Two-dimensional (2D) and three-dimensional (3D) batch electrochemical degradation (ECD) of raw cotton industry wastewater (CIWW) was adopted using stainless steel (SS) and aluminium (Al) electrodes. ECD as a treatment option was aimed at removing priority quality parameters, viz. chemical oxygen demand (COD), colour, chloride, nitrate, etc. COD removal of 85 and 80% were achieved by using 3D SS and 2D SS electrodes operated at 6 V (0.9 A) for a maximum electrolysis time (ET) of 30 min. Similarly, 76 and 70% COD removal were achieved for 3D Al and 2D Al electrodes, respectively. Simultaneous colour removal in the 2D ECD system using SS and Al electrodes was low by 12 and 11% compared to the 3D ECD system. Water quality parameters, viz. total dissolved solids, chloride, nitrate, phosphates, and sulphate were also removed by 3D (SS and Al) and 2D (SS and Al) electrodes. Higher pollutant removal efficiencies were observed at 30 min ET for 3D SS electrodes compared to 2D SS, 3D Al, and 2D Al. Post-ECD slurry showed good settling characteristics for SS electrodes generating dense and sturdy flocs giving a low sludge volume index values for 2D SS electrodes compared to other electrode options.

  • This novel research work is carried out on the use of electrochemical degradation (ECD) for the treatment of raw cotton industry wastewater (CIWW) using 2D and 3D ECD for Al and SS electrodes.

  • The results of the experimental work have applications for the removal of most pollutants/contaminants from raw CIWW.

  • ECD of CIWW offers low hydraulic retention time (HRT) and low spatial and energy footprint.

Only 3% of the total water resources available on Earth is freshwater, while the remaining 97% is saline water (Panagopoulos 2021). Extremities in pollution and inefficient utilisation of available freshwater resources cause a double effect resulting in freshwater shortages. In India, 160,000 tons of textile wastewater is generated each day. Worldwide, the textile industry consumes large quantities of water and involves a variety of wet processing operations using many hazardous substances. Among the various processes, dyeing, fixing, and washing processes consume large volumes of water of which only 15% is recovered for reuse. Approximately 40% of the dye used is not adsorbed to textile fibres and discharging untreated wastewater loaded with synthetic dyes to the aquatic environment cause serious complications. The textile processing industries contain suspended solids, dissolved solids, unreacted dyestuff, and other auxiliary chemicals used in various stages of the dyeing/other processes (Babu et al. 2007). Textile wastewaters are characterised by high concentrations of biodegradable and non-biodegradable organic compounds, dissolved salts and suspended solids, strong colour, pH from 2 to 12, and high discharge temperatures (55–90 °C). The discharge of wastewater containing salts degrades the water quality in receiving rivers/streams, so cannot be directly used as potable water, and is also rejected in industrial applications (Panagopoulos 2022). The most difficult waste stream in a fabric manufacturing unit is from the ‘degumming section.’ Changes in dyestuff used in dyeing cause considerable variations in the wastewater characteristics showing intense colour, high chemical oxygen demand (COD), total dissolved solids (TDS), and a highly fluctuating pH (Cerqueira et al. 2009). Natural degradation of artificial synthetic dyes in the aquatic environment is a slow process because of its persistence and therefore demands adequate treatment before discharge.

Currently, at global level there is no single robust technology applicable for the treatment of real textile wastewater that is technically sound and economically viable giving a small energy footprint (Mondal 2008). Recalcitrant wastewaters are not amenable to biodegradation as it contains over 80% insoluble pollutants/contaminants (Sujit et al. 2019). Conventional treatment methods show poor performance with high hydraulic retention time (HRT), large spatial footprint, and a high capital investment. In the wastewater containing low C/N ratio, the NAS may approach lower fouling and enhance the nitrification efficiency (Sepehri & Sarrafzadeh 2018). Novel technologies viz., forward osmosis, membrane distillation can treat high salinity waters and offer high recovery rates (Panagopoulos & Giannika 2022), but are high on cost investments. Recently the electrochemical treatment processes using 2D and 3D electrodes have found several applications for treating various industrial and domestic wastewater like landfill leachate (Zhang et al. 2010), silk textile wastewater (Hemalatha & Sanjay 2023), semi-urban agricultural soil (Shivaswamy et al. 2023), Ayurveda pharmaceuticals wastewater (Shriom et al. 2016), simulated domestic wastewater (Zhang et al. 2017), coffee processing (Sahana et al. 2018), health care wastewater (Shivaprasad & Mahesh 2022), etc.

2D and 3D electrochemical coagulation

The 2D electrode system requires larger HRT compared to the 3D electrode system (Sujit et al. 2019); the 3D electrochemical degradation (ECD) system offers large surface area for electrode dissolution (ED) and hence quick removal of pollutants compared to the 2D ECD system. It also improves mass (pollutant/contaminant) transfer and offers a good space time yield (STY) and enhances ionic charge distribution. In a 2D–3D ECD system, particle electrode materials like activated carbon, carbon aerogel, and graphite particles offer large specific surface area and high electro-activity contributing to an increase in the mass transfer coefficient with reduced energy consumption (Yan et al. 2011). The electrochemical coagulation (ECC) process is novel as it offers the total removal of most pollutants and contaminants from raw wastewater (Mahesh et al. 2006). The purpose of this research work was to electrochemically treat medium–high strength raw cotton industry wastewater (CIWW) in both 2D and 3D modes using aluminium and SS monopolar electrodes. The challenge in this research work was to focus on the maximum removal of priority pollutants simultaneously from the raw CIWW in short HRT of ∼30 min with the goal of achieving a low spatial footprint, and comparing two different modes of operation.

The various chemicals required for characterisation of wastewater and ECD sludge, characterisation procedures, experimental setup, and operating protocols of electrochemical treatment (ECT) are described in the following sections.

Chemicals and reagents

All chemicals used in the ECD studies were of analytical reagent grade. The chemicals reagents used were ammonium ferrous sulphate, standard buffer solution, stock nitrate solution, hydrazine sulphate, silver nitrate solution, Erichrome Black T, 1–10 phenanthroline, potassium chromate indicator, mercuric sulphate, ethylene diamine tetra acetic acid (EDTA), silver sulphate (Ag2SO4), potassium dichromate, manganous sulphate, magnesium sulphate (MgSO4) solution, potassium hydroxide solution, standard sodium thiosulphate (Na2S2O3), methyl orange indicator, alkali-iodide-azide solution, conditioning reagent, sodium chloride (NaCl), ferrous ammonium sulphate, starch indicator, phenol di-sulphonic acid, sulphuric acid, ferroin indicator, hydrochloric acid, standard sulphate solution, standard ferrous ammonium sulphate (FAS) solution, ferric chloride, hydrogen chloride (HCl), standard phosphate solution, ammonium molybdate solution, potassium iodide, ammonium vanadate solution, phosphate buffer solution, potassium chloride, mercuric sulphate (HgSO4), sodium thiosulphates, sodium hydroxide (NaOH), and many other chemicals. Double distilled water was used to prepare stock solutions, dilution, and to prepare various reagents for analyses of quality parameters.

Analysis of raw CIWW

Raw CIWW were analysed for various physico-chemical parameters following procedures described in Standard Methods (APHA 2017) before and after batch ECD experiments. All the quality parameter values were determined in triplicate and average values are reported. The various parameters and the relevant formulae/equation used in select wastewater quality parameter analyses are given in Table 1.

Table 1

Wastewater quality parameters and relevant equation

ParameterEquationParticulars
TDS  W1 is the empty weight of a crucible (g), and W2 is the empty weight of the crucible with solids (g) 
TSS  W1 is the weight of empty Petri dish + filter paper (g), and W2 is the weight of empty Petri dish + filter paper with solids (g) 
TS  TDS is the total dissolved solids (mg/L), and TSS is the total suspended solids (mg/L) 
Total colour  Iabs is the initial absorbance, and Fabs is the final absorbance 
COD  A is the FAS consumed for blank (mL), B is the FAS consumed for sample (mL), and M is the molarity of FAS 
Total alkalinity as CaCO3, mg/L   A is the H2SO4 required to bring the pH to 8.3 , B is the H2SO4 required to bring the pH to 4.5 , and N is the normality 
ParameterEquationParticulars
TDS  W1 is the empty weight of a crucible (g), and W2 is the empty weight of the crucible with solids (g) 
TSS  W1 is the weight of empty Petri dish + filter paper (g), and W2 is the weight of empty Petri dish + filter paper with solids (g) 
TS  TDS is the total dissolved solids (mg/L), and TSS is the total suspended solids (mg/L) 
Total colour  Iabs is the initial absorbance, and Fabs is the final absorbance 
COD  A is the FAS consumed for blank (mL), B is the FAS consumed for sample (mL), and M is the molarity of FAS 
Total alkalinity as CaCO3, mg/L   A is the H2SO4 required to bring the pH to 8.3 , B is the H2SO4 required to bring the pH to 4.5 , and N is the normality 

Electrochemical degradation

A laboratory scale electrochemical reactor (ECR) was designed as 2D to 3D convertible and fabricated using see through acrylic glass. All experiments were carried out at ambient water temperature of 25–28 °C. Two sheet metals, stainless steel (SS-304) and aluminium of 1 mm thick were used to serve as plate electrodes placed parallel to each other in the ECR with an inter-electrode gap of 10 mm. A magnetic bead in an inductive magnetic stirring unit was used to inductively stir the bulk solution in the reactor. The ECR had an effective wastewater retention volume of 1.5 L. Raw CIWW as is, was poured into the ECR with complete submergence of the electrodes. The lead wires were then connected to a DC power supply unit to supply current to the electrodes for initiating ED and generating in situ metal ions operated at a cell voltage of 6 V and the corresponding cell current of 0.6 A. The initial and final weights of the electrodes were noted down to assess the amount of electrode material dissolved into the bulk solution. The quantity of the electrode material released into the solution in each ECD cycle determines the life of the electrodes. Sample aliquots were drawn at regular time intervals, and filtered through Whatman 42 filter paper (in some cases) to determine certain priority water quality parameters like pH, electrical conductivity (EC), COD, colour, nitrate, phosphate, TDS, etc. In India, colour and TDS are the two prime parameters of environmental concern in the Pollution Control Boards exercising reactive EOP discharge standards.

Wastewater quality initiated reactions in an ECR during the ECD process

Electrochemical reactions during treatment undergo three sequential steps: (i) electrode-related reactions, (ii) water quality-related reactions, and (iii) manifest state. The electrode-related reactions are known by the generation of unstable monomeric and polymeric species which given a compatible bulk solution and a reaction-responsive liquid interface can bind, precipitate, and float the entrapped pollutants/contaminants to the top of the ECR. The microenvironment in the ECR is positively responsive for floc formation aptly ascribed to the water quality characteristics. A few parameters to mention are – pH manifest, alkalinity, chlorides, initial solids (organic) content, etc. SS electrode contains mostly iron which forms both monomeric and polymeric species. Aluminium electrodes show lower pHf (final pH after ECD treatment) while SS electrodes show higher pHf at the end of the prescribed electrolysis time (ET) during the ECD process. When the pHf of the solution during treatment crosses 8.0, ClO forms to disinfect the water lysing all the microbial mass enmasse (Sujit et al. 2019).

Water quality-related ECD reactions during the ECD process

Initially, the metal freshly oxidises to ferrous ions depending on the cell potential applied during ECD. In the second step, on the anode face, in the presence of other oxidants, the reactions (1) and (2) occur:
formula
(1)
formula
(2)
In the vicinity of the cathode face, the solution turns alkaline as the ECD process progresses with ET. The applied current forces the OH ions to migrate towards the anode and so the pH near the anode surface is more than that of the pH in the bulk solution. This condition favours the formation of ferric hydroxide following Equation (3):
formula
(3)
SS electrodes contain maximum iron content; the iron ions liberated into the solution forms iron species following Equations (4) and (5):
formula
(4)
formula
(5)
Kraft et al. (1999) reported that chloride concentration <100 mg/L is sufficient enough to produce free chlorine to efficiently disinfect water (Equations (6)–(8)):
formula
(6)
formula
(7)
formula
(8)
The removal of sulphate from the solution was 42% of its initial value. In the ECD process, sulphate removal is carried out by adsorption in which the polymeric metal hydroxide floc entraps the sulphate ions as shown by Equations (9) and (10):
formula
(9)
formula
(10)

3D batch ECD experiments

Al and SS particle electrodes were filled into the reactor space (meant for anode) with plate electrodes as cathode. The 3D particle electrodes were connected to the positive terminal of the DC power supply unit. The 3D particle microelectrodes were used as sacrificial anodes of size 3 mm × 3 mm × 1.5 mm. The same ECR used in the 2D experiments were again used in the 3D ECD system.

The initial characteristics of CIWW used in the batch ECD are shown in Table 2. It may be observed that the chloride values are high in raw CIWW. COD/BOD (biochemical oxygen demand) ratios were >3 showing that CIWW is not suitable for biological treatment. Nitrate in excess amounts together with phosphorous can accelerate eutrophication in receiving waters. Excess nitrate in waste discharges causes hypoxia and becomes toxic to warm-blooded animals at concentrations >10 mg/L. In raw CIWW, the initial concentration Co of nitrate was ∼18 mg/L before conducting the ECD treatment process. The natural levels of nitrate in surface water are typically <1.0 mg/L. The presence of sulphate favours the aggregation of hydrolysed metal species leading to the formation of larger polymers that binds the material as precipitates enhancing the floc aggregation sequence. The Central Pollution Control Board (CPCB) discharge standards are prescribed under the Environmental Protection Act of 1986, under the Ministry of Environment and Forests and Climate Change (MOEF & CC 2016) in India is shown for comparison in Table 2.

Table 2

Physico-chemical characteristics of raw CIWW

No.ParametersUnitDischarge standards
Raw CIWW values (Co)
Public sewerSurface water
pH — 5.5–9.0 5.5–9.0 7.34–8.33 
EC μS/cm a a 2,010–2,200 
BOD5 at 20 °C mg/L 350 30 600–650 
COD mg/L a 250 1,728–2,112 
COD/BOD Ratio a a 3.0–3.3 
Colour PCUb a a 2,200–2,600 
TDS mg/L 2,100 a 3,100–3,400 
Chloride mg/L a 1,000 1,800–2,100 
Phosphate mg/L a 38–44 
10 Nitrate mg/L a 10 6.92–18.27 
11 Sulphate mg/L a 1.5–1.8 
12 Total alkalinity as CaCO3 mg/L a a 800–880 
13 Total hardness as CaCO3 mg/L a a 2,240–2,362 
No.ParametersUnitDischarge standards
Raw CIWW values (Co)
Public sewerSurface water
pH — 5.5–9.0 5.5–9.0 7.34–8.33 
EC μS/cm a a 2,010–2,200 
BOD5 at 20 °C mg/L 350 30 600–650 
COD mg/L a 250 1,728–2,112 
COD/BOD Ratio a a 3.0–3.3 
Colour PCUb a a 2,200–2,600 
TDS mg/L 2,100 a 3,100–3,400 
Chloride mg/L a 1,000 1,800–2,100 
Phosphate mg/L a 38–44 
10 Nitrate mg/L a 10 6.92–18.27 
11 Sulphate mg/L a 1.5–1.8 
12 Total alkalinity as CaCO3 mg/L a a 800–880 
13 Total hardness as CaCO3 mg/L a a 2,240–2,362 

 aNot available.

bPCUs, platinum cobalt units.

COD removal

Figure 1 shows COD degradation curves for ECD of raw CIWW using SS and Al electrodes operated at 6 V (0.9 A) for both 2D and 3D modes of operation. In a 2D setup, COD removal for SS and Al were 78 and 72%, respectively, at the end of 30 min ET from its initial COD concentration of 1,780 mg/L. The post-ECD supernatant pH at the end of ET was 8.90–9.0 meeting the discharge standards.
Figure 1

COD degradation curves as a function of ET min in 2D and 3D ECD. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Figure 1

COD degradation curves as a function of ET min in 2D and 3D ECD. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Close modal

The 3D arrangement for the same operating current and cell voltage and discrete ETs showed better COD degradation compared to the 2D electrode arrangement. COD removal of 85 and 76% were achieved using 3D SS and Al particle electrodes at 25 min ET, ascribed to a condition, because particle electrodes offer larger anode surface area >by 40%, for the same cathode area facing the 3D anode surface. This arrangement provides greater STY and quicker mass transfer (Figure 1) giving larger surface area for capturing colloidal particles improving the pollutant removal efficiencies compared to the 2D process. During the 3D ECD, small variations in the current across the electrodes were noted. COD removal is achieved due to the formation of gelatinous insoluble metal hydroxides entrapping the oxygen demanding substances thereby removing the pollutants by flotation during ECD and by setting, and filtration thereafter. Low COD removal after 25 min ET is because at neutral pH the metal (SS and Al) ions in 2D mode readily form hydro complexes (Barrera Diaz & Rivas 2015).

TDS removal

Figure 2 shows simultaneous TDS removal for both 2D and 3D ECD of raw CIWW. TDS measurements were conducted by gravimetric analysis. Dissolved solids may be organic or inorganic. The organic dissolved solids in the waste stream are ascribed to the decaying materials in water and the inorganic constituents are either organic chemicals or gases. Removing these dissolved minerals, gases, and organic constituents is desirable as it can cause physiological effects and produce aesthetically displeasing colour, odour, and taste from water. In the 3D ECD process, the marginal increase in TDS value during treatment at 10 min is due to the release of innumerable M+ soluble metal ions into the solution ascribed to the larger surface area on the 3D electrodes. This variation in TDS was not observed in the 2D electrode configuration during the ECD process. In the case of 2D electrode arrangement, the residual TDS remaining in the solution at 30 min ET was little higher than the TDS residues in the 2D ECD system (Figure 2). TDS removal for the 3D ECD system was marginally high compared to the 2D ECD system for both Al and SS electrodes. TDS removal at 30 min ET in both 2D and 3D modes was 46–55%, respectively.
Figure 2

TDS removal for 2D, 3D Al and SS electrodes in 2D and 3D ECD. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, agitation speed: 300 rpm, electrode spacing: 1 cm.

Figure 2

TDS removal for 2D, 3D Al and SS electrodes in 2D and 3D ECD. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, agitation speed: 300 rpm, electrode spacing: 1 cm.

Close modal

Colour removal

The colour molecules in the ECD treatment follow a particular removal pattern in both 2D and 3D ECD treatment systems. Figure 3 illustrates colour removal of 2D and 3D ECD systems for Al and SS electrodes in discrete ECD experiments. It may be observed that an increase in colour occurs at 5–10 min ET for 3D SS electrodes and Al electrodes. As the treatment proceeds, the colour reduces to 280 PCU for 3D SS, and 240 PCU for 3D Al electrodes. The extent of colour removal in the 2D – ECD system using SS and Al electrodes was low by 12 and 11% compared to the 3D – ECD system. Higher solubility of ferrous ions can lead to delayed sedimentation of iron polymeric species and strongly depends on the concentration of dissolved metals in the bulk solution during ECC (Benekos et al. 2019) which has resulted in relatively less removal of colour in 2D ECD compared to 3D ECD for SS electrodes.
Figure 3

Colour removal for 2D, 3D Al and SS electrodes. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Figure 3

Colour removal for 2D, 3D Al and SS electrodes. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Close modal

Chloride utilisation

During batch ECD of raw CIWW, chloride values were monitored after 30 min ET in both 2D and 3D modes. Figure 4 shows chloride utilisation during the ECD process. Chloride is a mass parameter representing salts of Ca, Mg, Na, and K present in the wastewater. It was noted that chloride degradation was not prominent for varying cell voltages. The presence of salts enhances the release of M+ ions from the anode metal face and improves the mass transfer coefficient (Yasri et al. 2015). Two linking parameters TDS and EC also get reduced as the chloride value reduces. The 2D ECD system showed higher chloride removal compared to the 3D ECD system, confirming the saturation of more soluble salts in a 3D ECD system. Chloride induces certain reactions for its conversion into gas. Because of high chloride concentrations present in wastewater, ClO is formed at pH 6.5–9.5 due to anodic oxidation of chloride ions. Insufficient chloride removal is explained by the fact that hydroxide ions are less stable than chloride ions; the latter has a larger tendency to discharge over hydroxide ions formed during ECD because of its high concentration in the raw CIWW. Strong oxidants that are formed at pH 3–8 during ECD also aid in disinfection (Memelkina et al. 2017) where increase in the Fe coagulant dose during ECC experiments increases the chloride concentration in the wastewater. As shown in the curves, Al showed better removal of chlorides than SS in the 2D mode of operation.
Figure 4

Chloride removal for 2D, 3D Al and SS electrodes. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Figure 4

Chloride removal for 2D, 3D Al and SS electrodes. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Close modal

Nitrate removal

The level of nitrates in surface water is typically <1.0 mg/L. Figure 5 shows nitrate removal during 2D and 3D ECD for raw CIWWs using SS electrodes. SS electrodes showed larger removal of pollutants compared to Al electrodes. It may be seen from Figure 5 that nitrate removal stabilisers at 20–30 min ET. 3D SS particle electrodes showed higher nitrate removal compared to 2D SS electrodes. Nitrate removal is by cathodic reduction of nitrate ion to nitrite and ammonia. Nitrate removal is also because of the formation of ferrous hydroxide flocs (Xu et al. 2018). It is known that traditional treatment methods require 6–8 h of HRT to remove nitrate to <10 mg/L. Electro-reduction of nitrate using SS electrodes is given by Equations (11) and (12):
formula
(11)
formula
(12)
Figure 5

Nitrate removal for 2D, 3D SS electrodes. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Figure 5

Nitrate removal for 2D, 3D SS electrodes. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Close modal

Phosphate removal

The presence of phosphates in the ECD treated effluent can trigger algal blooms in the receiving waters. Figure 6 shows phosphate removal during ECD up to 30 min as a function of ET for SS and Al electrodes for 2D ECD and 3D ECD. SS-3D particle electrodes showed good removal of phosphates compared to Al-3D electrodes.
Figure 6

Phosphate removal as a function of ET for SS and Al 2D – 3D. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Figure 6

Phosphate removal as a function of ET for SS and Al 2D – 3D. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Close modal
Total phosphate removal is offered by the formation of FePO4 and Fe(OH)3 during ECD. The solubility of FePO4 tends to increase with the increase in the pH of the bulk solution, enhancing phosphate removal because at higher solution pH, Fe(OH)3 and Fe(OH)4 are formed in the dissolved state. Similarly, phosphate adsorption with Al(OH)3 and its precipitation forming AlPO4 remove PO4 from the wastewater when using Al electrodes (Kabdasli et al. 2012) following the reactions in Equations (13) and (14):
formula
(13)
formula
(14)

High phosphate removal in the case of SS-3D and SS-2D electrodes is ascribed to charge neutralisation by cationic hydrolysis products and pollutants entrapment in the amorphous hydroxide precipitate matrix produced by Al and Fe in SS electrodes.

Sulphate removal

Sulphate can cause laxative effects at >750 mg/L in freshwater (Peterson 1951). Sulphate removal was noted during 2D and 3D ECD using Al and SS electrodes:
formula
(15)

Equation (15) shows how sulphate ions are precipitated out into the gel floc matrix making the solution acidic for Al electrodes because of H+ ions. Sulphate removal during ECD was of the order of 62 and 67% for 2D and 3D Al electrodes.

Alkalinity utilisation and regulation

Adequate alkalinity in the wastewater influences the overall potential of the ECD process. It can cause deposition of metal hydroxide on the charged active electrode surfaces. The metal hydroxides formed during the ECD process escalate to final state of sweep flocculation increases (manifest state of the coagulation-flocculation process) and forms scum/sludge atop the ECR. Manganese (Mn) can undergo electro-reduction only with the change in electrons without any formation of solid precipitates. Each mole of alum uses 6 moles of alkalinity and produces 6 moles of CO2. This reaction shifts the carbonate equilibrium and decreases the pH of the solution. As long as sufficient alkalinity is present and maintained in the wastewater during the ECD process, the pH of the bulk solution drastically does not reduce and so is not an operational problem. When enough alkalinity is not present to neutralise the acid production, the pH may greatly be reduced. One mg of aluminium sulphate formed during ECD may consume 0.5 mg/L of alkalinity in water as CaCO3. Sufficient alkalinity (as CaCO3) should be present in the wastewater being treated by the ECD process. Treatment of low total alkalinity (TA) wastewaters using ECD causes devastating effects, because flocs do not form that aid in solids separation. The alkalinity as CaCO3 present in the raw CIWW before ECD was 800–880 mg/L. The bicarbonate ions present in the wastewater releases CO2 gas during ECD with ET increasing the prevailing pH. For Al and SS electrode materials, the reactions in Equations (16) and (17) apply:
formula
(16)
formula
(17)
Dubrawski & Mohseni (2013) mentioned alkalinity generation during the ECC process. The in situ coagulants released during ECC reduce the pH of the solution slightly, and consume the alkalinity for floc formation reducing the TA value after treatment. The net effect is a moderate alkalinity reduction after ECC:
formula
(18)
formula
(19)
formula
(20)
Total hardness and alkalinity as CaCO3 in the wastewater being treated by the ECD process can be controlled by the applied current. The reduction of Ca and Mg after ECD may be explained by the precipitation of carbonate and sulphate salts at pH > 8. (Equations (21)–(25)):
formula
(21)
formula
(22)
formula
(23)
formula
(24)
formula
(25)

Electrode dissolution

ED is a function of wastewater strength and applied cell voltage in an electrochemical treatment system. The extent of ED depends entirely on the amount of current passing through the electrode system catalysed by the presence of salts as chloride. Figure 7 shows the total ED in g/m3 at 30 min ET for 2D SS, Al and 3D SS electrodes. The ED during the treatment varies with ET depending on the pollutant concentration and the salt content in the bulk solution. It may be observed that the 3D–ECD system gives a lower ED compared to the 2D–ECD system treating CIWW. It was noted that the ED is lower at the end of the stipulated ET as recorded in the discrete ET-ECD experiments. Also, ED can be used as a control parameter in terms of sludge generated during ECD. Higher the applied current, larger will be the sludge production during the ECD process.
Figure 7

ED as a function of electrode material in a 2D and 3D ECD system using SS and Al. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Figure 7

ED as a function of electrode material in a 2D and 3D ECD system using SS and Al. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Close modal

Settling characteristics of ECD sludge

Figure 8 shows the settling pattern of batch electrochemical degradation (BECD) slurry for Al and SS for 2D and 3D electrodes. Before conducting the sludge settling process, the solid–liquid slurry was homogenised for uniform electro-floc distribution.
Figure 8

Settling pattern of BECD slurry for Al and SS 2D and 3D electrodes. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Figure 8

Settling pattern of BECD slurry for Al and SS 2D and 3D electrodes. Operating conditions: COD0: 1,780 mg/L, pH0: 7.56, SA/V: 13.32m2/m3, VECR: 1.5L, ET: 30 min, 2D and 3D monopolar, agitation speed: 300 rpm, electrode spacing: 1 cm.

Close modal
The sedimentation test was performed in a 1,000 mL graduated cylinder without stirring during the test. Al-generated slurry showed resistance to settling in both 2D and 3D modes of operation because of entrapped microbubbles in the sludge matrix. Al electrodes showed unusual settling pattern while SS electrodes (2D) showed discrete and compression settling patterns for 2D and 3D modes (Figure 8). The supernatant above the sludge interface was colourless clarified water marking the efficacy of the ECD treatment process. This condition gives a fair chance of clear water reclamation up to 80%. Poor sludge settling for Al electrodes increased the sludge volume index (SVI) values by over two times that of the SS sludge. The SVI was obtained using Equation (26):
formula
(26)
where H30 is the height of sludge interface after 30 min settling in the column, Ho is the initial height of the slurry, and Xo is the initial solids concentration in the slurry. Table 3 shows the SVI values for Al and SS electrodes. The 2D SS electrodes showed lower SVI values compared to other electrodes operated in 2D or 3D modes. In the case of 3D SS, a higher SVI was obtained by 22 units because of the three-dimensional particle electrodes release larger metal ions into the solution and so it is believed that a higher SVI is obtained. Contrastingly, 2D–3D Al electrodes showed relatively larger SVI values and as a result, larger sludge volumes are expected and hence was not viable. It was concluded that 2D arrangement using SS electrodes gives lower SVI values and is therefore a good option.
Table 3

SVI of post-ECD slurry for Al and SS for 2D and 3D modes

SVI after 30 min ETAl
SS
2D3D2D3D
370413180202
Standard deviation 15 22 12 12 
Remarks More sludge More sludge Less sludge Less sludge 
SVI after 30 min ETAl
SS
2D3D2D3D
370413180202
Standard deviation 15 22 12 12 
Remarks More sludge More sludge Less sludge Less sludge 

The ECD process was applied to treat real CIWW in a 3D and 2D ECD system using SS and Al electrodes focusing on removal of COD, colour, and other prime water quality parameters. Both SS and Al particle electrodes showed efficient COD removal, and colour removal with improved post-ECD supernatant water quality characteristics. COD removals of 85 and 78% were obtained using 3D SS and 2D SS electrodes, respectively. For 3D Al and 2D Al electrodes, COD removals were 76 and 72% from their initial value of 1,780 mg/L. Colour removal was significantly high using the 3D ECD system for SS and Al electrodes compared to the 2D ECD system by 12 and 11%. The 2D ECD system showed higher chloride and TDS removal compared to the 3D ECD system for both Al and SS electrodes. The 3D SS ECD system showed good removal of phosphate and nitrate compared to 2D SS, 3D Al, and 2D Al electrodes. The post-ECD water quality parameter values were well within the discharge standards prescribed by the MoEF & CC (2016) for the quality parameters COD, chlorides, nitrates, and phosphates. An additional polishing treatment of any kind downline the ECD unit for removal of residual dissolved constituents may be adopted for clear water reclamation of 75–85%. Raw CIWW was effectively treated using the ECD process operated in both 2D and 3D modes at bench scale. Future research work may be focussed on ECD obtained sludge for proximate and ultimate analysis, as also calorific value for taking decisions on further use/disposal.

The authors thank JSS Science and Technology University, Mysuru Karnataka State, India for providing research infrastructure in the specialised electrochemical treatment laboratory in the Department of Environmental Engineering, JSS S&TU, Mysuru to carry out this novel research work.

The authors declare there is no conflict.

All authors have agreed to this study.

T.M.S. performed laboratory research experimentation, wrote manuscript, and collected references. M.S. did research work plan, data interpretation, corrected the manuscript, organised text content and citation. M.M. corrected the manuscript, collected references, did data analysis, etc.

All relevant data are included in the paper or its Supplementary Information.

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