Assessment ofASTM D 4516 for evaluation of reverse osmosis membrane performance
Yu Zhao a, James S. Taylor b*
~URS Corporation, 7650 West Courmey Campbell Causeway Suite 700, Tampa, FL 33607-1462, USA hDepartment of Civil and Environmental Engineering, University of Central Florida,
Reverse osmosis membrane performance was evaluated using ASTM D 4516 and a modified form of the mass transfer coefficient (MTC) as described in the homogenous solution diffusion model (HSDM) on a common data set. Standardized salt passage and water production is used to compare both methods. ASTM D 4516 is based on normalized pressure over time for a given set of data, considers temperature only for water production but not for salt passage. The HSDM MTC is diffusion based and the HSDM considers variations in flux and recovery for salt passage. Normalization of the HSDM MTCs for temperature and pressure over time provided a universal assessment for the water and water quality for a specific diffusion controlled membrane. Assessment of water production was identical by either method, but assessment of salt passage was different. Salt passage determined by the ASTM method is dependent on actual net solute driving force, while HSDM determined salt passage is dependent on MTCs, flux, recovery, temperature, net solute driving force and changes in mass transfer over time. The HSDM MTC method of membrane evaluation is more versatile for assessment of membrane performance at varying sites and changing operation.
Reverse osmosis (RO) and nanofiltration (NF) are significant technologies for production o f drinking water. RO and NF performance is typically evaluated by the change of water pro-
*Corresponding author.
ductivity and salt passage over time. Standard- ization of productivity and salt passage (membrane performance) is required to compare inter and intra site membrane performance. The American Stan- dard for Testing Materials (ASTM) standard method, ASTM D 4516 M e t h o d - - Standard Prac- tice for Standardizing Reverse Osmosis Perform- ance Data, provides a procedure to normalize
232 Y Zhao, J.S, Taylor~Desalination 180 (2005) 231-244
permeate flow (Qp) and salt passage (SP) for an RO system [ 1 ] that can be used to assess mem- brane performance. ASTM D 4516 has been de- scribed in the literature for assessing long-term performance of RO membranes [2-4].
ASTM method of assessing membrane per- formance requires normalization of actual opera- ting conditions using standard operating con- ditions for pressure, temperature, conversion and feed concentration. Consequently, the ASTM specified Qp and SP [1], which would be inac- curate ifa significant concentration of ions passed the membrane were independent of pressure. Another error could be produced if there was a large difference in the actual and standard pres- sures required by ASTM D-4516 [1]. As mass transfer of ions through RO membranes are dif- fusion controlled, only low molecular weight and neutral solutes would be affected by the ASTM caveat. However, large pressure differences can be caused by changing temperature, feed stream water quality and membrane mass transfer characteristics during operation.
Other methods have been developed for assessment of diffusion controlled membrane performance [5-9]. These methods evaluate mass transfer coefficients (MTC) for water (K, also referred to as specific flux), and solutes (K,) over time of operation. Using these methods, MTCs are normalized for pressure, temperature over time of operation, and provide a more universal method of performance assessment. This paper presents assessment of membrane performance on a common data set using both methods.
2. Theory
2.1. Homogenous solution diffusion model (HSDM)
The HSDM describes water flux, J , and mass solute flux J , through diffusion controlled (RO/ NF) membranes as shown in Eq. (1) and Eq. (2) [10,11]. Eq. (3) is the HSDM and has successfully described steady-state permeation of water and solutes through diffusion controlled membranes [12-14]. A membrane flow diagram showing influent and effluent flow, concentration and pressure of the feed, permeate and concentrate streams is presented in Fig. 1.
The HSDM has been developed by mathe- matically relating the average feed stream concen- tration to system recovery using a mass balance approach [10]. The HSDM was the first model developed that considered recovery, which allowed the feed stream concentration to remain constant and allowed accurate prediction of permeate concentration for varying flux and recovery [15]. HSDM can be utilized to predict permeate con- centrations for any RO or NF membrane applica- tion given the feed stream concentration, flux, recovery and MTCs (K, K),
Jw=Kwx(AP-A=) =Qp (1) A
L=K.×aC=J.×C,, (2)
Cp K,C (3)
PRETREATED FEED(f) WATER
Qf, Cf, Pf
Fig. 1. NF or RO membrane flow diagram.
PERMEATE(p) Op, Cp, Pf
CONCENTRATE(c)
Qc, Cc, Pc
Y Zhao, J.S. Taylor/Desalination 180 (2005) 231-244 233
The osmotic pressure gradient is the difference between the feed-brine and permeate osmotic pressure, it can be estimated by the ASTM method as described previously, or by using total dissolved solids (TDS) as shown in Eq. (4) [11].
7t = Kms x TDS (4)
Permeate flow and salt passage increase as temperature increases [16,17]. K can be com- pensated for changes in viscosity and membrane film, e.g. membrane pore radius by normalization with respect to temperature [7,11]. Eq. (5) and Eq. (6) were used to normalize temperature. If Eq. (5) is developed only from the viscosity of water, 0 w will be equal to 1.03, which is what is used in ASTM D 4516 to adjust flux of water for changing temperature. Such adjustment assumes the membrane film does not change with tempera- ture. If this equation is developed by non-linear regression of actual operating data, 0 w will be normalized for changes in the membrane film as well as the viscosity of water, and a more accurate representation of the effect of temperature on water flux in an RO process will be obtained. K and K can be normalized for temperature using non-linear regression of a data set that includes
water and solute mass transfer at varying temperatures and Eq. (5) and Eq. (6).
Kwr = 0~ '-2s) x Kw25 (5)
K, r = 0 (y-zs) x Ks2 s (6)
2.2. ASTM standardization methods
ASTM D 4516 utilizes Qp and SP indices as shown in Eq (7) and Eq. (8). [1]. As shown in these equations, normalization of permeate flow is achieved using the standard and actual (a) net driving forces (b) temperature correction factors and (c) permeate stream flows. SP is normalized using standard and actual (a) net driving force, (b) feed and brine (concentrate) stream concen- trations and (c) SPs. Standard values of these variables are determined by average. Con- sequently large variations in temperature, con- version (recovery), pressure (flux or fouling) or feed stream concentration are compensated for by averaging the associated parameters, which limits the utility of this approach. This method is recommended for both spiral-wound and hollow fine fiber membrane systems.
2
QeS=(PY~ AP'°2 PP"-na'+nP~) T M (7)
Pyo 2 Pm -Irma +rcr~ s =(ps ×(seo)
Osmotic pressure is related to temperature and concentration as NaCI (in mg/L) using Eq. (9).
(8)
n =0.2654 × C ×(T + 273.15)/(1000- C/1000) (9)
234 Y. Zhao, d.S. Taylor~Desalination 180 (2005) 231-244
3. Methods
The simultaneous performances of 4"×40" Filmtec BW30FR, Filmtec BW30LE, Trisep X20 and Osmonics SG elements were evaluated in large-scale pilot study from July 16, 2002 to April 2, 2003. These four membranes were low-pressure reverse osmosis membranes. A flow diagram for the pilot study is shown in Fig. 2. The raw water was a highly organic and brackish surface water from St. Johns River at Sanford, Florida, USA. The surface water was pretreated by ferric sulfate coagulation using Super Pulsator and Actiflo tech- nologies and dual-media filtration. Sulfuric acid was used for pH adjustment. Chloramine addition and 5-micron cartridge filtration preceded mem- brane filtration. As shown in Fig. 2, a single-stage membrane system consisting of a single pressure vessel containing three 4"x40" spiral wound modules was used for the field study.
The permeate flux and system recovery were maintained at 22 L.h-l.m -2 (13 gsfd) and 30%. Feed flow was controlled by a valve on the feed stream, which was typical ly opened during opera- tion. Feed stream pressure increased as time of operation increased due to fouling. Pressure, flow and temperature of the feed, concentrate and per- meate streams were recorded twice daily. Con-
ductivity, pH, turbidity, UV254 and chloramine residual were recorded at the same time. Addit- ional water quality was measured weekly and included chloride, bromide, sulfate, silica, sodium, calcium, barium, strontium, total iron and NPDOC. The data shown in this paper is for X20 membrane. The membrane was replaced after 2370 h of cumulative operation.
4. Theoretical interpretations
The ASTM method of standardizing permeate flow is based on Eq. (7) and can be developed from the equations used to develop the HSDM. However ASTM method of standardizing salt passage (SP) as shown in Eq. (8) could not be developed from the solute mass transfer equations used to develop the HSDM and was likely deve- loped empirically. The actual and standardized permeate flows are shown in Eq. ( ! 0) and Eq. (11 ). Solving these two equations for Qr, produces Eq. (12) and demonstrates water productivity as described for RO or NF processes using either the ASTM or HSDM methods are identical.
Q m = J ~ . x A = K ~ z s x ( A P ~ - A 1 r . ) × A
H Ig h Booster SuP Transfer Pressure Break Transfer pump AF Pump Filter Tank Pump
Cartridge Pressure Membrane Filter Pu m p Vessel
Fig. 2. Integrated membrane system showing super pulsator (SUP) and actiflow (AF) pretreatment.
Y. Zhao, J.S. Taylor / Desalination 180 (2005) 231-244 235
Qr,, =J.', xA=Kwz, x (Ap., - ATt,)x A
x 0~;-25)
Cp Cp K, (II) C-- - -~AC:Kx(Ap-Arc ) (13)
( - A=., ) ×
= (aeo a=o )× × ° , "
SP = Cp K, x Cj~ - ( 1 4 )
(12) C: Kw×(AP-An)×C:
The ratio of the permeate concentration to the bulk concentration is shown in Eq. (13). The HSDM can be developed from Eq. (13) keeping the bulk feed concentration constant and incor- porating recovery as shown in Eq. (3). If the permeate concentration is dropped from the con- centration gradient, a similar ratio of permeate concentration to the average bulk concentration can be developed as shown in Eq. (13). A 1.5% concentration gradient error was produced in this work by dropping C , however the error would
P increase as C: or K (nanofiitration) increased. However, ASTM D 4516 is only meant for stan- dardization of SP and Qp from RO membranes, which have relatively low K s and Ks.
The left hand side of Eq. (13) is developed by assuming the permeate to average bulk ratio is simply the ratio of the solute mass transfer coef- ficient and water flux. The equation for SP cal- culation in shown in Eq. (14) and is a simple ratio of diffusion controlled solute flux divided by solvent flux. Note, HSDM solute flux is the pro- duct of the average concentration gradient and solute mass transfer coefficient. This is a rea- sonable approach initially but errors in that reco- very and permeate solute concentration is not con- sidered. Consequently a recovery term and solute MTC (K) appear in the denominator of Eq. (3) and are absent from Eq. (8). Consequently, the ratio of permeate and feed stream concentration or SP using the ASTM approach considers SP to be the ratio of diffusion controlled solute transport and flux and did not develop SP fully using mass balances. The associated degree of error depends on application and would be significant in some cases.
The ASTM approach does not rely on any MTCs and does not provide a means of predicting the impact of different membranes or operating conditions on SP. It does provide a means of com- paring permeate production and SP for any set of operating conditions and any environments. The membrane permeate production and SP would be normalized for comparison to any other membrane permeate and production in any environment. However, the actual SP for any given condition of temperature, flux, recovery or feed stream con- centration in a different environment could not be predicted unless the standard values specified in Eqs. (7) and (8) were known. The HSDM con- siders five different independent variables, is derived from a fundamental diffusion controlled mass transfer approach and offers a easy method of considering the impact of different membranes or operating conditions on SP. The ASTM ap- proach for evaluation of SP was likely postulated from rational thinking but not derived.
Temperature is not considered in ASTM D 4516 for SP, but it is considered for Q.. The as- sumption is that temperature does not a~ect solute diffusion. However, solute diffusion increases at a higher rate with respect to temperature than the viscosity of water decreases with respect to temp- erature. Consequently, salt rejection will increase as temperature decreases.
Temperature, normal use (water loading), organic loading (UV2s4), turbidity loading, and monochloramine loading are independent vari- ables that could potentially affect productivity and solute mass transfer. Eq. (15) and Eq. (16) are direct mass loading expressions that consider the impact of temperature and feed water quality on water and solute mass transfer. K and K are
236 Y Zhao, J.S. Taylor/Desalination 180 (2005) 231-244
empirically related to the change in membrane mass transfer by specific consideration of degra- dation (by water), oxidation (by NH2CI) and organic (by NPDOC or UV-254) or particulate (by turbidity) fouling. 0 w and 0 s as well as all of the water quality coefficients (x) are determined by non-linear regression. These equations much like the ASTM SP equation are postulated rational
equations for prediction of the impact of several variables on RO membrane mass transfer. But unlike the ASTM Eq. (7) and Eq. (8), these equations when combined with Eq. (3) provide a means of predicting productivity and SP or per- meate concentration at any time, temperature, flux, recovery, feed concentration or fouling conditions.
K~ = 0(~/'-25)
n n
K~25 + Xw~e~ ~ J~t, + Xuv y~ JwCuv2,_,t, i=1 i=1
n R
+XturbZJwCturb_iti q- XNH2CIZJwCNH2CI_iI , i = I i=!
(15)
K, = 0! r-z~)
n n
r 25+Xw=EJ.t,+XuvEJ.C v2 _,t, i=l i = I
?1 n
+x~ZJwC¢~eo_,t, + XN.,C.ZJ~CN.2C._,t, i=1 i=1
(16)
5. Results and discussion
5.1. Mass loading model development
The standard conditions for normalization are shown in Table 1 and were set to the average actual conditions as specified in ASTM D 4516. Since the X20 membrane was replaced at 2370 h, the average conditions were determined before and after 2370 h as shown in Table 1. The feed water quality, recovery, flow and flux are similar but the feed pressure and device pressure drop were different for each period. These standard condi- tions are averaged for more than 2000 h of opera- tion of each membrane.
The effects of temperature and water quality on mass transfer were assessed by regressing K and K over time using Eq. (15) and Eq. (16) and the water quality data set corresponding to periods one and two. Initially, all independent variables shown in Eqs. (15) and (16) were regressed in the mass loading models to determine their relative
significance. The most insignificant variable was determined by the highestp or ct values over 0.05, and was dropped from the model and the regres- sion was repeated. This continued until there were no insignificant terms remaining. Both 0 w and 0 were significant and are shown in Table 2. The model coefficients indicate K is more influenced by temperature than Kc
5.2. Standardized ASTM Qv and HSDM K w
The normalized K and ASTM standardized Qp vs. membrane run time for the X20 membrane are shown in Fig. 3. ASTM standardized Qpwas calculated from Eq. (7) and the normalized K was calculated from Eqs. (4) and (5). The X20 mem- brane used in period one is described as the first membrane. The X20 membrane used in period two is described as the second membrane. Clearly, both the ASTM PF and normalized K decreased with time. The ASTM PF and K changed propor-
Y Zhao, J.S. Taylor/Desalination 180 (2005) 231-244
Tablel Standard conditions for normalization of SP for LR2 system
~1 L.m-2.h -~ = 0.589 gsfd; 21 L'd-Lm -2 kPa -~ = 0.169gsfd/psi; 3Cumulative water flux in mS/m 2.
tionaily over time; there is no difference in the normalized HSDM K and ASTM Qp as shown in Fig. 3, which was stated previously in the theory section. Therefore, both methods provided equi- valent assessments of membrane productivity over time of operation.
The actual and predicted K s are shown in Fig. 4. K was predicted using Eq. (15) and as shown in Fig. 4 is accurately predicted. The model coeffi- cients for Eq. (15) are listed in Table 2. The varia- tion of K over time was caused by membrane fouling and deterioration and represents actual not normalized mass transfer of water.
5.3. A S T M and HSDM standardized SP
The predicted TDS permeate concentration m~d predicted SP vs. time are shown in Fig. 5. The HSDM as shown in Eq. (3) with incorporation of Eq. (13) for K and K was used to predict C and calculate predicted S~P. As shown in Fig. 5 p, the
HSDM accurately predicts permeate TDS and salt passage. There is significantly more variation in C than SP, which is due to Fig. 5 scale differences. ASTM SP is not shown in Fig. 5 because ASTM predicted SP methodology is not defined in ASTM D 4516 and is probably not developed.
Membrane replacement can be determined by SP. Membrane life can be estimated by predicting SP. Simple linear regression equations relating standardized SP to time are shown in Fig. 6 for each period of X20 operation. SP was standardized in order to compare the ASTM D 4517 and HSDM SP over time on an equivalent basis. The condi- tions for standardization are given in Table 1. Simple trend lines are shown in Fig. 6 for ASTM and HSDM SP with time for period I and period 2. The slope oftheASTM trend line was greater than the slope of the HSDM trend line in period one, both slopes were positive in period one. The infer- ence of increasing SP with time, is that the mem- brane will have to be replaced at some time due
238 Y. Zhao, J.S. Taylor~Desalination 180 (2005) 231-244
1 r J • Norm Kw
.
tA
0.6
'E 0.5 al
0.4
0.3
0.2
0.1
0.0 0
I
1st membrane i 2nd membrane I
I
._1 v
I
I
I
I I
I [ , l l l i l l l l l l l l l . . . . i i , , , t l . . . . 1 , 1 1 , 1 . . . . i , l , i
500 1000 1500 2000 2500 3000 3500 4000
Hours
Fig. 3. ASTM standardized PF and normalized K vs. membrane
15
14
13
12
11
10
92~, E
8 ~
7 0
5 ~ < 5
run time, X20 membrane .
4
3
2
1
0 4500
1.0
0.9
0.8 O~
I ~ .
d 0.7 II
"7 t~ o_ 0.6
E 0 5
._1
"7 0.4 O..
E 0.3 " O
_ . 1
ff 0.2
0.1
0.0
,A t=l,_ ~ - • ~ll I ~ • i " PredictedKw '=-aA ~. ~ , m ~ ' , l l ~ i l ' i A ActualKw
, ~ ~ p"= : . =
1st membrane 2nd membrane
500 1000 1500 2000 2500 3000 3500 4000 4500
Hours
Fig. 4. Actual K w and HSDM predicted K w vs. membrane run t ime for the X20 membrane .
Y. Zhao, J.S. Taylor / Desalination 180 (2005) 231-244 239
20
15
; T
1
t i
i
- i - ActuaIMembranesp Replacement
Predicted SP
• Actual TDS A Predicted TDS
ZX
! A SP tt~ ! ,
[ ] [ ] [ ]
i
0 500 1000 1500 2000 2500 3000 3500 4000 Hours
0 4500
Fig. 5. Actual and HSDM predicted TDS and SP vs. membrane run time, X20 membrane.
t i A ASTM Standardized SP - - - Membrane Replacement
----. HSDM Standardized SP
y = 9E-06x + 0.0074 ~ 1 ' \
r = 4E-06x + 0.0118 1' T ~ J y = -4E-07x + 0.0149
' \ l ~ ~ 1 1 ~ y=5E-O6x+O.OOQ5 ~ _
1st membrane I 2nd membrane | |
I I | , I I
1000 2000 3000 4000 Hours
Fig. 6. ASTM standardized SP and HSDM SP normalized for temperature vs. run time.
240 Y. Zhao, J.S. Taylor/Desalination 180 (2005) 231-244
to excess SP. In period two, the slope of the SP trend line was negative whereas the slope of the HSMD trend line was positive. A negative slope indicates the membrane can be used indefinitely to reject salt. The results of the two methods are different. Normalization is essential to see what is happening to SP during time, however the lack of consideration of temperature in ASTM D 4516 standardization of SP likely caused the negative trend of SP with time in period two. SP can de- crease with time, but it is noted that actual SP did not decrease with time as shown in Fig. 4.
5.4. Temperature effects on HSDM and ASTM SP
ASTM SP standardization is essentially based on actual and standard net driving forces and actual and standard feed and bulk stream salt con- centrations. Only osmotic pressure is corrected for temperature in the ASTM model; however temperature impacts solute more than water transport because of the relative changes in ion diffusivity and the viscosity of water. SP increases as temperature increases because ion diffusivity increases more rapidly than the viscosity of water decreases with increasing temperature. Although water flux increases as temperature increases, the diffusion of ions increases more rapidly and SP increases as temperature increases. AI-Bastaki and AI-Qahtani also noted that SP increases with temperature due to an increase in pore size with temperature [ 16]. As noted previously, feed stream pressure was varied in order to maintain constant Flux.
In order to calculate osmotic pressure terms in Eq. (8), a specific correlation between conduc- tivity and standardized NaCI concentration was established. Lab water quality data from April 2002 to March 2003 were analyzed in UCF ESI LAB that included all major cation and anion in feed water. Salt in NaCI was calculated using summation ofmolarities of Ba 2+, Ca 2÷, Mg v, Na +, SIO2, Br-, CI-, SO~- and Alkalinity in the feed water. Feed water conductivity and temperature were recorded twice every day in the field. A
correlation between field measured conductivity (in ms/m) and NaCI in mg/L was developed and is shown as Eq. (17).
NaCI =4.16 .~ (17)
where NaC1 = mg/L NaCi; p, = conductivity in ms/m @ 25°C
The effect of temperature on SP is shown in Table 3 for the HSDM, ASTM D 4516 and the ROSA program, which is distributed by Dow for Filmtec membranes. The same membrane and operating conditions are represented in all cases. Eq. (9) and Eq. (17) were used to determine the effect of temperature on ASTM D 4516 SP, which essentially did not vary with temperature. ROSA predicted SP was affected the most by temperature variations and varied from 0.55% at 5°C to 1.33% at 40°C. The data in Table 3 clearly shows that ASTM D 4516 normalization removes any sig- nificant impact of temperature on SP.
5.5. SP Comparison at varyingflux and recovery
The predicted ASTM D 4516 SP and HSDM predicted SP at different Ks of 0.5, 0.85 and 1.71 L.m-2.h -1 are shown in Fig. 7. Predicted SP using the HSDM model was calculated using a feed stream TDS of 400 mg/L at 25°C. Flux and recovery were varied from 8.5 to 51 L.m-2.h -1 (5-
Table 3 Comparison of ASTM and HSMD SP at varying tem- perature
ROSA-SP: Prediction using design software ROSA 4.3 for a BW30-4040 membrane, 3 vessels single stage.
Y. Zhao, J.S. Taylor / Desalination 180 (2005) 231-244 241
30 gsfd) and from 5 to 90 % respectively. K was assumed to be constant at 0.59 L.d-t.m -2 kPa -~ (0.1gsfd/psi), SP was calculated using a HSDM predicted C° which was developed from mass balance and did not neglect the permeate concen- tration. The required pressure and concentration terms for ASTM D 4516 were calculated from mass balances and water fluxes assuming the same K as for the HSDM. Neglecting the permeate concentration at higher recovery and lower flux introduces significant error in ASTM SP stan- dardization.
As shown in Fig. 7, the predicted SP using ASTM D 4516 does not change for varying flux and recovery, however the predicted SP using the HSDM does change with flux and recovery. The essential point ofASTM D 4516 is illustrated by the lack of variation of SP with flux and recovery. This method of standardization removes nearly all effects of feed concentration, flux and recovery on SP, and can be used to compare SP in different environments and for different RO membranes. ASTM D 4516 can be used accurately for deter- mining standardized SP on RO membranes, which are membranes with K s less than 0.59 L/d-m 2
~ 1 "d-l ~rn-2.d;l 0.30 to
0.25
. . . . .
1 , 0 . 1 ~
005
000t e
~u 45 50
Fig. 7. ASTM standardized SP and HSDM SP vs. recov- ery and flux.
(0.1 gsfd/psi) and Ks less than 0.51 L.d-~.m -2 kPa -j (0.3 gsfd), which represent the upper end of water and solute mass transfer coefficients for RO mem- branes. IfASTM D 4516 was used to standardize SP o fa NF membrane the resulting plane would not be fiat as shown in Fig. 8. Note, the plane generated by the lowest K and K~ is relative flat, but more plane curvature is generated in Fig. 8 when the highest and lowest K and K are used to determine ASTM SP. Hence, the effects of flux and recovery on SP using a NF membrane would not be removed using this method and SP stan- dardization would not be achieved. Moreover, if the predicted SP plane is not fiat, the SP does not meet the ASTM D 4516 criteria for standardi- zation. Consequently, NF SP using different mem- branes in different environments could not be accurately compared. Additionally ASTM D 4516 cannot be easily used if at all used to predict actual SP. The HDSM is more flexible than ASTM D 4516 and can be used to predict SP for varying concentration, flux, recovery and temperature for any membrane once the solute and water MTCs are known [ 17,18].
Fig. 8. ASTM standardized SP c, nd HSMD SP vs. flux and recovery for varying K.
242 Y Zhao, J.S. Taylor / Desalination 180 (2005) 231-244
In summary ASTM D 4516 provides a method of evaluating membrane productivity and salt passage for any environment and any set of oper- ating conditions by normalization. Production (Q,) is normalized to temperature, pressure and per- meate flow. Salt passage is normalized to pressure, salt concentration and salt passage. Hence, norm- alized production and salt passage can be com- pared for any environment and any set of operating conditions. ASTM D 45 i 6 is not meant to be used and can not be easily used to predict actual produc- tion for different feed stream concentrations, flux, recovery or temperature. ASTM D 4516 assess- ment of water production was verified by deriva- tion from basic mass transfer equations, but the assessment of salt passage was probably developed by rational postulation. Prediction of productivity and salt passage for varying temperature, foulants, feed concentrations, flux and recovery for dif- ferent membranes is easily done using the HSDM.
6. Conclusions
6.1. General conclusions
ASTM D 4516 and HSDM methods of asses- sing membrane productivity and solute mass trans- fer are different. The HSDM method considers water quality mass solute and water MTCs, fluxes, recoveries, foulants and temperatures which are directly transferable to any other water quality environment. ASTM D 4516 is based on a ratio of operating results to average operating condi- tions and does not consider major factors that influence mass transfer. However the ASTM D 4516 does provide standardized measures of pro- duction and salt passage that can be directly used to assess membrane performance among any en- vironments and operating conditions. ASTM D 4516just can not be easily used to predict actual performance.
6.2. Specific conclusions
• ASTM D 4516 should be modified to reflect a
mass transfer approach and develop solute and water MTCs that can be modified to accom- modate changes in feed water quality, foulants and temperature during operation and easily predict actual membrane performance. ASTM D 4516-85 consideration of tempera- ture significantly limits comparison of RO processes in cold and warm weather environ- ments. There was no difference in comparison of X20 productivity using ASTM D 4516-85 or HSDM predicted permeate flow. The trend of salt passage over time of produc- tion using ASTM D 4516 was negative and the HSDM trend of salt passage over time was positive. Negative trends of salt passage over time indicate acceptable membrane perform- ance indefinitely; positive trends infer mem- brane will be eventually required.
Acknowledgements
This pilot study was supported by the St. Johns River Water Management District and contracted directly to CH2M Hill, who subcontracted analy- tical, interpretation and modeling work to UCF.
Symbols
A C
C~,~ - -
C~s - -
-
C - - j P _ _
j _ W
Membrane surface area NaCI equivalent concentration, mg/L Feed concentration Actual feed concentration, mg/L NaCl Actual linear or log mean of feed- brine concentration, mg/L NaCI Standard linear or log mean of feed- brine concentration, mg/L NaC! Standard feed concentration, mg/L NaCI Permeate concentration Solute flux Water flux Solute MTC
Y Zhao, J.S. Taylor / Desalination 180 (2005) 231-244 243
K25 g T
KTDS n
g f m
K,25 K T
f _
SP a - -
S P - - T t TCF a - - TCF s - -
X i
A C m A P m
/2
ATt
0 s
0 w
pa
7t
~ w C UV254 / XJ,,C~,2clt
~,Jwft ur bt - - ~_J,t - -
Standardized solute MTC at 25°C Solute MTC at temperature T, °C 69 Pa/(mg/L TDS) or 0.01 psi/(mg/ L T D S ) Solvent MTC Standardized solvent MTC at 25°C Solvent MTC at temperature T, °C Actual feed pressure Standard feed pressure Actual permeate pressure Standard permeate pressure Feed stream flow Permeate stream flow Actual permeate flow Standard permeate flow Recovery = Qp/Qs Actual salt passage Standard salt passage Temperature, °C Time Actual temperature correction factor Standard tempera ture correct ion factor Appropriate regression coefficient Concentration gradient Pressure gradient Actual average device pressure drop Standard one half device pressure drop Osmotic pressure, kilopascals Osmotic pressure gradient = n ~ - n K temperature correction factor P K temperature correction factor Actual feed-brine osmotic pressure S t a n d a r d f e e d - b r i n e o s m o t i c pressure Actual permeate osmotic pressure Standard permeate osmotic pressure - - UV254 mass loading, M3/cmm 2 - - C o m b i n e d c h l o r i n e mass loading, g/m 2 Turbidity mass loading, m3ntu/m 2 Water mass loading, m3/m 2
R e f e r e n c e s
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