Separation and Purification Technology 126 (2014) 21–29

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Separation and Purification Technology

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Nanofiltration as tertiary treatment for the reuse of dairy wastewatertreated by membrane bioreactor

http://dx.doi.org/10.1016/j.seppur.2014.01.0561383-5866/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +55 (31) 34093669; fax: +55 (31) 34091879.E-mail address: lauraha@ymail.com (L.H. Andrade).

L.H. Andrade ⇑, F.D.S. Mendes, J.C. Espindola, M.C.S. AmaralDepartment of Sanitary and Environmental Engineering, Federal University of Minas Gerais, Av. Antônio Carlos, no. 6627, Pampulha, Belo Horizonte, Minas Gerais, Brazil

a r t i c l e i n f o

Article history:Received 5 July 2013Received in revised form 30 January 2014Accepted 31 January 2014Available online 13 February 2014

Keywords:Nanofiltration (NF)Operating conditionsWastewater reuseDairy wastewaterMembrane bioreactor (MBR)

a b s t r a c t

Due to the growth in the costs of collecting and treating water, and with the treatment and discharge ofeffluents, reuse has become an increasingly viable option for industries. This study aimed at evaluatingthe application of membrane bioreactor (MBR) as secondary and nanofiltration (NF) as tertiary treatmentfor the reuse of dairy wastewater, focusing on determining the best NF operating conditions. MBRshowed high removal efficiency for COD (mean of 98%) and nutrients (86% total nitrogen and 89%phosphorus). However, the concentration of dissolved solids in the permeate still prevented its reuse.In order to remove these solids, the MBR permeate was nanofiltrated, and three different cross-flowvelocities were evaluated. Due to lower external fouling and better quality of the permeate, the velocityof 7.8 m/s was selected as the most suitable for the NF system. The optimum permeate recovery rate wasdetermined to be 45% since, at values higher than this, the quality of the permeate dropped. The proposedtreatment system (MBR + NF) showed overall efficiencies of 99.9% for COD and 93.1% for total solids. Thefinal treated wastewater could be reused as water for cooling, steam generation, or washing of externalareas and trucks.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The dairy industry is considered, among the food industries, tobe the most polluting due to the high water consumption and thegeneration of large amounts of liquid waste which, in turn, consti-tute the main source of pollution of this type of industry [1]. Thesewastewaters are characterized by high concentrations of organicmatter and nutrients, and are composed mainly of carbohydrates,proteins and fats originated from milk, and of residual cleaningagents [2].

Conventional treatment systems for these wastewaters includethe use of primary treatment to remove solids, oils and fats;secondary biological treatment to remove organic matter andnutrients; and, in some cases, tertiary treatment such as polishing.However, several problems have been reported, such as highproduction of scum, poor sludge settleability, low resistance toorganic shock load, difficulties in removing nutrients (nitrogenand phosphorus) and problems in the degradation of fats, oilsand other specific types of pollutants [3,4].

Currently, the reuse of wastewater has become an environmen-tal and economically viable option for industry, due to increasingly

restrictive parameters for wastewater discharge, imposition ofcharges both for the collection of water as well as for the dischargeof effluents, and to the reduced availability and quality of water re-sources. In the case of the dairy industry, on the one hand, the useof treated wastewater should be avoided for washing equipmentthat receives products or for operations where there is a possibilityof direct contact with raw material, because there may be a risk ofcontamination; on the other hand, the reuse of water is encouragedas replacement in cooling or heating systems and for good manu-facturing practices, such as washing floors and trucks and rinsingexternal areas [5,6]. In this scenario, one of the most promisingtechnologies for wastewater treatment and reuse are membraneseparation systems and the combining of these systems with othertechnologies [7].

Membrane bioreactors (MBR) consist of biological reactorsassociated with membrane separation processes, usually withmicro or ultrafiltration. Among the advantages of MBRs, it isnotable that they are compact and modular systems, with lowsludge production, that show total removal of suspended solidsindependently of the characteristics of sedimentability of thebiomass, and that generate high quality treated wastewater [8].The wastewaters from MBRs may be reused directly for unre-stricted irrigation [7] or for recreational purposes after removalof the residual color [9]. However, if the water use requires a

22 L.H. Andrade et al. / Separation and Purification Technology 126 (2014) 21–29

higher quality, such as for indirect reusable drinking water or forindustrial reuse, a tertiary treatment with nanofiltration or reverseosmosis, for example, may be necessary [10,11].

Nanofiltration (NF) is an intermediate process of ultrafiltrationand reverse osmosis which carries advantages such as the efficientremoval of dissolved solutes, including multivalent ions and organ-ic compounds of high molar mass; however, with lower pressurerequirements and higher flows than reverse osmosis [12]. Studiesshow that NF is an efficient system for the secondary or tertiarytreatment of wastewaters, aiming at generating water for indus-trial, agricultural and/or indirect drinking reuse [11,13–15]. How-ever, the evaluation of the optimum operating conditions foreach specific NF system allows improvement of the overall perfor-mance of the process, both in terms of quality of the permeate andfouling control.

The use of NF for the treatment of several types of wastewatersis reported in the literature, e.g. tannery [16], textile [17,18], refin-ery [19] and municipal [20]. The application of NF for dairy waste-waters is also found. However most papers focus on the treatmentof real used cleaning-in-place (CIP) solutions [21–23], model CIPwastewater [24], flash cooler condensates from ultra high temper-ature treatments [25], milk whey [26], model dairy effluentprepared from commercial milk [1,27–29] or dairy chemical–bio-logical treatment plant effluent [30]. We found no references tothe treatment of real effluents from large dairy industries makinguse of MBR and NF.

Therefore, the aim of this study was to evaluate the applicationof MBR as secondary treatment and NF as tertiary treatment for thereuse of dairy wastewater. Emphasis was placed on evaluating thebest NF operating conditions that would generate a better qualitypermeate and provide less membrane fouling.

2. Materials and methods

2.1. Wastewater from the dairy industry

The wastewater that was fed into the MBR came from a largedairy industry located in the state of Minas Gerais, Brazil, whosemanufactured products are UHT milk, yogurt, cheese, cottagecheese and petit suisse. The wastewater was collected from theindustry wastewater treatment station, following the stages ofsieving and flotation with compressed air.

FT

LegendPump FlowmeterNeedle valve

P T

Permeate

Retentate

SC

P Manometer

T ThermometerSC Speed controller

FT Feed Tank

Fig. 1. Schematic of the NF system.

2.2. Experimental unit of the membrane bioreactor

The bench scale membrane bioreactor used to conduct the testswas built by PAM Membranas Seletivas Ltda (Rio de Janeiro Brazil).The MBR had one module of hollow fiber, submerged microfiltra-tion membranes (polyetherimide, average pore size of 0.5 lm,membrane area of 0.044 m2, packing density of 500 m2/m3). TheMBR system consisted of three acrylic tanks (a 40-liter feed storagetank, a 4.4-liter working volume biological tank, into which themembrane was inserted, and a 5-liter tank for permeate storing),a diaphragm pump, solenoid and needle valves, and flow and pres-sure gauges.

The MBR was initially inoculated with sludge coming from theactivated sludge reactor of the effluent’s supplier. After an initialphase of acclimatization of the microorganisms to the conditionsof the MBR and the effluent, which lasted 28 days and in whichthe hydraulic retention time (HRT) was set to 8 h and there wasno sludge discharge, the system operated continuously for 40 days.The operating conditions were: HRT of 6 h and sludge retentiontime of 60 days (the defined values were based on existing litera-ture and previous tests as explained elsewhere [31,32]. The flowrate was 0.80 L/h, and the permeate flux was 18.2 L/(h m2). The

air flow rates of the two aeration systems used for oxygenationand mixing of the biological tank and for membrane fouling controlwere both 0.5 Nm3/h. The backwash flow rate was adjusted to2.0 L/h and it was triggered automatically for 45 s, for every15 min of permeation.

The feed and permeate of the MBR were characterized daily asto COD and color (Hach DR2800 Spectrophotometer), and weeklyas to BOD, total nitrogen (Shimadzu TNM-1) and ammonia nitro-gen, phosphorus and total solids. Three times per week the concen-tration of mixed liquor volatile suspended solids of the sludge wasalso ascertained. All analyses were done according to the recom-mendations of the Standard Methods for the Examination of Waterand Wastewater [33].

2.3. Experimental unit for nanofiltration

The MBR permeate was nanofiltrated in order to generate finaltreated wastewater with quality for reuse. For NF testing, an NF90commercial membrane from Dow–Filmtec was used. The mem-brane was cut properly and inserted into an 8.9 cm diameter stain-less steel cell, providing a 62 cm2 filtration area, which simulated aprocedure with a flat membrane. The water permeability of themembrane used showed a mean value of 2.3 L/(h m2 bar), indicat-ing that this NF membrane had characteristics similar to those ofreverse osmosis.

The NF system comprised an feed tank (FT) where the MBR per-meate was stored, a pump connected to a speed controller, flow-meter for reading the feed flow rate, a pressure adjustmentvalve, a manometer and temperature gauge. Fig. 1 shows a sche-matic of this unit.

2.4. Determination of the best NF operating conditions

Initially, the best condition for the feed cross-flow velocity wasdetermined and next, the optimum permeate recovery rate of NF.

To determine the best flow regime, tests were conducted atthree different feed velocities, 4.4, 6.1, and 7.8 m/s, which corre-spond to the feed flow rates of 4.0, 5.6, and 7.2 L/min. These valuesare close to those applied by other authors, as Trägardh andJohansson [21], who studied the nanofiltration of dairy dischargedalkaline cleaning solutions at a cross flow velocity of 7.0 m/s, andLászló et al. [27], who treated model dairy wastewaters with ozone

Table 1Mean values of the main physicochemical parameters of feed and permeate and theremoval efficiencies of the MBR.

Parameters Raw wastewater MBR permeate Removal %

COD (mg/L) 2937.6 57.3 97.9BOD (mg/L) 1120 6 99.5Color (units Pt-Co) 2316.6 27.35 98.7TN (mg/L) 49.8 6.9 86.1N-NH3 (mg/L) 43.1 1.4 96.0Phosphorus (mg/L) 36.3 1.4 89.0TS (mg/L) 3.366 1.647 45.7TFS (mg/L) 1.527 1.473 0.7TVS (mg/L) 1.838 174 84.3

TN – total nitrogen; N-NH3 – ammonia nitrogen, TS – total solids, TFS – total fixedsolids, TVS – total volatile solids.

L.H. Andrade et al. / Separation and Purification Technology 126 (2014) 21–29 23

and nanofiltration at 4.6 m/s. Each test included measurements ofpermeability, evaluation of pollutant retention efficiency, anddetermination of resistance to filtration. To keep the conditionsconstant during these tests, all permeate and retentate were fedback into the feed tank, which was kept between 25 and 35 �Cby using an ice bath. At the end of each intermediate step, themembrane was chemically cleaned using a sodium percarbonatesolution at a concentration of 0.5 g/L in an ultrasonic bath for20 min.

Membrane permeabilities were calculated by measuring thepermeate flow at pressures of 10.0, 7.5, 5.0 and 2.5 bar. The perme-abilities with wastewater were compared to the permeabilitieswith distilled water for the clean membrane, previously measured.

To evaluate the retention capacity of the membrane, feedsamples (MBR permeate) and NF permeate were collected and ana-lyzed for conductivity (Hach 44600 Conductivimeter), color, totalcarbon (TC) and total organic carbon (TOC) (TOC Analyzer, Shima-dzu TOC-V CNP), and total solids (TS) [22].

Resistances to filtration were determined according to the ser-ies resistance model. Similar to the methodology used by Lászlóet al. [27], the total resistance (Rt) was divided into membraneresistance (Rm), external resistance (Re) and internal resistanceto fouling (Ri). To calculate these resistances, the flows Jw (flowof the clean membrane filtrating pure water), Ji (flow of the fouledmembrane filtrating pure water) and Jt (flow of the membrane fil-trating effluent) were determined at a fixed pressure of 10 bar. TheJw flow was determined by the permeation of distilled waterthrough the clean membrane. Following this step, the wastewaterwas nanofiltered for 30 min and the stabilization flow (Jt) wasmeasured. Subsequently, water was recirculated in the system ata flow rate of 3.2 L/min for 30 min and, following this procedure,the flow of permeation of distilled water was measured again,corresponding to Ji. The resistances were calculated according tothe following equations, considering the dynamic viscosity of thepermeate to be equal to that of the water:

Rm ¼ Pg� Jw

ð1Þ

Ri ¼ Pg� Ji

� Rm ð2Þ

Re ¼ Pg� Je

� Rm� Ri ð3Þ

in which P corresponds to the difference between the applied trans-membrane pressure and the osmotic pressure of the feed, and g tothe dynamic viscosity of the water at 25 �C.

To determine the optimum permeate recovery rate (RR), nano-filtration of the effluent was conducted with return of the concen-trate to the feed tank and continuous withdrawal of the permeate,using the optimum cross-flow velocity previously determined. Thepermeate flow was monitored and samples of the permeate werecollected periodically for TC, TS and conductivity analysis [33].The optimum RR was determined based on the results of permeatequality and flux decay.

2.5. Evaluation of the viability of reusing final treated wastewater

To evaluate the viability of reusing the effluent from the NF, thepermeate, collected with the RR and cross-flow velocity conditionsconsidered as the most appropriate for the system, was analyzedfor pH, alkalinity, total dissolved solids, COD and metals (Ca, Mg,Cu, Zn and Fe) according to Standard Methods for the Examinationof Water and Wastewater [33]. Metals were analyzed by an atomicabsorption spectrophotometer (Perkin Elmer 3300). The results

were compared with standards for both cooling and boiler waterfound in literature Asano et al. [5].

3. Results and discussion

3.1. Membrane bioreactor

The MBR operated with a mean MLVSS concentration of19,500 mg/L. This value, which may be considered high given thatthe mean MLVSS concentration in an MBR with a submerged mem-brane module fluctuates between 10,000 and 15,000 mg/L [34], isjustified by the fact of the wastewater having a high concentrationof organic matter which is highly biodegradable [35]. Therefore,there was sufficient substrate available for the microorganismsfor both the catabolism and the synthesis of new cells.

Table 1 shows the mean results of the concentration ofpollutants of the feed and the permeate of the MBR, and of therespective removal.

It can be observed that the MBR showed increased capacity forthe removal of organic matter, both in terms of COD and BOD,which may be justified by the high biodegradability of the waste-water, high concentration of biomass in the reactor and thepresence of the membrane in the system [32].

High removal of nutrients and color is also noted. The highsludge ages usually applied in MBRs contribute to the nitrificationthat occurs in these systems, since nitrifying bacteria, responsiblefor the conversion of ammonia to nitrate, are notoriously slow-growing microorganisms [8]. In addition, the tropical climate andhigh temperatures of the country also contribute to the nitrifica-tion that occurs systematically in biological treatment systemsimplemented in Brazil [36]. Thus, the high removal efficiencies ofammonia nitrogen were predictable. However, since the reactoris totally aerated and has no anoxic zones, the significant removalof NT, which indicates the occurrence of denitrification, was notexpected initially. Nonetheless, this phenomenon might haveoccurred due to the reduction in the oxygen transfer efficiencystemming from the sludge’s high viscosity. In this way, internal re-gions of the biological flocs possibly did not receive oxygen andtransformed themselves into anoxic zones, thus providing favor-able denitrification conditions [37].

According to Guadie et al. [38] incomplete air circulation in thereactor and/or the presence of biofilms, which create a shield fordenitrifiers, can lead to an anoxic microzone in the biofilm, result-ing in simultaneous nitrification and denitrification (SND) in thereactor even during aeration. In such cases, nitrification occurson the surface of the biofilm, whereas denitrification occurs inthe inner layers due to a dissolved oxygen gradient [39,40].

Moreover, since sludge growth was high, part of the total nitro-gen removal may result from a higher nutrient uptake. Accordingto Wang et al. [40], during the steady state, the mass of discharged

0%

20%

40%

60%

80%

100%

Eff

luen

t per

mea

bilit

y in

rel

atio

n to

wat

er p

erm

eabi

lity

4.4 m/s 6.1 m/s 7.8 m/s

Fig. 2. NF permeability with wastewater in relation to permeability with water forthree feed flow conditions.

24 L.H. Andrade et al. / Separation and Purification Technology 126 (2014) 21–29

sludge is equal to the mass of microorganism growth and forma-tion; and nitrogen consumption due to cell assimilation can be cal-culated with the cell formula (C5H7NO2), in which N accounted for12% of the total mass. Mean total nitrogen load removal was calcu-lated by the difference between feed and permeate TN concentra-tions multiplied by flow rate; and nitrogen removal forassimilation was estimated by emission of sludge, consideringsludge retention time of 60 days. The comparison of nitrogen re-moval for assimilation (171 mg N/day) and TN removal(719 mg N/day) shows that nitrogen loss in the system was largerthan nitrogen loss for assimilation. Thus, simultaneous nitrificationand denitrification must have occurred in the reactor.

The mean phosphorus removal was 86%. Traditionally, systemsprojected to remove phosphorus should contain aerobic and anaer-obic reactors in series for the selection and growth of phosphateaccumulating microorganisms (PAO) [36]. In the case of conven-tional biological treatment systems, the partial removal of phos-phorus takes place through its assimilation by the biomass forcellular synthesis. In this case, the discarding of excess sludgecan result in phosphorus removal that varies between 10% and30%, depending on the organic load of the effluent and on the oper-ation conditions [41].

Farizoglu et al. [42] evaluated the removal of nutrients in amembrane jet loop reactor treating whey and obtained total phos-phorus removal efficiencies between 65% and 85%, similar to thosein the present study and greater than those expected for systemsthat do not have specific configurations for advanced phosphorusremoval. For the authors, these elevated values result from consid-erable phosphorus assimilation for cellular synthesis, as the bio-mass concentration in the reactor was high (between 6000 and14,500 mg/L), which was also true for both MBRs in this study.Moreover, the authors assumed that part of the phosphorous re-moval was related to the precipitation of phosphates with Ca2+

and Na+ ions, present in great quantities in the effluent in question.The effluent used as feed in this work presented relatively highCa2+ concentration (near to 85 mg/L [31]). Given the low Kps valuefor calcium phosphate (1.3 � 10�32) the hypothesis of phosphorusprecipitation is quite plausible. Therefore, both justifications pre-sented by Farizoglu et al. [42] are applicable to the present work.

Moreover, although PAOs thrive under anaerobic/aerobic condi-tions, they do not necessarily require these operational conditionsin order to survive, persisting in bioreactors operated under strictaerobic conditions as well as other aquatic habitats [43]. In thatway, MBRs present a potentially suitable environment for PAO pro-liferation because they are slow growing organisms and the mem-brane may completely retain them, since their size is typicallylarger than microfiltration membrane pores [44 apud 38]. Addi-tionally, as discussed before, the high biomass concentrations nor-mally found in MBRs might lead to areas of anoxic or anaerobicmicro-niches within the sludge flocs, potentially providing PAOsa selective advantage [43].

Silva et al. [43] characterized the microbial diversity of the acti-vated sludge in a group of eight MBR plants fed with municipalwastewater, located in different regions of Europe. They found thatPAOs were presented in similar levels (10% ± 6%) in all studiedMBRs, even those without a defined anaerobic zone. The resultsfrom this study suggest that a defined anaerobic zone is not neces-sarily required for putative PAO growth in MBRs, since polyphos-phate storage may provide a selective advantage in fulfilling cellmaintenance requirements in substrate-limited conditions (lowF/M).

Thus, it is possible that a part of phosphorous removal found inthe presented study was related to biological assimilation by PAOs.

In relation to the solids, the highest removal was for volatile sol-ids, which are made of biodegradable organic matter. The removalof fixed solids may be related to the precipitation of salts and/or

the retention of inorganic particulate material by the membrane.It is important to note that MBRs show complete retention of sus-pended solids.

Thus, it can be stated that the MBR permeate shows high qual-ity, with low concentrations of organic matter and nutrients. How-ever, there is still a significant concentration of dissolved solids,since the MBR microfiltration membrane is not able to retain com-pounds in solution. Therefore, in order to remove these solids andto generate a final treated wastewater suitable for reuse, the MBRpermeate was sent to an NF unit.

3.2. Determination of the best NF operating conditions

Fig. 2 shows values of permeability of the NF membrane withwastewater divided by the permeability with water for the cleanmembrane at three different cross-flow velocities.

A positive impact on the permeability with wastewater, i.e. an18% raise, was observed by increasing the cross-flow velocity from4.4 to 6.1 m/s. This increase was responsible for an improvement inthe hydrodynamic conditions of the system, reducing the thicknessof the boundary layer near the surface of the membrane and thefouling [13]. In addition, according to Luo et al. [28], higher shearrates may reduce the accumulation of solutes on the membranesurface, minimizing the difference in osmotic pressure across it,and increasing the effective driving force for permeation, thusincreasing the flow.

An even greater increase in flow velocity, up to 7.8 m/s, also in-creased membrane permeability with the wastewater, althoughless significantly, which shows that there are types of foulingswhich cannot be removed by establishing better hydrodynamicconditions. The gain obtained in this operation was 4%, and thepermeability with wastewater reached a value corresponding to97% of permeability with water.

Table 2 shows the values of the physicochemical parameters ofNF feed and of the permeates obtained for the three conditionstested. It is noteworthy that NF tests were conducted in batchmode. Thus, the MBR permeate, collected during some hours,was fed into the NF system. Therefore, the concentration of someparameters of the NF feed shown in Table 2 may differ slightlyfrom those shown for the permeate of the MBR in Table 1, sincein the latter case they refer to average values obtained during40 days of monitoring.

Improvement in all parameters monitored was noted with theincrease in the feed velocity. Since increased flow rate reducesthe accumulation of solutes retained on the membrane surface, italso reduces their concentration gradient between the feed and

Table 2Values of the physicochemical parameters for the feed and the permeates obtained for the different cross-flow velocity and their respective retention efficiencies.

Parameter Feed Permeate

4.4 m/s 6.1 m/s 7.8 m/s

Value Retention (%) Value Retention (%) Value Retention (%)

Conductivity (mS/cm) 2.28 0.316 86.1 0.244 89.3 0.151 93.4Color (Pt-Co units) 36.8 8.9 75.7 6.4 82.7 3.3 91.0TS (mg/L) 1.482 767 48.3 523 64.8 488 67.1TC (mg/L) 231.2 6.7 97.1 4.4 98.1 4.1 98.2TOC (mg/L) 24.9 0.6 97.4 0.6 97.4 0.5 97.9

TS – total solids; TC – total carbon; TOC – total organic carbon.

20.0

16.114.0

2.1 2.9 3.5

7.0

1.70.2

02468

101214161820

4.4 m/s 6.1 m/s 7.8 m/sR

esis

tanc

e (

x1013

m-1

)

Membrane Internal External

6978 79

714

2024

81

0%

20%

40%

60%

80%

100%

4.4 m/s 6.1 m/s 7.8 m/s

Res

ista

nce

(%

)

Membrane Internal External

(a)

(b)

Fig. 3. Resistances to NF filtration: (a) values and (b) percentage.

L.H. Andrade et al. / Separation and Purification Technology 126 (2014) 21–29 25

the permeate, reducing the driving force for their transport. Inaddition, due to less fouling at high flow rates, the passage of sol-vent is favored, contributing to dilution of the permeate [28].

The reduction in the conductivity of the permeate in relation tothe feed was greater than that of the total solids, indicating thatmost of the retained solids correspond to divalent salts which con-tribute to conductivity to a greater degree. This is consistent withexpectations for an NF membrane. It is also noted that TOC concen-tration is well below that of TC, indicating that most of the carbonstill present in the MBR permeate is not due to organic compounds,but rather to the carbonates or other forms of inorganic carbonwhich were efficiently retained in the NF.

Fouling is the greatest problem facing most membrane separa-tion processes, causing significant reduction of permeate flow,increased frequency of cleaning, and reduced useful life of themembrane. Fouling is caused by adsorption of solutes in the mem-brane material, total or partial blockage of the pores, deposits ofparticles on the membrane surface (cake) and formation of a gellayer [23,45,46]. In the case of NF, the reduction of permeate flowdue to the effects of concentration polarization is also highly signif-icant. Due to convective transport, the concentration of retainedsolutes becomes greater near the surface of the membrane thanwithin the bulk solution, leading to concentration polarization. Insome cases, this concentration may reach threshold values, causingthe precipitation of inorganic compounds (scaling) and contribut-ing even more to the increased resistance to filtration [47].

Regarding membrane filtration of dairy wastewater, the proteinmaterials may act as powerful fouling agents, while the surfactantsmay alter the permeability of the membrane by concentrationpolarization or by the formation of micelles [48]. Furthermore, dur-ing the reclamation of wastewater, organic constituents containedin the biologically treated wastewater, such as polysaccharides,proteins, humic and fulvic acids, and nucleic acids, designated aswastewater organic matter, are found to play an important roleas membrane foulants, especially the hydrophobic fraction [49].

Fig. 3 shows the resistances to filtration for the three cross-flowvelocity that were analyzed. The resistances were divided intomembrane resistance (Rm); external fouling resistance (Re), whichincludes formation of a gel layer, concentration polarization andscaling; and, internal fouling resistance (Ri), which basically con-sists of adsorption and pore blockage. As the membrane used inthis study had characteristics closer to those of a reverse osmosismembrane, in this case the pores refer to the free volume betweenthe polymeric chains. Membrane resistance acquired different val-ues in the three tests performed, due to variations in the mem-brane permeability with water measured after chemical cleanings.

For the lowest velocity evaluated, the greatest contribution toresistance (without considering the resistance of the membrane it-self) came from the external fouling component (24% of the totalresistance). A concentration gradient of the retained solutes wasformed on the membrane surface, due to the low feed flow rateand the lower turbulence generated, causing a diffusive transport

in the opposite direction from the convective flow of the permeate,and increased resistance to filtration. In addition, the increase inthe solute concentration near the membrane may have led to theincrease in viscosity and the formation of a gel layer.

When the velocity was increased to 6.1 m/s there was a 76%reduction in external resistance, which corresponds to only 8% ofthe total resistance for this flow condition. This result justifiesthe permeability gain observed previously, and is related to the in-creased turbulence in the region near the membrane. Nevertheless,there was an increase in internal resistance, showing that theexternal fouling that was formed may retain solutes and reducethe availability of the membrane to internal fouling by adsorptionsince, according to Contrerasa et al. [50], dissolved organic com-pounds may be adsorbed by colloids that are deposited on themembrane.

The subsequent increase in cross-flow velocity to 7.8 m/sreduced external resistance even more, by 91%. Although there isalso an increase in internal resistance, the resistance due to the to-tal fouling (internal + external) was reduced from 4.6 to3.7 � 1013 m�1.

Kaya et al. [24] divided the total resistance of a nanofiltrationprocess into gel layer resistance, internal resistance, concentrationpolarization resistance, and membrane resistance. For the effluentevaluated by those authors, a synthetic wastewater based on

26 L.H. Andrade et al. / Separation and Purification Technology 126 (2014) 21–29

anionic and non-ionic detergents, dyes and sodium chloride, thetotal resistance ranged between 3 and 5 � 1013 m�1, valuesapproximately 5 times lower than those obtained in the presentstudy. On the other hand, László et al. [27] obtained resistancesfrom internal fouling and from the gel layer of 1.2 � 1014 and5.5 � 1014 m�1, respectively, for nanofiltration of model residualwater prepared with powdered skim milk and anionic surfactant.The differences observed between the results found in the litera-ture and those obtained in the present study may be related tothe different water permeabilities of the membranes used in eachstudy, and to the fact that the other authors used simulated waste-water based on basic compounds that, therefore, had much less in-ter-relational complexity among the constituents than the realwastewater studied here.

Thus, due to the significantly greater efficiency in the retentionof solutes and the lower fouling at the cross-flow velocity of 7.8 m/s, this was selected as the best operating condition among thethree evaluated.

Following determination of the best flow rate, the greatest per-meate recovery rate (RR) at which the system could operate with-out damage to its performance was determined. To accomplishthis, the wastewater was nanofiltered using the pressure of10 bar and the selected feed velocity of 7.8 m/s. The variation inpermeate flux resulting from the RR is shown in Fig. 4. The maxi-mum RR value attained was 70%, due to operational restrictionsof the system.

In Fig. 4, the bars represent the positive and negative values ofstandard deviation permeate flux. These standard deviations werecalculated according to the formula for determining the combinedstandard uncertainty:

ucðyÞ ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXN

i¼1

@f@xi

� �2

� uðxiÞ

vuut ð4Þ

where uc(y) is the combined standard deviation of the variable y, f isthe function y = f(x1, x2, . . . , xN) and u2(xi) is the uncertainty relatedto the parameter xi. If the flux of permeate (J) is calculated by:

J ¼ V=t

p� D2=4 ð5Þ

where V is the volume of permeate collected in a given time, t is thesampling time (assumed in this case as 60 s) and D is the diameterof the membrane, the combined uncertainty of J can be calculatedby:

0

5

10

15

20

25

0% 10% 20% 30% 40% 50% 60% 70%

Perm

eate

flu

x (

L/h

.m²)

Recovery rate

Fig. 4. Variation in permeate flux with the RR for feed cross-flow velocity of 7.8 m/sand pressure of 10 bar.

ucðJÞ ¼ J �

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiu2ðVÞ

V2 þ u2ðtÞt2 þ 4u2ðDÞ

D2

sð6Þ

The increase in RR may lead to reduction in permeate flux, dueto several factors: increase in the difference of osmotic pressurebetween the feed and the permeate, due to the increased concen-tration of retained solutes; thickening of the gel layer due to in-creased concentration of macromolecules and colloids on themembrane surface; increased viscosity of the fluid flowing throughthe membrane pores; increased fouling due to internal and exter-nal adsorption [46]. However, considering the standard deviation,no significant fluctuations or clear change in the permeate flux pro-file were observed with the increase of the RR, which may also berelated to the relatively low maximum RR attained.

Fig. 5 shows the results of conductivity, total solids (TS) and to-tal carbon (TC) of the NF permeate as a function of the RR applied.

There was little variation in the quality of the permeate for RRbelow 45%. However, for higher RR, the effect of the accumulationof retained molecules causes an increase in the driving force forpassing solutes through the membrane, and the conductivity andthe concentration of TS and TC of the permeate increase. Withthe RR at 70%, this effect was highly significant and a big drop inpermeate quality can be seen.

Therefore, despite there being no significant reduction in thepermeate flux with the increase in the concentration, the 45%recovery rate may be selected as the best, due to the drop in per-meate quality observed at the higher values. However, for largerscale applications, it is important to note that a higher recoveryrate can be achieved if a combination of modules in series is used,in several steps, instead of parallel modules operating at 45% RReach.

Furthermore, the data collected from the industry supplier ofthe wastewater show that 1300 m3/day of water are consumedin the industrial plant, of which 1000 m3/day are transformed intowastewater and 300 m3/day1 are lost to evaporation or are incor-porated into products. Of the volume collected (1300 m3/day),60% is used for cleaning operations (clean in place) and must meetpotability standards, 30% is used to replace water in cooling andheating systems, and 10% is used in good manufacturing practices(cleaning floors, bathrooms, external areas, etc.). Therefore, sincethe direct reuse of effluents as potable water is not recommendeddue to the associated risk [5], the aim was that the NF permeatecould meet the quality requirement of the 40% of water which is

0

50

100

150

200

250

300

350

0

10

20

30

40

50

0% 10% 20% 30% 40% 50% 60% 70% 80%

TS

(mg/

L)

C

ondu

ctiv

it y (

µS/c

m)

TC

(m

g/L

)

Recovery rate

TC TS Conductivuty

Fig. 5. Results of conductivity, total solids, and total carbon of the NF permeate as afunction of the RR.

Table 3Standards for cooling and boiler water, and values obtained for the NF permeate.

Parameter NF permeate Cooling watera Steam generationa

Low pressure (<10 bar) Medium pressure (10–50 bar) High pressure (>50 bar)

TDS (mg L�1) 233 500 700 500 200Alkalinity (mg L�1) 166 350 350 100 40pH 8.9 6.9–9.0 7.0–10.0 8.2–10.0 8.2–9.0COD (mg L�1) 4.0 75 5.0 5.0 1.0Calcium (mg L�1) 0.44 50 + 0.4 0.01Magnesium (mg L�1) 0.041 0.5 + 0.25 0.01Copper (mg L�1) 0.04 + 0.5 0.05 0.05Zinc (mg L�1) <0.1 + + 0.01 0.01Iron (mg L�1) 0.05 0.5 1 0.3 0.05

TDS – Total Dissolved Solids.+ Accepted as received, if other threshold values are met.a Source: Asano et al. [5].

Dairy WastewaterCOD = 3274 mg/LColor = 1802 Pt-Co unitsTS = 2323 mg/L

MBR PermeateCOD = 34 mg/LColor =35,5 Pt-Co unitsTS = 1783 mg/L

NF RetentateCOD = 73,4 mg/LColor = 75,1 Pt-Co unitsTS = 3087 mg/L

NF PermeateCOD = 4,0 mg/LColor = 15,0 Pt-Co unitsTS= 233 mg/L

MBR

NF

Fig. 6. COD concentration, color and total solids of the raw wastewater, MBR permeate, NF retentate and NF permeate.

Table 4Removal efficiencies of the MBR, of the NF and the combination of the two processes.

Parameter

COD (%) Color (%) TS (%)

MBR 99.0 98.4 47.0NF 88.2 57.7 86.9Overall 99.9 99.3 93.1

L.H. Andrade et al. / Separation and Purification Technology 126 (2014) 21–29 27

used for washing, cooling and heating (520 m3/day). Thus, if theindustry wastewater were treated by the proposed system, com-prising MBR and NF, and if 45% NF recovery rate were applied,450 m3/day of reusable water would be generated, supplying al-most all of the demand for non-drinking water in industrialfacilities.

Although the aim of this study is not the definition of a route forthe exclusive reuse by the industry in question, it is believed thatits water balance is representative of other industries in the sectorwhich fabricate the same products, and that similar decisionscould be made in more general settings.

However, direct comparison of water balance of different indus-tries can lead to erroneous interpretations, since water consump-tion levels are determined by production output and the appliedtechnologies and may significantly vary from one industry to an-other [6]. For example, according to Gleick et al. [51], water usein the dairy products industry is divided as following: 71% for cool-ing, 23% process, 3% restrooms, 3% landscaping; while otherauthors suggest a different distribution: 53% non-contact cooling/heating, 27% process, 19% sanitary use [52].

3.3. Reuse of the final treated wastewater

To verify the possibility of reusing the NF permeate, its physico-chemical properties were compared to quality standards for cool-ing and boiler water, as shown in Table 3.

It is observed that the quality of the NF permeate meets all thestandards for water used in cooling and low pressure steam gener-ation, enabling its reuse for such applications, as well as for wash-ing floors, external areas and trucks, which require a lower qualitywater. However, the only parameters that do not fall within theboiler water standard at medium pressure are alkalinity and cal-cium. Therefore, if there is interest also in reusing wastewater for

boilers which operate at pressures greater than 10 bar, optionssuch as the use of a subsequent degassing unit to remove CO2

and reduce alkalinity, exchanging nanofiltration for reverse osmo-sis, or implementing a final polishing system with ionic exchange,could be evaluated.

No studies were found in the literature that reported the use ofMBR and NF for treating dairy wastewater aimed at reuse. How-ever, some authors used NF as the only treatment system for thistype of wastewater [1,23,28]. Vourch et al. [1] used NF to treat asynthetic dairy wastewater, comprising whole milk, skim milkand milk whey, with COD concentration of 8200 mg L�1and con-ductivity of 700 lS cm�1. The NF permeate had a COD concentra-tion of 87 mg L�1, conductivity of 637 lS cm�1 and calciumconcentration of 3.2 mg L�1. Fernandéz et al. [23] evaluated theoperation of a pilot NF unit used for the recovery of clean in place(CIP) solution consumed in the dairy industry. The feed solutionhad a COD concentration between 3000 and 10,000, total dry ex-tract between 1.0% and 2.0%, and conductivity of 15 mS cm�1;while the permeate had 1500–2500 mgCOD L�1, conductivity of15–20 mS cm�1, and total dry extract of 0.9–1.0%. The concentra-tions of the permeate obtained in the present study were foundto be lower, which is probably due to the contribution of the highremoval of pollutants in the MBR in order to generate a final per-meate with high quality.

28 L.H. Andrade et al. / Separation and Purification Technology 126 (2014) 21–29

Fig. 6 shows the final result of the treatment route tested. Thevalues shown for the concentrations of pollutants in the rawwastewater and in the MBR permeate differ slightly from thoseshown in Table 1 because those were the mean concentrations ob-tained during the entire operating period of the MBR; while thoseshown in Fig. 6 are the measurements made on the day that thesamples to be used for conducting the NF tests were collected.

The MBR is found to function efficiently in removing organicmatter and color contained in the raw wastewater. However, theconcentration of total solids, predominantly composed of dissolvedsolids, was still high and had to be removed in order to providewater with quality for reuse. This was obtained by passing theMBR permeate through NF, which showed high retention of solidsand also of residual COD. As can be seen in Table 4, the overall sys-tem efficiencies were very high. It is emphasized that the NF reten-tate could be recycled in the industry as reused water forapplications that do not require very high quality, such as for irri-gating gardens, or could be discarded into bodies of water. The CODconcentration of 73 mg L�1 not only meets the discharge parame-ters of environmental legislation in effect in the state of Minas Ger-ais, Brazil (180 mgCOD L�1), but is also well below this standard,thus contributing to the release of better quality effluent and tothe preservation of water bodies.

4. Conclusion

� The MBR is a viable system for treating dairy wastewater, pro-viding high removal efficiencies not only of organic matter butalso of color and nutrients. The MBR permeate showed highquality; however, the concentration of dissolved solids is stillhigh to be reused as industrial water.� Cross-flow velocity of 7.8 m/s was selected as the most suitable

for the NF of the MBR permeate, once this condition led toincreased turbulence and, therefore, less fouling and better per-meate quality. The results of the resistances to filtration showedthat, when the cross-flow velocity increases from 4.4 to 7.8 m/s,the resistance due to external fouling significantly reduces;however, there is a slight increase in the resistance due to inter-nal fouling.� The permeate recovery rate was observed to have no influence

on permeate flux. However, the increased RR provided anincrease in the passage of pollutants to the permeate and a dropin quality. Based on this information and on the water balanceof the dairy industry, the RR of 45% was selected as theoptimum.� The quality of the NF permeate met all the standards for cooling

water and water for low pressure steam generation, provingthat it may be reused for these applications as well as for wash-ing floors, external areas and trucks, that require a lower qualitywater.

Acknowledgments

The authors gratefully acknowledge PAM Membranas SeletivasLtda for supplying the membranes, CNPq for the scholarshipawarded, FAPEMIG for the financial resources granted to theresearch and DESA for the resource for the article translation.

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