Nanofiltration and reverse osmosis of model process waters fromthe dairy industry to produce water for reuse

Nanofiltration and reverse osmosis of model process waters fromthe dairy industry to produce water for reuse

DESALINATION ELSEVIER Desalination 172 (2005) 245-256 www.elsevier.com/locate/desal Nanofiltration and reverse osmosis of model process waters from...

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DESALINATION

ELSEVIER

Desalination 172 (2005) 245-256 www.elsevier.com/locate/desal

Nanofiltration and reverse osmosis of model process waters from the dairy industry to produce water for reuse Mickael Vourch a, Bfatrice Balannec a*, Bernard Chaufer ~, Gfrard Dorange b "LARCIP, Universit~ de Rennes 1, 263, av. du G~n&al Leclerc CS 74205 35042 Rennes Cedex, France Tel. +33 (2) 23 23 57 62; Fax: +33 (2) 23 23 57 65; email: [email protected] bLARCIP, Ecole Nationale Supdrieure de Chimie de Rennes, Av. du Gdndral Leclerc, 35700 Rennes, France

Received 30 March 2004; accepted 15 July 2004

Abstract In dairy plants the process waters generated during the starting, equilibrating, interrupting and rinsing steps contribute to the production of effluents. They correspond to milk products (milk, whey, cream) diluted with water without chemicals. The treatment of these dairy process waters by nanofiltration (NF) or reverse osmosis (RO) operations was proposed to concentrate dairy matter and to produce purified water for reuse in the dairy plant. The study reports one-stage and two-stage (NF + RO and RO + RO) spiral-wound membrane treatments with five model process waters representative of the main composition variations observed in dairies. Performances (permeate flux, milk components rejection, purified water characteristics) of the different operations were compared. Discussion was focused on the comparison between quality of produced waters and vapour condensates (from product drying and evaporation processes) reused in dairy plants. Accordingly, both total organic carbon (TOC) and conductivity of water treated by a single RO or NF + RO operations were convenient for reuse as heating, cooling, cleaning and boiler feed water. With the two-stage RO + RO process, a more purified water complying with the TOC drinking water limit was achieved. Keywords: Effluent; Milk; Process water; Nanofiltration; Reverse osmosis; Water reuse

I. Introduction Within the framework o f limited water resources and increasing costs, industries - - and especially the food industry - - need to lower water consumption and to look at process water *Corresponding author.

and effluent treatment for water recycling or reuse. Several works dedicated to the treatment o f food industry streams show that using membrane operations for producing purified water from effluents or process waters is convenient. In the meat industry, nanofiltration (NF) was used for treating cooling water from sausage products to

0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.07.038

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M. Vourch et al. / Desalination 172 (2005) 245-256

produce drinking quality water for water reuse [1-3]. Ultrafiltration (UF) and NF were able to treat poultry slaughterhouse chiller tank effluent for reconditioned water recycling [4]. In the fish industry, microfiltration and UF were used for water recycling from commercial surimi process water [5]; NF of wastewater from the fishmeal industry permits recycling of the treated water [6]. In the beverage industry, water from bottle washing water processed by NF and low-pressure reverse osmosis (RO) is similar to water of drinking quality [7]; and a hybrid process using a membrane bioreactor and two-stage NF was developed to treat spent process water from a fruit juice company for purified water reuse [8]. In the vegetable industry, soybean soaking water was recovered using a combined NF-RO for water recovery [9]. The dairy industry has a high water consumption and generates a large amount of effluent from 0.2 to 10 L per litre of processed milk. The process waters generated during the starting, equilibrating, interrupting and rinsing steps of the different plant units contribute to effluent production. The treatment of the process waters to produce purified water for reuse could lower the effluent volume and the total water consumption of the dairy factory. Many membrane operations have been described for the treatment of dairy effluents: one- step operations based on UF [10], NF [11], RO [12] or two-stage operations such as U F + R O [13],NF + N F [3] or RO + RO [11]. It has been shown that NF and RO are convenient operations for the production of water for reuse in dairy plants from low contaminated process water [14]. In fact, in the dairy industry, vapour condensates from product drying and evaporation processes are largely used as heating, cooling, cleaning or boiler feed water [ 15-17], but French drinking water regulations do not allow reuse in contact with dairy products. The targeted microbiological and chemical quality of the treated water must be, at least, similar to that of used

vapour condensate [chemical oxygen demand (COD) <52 mg 0 2 . L -1 , conductivity <60/~S.cm-1]. It must approach the characteristics of water for human consumption [total organic carbon (TOC) <2 mg.L-1] to ensure larger reuse possibilities for applications where unexpected contact with the milk product may occur (e.g., cooling water for pump seal or plate heat exchangers); in this case, specific authority must be obtained from the French authorities. The aim of this work was to study NF and RO treatment of dairy model process waters for the production of reusable purified water. Data collected from several industrial plants showed that industrial dairy process waters, which are mixtures of water and milk products without chemicals, are variable in composition (fat content, whey/milk ratio, etc.) and concentration. Thus, an average model process water and four specific model process waters, taking into account the main composition variations, were studied. A previous feasibility study by dead-end filtration with a diluted skimmed milk solution allowed for the comparison of eight NF and RO membrane performances [ 18]. Crossflow experiments with one selected NF and one selected RO spiralwound membranes are in good agreement with the dead-end filtration experiments. However, due to the high COD level of the feed solution, the concentration by crossflow filtration does not produce a permeate stream that complies with the standard of drinking water quality (TOC <2 mg.L -1) in a single membrane stage. Therefore, in this study two-stage membrane treatments (NF + RO and RO + RO) were tested with the above-mentioned dairy model process waters. The choice of NF in the first stage was based on high water flux, i.e., a lower membrane area. Performances (permeate flux, milk components rejection, permeate and retentate compositions) of the different operations were compared and discussed. The composition of the permeate could be a key parameter for the choice of the first stage membrane.

M. Vourch et al. / Desalination 172 (2005) 245-256

2. Experimental 2.1. Model process waters from the dairy industry A study (inquiries and visits) of 10 French industrial dairy plants showed that dairy process waters are mixtures o f water and milk products without chemicals with various compositions and concentrations. The major composition variations concerned fat content, heat treatment, and the whey/milk ratio. An average composition, called "average model process water" (A), was defined, and the variation range for each o f the main parameters around the average value was estimated. Thus five different model process waters were defined, and their main characteristics are given in Table 1. The solutions were prepared from whole milk, skim milk and whey used fresh, with low heat treatment (63°C, 20 s) or reconstituted from highheat powders (100-130°C, 2-20 min). The dry matter (DM) of the model process waters was close to 6 g.L -~. The model process water (F) simulates a high fat content with a ratio o f whole milk to skim milk higher than that of A. The model process water noted HT represents a process water with high-heat-treated milk products, reconstituted from high-heat milk and whey powders. The model process water noted W

247

corresponds to a high whey/milk ratio process water in which whey was as high as 88% of DM. The model process water noted "aged" was an average model process water stored during 24 h at 25°C before treatment, for which pH decreased from 6.6 to 4.8 due to natural acidification. 2.2. Membranes Two commercially available spiral-wound membranes were used in this study: one NF membrane (Desal5-DL, Osmonics) with a 150300 g.mol-1 molecular weight cut-off and one RO membrane (TFC HR, Koch) with a NaCI rejection of 99.5%. Membranes are thin-film composite (TFC) with an active polyamide layer. The filtration area of the spiral wound modules is 2.5 m 2. Deionised water membrane permeabilities (at 25°C) were 3.3 and 7.3 L.h-l.m-2.bar -~ for Koch TFC HR and Desal5 DL, respectively. Before each experiment the membrane pure water permeability was measured with deionised water at 25°C. After each experiment the membrane was cleaned with a 0.04%w of alkaline detergent (Ultrasil 10, Henkel-Ecolab) for 1 h at 43°C. After cleaning membrane permeability was checked again. The membranes were stored in a 0.5% sodium metabisulfite solution.

Table 1 Main characteristics of the different model process waters Characteristics

Dry matter, g.L-1 COD, g O2.L-t Fat, g.L-j Conductivity, #S/cm pH

Model process waters Average (A)

High fat content (F)

High heat treatment (HT)

High whey/ milk ratio (W)

Aged (stored 24 h at 25°C)

5.3 8.2 0.9 700 6.6

6.1 9.0 1.5 500 6.6

5.9 8.3 1.0 670 6.6

5.9 7.6 0.2 730 6.6

5.8 8.2 1.0 700 4.8

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M. Vourch et aL / Desalination 172 (2005) 245-256

2.3. Operating conditions

2.4. Analysis

Crossflow filtration experiments were conducted with a filtration unit designed by TIA (Boll~ne, France). The feed tank can treat 50 L of solution. Temperature (7) of the solutions was regulated at 25°C, transmembrane pressure (TMP) and crossflow flowrate (QR) were set at 20 bar and 800 L.h-~, respectively. The aim was to concentrate the volume of treated solution and to reach a volume reduction ratio (VRR) = 50 (i.e., permeate volume close to 98% of feed volume). VRR vs. time was:

Feed, retentate and permeate samples were assessed for physicochemical and chemical properties: DM (accuracy +2%); Fat (normalised R6se-Gottlieb method, ISO 1211-1984, accuracy -4-0.05 g.L-1); TOC (OI Analytical TOC analyser, accuracy +2%); pH (accuracy +0.05). COD was determined with rapid test tubes (oxidation with potassium dichromate/sulphuric acid/silver sulphate at 148°C, accuracy ±3%) and photometric measurement (Nanocolor 300D) provided by Macherey Nagel. Cation (Na+, K+, Ca2+, Mg 2+) concentrations were measured by atomic absorption (Spectra 300, Varian Associates) [19] and anion (phosphate, CI-) concentrations by ion chromatography (Dionex, DX 500) according to the method described by Gaucheron et al. [20]. Accuracy was about 1% for all ions. The conductivity ~ was measured (CDM 210, Radiometer Analytical) with an accuracy of 2%. For the different components, the observed rejection of the membrane was obtained by the following equation:

VRR (t) -

v0 VR(0

+

v0 Vo - Vp(0

(1)

where V0 is the feed volume, and VR(t) and Vp(t) are volumes of retentate and permeate at time t, respectively. The filtration unit can concentrate the solutions up to VRR 10 due to dead volume. Thus, to reach a higher VRR, two steps were performed: filtration of the feed solution up to VRR 10, addition of a 10-times concentrated solution in the feed tank, and following of the filtration up to the targeted VRR. This two-step procedure was quite satisfactory, provided the component rejection is close to 100%. This condition is fulfilled for RO where complete rejections are reached. Results at VRR > 10 are disturbed by the two-step procedure for poorly rejected components, i.e., monovalent ions in NF. For the latter, at the end of the first NF concentration step, there is a depletion ofmonovalent ions in retentate (due to low rejection) and before the beginning of the second step, a 10-times concentrated solution is added with a higher monovalent ion content. It must be kept in mind that both the retentate and permeate of the second concentration step will have a higher monovalent ion concentration than with a process in which concentration from 1 to VRR 50 is performed in a continuous way.

R ( % ) - - ( 1 - Cv

CR

xl00

(2)

where Cp and CR are the permeate and retentate concentrations, respectively. The removal efficiency was calculated using the following equation: E(%) = (1--~oe ) x l 0 0

(3)

where Co is the feed concentration. 3. Results and discussion 3.1. Membrane operation performances with average model process water

Two-stage operations (NF+RO and RO+RO) were performed on the average model process

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M. Vourch et al. / Desalination 172 (2005) 245-256 4O O NanofiltratJon 35

O

• Reverse osmosis

'E 30

~2s Im

x -' 20

0

O

0

O

0

E



0

Ck w

O

O •

0

25

50

75

100

125

150

O 175

200

225

250

Dry m a t t e r ( g . L 1)

Fig. 1. Permeateflux vs. the retentate dry matter during averagemodelprocess water A concentrationwith NF (o) or RO (•) membrane. water A. The first operation concentrated the DM dairy solution up to 169 g.L -1 (VRR 33) with RO and 224 g.L -1 (VRR 43) with NF, respectively (Fig. 1). During NF concentration, permeate flux decreased from 34.8 L.h-t.m -2 (at VRR 1) to 1.2 L.b-l.m2 (at VRR 43) with a DM content of retentate increasing from 5.3 to 224 g.L -1. RO concentration of process water A showed a decrease of permeate flux from 33.8 L.h-1.m-2 (at VRR 1) to 1.0 L.h-~.m 2 (at VRR 33) while the DM content of retentate increased from 5.3 to 169 g.L -1. Unfortunately, permeate flux from the NF membrane was not higher than RO membrane permeate flux, likely due to membrane fouling by the feed components (proteins). In the last part of filtration (DM >125 g.L-I), RO permeate flux declined faster than NF flux. At the end of the concentration stage, the combined phenomena of osmotic pressure increase, polarisation concentration and membrane fouling were involved for fluxes close to zero. The permeate stream of the first operation (NF or RO) was processed with a second stage (RO) to improve purification. Permeate flux of the second RO stage was high and remained almost constant at 52 L.h-].m-2 for both NF and RO permeate filtration.

Permeate and retentate characteristics of the two-stage operations are given in Figs. 2 and 3. In Fig. 2, for the NF stage, conductivity between brackets is over-estimated due to the limitation of the feed tank capacity and the two-step NF concentration process at VRR >10 (see Operating Conditions). In the first operation (NF or RO concentration stage), all the components were concentrated in retentate by a factor equal to VRR, except monovalent ions in NF. Monovalent ions had a low rejection in NF that explained the low conductivity value in NF retentate and high value in NF permeate. During NF concentration, TOC in permeate fraction increased from 1 to 548 mg.L -~, and TOC of all collected NF permeate was 42 mg.L -~. For the RO concentration stage, TOC in permeate ranged from 1.5 to 30 mg.L-~ and TOC of all collected RO permeate was 3.2 mg.L -1. TOC rejection remained almost constant, around 99.7% for NF and 99.95% for RO whatever VRR. Ion rejection was higher than 99% in the first RO stage, and RO permeate was nearly demineralised (X = 10 #S.cm-l). The second RO stage improved purification and highly demineralised NF+RO and RO+RO permeate streams (X < 10 #S.cm-1) were obtained

250

M. Vourch et al. / Desalination 172 (2005) 245-256 Feed TOC = 3A g/L COD = 8.2902/L

~ R R

NF Permeate

~,

RO P¢~lz/aeate I TOC = 3 3 mg/L Z=9BS/em [

TOC = 42 mg/L . COD = 87 mgO 2/L ~ (~ =_.,.._.__637/aS/em)

D M =5.3 g/L Fat = 0.9 g/L

v

= 700 tXS/em

43 NF Retentate TOC = 133 g/L COD = 369 g O 2/L DM = 224 g/L Fat = 38,7 g/L (X = 5500 gS/em )

~ V R R

9.5

...... RO Retentate TOC = 312 mg/L COD = 803 mgO~/L [ ~ = 4300 ~tS/crn

Fig. 2. Permeateand retentate characteristicsofNF + RO operations with the average model process waterA.

TOC =Feqd 3.3 g/L I [ S O Permeate [ COD = 8.2 g 0 2/L c--------1 [ TOC = 3.2 mg/L [ - DM = 5.3 g/L ~ C O D = 6 mgO 2/L[ ~

Fat =0.9 g/L

~

IX=l°l~s/cm

X = 700 ktS/cm

/

I VRR33 RO Retentate TOC = 114 g/L COD = 270 g O 2/L DM = 169 g/L Fat ~ 29,2 g/L = 8000 gS/cm

I

RO Permeate I TOC = 1.5 mg/L *l g = 3 B S / c m

/~VRR8.5 RO Retentate

TOC = 21.2 mg/L COD = 48 mgO:/L [ ~ = 74 txS/em

Fig. 3. Permeateand retentate characteristics of RO + RO operations with the average model process water A.

with very low TOC (3.3 mg.L -1 and 1.5 mg.L -1, respectively). The TOC removal efficiency was 99.9% for the NF + RO operation and 99.95% for RO+RO. The first RO stage gave the same removal efficiency as the two-stage NF+RO operation (99.9%). The TOC removal efficiency of the second stage of RO+RO operation was significantly lower (53%), probably because of low molecular organic substances that were poorly rejected by the RO membrane (similar to small organic compounds of vapour condensate going through RO membranes [15]).

3.2. Influence of composition of model process waters at p H 6. 6

The two-stage membrane treatments were performed with four dairy model process waters that simulate the main composition variations of the industrial process waters. In Fig. 4, permeate fluxes of the RO concentration stage were plotted vs. retentate DM content. The flux decline was similar for all process waters until 125 g.L-' DM. For high concentrations, process water W with a high whey/milk ratio showed the fastest flux decline. For a given retentate DM value, W has

M. Vourch et al. / Desalination 172 (2005) 245-256

251

40 OHT 35

OA AF OW

x

[]

~2o

Ao

o

<>~.

<> .,,,

I1.

0 D

0 DD

0

,,,,, 0

25

50

75

100

125

150

~ 175

200

&,

,--

225

250

Dry matter (g.L "I)

Fig. 4. Permeateflux vs. retentatedry matterduring concentrationwith a RO membranefor A, HT, F and W model process waters. 40 QA

35

O HT

~o ~25

aged QW

1,( :320

o

o, 0 0

a.

OX

0

0

0

X

[]

X

o n

0

25

50

75

100

125

150

175

200

° 225

250

Dry matter (g.L "~) Fig. 5. Permeate flux vs. the retentate dry matter during concentration with a NF membrane for A, HT, W and aged model process waters.

higher osmotic pressure than process water A, due to higher conductivity, so the RO permeate flux was lower. Permeate fluxes o f the NF concentration stage vs. retentate DM content are plotted in Fig. 5. Fluxes were similar for all model process waters; only slight differences were observed. Permeate fluxes o f the second RO

stage were high and remained almost constant, ranging between 48 and 52 L.h-'.m -2 in all cases with VRR ranging from 1 to 10. The composition o f permeate and retentate was determined, and rejection of TOC and ions was calculated. For RO in the first concentration stage, TOC rejection was at least 99.9%, regard-

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M. Vourch et al. / Desalination 172 (2005) 245-256

100

100 _ilol in wnn u

~

60

--

40

•~

20

0

I

• •

:

;N--a ~> 0

o

0 -20

o

o

O

, 0

10

oK

O

20

30

40

uu

~;

-so O0

ne -100

oK

i

50 60



oCI iiNa

-150

O 50



0

o Cl





100

150

200

250

Retentate Dry Matter (g/L)

R e t e n t a t e Dry M a t t e r (g/L)

Fig. 6. Rejection ofmonovalent ions vs. the retentate dry matter during concentration with the NF membrane for average model process water A (VRR 1 to 10).

Fig. 7. Rejection ofmonovalent ions vs. the retentatedry matter during concentration with the NF membrane for average model process water A (VRR 10 to =40).

less of the model process water used. Ion rejection was high (>99%), at the same level for the process water F, HT, W as for A, so conductivity of all the collected RO permeates ranged between 9 and 12/~S.em -I. For the NF concentration stage, TOC rejection for process water W and HT was similar to TOC rejection with process water A, between 99.3 and 99.7%. Ion rejection in the NF concentration stage with process water W and HT was the same as ion rejection with process water A. For Mg 2+and Ca 2+ rejection was higher than 97.5%; for phosphate it was higher than 95%. Rejection of Na +, K + and CI- vs. retentate DM is plotted in Fig. 6 for the first concentration step (VRR 1 to 10), corresponding to diluted model process water, and in Fig. 7 for the second concentration step (VRR 10 to =40), corresponding to concentrated model process water. For diluted model process water, the mean Na + rejection was about 72% (ranging between 65 and 75%) and the K + average rejection value was about 76% (ranging between 63 and 85%). Chloride rejection decreased during concentration and became negative in order to ensure permeate eleetroneutrality in accordance with membrane co-ion exclusion [21]. For concentrated model process water (Fig. 7), Na + and K + rejection

decreased to about 50% and 56%, respectively, unlike chloride rejection, which was always negative and decreased with concentration from 0 to -120%. Consequently, chloride concentration was higher in the collected permeate than in the feed model process water. It must be kept in mind that at VRR 10, the process became discontinuous by the addition of 10-times concentrated feed solution, which differs significantly from NF retentate content in monovalent ions.

3.3. Influence of 24-h storage before processing model process water An average model process water (called aged) was stored during 24 h at 25°C before treatment to simulate a possible storage effect on filtration performances. Natural acidification took place, with the pH decreasing from 6.6 to 4.8 during storage. Low molecular weight organic matter (such as lactate and ethanol) was produced by lactose degradation during the acidification process. This aged model process water was processed by two-stage NF + RO operations. Permeate flux of the NF concentration stage was similar to fluxes with the other model process waters (Fig. 5). In this case, TOC of the collected NF

M. Vourch et al. / Desalination 172 (2005) 245-256 permeate was 184 mg.L -l, significantly higher than the TOC of the NF permeate from process water A (42 mg.L-l). During the RO purification stage, this organic matter was concentrated in the retentate, inducing the permeate flux to decline from 48 to 40 L.h-~.m-2. TOC rejection of the NF stage was significantly reduced, ranging from 97 to 95%, when it was higher than 99.3% with the other process waters. Rejections of the low molecular weight molecules (lactate 89 g.mol -~, ethanol 46 g.mol-1) were low compared to that of lactose (342 g. mol -~) because molecular weight cut-off of the NF membrane is about 150-300 g.mo1-1. Fig. 8 shows a comparison of ions rejections between aged and A process waters after coneentration to DM of 150 g.L -1. Multivalent ion rejection was not affected, but monovalent ion rejection was significantly changed for aged process water compared to average process water. The rejection of Na ÷ and K ÷ was lower. The rejection of CI- was then positive instead of negative for the A process water, likely due to lactate ions that were mainly balanced by K ÷. It was checked (by charge balance) that permeate electroneutrality was achieved with a lactate molar concentration close to that of C1-.

253

3.4. Quality o f purified produced water and possible reuse applications Quality of the produced water (permeate) was depending on the number (one or two) and the type (NF or RO) of membrane stages and on the type of treated model process water. Table 2 summarises the main permeate characteristics for the different process waters after one-stage or two-stage operations. With one NF stage, permeate quality was similar for process water A, W and HT - - with TOC between 35 and 53 g.L -1, conductivity ranging from 580 to 660 #S.cm -~ due to monovalent ions concentrations (around 150 mg.L -1 for CI-, 100 mg.L -1 for K + and 30 mg.L -1 for Na+); multivalent ion concentrations were low (<5 mg.L-l for Ca 2+,<1 rag.L-1 for Mg 2÷ and <16 mg.L -~ for phosphate). NF permeate from aged process water was of lower quality than other process water - - with TOC 3 to 4 times higher (184 mg.L -~) because aged process water contained small organic molecules (likely lactate and ethanol) which were poorly rejected. For one RO stage, all the process waters gave approximately similar results: permeate TOC value ranged from 3.2 to 5.6 mg.L -~ and permeate was well demineralised (conductivity

100 80 60-

40

,--

.....

20

O Q ...... 4_

-~I

40

-6(] ................ i

-8C P04

--~ . . . . . . . . . . . . . . ~_/aged

Ca

Mg

Fig. 8~ Ion rejection for model process waters A (pH 6.6, closed bar) and aged (pH 4.8, open bar) after concentration to dry matter 150 g.L-I with the NF membrane.

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M. Vourch et al. / Desalination 172 (2005) 245-256

Table 2 Main characteristicsof water (collected permeate) produced by one-stage or two-stage membrane operations from different model process waters Feed A One-stage

HT

W

Aged

NF

A

HT

W

F

5.6 -12 6 1.6 <1 1.2 <1 --

4.8 -9 3 1.2 <1 <1 <1 <1

3.6 -12 5 1.4 <1 <1 <1 <1

2.5 2

1.7 2

1.3 2

RO

operation:

TOC (mg.L-t) COD (mgO2.L-1) X (#S.cm 1) CI (mg.L-~) K÷(mg.L-~) Na÷(mg.L-l) Ca2+(mg.L-1) Mg2+(mg.L-1) PO43 (mg.L-I)

42 87 637 158 105 32 3.2 <1 16

Two-stage

NF + RO

35 76 580 136 90 26 5.0 <1 10

53 115 660 149 120 32 2.3 <1 14

184 640 715 94 144 33 5.5 <1 31

3.2 6 10 4 1 <1 1.5 <1 -RO + RO

operation: TOC (mg.L-~) (#S/cm)

3.3 9

3.2 4

3.1 7

<12 #S.cm -I, ions concentrations all under 6 mg.L-1). With a two-stage NF+RO treatment o f process waters A, W or HT, purified water was produced with roughly the same quality as with the single RO stage: TOC <3.3 mg.L -1 and conductivity <9/~S.cm -1. For aged process water, water produced by NF+RO was less purified: conductivity was 88/tS.cm-t because conductivity rejection by RO stage ranged from 92 to 96% only and TOC was 27 mg.L -I because small molecules like ethanol were not well rejected by the RO membrane. For the four process waters processed by RO+RO operation, purified water was quite significantly demineralised (conductivity <3/tS.cm -1) and TOC was <2.5 mg.L -1. These results show that, except with the aged process water, the different process waters gave roughly the same purified water quality, depending only on the number and type o f membrane used. Three qualities o f water could be produced:

27 88

1.5 3

the poorest quality by a single NF, good quality for reuse in dairy plants by single-stage RO or by a two-stage NF+RO process, and very pure water by a two-stage RO+RO. The quality of these purified waters is similar to the quality ofvapour condensate produced by the dairy industry [15]. Vapour condensates with conductivity <25 #S. cm -1 were often used as boiler feed water. For conductivity ranging between 25 and 60 #S.cm- l, water was used for cleaning in place: preparation of dilute acid or alkaline solutions, pre- and intermediate rinsing, for washing floors and the outside of plant and vehicles, for heating and cooling applications. For such applications purified water produced by a single RO operation or by two-stage operations (NF+RO or RO+RO) can be reused as well as vapour condensates. For applications where unexpected contact between milk products and water can occur, when there is a risk o f leakage (e.g., in plate heat exchangers or

M. Vourch et al. / Desalination 172 (2005) 245-256

255

pump seals), drinking water quality is required (TOC <2 mg.L-~). Highly purified water produced by RO+RO, close to drinking water quality, could be used for such applications provided that specific permission has been obtained from the French authorities.

to M. Vourch and BBA (Bretagne Biotechnologies Alimentaires) for financial support. M. Clement (LERES, ENSP, Rennes, France) and J. Cabon (ENSC, Rennes, France) are gratefully acknowledged for their technical contribution to the analysis.

4. Conclusions

References

Five model process waters, representative of the main composition variations of 10 industrial dairy plants process waters, were treated by onestage and two-stage membrane operations. It was shown that, except with aged process water (process water naturally acidified due to 24-h storage at 25°C), the treatment of the four different model process waters gave similar performances: permeate fluxes were almost similar, composition of permeates and retentates, just as rejectionofthe different components, were nearly identical. It can be concluded that, in the range of the study, heat treatment, fat content and whey/ milk ratio of the process waters do not have a significant effect on the performance of the treatments. A drawback was also shown: process water must be treated within a few hours before degradation of milk components occurs because membrane operations afford moderate quality permeates. After a single RO or NF+RO operations, the water could be reused in replacement for cleaning, heating or cooling purposes and for boiler feed water. With a RO+RO treatment, highly purified water complying with TOC drinking water regulation was obtained and could be reused, with specific and special permission, for applications where unexpected contact with products may occur (risk of leakage) in order to prevent possible contamination.

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Acknowledgements The authors wish to thank the Councils of Regions Bretagne and Pays de la Loire for a grant

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