Abstract

Supplying fresh water has become an important environmental and economic issue. Desalination with reverse osmosis (RO) represents an effective solution to meet the challenges of cleaning brackish water and seawater for drinking and industrial purposes. The principal problem of RO plant operation is the precipitation of low soluble salts (scaling). There are some well-known techniques to reduce scaling; one of the most widespread is antiscalant dosing. In this study, 11 samples of new prototype chemicals based on co-polymers of acrylic and methacrylic acids, polyaspartic acid, maleic anhydride and different cross-linking agents is tested. A wide range of commercial phosphorus-based and green inhibitors were used as reference antiscalants. The testing procedure included circulation experiments on a RO laboratory unit with continuous concentration of feed water – tap water and model solution with elevated hardness. The amount of calcium carbonate accumulated in the membrane module was determined and the inhibitor efficiency was calculated. The best results were obtained for scale inhibitors based on polyaspartic and (poly)acrylic acids. The most promising was the reagent based on a mixture of polyaspartate and nitrilotrimethylphosphonic acid.

INTRODUCTION

A serious environmental problem connected to the eutrophication of water bodies appears with the discharging of reverse osmosis (RO) plant concentrates into surface reservoirs. Wastewater discharge containing phosphonic acid or phosphonate functional groups from conventional antiscalants are regulated by special standards; at the same time, requirements for admissible content of phosphates in drains are constantly becoming tougher. This forces scale-inhibitor chemistry to move toward ‘green’ antiscalants that do not contain phosphates and other nutrients, as well as being biodegradable in the natural environment. The application of these inhibitors should be economically attractive for implementation in water treatment. A wide number of polymers and co-polymers are used for water pretreatment before desalination by RO. These chemicals can be classified into the following groups:

  • 1.

    Polyphosphates (sodium triphosphate, etc.).

  • 2.

    Phosphonates (HEDP, aminomethylene phosphonic acids, polyethylenepolyamine-N-methylphosphonic acids, 2-phosphonobutane 1,2,4-tricarboxylic acid, etc.).

  • 3.

    Acrylic/maleic based polymers (polyacrylic acid, polymethacrylic acid, polyethyl methacrylate, polyisobutyl methacryalte, polymaleic acid, polymethyl methacrylate, co-polymer of maleic anhydride and methacrylic acid, etc.).

The most promising biodegradable polymers are currently based on polyaspartic acid (Hasson et al. 2011). They are also used for sludge formation, corrosion inhibition and water softening. Poly(aspartic acid) does not contain nitrogen and phosphorus, and is sufficiently biodegradable (Thombre & Sarwade 2005; Gao et al. 2009). Numerous studies carried out under various conditions have also shown the high efficiency of this antiscalant for calcium sulfate scale inhibition (Quan et al. 2008; Shemer & Hasson 2014; Ali et al. 2015; Chaussemier et al. 2015). The best results are shown for antiscalants with lower molecular weights (Ali et al. 2015). Modified polyaspartic acid was tested with respect to calcium carbonate, sulfate and phosphate, demonstrating a scale inhibition efficiency close to 100% at a dose of 4 mg/L for calcium carbonate. The second popular green antiscalant is polyepoxysuccinic acid. It also does not contain nitrogen and phosphorus, as well as being biodegradable. Polyepoxysuccinic acid shows high results synergised with polyacrylamide acid (Lei et al. 2015). Liu et al. (2012) indicated that polyepoxysuccinic acid reveals high efficiency against CaCO3 and SrSO4 and polyaspartic acid – with respect to CaSO4·2H2O and BaSO4. One more promising class of inhibitors is polysaccharide-based polycarboxylates. Thanks to carboxylic acid, groups in the structure of this compound obtain a valuable inhibition effect on calcium carbonate crystallisation (Martinod et al. 2009; Chaussemier et al. 2015).

Some newly synthesised dendritic polymers with a highly branched three-dimensional structure exhibit a rather high inhibition effect and environmentally friendly properties (Demadis et al. 2005; Mavredaki et al. 2007).

A high inhibitory ability was demonstrated by maleic acid-based antiscalants in numerous studies. Amjad & Koutsoukos (2014) showed the highest inhibitory ability at low doses (about 2 mg/L) was for low molecular weight (∼2,000 MW) compounds: polyacrylic acid, polymaleic acid and other polymers with free carboxyl groups.

Based on a literature review, a variety of prototypes of inhibitors was chosen for this research. The main purpose of this study was to compare different types of new ‘green’ antiscalants – commercial and newly synthesized with traditional phosphonic inhibitors, and to make preliminary results of their efficiency with different kinds of water. This survey will prepare the background for future studies that will be conducted with the most promising and effective inhibitors. Moreover, widespread tests following NACE Standard TM0374-2007 show calcium carbonate as well as calcium sulfate precipitation (Popov et al. 2016, 2017). However, it is known that one type of chemicals prevents calcium carbonate deposits, while another works better for calcium sulfate.

MATERIALS AND METHODS

The study was divided into two stages. The goal of the first stage was to find the best composition of newly synthesized ‘green’ inhibitors with a complex molecular composition. Different special structures of polymers that met the required properties of the polymerization reactions were tested. Polymers containing carboxyl groups have been shown to be more effective for preventing calcium crystals growth (Chen et al. 2015) and the purpose was also to check this out in the current tests.

The second stage of the experiments was devoted to a deeper study of the most promising synthesized samples with simpler molecular formula – polyacrylates and polyaspartates. A wide range of different kinds of inhibitors as reference antiscalants were tested in two stages: traditional commercial antiscalants based on acrylic and methacrylic acids, polyaspartic acid, polyepoxysuccinic acid, maleic anhydride and several conventional compounds based on phosphonic acid.

During the first stage, newly synthesized inhibitors supplied by PJSC ‘Fine Chemicals R&D Centre’ (Russia) were tested:

  • RPAC-1 (rarely cross-linked polymer based on acrylic acid and N,N′-methylene-bis-acrylamide as a cross-linking agent).

  • RPAC-2 (rarely cross-linked polymer based on methacrylic acid and N,N′-methylene-bis-acrylamide as a cross-linking agent).

  • RPAC-3 (rarely cross-linked co-polymer based on methacrylic acid and allyl ether of pentaerythritol as a cross-linking agent).

  • RPAC-4 (rarely cross-linked co-polymer of methacrylic acid and allyl ether of sucrose as a cross-linking agent).

  • FPA (phosphorus compound based on derivatives of polymethacrylic acid).

  • MAAC (co-polymer of maleic anhydride and methacrylic acid).

  • RPAC-5 (mixture of rarely cross-linked polymer based on methacrylic acid and N,N′-methylene-bis-acrylamide as a cross-linking agent and polyhexamethyleneguanidine-based compound).

  • RPAC-6 (mixture of RPAC-3 and polyhexamethyleneguanidine-based compound).

  • SAAC (rarely cross-linked copolymer of acrylic acid and allyl ether of sucrose as a cross-linking agent).

The reference antiscalants were ATMP (nitrilotrimethylphosphonic acid), OEDP (oxyethylidenediphosphonic acid: OAO ‘Khimprom’, Russia), and Aminat-K (a mixture of sodium salts of nitrilotrimethyl-phosphonic and methyliminobis-methylenephosphonic acids (ZAO ‘EKOS-1’, Russia).

For the second stage, inhibitors supplied by Shandong TaiHe Water Treatment Co. Ltd were used as reference antiscalants:

  • PASP – polyaspartic acid;

  • MA/AA – co-polymer of maleic and acrylic acid;

  • PESA – polyepoxysuccinic acid;

  • PAAS – sodium polyacrylate.

Three new green antiscalants were prepared by PJSC ‘Fine Chemicals R&D Centre’:

  • PAC-3 – low molecular weight polyacrylate;

  • PASP-1 – polyaspartate;

  • RO-1 – complex inhibitor based on PASP-1 and ATMP.

The reference antiscalants were: ATMP (nitrilotrimethylphosphonic acid (OAO ‘Khimprom’, Russia), DTPPH (diethylenetriaminepentamethylphosphonic acid), EDTP (ethylendiaminetetramethylenephosphonic acid), and PBTC (2-phosphonobutane 1,2,4-tricarboxylic acid) (‘IREA’, Russia).

For the first stage the scaling experiments were conducted using tap water and for the second stage the model solution was prepared by adding salts CaCl2, MgSO4 and NaHCO3 into the tap water. The compositions of the tap water and model solution are presented in Table 1.

Table 1

Feed water quality for scaling experiments

ParameterTap water (stage 1)Model solution (stage 2)
TDS (ppm) 246–266 960–1,040 
Total hardness (mEq/L) 3.4–3.8 7.4–8.2 
Total alkalinity (mEq/L) 3.0–3.65 7.8–8.1 
Calcium (mEq/L) 2.2–2.6 5.4–5.8 
Chloride (mg/L) 8–10 180–190 
Sulfate (mg/L) 10–13 55–65 
pH 7.6–8.2 8.6–8.8 
ParameterTap water (stage 1)Model solution (stage 2)
TDS (ppm) 246–266 960–1,040 
Total hardness (mEq/L) 3.4–3.8 7.4–8.2 
Total alkalinity (mEq/L) 3.0–3.65 7.8–8.1 
Calcium (mEq/L) 2.2–2.6 5.4–5.8 
Chloride (mg/L) 8–10 180–190 
Sulfate (mg/L) 10–13 55–65 
pH 7.6–8.2 8.6–8.8 

Membrane scaling tests were carried out using a commercial 4040 spiral wound membrane module (model ERN-B-45–300, ZAO STC ‘Vladipor’, Russia, manufactured using ESPA RO membranes by Hydranautics with selectivity up to 98.5%) and a laboratory membrane unit. Scaling experiments were conducted by a series for new antiscalants and selected reference scale inhibitors with doses from 0 to 10 mg/L. At the first stage most green inhibitors were tested at a dose of 5 mg/L because of the small amount of the synthesized substance. All scaling tests were conducted in circulation mode whereby rejected flow (i.e. concentrate) is returned to the feed water tank, and permeate is collected in a separate tank. Three (sometimes two) replicates were made for each test.

Samples were taken from the initial feed solution and circulated solution (for various concentration ratios) – from the tank (1) and from samplers on the feed water line and circulation line (15); for the permeate, they were taken from the permeate tank (4) (one sample characterized the average quality of product water) and from a sampler on the permeate line (15) (Figure 1).

Figure 1

Schematic diagram of laboratory RO unit for membrane scaling tests: 1 – feed water tank; 2 – pump; 3 – spiral wound membrane module; 4 – permeate tank; 5 – heat exchanger; 6 – pressure-gauge; 7 – feed water rotameter; 8 – permeate rotameter; 9 – concentrate rotameter; 10 – bypass adjusting valve; 11, 13 – feed water adjusting valve; 12 – concentrate adjusting valve; 14 – cooling water adjusting valve; 15 – sampler.

Figure 1

Schematic diagram of laboratory RO unit for membrane scaling tests: 1 – feed water tank; 2 – pump; 3 – spiral wound membrane module; 4 – permeate tank; 5 – heat exchanger; 6 – pressure-gauge; 7 – feed water rotameter; 8 – permeate rotameter; 9 – concentrate rotameter; 10 – bypass adjusting valve; 11, 13 – feed water adjusting valve; 12 – concentrate adjusting valve; 14 – cooling water adjusting valve; 15 – sampler.

In all samples, the following were determined: temperature, total dissolved solids (TDS) (conductivity), pH, total hardness, total alkalinity, and calcium. Conductivity and temperature were tested by a laboratory conductivity meter Cond 730 (WTW inoLab®); pH value – using a laboratory pH meter HI 2215 (Hanna Instruments); total alkalinity – by titration with HCl; and total hardness and calcium – by complexometric EDTA titration.

To restore the membrane element's productivity and to remove accumulated precipitation, chemical washing was conducted between experiments using a 2% solution of citric acid.

The transmembrane pressure was maintained at 7 bar for tap water in the first stage of the experiment, and at 16 bar for the imitation of hard water in the second stage.

The amount of CaCO3 and CaSO4 scale accumulated in the membrane module was calculated as the difference between the initial amount of calcium in the feed solution and the sum of the amount of calcium in the concentrate (circulating solution) and permeate (Pervov 1991). This difference was calculated for concentration ratios of 2, 3, and 5. The total hardness and total alkalinity were determined to control the determination of the other parameters:  
formula
(1)
where is the amount of calcium accumulated in the membrane module, mEq; V is the volume of feed solution, L; is the concentration of calcium in the feed solution, mEq/L; is the volume of circulating solution and total permeate, respectively, for time t, L; is the concentration of calcium in the circulating solution and total permeate, respectively, for time t, mEq/L.
Antiscalant efficiency as a calcium carbonate and calcium sulphate inhibitor was calculated as:  
formula
(2)
where and are the mass of calcium accumulated in the membrane module, mEq, without and with antiscalant dosing, respectively.

RESULTS AND DISCUSSION

For the first stage of the experiments, the best results on tap water was achieved with co-polymer of methacrylic acid and maleic anhydride (MAAC), co-polymer of methacrylic acid and allyl ether of sucrose (RPAC-4), polymer based on methacrylic acid and N,N′-methylene-bis-acrylamide (RPAC-2), copolymer of acrylic acid and allyl ether of sucrose (SAAC) and phosphonates – ATMP and OEDP. RPAC-5 and RPAC-6 with extended structure using a cationic polyelectrolyte polyhexamethyleneguanidine was not revealed to increase efficiency, but obviously diminished biological activity on the membrane (Wei et al. 2012). Higher antiscalant efficiency was achieved for the lower dose (3 mg/L) for the co-polymer of methacrylic acid and maleic anhydride. The characteristic relationships between accumulated scale and concentration ratio for selected antiscalants are shown in Figures 2 and 3. Simultaneous TDS increase and a decrease in the relative concentration of the calcium ion in the circulating solution causes a gradual decline of scaling rate with feed water concentrating. Some inhibitors (PBTC, DTPPH, RPAC-3, etc.) demonstrate a subsequent deterioration in efficiency with concentration ratio increase, for them the graphs are closer to a straight line. Inhibitors were tested using varying dosage levels, but the control dosage for comparison and evaluation was 5 mg/L. Summary results of antiscalant efficiency are listed in Table 2.

Table 2

Antiscalant efficiency in RO tests (concentration factor 5.3)

AntiscalantDosage, mg/LInhibitor efficiency, %AntiscalantDosage, mg/LInhibitor efficiency, %
RPAC-1 47 MAAC 62 
RPAC-2 2.5 46 75 
54 10 69 
RPAC-3 40 Aminat-K 32 
RPAC-4 4.35 57 50 
RPAC-5 47 10 56 
RPAC-6 39 OEDP 45 
FPA 2.5 32 62 
51 10 64 
SAAC 56 ATMP 46 
   62 
   10 49 
AntiscalantDosage, mg/LInhibitor efficiency, %AntiscalantDosage, mg/LInhibitor efficiency, %
RPAC-1 47 MAAC 62 
RPAC-2 2.5 46 75 
54 10 69 
RPAC-3 40 Aminat-K 32 
RPAC-4 4.35 57 50 
RPAC-5 47 10 56 
RPAC-6 39 OEDP 45 
FPA 2.5 32 62 
51 10 64 
SAAC 56 ATMP 46 
   62 
   10 49 

The accuracy of the inhibitor efficiency calculations is to within 5%.

Figure 2

Relationships between the accumulated scale and concentration ratio (1st stage).

Figure 2

Relationships between the accumulated scale and concentration ratio (1st stage).

Figure 3

Relationships between the accumulated scale and concentration ratio (2nd stage).

Figure 3

Relationships between the accumulated scale and concentration ratio (2nd stage).

The shape of the obtained relationships of accumulated scale on concentration ratio indicates that at relatively low water hardness (equal to 7–8 mEq/L for concentration ratios of up to 3) the performance of antiscalants at low concentrations is close to that for higher concentrations. However, with further increase in the concentration ratio (and the hardness of the circulating solution up to 12–14 mEq/L) the mass of accumulated scale becomes inversely proportional to the antiscalant concentration.

For the second stage, the investigated antiscalants can be arranged in descending order of effectiveness:

  • 2 mg/L: PBTC > PASP ∼ PAC-3 > PAAS > DTPPH > МA/AA > PASP-1;

  • 3 mg/L: PASP ∼ МA/AA ∼ EDTP ∼ PBTC > DTPPH ∼ PAAS > PESA;

  • 5 mg/L: ATMP ∼ PAAS > PASP-1 ∼ RO-1 ∼ PASP > EDTP > DTPPH > МA/AA > PAC-3 ∼ PESA ∼ PBTC.

This sequence is different from the one that was obtained by the NACE methodology (Popov et al. 2017). This could be foreseen taking into account that the operating conditions of the same inhibitor distinguish significantly depending on the degree of solution supersaturation.

Summary results of antiscalant efficiency of the second stage inhibitors are listed in Table 3. This sequence and the data of Table 3 show that inhibitors PASP-1, PAC-3 and RO-1 demonstrated efficiency comparable with that of commercial samples and conventional reagents, but need further improvements. Generally, the best efficiency was obtained at the dose of 5 mg/L for antiscalants based on polyaspartic and (poly)acrylic acid, as well as co-polymers of maleic and acrylic acids. A dose below 3 mg/L was, as a rule, unsatisfactory. The addition of 10 mg/L of antiscalant instead of 5 mg/L had no significant effect. Promising results were obtained from the complex inhibitor based on PASP-1 and ATMP, which showed sustainable results over different dosages. ATMP has a better antiscale performance than other polyphosphates through its excellent chelating ability and the presence of a negative charged ion in its molecular structure.

Table 3

Antiscalant efficiency in RO tests for second stage

AntiscalantDosage, mg/LInhibitor efficiency, %
Calcium carbonate and calcium sulfate scale formationTotal scale formation
PAAS 68 49 
75 59 
10 66 46 
MA/AA 43 34 
72 57 
63 48 
10 73 58 
PESA 63 52 
59 48 
10 75 58 
PASP 59 51 
71 63 
72 62 
DTPPH 55 39 
67 56 
62 56 
EDTP 69 63 
69 54 
PBTC 67 56 
68 57 
48 46 
ATMP 75 61 
PASP-1 39 25 
71 57 
10 65 48 
RO-1 (PASP-1 + ATMP) 5 + 0.5 71 72 
5 + 1.0 72 78 
10 + 0.5 66 48 
PAC-3 60 57 
64 45 
10 58 50 
AntiscalantDosage, mg/LInhibitor efficiency, %
Calcium carbonate and calcium sulfate scale formationTotal scale formation
PAAS 68 49 
75 59 
10 66 46 
MA/AA 43 34 
72 57 
63 48 
10 73 58 
PESA 63 52 
59 48 
10 75 58 
PASP 59 51 
71 63 
72 62 
DTPPH 55 39 
67 56 
62 56 
EDTP 69 63 
69 54 
PBTC 67 56 
68 57 
48 46 
ATMP 75 61 
PASP-1 39 25 
71 57 
10 65 48 
RO-1 (PASP-1 + ATMP) 5 + 0.5 71 72 
5 + 1.0 72 78 
10 + 0.5 66 48 
PAC-3 60 57 
64 45 
10 58 50 

The accuracy of the inhibitor efficiency calculations is to within 5%.

Comparing the first and second stages, it is possible to see lower total inhibitor efficiency in the first one. The extended derivative structure of the newly developed co-polymers and water with less hardness has no influence on the final result. However, the co-polymer of maleic anhydride and methacrylc acid and co-polymer of maleic and acrylic acid achieved superior performance in both stages.

CONCLUSIONS

The latest trend in RO pretreatment is the development of ‘green’ antiscalants, low in phosphorus and biodegradable. The most promising compounds for synthesis of new inhibitors are polyaspartic, methacrylic, polyepoxysuccinic, polymaleic and maleic acids, as well as polysaccharide-based polycarboxylates. Twelve samples of pilot inhibitors based on these compounds were prepared and tested, to prevent calcium carbonate and calcium sulfate compared to conventional antiscalants. Several green inhibitors showed quite good inhibitory capacity. In the first stage, testing MAAC, RPAC-4, RPAC-2, SAAC on tap water, inhibitors based on methacrylic acid and maleic anhydride were the most effective. During the second stage with hard water the best quality of inhibition was revealed to be with PAAS, RO-1, PASP-1, MA/AA. Among the others, MAAC was efficient at the low dosage of 3 mg/L. To sum up, the best results at typical doses of about 5 mg/L were for scale inhibitors based on polyaspartic and (poly)acrylic acids. The most promising is the reagent based on a mixture of polyaspartate and nitrilotrimethylphosphonic acid. Efficiency correlation of polymer ‘green’ inhibitors with reference to traditional phosphonates proved to be at approximately the same level in the second stage. But in the first stage phosphonates revealed higher results. This data helps to narrow future deeper investigations into the most efficient chemicals. A comparison of molecule structures and other chemical parameters of the most promising antiscalants with those less suitable for RO treatment reflects the strong and weak characteristics for effective inhibition process.

Protection of the environment, energy cost reduction and increasing product outcome will be a result of an optimal and proven full-scale desalination process and in particular of efficient inhibitor application.

ACKNOWLEDGEMENTS

This research was financed by the Ministry of Education and Science of the Russian Federation, Project ID RFMEFI58214X0007; Grant agreement 14.582.21.0007, and partly by the Russian Foundation for Basic Research (Project No. 17-08-00061). We are grateful to M.S. Oshchepkov and S.D. Kamagurov for the synthesis of MAAC, SAAC, RPAC-1–6, FPA, PAC-3, PASP-1 and RO-1 inhibitor samples.

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