Engineering Innovations, ISSN: 2813-1002, Vol. 3, pp 13-21

doi: 10.4028/p-g0witu

© 2022 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2021-10-23

Revised:
2022-01-12

Accepted:
2022-05-24

Online:
2022-09-01

Pretreatment Process on Reverse Osmosis Brine as Electrodialysis Feed

The original version of this paper is available on https://www.scientific.net/EI.3.13.pdf

Fadlilatul Taufany , Rahmasari Nur Setyono , Abdul Wasi , I Wayan Restu Surya Krishna , Yeni Rahmawati , Ali Altway , Susianto , and Siti Nurkhamidah

¹Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Keputih, Surabaya 60111, Indonesia

susianto@chem-eng.its.ac.id, dst_eureka@yahoo.uk

Keywords: Pretreatment, Electrodialysis, RO Brine

Abstract

Reverse Osmosis (RO) Brine is waste generated from the desalination process using the RO method. RO Brine is generally directly thrown back into the sea, even though it has the potential to be reprocessed because it still contains a variety of ions in it. The best method in RO Brine processing is Electrodialysis. But it has a problem of decreased membrane performance caused by the formation of fouling. The fouling problem can be overcome by doing a pretreatment process to eliminate impurities contained in RO Brine, one of which is . The existence of can trigger the formation of deposits. Therefore, it needs excess reagent Na 2 CO 3 with a certain amount to eliminate the whole . Currently, it isn't yet known the best pretreatment conditions that can eliminate impurities ions and produce high concentrations of NaCl . Pretreatment trials are needed in various variations of reagents amount to reduce impurities. The purpose of this study is to find out the best RO Brine pretreatment process that will later be used for the electrodialysis process to produce high NaCl recovery. The best results were obtained in the pretreatment process with variations NaOH excesses by and by from the ideal stoichiometry.

Introduction

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The process of desalination of water in the power plant industry uses many Reverse Osmosis (RO) methods [1]. Seawater is processed to produce freshwater that will later be used as boiler feedwater [2]. But of the processed RO becomes freshwater while the remaining is wastewater (water that still contains some ions in it) that is not used and discharged into the sea. The ions contained in such wastewater are [3]. The disposal of RO waste into the sea is a very unfortunate action because if processed, the RO Brine can produce industrial salt ( NaCl ) [4].

RO brine processing can be done by several methods that are broadly divided into membrane and thermal technology [5]. Thermal technology is a method by which saltwater is heated and then produced water vapor that will be condensed into clean water. This process can be done without the need for a pretreatment process and can eliminate existing biological impurities, but this process requires high energy, expensive tool manufacturing costs as well as serious corrosion, scaling, and fouling problems [6]. While membrane technology is the process by which saltwater will be separated into two phases, then there will only be one phase that can pass through the membrane. The phase shift is affected by the driving force, which occurs due to differences in pressure, concentration, temperature, and potential differences [7]. In its application, membrane processes require a pretreatment process and cannot eliminate biological impurities, but membrane processes require less energy and the cost of making tools is not too high when compared to the thermal process [8]. Compared to some other methods for processing RO Brine, processing methods utilizing membrane technology are often used and developed [9].

One membrane technology that can be used is electrodialysis (ED) [6]. ED was first developed in the 1950s. The function is to demineralize brackish water. Nowadays ED has been applicated in many
functions, for example, to reduce inorganic substances in water, in-home water treatment and water recovery for reverse osmosis reject [10]. Electrodialysis (ED) is a method to reduce ion or salt that is contained in solution by conducting electric current to the process. There are two different types of electrodes (cathode and anode) that will be used to transfer the electric current and each of it has a different electrical pole [11]. In between anode and cathode, there are stacks of specific membrane for the transfer of positive and negative ions [12]. Electrodialysis methods can eliminate TDS up to [13]. It has lower energy consumption [14] and higher membrane life [6][15]. However, in membrane technologies such as ED, major types of IEMs fouling that are commonly found are: biofouling and colloidal [16][17]. Membrane biofouling is made up by the attachment of microorganisms which are mainly bacteria to the membrane surface, followed by their growth and also multiplication, in this way it can eventually begin to form a biofilm on the membrane surface [18]. The electrical efficiency used to decrease over time caused by the formation of deposits on one side of the membrane (fouling). Deposits on the membrane (fouling) will decrease the limiting current, causing a decrease in electrodialysis performance. Fouling can be caused by a buildup of neutral or colloidal material resulting from ion exchange [19]. With reduced fouling in the membrane, the performance of the membrane will become lighter so that it will reduce the energy consumption used and can obtain higher NaCl purity [13]. To prevent fouling, an effective pretreatment process is needed in the solution that will be used as a feed in the electrodialysis process to remove ion impurities.

Materials and Methods

The original version of this paper is available on https://www.scientific.net/EI.3.13.pdf

Materials. RO Brine is obtained from Paiton Power Plant, East Java, Indonesia. Reagents for the pretreatment process consist of sodium bicarbonate Pro Analyst, sodium hydroxide Pro Analyst, barium chloride Pro Analyst, Poly Aluminum Chloride (PAC) , flocculant accofloc type A310, filter paper Whatman which sized , conventional filter paper and HCl solution 0.1 M .

Methods. Before pretreatment, the ion content in the solution RO Brine is analyzed to find out the amounts of reagents that must be added to the feed (RO Brine). The pretreatment process consists of 3 stages [20]. The first stage is the addition of NaOH solution with a variation of excesses of in the feed until the formation of deposits and with stirring for 1 hour. Then the results of the first stage are filtered with Whatman filter paper . The second stage of pretreatment is the addition of a solution of with an excess amount of on the feed in a temperature condition of to precipitate ions into deposits with stirring for 1 hour. PAC is added slowly as a coagulant on a feed that has gone through the second pretreatment stage with stirring for 15 minutes. The precipitation process is carried out for 30 minutes until the sediment collects below. Then the solution of the results of the second stage of pretreatment is filtered with conventional filter paper. The third pretreatment stage is the addition of solution with the amount according to the calculation of stoichiometry on the feed that has gone through the second pretreatment stage with stirring for 50 minutes until deposits are formed. Once the precipitate is formed, the solution is filtered using conventional filter paper. Then do a pH check on the solution of pretreatment results. If obtained a pH value of , it is necessary to add HCl to the solution of pretreatment results so that the pH of the solution becomes 5 . For the addition of NaOH and reagents, variations were made with excesses of and for NaOH and excesses of and for solutions. This variation aims to determine the amount of the most effective reagent that must be added to the pretreatment process in order to produce the least impurities. For every reagent, the excesses were calculated from the ideal stoichiometry. At the end of the pretreatment process, an analysis of the content of ion impurities in the solution of pretreatment results and these results are compared to all variables to determine the best pretreatment conditions.

Results and Discussion

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Analysis of RO Brine's ion composition. The composition of impurities of RO Brine as feed pretreatment process is analyzed by several methods [21] [22]. Some of the impurities ions that have large amounts on RO Brine are and . The results of the analysis of the ion content are described in Table 1. It can be seen that the largest composition of RO Brine is and ions where both ions will later be recovered by electrodialysis process to produce a solution with high NaCl content as an industrial salt feed. The results of the analysis of the RO Brine solution to be used have a lower composition of ion impurities compared to other authors' reports [23].

Table 1. Composition of RO Brine

Component | Composition [ppm] | Method of Analysis
Ca²⁺ | 116,67 | Complexometric
Mg²⁺ | 417,56 | Complexometric
Na⁺ | 8725 | Flame Photometry
Cl⁻ | 27550 | Argentometry
SO₄²⁻ | 348,49 | Spectrophotometry
K⁺ | 132,78 | Flame Photometry
CO₃²⁻ | 0,6 | Alkalimetry
TDS | 38500 | Gravimetry

Electrodialysis is a process of separation using a membrane [24]. In general, membrane electrodialysis has certain specifications and restrictions for each ion impurities as described in Table 2. The purpose of the pretreatment process is to eliminate the ion impurities content to close to the requirements of the main composition of membrane electrodialysis cells [20]. From Tables 1 and 2, it can be seen that the ion content of and contained in RO brine not met the requirement. So, it needs to be processed by pretreatment with several variables to reduce impurities by the addition of NaOH and reagents.

Table 2. Main Requirements Composition of Membrane Electrolysis

Ca²⁺ | Mg²⁺ | Na⁺ | Cl⁻ | SO₄²⁻
Ca²⁺ + Mg²⁺ < 20 ppb | 118 g/L | 182 g/L | 8 g/L |

(Casas et.al, 2012, [23])
The influence of reagent amount for impurities degradation. The pretreatment process is carried out with three stages of the process. In the first stage, NaOH reagents will be added to remove and impurities [25]. Then the second stage is the addition of reagents to remove impurities [25]. And the third stage is the addition of reagents to remove impurities [20]. Variations in the addition of reagent excesses are carried out in the first and second stages of pretreatment, namely in the addition of NaOH solution and solution. For NaOH solutions, excesses vary by (without excesses), , and . As for the solution carried out excess variations of (without excesses), 20%, and 30%. In previous research data [20] found that
to eliminate calcium content, it takes the addition of excess reagent by above stoichiometric calculations. The effect of adding excesses of NaOH and reagents to ion impurities is shown in Figures 1, 2, and 3.

The addition of NaOH to the pretreatment process aims to remove calcium carbonate solids and to neutralize carbonic acid. The addition of this reagent is also to eliminate the impurities of . The reactions that occur are as follows:

This addition can be used to handle RO Brine that has a high carbon and low calcium. In addition, the addition of NaOH also aims to increase the pH of the solution to achieve the formation of solids when the solution is added with the reagent [20]. For NaOH reagents, the analysis's results of the first stage pretreatment can be seen in Table 3.

Table 3. Effect of NaOH Reagent Addition on Ion Reduction

RO Brine | NaOH Excesses 0% | NaOH Excesses 15% | NaOH Excesses 25%
417,56 | 116,28 | 73,05 | 100,94

From Table 3, it can be known the value of the ion content of after the addition of NaOH reagents with variations in excesses of , excesses of , and excesses of . When compared with the value of the content of before the addition of NaOH reagents, it was obtained that the ion content of in all excess variations decreased. But from the variation given, the best results obtained on the variation in the addition of NaOH reagent excesses , this is indicated by a decrease in the ion content of from the beginning of 417.56 ppm to 73.05 ppm . This value is the smallest value when compared to the value of the ion content of in the variation in the addition of NaOH reagents excesses and excesses .

Figure 1. Decrease of Ion with Variations of NaOH Addition

Figure 1. Figure 1. Decrease of Ion with Variations of NaOH Addition

Based on Fig. 1, it can be compared the percentage of the decrease in ions where NaOH excesses can decrease the ion by , for variations NaOH excesses can decrease ion to and for variations NaOH excesses can reduce ion to . According to Table 2, it has been explained that the impurities content for the ion to match the main composition requirements of the membrane electrolysis cell by , therefore a
further pretreatment process is needed to reduce the content of . The solubility of magnesium salts is high in seawater [26][27]. So that the magnesium is not completely precipitated when the NaOH solution is added and the remaining ion impurities content is higher compared to the target that must be achieved.

In addition to reducing the ion, NaOH reagents also react with ions. The effect of adding NaOH reagents on the amount of content in RO Brine solution is presented in Table 4.

Table 4. Effect of NaOH Reagent Addition on Ion Reduction

RO Brine | NaOH Excesses 0% | NaOH Excesses 15% | NaOH Excesses 25%
0,6 | 0,0003 | 0,0000 | 0,0000

From Table 4, it can be known the value of the content of ions after the addition of NaOH reagents with variations in excesses , excesses , and excesses . When compared with the value of the content of ions before the addition of NaOH reagents, it is obtained the result that the content of ions decreased, even obtained the results of ion content dropped to reach 0 ppm. From the variation given, the best results obtained on the variation in the addition of NaOH reagents excesses of and , this is indicated by a decrease in the content of ions until reach 0 ppm . Meanwhile, in the variation of the addition of NaOH reagents excesses of obtained results of 0.00009 ppm .

Figure 2. Decrease of Ion with Variations of NaOH Addition

Figure 2. Figure 2. Decrease of Ion with Variations of NaOH Addition

Based on Fig. 2, it can be compared the percentage of the decrease in ions where NaOH excesses can decrease ions by , for variations NaOH excesses can decrease ions to and for variations NaOH excesses can decrease ions to . Due to the relatively low initial amount of impurity ions, the addition of reagents with excess variations did not have a significant difference. However, the NaOH reagent is used to remove 2 types of impurity ions ( and ), the best NaOH reagent excess is in the excess variation. This is due to the presence of sedimentation (equation 1 and 2) with the addition of NaOH excess , the impurities ions can react more due to the presence of ionic force between the RO Brine particles [20].

According to Table 2, a pretreatment process is needed to reduce the content of . One of the ways to reduce the content of is by adding reagents . The addition of to react with calcium ions to form thus avoiding the formation of deposits. The reactions that occur are as follows:

This addition can be used to handle RO Brine with high alkalinity and low carbon content [20].

Table 5. Effect of Reagent Addition on Ion Reduction

RO Brine | Na₂CO₃ Excesses 0% | Na₂CO₃ Excesses 20% | Na₂CO₃ Excesses 30%
116,67 | 19,16 | 17,11 | 14,99

From Table 5 can be known the value of the ion content of after the addition of reagents with variations in excesses , excesses , and excesses . When compared with the value of the ion content of before the addition, it is obtained that the ion content of decreased. From the variation given, the best results obtained on the variation in the addition of reagents excesses , this is indicated by a decrease in the ion content of to 14.99 ppm. This value is a smaller value when compared to the value of the ion content in the variation of the addition of reagent excesses and excesses .

Figure 3. Decrease of Ion with Variations of Addition

Figure 3. Figure 3. Decrease of Ion with Variations of Addition

Based on Fig. 3, it can be compared the percentage of the decrease in ions where excesses can decrease the ion by , for variations excesses can decrease the ion to and for variations excesses can decrease the ion to .

Then in the third stage of pretreatment process is done the addition of reagent to reduce the content of ions. The addition of is done to remove the sulfate content thereby reducing calcium solids or non-carbonate solids by forming deposits. The reaction is as follows [20]:

Table 6 presented the results of analysis of content in results solution of pretreatment:

Table 6. Effect of Reagent Addition on Ion Reduction

Experiment | SO₄²⁻ content [ppm]
1 | 13,8203
2 | 14,0484
3 | 13,4735
Average | 13,7807

From Table 6 can be known the value of the ion content of after the addition of reagent with excesses, decreased when compared to the initial content of in Table 1. In the third stage pretreatment, the addition of reagent was not using some variations of excesses because the effectiveness of Reagent to reduce impurities has reached .

Summary

The original version of this paper is available on https://www.scientific.net/EI.3.13.pdf

The addition of , and as a reagent in the pretreatment process successfully decrease the impurities ion in RO Brine. To prevent fouling, an effective pretreatment process is needed in the solution that will be used as a feed in the electrodialysis process to remove ion impurities. The best pretreatment methods are obtained with the excess variation of NaOH reagents by by using PAC coagulants, and flocculant and excesses by (without excesses). This is indicated by a decrease in the ion content of by and by after the addition of NaOH reagents with excesses of . Ion experienced a decrease of after the addition of reagent with excesses using PAC coagulant and flocculant. Then ion decreased by after the addition of reagent (without excesses).

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The original version of this paper is available on https://www.scientific.net/EI.3.13.pdf

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