Continuous electrocoagulation treatment of pulp and paper mill wastewater: operating cost and sludge study

Abstract

The present research deals with the treatment of agri-based pulp and paper mill wastewater by continuous electrocoagulation (CEC) process using iron (Fe) as an electrode material. Effects of flow rates (dm³ h⁻¹): 0.5–4.0 and residence time (τ): 0.5–4.0 h were investigated on degradation of chemical oxygen demand (COD), color, total solid (TS), turbidity, specific energy consumption (SEC), instantaneous current efficiency (ICE) and electrochemical degradation index (EDI). At flow rates of 1.0 and 0.5 dm³ h⁻¹, COD removal efficiency of 78.20 and 82.15%; and color removal efficiency of 79 and 90%, respectively, was achieved. TS concentration of wastewater slurry was also reduced by 65% after 3 h residence time with flow rate of 1 dm³ h⁻¹. The specific energy consumption (kW h per kgCOD_removed) was decreased from 16.3 to 14.3 with decrease in τ from 4 to 1 h. At a supply charge concentration of 0.62 A h dm⁻³, the current efficiency (CE) values were 310% and 274% after τ = 2 and 1 h, respectively. Dissolution and consumption of electrodes were also studied with a change in flow rates. Sludge obtained after the CEC process was analyzed for settling and filterability characteristics, morphology and elemental analysis, point of zero charge, physicochemical and elemental characterization, and TS concentration. The operating cost of the process was also calculated based on the electrical energy and electrode consumption and was found to be 61.0 Indian Rupees (0.9 USD) for the treatment of 1 m³ of wastewater.

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Introduction

Pulp and paper industries require a large amount of fresh water for the production of paper (250–300 m³ per tonne).^1,2 Therefore, these industries are facing the problem of a shortage of water. This problem can be resolved by reducing the consumption of water or by recycling of treated wastewater.^3,4 However, wastewater or effluent generated in pulp and paper mills exhibits high chemical oxygen demand (COD), suspended solids (SS), color, etc. and contains a number of other recalcitrant compounds.⁵ The discharge of high COD and colored wastewater into the environment causes loss of environmental aesthetics, death of aquatic animals as well as profoundly affecting the terrestrial ecosystem.^6–9 Pulp and paper industries are one the highly polluting industries as they use woody and non-woody lignocellulose compounds as a raw material and generate lignocelluloses and other toxic compounds during reaction between lignin and organic compounds.^10–12 The ministry of environment, forest, and climate change (MOEFCC) and central pollution control board (CPCB) have listed the pulp and paper industry wastewater in the “red category”, which provide a serious pollution threat.¹³ Therefore, the treatment of pulp and paper industry wastewater requires the utmost attention today.

Various technologies such as adsorption, chemical coagulation, aerobic and anaerobic biological processes, advanced oxidation, electrocoagulation (EC), and membrane separation process are available for treatment wastewater.^14–25 Although, treatment of wastewater is easy in large paper producing mills, since they can afford modern facilities and expensive equipments however, small industries are unable to treat their wastewater properly due to financial constraints.^2,26 Some of the small paper mills do not have any chemical recovery units (CRU) even now.^27–29

EC process is the one of the most promising technologies in the area of wastewater treatment because of the requirement of simple equipment with ease of automation, no requirement of any addition of chemicals, formation of O₂ and H₂ bubbles during the process which enhances the efficiency of separation via electroflotation (EF) and the capability of handling wide variety of pollutants.^30–32 The basic steps of EC process are: (i) electrolytic reactions at electrode surfaces, (ii) formation of OH⁻ ions and H₂ at the cathode, (iii) oxidation of generated metal ions and subsequent precipitation of metal hydroxides in aqueous phase, (iv) adsorption of pollutants at metal hydroxide surface and charge neutralization, and (v) removal of pollutants by settling.^33,34

Configuration of electrodes in EC reactor is very important and can be arranged in series or parallel in monopolar or bipolar system. In the monopolar parallel mode, all anodes are connected to each other, and similarly all cathodes connected to each other. In the monopolar series system, the outermost electrodes are connected to a power source and the current passes through the other electrodes, thus polarizing them.^35,36 Since higher resistance is encountered in the electrodes connected in a series mode, higher potential is required for a given current. In bipolar parallel connection, where two parallel electrodes are connected to the electric power source with no power connection to the sacrificial electrodes, maintenance of system becomes easier due to the simpler set-up.^37–39

Few studies have been reported for the treatment of pulp and paper mill wastewater by batch EC process^27,40–44 and by continuous EC (CEC) process.^23,45 However, in the reported CEC studies, no information has been given regarding the analysis of sludge, rate of dissolution and consumption of electrodes and operating cost of CEC process. For the application of the EC process on industrial scale, studies on CEC process are of utmost importance. It is necessary to study the effect of various operating parameters such as pH, flow rate, residence time, etc.^46,47 All these aspects have been covered in the present study.

The aim of the present study is to investigate the CEC process for the treatment of paper mill wastewater using parallel plate electrodes arrangement. The effects of the residence time (hydraulic retention time, τ) and flow rates were studied on the removal of COD, color, TS, and turbidity; specific energy consumption (SEC), instantaneous current efficiency (ICE) and electrochemical degradation index (EDI). Sludge obtained after CEC process was analyzed by settling, filterability, point of zero charge, physicochemical and elemental characterization.

Experimental

Wastewater and chemicals

The wastewater as obtained from the cooking washing section of an agri-based (wheat stock) small pulp and paper mill manufacturing Kraft paper was used in the experiments. The wastewater had a very low transparency and was dark yellow-brown in color due to presence of lignin fragments forming chromophores. The characteristics of wastewater are given in the Table 1.

Table 1 Characteristics of pulp and paper mill wastewater

Waste water characteristics
Parameter	Range
pH	6.86–7.12
BOD (mg dm^−3)	615–670
COD (mg dm^−3)	2000
Total solids (mg dm^−3)	2200
Color (platinum cobalt unit)	1750
Turbidity (NTU)	182
Chlorides (mg dm^−3)	48–62
Adsorbable organic halogens (AOX) (μg dm^−3)	540
Total alkalinity (mg dm^−3)	380–410

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All the chemicals used in the study were of analytical reagent grade. Ammonium ferrous sulphate, 1–10 phenanthroline, mercuric sulphate, and silver sulphate were obtained from HiMedia Laboratories, Mumbai (India). Potassium dichromate was obtained from Ranbaxy Chemicals Ltd, New Delhi (India). Potassium chloroplatinate and cobalt chloride were obtained from Qualigen Fine Chemicals, Mumbai.

Experimental procedure

The schematic diagram of the laboratory scale experimental set-up used in the CEC treatment of pulp and paper mill wastewater is shown in Fig. 1. The CEC reactor and electrodes configuration details are given in Table 2. CEC experiments were performed at optimum operating conditions obtained in batch study of present wastewater^43,44 with different flow rates and residence times (Table 2). The feed tank and the CEC reactor were stirred continuously by a magnetic stirrers to maintain uniform concentration of the feed. The flow rate of the feed to the reactor was maintained through a peristaltic pump (Miclins-20 PP, India). The voltage across the cell was measured using a digital multi meter (Keithley, Germany). Samples were collected at regular time intervals from the CEC reactor outlet, filtered, and analyzed for color and COD.

Fig. 1 Laboratory scale experimental setup for CEC treatment.

Table 2 Continuous EC reactor, electrode characteristics, and optimized operating conditions

CEC reactor specification		Electrode characteristics		Optimized operating conditions
Material	Perspex (organic glass)	Material and shape	Iron	Cell voltage	3.1–3.2
Dimensions (cm)	11.6 cm × 11.7 cm × 16.6 cm	Shape	Rectangular	No. of electrodes and electrode gap	6 and 10 mm
Volume (dm^3)	2.0	Size of each plate	9 cm × 10 cm	Active electrode SA/V ratio (m^2 m^−3)	54
Type	Up flow	Thickness (mm)	1.5	Current density (A m^−2)	55.56
Stirring mechanism	Magnetic bar	Plate arrangement	Parallel	pH	∼7

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Analytical procedure

pH was measured using a digital pH meter (NIG 333, Toshniwal, Delhi). The color intensity was measured as per standard methods given by American Public Health Association (APHA) using a spectrophotometer (Perkin Elmer, Switzerland).⁴⁸ Adsorbable organic halides (AOX) was measured using a Thermo-Scientific AOX analyzer. COD of the solution was determined as per standard methods given by APHA.⁴⁸ The decolorization of the samples was determined by monitoring the decrease in absorbance of the visible spectrum from 190–480 nm. The percent decolorization was estimated by following equation:where, D is the decolorization in%, A_initial and A_observed is the area under the curve of the absorption spectrum before and after time t.⁴⁹

Reaction mechanism

The sacrificial Fe electrodes generate Fe²⁺ and OH⁻ ions according to the Faraday's law. Electrochemical degradation is a complicated time-dependent process, which completed in steps i.e. hydrolysis, polymerization, and precipitation.^50–52 During these steps, three classes of products of different species are involved, i.e. (a) low molecular weight iron hydroxides Fe(OH)²⁺, Fe(OH)₂⁺, Fe(OH₂)⁴⁺ etc.; (b) hydrolytic iron polymer (Fe_n(OH)_m(H₂O)_x^(3n−m)+), or Fe_mO_n(OH)_x^{(3m−2n−x)+}; and (c) precipitated oxides (amorphous(am)-Fe(OH)₃), FeOOH and Fe₂O₃). Fe³⁺ ions may undergo hydration depending on the pH of the solution.

In fact, for pH ≤ 5, at least four different Fe(III) ions coexist: Fe³⁺, Fe(OH)²⁺, Fe(OH)₂⁺ and Fe₂(OH)₂⁴⁺. In the range of pH ≥ 5, Fe(OH)₆⁻, Fe(OH)₄⁻ and other species may also be formed.^40,43,44,53 Following anodic and cathodic reactions take place in an EC reactor having iron electrodes, over a pH range of 6–9:⁵⁴

Anode: Fe → Fe²⁺ + 2e⁻ (2)

Fe → Fe³⁺ + 3e⁻ (3)

Iron undergoes hydrolysis by following reactions:

Fe + 6H₂O → Fe(H₂O)₄(OH)_2(aq.) + 2H⁺ + 2e⁻ (4)

Fe + 6H₂O → Fe(H₂O)₃(OH)_3(aq.) + 3H⁺ + 3e⁻ (5)

Fe(H₂O)₃(OH)_3(aq.) → Fe(H₂O)₃(OH)_3(s) (6)

Fe(H₂O)₄(OH)_2(aq.) → Fe(H₂O)₄(OH)_2(s) (7)

Cathode: 2H⁺ + 2e⁻ → H_2(g)↑ (8)

The pH for minimum solubility of Fe(OH)_n is in the range of 7–8. More hydrogen is evolved due to iron hydrolysis.

Results and discussion

Effect of flow rates and residence time

Removal of COD and color. Degradation of COD as a function of electrolysis time (ET) at the residence time τ (0.5–4 h) and flow rate (0.5–4 dm³ h⁻¹) is shown in Fig. 2a. Percentage removal of COD was 78.2, 80 and 82.2% at τ = 1, 2 and 4 h, respectively. As the residence time in the CEC reactor decreases, a slow but prolonged (30 and 75 min at τ = 4 h and 0.67 h, respectively) formation of flocs takes place. At τ < 1 h, large scale flake (golden yellow color on drying-due to rusting) start to deposit over the anode plates which results in the current drop by ∼0.05 A. At lower residence times (t ≤ 0.67 h), the suspended solid particles tend to cling and deposit at the rim of the dents on the anode plate surface and as the electrolysis proceeds, these particles act as the nucleus for further particle deposition, leading to the spread of the flake formation over the entire plate area. The COD degradation rate decreased in accordance with the increased residence time at a steady state. The increase in residence time improved the COD and color removal; however, the degradation byproducts tend to accumulate gradually atop the reactor as a foamy green gel.^23,43,44

Fig. 2 (a and b) Degradation of COD and variation in color intensity at different flow rates (c) Color of wastewater before and after CEC treatment. Flow rate, dm³ h⁻¹; residence time, h: (0.5 dm³ h⁻¹; 4 h), (1 dm³ h⁻¹; 2 h), (2 dm³ h⁻¹; 1 h), (3 dm³ h⁻¹; 0.67 h), (4 dm³ h⁻¹; 0.5 h).

Fig. 2b demonstrates the change in color intensity of the reactor effluents at different τ. At any τ, an abrupt increase in color intensity from 1750 to ∼3750–4300 PCU was observed during the initial 30 min ET. This occurred due to the oxidative polymerization of wastewater, which yields dark colored organic compounds.^23,55 After 30 min, the color removal depends on the flow rate. The decolorization reaches a constant value at ∼135 min ET for 1 ≤ τ ≤ 4 h and maximum 89.9% removal (183 PCU) at τ = 4 h. It was 78.7% removal at τ = 0.67 h. The iron levels of 200–500 mg dm⁻³ in the CEC reactor can remove color by ∼90%. As seen from the Fig. 2, the color removal begins to “bottom off” and further treatment produces no color reduction. In contrast, for τ = 0.5 h, the color intensity reached a value of 2450 PCU at the end of ET with a concomitant COD reduction of <14%. A decrease in turbidity from 182 NTU to 3–7.2 NTU was observed for flow rates of 0.5–1 dm³ h⁻¹ (τ = 2 to 4 h) at 150 min ET. At a flow rate of 4 dm³ h⁻¹ (τ = 0.5 h), the treated wastewater had a turbidity of ∼418 NTU. Fig. 2c shows color intensity of the wastewater before and after CEC process. At the beginning of each run, the wastewater solution was translucent, dark brown in color. The solution first became darker and more opaque as the reaction proceeded. A head of creamy foam was formed on the top of the solution due to the production of hydrogen gas. As the electrolytic reactions progress, a forest green viscous gel like flocs start accumulating at the top of the CEC reactor which gradually turns to pleasing dark brown color. As can be seen, the intensity of the wastewater is brought down from an initial value of 1750 PCU to <175 PCU.

pH of wastewater, cell voltage and percentage variation in COD removal. Fig. 3a shows the variation in the pH of the CEC reactor treated effluent as a function of ET at different flow rates and residence times. At all the flow rates and up to ∼40–60 min ET, a quick increase in the pH was observed after which the change in the pH was marginal. The initial increase in the pH during electrolysis is attributed to the production of hydroxyl ions in the solution. As the EC process proceeds (at τ < 1 h) the bubble coalescence is inhibited probably because of the alkaline pH of the solution. Therefore, a reduction in COD/color removal occurs and the active nucleation sites of the anode plates, which produce bubbles, tend to be blocked by micro-flocs, which gradually spread over the anode plates.^40,43,44,56 In addition, the chemical dissolution of Fe consumes H⁺ resulting in the pH increase. Short residence time ended up with lower final pH values. Long residence time caused increased final pH values. Since pH is a logarithmic function, the pH change is less at higher pH values. A slight drop in the pH at flow rates > 2 dm³ h⁻¹ was observed, particularly after 60–75 min ET. This was because of the gradual consumption of hydroxyl anions as well as the production of hydrogen cations in the dissociation reactions of HOCl and OCl⁻ along with the formation reaction of HOCl in the treated waste stream.

Fig. 3 (a) Change in pH of wastewater, (b) cell voltage and (c) COD removal efficiency at different flow rates. Flow rate, dm³ h⁻¹; residence time, h: (0.5 dm³ h⁻¹; 4 h), (1 dm³ h⁻¹; 2 h), (2 dm³ h⁻¹; 1 h), (3 dm³ h⁻¹; 0.67 h), (4 dm³ h⁻¹; 0.5 h).

Fig. 3b shows the variation in cell voltage during electrolysis and Fig. 3c shows the corresponding COD removal for flow rates in the range of 0.5–4 dm³ h⁻¹. With a starting cell voltage of ∼3.1–3.2 V and ET up to 45–60 min, a decline in cell voltage was observed irrespective of the flow rate. Up to this time, flake deposition occurs over the anode plates simultaneously adding to the increase in color intensity along with a small amount of COD removal (Fig. 3c). At this time, for flow rates <2 dm³ h⁻¹, the cell voltage again begins to increase gradually and reaches the near starting value at the end of the ET. During this period, flocs start forming in the reactor, degrading COD, color, turbidity and other organic constituents. Flakes that were deposited along the edges of the anode plates are released back to the solution with an increase in cell voltage. In contrast, at flow rates >3 dm³ h⁻¹, the cell voltage continues to decline and do not recoup back to the starting value with large scale flake deposits all over the anode causing reduced removal efficiencies. At any flow rate, the voltage across the cell gradually decreases to a minimum and then increases to its initial value.^42,57 The degradation starts when the cell voltage begins to increase gradually. The color and turbidity show an increase with ET, reach the maxima and then decrease.

SEC, normalized COD value as a function of charge and instantaneous charge efficiency (ICE). In the present study, the energy consumption (kW h per kg COD removed) was calculated on the basis of amount of energy consumed per unit mass of COD removed and expressed by following equation:where, V is the voltage across the electrodes (V), I is the current (A), t is the time (h) and ΔCOD is the amount of COD removed. Fig. 4a shows the SEC as a function of ET at different flow rates and residence times. The SEC values were found to be 16.3, 15.2 and 14.3 kW h per kg COD removed at 180 min ET for τ = 4, 2 and 1 h, respectively. SEC for τ = 0.5 h and 0.67 h were 4.5 and 1.5 times that for τ = 1 h.

Fig. 4 Effects of flow rates on (a) specific energy consumption as a function of ET, (b) normalized COD values as a function of charge, and (c) ICE values as a function of charge. Flow rate, dm³ h⁻¹; residence time, h: (0.5 dm³ h⁻¹; 4 h), (1 dm³ h⁻¹; 2 h), (2 dm³ h⁻¹; 1 h), (3 dm³ h⁻¹; 0.67 h), (4 dm³ h⁻¹; 0.5 h).

Fig. 4b shows the normalized COD values as a function of electric charge at different flow rates. At a charge of 3.75 A h dm⁻³ and flow rate of 0.5–1.0 dm³ h⁻¹, removal of COD was ∼78–80%. After a charge input of ∼6.19 A h dm⁻³, at lower flow rates of 0.5–2 dm³ h⁻¹, COD removal was ∼82.1, 80.2 and 78.2%, respectively. Removal of COD was 51 and 13.2% at flow rates of 3 and 4 dm³ h⁻¹, respectively. These results clearly demonstrate the effectiveness of iron electrodes for anodic mineralization of organic pollutants in wastewater. However, in order to have a quantitative estimation of the efficiency of the degradation reaction, the instantaneous current efficiency (ICE) was calculated. ICE is measured at a particular time or at constant time intervals during EC degradation of wastewater. ICE provides information about the formation of the polymeric products at the anode plates. The ICE values were calculated by COD method using the following relation:⁴³

where, (COD)_t and (COD)_t+Δt are the COD values at time t and t + Δt (gO₂ dm⁻³), respectively, V is the volume of the wastewater (dm³), 4 is the number of electrons exchanged per mole of oxygen consumed, and 32 is the molecular mass of oxygen (gO₂ mol⁻¹). The plot of ICE versus charge at different flow rate is shown in Fig. 4c. Best removal of COD was found at flow rates of 0.5 and 1 dm³ h⁻¹ with ICE values of 310% and 274%, respectively for a charge of 0.62 A h dm⁻³. It may also be observed that the ICE values decrease with ET giving a clear indication of flake deposition/film formation and electrode passivation.

The electrochemical oxidation of the organic species can be estimated by using the electrochemical degradation index (EDI) which gives an idea of the average current efficiency during the process. EDI was calculated by following equation:

where, Δt is the total elapsed ET. The estimated EDI values at the flow rates of 4, 3 and 2 dm³ h⁻¹ were 0.21, 0.72 and 1.16%, respectively. Similarly, at a flow rate of 1 and 0.5 dm³ h⁻¹, the EDI values were 1.5 and 1.73%. The larger the EDI value, the more easily the species are oxidized.

Anode dissolution and total solids. Fig. 5a shows the actual amount of anode consumption in the formation of iron hydroxides at different τ. As the τ increases, the anodic dissolution also increases. Fig. 5b shows the actual amount of iron consumed at different ET for an optimized flow rate of 1 dm³ h⁻¹ (τ = 0.5 h). As the electrolysis proceeds, a decrease in TS was observed. The increase in the TS concentration initially is ascribed to the dissolution of iron into the bulk solution and as the EC proceeds, the flocs begin to increase in size and accumulate atop the CEC reactor, while a small portion of the flocs flows along with the treated wastewater at the CEC reactor outlet. In addition, the anode dissolution increased up to ∼110 min, after which the anodic dissolution was very small. A larger anodic dissolution occurs up to ∼120 min ET. After 120 min ET, the anodic dissolution was ∼240 mg for every dm³ of wastewater treated.

Fig. 5 (a) Anode consumption and sludge volume at different residence times, (b) total solids concentration and anode dissolution as a function of ET for residence time (τ) = 2 h, (c) ΔT/ΔV as a function of cumulative filtrate volume as a function of residence time, τ. Flow rate, dm³ h⁻¹; residence time, h: (0.5 dm³ h⁻¹; 4 h), (1 dm³ h⁻¹; 2 h), (2 dm³ h⁻¹; 1 h), (3 dm³ h⁻¹; 0.67 h), (4 dm³ h⁻¹; 0.5 h).

TS concentration in the treated wastewater as a function of ET is given in Fig. 5b. After 180 min, the treated wastewater supernatant had a TS concentration of 0.76 g dm⁻³, while the original TS was ∼2.10 g dm⁻³. TS concentration reduced by ∼63% before and after EC treatment at a flow rate of 1 dm³ h⁻¹. The CEC reactor slurry (reactor content after terminating the experiment at 180 min ET) had TS of 6.68 g dm⁻³. The CEC reactor slurry had higher TS due to the partial accumulation of the flocs precipitate atop the CEC reactor during the EC process.

Slurry, sludge and electrodes analysis

Settling. The CEC runs were terminated at 180 min ET. For every run, a fluffy bubble filled jelly like viscous sludge got collected atop the CEC reactor. The reactor contents-the top scum and the 2 dm³ of the effluent were mixed slowly in a 2 dm³ glass container after removing the electrodes for weighing. After a while, the sludge settled to the bottom of the container thereafter the reactor contents were again homogenized and used for settling test. At 30 min settling time in the column, for the effluent obtained at τ = 4, 2, 1, 0.67 and 0.5 h, the sludge volume occupied was 0.60, 0.56, 0.38, 0.30 and 235 dm³, respectively in a 1 dm³ settling column (Fig. 5a). The results show that higher residence time of the slurry in the CEC reactor caused larger formation and accumulation of sludge.

Filterability. The formation of large and dense flocs during wastewater treatment in a CEC reactor using iron electrodes produced sludge showed good filtration characteristics. The dewaterability of the treated wastewater slurry containing the aggregated sludge flocs was studied using the gravity filtration technique; being a constant pressure filtration, neglecting the effect of change in the hydrostatic head on the total pressure as described earlier. The Buchner funnel with a pre weighed filter paper wetted at its peripheries was filled up to 50–60% of its volume (185 mL). The volume of the filtrate collected in the graduated cylinder was observed as a function of time and a plot was made between Δt/ΔV versus V. Fig. 5c shows such plots for 0.5≤ τ ≤ 4 h. At lower flow rates, the sturdy sludge in the supernatant slurry gives a clear filtrate with ∼50–60 PCU for 1≤ τ ≤ 4 h. On the other hand, as the flow rate increased, the effluent containing the sludge was murkier showing poor filtration characteristics. The filtration resistance of the filter medium (R_m) and the filter cake (α) were determined according to previous study.^43,44 The values of the slope, intercept, density of solids (ρ), α and R_m are shown in Table 3 for 0.5 ≤ τ ≤ 4 h. Lower the value of α, the better is the filtration. It may be seen from the values of α that as the flow rate increases (or decreases), the value of α increases.

Table 3 Filterability of the slurry

α	ρ	Slope × 10^12 (s m^−6)	Intercept × 10^6 (s m^−3)	α (m kg^−1)	Rm (m^−1)
4	1064.64	0.06	1.24	1.20 × 10^12	3.07 × 10^9
2	1064.68	0.07	1.61	1.33 × 10^12	3.98 × 10^9
1	1064.96	0.18	1.77	3.27 × 10^12	4.36 × 10^9
0.67	1064.80	0.26	2.90	5.62 × 10^12	7.17 × 10^9
0.5	1063.88	1.76	1.76	3.79 × 10^13	3.70 × 10^8

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Physico-chemical and elemental characterization of sludge. Table 4 shows the results of the proximate analysis, chemical analysis, heating value and CHNS composition of the settled sludge/precipitate formed after the CEC treatment of wastewater at optimal conditions without any additives. The proximate analyses (as per IS-1350; part-I, 1984) show a considerably lower ash content in the EC generated sludge. The chemical analyses show ∼15% of insoluble residue and ∼76% Fe₂O₃ and Al₂O_3. The settled sludge is a little leaner in carbon and hydrogen content as compared to that of the top scum. The elemental analysis shows a decrease in the CHNS composition of the settled sludge than that of the top scum generated during the CEC process. As can be seen, there is enhancement in the carbon content in the top scum as compared to that of the settled precipitate.

Table 4 Characteristics of EC generated sludge under optimal conditions

Proximate analysis		Chemical analysis		Elemental analysis
				Element (%)	Top scum	Settled sludge
Inherent moisture (%)	12.87	Insoluble residue (%)	14.63
Volatile matter (%)	42.39	SiO2 (%)	6.30	C	25.8	17.40
Ash (%)	17.69	Fe2O3 and Al2O3 (%)	75.99	H	3.27	2.94
Fixed carbon (%)	27.11	MgO (%)	3.08	N	0.82	0.61
Heating value (MJ kg^−1)	11.33	Sludge particle density (kg m^−3)	1067	S	0.42	0.21

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Points of zero charge are pH values associated with specific requirements on surface charge.⁵⁸ pH_PZC has been an important parameter characterizing the adsorption properties of the sludge material. The pH value at which the surface charge is zero is called the point of zero charge (PZC). The pH_PZC of the sludge samples obtained after EC treatment of wastewater were obtained by solid addition method. The difference between the initial and final pH values (pH₀ − pH_f = ΔpH) was plotted against the initial pH (pH₀). The cross-over point on the resulting curve at ΔpH = 0 gives the pH_PZC. Fig. 6a shows the PZC for two ionic strengths (0.1 and 0.01 M). As seen in Fig. 6a, the zero value of ΔpH lies at the pH₀ value of 8.4, which is considered as the pH_PZC of the untreated dried sludge obtained after the CEC treatment of wastewater.

Fig. 6 (a) Point of zero charge (pH_pzc), (b) SEM micrographs of anode and sludge, (c) EDAX of scum and sludge.

SEM micrographs of iron electrodes (anode), before and after CEC process, were obtained to compare their surface texture. Fig. 6b(1) shows the SEM micrograph of the same anode plate after several cycles of its use in EC experiments for a total duration of ET ∼6 h. The anode plate surface was found to be rough, with a large number of small sized three-dimensional dents of ∼100–200 μm in width and depth. After repeated cycles of EC runs, these dents increased in size all over the active side of the plate leaving behind an eroded surface. The flake deposits on anode plates are inevitable after a few cycles of use. These deposits, once they reach a thickness of over 1–1.2 mm (over the edges of the anode plates) slough off again in the bulk solution.⁴⁴ Also at τ < 2 h, the flake deposits tend to remain on the anode plates. One such flake is shown in Fig. 6b(2). The jelly like bubble loaded viscous top scum with unusual beehive structures is shown in Fig. 6b(3). The voids/bubbles entrapped in the sludge flocs structure can be seen in the micrographs in Fig. 6b(4).

EDAX analyzer was used to determine the elemental composition of the Fe–EC sludge samples in the top scum (viscous jelly like flocs) as well as the sludge settled after the CEC process of wastewater. Fig. 6c displays the spectrum obtained by the elemental microprobe analysis of EDAX for the top scum and the settled sludge. The results indicate that the top scum contains a larger fraction of carbon than the settled sludge by 14.73%. Similarly, the settled sludge contains a larger percentage of iron (67.46%) when compared to that of the top scum (32.24%). The increased peak intensity for Fe in the settled sludge was because of the formation of iron hydroxides in the EC reactor.⁵⁹

Operating cost. Electricity (5 Indian Rupees (INR) kW h⁻¹) and iron electrodes consumption (50 INR kg⁻¹) were considered two major cost-involving factors when applying CEC process to this study. Costs may also be dependent on the wastewater conductivity, type, and characteristics of wastewater, fluctuation in the global parameters, and the extent of destruction, and the type of sludge desired as well. Operating cost of the process for treatment of 1 m³ wastewater is calculated by the following equation:

Operating cost = C_energy + C_electrode (12)

The flow rate of 2 dm³ h⁻¹ showed average anode dissolution of ∼240 mg dm⁻³ of wastewater treated with maximum COD removals of 76% and color by ∼74% at 90 min ET. As the CEC reactor performed reasonably well at 2 dm⁻³ h⁻¹, with a cell voltage of ∼3 V and an applied current of 5 A, the power consumption has been estimated at these conditions.

Cost of iron

∼240 mg of iron gets dissolved per dm⁻³ of wastewater at the flow rate of 2 dm³ h⁻¹.

Iron required per m³ of wastewater (COD₀ = 2000 mg dm⁻³ at 2 dm³ h⁻¹) = 240 g.

Cost of iron for per m³ of wastewater treated = 0.24 kg × 50.0 INR kg⁻¹ = 12.0 m⁻³.

Cost of energy

Energy required for treatment of 1 m³ of wastewater: ∼9.801 kW h.

Cost of energy/m³ wastewater treated = 5 INR kW h⁻¹ × 9.801 kW h = 49.00 INR.

Total cost = cost of (electrode + energy) = 12.0 + 49.0 = 61.0 INR per m³ of wastewater treated.

Conclusions

Results of the present study show that the electrocoagulation (EC) process has good potential to remove COD, TS, turbidity and color from the pulp and paper mill wastewater. During continuous electrocoagulation (CEC) process using iron (Fe) electrodes, maximum COD removal of 82.2% with corresponding color removal efficiency of 89.9% was observed at a flow rate of 0.5 dm³ h⁻¹ (residence time = 1 h) with other operating conditions being pH = 7.1, current density = 55.56 A m⁻² and number of electrodes = 6. At this operating condition, the specific energy consumption was minimum (=14.3 kW h per kg COD removed). Maximum and minimum values of current efficiency (CE) values were 310% and 274% with supplying charge of 0.62 A h dm⁻³ after residence time (τ) = 2 and 1 h, respectively. Sludge produced after CEC process showed good settling and filterability characteristics at near neutral pH (∼7.0). The chemical analysis of sludge showed ∼15% insoluble residue, which contained ∼76% Fe₂O₃ and Al₂O₃. Settled sludge showed higher content of carbon and hydrogen in comparison to scum. Operating cost was found to be 0.9 USD (61.0 INR) for treatment of 1 m³ of wastewater. Overall, the CEC process can be used for the treatment of pulp and paper mill wastewater in combination with other treatment processes so as to reduce the COD and other pollution parameters within permissible limits.

Abbreviations

AOX	Absorbable organic halides
BOD	Biological oxygen demand
CEC	Continuous electrocoagulation
CHNS	Carbon, hydrogen, nitrogen, sulfur
COD	Chemical oxygen demand
CRU	Chemical recovery units
EDAX	Energy dispersive X-ray spectroscopy
EDI	Electrochemical degradation index
EF	Electroflotation
ET	Electrolysis time
ICE	Instantaneous current efficiency
INR	Indian rupees
PCU	Platinum cobalt unit
PZC	Point of zero charge
SEC	Specific energy consumption
SEM	Scan electron microscopy
SS	Suspended solids
TS	Total solid

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