Environ Sci Pollut Res (2014) 21:2876–2887 DOI 10.1007/s11356-013-2237-1


Precipitation softening: a pretreatment process for seawater desalination George M. Ayoub & Ramez M. Zayyat & Mahmoud Al-Hindi

Received: 1 July 2013 / Accepted: 10 October 2013 / Published online: 23 October 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract Reduction of membrane fouling in reverse osmosis systems and elimination of scaling of heat transfer surfaces in thermal plants are a major challenge in the desalination of seawater. Precipitation softening has the potential of eliminating the major fouling and scaling species in seawater desalination plants, thus allowing thermal plants to operate at higher top brine temperatures and membrane plants to operate at a reduced risk of fouling, leading to lower desalinated water costs. This work evaluated the use of precipitation softening as a pretreatment step for seawater desalination. The effectiveness of the process in removing several scaleinducing materials such as calcium, magnesium, silica, and boron was investigated under variable conditions of temperature and pH. The treatment process was also applied to seawater spiked with other known fouling species such as iron and bacteria to determine the efficiency of removal. The results of this work show that precipitation softening at a pH of 11 leads to complete elimination of calcium, silica, and bacteria; to very high removal efficiencies of magnesium and iron (99.6 and 99.2 %, respectively); and to a reasonably good removal efficiency of boron (61 %). Keywords Pretreatment . Desalination . Calcium carbonate . Magnesium hydroxide . Precipitation . Seawater Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-013-2237-1) contains supplementary material, which is available to authorized users. G. M. Ayoub (*) : R. M. Zayyat Civil and Environmental Engineering Department, American University of Beirut, Beirut, Lebanon e-mail: [email protected] M. Al-Hindi Chemical Engineering Program, American University of Beirut, Beirut, Lebanon

Introduction With the increasing demand on the declining world natural freshwater resources, alternative water supply options are being sought after. Among such options, seawater desalination stands out as one of the most promising and sustainable water supply solutions. Desalination technologies are broadly classified into two categories: thermal-based processes and membrane-based processes. Thermal-based units include multi-stage flash (MSF) and multi-effect distillation (MED) units, with the former dominating the desalination market during the latter part of the last century. On the other hand, membrane-based technologies, especially the reverse osmosis process, have grown at a tremendous rate over the past 40 years and at present account for approximately 44 % of the market share and 80 % of the total number of installed desalination plants worldwide (Greenlee et al. 2009). Membrane fouling stands as one of the major issues in maintaining sustainable operation of seawater reverse osmosis (SWRO) systems as fouling leads to the deterioration of the basic membrane functions such as salt passage through the membrane, reduction in permeate flux, increase in pressure drop across the membrane due to membrane pore plugging as well as to higher operation costs due to higher energy demand, increase of cleaning frequency, and reduced lifetime of the membrane elements (Valavala et al. 2011). Membrane fouling is normally associated with particulate matter and colloids, organic and inorganic compounds, and biological growth. Colloidal particles are typically composed of clay, organic materials (where humic substances constitute the major portion of the organics in seawater; Dalvi et al. 2000), and metal inorganics, while biological fouling is related to the presence of bacteria, fungus, and algae where the microbial cells accumulate and attach to the surface of the membrane, thus promoting biofilm growth (Matin et al. 2011). Membrane autopsies carried out by various researchers have

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demonstrated that membrane fouling deposits include inorganic species as well as biological and organic matter (Fernandez-Álvarez et al. 2010; Melián-Martel et al. 2012). Several materials have been reported to have an influence on the fouling of SWRO membranes; these include, but are not limited to, calcium, magnesium, bacteria, iron, silica, organic, and colloidal matter. Boron, although not reported to be a fouling species, usually poses a problem in SWRO treatment because of the difficulty of its removal by reverse osmosis (RO) membranes (Nadav et al. 2005) and consequently presents a challenging issue to RO treatment. To alleviate membrane fouling problems, effective pretreatment is most critical for the successful long-term performance of a SWRO desalination plant (Brehant et al. 2002, 2003). Existing pretreatment systems are classified into conventional and membrane-based processes or a combination of both. Conventional pretreatment in SWRO generally includes coagulation/flocculation/sedimentation or flotation followed by granular media filtration (Greenlee et al. 2009; Isaias 2001). The control of bacteria and other microorganisms is performed by disinfection, which involves the use of chlorine, ozone or UV light, and other agents. However, depending on the quality of the feed seawater, all or some of these processes may be applied. Acids such as sulfuric acid or hydrochloric acid (Bonnélye et al. 2007) and/ or anti-scalants are used to control alkaline and non-alkaline metallic scaling (Antony et al. 2011). The membrane processes used in SWRO pretreatment include ultrafiltration/microfiltration. These normally provide silt density index values below 2 (Pearce 2008; Vedavyasan 2007). Also, nanofiltration has been successfully used in RO pretreatment for the removal of particulate and colloidal materials (Greenlee et al. 2009; Choi et al. 2009). The problem of scale deposition is also encountered in thermal desalination plants where alkaline (CaCO3 and Mg(OH)2 either individually or in combination) and nonalkaline scale (mainly calcium sulfate) are reported to occur on the heat transfer surfaces of MED and MSF units (AlRawajfeh et al. 2008, 2011; Al-Sofi 1999; Wildebrand et al. 2007; Shams El Din et al. 2005). Scale control in thermal desalination units can be achieved by several means including one or more of the following: the addition of acid to reduce the propensity of alkaline scaling and the addition of anti-scalants to mitigate the potential of alkaline and non-alkaline scaling and maintain the top brine temperature below 120 °C for MSF and 70 °C for MED (AlSofi 1999; Shams El Din et al. 2005; Wildebrand et al. 2007; Al-Rawajfeh et al. 2011). Other methods such as the use of nanofiltration to reduce the concentrations of calcium, magnesium, and sulfate (Hassan et al. 1998); ion exchange for the reduction of sulfates (De Maio et al. 1983); and partial softening combined with nanofiltration (Al-Rawajfeh et al. 2011) have been used on a limited scale.


Precipitation softening through the use of a variety of chemicals, such as lime, sodium bicarbonate, sodium carbonate, and sodium hydroxide, whether individually or in combination, has been utilized for the removal of scalecausing ions, such as calcium, magnesium, barium, strontium, and silica, from surface water, well water, mine water, wastewater, RO/NF concentrates, and seawater (Gilron et al. 2000; Rahardianto et al. 2007, 2010; Gabelich et al. 2011; Mohammadesmaeili et al. 2010; Comstock et al. 2011; Subramani et al. 2012; Sanciolo et al. 2012; Hussain 2007). Most of this literature describes methods employed for the treatment of the concentrates from brackish water RO/NF plants using conventional softening techniques (Subramani et al. 2012; Comstock et al. 2011) and accelerated seeded precipitation (Rahardianto et al. 2007, 2010; Gabelich et al. 2011; Sanciolo et al. 2012) and the pretreatment of wastewater (Sanciolo et al. 2012; Al-Rawajfeh and Araj 2013) with the aim of improving the overall recovery of the RO system by removing the main fouling species, namely, calcium, silica, phosphates, and sulfates and of reducing discharges to the environment. Precipitation softening of seawater has, to the authors’ knowledge, received limited attention in the literature where the process was investigated as a pretreatment step, either on a stand-alone basis (Irving 1926; Mavis and Checkovich 1975; El-Manharawy and Hafez 2003) or in combination with NF membranes (Al-Rawajfeh and Zarooni 2008; Al-Rawajfeh 2012). For processes utilizing precipitation softening only, the maximum reported removal efficiencies of calcium and magnesium were found to be below 90 and 86 %, respectively (Irving 1926; Mavis and Checkovich 1975; El-Manharawy and Hafez 2003). Conventional coagulation/flocculation forms the core of the proposed pretreatment process in which softening and disinfection are induced as a result of the chemical reactions that occur during the development of the coagulant flocs. The basic coagulant used in this process is magnesium hydroxide, which has proved its efficacy in removing suspended (and in some cases dissolved) solids in the form of clay, silica (Sheikholeslami et al. 2001), organic matter, iron, and aluminum salts and partial removal of bacteria (Gilron et al. 2000), as well as oil and algae (Ayoub and Koopman 1986; Ayoub et al. 1986, 1992). Softening will remove the divalent metallic salts of calcium and magnesium, while the high pH will lead to the inactivation of bacteria, viruses, and other microorganisms (Lechevallier et al. 1988; Zebger et al. 2003; Rincón and Pulgarin 2004). Boron removal has been reported to occur when process water is treated with magnesium hydroxide coagulation (Parks and Edwards 2007). Moreover, operation at increased pH is reported to induce higher boron removal rates (Koseoglu et al. 2008a, b). As a high pH will inactivate the bacteria, the process of chlorination and dechlorination will be eliminated from the treatment system. The rationale leading to the adoption of this


process lies in the fact that seawater harbors large amounts of magnesium, which, at raised pH levels of 10.5–11.5, will coagulate to form magnesium hydroxide flocs. This will provide an extremely cheap source for a coagulant. Raising the pH is normally induced by the addition of lime and caustic soda. By supplementing these chemicals with soda ash, a softening process will be produced. The present study aims to demonstrate the use of precipitation softening as a pretreatment step for seawater desalination. The effectiveness of the process in removing several scale-inducing materials such as calcium, magnesium, and silica is investigated under variable conditions of temperature and pH. The efficiency of removal of boron from seawater is also evaluated. The treatment process is also applied to seawater spiked with other known fouling species such as iron and bacteria to determine the efficiency of removal.

Materials and methods Seawater used in testing Seawater samples used in the testing procedures were collected from the beach at the American University of Beirut which is located on the Eastern Mediterranean shore. Forty-liter samples of seawater were collected on a bimonthly basis and were stored at 24 °C. Regular tests were conducted to characterize the collected samples and to ensure that the water quality remained consistent. To prevent any contamination of the collected samples, the containers were cleaned properly in the laboratory prior to sampling and were rinsed at least twice with seawater at the site prior to sample collection. The tests conducted on the seawater samples included the following parameters: pH, total dissolved solids (TDS), total suspended solids (TSS), volatile suspended solids (VSS), total organic carbon (TOC), Ca, Mg, turbidity, iron, Na, SO4, potassium, silica, boron, and total and fecal coliforms. The averaged results of the various parameters are shown in Table 1. Selection of alkalizing agent In order to achieve maximum removal efficiencies, it was deemed necessary to perform a preliminary set of experiments to select the proper alkalizing agents. For this purpose, 10 % (w /v ) Ca(OH)2 (prepared by adding 100 g of 99 % pure CaO, supplied by Riedel-Dehaëu, to 1 L of distilled water), 5 N NaOH (prepared by adding 200 g of 99.2 % pure NaOH, supplied by High Media, to 1 L of water), and 1.85 N Na2CO3 (prepared by adding 100 g of 99 % pure Na2CO3, supplied by Boekr, to 1 L distilled water) were tested to determine their

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efficiency in removing scaling precursors such as calcium and magnesium using standard jar tests. When Ca(OH)2 was used, the removal efficiencies recorded were 20±1.2 % for Ca, 65±3.8 % for Mg, and 30±0.8 % for B, thus indicating that the use of Ca(OH)2 as an alkalizing agent was not effective for the removal of Ca. A second set of tests was conducted using NaOH, which resulted in substantially better removal efficiencies for Ca (67±2 %) and Mg (84.2± 1.9 %), but not for boron (21±1.2 %). Further tests were conducted where Na2CO3 was used on its own, which resulted in improved Ca removal (90±2.5 %), reduced Mg removal (15 ±4.55 %), and slightly improved B removal (28±1 %). Finally, tests were conducted where Na2CO3 was added to NaOH to evaluate their combined effect. The results showed that this combination was successful in achieving the highest removals. Consequently, further tests were conducted to determine the most effective ratio of NaOH/Na2CO3 which will achieve the highest desired removal levels. Ratios of 1:1, 2:1, and 5:2 were used, which resulted in the following removal percentages: Ca of ≈100 %, ≈100 %, and 95±0.077 %, respectively; Mg of 99.82 ± 0.0042 %, 99.7 ± 0.008 %, and 96.4 ± 0.056 %, respectively; and B of 41±0.8 %, 75±3 %, and 60±3 %, respectively. The ratio of NaOH/Na2CO3 of 2:1 was thus selected for performing the testing procedures. The titration curve for NaOH/Na2CO3 of 2:1 is presented in Fig. 1. Two equivalence points were obtained at dissociation equilibrium constant values of pK a =9.5 and 10.8 with corresponding solution volumes of 3 mL and 35 mL/1 L samples, respectively. Carbonate precipitation started at pH 9.3 and ended at pH 10.0, while hydroxide deposition started at pH 10.5 and completed at pH 11. It is worth noting that each reported result represents the average of four tests. Experimental setup A standard jar test apparatus (model 300, Phipps and Bird, Inc., Richmond, Virginia) was used in conducting the experimental work. The apparatus contained multiple stainless steel paddles and stirrers and was equipped with a variable speed drive motor and control to allow operation at paddle speeds in the range of 0–100 rpm. A set of six jars made of acrylic plastic with dimensions of 115×115×250 mm and designed to hold 2-L samples was also used. The jars were fitted with sampling ports located at 10 cm from the bottom for the purpose of withdrawing samples without disturbing the settled sludge. Experimental procedure Tests were conducted by placing 2-L seawater samples in the jars with subsequent addition of alkalizing agents, as determined from respective tests, until the desired pH was achieved. The samples underwent rapid mixing (100 rpm) for

Environ Sci Pollut Res (2014) 21:2876–2887 Table 1 Characteristics of the collected seawater used in the experimental procedure

SD standard deviation, N/A not applicable




No. of observations




pH units mg/L mg/L mg/L mg/L

26 4 4 4 4

8.02±1.10 37,701±718 206±13 52.6±5 5.10±0.44

7.82–8.15 35,500–39,100 198–220 46.67–58.00 4.58–5.20

Mg Alkalinity (CaCO3) Ca Sodium Boron Turbidity Iron Sulfate Chloride Potassium Silica Total coliforms Fecal coliforms

mg/L mg/L mg/L mg/L mg/L NTU mg/L mg/L mg/L mg/L mg/L CFU CFU

20 5 20 5 10 4 7 3 1 1 10 6 6

1,535.8±55 267±12 521±9.2 11,683±81 5.1±0.3 3.2±0.1 0.29±0.03 2900±74 21200 510 1.5±0.05 0 0

1,460–1,623 250–289 502–530 11,600–11,792 4.8–5.6 3.0–3.4 0.23–0.31 2,810–3,100 N/A N/A 1.4–1.7 N/A N/A

1 min followed by slower paddle velocities of 30 rpm for a period of 20 min to allow for floc formation, followed by a settling period of 60 min. Temperature and pH were monitored at all times during each experiment. Six sets of experiments were conducted for four preset pH values of 10.5, 11, 11.5, and 12 and a fixed temperature of 20 °C. Removals were also assessed at temperatures of 10, 25, and 30 °C and at pH values of 10.5, 11, and 11.5 for each temperature. When carrying out tests at 10 and 30 °C, it was deemed necessary to maintain these temperatures with the least possible variation. For this purpose, polystyrene foam jackets were used to insulate the jars. The supernatant withdrawn after the jar testing procedure was analyzed for the parameters listed in Table 1. Removal percentages and standard deviations were determined.

Further series of tests were conducted by maintaining the pH at 11 and the temperature at 20 °C and spiking the seawater samples with specific concentrations of Si, iron, and bacteria through the addition of sodium silicate, ferrous sulfate, and wastewater, respectively. A testing matrix for the experiments conducted is presented in Electronic supplementary material (ESM) Table S6. Analytical procedures For each set of experiments, the parameters listed in Table 1 were measured at the start of the test and at 5 min intervals throughout the duration of the test. Parameters were determined according to standard methods (APHA, AWWA, WEF 2012) and HACH methods as detailed in Table 2. The statistical analysis methods used in this study included one-way and two-way ANOVA, F testing, hypothesis testing, and regression analysis using Excel and Minitab 16. In addition, interaction plots were used, where applicable, to elaborate the effects of pH and temperature on the removal of certain parameters.

Results and discussion The effect of pH on removal efficiencies

Fig. 1 Titration curve for NaOH/Na2CO3 of 2:1

To assess the effects of pH variation on the removal efficiencies of scale-causing constituents, four pH values (10.5, 11, 11.5, and 12) at a controlled sample temperature of 20 °C were tested. The mean removal efficiencies and the


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Table 2 Respective analytical procedures adopted in parametric analyses Parameter

Type of analysis

APHA reference method or HACH method

pH Temperature Turbidity TOC


5400-H+B 2550 2130 B 5310

TDS TSS VSS Sodium Calcium hardness Total hardness Boron Iron Silica Fecal coliforms Total coliforms Alkalinity Chloride Potassium Sulfates

Gravimetric Gravimetric Gravimetric AAS EDTA titration EDTA titration Spectrophotometric AAS Colorimetric Membrane filtration Membrane filtration Acid titration Argentometric AAS Spectrophotometric


2540 C 2540 D 2540 E 3500-Na D 3500-Ca D 2340-C 4500-B.C/HACH 8015 3500-Fe+B HACH method 8185 9222D 9222B 2320-B 4500-Cl-B 3500-K+D HACH method 8051

AAS atomic absorption spectrophotometry

standard deviations for calcium, magnesium, and boron at the selected pH values are presented in Table 3. An analysis of variance for the Ca, Mg, and Boron removal efficiencies was prepared using the experimental data generated at the various pH values (presented in ESM Table S1). In order to relate and assess the significance of pH on the removal of Ca, Mg, and B, a polynomial regression analysis was also Table 3 Treated seawater constituents at different pH values and T =20 °C

The numbers of observations were 4 for each pH value BDL below detection limit (Ca detection limit = 0.5 mg/L, Si detection limit=0.3 mg/L), N/A not applicable, NM not measured a

pHf represents the pH recorded at the end of the jar test procedure and is compared to the pH to check for pH variation due to CO2 escape or entry


performed (as shown in ESM Table S4). The respective values of F and P confirm that the pH variation impacts the removal of Ca, Mg, and Boron at 100 % significance. From Table 3, it can be seen that the pH value of 10.5 resulted in the highest removal efficiency for boron and that the increase in pH beyond 11.0 resulted in a decrease in the percentage removal of boron. This result is consistent with the observation reported by Parks and Edwards (2007) who also noted that the optimal removal efficiency of boron was at a pH of 10.8 (the efficiency of removal recorded for seawater was at 59 %, while for brackish water the removal efficiency was 70 %). The removal of boron is closely associated with the precipitation of magnesium, where it has been hypothesized that at pHT 1), leading to higher precipitation and therefore higher calcium removal efficiencies.

The effect of temperature on removal efficiencies The impact of temperature on the removal efficiencies was evaluated by conducting a series of experiments relating species removal to temperature (10, 20, 25, and 30 °C) at different pH values (10.5, 11, and 11.5). The results are presented in Table 5.

Fig. 4 Percent removal of boron (with temperature and pH)

2884 Table 6 Effect of Si concentration on the mean percent removal of boron

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Si concentration (mg/L)

No. of observations

Mean % removal


Average temperature (°C)

Average pH

3 5 10 15 20

4 3 3 3 3

72.55 79.74 80.27 80.595 85.196

3.58 2.26 1.79 0.351 0.995

20±0.02 19.5±0.1 19.7±0.11 19±0.015 19.5±0.008

10.49±0.001 10.49±0.08 10.51±.0.012 10.48±0.011 10.49±0.009

On the other hand, and from the data shown in Table 5, no firm conclusions can be made with regard to the effect of temperature on the removal of magnesium. However, it is important to highlight that co-precipitation of two or more scaling species is not a very well-understood process, and therefore, the dominance of kinetics over solubility or vice versa cannot be established. As for boron removal, the optimal pH at which maximum removals was recorded for all temperatures was 10.5. A definite trend denoted that boron removal decreased with an increase in pH; however, the lowest recorded values occurred at 10 °C and the highest at 20 °C. At 25 and 30 °C, the values were close to but lower than those attained at 20 °C; a possible explanation for this behavior was given earlier. Very high removal efficiencies were recorded at all temperatures for Ca and Mg at pH values of 11.0 and 11.5. At pH 10.5, the removal efficiencies were lower than those recorded at the higher pH values. However, improved Ca removal efficiencies were noted with increased temperature at pH 10.5, where removal percentages of 71.3, 76.9, and 81.5 % were recorded for temperatures of 10, 25, and 30 °C, respectively, suggesting an average 5 % increase in removal efficiency per 10 °C increase in temperature. Combined effect of pH and temperature on the removal efficiency of boron In order to establish the significance of the combined effects of temperature and pH variations on boron removal, a regression analysis was performed for all the experimental data. Intervals of removal at 95 % confidence for boron illustrate the removal range at specific pH and temperature values (see ESM Fig. S2). Table 7 Iron concentrations and percent removal at pH 10.5, 11, and 12

The plot establishes that the combined effects of pH and temperature are significant. In order to obtain an effects plot, a statistical analysis was performed using two-way ANOVA (see ESM Table S2). From the p values, it may be concluded that the effect of temperature and pH, as well as the effect of their interaction, is highly significant at any α (p is used to determine whether a factor is significant, typically compared against an α of 0.05 which represents a maximum acceptable level of risk for rejecting a true null hypothesis). A scatter plot comparing the effects of pH and temperature on the mean percent removal of boron is presented (Fig. 4) in support of the hypothesis. Effect of spiked samples on parametric removal efficiency Silica removal It has been established that the presence of Mg and Si in seawater induces the formation of an MgSi compound which tends to attract borate at high pH and, thus, enhances boron removal. To test this hypothesis, boron removal, in the presence of Si, was evaluated by the application of a oneway ANOVA at the test temperature and pH (T =20 °C and pH 10.5). The effect of Si concentration on Mg and Ca removal efficiencies was negligible; in addition, all the residual Si was completely removed (results for the one-way unstacked ANOVA are presented in ESM Table S3). On the other hand, from the results shown in ESM Table S3, it can be concluded that the ANOVA provided enough evidence to support the claim that there is a significant effect imposed by Si spiking on the removal efficiency of boron; this is illustrated in Table 6. The addition of Si to seawater samples increased the efficiency


Initial (control jars)

pH 10.5

pH 11

pH 11.5

No. of observations Temperature (°C) pH Iron (mg/L) % Iron removal

7 19.2±0.01 8.13±0.04 3.6±0.85 N/A

7 20.14±0.05 10.48±0.001 0.029±0.003 99.2±0.08

7 19.8±0.12 11.2±0.11 0.03±0.0025 99.16±0.07

7 20.05±0.2 11.57±0.05 0.049±0.0026 98.6±0.072

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of removal of boron up to 86 % in comparison to 72 % when Si was not spiked. Table 6 indicates that the higher the Si concentration, the more efficient is the boron removal. Table 6 also indicates that, statistically, there is very little difference on boron removal when the silica concentration is varied; for example, at 3 mg/L, the error may be sufficiently large that it may be within the error range of boron removal at 5–15 mg/L Si. Therefore, a definitive explanation for this behavior is not possible suffice it to say that the results of a study reported by Parks and Edwards (2007) concluded that reaching a high Si/Mg ratio is essential in removing boron. Iron removal Iron is present in water in two forms: ferrous and ferric. The ferric form is easily removed via filtration. On the other hand, ferrous iron is more water-soluble and cannot be easily removed. Ferrous iron in the form of ferrous sulfate (FeSO4) was used to spike the seawater samples prior to jar testing in order to assess the efficiency of removal of dissolved iron at concentrations ≈3.6 mg/L. Removal was assessed at pH 10.5, 11, and 11.5 and at sample temperature of 20 °C. A control jar experiment was also conducted to check for settling of oxidized iron (ferric form) throughout the duration of the experiment. The initial concentrations of Fe were recorded based on the control jars at the end of the experiments, and the percentage removals were calculated at room temperature of 23.4 °C, with the results presented in Table 7. From Table 7, it can be deduced that iron removal is related to its adsorption by CaCO3 and Mg(OH)2 flocs starting at pH 10.5, with negligible change in removal resulting from pH variations. Bacteria removal A multitude of microorganisms, and specifically bacteria, could be found in seawater as a result of wastewater discharge into the marine environment. If left unchecked, these microorganisms may lead to fouling of membranes and lowtemperature heat transfer surfaces. To assess the effectiveness of bacterial inactivation at high pH values, a series of experiments were conducted whereby seawater

Table 8 Total and fecal coliform removal

TNTC too numerous to count

samples were spiked with 1 % municipal wastewater in order to increase the bacterial population in the samples, which will assist in properly evaluating the impact of bacterial inactivation. The effect of wastewater addition on inorganic seawater constituents was minimal and within the provided standard deviations for Mg, Ca, Na, B, Si, and turbidity. Iron concentrations were recorded as 1.2±0.1 mg/L, whereas total and fecal coliforms, which were used as indictor organisms, were too numerous to count. Both iron and coliform removals were assessed at room temperature of 24.1 °C. It is worth noting that the removal efficiencies for Ca, Mg, Br, and Si for the spiked seawater samples were also assessed, and the results were found to fall within the standard deviations of the previously recorded removal efficiencies. Table 8 shows that the total and fecal coliforms were removed completely, either through inactivation or settling or both, at all of the tested pH levels.

Advantages and limitations of the treatment process Removal of calcium and magnesium ions will allow the operation of thermal desalination units at higher top brine temperatures, thus leading to increased performance ratios, lower energy consumption, reduced need of anti-scalants and primary chlorination, lower volumes of brine reject, smaller specific area (area per distillate flow rate), and, ultimately, to lower desalinated water costs. For membrane processes, the elimination of Ca, Mg, and Si and the inactivation of bacteria will significantly reduce the risks of inorganic, colloidal, and biological fouling, thus leading to reduced need of anti-scalants and primary chlorination and consequently resulting in reduced membrane replacement, operation, and maintenance costs. The drawbacks related to the proposed process lie in the costs induced by the use of alkalizing agents and in the increased amounts of sludge resulting from the precipitation of magnesium hydroxide compared to the amounts of sludge produced by other coagulants used in conventional pretreatment processes. However, the sludge drawback may not pose serious problems as the sludge produced is overwhelmingly composed of magnesium hydroxide, which could be recycled or used for the extraction of magnesium.


Initial (spiked sample)

pH 10.5

pH 11

pH 11.5

No. of observations Temperature (°C) Total coliforms (CFU) Fecal coliforms (CFU) Iron (mg/L)

4 20.1±0.5 TNTC TNTC 1.2±0.1

4 19.43±0.01 0 0 0.028±0.0015

4 21±0.5 0 0 0.022±0.004

4 19.8±0.04 0 0 0.031±0.01


Conclusions In this study, precipitation softening is investigated as a pretreatment step for seawater desalination. Seawater off the coast of Beirut city on the Eastern Mediterranean is alkalized by the addition of 2:1 NaOH/Na2CO3 solution, thus providing CO32− and OH− ions which react with seawater constituents, inducing precipitation and leading to the removal of a number of scale-inducing materials. The major findings of this work are as follows: &




pH variation had a significant effect on the removal efficiencies of Mg and Ca, whereas at high pH values, temperature variations were found to be less significant. The following removal efficiencies were recorded at 20 °C: Ca removal at pH 10.5 was 77.2 %, whereas at pH values of 11, 11.5, and 12, Ca removal efficiencies were almost 100 %, i.e., the effluent’s concentration of Ca was below the detection limit. On the other hand, Mg removals at pH 10.5, 11, 11.5, and 12 were found to be 84.5, 99.6, 99.37, and 99.13 %, respectively. For the range of pH values tested, boron removal was highest at pH 10.5 (72.5 % was recorded) and decreased with the increase in pH values (60.78, 51, and 37.25 % at pH 11, 11.5, and 12, respectively). The addition of Si, in the form of sodium silicate, improved the percent removal of boron significantly. At a pH of 10.5 and temperature of 20 °C, spiking of Si at concentrations of 3, 5, 10, 15, and 20 mg/L resulted in boron removals of 72.6, 79.7, 80.3, 80.6, and 85.2 %, respectively. Furthermore, very high removal efficiencies were also recorded for Si. Spiking with dissolved iron up to a value of 5 mg/L resulted in high removal efficiencies for iron. At pH 10.5, 11, and 11.5, the removal efficiencies of iron were found to be equal to 99.2, 99.1, and 98.6 %, respectively. Complete removal of bacterial content (either by inactivation, precipitation, or both) was achieved at pH values of 10.5, 11, and 11.5.

Acknowledgment The authors acknowledge the Environmental Engineering Research Center at the American University of Beirut for providing their facilities to conduct the research.

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Precipitation softening: a pretreatment process for seawater desalination.

Reduction of membrane fouling in reverse osmosis systems and elimination of scaling of heat transfer surfaces in thermal plants are a major challenge ...
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