A hybrid treatment process for product recycling from tannery process effluent and soak liquor

Journal Pre-proof A hybrid treatment process for product recycling from tannery process effluent and soak liquor C. Karthikeyan, S. Hosimin, G. Hema Sindhuja, S. Maruthamuthu, T.M. Khaleel

PII:

S2213-3437(19)30639-6

DOI:

https://doi.org/10.1016/j.jece.2019.103516

Reference:

JECE 103516

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

4 October 2019

Revised Date:

30 October 2019

Accepted Date:

2 November 2019

Please cite this article as: Karthikeyan C, Hosimin S, Sindhuja GH, Maruthamuthu S, Khaleel TM, A hybrid treatment process for product recycling from tannery process effluent and soak liquor, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103516

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A hybrid treatment process for product recycling from tannery process effluent and soak liquor C. Karthikeyana, b *, S. Hosimina, G. Hema Sindhujac, S. Maruthamuthua and T. M. Khaleeld a

Corrosion and Material Protection Division, bAcademy of Scientific and Innovative Research,

CSIR-Central Electrochemical Research Institute, Karaikudi, Tamil nadu, India. cDepartment of Chemistry,

E.K.M. Leather Processing Company, Erode, Tamil Nadu, India.

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Jamal Mohamed college, Tiruchirappalli, Tamil Nadu, India.

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*Corresponding author at: Academy of Scientific and Innovative Research (AcSIR), CSIR-

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Central Electrochemical Research Institute (CECRI), Karaikudi 630 003, India.

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Email address: [email protected] (Karthikeyan Chandrasekaran).

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Graphical abstract

Highlights

Hybrid treatment method for tannery process effluent and soak liquor

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Accessory nutrients were not used to enhancing bio degradation in the present study.

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Effective treatment method for sulfate and chloride containing tannery wastewater.

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The impact of secondary oxidant (hypochlorite) on bacterial growth was avoided.

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Hydrogen sulphide liberation, bad odour and chloride loss were not occurred.

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ABSTRACT

The sulfate rich saline tannery effluent (SRSTE) was treated effectively using halophilic

bacterial pretreatment followed by electrochemical oxidation (hybrid treatment). Aerobic degradation was done using halophilic bacterial consortia isolated from the tannery soak effluent, resulted in 76% COD removal after 6 days. The remaining COD was reduced by electrochemical oxidation (EO). In electro oxidation process (without aerobic pretreatment), 24 h of electro2

oxidation was needed to complete COD removal. The process efficiency was analysed through Fourier transform infrared spectroscopy (FTIR), high performance liquid chromatography (HPLC), total organic carbon (TOC) and chemical oxygen demand (COD). The reduction of sulfate could not be noticed in presence of halophilic bacteria in aerobic treatment. The energy consumption for COD removal was about 0.343 and 0.020 kWh/g1 in electrochemical oxidation and hybrid treatment process respectively. Besides, chloride loss was also higher in EO when

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compared to hybrid approach. Hybrid treatment process was proved to be an effective method for

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treating SRSTE. The mixed salt was recovered and used for dyeing purpose.

Keywords: Biological treatment; Electro-chemical treatment; hybrid approach; Halophiles; Total

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organic carbon

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1. Introduction

Leather processing effluent and soak liquor are treated separately in most of the tannery

industries in Tamilnadu, India. Leather process effluent is the mixed effluent generated from various steps of tanning process viz. liming, deliming, tannage operation, retanning, dyeing and finishing. This effluent contains sulfate, sulfide, sulfite, chloride, hardness, organic compound, 3

lipids, proteins etc. Soak liquor contains high saline condition with organic load. The treatment of this type of organic effluent is the main challenging task in tannery industries. The industries require large areas to dispose the soak liquor for making as solid waste and the recovered waste salt cannot be reused because of its high organic load [1], which leads dumping of waste salts in treatment plants. Another treatment options for tannery effluents are chemical coagulation, electrocoagulation, biological treatment, ozonation, sequential batch reactor

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and advanced oxidation processes etc. [2-7]. Chemical coagulation has been done in tannery

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effluent using various inorganic compounds like aluminium sulfate and ferric chloride to reduce suspended solids and chemical oxygen demand [8]. Preethi et al. (2009) studied the ozonation of

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tannery effluent for removal of COD and color, where the efficiency of COD reduction decreases while increasing the concentration of COD in the effluent. It should be considered that the ozone

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treatment is costlier [5].

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Aerobic and anaerobic treatment processes are generally used in the treatment plant in most of the tannery industries. Halotolarant and halo-dependent microorganisms can grow up to 30% of

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NaCl concentration. Halophilic bacteria are having great potential in the field of biotechnology, especially in bioremediation due to its ability to degrade organic pollutants [9]. Biological treatment of saline tannery wastewater using halophilic aerobic bacteria was done by many investigators [1,10-13] where soak liquor was selected for treatment process. Biological anaerobic

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treatment is the predominant treatment for sulfate rich tannery effluent, which results in the emission of obnoxious odor gases due to sulfate reducing bacteria [14,15]. Durai and Rajasimman (2011) reported that the decomposition rate was higher in aerobic process without unpleasant odor when compared to anaerobic process [16]. In India, most of the common treatment plants prefer both aerobic and anaerobic treatment processes, where the bacterial activity in high chloride and sulfate is an important factor. Hence, intensive research is needed to find out the effective method 4

to treat such a complex effluent (tannery process effluent and dyeing) without any secondary pollutants. The electrochemical treatment method is potentially a powerful tool to degrade the organic compounds in industrial wastewater. In-situ generation of oxidizing agents in electrochemical reactor like hypochlorite, oxygen based radicals, ozone and nitrogen oxides are used to degrade the organic compounds [17]. A major advantage of the electrochemical oxidation process is to avoid

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sludge and secondary pollutant formation. Chloride mediated electrochemical oxidation of tannery

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organics was studied by many investigators using several anodes [18-21]. Toxic chlorine is released in electrochemical process, which can be converted as chloride by solar treatment [22].

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Nevertheless, the implementation of electrochemical method in treatment process is not available

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in tannery industries.

While discussing with staff of a tannery effluent treatment plant, they suggested for

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degrading mixed soak liquor and processing effluent (wastewater generated from liming, deliming, tanning process, neutralization, retanning, dyeing processes) by biological and electrochemical

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method. In the existing process, soak liquor (chloride 35,000 mg/L and sulfate 20,000 mg/L) is treated by chemical and biological (aerobic) methods, where the process effluent (chloride 8,800 mg/L and sulfate 7,600 g/L) is treated by chemical and biological (anaerobic and aerobic) processes. In existing tannery treatment process, Reverse osmosis (RO) is facing agglomeration or

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blocking of organic on pores in the membrane, which affects the RO recovery over a period of operation. Since soak liquor has rich chloride content, it was requested to add in the process effluent for this treatment process. The mixing of both soak liquor and process effluent for treatment process may reduce the utilization of space and labor. Limitations of the electrochemical oxidation prior to biological treatments are removal of secondary oxidant (hypochlorite) from the EO treated effluent. In our previous work, we did hybrid approach via electrochemical oxidation 5

with the aerobic pretreatment process for sulfate rich tannery effluent. Anaerobic feed of tannery effluent (COD 3300 mg/L; chloride 8000 mg/L and sulfate 7500 mg/L) was used from leather processing in the study. The existing bacteria in the anaerobic feed of tannery effluent were used for aerobic degradation process. Due to the availability of non-biodegradable matters, COD reduction was not observed significantly after fourth and fifth days. The non-biodegraded organic matter was treated by electrochemical oxidation where COD was nil within an hour [23]. Hence,

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soak liquor (35000 mg/L chloride; 20000 mg/L sulfate; 2000 mg/L COD) was mixed with process effluent (8800 mg/L chloride; 7600 mg/L sulfate; 3300 mg/L COD) to improve the efficiency of

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pretreatment by halophilic bacteria.

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In continuation of the previous work, the present authors introduced a new approach on treatment process using halophilic aerobic pretreatment followed by electrochemical oxidation

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without addition of “external nutrients” for tannery mixed effluent (process effluent and soak

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liquor) to degrade the complex or organic matters. Besides, the aerobic pretreatment will improve the recovery of sulfate and reduce the formation of sulfide in the effluent. Impact of hardness on electrochemical process was also considered where hardness was removed by chemical method.

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The effluent after hardness removal was used for biological and electrochemical treatment. The available tannery effluent treatment literature are listed in Table 1. Based on our knowledge, there is no literature available related to a treatment method for tannery process effluent with soak liquor

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using biological pretreatment and electrochemical processes. 2. Materials and methods 2.1 Tannery effluent collection The tannery process wastewater and treated soak liquor were collected from EKM tannery effluent and treatment unit located at Erode, Tamilnadu, India (latitude 11.366125, longitude 6

77.701684). The wastewater and soak liquor were collected using sterile containers and stored at 5 °C for future study. The flow chart of the sample collection point is presented in Fig. S1. 1:1 ratio of tannery process wastewater and soak liquor were mixed for biological and electrochemical treatment. The mixed effluent was named as sulphate rich saline tannery effluent (SRSTE) 2.2 Microorganism isolation and identification Nutrient medium with high sodium chloride concentration was used to isolate dominating

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strains from the tannery soak liquor. The isolated two dominating strains were enriched using

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modified nutrient medium. The modified nutrient medium consists of the following compounds in g/L: Peptone 5 g, Beef extract 1.5 g, Yeast extract 1.5 g, Sodium chloride 60,100 g and pH 7.5 (at

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25°C). Biochemical test was carried out to characterize the isolated dominating strains. viz. NaCl tolerance, temperature tolerance, methy red, voge’s proskauer, indole, catalase, urease and citrate.

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The bacterial growth was evaluated by the following changes in the optical density (O.D) at a

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wavelength of 600 nm by UV-Visible spectrophotometer (EVOLUTION201). 16s rRNA gene sequencing was carried out to identify the bacterial strains. The bacterial cell morphology was

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observed using scanning electron microscope (Model- TESCAN LF).

2.3 Biological pretreatment

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The isolated two strains were collected separately as pellets using centrifugation. The collected pellets of two isolates were used for biological pretreatment study. Pellets of two isolates (1%) were added to the 10 numbers of 250 mL flasks containing 100 mL of SRSTE. External nutrients were not used in the present study. All the flaks were kept in the shaking condition (120 rpm) at 37°C for 10 days. The chemical oxygen demand (COD) and optical density were monitored regularly up to 10 days. 7

2.4 Physico-chemical characterization Sulfate, Sulfite and Sulfide were estimated by Spectroquant® Pharo 300 (MERCK). Chloride and hardness were estimated using volumetric method. Conductivity and TDS were measured using Eutech model-COND610. Total lipid concentration and protein content were estimated by gravimetric method and Lowry’s method respectively [21]. Total organic carbon, and chemical oxygen demand were analyzed using SHIMADZU TOC-L CPH and spectro quant

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Picco (Merck), instruments respectively. UV-Vis spectrometer (Thermo scientific evolution 201)

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was used to monitor the biological and electrochemical degradation of organics present in the effluent.

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2.5 Electrochemical reactor design

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The cylindrical type electrochemical reactor was used in the present work. Titanium tube was used as cathode and mesh of mixed metal oxide (Ti/TiO2/IrO2/RuO2) tube was used as anode.

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Anode was kept inside the titanium tube and the total reactor volume was about 250 mL (Fig. S2). The electrolyte was circulated with constant flow rate (100 mL/min). 350 mL of aerobic treated

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and untreated SRSTE were used as an electrolyte. The standard equations were used to evaluate the performance of electrochemical process like COD removal energy consumption and energy consumption of the process [23].

2.6 Mixed salt recovery and characterization

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After electrochemical oxidation and hybrid treatment process, (biological pretreatment and

electrochemical) solutions were collected and exposed to solar light (sun light) treatment to remove hypochloride. The hypochloride free sample was condensed using Rotavapour R3 and mixed salt was recovered. The recovered salts were analyzed using EDAX analysis. 2.7 Hardness removal of SRSTE

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Impact of total hardness in the recover salt from SRSTE was investigated using chemical hardness removal process. Besides, SRSTE contains 1500 mg/L of total hardness, which should be removed before treatment process. The total hardness affects purity of the desired end product (mixed salt) and electrochemical process by deposit over the cathodic surface. 1680 mg/L of Na2CO3 was weighed and added into 1L of SRSTE and stirred vigorously for 10 min followed by addition of 2190 mg/L of NaOH with continuous stirring. The hardness of the precipitate was

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allowed to settle down for 25 min, which was filtered using normal filter paper. The filtrated/ hardness removal solution was used for treatment process. The aerobic pretreatment was done in

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presence and absence of hardness SRSTE followed by electrochemical oxidation.

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2.8 Dyeing application

The recovered salt purity and dye fixing capability were analysed at South Indian Textile

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Research Association (SITRA), Coimbatore, Tamilnadu, India. The dye fixation study was performed by following method: Dyestuff used – Remazol Black B133, Depth of shade – 0.5 %,

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M : L ratio – 1 : 20, recovered salt used – 60 g/L, Soda ash – 5 g/L, temperature – 55 °C, and Time – 30 min. After dyeing process, the samples were washed, neutralized, hot soaped and finally

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washed with soft cold water. Macbeth 7000A spectrometer was used to measure the colour difference (ΔE) of the dyed sample. All the above test of the recovered salts was compared with laboratory grade salts. Coloured fastness to washing and rubbing were also tested for the dyed

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samples.

3. Results and discussion The mixing of effluents (soak liquor and process effluent) was used in treatment process,

where hardness was also removed (100 %) to improve the efficiency in biological pretreatment with electrochemical oxidation. The novelty of the work is degradation of the mixed effluent (soak liquor and process effluent) collected from industry by halophilic bacteria, where the presence of 9

non-biodegraded compounds was degraded by electro oxidation process. Karthikeyan et al. (2019) did hybrid approach via electrochemical oxidation with the aerobic pretreatment for anaerobic feed of tannery effluent collected from industry by existing bacteria, where chloride concentration was 8000 mg/L [23]. Besides, hardness removal and product recycling was not studied. Hence, in continuation of this study, halophilic bacteria were selected to reduce COD in high chloride (21,000 mg/L) and sulfate (19,800 mg/L) containing mixed effluent without hardness. In the

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present work, three systems were used to evaluate the efficiency of the treatment process (a) aerobic halophilic bacterial degradation (BIO) (b) electrochemical oxidation (EO) (c) aerobic

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halophilic bacterial degradation with electrochemical oxidation (Hybrid treatment process) (BIO-

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EO). 3.1 Bacterial isolation and identification

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Fig. 1a & b shows the morphological identification of HB1 and HB2 isolated from tannery

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soak liquor. The morphology was identified through the scanning electron microscope at 10KX magnification. Destined well-defined individual cells can be noticed in both the images. Both the bacteria were rod shaped, whereas, the HB1 was small rod shape when compared to HB2. Table

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S1 shows the biochemical characteristics of isolated bacterial strains. Based on the biochemical tests, the isolates were urease positive, which can grow in the pH range between 5 and 10. HB1 can tolerate at 1-12% of NaCl concentration and HB2 can tolerate at 2-20% of NaCl concentration

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(CSIR - Institute of Microbial Technology, Chandigarh). Fig. S3a & b shows the growth curve of both isolated species at modified nutrient agar with different salt (NaCl) concentrations of (a) 6% (60 g/L) and (b) 10% (100 g/L). In tannery process (EKM tannery, Erode, Tamilnadu, India.) the chloride concentration range was about 40 to 60 g/L. Hence, 60 and 100 g/L of chloride concentrations were used to evaluate the growth condition of the isolated halophilic strains.

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In 60 g/L sodium chloride concentration HB1 and HB2 reached stationary phase 2nd and 4th day respectively. The stationary phase was maintained up to 8th day by both the stains. Besides, in 100 g/L concentration, short stationary phase was observed in both the bacterial strains. At 6% salt concentration, the maximum optical density (1.8976) was observed during the duration of 10 days whereas, at 10% salt concentration, the maximum optical density observed was only 1.5673. In both the cases, the bacterial growth was appreciable. Exponential growth was observed in both the

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cases. The lag phase was followed by log phase in which appreciable growth was observed. The growth curve results concluded that the isolated strains could grow well at high salt concentration.

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It slowly reaches death phase, which is due to the organic depletion in the modified nutrient broth.

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16S rRNA genes sequences were compared with genbank database. Based on the blast analysis and homology of sequence, the isolated dominating strains HB1 and HB2 were 100%,

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99% similarity with the gene sequence of Halomonas maura and Halomonas pasifica respectively.

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The dominating strains were registered in NCBI and the accession numbers are MH220207.1 (HB1) and MH220216.1 (HB2). Phylogenetic analysis of 16S rRNA gene sequence of the isolated

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strains in the tannery soak liquor is presented in Fig. S4. 3.2 Tannery effluent characterization

Table 2 shows physico-chemical characterization of mixed effluent (SRSTE) of tannery wastewater, where treated soak liquor and effluent from processing unit (1:1) used for the

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preparation of SRSTE located at Erode, Tamilnadu, India. Tannery process wastewater contains 8,800 mg/L, 7,600 mg/L and 3,300 mg/L of chloride, sulfate and COD respectively. 35,000 mg/L of chloride, 20,000 mg/L of sulfate and 2,000 mg/L COD were noticed in treated soak liquor. The initial pH of SRSTE was about 7.8. Initial chloride and sulfate concentration was about 21,270 and 19,800 mg/L respectively. Besides, sulfide and sulfite concentration was very low. 3,750 mg/L and

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1,200 mg/L of COD and BOD were observed respectively. The BOD/COD ration was about 0.32. The total hardness was 1,500 mg/L with the calcium hardness of 350 mg/L and the magnesium hardness of 1,150 mg/L. The total dissolved solids were 56,000 mg/L, which indicates the presence of pollutants and unwanted solids in the effluent. 3.3 Aerobic pretreatment of SRSTE In the present study, aerobic degradation was done instead of anaerobic degradation to

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reduce the sulfide formation. Fig. 2a shows the bacterial growth curve of isolated bacterial strains

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HB1 and HB2 in the untreated raw effluent. The maximum OD value was 1.4, where the isolated strains utilized the organics present in the SRSTE without addition of external carbon / nitrogen

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sources. Fig. 2b shows the COD reduction during aerobic degradation. The initial COD

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concentration was about 3,750 mg/L, after 6 days of incubation period, COD concentration was about 875 mg/L. After 6th day, there was no significant COD reduction in the presence of HB1 and

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HB2. It can be assumed that after 6th day, bacteria could not degrade the non-biodegradable organics present in the SRSTE. In absence of halophilic strains, COD concentration was about

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1790 mg/L after 6th day, which indicates that the existing microorganisms show poor degradation efficiency due to the saline conditions. The above study confirms that significant COD reduction was observed in presence of HB1 and HB2. The bacteria living in saline or hypersaline environment might have greater potential to degrade pollutants [33]. Due to high concentration of

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chloride, halophilic bacterial strains were used in the present work without any additional nutrients.

3.4 Electrochemical oxidation The electro-oxidation of tannery effluent was done at a current density of 0.020A/cm2 to compare COD reduction efficiency with biological process using cylindrical electrochemical 12

reactor with titanium cathode and Ti-TiO2/IrO2/RuO2 anode. Sundarapandiyan et al. (2010) studied COD reduction in soak liquor using graphite electrode under various current densities viz. 0.006, 0.012, 0.018 and 0.024 A/cm2 respectively for 2 h [34]. Rajeswari et al. (2016) also investigated the electrochemical oxidation of soak liquor using Ti-TiO2/IrO2/RuO2 coated anode at current density of 0.012A/cm2 for 30 min. The experiment was carried out at 7.8 pH. It was concluded that near neutral pH, the formation of hypochlorite is higher during electrochemical oxidation of

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tannery effluent [21].

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Fig. 3a shows the formation of hypochlorite during electro oxidation. The concentration of hypochlorite was on the range between 600 and 3,750 mg/L. In general, two types of

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electrochemical oxidation processes are used in organic pollutant removal namely (i) direct oxidation (ii) indirect electro oxidation. In direct anodic oxidation catalytic electrode directly

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involved in the organic pollutant degradation. The process involves direct charge transfer reactions

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between the organic pollutants and the anode surface. In indirect electrochemical oxidation, high oxidizing species are involved in the degradation process, which are present in the electrolyte. The formation of chlorine on the anode surface due to chloride oxidation plays an important role in

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organic degradation [23,35]. In the present study, indirect electrochemical oxidation is the responsible for the oxidation of the organic complex using Ti-TiO2/IrO2/RuO2 anode. It is well known that the higher concentration of sodium chloride (21 g/L) in the tannery effluent makes

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more favorable for the TSIA electrode with 0.020A/cm2 current density, which is necessary for indirect electrochemical oxidation. During electro oxidation, the requirement of time for COD reduction was higher (24 h) and loss of chloride was higher which is due to conversion of chloride into chlorine (Fig. 3b). While using electrochemical oxidation process, the energy consumption for COD removal was about 0.343 kWh/g1 and energy consumption was about 1286 kWh/m3. To

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overcome the disadvantages, hybrid approach has been studied to degrade the organic contaminants present in SRSTE. 3.5 Hybrid approach In this study, after 6 days of biological degradation, the effluent was used as an electrolyte for electrochemical oxidation process. After biological degradation, COD was about 875 mg/L (Table 3) where initial COD was about 3,750 mg/L. Fig. 4 shows the COD reduction and

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formation of hypochlorite during electro-oxidation after biological pretreatment. The maximum

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hypochlorite formation was about 4,836 mg/L. The COD reduction during EO after biological process within 120 minutes reached <25 mg/L of COD concentration. The loss of chloride was

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also very lesser when compared to electro oxidation process.

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Table 3 shows the tannery effluent parameters after various treatment process of SRSTE. In aerobic halophilic bacterial degradation, there was no significant variation in chloride and

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sulfate concentration before and after biological treatment. During biological degradation, being a halophilic bacteria they utilized the organic as the carbon source. Due to aerobic process, sulfide

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and sulfite were oxidized and converted as sulfate. As a result, COD, BOD values were decreased by biodegradation process. The concentration of lipid also reduced to 6000 mg/L. After 6 days of biological process, there was no significant reduction in COD due to very low concentration of BOD present in the effluent after biological degradation. The initial protein concentration was 206

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mg/L and after biological treatment, the concentration of protein was about 86 mg/L. Total hardness was 1,000 mg/L. The value of calcium and magnesium was 250 and 750 mg/L respectively. It is understood that the reduction of COD by biological pretreatment is due to degradation, where COD was about 875 mg/L. The BOD value was reduced from 1200 to 150 mg/L, which is due to the presence of non-biodegradable compounds. The BOD/COD ration was

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about 0.17 after 6 days of aerobic halophilic treatment. Hence, it is believed that due to the presence of COD after aerobic pretreatment, EO could be used to degrade the non-biodegradable compounds and reduced the COD significantly within 2 h. In electrochemical oxidation process, the chloride concentration was about 945 mg/L after treatment (24 h). Besides, chloride loss was very high due to formation of hypochlorite. The generation of OH· and OCl· radicals during electrolysis was established by previous researchers

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[35]. Sulfite and sulfide was nil because sulfide and sulfite got oxidized and attained maximum

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oxidizing state of sulfate. Total hardness and BOD was around 250 and 50 mg/L respectively. TDS also reduced from 55,500 mg/L to 48,733 mg/L after electrochemical oxidation. There was

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no significant change in the sulfate concentration.

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In hybrid approach, the chloride concentration was about 19.5 g/L (after BIO-EO process). The concentration of BOD, total hardness and TDS were 100, 500 and 55,500 mg/L respectively.

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TOC also confirmed the advantages of the hybrid approach (Fig. S5). The initial TOC was about 1,112 mg/L, during electrochemical oxidation for 24h, there was no significant reduction in the

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TOC (1033 mg/L). After 6 days of biological degradation, the TOC was dramatically reduced up to 297 mg/L due to organic degradation by bacteria. After BIO-EO process, the TOC was about 164 mg/L. It can be claimed that the relationship between COD and TOC in EO and BIO-EO process is debatable. The carbon content (TOC) cannot be reduced significantly by

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electrochemical oxidation whereas COD and TOC can be reduced significantly by biological process, which supports with our previous work [23]. It may be due to conversion of carbon into carbon dioxide by bacteria. The energy consumption for COD removal was about 0.020 kWh/g1and the energy consumption was about 107 kWh/m3. The energy consumption for COD removal and process energy consumption rates were lower than the separate electrochemical treatment process. 15

The FT-IR spectrums of SRSTE before and after various treatments are explained in Fig. S6. Raw effluent was highly contaminated with organics where the appearance of high noise was noticed. The functional group prediction was impossible. After biological degradation, the number of peaks reduced, which confirmed the degradation of organics. In all four cases, a strong peak was observed at 1629 cm-1, 990 cm-1which represent the c=c group. FTIR results concluded that based on the raw SRSTE, the peak intensity in IR spectra of treated sample was reduced. The

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maximum peaks were disappeared after hybrid and EO processes. Besides, there is no significant

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variation in IR peaks in both the processes.

The SRSTE was analyzed by HPLC before and after various treatments processes (Fig. 7).

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The untreated effluent has six major peaks at retention times around 2.539, 3.800, 4.303, 5.035, 6.676, 6.823 min. After biological degradation, the intensity reduced and little shifts were

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observed. During electrochemical oxidation and hybrid treatment, only three peaks were observed

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2.525, 6.708, 6.939 and 2.512, 6.644, 6.802 min respectively. These results also support that hybrid treatment and electrochemical oxidation promotes significant degradation of organics

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present in tannery effluent. But EO alone takes more energy. 3.6 Salt recovery

After electro oxidation and hybrid treatment, the mixed salts were recovered from the treated effluent using rotary evaporator. Recovered salts were analyzed using EDS analysis (Fig.

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8). Sodium, sulfur, chloride and oxygen are the major element present in the recover salt in both the process. Fig. 6a & b shows the spectrum of elemental composition of mixed salt recovered from SRSTE after EO (oxygen 45 %, sodium 30 %, chloride 17 % sulfur 7 %) and hybrid treatment (oxygen 46 %, sodium 31 %, chloride 17 % and sulfur 5 % ) systems. 3.7 Effect of hardness in dye fixation 16

In hybrid SRSTE treatment process, the concentration of the total hardness was about 500 mg/L. The presence and absence of total hardness, hybrid treatment process was carried out and recovered salts were used for dye fixation study. The presence and absence of hardness samples were named as AF1 and HAF1. Total hardness was removed from the SRSTE using sodium hydroxide and sodium carbonate before hybrid treatment process. The purity of NaCl in presence and absence of hardness was about 84% and 92% respectively. 15 and 7 % of Na2SO4 purity was

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observed in presence and absence of hardness in the recovered mixed salt. The samples (AF1 and HAF1) were taken for dyeing of cotton fabric with Ramazol dye and compared with laboratory

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grade salt. Based on the colour difference (ΔE) observation HAF1 sample showed lower ΔE value

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when compared with AF1 sample (Table 4). This observation reveals that after removal of hardness, the hybrid approach enhances the purity of NaCl and the efficiency of the dyeing also.

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4. Conclusion

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A hybrid approach, electrochemical treatment of saline tannery wastewater with halophilic bacterial pretreatment and salt recycling were investigated. The following conclusions can be

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drawn.

1. Biological degradation, electrochemical oxidation and hybrid approach had been selected to reduce COD and to recover salts from SRHSTE. The halophilic bacteria were used to degrade mixed tannery effluent and subsequently the non-biodegradable compounds were degraded by

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electrochemical oxidation.

2. Due to presence of non- biodegradable organic compounds, 100% of COD reduction was not possible in biological treatment process alone. Treating the effluent with electrochemical approach is power consuming process, it takes 24 hours to completely degrade the organics (COD). Loss of chloride was very high during electro oxidation.

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3. The hybrid approach overcomes the disadvantage of biological and the electrochemical approach. Aerobic treatment using halophilic strains reduces the sulfide formation in SRSTE. Whereas, treating the effluent by electro oxidation with biological pretreatment takes only 120 minutes. The energy consumption was lower in hybrid approach when compared to electrochemical oxidation. COD reduction during aerobic pretreatment was about 76% at 6th day. It favors the electrochemical oxidation process to break all the organics with in 120 min.

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4. Hybrid treatment process gave 98% COD and 85% TOC reduction without any significant loss of chloride and sulfate. Removal of hardness in mixed salt increases the quality of the dyeing

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application.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

CSIR-HRDG, New Delhi is gratefully acknowledged for the senior research fellowship

(SRF) of Karthikeyan Chandrasekaran. Authors are grateful to Academy of scientific and innovative research (AcSIR), CSIR-CECRI.

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Figure captions Fig. 1: SEM view of the isolated bacterial strains (a) HB1 (b) HB2 Fig. 2: (a) Halophilic bacterial consortium growth in SRSTE (b) COD reduction during aerobic treatment of SRSTE

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Fig. 3: Electrochemical oxidation (a) Hypochloride formation and (b) COD reduction Fig. 4: Integrated process (a) COD reduction and (b) hypochloride formation

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Fig. 5: HPLC spectrum of the tannery effluent after various treatment processes (a) Raw SRSTE (b) After aerobic treatment (c) After electrochemical oxidation (d) After integrated treatment process.

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Fig. 6: EDS analysis of recovered salt from SRSTE (a) Electrochemical oxidation treated (b) Integrated treatment

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Table 1: Comparison of available literature of tannery effluent treatment

Equalization tank Tannery waste water Soak liquor

Reference

[24] [25]

[26]

[21]

Soak liquor

Post tannery waste water

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Raw soak liquor

Tannery wastewater

[27] [28]

[29]

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Leather industry waste water

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Tannery waste water Tannery effluent

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Soak liquor

ClSO4-2 COD Remarks (mg/L) (mg/L) (mg/L) Electrochemical oxidation and advanced oxidation processes Electro-Fenton 6400 2810 60-70% of COD oxidation process removal efficiency Advance oxidation and 1497 1556 Maximum TOC electro oxidation removal efficiency was 86% Electrochemical 41200 1700 5800 94.8% COD treatment removal efficiency (current density of 5.8 A dm-2, and a time of 7.05 h) Biological and electrochemical oxidation / advanced oxidation processes Electrochemical and 17080 7300 Current density: 0.012 biological A cm-2; duration (EO – 30 min and biodegradation – 7 Days) 96% COD reduction Electrochemical and 35004600-6000 COD removal biological 4000 efficiency 66% Combined advanced 1360– 2100– 890–1600 COD removal oxidation and 1740 2760 efficiency 64% biological treatment Anaerobic biological 6528 2533 Duration: 30 min and Fenton oxidation Fenton’s oxidation process followed by 72 h biological treatment (93% COD removal) Biological treatment process Biological method 1500-4400 Experimental duration: (aerobic and 300 anaerobic) days Batch anaerobic 4853 Effect of COD/ digestion sulphate ratio on posttanning wastewater treatment Biological 16559 1332 Saline tannery effluent treatment by halotolerant bacterial consortia Aerobic Sequencing 6240, 4680, Using salt-tolerant batch reactor (SBR) 3220 and bacterial strains 1560 mg/L Process type

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Type of effluent

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[30]

[31]

[15]

[32]

Table 2: Physico-chemical properties of 1:1 ratio of mixed effluent (soak liquor + process effluent) Units pH 7.8 ± 0.2 Colour Straw yellow Odour Unpleasant Chloride (g/L) 21.2 ± 1 Sulphate (g/L) 19.8 ± 0.5 Sulphite (mg/L) 210 ± 5 Sulphide (mg/L) 13 ± 2 COD (mg/L) 3750 ± 10 BOD (mg/L) 1200 ± 5 Lipid (g/L) 39.6 ± 1 Protein (mg/L) 206 ± 4 Total Hardness (mg/L) 1500 ± 10 Ca Hardness (mg/L) 350 ± 5 Mg Hardness (mg/L) 1150 ± 5 Total dissolved solids (mg/L) 56000 ± 25 Conductivity ( mS/cm) 61.3 ± 5 Salinity 40.1 ± 0.5

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Parameters

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S.NO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

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Table 3: SRSTE parameters after various treatment processes

EO Units pH 8.4 ± 0.2 6.9 ± 0.4 Colour Colloidal Yellow Colourless Odour Unpleasant Odour less Chloride (g/L) 21.2 ± 0.8 0.945 ± 0.4 Sulphate (g/L) 19.8 ± 0.5 22.2 ± 0.5 Sulphite (mg/L) 19.5 ± 1 Nil Sulphide (mg/L) 1.52 ± 2 Nil COD (mg/L) 875 ± 5 <25 BOD (mg/L) 150 ± 5 50 ± 3 Lipid (g/L) 6 ± 0.1 Nil Protein (mg/L) 86 ± 5 Nil Total hardness (mg/L) 1000 ± 10 250 ± 5 Ca hardness (mg/L) 250 ± 5 50 ± 4 Mg hardness (mg/L) 750 ± 10 200 ± 5 TDS (mg/L) 58,200 ± 25 48,733 ± 50 Conductivity 58.5 mS/cm 48.73mS/cm Salinity (mg/L) 39.2 ± 2 12.7 ± 2.5

BIO-EO 7.1 ± 0.2 Yellow Odour less 19.4 ± 0.5 20.2 ± 0.6 2.8 ± 0.5 0.58 ± 0.1 <25 100 ± 5 Nil Nil 500 ± 10 150 ± 5 350 ± 5 55,500 ± 50 55.2mS/cm 37.7 ± 2

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Parameters

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Table 4: Dye fixation study in presence and absence of hardness in SRSTE after hybrid treatment process.

1.38 4

Nylon Polyester Triacetate Cotton

4 4-5 4-5 4

Dry Wet

4-5 4-5

4 4-5 4-5 4-5 4-5 4-5 4 4 Color fastness to rubbing 4-5 4-5 4 4-5

4-5 4-5 4-5 4-5

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Change in colour

HAF1 Na2SO4 0.66 4-5

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*

Delta E

AF1 HAF1 Na2SO4 NaCl 3.12 0.32 Color fastness to washing 3-4 4-5

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AF1 NaCl

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Conditions

4-5 4-5

*The Delta E value upto 0.5 is acceptable and upto 1.0 is tolerable (SITRA chemical

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Laboratory report number: G1801022)

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