle size = 53–212 µm, V7 type; Mikawakeiseki Co., Japan) was used as ballast in microsand. We selected six types of anionic polymers (AP199, AP335B, AP825B, AP335PWS, AP120PWS, and AP410PWS; Mitsubishi Chemical Corporation, Japan) and one nonionic polymer (NP800PWS, Mitsubishi Chemical Corporation) as polymer flocculants. The properties of polymer flocculants are shown in Tables S1. The stock solution of each polymer flocculant was dissolved in distilled water to adjust the concentration to 0.1 (w/v%).

3.4. Softening treatment

In a 500- or 1,000-mL beaker, 500 or 1,000 mL of water sample was placed, and the beaker was set in a jar tester (JMD-8E, Miyamoto Riken Ind., Japan). A prescribed amount of each alkaline agent, such as single Ca(OH)₂, single NaOH, Ca(OH)₂ + Na₂CO₃, and NaOH + Na₂CO₃, was added to the sample with stirring. The sample was stirred rapidly (100 rpm) for 1 min and then slowly (30 rpm) for 20 min. After allowing the mixture to stand for 60 minutes, a precipitate was formed, and the supernatant water (called the treated water) was gently collected using a syringe.

In addition, to determine an appropriate dosage of alkaline agent for seawaters with different salinities, the artificial seawater was diluted to the salinities of 0.34–3.4% using distilled water. The appropriate amount of added alkaline agent was determined for each salinity value.

3.5. SBF

To ensure uniform stirring and mixing of the insoluble salt suspensions produced via softening and ballasted flocculation (using microsand and polymer flocculant), four baffle plates (width = 15 mm and height = 140 mm) were fixed inside the beaker. An alkaline agent was added to 1,000 mL of the sample placed in the 1,000-mL beaker set in a jar tester with rapid stirring (200 rpm) for 1 min. Subsequently, silica sand (0–20 g/L) was directly added, and after stirring at 200 rpm for 2 min, a prescribed dosage (0–20 mg/L) of polymer flocculant was injected into the samples. After injecting the polymer flocculant, the stirring speed was adjusted to 140 rpm and the mixture was further stirred for 3 min. After allowing the mixture to stand for 3 min, 100 mL of the supernatant water (treated water) was gently collected using a syringe (5 cm from the top of the supernatant).

3.6. Water-quality analysis

The pH values and electric conductivities of the samples were determined using a water-quality analyzer (LAQUA F-74, Horiba, Japan), and the turbidities were determined via a turbidity meter (PT-200, Nittoseiko Analytech, Japan). The Na⁺, K⁺, Mg²⁺, and Ca²⁺ concentrations in the sample (diluted100 fold with distilled water) were analyzed using an inductively coupled plasma emission spectroscopic analyzer (ICPS-8100, Shimadzu, Japan). The SiO₂ concentration was measured using a portable absorptiometer (DR-2800, Hach, U.S.) based on the silicate molybdic acid method according to the manufacturer's instructions. To obtain suspended solids (SS), 50 ml of the sample was passed through a glass fiber filter (GF/F; diameter = 47 mm; pore size 0.7 µm, Whatman, Merck, Germany) and dried at 105°C for 2 h. Bacterial counts were measured in the actual seawater. Escherichia coli and total coliform colonies were counted after growth on CHROMagar ECC agar plates (CHROMagar, Paris, France). For this, 100 mL of each water sample was filtered through a 0.45-µm pore size–membrane filter (47-mm diameter, sterile, mixed cellulose ester; Advantec, Tokyo, Japan) and incubated on ECC agar plates for 24 h at 37°C. Blue colonies were designated as E. coli isolates and mauve colonies were designated as the isolates of other coliform species. Heterotrophic bacteria were counted on agar plates (1.5% agar, Difco Marine Broth 2216; Becton, Dickinson and Company, MD, USA). The seawater and treated-water samples were spread onto the plates and incubated for seven days at 22.5°C ± 2.5°C. After incubation, all colonies were counted as heterotrophic bacteria. The numbers of coliform bacteria, E. coli, enterococci, and heterotrophic bacteria in all samples were determined based on the mean number of colony-forming units (CFUs) of three replicates. The bacterial count was expressed as CFU/100 mL water.

3.7. Determination of floc-settling velocity and sludge volume

Immediately after slow stirring in a jar tester, the settling velocity of the flocs was measured as the settling depth (cm) at the interface between the floc suspension and supernatant water per unit time (cm/s). The settling velocity was reported as the average of three measurements of the jar test. If the floc-settling velocity exceeded 2.0 cm/s (i.e., it was too fast to measure using this approach), a 2,000-mL glass cylinder (height = 50 cm) was filled with treated water and a portion of the sludge floc was separated and gently poured onto the water surface. The settling velocity was again measured as the settling depth per unit time. In the cylinder, the settling velocity was determined as the average of five measurements.

The sludge volume (SV) after a set time (settling for 3 min, SV_3min; settling for 60 min, SV_60min) was calculated as follows:

SV (%) = sludge volume (mL)/sample water volume (mL) × 100.

3.8. Neutralization of treated seawater

The softened water is strongly alkaline. Direct passage of strongly alkaline water through the RO membrane increases the burden on the membrane and corrodes the water pipes⁴⁸. To avoid these problems, the water must be neutralized before being subjected to the RO membrane process. In this study, acid and carbon dioxide⁴⁹ were used to neutralize the alkaline water. In the neutralization with acid, the pH of water was measured while dropwise adding a 1-mol/L sulfuric acid (H₂SO₄) solution to 1,000 mL of treated water. In the neutralization with carbon dioxide (CO₂), the pH was measured while passing CO₂ gas through 1,000 mL of softened water (flow rate = 0.5 L/min).

3.9. SDI measurement

The SDI method is the usual method for evaluating the quality of water used for the RO membrane process and is stipulated in the American Society for Testing and Materials (ASTM)⁵⁰. After neutralizing the treated water with CO₂, SDI₁₅ (measured during 15 min of water passage) was determined using an SDI automatic measuring device (Simple SDI 2.0, Tosc Japan, Japan) based on the ASTM regulation.

3.10. Buchner funnel test for determining specific resistance

The specific resistance of the sludge was evaluated via the Buchner funnel test using a Buchner funnel (diameter = 80 mm, height 160 mm) and a standard filter paper (diameter = 90 mm; No. 1, Advantec, Toyo Roshi Co., Japan). Buchner funnel test was performed using gravity filtration. A sludge sample generated from 1,000 mL of seawater was added to the funnel, and the filtration duration and amount of filtrate were determined. The dehydration tolerance of the sludge was evaluated in terms of the filtration constants K (cm⁶/s) and C (cm³) as defined by Ruth⁴⁴.

Acknowledgments

Nishihara Environment Co., Ltd. provided the experimental materials. We would like to thank all the people concerned for their cooperation.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Author Contributions

Tomohiro Yadai: Investigation, Data Analysis, Writing – Original Draft. Yoshihiro Suzuki: Conceptualization, Methodology, Investigation, Writing – Original Draft, Writing — Review & Editing, Supervision.

Competing Interest Statement

The authors declare no conflicts of interest associated with this manuscript.

Data availability statement

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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You are reading this latest preprint version

Development of softening and ballasted flocculation as a pretreatment process for seawater desalination through a reverse osmosis membrane

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Results And Discussion

2.1. Removal of Mg²⁺ and Ca²⁺ using the softening agents

2.2. Generation amounts of SS and sludge

2.3. Relation between salinity and dosage of the alkaline agent

2.4. Treated-water quality of the actual seawater samples after softening

2.5. Optimization of SBF

2.6. Relation between floc-settling velocity and sludge-generation volume at different microsand dosages

2.7. Neutralization of the alkaline agent–treated water

2.8. Application of SBF to actual seawater

2.9. Evaluation of SDI of actual seawater

2.10. Sludge dehydration

2.11. Advantage of SBF

3. Materials And Methods

3.1. Seawater preparation

3.2. Alkaline agents

3.3. Microsand and polymer flocculants used for ballasted flocculation

3.4. Softening treatment

3.5. SBF

3.6. Water-quality analysis

3.7. Determination of floc-settling velocity and sludge volume

3.8. Neutralization of treated seawater

3.9. SDI measurement

3.10. Buchner funnel test for determining specific resistance

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

Development of softening and ballasted flocculation as a pretreatment process for seawater desalination through a reverse osmosis membrane

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Results And Discussion

2.1. Removal of Mg2+ and Ca2+ using the softening agents

2.2. Generation amounts of SS and sludge

2.3. Relation between salinity and dosage of the alkaline agent

2.4. Treated-water quality of the actual seawater samples after softening

2.5. Optimization of SBF

2.6. Relation between floc-settling velocity and sludge-generation volume at different microsand dosages

2.7. Neutralization of the alkaline agent–treated water

2.8. Application of SBF to actual seawater

2.9. Evaluation of SDI of actual seawater

2.10. Sludge dehydration

2.11. Advantage of SBF

3. Materials And Methods

3.1. Seawater preparation

3.2. Alkaline agents

3.3. Microsand and polymer flocculants used for ballasted flocculation

3.4. Softening treatment

3.5. SBF

3.6. Water-quality analysis

3.7. Determination of floc-settling velocity and sludge volume

3.8. Neutralization of treated seawater

3.9. SDI measurement

3.10. Buchner funnel test for determining specific resistance

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

2.1. Removal of Mg²⁺ and Ca²⁺ using the softening agents