Desafhation, 33 flS80) 21-41 Q Elsetier Scientific Pubbhmg Company, Amsterdam - printed in The Nethr?riands

ENERGY AND WATER CG~S~VA~G~ TFZR0UGH RECYCLE GE’ DYEING WAS~WATEIR USING DYNAMIG Zr(IV)-PAA MEMBRANES

AL1 M. EL-NASHAR

(Received October 2?,Z9?9)

SUMMARY

The textile dyeing operatimxs coxm.mes large anxmnt~ of process water and thermal energy. The wastewater resulting from these operations contains various types of contaminants of organic and inorganic origin. This waste- water has an above average ~~pe~t~. The puss%Nity of remvat5ng and recycling a major por#zion of thii wastewater would mduce the: water and energy consumption by these dyeing operations.

This paper presents pilot plant test results of using Z@V)-PAA dynamic EL0 membrane modules for treating dyeing wastewater. Using these results, a mathematical model was presented to evaluate the design parameters and economic feasibiity of a 1000 m3/day plant for treating and recyciing dyeing wastewater- The economic results show that a net profit could be obtained by recycling product water_

SYMElQLS

c - composition, ppm salts d - internal diameter of tube D - discharge flow rate F - feed flow rate

f - Fanning friction factor x - inlet flow rate J - water flux

i - section number L - water pernxeabiity constant

&? - Schmidk mmher R - total number of sections

P - pZX?Sti~@

22

P -

R - -

z” -

T -

u -

VW - P -

Y -

;’ - -

hl - v -

P -

k M. EL-NASHAR

product flow rate Reynolds number inlet Reynolds number membrane area temperature velocity velocity pe~end~c~~ to wall recirculation ratio overall plant recovery section recovery salt rejection observed rejection kinematic viscosity density of stream

2X3.4 &Scrip t-S

D - related to discharge stream F - related to feed stream i - index to summation I - related to inlet stream

i - related to sectioni P - related to product stream

Superscripts - related to average value

, - related to section recovery

INTRODUCTION

In 1975 the total water withdrawal by the U.S. textile industry was 2.1 x 10” m3/d (561 mgd). About 99% of the water used by this industry is used for process purposes which include: washing, scouring, dyeing, printing, bleaching, and finishiig the cloth.

Water used to process textiles should have a low concentration of sus- pended solids, turbidity, color, iron, manganese, hardness, and organic matter. Concentrations of iron and manganese as low as 0.2 mg/l can cause staining in fabrics [II].

The textile industry in the U.S. consumes also a substantial amount of energy mainly in its thermal form. In 1973 it consumed 2.8% of the total industrial energy consumption [Z]. Most of this energy is used for water

RENCLE OF DYEING WASTEWATER WITH DYNAMIC MEMBRANES 23

heating for the different wet finishing processes and a substantial portion of this energy is discharged with the wastewater.

The recycle of textile dyeing wastewater may result in sub.stantiaI savings in water and energy resources. The economic feasib%ty of using reverse osmosis technology for the renovation of this wastewater has to be deter- mined separately after the technical feasibility has been established.

This paper presents soxne of the experimental results obtained during previous studies /3,4, 53 on the use of Zr(EV)-polyacrylate reverse osmosis membranes for the treatment of dyeing wastewaters. Using these results a detailed design analysis of a 1000 m3/d KU plant is made and is presented here_ The results of this design analysis will be used to make relevant economic estimates.

THE DYEING SYSTEM

The parameters of greatest significance in providing real variety to dyeing systems are the fiber dyed, the application class of the dye used, and the application method. Fibers may include cotton, rayon, wooi, poIyamide, polyester, and polyacrylic. Yarn, woven, knit, and carpet construction may be used. The application classes of dye may be acid, basic, direct, reactive, vat, disperse, su.Iphur, naphthol, direct developed, acid premetallized or acid chrome. Application methods may include: beck package machine, continu- ous range or Keuster range. In addition to hundreds of dyestuffs used in the dyeing operation many other au.xiEary ehemicak axe used as carrying agents, antifoam agents, suirfactants, leveling agents, etc.

Dyes in solution or in a finely dispersed state are preferentially absorbed by a fiber substrate in the dyebath. Such factors as time, temperature, pH, and the presence of auzrihary chemicals (e.g_ electrolytes, solvents, surface active agents) may affect the efficiency with which the dye is absorbed.

To bring about dyeing in a reasonable length of time, the rate of diffusion of dye molecules into the substrate fiber is frequently accelerated by raising the dyebath temperature, Elevated temperatures also facilitate uniform distribution of dye throughout the material being dyed. In the case of the more hydrophobic synthetic fibers accelerated dyeing may be accompiiihed by (a) the use of organic substances known as carriers and/or (b) the use of closed dyeing systems allowing dyeing temperatures as high as 15U*C [6j- Fig. 1 shows the dyeing procedure for a typical high-pressure beck in a textile dyeing mill. The dyeing procedures for cotton, wool, and acrylic fabrics on a Rodney Hunt 20” Sample Dye Reck is shown in ‘Fables I, II, III f6l-

24 A M. ELNASKAR

hod Ad just Cloth BH

‘Patch MC 5’ Test Dye Tea

I 1 c

Tlma (hours)

Fig. 1. Cycle procedures for high pressure Beck.

Eh EZOF Unload Cloth Cloth

JI

Chamcterization of dyeing wastewater The characteristics of the dyeing wastewater depend mainly upon the dye

class and the fiber substrate, and to a lesser extent on the previous finishing operations that took place before dyeing. The wastewater characteristics resulting from twenty different types of dyeing operations are shown in Table IV [6] for a variety of dyestuffs and fiber material. It can be seen that dyeing wastewater is characterized by a high color content, high total dissolved solids (except for disperse, acid/cationic dyes on polyamide), and medium BOD values.

DYNAMIC Zr( IV)-PAA MEMBRANES

Dynamic Zr(IV)-PAA membranes are formed by flowing a feed solution containing 50 to 100 mg/liter of membrane forming material tangential to a clean porous surface at velocities from 1.5 to 15 m/s under pressures from

RECYCLE OF DYEING WILf’JXWATER WITH DYNAMK MEM3RA.NES

TABLE I

DYEING PROCEDURE FOR COTTON (Adapted from Ref. [S] )

Dyes:

Procedure:

equipments

Material:

Reactive

Exhaust

Rodney Hunt 20” Sampfe Dye Beck

Bleached, mercerized cotton

Liquor ratio: 2O:l

Dyeing procedure: Z&&bath a~roomtemperahrewiEbr 2-O g/I Ludigol (anti-reducing agent) 3.0% CL Reactive Red 120 Raise temperature to 120°F and hold fur

10 minutes, then add: 100.0 g/l salt (calcium and magnesium free) Add salt in 4 portions over 40 minutes while

raising temperature to l&F Dye at lX?F fC%F 20 minutes, then add: 20 g/l soda ash 1.2 g/l caustic soda Run at l?!?F for 50 minutes Drop bath

Rinse: Cold rinse (original volume) Drop bath Hot rinse at 150’“F (original vdunte) Drop bath

Soaping: Set bath at lOOOF dtht 1 g/I Bark& BRM (anionic su.rfactaBt) 1 g/l soda ash Run bath for 15 minutes at 212OF Cool to 160°F Drop bath

Rinse: Give hot rinse at 150°F (original v~~~rne} Give cofd rinse (original vobxme)

Dry: Clip tenter frame

26 A. M. EL-NASHAR

3.5 MPa to ‘7.5 MPa. Within minutes after starting measurable salt rejection occurs [ 71. The principle of dynamic membrane was first reported in 1966 [71_ Since then the characteristics of hydrous zirconium oxide-poIyac.ryIk acid (Zr(IV)-PAA) membranes have been extensively studied on brackish water, effluents from sewage plants, paper mill wastes, food wastes, low level radioactive wastes, laundry wastes, and, more recently, dyehouse wastes [Sl . These membranes can be formed on several types of porous supports having pore sizes less than 1 pm. Among the porous supports which has been used recently are ceramic and stainless steel.

TABLE II

DYEING PROCEDURE FOR WOOL FABRIC (Adapted from Ref. [6 J)

Dyes: Acid

Procedure: Exhaust

Equipment: Rodney Hunt 20” sample dye Beck

Material : Wool fabric

Liquor ratio: 309

Dyeing procedure: Set bath at lOOoF and add: 2.0% Acetic acid (56%) 5.0% anhydrous sodium sulfate 5.0% C.I. Mordant Black 11 Circulate for 10 minutes at 120°F Raise temperature in 30 minutes to 21ZbF Run at 212OF for 30 minutes, then add: 2.0% formic acid (pH 3.5-4) Run for 30 minutes at 212*F Cool to 170°F; add: 3.0% sodium bichromate Raise temperature rapidly to 2B°F Run at 212*F for 30 minutes Coo1 to 140°F

Drop bath

Rinse: Give two warm rinses ( 120°F) (original volume) Drop bath

Dry: Clip tenter frame

RECYGLE OF DYEING WA.S’.FEWAmR WITH DYNAMSC SB~RAN3XS 21

TABLE Kll

DYEING FROGED~B FOR ACRYLIC FAM?W (Adapted &om Ref, ES] )

Dyes: Basic

Procedure :

Equipment:

E&au&

Rodney I&nt 20” Sample Dye Beck

Mater%&

Liquor ratio:

Dyeing procedure 2

Acrylic fabric

30:x

Set bath at l@F and add: W.O% auhydrous sodium suIf&e 2.0% acetic acid (56%) pH 4.5 20% Retarder HP (eationic retarder) CitcuIate fur 10 minutes at 120°F then add: 3.0% C-1. Basic Red 23 Circulate 10 minutes at 120°F Raise temperature in 46 minutes to 190°F Hold at 1905F fur 16 minutes Raise temperature to 212°F at 1*F per minute Run at 212*F for 60 minutes Coo1 to 14O’F at 4OF per minute Drop bath

Riir Give two warm rinses 1109 (original volume) Drop bath

I&T: Clip tenter frame

Johnsm, Mintum and Wadk [S] describe the folkwing procedure for forming composite Zr(IV)-PAA membranes on porous plastic supports:

1. Adjust to pH -4 with HCl a solution of 0.05&I NaCl 10+&f hydrous Zr(IV) aud intraduce into loop.

2. !3tart preswriziug, as the hydrous Zr(EV) begins to fc~rm a layer 011. the porous support increase pressure to 6.2 MPa (900 psig) to 6.9 ma (1000 psig)-

3. Monitor chloride rejection lantil it is in the range 30% to 50%; at that time rinse the T?&(N) from the loop and rinse with distilled water adjusted to pH -4 with HCl.

28 A. M. EL-NAS-HAR

TABLE IV

3915 Iii0

- PO - LVG

--

<I < I .i

315

__

300

3%

in

33

39

91.I

-

190

35

07

535

I380

563

190

-6

-

V6

3890 100

- -

150 130

No re-9.punrr it!

11.3 x3

9HOO 28

32 1.I

1 L.500 396

19 70

67 1-I 5

.1 0 10

09 04

3300 5600 215

- 12.000 315

PI0 25i 240

I35 “IO 1.59

10 15 7.1

33 27 27

9 !3 101

1066 I169 771

1i 5t Oi

160 95 15

IO V 75 30

Vfi 06 05

4000

-

315

Z-10

6.1

14

14

20%

<V5

10

mtcrhr

<03

4. Introduce a 0.05&f NaCl solution (ph “2) containing 50 ppm PM to the loop and circulate for -30 minutes.

5. Add NaOH solution at -30 min intervals to raise pH a unit or so at a time until the circulating stream is near neutrak

6. Rinse PAA solution from loop with distilled water, at that time the membrane is considered formed and the rejection of 0.05N NaCl ranges between 90% and 95% while the flux at 6.9 MPa will range between 6 to 9 m3/m2 day.

Chamcteristics of Zr(IV)-PAA membranes Several factors affect the performance of Zr(IV)-PAA, as they affect other

RO membranes, among which are: operating pressure, solute concentration, and solution temperature.

System pressure As with other RO membranes, both flux and rejection from dynamic

membranes increase with the operating pressure. Thomas and Mixon [lb]

RECYCLE OF BYEIN WASTEW ATER WITH DYNAME l&EMBRANRS 29

TABLE IV

RAW WASIBWATER CHARAfXRRl6TSC6 konttnued)

ADMI color l2.506 2730

A,w.rrnt - -- ADMI Color*

TDC. Inall 140 65

ROD. n&l 1s 12

PH 66 32

cr. nwll 61 130

Svsp yIiids. 26 13 m$!ll

To(ll DafoL 2669 916 Solids. mgll

To&l P. m&I 13 06

KteIdahl N. 110 60 mgll

NH,N.rn~lt - -

N0xiN01.N. t55 165 mgll

210

720

1 io 360 4CO

32 198 990

67 10.2 3.7

10 1680 12

8 56 31

IS0

60

50

1245

1700

07

13 5

intcrfcr

03

450

-

2000

--

90

interfcr

26

1390 365

- 1100

230 350

102 360

91 10 1

57 161

S 27

691 2292

16.5 165

197.5 14 0

10.0 12.0

0.5 09

I.300

2010

1120

1470

50

I7

4

1360

23

12.5

3.5

co3

CE.0 241s

190 -

160 150

130 200

6.5 93

22 7630

.t9 387

258 t 0.900

24.0 51

5.5 11.5

< 1.0 1.5

0.1 1.3

‘Apparent ADMI Calor Valua IPR obtained by ormtting tic Cclite tikatroe step from tJw publkhed ADMI procedure.

presented results for Zr(IV)-PAA membranes formed at a system pressure of 6.9 MPa (1000 psi) and with a circulation velocity of 12 m/s (35 f&/s), see Fig. 2. Xt can be seen that at a pressure of 6.9 MPa (1000 psi) the membranes possessed fluxes of -7.5 x 10W3 cm/s (160 gal/ft2day) with observed rejections of -90%.

Dynamic Zr(IV)-PAA membranes possess ion exchange properties and their intrinsic rejection is a strong function of solute concentration. Thomas 181 reported results which indic&e that the observed rejection X0 could be expressed by

(I. -ho) = con&. (c&Jps

where CW = solute concentration at the TDS).

(1)

high-pressure membrane side (ppm

k M. EL-NASHAR

(r: so- /

/I

CirculatiOn s 12m/S

$40

~-%I 3 / 20

I ‘0 -/

cl/ * CJ 1 2 3 4 55 6 7

Circulation cl2 m/S

66

“&

%-

=’ 4

ii! 2

Fiii; 2. Typical performance of a Zr(IV)-PAA membrane 2000 ppm NaCI, 5.8 MPa, and 25 c.

Dynamic membranes has the definite advantage that they cafl be US& to treat high-temperature effhrents such as wastewai~rs from the dyeing opera- tions. Short-term tests carried out on high-temperature, 88OC (190*F) dyeing wastewater [3] using a Zr(IV)-FAA membrane showed that while the flux increased substantially over its value at 25% (77OF), the observed rejection dropped slightly below its low-temperature valve which makes dynamic membranes most suitable for energy as well as material conserva- tion_

CLEMSON PILOT PLANT TEST RESULTS

Extensive research work on the renovation and recycling of textile waste- water using reverse osmosis and ultrafiltration membranes has been con- ducted by Clemson University (CU) in cooperation with the South Carolina Textile Manufacturing Association, the U.S. Environmental Protection Agency and the US. Department of Energy [2, 4, 51. We will refer here to some pilot plant test results on high-temperature dyeing wastewater using dynamic ZrffV)-PAA membrane which were obtained during these studies.

Description of pilot plant A schematic of a skid-mounted test unit used by CU is shown in Fig, 3.

Dyeing wastewater is pumped through a float valve which zmintains the fare1 of fluid in the reservoir. It is mixed in the reservoir with concentrate return-

31

A1r Bleed

bhste feed

IJverf IOU

Feed Tank

Fig. 3. Ftow schematic of skid mounted unit.

Ilo Florr-Full Flow valve w I

ing from the RO module and pH correction-fluid from the reagent tank. The mixed fluid enters a small boost pump which forces flow through the particulate filter and supplies the net positive suction head of the high- pressure pump. Eiigh-pressure flow passes through the heat exchanger where its temperature is moderated by cooling water. The flow then is split between the primary module flow and a bypass flow which allows velocity control in the module_ A turbine meter in the module Bow circuit allowe measurement of fIow. The rejoined flow then normaliy passes tbrougb the automatic pressure control valve and is split into a small concentrate fIow and the concentrate which returns to the reservoir. Changes in concentration are only produced slowly by controlling the relative amounts of concentrate and permeate released.

The RO modules used Two types of modules were used in tbis study with dynamically formed

Zr(Iv)-PAA membranes. One consisting of a bank of porous ceramic tube modules and a module incorporating porous stainless steel tubes+ The basic ceramic tube module and its coupling arrangement used is shown in Fig. 4. The stainIess steel module assembly is shown in F’ig. 5.

A. M. EL-NASKAR

Potting Material

7 Tubes With

Filtrat Feed Out 1111)

I Feed in (Out)

CERAMIC TUBE MNUCE ELEIIENT

out

Product Collecti Manifold

Hethod of Joining Elements to Assemble Module

Couplinq

in

Fig. 4. Ceramic tube module arrangement.

Flat 7 Gaskets \T-

Tube Header Sheet

\ Flow Kouting Sheet

End Cap

Basic Hexagonal Arrangement 13 Tubes

Fig_ 5. Stainless steel tube module.

RECYCLE OF DYEING WASVI’RWATER WFFH lDYNAMiC MEMBRANES 33

Test results an atmospheric beck dyeing wustewater

(a) Opemtion at 66OC Fig. 6 shows the average flux and conductivity rejection of a bank of

twelve 74ube ceramic bun&es when operated for 500 hours at a pressure of 5.5 MPa (800 psig) on Beck dyejng wastewater [Z] - similar results were obtained from the stainless steel module. The flux has suffered a substantial decline but appeared to stabilize at a value of about 0.28 m3/m2 day (6 gal- lon/ft2 day). The conductivity rejection appeared to be steady throughout the operating period.

(b) Opemtion at 70°C and 88°C Using a different pilot plant but similar to that shown in Fig, 3 except

that no particulate filter was used, dyeing wastewater was treated by a porous ceramic tube module containing two tube bundles. Each bundle contains 19 tubes, each having a 5.5 mm outsid? diameter. The dynamic membrane was deposited on the outside of these tubes [3] . The module was &I operation for 550 hours on dyeing wastcwater having temperatures of 70°C and 88OC.

The product flux for this module is shown in Fig. 7 for an adjusted opera- ting pressure of 5.9 MPa (850 p&g). It is apparent from this flux history record that, in spite of two membrane washings, the flux has dropped to

toor I 0 Oo 3 0 o” @ OQQO 000 0 000 00

“0

0 100 200 300 400 500

Operating Tima (hourr~

Pig. 6. Permeate flow results -atmospheric dye Beck

TIME (HOURS)

unacceptable low values_ This suggests that wastewzzter pretreatment ia necessary to reduce feed turbidity and suspended, and possibly iron and ma~esiu~ to acceptable values,

The rejection of the Zr(IV)-PAA membrane to conductivity; color, tur- bidity, COD (chemical oxygen demand), TOC (total organic carbon), end TDS (total dissolved solids) is shown in Fig. 8,9. These results indicate that the membrane rejection was satisfactory with no major deterioration taking place.

DESIGN AIULXZS

The results from the pilot plant test programs mentioned above will now be used to carry out a design analysis for a 1000 m3/d dyeing wastewater recycling plant. The objective of this analysis is to determine the phmt arrangements and parameters which woUld be best suited to meet particuk sets of requirements. The pammeters which will be allowed to vary include the plant recovery, the number of sections per stege, the operating pressure, the feed temperature, and the recirculation ratio.

The principal assumptions used in this analysis are: 1. The TDS of the wastewater feed is 2000 ppm,

RECYUZ OF DYEING WASTRWAmR WM’H DYNAMIC MEMBRANES

100

90

80

loo

90

80

100

90

80

70

60

50 1.0 10 100 1000

TIME (HOURS)

Fig. 8. Conductivity, color and turbidity rejections

T=88OC -AT=

A

ii

. loo- c a .

w

1

+

. 70 .

I 1 I , , I 1.1 I * I , , ,,*, I I * ,m,,,t 1.0 10 100 1000

TIME (HOURS)

Fig. 9. COD, TOC and TDS rejections.

36 A. M. EL-X’iASHAR

2. The design value for the water permeability constant L in the fhur equation J = L (Ap = Ar) will be half the initial value of L for the membrane used,

3. The salt concentration ratio between its value on the membrane sur- face and in the hulk of the solution on the high pressure side of the membrane, is C, /G = l-2,

4. The m-m opemting pressure is 6.9 MPa (1000 psig) and the maxi- mum feed temperature is 90°C (194*F),

5. Only one tube diameter will be used, d = 1.3 cm. No attempt is made to vary the tube diameter.

The plant arrangement used in the design analysis is shown in Fig. 10. The plant is made up of several sections- Each section consists of a number of tubes which are operated in parallel. Succeeding sections will generahy have a sznaffer number of tubes in parallel in order to maintain the Reynolds number at an acceptable level [ 111.

Fig. 10. Multisection unit.

Material balances and concentrations Consider the jth section of an RO plant as shown in Fig. ll,_The flow

K&S of the VZSI~OUS streams ECE indicated by Fi, 4, Pi, L)i, atld &S’j, where & is the ratio of recirculation to feed. The concentration of the various streams in ppm or mg/lit are indicated by subscripted C’s.

The section recovery, designated by 7) is defined as:

and flow rates in this section are; therefore;

Pj = 7)Fj

Ij = (1 + &) Fj

Dj = (l-$j)Fj

Recircubtion

(a) Diaqram of Section i

Fig. 11. Flaw r&e and concerntration designatirm for sectionj.

Overall section material balances on salt can be written as:

E; GF,j = Pj Gpi f Di GD,j W

Jj GXJ = FJ GF,~ f @ Fj GB,~ m

Et is necessary to consider the process bide one section in order to relate concentrations to membrane properties and concentration polarization effects. Consider the portion of the section fkom the inlet, where section recovery is zero, to a point where the section recovery is 6, with 6 v-g from zero to 7; in the complete section.

The salt permeability (1 - X) of the dynamic membrane is defiwd as the ratio of the concentration of product passing through the membrane to the ccmcentration C, of the saline water at the membrane su.rface_ The concen- tration of the product passing locally through the membrane is therefore (1 - k) CU. For the streams emerging in Fig, 11 we have:

Product flow rate = S$‘j (8)

&I% fiOW m&e h PlYOdUC~ = .f’ (I - $1 Gwr (Fj d&i); (9)

Goncentration of product % = i of (l-%j)G,J d5 (10)

i

Eq_ (13) cz5n be writkm as

or

fjj = Qj ~-~)~-= (W

The discharge concentmtion is obtakI& from Eq. (18) by setikg Sj = ~3 ~XI give

Aj = 1 7) --

1 +Pj

RECYCJX OF DYEING WASTEWATER WlTH DYNAMIC lKlB5RANES

Using Eqs. (4) and (21) in Eq. (7) we get

CIS = CF& Ai

(I- r)I Ej

WithEj = l+ si

l +_ (1 --A?) I

And using Eqs. (3) and (22) we obtain

c&j = CF,j A;

(I- +~‘j) J3j

Also, using Eqs, (3), (51, (6), and (24) we get

39

(21)

(22)

(23)

(241

(25)

Section j is related to the proceeding one by

Fj = Dj-1 (53%

CFj = CD, j-1 (27)

Using Eqs. (26) and (27) with Eqs. (2), (3), (4), (5), (221, (24) and (25), it is possible %o develop CtXEWtSSk3nS for Fj, Pj, Ij, DJ, cF,j, C&j, cpj, and CDj in terms of F, and cF,l. For the sake of brevity these relations will not be written here.

Section pressure drop The pressure gradient in a tube of circular cross section is given by

dp 4PU2 f A_=- -

0 dx d 2 (2W

in which the Fanning friction factor is used. For smooth tubes in the range of Reynolds number of lo4 to lo5 the fkiction factor is well fit by the equa- tion

f - = 6 032 ~-0.23

2 -

in which the customary definition of Reynolds number is used:

(29)

40 A. M. EL-NASHAR

R du

=- LI

c3m

The water flux u, wiIl be taken as constant in any section for the purpose of estimating the section pressure drop. At a point x fkom the section enhance the section recovery is taken as 6 and for x = L, 6 = y’. With sub- script zero designating values at the entrance to the section,

R (321

(33)

Substituting Eqs, (29) and (33) in Eq, (23) gives

dP 4 (0.032) p Y2 (R*)“*‘7 8 1.77 --=

d3

~ --

dx 1+P (341

To be able f;u integfa%e Eq. (341, a refationship betieen x and 6 is needed.

SF = sdxvwp (3%

and the inlet sfxmz~~ weight is

I = ;dZ uop=(l+p)F

Ekninating F between Eqs. (35) and (36) gives

(36)

(371

which can be differentiated and used in Eq. (34) to give

0.032 -Ap 2= - 2.77

(33)

~b~tu~g Eqs. (29) and (31) for a point in the middle of the section, where 6 = $12 gives

Membrane transport model The rejection of dynamic S%(N)-PkA membranes wiJl be approximated

by the relation ES] :

(1 -W = Coust~ (C, )0-S Id21

The Membrane flux u, for J) ti be given by

VW = Const. @ - n) (43)

where n is the osmotic pressure. The effect of operating temperatue on the membrane flux is given by

I51 :

where k is a constant and Jo is the flux at a standard temperature Z’$ Kelvin, and Jis the flux as any temperature T.

The effect of the operating temperature on the membrane rejection FpiII be assumed to be negligible as was supported by experimental evidence.

The dyeing wastewater wiIl be pretreated prior to its intruduction to the RO unit by chemical addition, mixing, coagulation, 20 minutes flocculation, and filtration at 0.2 &/day M2 (5 gpm/ft=)_ A schematic of the pretreat- ment system is shown in Fig. 12,

The design parameters used for the pretreatmenk process are shown befow,

Des&n criten’a lhzs@ parameters Alum operating dose 30 mgfit Polymer operating dose 0.2 mg/lit Rapid mix 30 seconds 3?lOCCbtiOXl 20 minutes Filtmtion ~du~-rne~~ 0.2 m3!day m2

PRODUCT

CHEMICJALS

1 t

I f F?APIFAMFING _ SLOW MIXING

TANK (FLOCCU~.ATOR)

DUAL-MEDIA REVERSE FILTER - OSMOSIS

PLANT L

I

i

CONCENTRATE

Fig. 12. Schematic diagram of the coagulation-filtration process used for pretreatment before the RO plant.

ECONOMIC ESTIMATES

An economic study was conducted in order to estimate the net yearly cost or profit resulting from recycling hot product water (90°C) from a dynamic membrane plant back to the dyetig system. The results presented here are for an RO plant capacity of 1000 m3/d. The product water salinity ranges from 100 to 500 ppm (TDS) and its color content ranges from 20 to 50 on the platinum-cobalt scale.

The following economic ground rules and assumptions are used:

Interest rate on capital = 10% Amortization period = 20years Cost of RO modules (5 nazImodule) in = $1000/m*

small numbers Cost of RO modules in bulk 0.8 power law used Cost of pumps (small quantities) = $l?O/kW Cost of pumps in bulk 0.8 power law used Cost of water = $0.079/m3 Cost of electric energy = !$O.OS/kWh Cost of thermal energy = $3_O/GJ Plant on-stream factor = 90% Feed water salinity = 2000 ppm (TDS) Product water salinity = 100-500 ppm (TDS)

The total cost of the plant is made up of the capital cost and the operating cost. The capital and operating cost of each of the RO unit and the pretreat-

ment unit were estimated separately and added together in order lm obtain the total capital and operating costs for the plant.

The capital cost of the RU unit is divided into the following cost items: (a) Cost of membrane modules (b) Cost of primps (c) Cost of piping, valves, fittings, etc. (d) Cost of measuring and electrical instruments The capital cost items considered for the pretreatment unit are: (a) Cost of alum feed system (b) Cost of polymer feed system (c) Cost of rapid mix tank (d) Cost of fioccnlation tank (e) Cost of dnal-media (anthmcito coal-silica Saud) The open&ion and maintenauce cost for the whole plan inchxde the

following elements ; (a) Operating and ~~n~c~ labor (b) General and administrative overhead (c) Supples and maintenance materials (d) Membrane assembly replacement (e) Chemicals, and (f) Electric power The method of estimating the capital and operating costs of the RQ unit is

based to a large extent on OSW Research and Development Report No. 555 titled “Desalting Cost Calculating Procedures” [12]_ The EPA (Euviron- mental Pro&&ion Agency) publication titled “Estimating Costs for Water Treatment as a Function of Size and Treatment Plant Efficiency” [X3] was used to estimate the cost for the pretreatment unit. Cost adjustments were made when apprupriate using the cur&t valnes of the Engineering News Record Building Cost Index and Labor Wage Index.

RESULTS

The effect of the plant recovery on the membrane area and pumping power requirement is shown iu Figs. 13 and 14 for a plant having either one saction (n = 1) or three sections (ra. = 3). The operating pressure and tempera- ture are kept comtant at 6.9 MPa (1000 psig) and 40°C (104OF). The re- cirenlatian parameter was allowed to vary between 0 aud 0.5. Iucreasing plant recovery causes a substantial increase in membrane area and a corres- ponding decrease in pumping power requirement. The increase in area with hxxeasing plant recovery can be explain& by noticing that high reooveries are accompanied by an increase in the average salt concentration aud thera- fore lower flux.

PLANT flECQVERY % PLANT Rt5Ct3JERY%

Fig. 13. Effect of plant recovery on the membrane area and power requirement. No* of sections = 1. Plant capacity = 1000 m3 fd, Operating pressure = 6.9 MPa, Feedwater temp. = 4oac.

PLANT RECOVERY % PLANT RECoVERY %

Fig. 34. Effect of plant recovery on the membrane area and power recp&ement. NO. of sections = 3. Plant capacity = 1000 m3fd, Operating pressure = 6.9 l&Pa, Feedwater temp. = 40%.

The lower pumping power requirement for increasing plant recovery is expected because of the Iower pumping capacities at high recovery-

tiereasing the number of sectknw for a given plant recovery causes a decrease in the membrane area but at tbe expense of higher pumping power (see Fig. 15). This is due to the higher average pressure prevailing with a

RECYCLE OF DYEmG WASTEWATER WITH. DYlMMIC MEMBRANES 45

loo- 5-

r 0 =.

1 2 3 4 5

NUMBER of SECTIONS, n NUMBER UF SECTION’S, n

Fig. 15. Membrane area and power requirement versus number of se$tiom~ PSant capacity = 1000 m3/d, Operating pressure = 6.9 MPa, Feedwatcr temp. = 40 C!, Plant recovery =

80%.

larger number of sections since the lower pressure discharge from one section is boosted to the higher operating pressure before entering the following section.

An example of tbe effect of the operating pressure on the area and power is shown in Fig. 16. A reduction iu pressure, as expected, causes an kmease

05t ; ; ;, ; a’ 091 - * * f 5 6 7 8

OPERATING PRESSURE m MR PRESSURE, MPa

Fig. 16. Effect of opemtinet, pressure on the membrane area and power requirement.

Operating temperature = 40 C. Plant capacity = 1000 m3/d, Plant recovery = 80%. No. of sections = 3.

46

60 a0

FEED WATER TEhqP. C*

100

Fig. 17. Effect of feedwater temperature on the area. No. of sections = 3, Plant capacity = 1000 rn31d, pressure = 6.9 MPa, Plant recovery = 80%.

in membrane area due to the lower average flux and a decrease in pumping power for a certain value of plant recovery. Fig. 17 shows the effect of the feed water temperature on membrane area requirement and indicates a sub- stantial reduction in area requirement and cost of modules when the feed water is a high temperature as compared to a relatively lower temperature feed.

Fig. 18. Effect of tbe operating pressure on the net annual expense or benefit. NO. of sections = 3, Capacity = 1000 m’/d, Temp. = 90°C, No recirculation.

RECYCLE OF DYEING WASTEWATER WITH DYNAMXC; MBMBRANBS 43

The net economic result of constructing such a wastewater recycling pIant is shown in Fig. 18 for plants of different recovery and operating pressure processing dyeing wastewater at 90°C (194OF). This figure shows the net annual expense or benefit resulting from such a plant. This plant is designed with three sections with no recirculation. It is clear fkom this figure that for plants with recoveries of 60% to 70% no eccmomic benefit can be a~rued. However, with plant recoveries of 80% and 95%, net benefit could be achieved at operating pressure s of 4.6 &lPa (670 psig) and 5.6 MPa (800 psig) respectively. A net benefit results when the annnal savings in water and energy which result from recycling high-temperature product water is greater than the annual plant cost which includes the cost of capital plus the 0 & M costs. Energy savings are estimated based on an ambient water tempera-e ofHm (42°F).

1.

2.

3.

4,

5.

6.

7.

8.

9. X0.

Il.

12,

13.

R, L. Gulp, A, A. Wesner and Et, L. Gulp, Water Reuse and Recycling, Vol. 7, Evalu- ation of Needs and Potentials, OWRT Report No_ OWRT@U-141, April 1979. J, L. Gaddii, C. A_ Brandon, and J_ 3. Porter, Energy Conservation Through Point Source Recycle with High Temperature Hyperfiltratiox, EPA Report No. EPA-600/7- 74131, June 1979; C_ A. Brandon, Personal communication, Oct. 1979. A_ M. El-Nashar, Dynamic Zr(N)-PAA Membrane Hyperfdtration of Textile Dyeing Wastewater at Elevated Temperature, Proceedings, Fifth Annual Conference NWSLA, July 1977. C. A. Brandon, and 3. J. Porter, Hyperfdtmtion for Renovation of Textile Finishing Plant Wastewater, EPA Report No. EPA-600/2-76-060, March 1976. C. A. Brendon, I J_ Porter, and D. EC. Todd, ~y~e~~a~on for Renovation of Com- posite Wastewater at Eight Textile Finishing Plants, EPA Report No. EPA-600/2- 78-047, March 1978. R. H. Horn’ng, Textile Dyeing Wastewaters: Charscterixation and Treatment, EPA Report No. EPA-69012-78-098, May 1978. A. E. Marcinkowsky, K. A. Emus, H. 0. Phillips, J, 6. Johnson and A. J. Shor, J. Am. Chem. Sot. 88 (1966) 5744. D. G. Thomas, Dynamic Membranes -Their Technological and Engineering Aspects, in: Reverse Osmosis and Synthetic Membranes, Ed_ S. Sourirajan, National Research Conneii Canada, 1977. J. S. Johnson, R. E. Mmturn, and P. Wadia, J. ElectroanaL Chem., 37 jlQ72) 267. D_ G. Thomas and W. Et. Mixon, Ind. Eng+ C&em Process Design Devel., 11 (1972) 339. P. A. Longweii, Composition and Flow Relatiouships for Tubufar Reverse Usmosis Unita Operating Under Turbulent Flow Conditions, Office of Saline Water, Res. Devel, Progr. Rept. No. 426, May 1969. W_ L. Prehn Jr., and J. L. McGaugh, Desaiting Cost Calculating Procedures, Offsee of Sahne Water, Res. Dew& Progr. Rept. No. 555, May 1970. R. C. Gumerman, R. L. Gulp, and S. P- Hansen, Estimating Costs for Water Treat- ment as a Fuuction and Size and Treatment Efficiency, RPA Report No. EPA-606/2- 78-182, Aug. 1978.