Anion rejection in a nitrate highly rejecting reverse osmosis thin-film composite membrane

DESALINATION ELSEVIER

Desalination 104 (1996) 165-174

Anion rejection in a nitrate highly rejecting reverse osmosis thin-film composite membrane L. Panyor a, C. Fabiani b * aSeparem S.p.A., Via per Oropa 118, 13051 Biella, Italy bEnvironment Department, ENEA, CE-Casaccia, Via Anguillarese 301, 00060 Rome, Italy Received I 1 July 1994; accepted 5 October 1994

Abstract

A composite polyamide/polysulfone thin-film membrane with a high rejection towards nitrate ions has been selected to develop a reverse osmosis denitrification treatment of natural water polluted by inorganic nutrients. The model was chosen to characterize the membrane under a broad range of different operating conditions (temperatures, pressures, recycling rates and electrolyte concentrations). The Kimura-Sourirajan model can simplify the experimental work needed to select the membrane suitable for the studied process. Numerical equations to calculate the main transport parameters of the membrane (pure water permeability, mass transfer coefficients and solute transport parameter) have been obtained by a multiple regression procedure and checked with NaCI, NaNO3, KC1 and K2SO4 solutions. Keywords: Reverse osmosis; Nutrients; Kimura-Sourirajan model

1. Introduction High concentration levels of inorganic nutrients (nitrates and phosphates) in natural water have been detected wherever intensive fertilization is used in fanning practice [1]. Nutrients strongly contribute to the eutrophication phenomenon observed in many surface waters, Nitrates are produced from nitrous or ammonia nitrogen through aerobic oxidation. Because *Corresponding author.

of their high solubility they are easily transported into surface and ground waters. The negative effects of nitrates on human health are still under discussion, but the WHO and the European Cornmunity have stated the absolute maximum standard level at 50 mg/1 and a recommended maximum level at 25 mg/l. These levels are exceeded in many places. Several water treatment methods for nitrate removal are available (Table 1) [2], but R&D work is still needed to improve both process

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166

L. Panyor, C. Fabiani / Desalination 104 (1996) 165-174

Table 1 Water denitrification methods [2] Method Biodenitrification

Algae harvesting Ion exchange Electrodialysis Chemical reduction Reverse osmosis Distillation Land application

Removal efficiency (%) 70-95 50-90 80-99 30-50 33-90 50-96 90-98 5-15

performances and economics [3-5]. In fact the challenge between different technologies depends on desired water quality, plant capacity, process automation or access to manpower of suitable technical skill, Biological denitrification is usually the preferred solution for nitrate removal because it is transformed in gaseous nitrogen with a very high yield and low process costs. However, biological denitrification shows some drawbacks in process control and output water quality. Nitrites are formed if insufficient carbon or energy is available and substrate is in excess. This problem, especially when random changes in the feed composition occur, can also be complicated by the presence of excess biomass (bacteria) or dead biomass in the final water. Therefore post-treatmerit, disinfection and oxygenation of product water are generally needed. Biological treatment is preferred for large plants [2]. Other processes based on ion exchange [6], RO and electrodialysis have a lower efficiency if compared to biological denitrification, but they seem very interesting for medium and small applications or under emergency conditions. Better economics, larger automation possibilities, lower level in feed and process parameters control and no need of extensive post-treatment (in the RO case) are advantages of these processes, A comparison made among ion exchange and RO states the higher cost of the latter process [2], but data refer to traditional membranes for desali-

nation with a nitrate rejection as low as 70%. New membranes with high nitrate rejection can improve RO costs. A thin-film composite polyamide-polysulfone commercial membrane with high nitrate rejection has been selected and characterized for nitrate removal. T o reduce the experimental work needed to characterize the selected membrane, the preferential sorption-capillary flow model (the Kimura-Sourirajan model, K-S) was assumed to account for the membrane performance. According to this model [7], the knowledge of a reduced set of transport parameters for a given electrolyte makes it possibile to predict the membrane separation efficiency and productivity for different electrolytes and binary electrolyte mixtures. Therefore a detailed study of the NaCl-membrane system was undertaken in many different experimental conditions, and numerical equations for the main transport parameters were obtained. These equations were used to predict the behaviour of NaNO 3, KC1 and K2SO 4 solutions. The numerical results were then experimentally verifled. The set of experimental results is the basis for the development of a RO denitrification process. 2. Experimental 2.1. The model

The Kimura-Sourirajan approach to membrane transport is described in many papers [7,8], and here only the basic equations are reported for the sake of discussion. The A constant, which defines the solvent (water) flux, Jw = A ( A P - A H )

(1)

being AP, the osmotic pressure difference between the feed (at the membrane surface) and permeate solutions, can be calculated from equation A -~

PR M w. S. 3600. AP

(2)

L. Panyor, C. Fabiani / Desalination 104 (1996) 165-174 where PR is the pure water permeability (g/h) and S the membrane surface. The solvent flux, when an electrolyte solution is processed, is given by

Jw - DAM K.d 1-Xp 'r'~P"!IC'~'-C~Pl (3)

Cf(Xp) = K

Xk-XP

1-

In

Xf-Xp

The mass transfer parameter k and the solute transport parameter (DAM/Kd) of any electrolyte are related to the NaCI similar quantities by means of the relations

ksolute kNaCl

---(DAB)solute l_

(4)

\

II)AB]NaC1

(DAM/Kd)NaCI

S(-AAG/RT)iNaCI

where Z(-AAG/RT)i is the sum of the values of the energy parameter for each ion in the solution,

2.2. The membrane and the procedure A spiral wound polyamide-polysulfone thinfilm composite membrane supplied by Separem (Biella, Italy) was used in a pilot unit equipped with two 4" modules. The membrane can be used at a maximum pressure of 44 bar and below 50°C. The useful pH range in normal operation is 4-11. The maximum chlorine exposure for this polymer is 1000 ppm/h while 0.1 ppm of free chlorine is suggested in operating conditions, The clean membrane volume permeability as measured with 1000 ppm NaC1 solution at 28 bar and 25°C (pH 5-8) is typically 250 dm3/h for the 4" modules used in the experimental work. These modules have a 6 m 2 membrane, and then the

167

total membrane surface area in the unit was 12 m 2. In the above conditions the observed minimum salt rejection was 97.5%. Experiments were performed at temperatures, pressures and feed flow rates within the following ranges: 18-36°C, 10-40 bar, 500-3000 dm3/ h. The experimental set up is shown in Fig. 1. The "pure" water permeability of the modules was measured as a function of the reported parameters by using normal tap water as feed. The A constant values were obtained from Eq. (2). From the set of data representing the dependence of A from T, P, and Jfeeci, an equation was obtained by means of a multiple linear regression to predict the value of the constant under any other condition. The NaCl-membrane system was studied in the of 0.04-0.55 mol/dm 3 concentration range. The separation efficiency of the membrane was calculated as rejection percent:

Each experiment was performed by assuming that the membrane had reached a "pseudo-equilibrium" state after 1 h of solution membrane contact under the selected operating conditions. Rejection was calculated by conductivity measurements on the feed and permeate solutions. From the data of the NaC1 experiments the pure water permeabilities were determined by using the obtained empirical equation (Eq. 7), and then the osmotic pressure at the membrane/feed solution interface was evaluated through Eq. (1). From these data the concentration at the membrahe surface was calculated, and the mass transfer coefficient, k, and the solute parameter are obtained. Both parameters together with a numerical equation for each of the two membrane parameters, k and DAM/Kd, were finally derived by means of multiple regression analysis. The results of the KCI, NaNO 3 and K2SO 4 experimental separation tests were compared with the predicted values derived from the NaC1 data.

168

L. Panyor, C. Fabiani /Desalination 104 (1996) 165-174 F2

FI

Pl

?

4" MODULE pl

RI

STORAGE TANK R2

T

Cooling

w a ~ ~ V

P2

5 micron

7 k

Heat

F e ~ ,,)

_

Feeding ~

pump

High pres= pump

3. R e s u l t s a n d d i s c u s s i o n

2,0

The pure water permeability constant A changes with temperature, pressure and feed recirculation rate. Temperature significantly affects the A value (Fig. 2); the other two parameters have a reduced effect (Fig. 3). The empirical equation obtained from the whole set of experiments, A = 1.41 10 -5+ 1.48 10 -7 - 2.47 10 -6 In T

~ ,,8 ~ ,.e

~. ' " ~ 1,2 <

+ 1.44 10-8p 1.94 10 -7 In P

Fig. 1. Layout of the RO test unit. Merebranes are spiral-wound thin-film polyamide/polysulfone composite by Separem (Biella, Italy). V, valves; F, flow meters; P pressure gauge; T, temperature control; R, heating resistances.

(7)

- 1.04 10 -6 In Jfeed

~K/"

P = 10 bar

1,o. o.,

0

J

20

30

T (°(3)

40

agrees quite well with the measured quantities (Fig. 4).

Fig. 2. Dependence of the water permeability on temperature and applied pressure.

To check the K-S model, a full set of experiments with NaC1 solutions was performed according to the described procedure at a fixed temperature of 25°C. As shown in Figs. 5 and 6, permeate fluxes and NaC1 rejection increase with

pressure and decrease strongly concentration. On the contrary, had little effect on permeation using Eq. (7), the pure water

by increasing salt the feed flow rate and rejection. By permeability was

L. Panyor, C. Fabiani / Desalination 104 (1996) 165-174

169

~.3 "G" u~

100

-~ 1,2 e~

.,/~-

E

J feed (dm3/.h) . 2700

2800

2900

3000

3100

3200

~'x

~ - - / ~ 3300

Fig. 3. Water permeability as a function of the feed flow rate at different applied pressures,

10

~"

.

"

~.o,o.,

P (bar) 2'0

3~0

40

Fig. 5. Permeate flux for NaC1 solutions at different molar concentrations as a function of applied pressure. NaCI

r,~ 2,07

100.

1,8'

::P~o

v~ 1,6'

g9.

¢. 1,4' d

~

a


I"1

O

~ "°1.o

g9.

A exp. (mole/cm2.s.bar)xlO +6 1.~

1.,

,:,

1:~

~:o

Fig. 4. Pure water permeability A constant: correlation of measured and calculated (Eq. 7)values in different conditions of temperature, applied pressure and feed flow rate.

97

.

10 20 30 40 Fig. 6. Salt rejection in NaCI solutions at different molar concentrations as a function of applied pressure.

calculated and, according to the model procedure, the surface membrane concentration X k was obtained. By means o f these results the mass transfer coefficient (k) and the solute transport parameter (DAM/Kd) were calculated. In Table 2 some typical results for the NaC1 model system are collected. As can be seen from the reported data, k strongly depends on the feed flow recirculation while is slightly affected by pressure and concentration. On the contrary, the main effect on the solute transport is due to the feed concentration,

Finally in the same table two empirical equations, obtained by multiple linear regression on the whole set o f experimental data, are reported. These equations can be used to predict the transport parameter values under whichever experimental condition. These two equations, together with Eq. (7), allow for prediction o f the membrane performance with different electrolytes through Eqs. (4) and (5). In Table 3 the main transport and selectivity parameters for the examined salts are reported with the k and DAM/Kd values obtained according to

L. Panyor, C. Fabiani /Desalination 104 (1996) 165-174

170

Table 2 Transport parameters for the NaCI composite membrane system: selected experimental results obtained at 25°C and empirical equations for their calculation under different operating conditions (P, bar; C, mole/dm3; J, dm3/h; k, cm s-l; DAM/KS, cm s -l) AP

Cfeed

"]feed

'Jp

R (%)

kxl0 2 a

(DAM/K~)×10 6 b

35.2 35.4 35.1 35.0 35.1 35.1 35.0 35.0 35.1 35.0 35.1 30.2 25.3 20.4 15.6

0.044 0.115 0.249 0.409 0.534 0.507 0.511 0.507 0.490 0.494 0.242 0.223 0.230 0.234 0.231

2950 2915 2840 2750 2716 2245 1743 1194 731 694 2825 2770 2770 2717 2623

175 158 120 66 53 59 58 46 66 46 118 92.5 74 48 26

99.73 99.54 99.25 98.26 98.04 98.18 98.13 97.63 98.69 97.47 99.44 99.39 99.23 98.86 97.86

5.36 4.26 3.54 4.07 3.22 2.08 1.56 0.75 1.08 0.62 2.42 1.61 4.47 3.72 3.58

1.88 2.83 3.57 4.69 4.49 4.43 4.30 3.89 3.56 3.91 2.44 2.01 2.45 2.40 2.53

aln k = -4.8428+0.7 10 -3 Jfeed +0.5909 Cfeed - 0.440 P -0.5378 In P bD/u~/K5 = 4.65 10 -5 --4.94 10 -7 T+2.51 10 -9 "]feed -4.52 10 -6 In "/feed +5.31 10 -6 Cfeed --1.14 10 -6 In P. the K-S model. Similar data for other m e m b r a n e s are also reported for the sake o f comparison (Table 4). With this set o f data the permeate fluxes and the rejection o f any tested electrolyte can be calculated and c o m p a r e d with experimental results (Figs. 7 and 8). The calculated results 200

for N a N O 3, KC1, and K2SO 4 solutions were obtained neglecting the dependence o f the mass transfer coefficients and solute transport parameters on pressure, feed concentration and feed flow rates. In spite o f this simplification, the m e a s u r e d and predicted fluxes show very g o o d agreement. 100.0

-

oo/ •

dp,calc(K2.S04)

[] gg,o

o /

.

, 100

Jp exp (dm3/h) , 200

98,5

98

2 ~ .

R%exp

Q

,

9g

100

Fig, 7. Correlation of measured and calculated permeate

Fig. 8. Correlation of the measured and calculated salt

fluxes for NaNO 3, KCI and K2SO 4 solutions according to the Kimura-Sourirajan model,

rejection for NaNO3, KCI and K2SO4 solutions according to the Kimura-Sourirajan model.

L. Panyor, C. Fabiani / Desalination 104 (1996) 165-174

171

Table 3 Transport parameters for three different electrolyte-membrane systems at 25°C (P, bar; C, mole/dm3; J, dm3/h; k, cm s-l; DA,vt/KS, cm s -l) Cfeed

Jfeed

Jp

R (%)

kx 102

35.1 35.1 35.0 35.0 35.0 30.3 20.4 15.6 20.4 20.4 20.4

0.128 0.127 0.124 0.121 0.118 0.113 0.133 0. ! 27 0.041 0.071 0.047

2885 2580 2095 1595 1100 2845 2769 2721 2794 2802 2781

148 145 150 150 153 118 73 51 93 87 91

99.26 99.27 99.24 99.22 99.16 99.15 98.57 98.03 99.43 99.12 99.43

2.30 1.16 0.86 0.64 0.55 1.87 7.38 30.17 2.35 5.71 1.84

3.77 2.27 2.38 1.86 1.67 3.48 4.64 4.64 2.05 3.30 1.91

KCI: 35.1 30.2 25.3 20.4 i 5.6 35.1 35.1 35.1

0.120 0.122 0.120 0.121 0.071 0.071 0.035 0.020

2781 2712 2686 2698 2722 2722 2720 2725

148 122 98 75 48 163 170 175

99.44 99.37 9.26 99.00 98.60 99.62 99.68 99.69

1.42 1.55 1.83 1.77 1.77 1.15 0.69 0.54

2.38 2.47 2.63 2.77 2.77 1.49 0.80 0.57

K 2SO4: 35.1 30.2 25.3 20.4 15.6

0.066 0.065 0.64 0.064 0.065

2722 2704 2686 2698 2671

153 128 100 83 58

99.88 99.87 99.85 99.82 99.76

0.95 0.94 0.78 1.70 1.41

0.40 0.41 0.38 0.55 0.52

Z~°

a

(DAM/KS)x 106 b

N a N O 3:

aln k = -4.8428+0.7 10 -3 "]feed +0.5909 Cfeed - 0.440 P -0.5378 In P bDAM/K8 = 4.65 10 -5 -4.94 10 -7 T+2.51 10-9 Jfeed -4.52 10 -6 In Jfeed +5.31 10 -6 Cfeed --1.14 10 -6 In P. This is an interesting result for process evaluation. In fact, the feed concentration for N a N O 3 and KC1 solutions was varied in a broad range. In the case o f N a N O 3 also the feed recirculation varied f r o m 1100 to 2885 dm3/h. The agreement between the calculated and measured rejections was quite g o o d too. Some data scatter from the linear correlation, but these rejections are still v e r y high - - 99.87% and 99.88%. Errors in the conductivity measurements, for example the CO 2 dissolution from air, can produce the observed differences,

The working hypothesis for the R O denitrification process is based on the assumption that water, as permeate stream, must be obtained with a fairly constant composition with a nitrate concentration fixed below 10 ppm during a daily working cycle. To guarantee a nearly constant performance, a feed-and-bleed operation m o d e following a feed preconcentration has been chosen (Fig. 9). The actual water to be treated will be collected from a well (60 m deep). This water at given periods o f the year shows an unacceptable composition. A m o n g the inorganic species

172

L. Panyor, C. Fabiani / Desalination 104 (1996) 165-174

Table 4 Mass transfer and solute transport parameter ratios for a selected group of membranes [7] and the tested composite polyamide-polysulfone thin-film membrane Membranes

Solute

ksolute/kNacl Cellulose acetate Composite (DAM/KS)solute/(DAM~KS)NaCI Cellulose acetate Cellulose acetate-propionate Polyamide aromatic Polyamide hydrazide Composite

NaCI

NaNO 3

KC1

K2SO4

1.00 1.00

0.98 0.95

1.16 0.65

1.00 0.35

1.00 1.00 1.00 1.00 1.00

2.14 -3.29 3.29 1.65

1.13 1.03 0.97 1.07 1.125

0.06 -0.26 0.68 0.23

%+++° Multimedia ~ ] filter I

~

llI[llll flInl11111111tlllllJ

1

~'~'~.0o

or

v]~ 800I/h~ t -

,

....... ++1+

UTILITY SPILL POND

200 I/h

NO3 70 ppm Fe 190 ppm CI 227 ppm

NO3 <10 pm

Fig. 9. Denitrification process scheme. 1, flocculant; 2, anticoagulant, chlorine, pH, NaHSO3; 3, antiscalant, pH. exceeding legal regulations for use as potable water, nitrates (70-100 ppm), chloride (220250 ppm) and iron (150-190 ppm) are the principal ones. The pH is 7.4. Iron can produce troubles both damaging or blocking (metal oxides) the RO membrane during concentration. The assumed pre-treatment scheme (Fig. 9) is based on filtration (multimedia filter and a 5 g filter), iron removal and chlorination. The fixed residue at 180°C (1230 ppm) is essentially due to inorganic dissolved species. Their overall concentration corresponds to a NaC1

equivalent salinity o f 0.02 mole/dm 3 (0.97 bar as osmotic pressure). Previuosly to the RO step the feed water is threefold pre-concentrated: the nitrate concentration becomes 2.4 mM and the expected solution osmotic pressure is about 3 bar. This means that a feed pressure o f 38 bar is needed to operate with a feed-and-bleed mode with an effective pressure differential o f 35 bar. By selecting a feed recycling rate o f 3000 dm3/h to reduce polarization, the product can be obtained with a rate of about 0.2 m3/h with a module o f 6 m 2 as those tested in this study. In

L. Panyor, C. Fabiani / Desalination 104 (1996) 165-174

173

Table 5

product yield is obtained when the volume pre-

Nitrate concentration in the permeate as a function of the membrane rejection (R%) and the feed volume preconcentration ratio, r

concentration ratio is too high.

4. Conclusions r

Creed (mg/1)

0

70

3

210

5

350

10

700

R%

Cp (mg/l)

99.5 99.0

0.4 0.7

99.0 98.0 99.5 99.0 98.0 99.5

2.1 4.2 1.8 3.5 7.0 3.5

99.0 98.0

7.0 14.0

98.0 99.5

1.4 1.1

The results show that the Kimura-Sourirajan analysis accurately describes the performance of the selected thin-film composite membrane. The mass transfer coefficient and the solute transport parameter are function of the solutes and merebrane material. This means that the full characterization of the NaC1 membrane system alone cannot be sufficient to clearly predict the membrane behaviour towards other salts. However, very few additional experiments were necessary with NaNO3, KC1 and K2SO 4 to predict their

Table V the nitrate concentration in the permeate at different preconcentration ratios and rejection values corresponding to those previously discussed is reported. At a preconcentration ratio r=3 and with a nitrate rejection above 98%, the nitrate concentration in the permeate does not exceed 2 ppm. During membrane operation, the nitrate concentration in the recirculating volume will grow, and as a consequence permeate concentration also increases. To evaluate this effect a simple calculation can be worked out if a constant rejec-

permeation and rejection in a large range of different experimental conditions. The obtained empirical equations and the model analysis seem well suited as a basis for the preliminary process design. Further work on real water to assess the optimum set of operating parameters as well as effective process cost is in progress.

5. Symbols A - - pure water permeability, mole/cm2.s bar

Cf, Cp,

tion R (99.5%) is assumed. As a first approximation, the concentration in the recirculating feed C at time t is given by

C/Co = r+(Qin/V)t

(8)

where C O is the initial concentration, Qin the input flow rate and V the feed volume. Under the assumed conditions for the denitrification process (r=3), the expected output (permeate) nitrate concentration, Cp, calculated by means of Eq. 8 and the rejection equation can grow after 10 h by 100% still remaining under 10 ppm as requested. If there is high preconcentration, an increase in the feed osmotic pressure and a decrease in the

C k - - molar concentration of the solute in the feed, permeate and at the membrane feed solution interface, mole/cm 3

DAM --diffusion coefficient of the solute in Jw

--

k K

---

Mw

-----

P

PR R

the membrane material, cm2/s solvent flux through the membrane, mole/cm2.s mass transfer coefficient, cm/s partition coefficient of the solute between the solution and the membrane material solvent molecular weight, g/mole pressure, bar product rate, g/h membrane rejection, %

L. Panyor, C. Fabiani / Desalination 104 (1996) 165-174

174 S

--

Xp Xp, Xk --

membrane surface, cm 2 solute mole fraction in the feed, in the permeate and in the feed solution at membrane surface

Greek 5

References [1] [2] [31 [4] [5]

--

thickness o f the boundary layer between feed solution and membrane, cm rI - - osmotic pressure, bar Y.(-AAG/RT)i - - sum of the values of the free energy parameter for each ion in the solution

[6] [7] [8]

G. Solt, Chem. Engineer, May (1987) 33. J.W. Patterson, Industrial Wastewater Treatment Technology, Butterworth, Boston, 1985. A.F. Miquel, Membrane Technology, 17 (1991) 7. G. Morel, A. Graciaa and J. Lachaise, J. Membr. Sci., 56 (1991) 1. F.T. Awadalla, C. Striez and K. Lamb, Sep. Sci. and Technol., 29 (1994) 483. Rohm and Haase, Nitrate removal, Technical Report ASP 860LA, 1988. S. Sourirajan, Reverse Osmosis, Logos Press, London, 1971. S. Sourirajan, ed., Reverse Osmosis and Synthetic Membranes, Nat. Res. Council, Canada, 1977.