Electrocoagulation vs. Integrate Electrocoagulation-Natural Zeolite for Treatment of Biowaste Compost Leachate—Whether the Optimum Is Truly Optimal

Abstract

Natural zeolites are well-known materials widely applied in the environmental remediation treatment process. However, the integration of various treatment methods is exceedingly investigated for achieving satisfactory effluent quality. In this paper, the integration of electrocoagulation and natural zeolite was evaluated in the treatment of biowaste compost leachate in a single step. The influence of different distances of electrodes (1.5, 3, and 4.5 cm), stirring speed (70, 200, and 400 rpm), the addition of natural zeolite and electrolyte NaCl on the efficiency of treatment of biowaste compost leachate has been carried out. Process efficiency was evaluated by measuring the change of pH value, electrical conductivity, temperature, turbidity, chemical oxygen demand (COD), total Kjeldahl nitrogen (TN_K), total solids, and sludge settling test. The Taguchi method was applied to optimize biowaste compost leachate treatment. Experiments are planned according to Taguchi’s L8 (2⁴ 4¹) orthogonal array. The stirring speed, electrode distance, electrolyte and zeolite addition, solution initial pH adjustment were chosen as controllable factors, and their impact on COD, turbidity, TN_K, settling rate, and electrode consumption were studied. Results show that optimal conditions depend on the parameter of interest and that optimal values for a particular parameter are not always the optimum if the desired goal is considered.

1. Introduction

Even composting is recognized as an effective way of managing organic waste, composting on industrial scales creates a significant amount of leachate, which contains a variety of hazardous substances that can have potential adverse effects on the environment. Therefore, leachate treatment prior to discharge into the sewer or natural recipient is necessary. Today, various physicochemical treatment methods (coagulation/flocculation, electrocoagulation, nanofiltration, reverse osmosis), biological treatment methods (biofilters, anaerobic bioreactors, membrane bioreactors, MBR), advanced oxidation technologies (Fenton, ozone, UV/TiO₂, etc.) are used to treat leachate generated during composting. Still, they do not achieve a sufficient purification effect when applied as a single process [1]. Thus, many scientists are focused on the integration of two or more single process treatments conducted simultaneously or in succession to achieve satisfactory leachate effluent quality. Such processes are commonly referred to as hybrid, combined, or integrated [2,3,4]. Our intention here was to investigate the combination of integrated adsorption and electrocoagulation (EC) in wastewater treatment because only several published papers considered this combination in wastewater purification, most of them without mathematical analysis, such as Taguchi optimization. Regarding the previously published papers on this topic, Linares-Hernández et al. (2007) [5] reported that the integration of EC with biosorption on bioadsorbent Ectodemis of Opuntia improves the efficiency of pollutant removal from mixed industrial wastewater. The processes were carried out sequentially, i.e., EC as pretreatment and biosorption after EC. The parameters monitored in the purification process were color, turbidity, COD, BOD₅, and fecal coliform content. The efficiencies of removal of color, turbidity, and fecal coliforms were 97%, 98.8%, and 99.9%, respectively, while removals of COD and BOD₅ recorded lower removal values of 84% and 78%, respectively. Narayanan and Ganesan (2009) [6] apply a hybrid of EC and adsorption process to remove chromium from synthetic wastewater using activated carbon as an adsorbent. It was found that the addition of adsorbent increases the rate of removal of chromium at lower current density, and increased chromate removal is due to the combined effect of precipitation, coprecipitation, sweep coagulation, and adsorption. Chang et al. (2010) [7] also combine EC with activated carbon as an adsorbent to remove the azo dye Reactive Black 5 (RB5) from the synthetic solution. The treatment was conducted as a two-stage, first EC with Fe-electrodes and NaCl electrolyte, and then adsorption on activated carbon. Although visually very rapid removal of the staining solution was achieved, COD removal was only 39%, and the effluent after treatment was highly toxic due to the resulting RB5 intermediates and chlorine byproducts from added NaCl electrolyte. Ouaissa et al. (2012) [8] investigated the treatment of tannery wastewater pollutants by EC using aluminum plates coupled with adsorption onto granular carbon. The monitored parameters in the purification process were COD, turbidity, and chromium content. When applying only the EC process, the removal efficiency of COD, turbidity, and chromium was 25%, 94.25%, and 24.6%, and when combining EC and adsorption, 75%, 96%, and 92%. Elabbas et al. (2020) [9] used an integrated process of EC and adsorption to remove chromium from the wastewater from the leather industry, using the eggshell as an adsorbent. This type of material, as adsorbent, showed high efficiency of chromium removal of 99%, while the stand-alone EC process was 63%.

The EC found its application only in the 21st century due to the high price of electricity in previous times. However, as the price of electricity is rising again, combining renewable energy sources with electrocoagulation, these processes will maintain a sustainable alternative for water and wastewater treatment [10,11,12,13]. The advantages of EC are related to relatively low investment costs, energy, and treatment costs due to the small size of used equipment [14,15]. EC is a complex process that can be divided into four steps: (1) electrochemical reactions on the electrode surfaces, (2) formation of coagulant in the aqueous phase by hydroxides of anodic metal, (3) adsorption of pollutants in flocks, and (4) removal by electroflotation, sedimentation, and adhesion to bubbles [16,17]. On Al electrodes, the basic electrochemical reactions on the anode and cathode can be written as follows:

$$\mathrm{Main}\mathrm{anodic}\mathrm{reaction}:\mathrm{Al}\to {\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}$$

(1)

$$\mathrm{Acidic}\mathrm{medium}:2{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{H}}^{+}+{\mathrm{O}}_2+4{\mathrm{e}}^{-}$$

(2)

$$\mathrm{Basic}\mathrm{medium}:4{\mathrm{OH}}^{-}\to {\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}$$

(3)

$$\mathrm{Chatodic}\mathrm{reaction}: 2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to 2{\mathrm{OH}}^{-}+{\mathrm{H}}_2$$

(4)

$$\mathrm{Solution}\mathrm{medium}:{\mathrm{Al}}^{3+}+3{\mathrm{H}}_2\mathrm{O}\to \mathrm{Al}{(\mathrm{OH})}_3+3{\mathrm{H}}^{+}$$

(5)

During the oxidation process, the aluminum anode dissolves and generates Al³⁺ cations, which are further reacted and transformed into various monomeric and polymeric ionic forms such as Al(OH)²⁺, Al(OH)₂⁺, Al₂(OH)₂⁴⁺, and Al(OH)₄⁻, Al₆(OH)₁₅³⁺, Al₇(OH)₁₇⁴⁺, Al₈(OH)₂₀⁴⁺, Al₁₃O₄(OH)₂₄⁷⁺, and Al₁₃(OH)₃₄⁵⁺ and also low soluble amorphous species such as Al(OH)₃ and Al₂O₃, which depend on the initial pH of the solution [17,18,19].

Electrolysis of water at the cathode generates hydroxide ions that combine with dissolved aluminum from the anode, and form hydroxide precipitate at a suitable pH which removes pollutants by sweep-floc mechanism. The formation of H₂ due to water electrolysis at the cathode may promote the flotation of some portion of coagulated pollutants to the surface [20]. Besides aluminum dissolution from the anode, aluminum dissolution from the cathode is also recorded, confirming that both electrodes are influencing on formation floc of Al(OH)₃ during electrocoagulation [14]. Thus, the mechanism of EC takes into account aluminum dissolution at the cathode according to Equation (6):

$$\mathrm{Al}\left(\mathrm{s}\right)+4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)-3{\mathrm{e}}^{-}\to \mathrm{Al}{\left(\mathrm{OH}\right)}_4^{-}\left(\mathrm{aq}\right)$$

(6)

Aluminum dissolution at the anode (1) and cathode (6) produces Al³⁺ and Al(OH)₄⁻ (aq), respectively. In solution, Al³⁺ reacts with OH^- and forms floc Al(OH)₃ (s), but also contributes with Al(OH)₄⁻ to produce floc of Al(OH)₃ (s), according to Equation (7) [21]:

$$\mathrm{Al}{\left(\mathrm{OH}\right)}_4^{-}\left(\mathrm{aq}\right)\to \mathrm{Al}{\left(\mathrm{OH}\right)}_3\left(\mathrm{s}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$$

(7)

The application of EC allows avoidance of chemical additives (except possible NaCl as electrolyte), which makes the EC a “green technology” due to the prevention of secondary pollution. Compared to classical coagulation, EC can remove much smaller particles as the electric field causes faster movement and collision, increasing their aggregation process’s efficiency [22]. Bubbles formed in the EC process carry the substances toward the surface, where they are more concentrated and easier to coagulate and remove [23]. Compared to classical coagulation, EC significantly reduces the volume of sludge produced. This is particularly key regarding the economy of the process since generated sludge in any water treatment method must be further treated [22]. The obtained sludge is of better quality: lower water content, much larger and more stable accumulations (flocs) with better settling characteristics [22,23].

Like every other process, EC also has some disadvantages. Periodically replacement of the electrodes is needed because they oxidize and consume water [22]. Uneven anode dissolution can interfere with EC efficiency. Electrode inertness can cause oxide protective layer formation on electrode surfaces. This prevents electron exchange and dissolution of metals, resulting in increased electricity consumption. Although there is still no universal solution to this problem, periodic electrode cleaning and the use of alternative pulse current (APC) can prevent passivation [22]. In addition, there is a need for adequate water conductivity. However, natural water and slightly polluted wastewater do not have this feature, so their conductivity must be increased by using a supporting electrolyte. Due to its low cost, availability, and non-toxicity, NaCl is the most used support electrolyte, but others have occasionally been used, such as Na₂SO₄ [22,23].

Zeolites can be described as aluminosilicates of specific spatial structures that have the properties of ion exchange, adsorption, and molecular sieves. Natural zeolites and their modified forms are used to separate, bind, and chemically stabilize hazardous inorganic, organic and radioactive species in soils and aqueous systems. After application as adsorbent and ion-exchanger, zeolite can be easily regenerated and used again without losing its original capacity [24,25,26,27,28,29,30]. According to Syafalni et al. [31], the combination of modified zeolite-ferric chloride and modified zeolite-bentonite was found to be a suitable environment-friendly coagulant in river water treatment. Besides modified zeolite, other coagulants such as FeCl₃·6H₂O, Al₂(SO₄)₃·18H₂O, as well as chitosan, powder-activated carbon, starch, etc., are also investigated, mostly in powdered form, to enhance coagulation [32,33,34]. Coagulation is defined as the destabilization of colloidal particles by reducing repulsive forces between them, while flocculation represents the binding of destabilized particles into larger agglomerates flocs as a result of flocs collision due to Brownian motion (perikinetic flocculation) or velocity gradients (orthokinetic flocculation) [35].

Today, only a few papers are published on the topic of integration of EC and natural zeolite. Hamid et al. (2020a, 2020b, 2021) [36,37,38] investigated the ammonia removal from landfills leachate at a zeolite/solution ratio of 120 g/L in a single treatment reactor. Our preliminary investigation has confirmed that EC combined with natural zeolite is efficient for the treatment of biowaste compost leachate in a single step and with a significantly smaller amount of zeolite of 20 g/L [39].

The Taguchi method uses a specifically designed orthogonal array consisting of controllable factors and their variation levels to optimize experimental conditions. The advantage of the Taguchi method is assessing optimal experimental conditions without a high number of experiments [40,41].

This paper examines the efficiency of biowaste compost leachate treatment using an electrocoagulation process with and without the addition of zeolites and electrolytes at different distances of electrodes and stirring speeds. The Taguchi method was applied to achieve multiple parameters optimization. The stirring speed, electrode distance, electrolyte and zeolite addition, and solution initial pH adjustment were chosen as controllable factors, and their impact on COD, turbidity, TN_K, settling rate, and electrode consumption were studied.

2. Materials and Methods

Biowaste compost leachatesolutions were used for treatment. The leachate was obtained from composting mass in an open container. The characterization of initial biowaste compost leachate was performed through the determination of the pH, el. conductivity, turbidity, COD, TN_K, and TS were performed according to standard methods for examining water and wastewater [42].

Natural zeolite used in this study was originated from the Zlatokop deposit, Vranjska Banja, Serbia, with granulation of 0.1–0.5 mm. The XRD and semiquantitative mineralogical analysis have confirmed that the main mineralogical component is clinoptilolite with quartz as an impurity [26,43]. Based on chemical composition, the theoretical exchange capacity equals 1.411 mmol/g [44]. Si/Al ration equals 4.08, while the content of exchangeable cations indicates that Ca > (Na + K), which confirms the Ca-form of clinoptilolite. The characterization of the raw zeolite sample by the X-ray powder diffraction method (XRPD, Phillips PW-1710 diffractometer, Amsterdam, The Netherlands), scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDS, JOEL JSM-6610 microscope, Tokyo, Japan), a thermal analysis (TG-DTG, Perkin Elmer STA 6000, Waltham, MA, USA) and Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet 6500 spectrometer, Waltham, MA, USA) has been published previously [45].

Al electrode material used in this study was aluminum alloy AA2007 series 2000, in which the main alloying element is copper. Analysis of chemical composition is showing that Al = 92.58%, Cu = 3.39%, Pb = 1.02%, Mg = 0.88%, Mn = 0.67%, Fe = 0.62%, Si = 0.21%, while Zn, Ti, Cr, Ni, Sn, Bi, Zr, and Be are present in less than 0.1%. The total surface area of Al-alloy electrodes immersed in biowaste compost leachate was 44 cm².

Electrocoagulation experiments (EC) were performed in a batch-type electrochemical cell of 300 mL, with immersed aluminum AA2007 alloy electrodes. In all experiments, current intensity and applied voltage were in the range of 0.1–0.3 A and 8.3–29.9 V, respectively. A total of 16 experiments have been performed to investigate the influence of different distances of electrodes (1.5, 3, and 4.5 cm), stirring speed (70, 200, and 400 rpm), the addition of natural zeolite (in the amount of solid to liquid ratio 20 g/L), the addition of electrolyte NaCl (NaCl, p.a. was added in a concentration of 1 g/L), at a contact time of 40 min, without adjustment of initial pH of solutions.

Table 1. Overview of experimental conditions during experiments.

Exp. no.	Experiment Mark	Stirring Speed, rpm	Electrode Distance, cm	Zeolite Addition, 20 g/L	NaCl Addition, 1 g/L	Voltage, V	Current Intensity, A
1	EC70rpm+EL	70	4.5	no	yes	16.1	0.3
2	EC200rpm+EL	200	4.5	no	yes	16.1	0.3
3	EC400rpm+EL	400	4.5	no	yes	16.1	0.3
4	EC70rpm	70	4.5	no	no	29.9	0.1
5	ECZ70rpm+EL	70	4.5	yes	yes	16.1	0.3
6	ECZ200rpm+EL	200	4.5	yes	yes	17.8	0.3
7	ECZ400rpm+EL	400	4.5	yes	yes	16.1	0.3
8	ECZ70rpm	70	4.5	yes	no	29.2	0.1
9	EC1.5cm+EL	70	1.5	no	yes	8.3	0.3
10	EC3cm+EL	70	3.0	no	yes	13.4	0.3
11	EC4.5cm+EL	70	4.5	no	yes	16.1	0.3
12	EC1.5cm	70	1.5	no	no	22.8	0.1
13	ECZ1.5cm+EL	70	1.5	yes	yes	9.3	0.3
14	ECZ3cm+EL	70	3.0	yes	yes	12.8	0.3
15	ECZ4.5cm+EL	70	4.5	yes	yes	16.1	0.3
16	ECZ1.5cm	70	1.5	yes	no	21.2	0.1

gives an overview of experimental conditions during the 16 experiments.

Between each experiment, the electrodes were ground and polished with successive wet SiC emery papers (up to 800 grit), ultrasonically cleaned in 70% ethanol, and deionized water. The pH value, temperature, and el. conductivity were recorded during the experiments, while the turbidity, COD, TN_K, and TS were determined in solution at the end of the experiments.

Sludge settling tests were performed after each electrocoagulation experiment. The suspensions were transferred into the cylindrical vessel, and a sludge settling test was performed by applying the Kynch theory of sedimentation. The settling curves were determined for all 16 experiments.

Taguchi Optimization

In order to optimize the integrated electrocoagulation process with natural zeolite, a set of eight experiments were performed, according to Taguchi’s L8 (2⁴ 4¹) design.

Table 2. Experimental plan and factor used for L8 (2⁴ 4¹) Taguchi design.

Exp. no.	Experiment Mark	Stirring Speed	Electrode Distance	Electrolyte Addition	Zeolite Addition	Solution Initial pH Adjustment
1	ECZ1.5cm+EL+pH not adjusted+0rpm	S1	D1	E1	Z1	pH1
2	EC4.5cm+pH=4+0rpm	S1	D2	E2	Z2	pH2
3	EC1.5cm+EL+pH=4+70rpm	S2	D1	E1	Z2	pH2
4	ECZ4.5cm+pH not adjusted+70rpm	S2	D2	E2	Z1	pH1
5	ECZ1.5cm+pH=4+200rpm	S3	D1	E2	Z1	pH2
6	EC4.5cm+EL+pH not adjusted+200rpm	S3	D2	E1	Z2	pH1
7	EC1.5cm+pH not adjusted+400rpm	S4	D1	E2	Z2	pH1
8	ECZ4.5cm+EL+pH=4+400rpm	S4	D2	E1	Z1	pH2

Note: S1, S2, S3, and S4 correspond to steering speeds of 0, 70, 200, and 400 rpm; D1 and D2 correspond to electrode distances of 1.5 and 4.5 cm; E1 and E2 correspond to NaCl electrolyte addition (with and without); Z1 and Z2 correspond to zeolite addition (with and without); pH1 and pH2 correspond to solution initial pH adjustment (pH not adjusted and adjustment at pH = 4).

gives an overview of experimental conditions during these eight experiments. For that purpose, experiments were performed with (Z1) and without (Z2) the addition of natural zeolite, with (E1) and without (E2) addition of electrolyte solutions of NaCl, in concentration 1 g/L, with adjustment of initial solutions pH values pH = 4, and without initial pH adjustment. Distances of electrodes were 1.5 (D1) and 4.5 cm (D2), while stirring speeds were 0 (S1), 70 (S2), 200 (S3), and 400 (S4) rpm, respectively. The experiment duration time was set at 30 min. The addition of natural zeolite was 20 g/L. Initial pH values were adjusted with 0.1 mol/L HCl.

Experiments in this study were planned according to Taguchi’s (2⁴ 4¹) design (Table 2). This design has eight rows representing the number of experiments and five columns representing the controllable factors. The electrode distance (D), the addition of electrolyte (E), the addition of zeolite (Z), solution initial pH adjustment (pH), and stirring speed (S) were chosen as controllable factors, and their impact on COD, TN_K, turbidity, settling rate, and electrodes loss were studied. As shown in Table 2, four factors used had two levels of testing conditions and fifth factor had four levels.

As the studied integrated process has a few output parameters, and in this study, the effect of experimental conditions for five of them are of interest, the two quality characteristics, the smaller-the-better or the larger-the-better, were used depending on the parameter to be optimized.

Smaller-the-better quality characteristic is represented by Equation (8) [46]:

$$S/N_{\mathrm{SB}}=-10log\frac{{\sum }_{i=1}^n{y}_i^2}{n}$$

(8)

where S/N represents signal-to-noise ratio, subscript SB represents smaller-the-better, n is the number of repetitions under the same experimental conditions, and y is a measurement value.

Larger-the-better quality characteristic is represented by Equation (9) [40,41]:

$$S/N_{\mathrm{LB}}=-10log\frac{{\sum }_{i=1}^n\frac{1}{y_i^2}}{n}$$

(9)

where subscript LB represents larger-the-better.

Afterward, it is urged to determine the optimal conditions. For that purpose, the average S/N_SB or S/N_LB ratio of each controllable factor at level i, marked as S/N_FL, was calculated:

$$S/N_{\mathrm{FL}}=\frac{{\sum }_{j=1}^{n_{Fi}}{\left[{\left(S/N\right)}_i^F\right]}_j}{n_{Fi}}$$

(10)

where ${\left(S/N\right)}_j^F$ represents S/N_SB or S/N_LB ratio for factor F on level i, the superscript j is the j-th appearance of the i-th level.

3. Results

3.1. Physical-Chemical Characterization of Biowaste Compost Leachate

Table 3. Biowaste compost leachate characterization and the admissible values according to Croatian regulation (NN 26/2020) [47].

Parameter	Biowaste Compost Leachate	Natural Surface Waters	Public Sewage System
pH	7.95	6.5–9.5	6.5–9.5
El. Cond., μS/cm	205.00	-	-
Turbidity, NTU	87.20	-	-
COD, mg/L	278.32	125	700
TNK, mg N/L	47.62	15 *	50 *
TS, mg/L	275	35 **	-

Note: *—values for total nitrogen are compared since values for Kjeldahl nitrogen are not specified by Croatian regulation; **—values for total suspended solids TSS are compared since values for total solids (TS) are not specified by Croatian regulation.

gives a physical-chemical analysis of biowaste compost leachate compared with maximal allowed values according to the Croatian regulation (NN 26/2020) [47]. The pH value of the biowaste compost leachate is 7.95 and does not exceed the limit values for discharge into surface waters and into the public sewage system. The el. conductivity, as an indication of the presence of solutes that have ionic conductivity properties, is 205.00 µS/cm. However, to promote the EC process, it is necessary to add electrolytes to ensure better conductivity and lower resistance. The value of COD in the sample is 278.32, which indicates the presence of both biodegradable and inorganic substances susceptible to oxidation with bichromate. The value is above the limit prescribed by the Croatian regulation (125 mg O₂/L). The TN_K equals 47.62 mg N/L. The obtained value is compared with the values for total nitrogen since the values for TN_K are not defined by Croatian regulation. It can be seen that the TN_K value is higher than the limit value stated by the Croatian regulation for discharge into surface waters (15 mg N/L). The turbidity is very high and amounts to 87.20 NTU (Nefelometric Turbidity Units) due to the presence of dissolved and dispersed substances in the leachate. Although the values for turbidity are not stated, the discharge of water of this turbidity into a natural recipient would cause detrimental effects on the photosynthesis process and the dissolved oxygen content. The TS equals 275 mg/L, indicating the presence of suspended, dissolved, and settleable solids in the biowaste leachate and refer to turbidity and el. conductivity. The obtained value is compared with the values for total suspended solids (TSS) since the values for TS are not defined by Croatian regulation. It can be seen that the TS value is significantly higher than the TSS limit value prescribed by the Croatian regulation for discharge into surface waters (35 mg/L), which indicates the presence of suspended and settleable solids.

3.2. Biowaste Compost Leachate Treatment with Electrocoagulation and Electrocoagulation Integrate with Zeolite

The biowaste compost leachate was treated by EC without and with the addition of zeolite and electrolyte at different distances between the electrode (1.5, 3, and 4.5 cm) and stirring speed (70, 200, and 400 rpm). Results of continuous monitoring of pH, el. conductivity and temperature at different experimental conditions during the EC process are compared in Figure 1, Figure 2 and Figure 3.

3.2.1. pH Values Change during the Process

According to Tahreen et al. (2020) [3], the pH tends to change throughout the EC process, which depends on the type of electrodes used and their interaction with the wastewater being treated.

From Figure 1, it is evident that the curves of pH change in all experiments show a sharp increase in the first 5–10 min, after which the increase is less pronounced. The increase in pH value during the electrocoagulation process is a consequence of the hydrolysis of water on the cathode, which results in the formation of hydrogen and hydroxide ions. With an increase in the stirring speed from 70 to 400 rpm (Figure 1a), the pH increases faster, the sharp increase is within the first 5 min, followed by an insignificant increase, and the final pH is in the range of 9.21–9.31.

The addition of zeolite (Figure 1b) affects the trend of pH change, and the pH curves follow the form of continuous growth. The reason for this is the ability of zeolites to neutralize the solution. The largest increase is at the lowest stirring speed (70 rpm), while increasing stirring speed decreases the final pH value. If we compare the experiments with and without the addition of electrolytes at the same stirring speed (70 rpm), it can be seen that in the experiments without the addition of zeolite (Figure 1a), the addition of electrolytes has no significant effect on the pH change in the second part of the increase, but it affects the initial value and sharper part of the increase. With the addition of zeolite (Figure 1b), the addition of electrolyte has the effect of the increase in pH; its values are slightly higher compared to the experiment without the addition of electrolyte.

With the reduction in the electrode distance from 4.5 to 3 and 1.5 cm (Figure 1c), the pH increases faster. With the addition of zeolite (Figure 1d), the shape of the pH curves is almost the same. If we compare the experiments with and without the addition of electrolytes at the same electrode distance of 1.5 cm, in experiments without zeolite addition (Figure 1c), electrolyte addition affects only the sharp part of the increase. With the addition of zeolite (Figure 1d), the addition of electrolytes affects the shape of the pH curve. Without the addition of electrolytes, the shape of the curve is flatter.

3.2.2. Electrical Conductivity Changes during the Process

Figure 2 shows significantly increased values of el. conductivity in experiments with the addition of electrolytes. Evidently, the addition of electrolyte boosts the el. conductivity [3]. It is also evident that curves of el. conductivities oscillate during the EC process, with a slightly decreasing trend (up to 10%) attributed to the binding of harmful substances into the resulting flocs. The increase in the stirring speed from 70 to 200 and 400 rpm and the distance of the electrodes from 1.5 to 3 and 4.5 cm does not show a significant impact on the el. conductivity. In experiments without the addition of electrolytes, the conductivity was significantly lower (205 µS/cm), and the addition of zeolite did not show a significant effect on change in el. conductivity. This is connected with behavior of zeolite and its exchangeable cation in solutions. Namely, among exchangeable cations in zeolite structure, the Ca²⁺ ions predominate. During the EC process, the Ca²⁺ ions in solutions hydrolyze and form H⁺ ions. The resulting H⁺ ions neutralize the already present OH⁻ ions formed by the reduction in water at the cathode, which is why the final pH of suspensions is slightly lower in experiments with added zeolite (see Figure 2). Thus, even the addition of zeolite contributes to the reduction in COD (see

Table 4. Final values and removal efficiency of turbidity, chemical oxygen demand (COD), total Kjeldahl nitrogen (TN_K), and total solids (TS) in suspension after EC experiments.

Exp. no.	Experiment	Final Values				Removal Percentage, %
		Turbidity, NTU	COD, mg/L	TNK, mg/L	TS, g/L	Turbidity	COD	TNK
1	EC70rpm+EL	3.38	88.47	14.01	1.165	96.12	68.21	54.55
2	EC200rpm+EL	2.29	62.67	0.56	1.175	97.37	77.48	98.18
3	EC400rpm+EL	0.77	25.81	5.60	1.165	99.12	90.73	81.82
4	EC70rpm	3.26	51.61	14.01	0.16	96.27	81.46	54.55
5	ECZ70rpm+EL	0.88	47.92	30.82	1.045	99.00	82.78	0.00
6	ECZ200rpm+EL	0.99	22.12	8.40	0.98	98.86	92.05	72.73
7	ECZ400rpm+EL	1.43	22.12	11.21	1.08	98.37	92.05	63.64
8	ECZ70rpm	2.78	7.37	8.40	0.30	96.82	97.35	72.73
9	EC1.5cm+EL	0.50	14.75	30.82	1.7	99.43	94.70	0.00
10	EC3cm+EL	0.59	0.00	5.60	1.53	99.33	100.00	81.82
11	EC4.5cm+EL	3.38	88.47	5.60	1.165	96.12	68.21	81.82
12	EC1.5cm	1.61	3.69	5.60	0.295	98.15	98.68	81.82
13	ECZ1.5cm+EL	0.38	147.46	5.60	1.545	99.56	47.02	81.82
14	ECZ3cm+EL	0.39	40.55	19.61	1.68	99.56	85.43	36.36
15	ECZ4.5cm+EL	0.88	47.92	30.82	1.045	99.00	82.78	0.00
16	ECZ1.5cm	0.45	7.37	2.80	0.32	99.48	97.35	90.91

), significant oscillations in el. conductivity are not so visible.

3.2.3. Temperature Changes during the Process

From Figure 3. the increase in solution temperature from 21 to 33 °C is evident in all experiments. The highest increase in temperature is obtained at the highest distance between the electrodes of 4.5 cm (Figure 3c,d). However, an increase in stirring speed does not significantly influence temperature change (Figure 3a,b).

If we compare the experiments with and without the addition of electrolytes at the constant stirring speed of 70 rpm (comparison of Figure 3a,b) and at the same electrode distance of 1.5 cm (comparison of Figure 3c,d), it is evident that slightly higher increase in temperature is obtained in the experiment with the addition of electrolyte. However, the addition of zeolite reduces the effect of temperature rise.

The temperature during the electrocoagulation process tends to increase due to electrolysis reactions, depending on electrode type and the electricity applied [48]. However, thermal pollution can be caused by wastewater discharge with elevated temperature into natural recipients. This can be controlled by reducing the ratio of the electrode surface to the volume of leachate to be treated.

3.3. Analysis of the Efficiency of Electrocoagulation

At the end of each experiment, the turbidity, COD, TN_K, and TS were analyzed in suspension, and their results are summarized in Table 4. In addition, treatment efficiency during experiments was assessed for the turbidity, COD, and TN_K and are summarized in Table 4.

3.3.1. Turbidity Removal

After implementing the EC and ECZ in all experiments, almost entire turbidity removal was achieved regardless of the addition of zeolite, electrolyte, stirring speed, and electrodes distance, and was in the range from 96.27% to 99.56%. According to Sutanto et al. (2019) [49], reduced water turbidity was due to the formation of higher amounts of Al³⁺ and OH⁻ ions resulting in the formation of coagulation compound Al(OH)₃, responsible for adsorption of pollutants into flocs at the bottom of the reactor.

3.3.2. Total Solids Removal

In experiments with the addition of the electrolyte, the final values of TS are significantly higher than the initial values (0.275 g/L). However, it can be seen that increase in the stirring speed from 70 to 20 and 400 rpm (experiments no. 1–3) has no significant effect on the final values of TS. The addition of zeolite is slightly lowering the values of the TS (experiments no. 5–7), which can be attributed to the ability of zeolite to bind contaminants.

Decrease the electrode distance from 4.5 to 3 and 1.5 cm without the addition of zeolite (experiments no. 9–11) increase the TS values even more (up to 1.7 g/L). In the experiments conducted with zeolite (experiments no. 13–15), the TS values are slightly lower than those obtained in experiments without zeolite. However, in experiments without electrolyte addition (experiments 4, 8, 12, and 14), the TS values are significantly lower compared to experiments with electrolyte addition.

3.3.3. COD Percentage Removal

With an increase in stirring speed from 70 to 200 and 400 rpm (experiment no. 1–3), and with the addition of electrolytes, the COD removal percentage is growing from 68.21% to 90.73%. With the addition of zeolite (experiment no. 5–7), COD removal percentage is growing even more from 82.78% to 92.05%. Without the addition of electrolytes (comparison of experiment no. 4 vs. 8), the removal percentage expressed thought COD is better with the addition of zeolite, reaching 97.35%. This indicates that the integration of EC and zeolite enhance the COD removal compared to the stand-alone EC process. Even zeolite possesses the ability for COD reduction, its role as a natural coagulant must not be neglected [31].

Regarding the influence of electrode distance with the addition of electrolyte (experiment no. 9–11), the complete COD removal efficiency was achieved at an electrode distance of 3 cm (100%). Decreasing the electrode distance to 1.5 cm decreases the removal efficiency to 94.70%. At an electrode distance of 4.5 cm, there is a significant reduction to 68.21%.

For experiments with the addition of zeolite and electrolyte (experiments no. 13–15), removal efficiency is lower compared to samples without zeolite addition, and the lowest value is obtained at an electrode distance of 1.5 cm, and equals 47.02%. However, at the highest electrode distance of 4.5 cm, the removal efficiency is higher in the experiment with the addition of zeolite and electrolyte and equals 82.78%.

For samples without the addition of electrolytes at an electrode distance of 1.5 cm (comparison of experiment no. 12 vs. 16), the COD removals are high in both experiments, with and without the addition of zeolite.

Evidently, electrode distance influences COD removal efficiency and must be kept optimum to avoid higher power consumption. According to Khaled et al. (2019) [50], by increasing the distance between the electrodes, the efficiency decreases due to increased resistance from the formation of aluminum hydroxide film at the anode, but also due to slower charge transfer and oxidation of aluminum, which results in less formation of aluminum cations and therefore coagulants. According to Tahreen et al. (2020) [3], the distance between the electrodes determines the electrostatic field between the anode and cathode. The electrostatic field increases when the distance between the electrodes decreases. Therefore, metal hydroxides that help form flocs that support coagulation are degraded due to strong collisions caused by high electrostatic attraction. Therefore, the EC efficiency is low at the minimum distance between the electrodes.

3.3.4. TN_K Percentage Removal

With an increase in steering speed from 70 to 200 and 400 rpm with the addition of electrolyte and zeolite (experiment no. 1–3 vs. experiment no. 5–7), the TN_K removal percentage oscillates. The best removal is at a steering speed of 200 rpm with the addition of electrolyte (98.18%), while the addition of zeolite decreases the TN_K removal percentage.

Without the addition of electrolytes (comparison of experiment no. 4 vs. 8), the removal percentage expressed thought TN_K is better with the addition of zeolite, reaching 72.73%. At an electrodes distance of 1.5 cm, better results are obtained in the experiment without the addition of electrolyte and zeolite. Increasing electrode distance at 3 and 4.5 cm, the addition of zeolite and electrolyte also compete with each other; thus, better results are obtained with the addition of electrolyte.

Results also indicated that at certain experimental conditions, none of TN_K is removed, at the lowest steering speed of 70 rpm and highest electrode distance of 4.5 cm and addition of zeolite and electrolyte (experiments no. 5 and 15), and at the lowest electrode distance with addition of electrolyte (experiment no. 9).

Based on results given in Table 3, additions of zeolite, electrolyte, selected electrode distance, and steering speed have mutually opposite influences on decreasing turbidity, COD, and TN_K, which is also connected with the mass of aluminum electrodes consumed during experiments.

3.4. Comparison of the Mass of Al Electrodes Consumed during the Experiment

Before and after immersion, the electrodes are weighed on an analytical balance, and the results are given in

Table 5. Mass of Al electrodes consumed during the experiment.

Exp., no.	Experiment, Mark	Mass of Al Electrodes Consume during the Experiment, g
		Anode	Cathode
1	EC70rpm+EL	0.0648	0.0329
2	EC200rpm+EL	0.0613	0.0379
3	EC400rpm+EL	0.0584	0.0335
4	EC70rpm	0.0029	0.0109
5	ECZ70rpm+EL	0.0528	0.0318
6	ECZ200rpm+EL	0.0595	0.0342
7	ECZ400rpm+EL	0.0561	0.0251
8	ECZ70rpm	0.0054	0.0067
9	EC1.5cm+EL	0.0342	0.0245
10	EC3cm+EL	0.0397	0.0188
11	EC4.5cm+EL	0.0648	0.0329
12	EC1.5cm	0.0072	0.0075
13	ECZ1.5cm+EL	0.0568	0.0218
14	ECZ3cm+EL	0.0614	0.0280
15	ECZ4.5cm+EL	0.0528	0.0318
16	ECZ1.5cm	0.0096	0.0119

.

In experiments without the addition of electrolytes, the consumption of electrodes was significantly reduced. Increasing the steering speed does not lead to increased wear of the electrodes. The highest consumption of both electrodes is visible in experiment 2 with 200 rpm with electrolyte. Anode consumption was expected, but cathode consumption confirms a different mechanism of the EC process, which is in accordance founding of Ghernaouta et al. (2009) [21]. The cathode dissolves due to OH^- ions formed in the hydrogen evolution reaction. As the distance between the electrodes increases, so does the consumption of the anode and cathode. The addition of zeolite increases the consumption of electrodes only at lower electrode distance, while at higher electrode distance, this effect is less pronounced. Evidently, the abrasive effect of zeolite depends on electrode distance.

3.5. Analysis of Sludge Settling Curves

Sludge settling was evaluated by applying the Kynch theory of sedimentation, and the sludge settling curves were determined for all 16 experiments and are compared in Figure 4.

It was evident that in the suspensions in which the process was carried out without the addition of electrolytes, the sludge settling was very slow, and in the time interval of 30 min, there was no separation of the precipitate (sludge) from the suspension. For experiments without the addition of zeolite, the influence of stirring speed is very pronounced. The settling rate decreases by increasing the stirring speed from 70 to 200 and 400 rpm. The lowest settling rate is observed in the experiment in which the process was carried out with a stirring speed of 400 rpm, probably due to breaking the flocs at high steering speeds. With the addition of zeolite, the effect of stirring is less pronounced but present. Floc properties are affected by zeolite addition, preserving their breaking. Probably addition of zeolite increases the probability of collision during electrocoagulation and contributes to the formation of larger flocs with a better settling rate [31]. Hoverer, further experiments should be focused on analyzing the floc size to confirm these findings.

The distance of the electrodes does not significantly affect the difference in settling rate. The slope of the settling curves, and thus the settling rates, is slightly higher for suspensions in which the process was carried out with the addition of zeolite compared to suspensions in which the process was carried out without the addition of zeolite.

3.6. Estimation of Operating Costs

Estimation of operating costs for EC and ECZ processes depends on material cost (electrodes, zeolite, and electrolyte costs), electricity, operation, maintenance, and sludge disposal. The price of the electrodes (kg Al/m³) can be calculated from Faraday low or from electrode mass consumption (anode plus cathode), according to Equations (11) and (12) [51,52].

$${\mathrm{C}}_{\mathrm{electrode}}=\frac{I·t·\mathrm{M}}{\mathrm{z}·\mathrm{F}·V}$$

(11)

$${\mathrm{C}}_{\mathrm{electrode}}=\frac{m}{V}$$

(12)

The cost of electrical energy (kWh/m³) is calculated from Equation (13) [52,53]:

$${\mathrm{C}}_{\mathrm{energy}}=\frac{U·I·t}{V}$$

(13)

where: U—cell voltage (V), I–current (A), t—the operating time (s), V—the volume of treated biowaste compost leachate (m³), z—number of electron transfer (z = 3), F—Faraday constant (96 485.33 C/mol), M—molecular mass of aluminum (26.98 g/mol), m—the mass of consumed electrode (anode and cathode), g.

Considering the used values of operational parameters described in experimental section (I = 0.1–0.3 A, U = 8.3–29.9 V, t = 40 min = 2400 s, V = 0.25 m³), calculated values of electrode and electrical energy consumption are compared in

Table 6. Comparison of the electrode and electrical energy consumption.

Experiment Mark	U, V	I, A	From Faraday Low		From Electrode Consumption
			Cenergy, kWh/m3	Celectrode, kg/m3	Celectrode Anode, kg/m3	Celectrode, Cathode, kg/m3	Celectrode, Sum of Anode and Cathode, kg/m3
EC70rpm+EL	16.1	0.3	12.751	0.270	0.137	0.098	0.391
EC200rpm+EL	16.1	0.3	12.751	0.270	0.159	0.075	0.397
EC400rpm+EL	16.1	0.3	12.751	0.270	0.259	0.132	0.368
EC70rpm	29.9	0.1	7.894	0.090	0.029	0.030	0.055
ECZ70rpm+EL	16.1	0.3	12.751	0.270	0.227	0.087	0.339
ECZ200rpm+EL	17.8	0.3	14.098	0.270	0.246	0.112	0.375
ECZ400rpm+EL	16.1	0.3	12.751	0.270	0.211	0.127	0.325
ECZ70rpm	29.2	0.1	7.709	0.090	0.038	0.048	0.048
EC1.5cm+EL	8.3	0.3	6.574	0.270	0.137	0.098	0.235
EC3cm+EL	13.4	0.3	10.613	0.270	0.159	0.075	0.234
EC4.5cm+EL	16.1	0.3	12.751	0.270	0.259	0.132	0.391
EC1.5cm	22.8	0.1	6.019	0.090	0.029	0.030	0.059
ECZ1.5cm+EL	9.3	0.3	7.366	0.270	0.227	0.087	0.314
ECZ3cm+EL	12.8	0.3	10.138	0.270	0.246	0.112	0.358
ECZ4.5cm+EL	16.1	0.3	12.751	0.270	0.211	0.127	0.338
ECZ1.5cm	21.2	0.1	5.597	0.090	0.038	0.048	0.086

.

Electrical energy consumption at different operational conditions ranges from 5.597 to 14.098 kWh/m³. Electrode and energy consumption calculated from Faraday low does not oscillate significantly with increasing the electrode distance or steering speed. However, oscillations are evident when electrode consumption is evaluated from the cathode and anode consumption results and depends on operating conditions. Increasing the stirring speed, the distance between the electrodes, addition of zeolite and electrolyte mainly increase the electrode consumption. In addition, electrode consumption calculated from electrode mass is mostly higher than the values calculated from Faraday low. The exception is experiments without electrolyte addition. However, the operating costs of the ECZ process include energy consumption, electrode material, electrolyte, and zeolite consumption. With the average price of natural zeolite equaling 0.25 USD $/kg [37] and zeolite consumption of 20 g/L, the total cost of the ECZ process is increased by 5 USD $/m³ compared to the stand-alone EC process. However, it is well known that zeolite can be regenerated and reused again, which will influence the decrease in the total cost of the ECZ process.

3.7. Taguchi Optimization

For the optimization purposes of integrating the process of natural zeolite and EC, a new set of eight experiments were performed to evaluate the effect of the electrode distance (1.5 and 4.5 cm), the addition of electrolyte (NaCl), the addition of natural zeolite (Z), solution initial pH adjustment (pH = 4 and without pH adjustment) and steering speed (0, 70, 200, and 400 rpm) on COD, turbidity, TN_K, settling rate, and electrode consumption. The experiments’ plan and the experimental conditions are given in Table 2. Change of pH, el. conductivity, and temperature during investigations and comparison of total solids in initial and final solutions are given in Figure 5. Data show increased pH and temperature values, a slight downward trend of el. conductivity, and increased values of TS in experiments with the addition of electrolyte compared to the initial solution.

In this work, for Taguchi optimization, the process removal capacity for COD, turbidity, and TN_K are calculated in the form of a percentage of removal. For this reason, these parameters and settling speed are optimized using larger-the-better characteristics. Only the electrodes loss (anode and cathode) is optimized using smaller-the-better characteristics.

The experimental results and calculated values of S/N_LB and S/N_SB ratios determined using Equations (8) and (9), along with experimental values for all parameters, are presented in

Table 7. Results of experimental part of Taguchi design.

Test	Average COD Removal, %	Average TNK Removal, %	Average Turbidity Removal, %	Average Settling Speed, cm/min	Average Electrodes Loss, g
1	84.77	81.82	99.39	0.92	0.084
2	99.34	81.82	99.82	0.01	0.003
3	86.09	90.91	99.30	0.83	0.058
4	99.34	0.01	96.31	0.01	0.012
5	88.74	63.64	95.18	0.01	0.023
6	71.52	100.00	99.27	1.02	0.069
7	78.15	27.27	97.44	0.01	0.043
8	79.47	54.55	99.38	0.65	0.040

and

Table 8. S/N_LB and S/N_SB ratios.

Test	S/NLB Ratio COD	S/NLB Ratio TNK	S/NLB Ratio Turbidity	S/NLB Ratio Settling Speed	S/NSB Ratio Electrodes Loss
1	38.56	38.25	39.94	−0.724	21.57
2	39.94	38.25	39.98	−40.00	50.46
3	38.70	39.17	39.94	−1.618	24.79
4	39.94	−40.00	39.67	−40.00	38.64
5	38.96	36.07	39.57	−40.00	32.84
6	37.09	40.00	39.94	0.172	23.31
7	37.86	28.71	39.77	−40.00	27.35
8	38.00	34.74	39.94	−3.74	27.98

. To apply Taguchi optimization in experiments in which results are the same as initial data (the percentage removal or settling speed are zero) for the purposes of calculation due to the program limitations, these values were set as 0.01.

The initial value of COD is reduced by 71.52–99.34%, generating values of S/N_LB ratios in the range of 37.09–39.94. The initial TN_K is reduced in the range of 0.01–100%, generating values of S/N_LB ratios in the range of −40.00–40.00. Average turbidity removal and related S/N_LB ratios do not significantly differ between experiments. Average electrodes loss is in the range of 0.003 to 0.084 g.

The highest value of COD reduction is obtained for experiments 2 and 4, for TN_K in experiment 6, for turbidity in experiment 2, the highest settling speed is obtained in experiment 6, and the lowest loss of electrodes in experiment 2.

Calculated S/N_FL values for all controllable factors are presented in

Table 9. S/N_FL values and factors rank for COD, TN_K, turbidity, settling speed, and electrodes loss. The bold values represents optimal choice.

Parameter	COD
Factor	S	D	E	Z	pH
Level 1	39.25	38.52	38.09	38.87	38.36
Level 2	39.32	38.74	39.18	38.40	38.90
Level 3	39.03
Level 4	37.93
Range	1.39	0.22	1.09	0.47	0.54
Rank	1	5	2	4	3
Parameter	TNK
Factor	S	D	E	Z	pH
Level 1	38.26	35.55	38.04	17.27	16.74
Level 2	−0.42	18.25	15.76	36.54	37.06
Level 3	38.04
Level 4	31.72
Range	38.67	17.31	22.28	19.27	20.31
Rank	1	5	2	4	3
Parameter	Turbidity
Factor	S	D	E	Z	pH
Level 1	39.96	39.81	39.94	39.78	39.83
Level 2	39.81	39.89	39.75	39.91	39.86
Level 3	39.75
Level 4	39.86
Range	0.21	0.08	0.19	0.12	0.03
Rank	1	4	2	3	5
Parameter	Settling Speed
Factor	S	D	E	Z	pH
Level 1	−20.36	−20.58	−1.48	−21.12	−20.14
Level 2	−20.81	−20.89	−40.00	−20.36	−21.34
Level 3	−19.91
Level 4	−21.87
Range	1.96	0.31	38.52	0.76	1.20
Rank	2	5	1	4	3
Parameter	Electrodes Loss
Factor	S	D	E	Z	pH
Level 1	36.01	26.64	24.41	30.26	27.72
Level 2	31.71	35.10	37.32	31.48	34.02
Level 3	28.08
Level 4	27.67
Range	8.35	8.46	12.91	1.22	6.20
Rank	3	2	1	5	4

, along with the data that define optimum parameters. The range is calculated as the difference between the highest and lowest S/N_FL of individually controllable factors. As a result of the calculated range, every factor was related to rank. The most extensive range has a rank of 1, so it is the most influential factor and should be used first. In this study, the stirring speed is the most significant factor for COD, TN_K, and turbidity, while electrolyte addition is the settling rate and electrode loss. In most cases, the electrode distance was the least important.

The optimal stirring speed for optimum values of TN_K, turbidity, and electrode loss is S1 (0 rpm). For COD and settling speed is S2 (70 rpm) and S3 (200 rpm), respectively. However, as can be seen from the values of S/N_FL for these factors, if S1 is chosen as an optimum level, it will not make a significant change in the result.

The optimal electrode distance for COD, turbidity, and electrodes loss is D2 (4.5 cm) and for TN_K and settling speed is D1 (1.5 cm). The only parameter for which the change of electrode distance has a significant effect is electrode loss. For other parameters, this factor is relatively unimportant (rank 4 or 5), so the optimal level, if all parameters are taken into account, is D2.

The optimal electrolyte addition for COD and electrodes loss is E2 (without the addition of electrolyte) and for TN_K, turbidity and settling speed is E1 (addition). Although, according to the rank, this factor is essential for all parameters, significant change in results is not observed for COD and turbidity for various levels. As the presence of electrolyte has a significant positive effect on two parameters and the absence for only one and a bit less pronounced, the overall optimal level is E1.

The optimal zeolite addition for COD is Z1 (addition of zeolite), and for TN_K, turbidity, settling speed and electrodes loss is Z2. Considering the factor’s rank, this is not a particularly influential factor. The “best” ranking for this factor is third but for a parameter that shows almost the same removal value in all experiments. For other parameters, it is ranked as fourth or fifth. The highest effect on the difference in S/N_FL has for TN_K indicates that the final selection should be level Z2 (non-addition of natural zeolite).

The optimal pH for COD, TN_K, turbidity, and electrodes loss is pH2 (adjustment of pH). The better results without pH adjustment are obtained only for settling speed. As pH adjustment has an impact on more parameters, and this impact is more pronounced, a better choice is pH2.

4. Conclusions

This paper compares the treatment efficiency of biowaste compost leachate with EC and ECZ at different stirring speeds, electrode distances, and the addition of electrolytes. The results show that the stirring speed, electrode distance, the addition of zeolite and electrolyte influence the efficiency of removal of COD and TN_K, while turbidity removal is very high, independently of investigated experimental condition. The final TS values increase in experiments with the addition of electrolytes. An increase in temperature and pH is visible, while the electrical conductivity oscillates with a slight downward trend. El. conductivity significantly increases in experiments with the addition of electrolytes. The settling was very slow without the addition of electrolytes (no separation of the sludge from the suspension). Increased electrodes consumption is observed by increasing the electrode distance and zeolite addition, while adding electrolyte decreases the electrode consumption. Electrical energy consumption varies in the range of 5.597–14.098 kWh/m³. The total cost of the ECZ process is increased by 5 USD $/m³ compared to stand-alone EC process.

Taguchi optimization shows that the stirring speed is the most influential factor for COD, TN_K, and turbidity, while electrolyte addition is for the settling rate and electrodes loss. In most cases, the electrode distance was the least important. The only parameter for which the change of electrode distance has a significant effect is electrode loss. Zeolite addition enhances COD removal but is not suitable for TN_K, turbidity, settling rate, and electrode loss. Adjusting pH at pH = 4 is suggested to achieve optimal removal of COD, TN_K, turbidity, and lower electrode loss. From the results obtained in this work, it is evident that several factors need to be considered and compensated when evaluating the optimal conditions, as our result confirms that optimal values are not always optimum. For some parameters, better results are obtained for EC and for some for ECZ. For final selection, the goal should be defined.

Due to the requirements related to minimizing the ecological footprint, environmental sustainability, and the possibility of implementing the process without the need for extensive control, the integrated EC with the addition of natural zeolite undoubtedly stands out as the future of wastewater treatment. Since the cost of zeolite increases the cost difference between EC processes carried out with and without zeolite, it is important to emphasize that zeolite can be regenerated after implementing the ECZ process.