**3. Materials and Methods**

### *3.1. Whey*

The nanofiltrated acidic whey (NFW) was obtained in curd producing and provided by the Madeta milk factory (Jindˇrich*u*˚v Hradec, Czech Republic) which specializes in the production of milk-based desserts as well as yogurts, fermented milk products, curd and yogurt deserts. (see Table 2).

**Table 2.** Feed (nanofiltrated acid whey) composition and physicochemical characteristics.



**Table 2.** *Cont*.

#### *3.2. Reagents*

The chemicals used in the experiments were of analytical grade and purchased from Sigma Aldrich (Germany). The demineralized water (қ<sup>≤</sup> 10 <sup>μ</sup>S·cm−1) is produced in MemBrain Ltd., (Stráž pod Ralskem, Czech republic) by reverse osmosis.

#### *3.3. Membranes*

The food grade membranes were used in ED. The monopolar membranes used in ED processes for demineralization of whey were CEM-PES and AEM-PES cation and anion exchange membrane, respectively. These are heterogeneous membranes based on polyethylene as polymer and sulfonated groups as cation exchanger and quaternary ammonium groups as anion exchanger groups. Furthermore, both types of the membranes were reinforced with two polyesters (PES) fabrics. The reinforcement was performed by repressing at 150 ◦C and 5.06 <sup>×</sup> 10−<sup>6</sup> Pa. The membranes were produced in MemBrain s.r.o., (Stráž pod Ralskem). The membranes properties including the resistivity and the permselectivity were studied and reported in previous works and presented in Table 3 [24].

**Table 3.** Properties of the membranes used in ED.


1d: the thickness of the membrane; <sup>2</sup>ρ: specific resistivity; 3P: the apparent permselectivity.

The electromotive force emf method [8], was used to measure the apparent permselectivity of the membrane. To briefly explain, a two-compartment cell was used whose chambers were filled with 0.5 and 0.1 M KCl, respectively. The membrane was placed in a hole between the compartments. Two Ag/AgCl (1 M/KCl) reference electrodes were inserted into the solutions close to the membrane. After 1 h of stirring the solution with magnetic stirrers the potential between two electrodes was measured and the apparent permselectivity was calculated as a ratio of the measured potential to the theoretical potential which corresponds to a 100% permeselective membrane Equation (1):

$$P\% = \frac{U\_{\text{measured}}}{U\_{\text{theoretical}}} \times 100\tag{1}$$

where (*P*) is the apparent permselectivity, (*Umeasured*) is the measured potential across the membrane and (*Utheoritica*l) is the theoretical potential which is calculated for a membrane with 100% permselectivity [27,28].

For measuring the resistance of the membrane the same type of cell was used despite that both compartments were filled with 0.5 M NaCl. Two Pt wire electrodes were inserted into the solution while two Ag/AgCl (1 M/KCl) reference electrodes were placed next to the membrane on each side. The dc current of 10 mA amplitude was applied to the Pt electrodes and the resulted potential drop between the reference electrodes was measured. The same measurement was carried out without the membrane. The resistance of membrane was calculated using the ohm law, Equation (2):

$$
\rho\_{sm} - \rho\_s = \rho\_m \, (\Omega.cm) \tag{2}
$$

where (ρ*sm*) is the specific resistivity of the membrane and solution layers trapped between the membranes and the references electrodes, (ρs) is the specific resistivity of the solution and (ρm) indicates the specific resistivity of the membrane [27,28].

#### *3.4. Electrodialysis*

The ED processes is shown in Figure 4. The ED was performed with modified electrodialysis unit P1 EDR-Y/50-0.8 (manufactured by MemBrain s.r.o.). The pH and the conductivity of the solutions were measured by SenTix® 940 glass electrode and TetraCon 925 conductivity cell, respectively. The probes were connected to the WTW multi 3420. It must be mentioned that the conductivity cell also possesses the temperature sensor. The stack contained 50 pairs of membranes AEM-PES and CEM-PES assembled in C-A-C (cation exchange membrane–anion exchange membrane–cation exchange membrane) configuration. The active area of each membrane was 400 cm2. The unit was additionally equipped with a device which introduces the potential pulse and pause. The minimum length of a pulse which could be applied was 1 s. The pulse consisted of a working period and a cleaning (reverse) period. Diluate was desalinated during working period, whereas fouling was expected to be removed during cleaning period.

**Figure 4.** The configuration used for electrodialysis of whey.

Fouling could be removed due to diffusion and electric migration in electric field of reverse polarity in PER. The potential in working period was 50V (1.0V/pair). Three different regimes were applied differing in the length of working and cleaning periods and the applied potential during cleaning period. Total voltage (voltage on the whole unit) and the voltage on the stack without electrode compartments were monitored. The voltage on polarizing electrodes was adjusted so that the voltage on the stack was (50 ± 1) V. In ED of whey, diluate container was filled with the 30.0 kg of nanofiltrated whey (NFW) while 7.0 kg of tap water was poured into the concentrate chamber. The flow rate and

the linear velocity through the membranes of solutions are given in Table 4. The electrodes solution was 10 g·L−<sup>1</sup> NaNO3 7.0 kg. The ED was performed in batch mode. The ED regimes which were used for desalination of whey are shown in Table 5. To compare the effect of the different regimes on the membrane fouling two consecutive runs of each regime were performed without any cleaning step between. It must be mentioned that each regime was continued until the conductivity in diluate reached 1.0 mS·cm<sup>−</sup>1.


**Table 4.** Process conditions.



#### *3.5. Fouling Analysis*

The membranes samples were submitted for the SEM, immediately after the second run of each ED regime. Images were taken on an uncoated sample with a scanning electron microscope (SEM) (Quanta FEG 450, FEI, Hillsboro, OR, USA). The potential of 5 KV was applied and the working distance was 15 mm. The hydrophobicity of the membrane was studied by measuring the contact angle using (Theta QC, Attension, Espoo, Finland). For measuring the contact angle, a drop of distilled water was placed on a surface and the contact angles between the drop and the membrane surface were measured. The contact angles ranged from 0◦ to 180◦.

#### *3.6. Calculations*

The Degree of demineralization in ED was obtained as Equation (3) [18]:

$$\text{Degree of demineralization } \%= \left(1 - \frac{\kappa\_{\text{final}\ of\ diluate} \left(\text{S } m^{-1}\right)}{\kappa\_{\text{initial}\ of\ diluate} \left(\text{S } m^{-1}\right)}\right) \times 100\tag{3}$$

where *(*қ*initial)* and *(*қ*final)* are the initial and final conductivity of the diluate.

Ash content %ODB (on dry basis) was calculated as Equation (4) where the ash content and the total solids are unit less parameters:

$$\text{Ash content } \% \text{ODB} = \frac{\text{Ash content } (\%)}{\text{Total solids } (\%)} \times 100 \tag{4}$$

The electrodialysis capacity is defined as Equation (5):

$$\mathcal{C}\_F = \frac{m\_F}{N\mathcal{A}.t} \tag{5}$$

where (m*F*) is the mass of the feed, (*A)* is the active surface of the membranes; (*N)* is the number of membrane pairs, and (*t)* is the total time of electrodialysis process.

Average ash transfer rate was determined using Equation (6):

$$J\left(\text{kg}\,\text{m}^{-2}\,\text{h}^{-1}\right) = \frac{\left(m\_F \times \,\text{W}\_F\right) - \left(m\_{D,\,final} \times \,\text{W}\_{D,\,final}\right)}{N \times S \times t} \tag{6}$$

where (*mF)* and (*mD,final)* are initial and final mass of diluate, (*wF)* and (*wD,final)* are initial and final ash concentration (*g*/*kg*), (*N)* number of membrane pairs, (*S)* effective membrane area (*m2*) and *t* time (*h*).

Energy consumption was calculated Equation (7):

$$E = \frac{\int\_{t0}^{t1} \mathcal{U}\_{\text{avg}} I dt}{m\_F} \sim \frac{\sum\_{t0}^{t1} \mathcal{U}\_{\text{avg}} I \Delta t}{m\_F} \text{ (Wh/kg}\_F\text{)}\tag{7}$$

where (*Uavg)* is average voltage on the stack (*V*), *t*<sup>1</sup> *<sup>t</sup>*<sup>0</sup> *I*Δ*t* amount of transported charge (*Ah*) and (*mF)* initial weight of diluate (*kg*).

#### **4. Conclusions**

Comparing the conventional ED and PER in two consecutive batch experiments without cleaning in place (CIP) between them, the electrodialysis parameters are almost the same in the first and second runs of conventional ED while in PER the parameters of the second run are evidently worse than the parameters of the first run. Since CIP was not applied, deterioration of the ED parameters such as the electrodialysis capacity and the energy consumption in PER might be attributed to the fouling on the surface of the membranes and/or to the back transfer of mass during the reversal period. Considering that the SEM analysis and the contact angle values indicate that fouling on cation exchange membranes and on concentrate side of anion exchange membranes were comparable under all regimes and fouling on diluate side of anion exchange membranes was even reduced under PED regimes, it can be concluded that the back transfer to the diluate compartment when the reverse pulse was applied is dominant. However, due to the PER the fouling (scaling) was reduced in PER regimes without significant prolongation of the ED process. As shown [15] in PEF, the efficiency of the ED was improved and the fouling/scaling was decreased. However, to achieve this the time of the ED was prolonged to around 6 × that of conventional ED. In the present work, even though the parameters of the ED efficiency were slightly decreased in PER compared to the conventional ED, the duration of the ED process was only slightly increased. The decrease in ED efficiency in PER was mainly because of the back transport of minerals in reverse pulse which occurs due to the applying a high magnitude of voltage (−50 V). The back transport of minerals in a solution containing multivalent ions was also pointed out by Tufa *et al.* [29]. Despite this, in long term application the effect of the PER can be highlighted further due to a reduction of fouling and scaling on the membranes. Therefore, research must be continued to find an optimum regime in which the back transfer does not play an important role and the fouling also decreases when the optimum pulse/pause is used.

**Supplementary Materials:** Supplementary materials can be found at http://www.mdpi.com/1422-0067/20/8/1918/s1.

**Author Contributions:** The experiments were designed and carried out by both Authors (A.M.; A.M.A.). The obtained results were discussed and the manuscript was written by both authors.

**Funding:** This work was supported by the program NPU I Ministry of Education Youth and Sports of the Czech Republic [project No. LO1418]; Progressive development of Membrane Innovation Centre using the infrastructure of the Membrane Innovation Centre.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **Abbreviations**


