Treatment of wastewater from the tannery industry

July 1, 2021 (Reading 21 mins)
Jordi Fabregas

1. Introduction: tanneries and wastewater

Leather production and trade has increased considerably in recent years. This increase in activity has led to an increase in wastewater produced by leather producers, which requires more efficient systems for the treatment of effluents generated in this sector.

TANNING is the process of treatment and transformation of the skin of various animals into LEATHER. This process avoids deterioration caused by environmental conditions and the degrading action of microbes, fungi, insects and other microscopic life forms.

The process consists of adding a series of tanning products to the hides, mainly chromium salts and/or vegetable tannins. These products penetrate the leather and fix themselves to its structure, blocking chemical and biological degradation reactions, which are enhanced by humidity, and obtaining a structure that is inert and resistant to these processes.

After the skin is separated from the slaughtered animal, it is generally treated with salt to prevent putrefaction and to preserve it until it is processed. Tanning is carried out in a sequence of stages, in which water consumption is very high, generating polluting gases, wastewater and solid waste.

What is the most effective treatment for effluents generated in tannery production?

Due to the variety of contaminants that are present in the wastewater generated in tannery production, the processes for their treatment usually combine different technologies, which may vary depending on the characterization of the wastewater and the client's objectives.

The most common process starts with a homogenization stage, followed by pretreatment by clarification with DAF systems. The clarified water is subjected to biological treatment for the separation of contaminants and, finally, to a sludge management process for which there are a variety of alternatives. In addition, membrane processes are applied when the treated water is to be reused for reincorporation into the production process.

The four main stages of tanning are:

  • Ribera: soaking, separation of the coat, dehairing and rendering.
  • Tanning: pickling and chrome tanning.
  • Post-tanning: repetition of the previous stage to improve the treatment.
  • Finishing: dyeing, drying and conditioning.

The wastes generated in the tanning processes are mainly SOLID WASTE and WASTEWATER.

Solid waste:

These wastes consist of salt, shavings, tanned leather trimmings, untreated sludge from the wastewater treatment plant, chemical and packaging residues, etc.

These are products that are NOT recoverable (with the exception of sludge) and must be disposed of in controlled landfills, which is another environmental problem.

Non-tanned by-products (raw hide trimmings, hair, wool, fats and tallow, fleshings, etc.) can be diverted to industrial applications such as the manufacture of gelatine, glues and collagen.


Water consumption is between 25 - 60 m3 per ton of fresh leather, and the yield is 500 kilograms of finished leather per ton of leather. Depending on the process, which is unique to each industry, more than 440 kilograms of chemicals can be used per ton of leather. These chemicals are dissolved in process water, which generates an effluent with a very high organic load and polluting inorganic compounds (chromium, chlorides, ammonium, sulfides, sulfates).

There are two wastewater collection and treatment options:

  • Separate treatment of wastewater generated at each stage of the production process. Each plant is specially designed according to where the wastewater comes from: riverside, tanning or finishing.
  • Mixing and homogenization of all the discharges generated in each stage and joint treatment of the overall wastewater. This is the most common option, since it involves the installation of a single water treatment plant instead of one plant per discharge.

2. Characterization of tannery wastewaters.

Typical tannery wastewater has a very high concentration of biodegradable organic load (COD) and a high nitrogen content, since the leather is made up of proteins, keratins, fats, etc. In addition, organic compounds (tanning agents, synthetics, fats, dyes, etc.) are applied in the tanning process.

Water also contains inorganic compounds (chromium, chlorides, ammonium, sulfides, sulfates, etc.), of which chromium is the most concentrated and problematic. There is also a very high content of salts (salts are used in the tanning process to preserve the leather, sulfides, chromium salt, etc.), which results in high levels of alkalinity, with a pH around 10 in the mixed and homogenized wastewater. The content of oils and fats is usually depreciable.

The characteristic composition of a homogenized wastewater from a tanning industry is as follows:

Table 1. Composition of homogenized effluents from the tanning industry. Adapted from Artiga 2005.

Parameter (mg/L)General effluentsEffluents previously subjected to hair recovery, chromium and desulfurization
COD7000 - 80005000 - 5500
DBO54000 - 45003000 - 3500
NH3200 - 250<200
SS3500 - 40002500 - 3000
Cr3+200 - 30080 - 100
S2-200 - 250<2
Cl-5000 - 60005000 - 6000

The characteristics of the various contaminants present in this wastewater can be summarized as follows:

  • Salts: salts and the high conductivity they cause are not easily eliminated. In addition, they hinder biological treatment when they are in excess.
  • Organic matter: it is found in very high concentrations, being mostly biodegradable. A properly designed biological treatment, including an effective pretreatment, can achieve the removal of up to 99% of COD.
  • Sulfides: can be removed by an oxidation process as part of the pretreatment.
  • Total nitrogen and ammoniacal nitrogen: they come from the leather and from the ammonium-based products added during the tanning process. They are removed in biological treatment by nitrification-denitrification.
  • Chromium: it is mostly found as Cr3+. It generally does not affect biological treatment, but its recovery in pretreatment is recommended. The main challenge of pretreatment design is to select an appropriate technology for chromium reduction in the effluent.

Installing different customized treatment plants for the water from each tanning stage allows better adaptation to the typical characteristics of each effluent, which facilitates its treatment and increases the recovery yield of components of interest and value, such as chromium. However, this method is not common given its high management cost compared to that of managing homogenized wastewater.

Table 2. Characterization of effluents from each stage of the tanning process. Values are shown based on tons of raw cowhide. Adapted from Artiga 2005.

Volume (m3/ton)20-251-33-8124-37
COD (kg/ton)120-16010-2015-4010145-230
BOD5 (kg/ton)40-603-75-15448-86
SS (kg/ton)70-1205-1010-20585-155
Cr3+ (kg/ton)-2-51-2-3-7
S2- (kg/ton)8-12---8-12
Nitrogen (kg/ton)10-20-1-2-11-22
Chlorides (kg/ton)120-20050-605-10-175-270
Sulfates (kg/ton)5-2030-5010-40-45-110

3. Effective treatment of tannery wastewater.

The processes most commonly used in the treatment of tannery water are PHYSICAL-CHEMICAL PROCESSES and BIOLOGICAL TREATMENTS. MEMBRANE TECHNOLOGIES are also common nowadays, as they allow the reuse of treated water in the process.

The inorganic compounds present in the wastewater can have toxic and inhibitory effects for the microorganisms of the biological treatment, for which reason a physical-chemical pretreatment is carried out for their elimination.

When effluents are not separated and the mixed and homogenized wastewater is to be treated, the following points must be taken into account during the design of the treatment process:

  • High chromium content (mostly as Cr3+ although it can occasionally be found as Cr6+). This reduces the possibility of reusing and revaluing the sludge generated in biological treatment.
  • High concentration of organic load. If this load is too high and the biological treatment does not reach the required performance, a post-treatment stage has to be installed (membrane technologies are generally applied).
  • High sulfate content. This makes the application of anaerobic biological treatment not recommended. An alternative is the application of desulfurization technologies during pretreatment.
  • High solids content. Proper pretreatment design can significantly reduce solids. Coagulation-flocculation and primary clarification processes (decanters or DAF disolved air flotation equipment) are highly effective in removing solids. An efficient pretreatment design can protect the equipment against these solids and considerably reduce the volume of influent to the biological treatment, resulting in savings in equipment, space and energy.

If, in addition to wastewater treatment, it is desired to reuse the treated water, the following points should be considered:

  • Chromium recovery during pretreatment. A properly designed pretreatment for this purpose can remove 95-100% of chromium from the water. If the chromium is removed, the sludge generated during biological treatment can be applied as fertilizer.
  • The installation of an anaerobic biological treatment allows the generation of biogas. If this type of treatment is to be installed, sulfides and sulfates must first be removed. After the anaerobic treatment, an aeration treatment must be installed to achieve an organic load removal that complies with the discharge limits.

The PHYSICAL-CHEMICAL PROCESS consists of treating the homogenized effluent by adding precipitating agents for chromium removal(sodium hydroxide NaOH), coagulants(iron chloride FeCl3 or aluminum salts) and flocculants(polyelectrolytes).

3.1. Physical-chemical treatments for pretreatment

Pretreatment of wastewater from the tanning industry is carried out by physicochemical processes, which remove solids, sulfides, chromium, etc. and improve the efficiency of the subsequent biological treatment.

The most commonly used operations are as follows:

3.1.1. Homegeneization

Uniform mixing of the different wastewater streams generated at each stage of the tanning process. This allows to obtain a constant flow rate and concentrations of the influent to the treatment.

3.1.2. Roughing

Physical separation of larger coarse solids using screens and sieves.

3.1.3. Sulfide removal

Air oxidation or other more powerful oxidants are applied. Air oxidation, which is carried out at pH around 11, is slow and requires a catalyst for acceleration. Sulfide removal efficiencies can be achieved with concentrations below 1 mg/L in the effluent. Other agents applied as strong oxidizers are: hypochlorous acid HClO, chlorine Cl2, hydrogen peroxide H2O2 and ozone O3.

3.1.4. Coagulation-flocculation

In addition to naturally settling solid particles, there are also colloidal particles and particles that do not settle easily, since due to their electrostatic charge they repel each other and remain in suspension. In these cases, coagulants are applied to destabilize this charge so that the suspended and colloidal particles can precipitate.

Occasionally, a polyelectrolyte can be applied as a flocculant to agglomerate these precipitated particles and form flocs of higher sedimentability or buoyancy, which will optimize the subsequent clarification stage. In addition, this precipitation allows the reduction of chromium, sulfates, and organic matter associated with the separated solids, which is slowly biodegradable material that hinders biological treatment.

The percentages of elimination that can be achieved are as follows:

  • >60% COD
  • >55% of BOD5
  • >30% sulfates
  • >95% chromium.

The most commonly applied coagulating and precipitating agents are aluminum and iron salts, mainly iron chloride FeCl3, aluminum chloride AlCl3 and aluminum sulfate Al2(SO4)3, and precipitating bases such as calcium hydroxide Ca(OH)2 or sodium hydroxide NaOH. Chromium removal with these precipitating and coagulating agents requires a controlled pH around 7.5.

In some tanning processes, chromium is not applied, but tannins are applied instead. These substances are difficult to biodegrade and are eliminated in the pretreatment by precipitation with aluminum or iron salts, which can reduce the COD by approximately 50%.

3.1.5. Separation of solids by flotation

Dissolved air flotation (DAF) systems are widely used in the pretreatment of wastewater from the tanning industry.

In a DAF unit, the flotation and sedimentation of the particles and flocs generated in the coagulation-flocculation process are carried out together.

The clarified water is pressurized and saturated with dissolved air. When this saturated stream enters the flotation chamber, depressurization occurs and micro-bubbles of air are generated that carry to the surface the particles and flocs that could not settle or float. The particles that have sufficient sedimentability are accumulated at the bottom of the DAF unit.

This equipment allows extracting floated and settled sludge with very high solids content (up to 5%) and obtaining a high quality clarified product.

The sludge generated in a DAF plant is sent to a dewatering treatment for which several techniques are available: centrifugation, pressure filtration, thermal drying or vacuum filtration.

There are also decanting units that allow the separation of settleable solids; these units do not separate solids that cannot settle and take up more space than DAF clarifiers.

3.1.6. Alternative Pretreatments

There are other technologies capable of achieving considerable yields in the removal of chromium, sulfides and solids. However, these technologies are less applied due to their high cost: ion exchange, activated carbon adsorption...

Combined application of coagulation-flocculation and DAF flotation: technology offered by Sigmadaf

This sequence allows the separation of contaminants by coagulation-flocculation and the subsequent removal of flocs and flocculent solids by the application of a DAF system.

The flocs formed in the coagulation-flocculation process are ideally sized to be separated from the water in a dissolved air flotation unit. The DAF technology developed by Sigmadaf combines the principles of dissolved air flotation and sedimentation with optimal equipment design.

SIGMA offers several designs for the coagulation-flocculation process: a stirred tank process or the SIGMA PFL continuous process.

SIGMA DAF technology is an efficient and robust separation process for oils, fats, greases, colloids, ions, macromolecules, microorganisms and fibers.

The coagulation-flocculation sequence followed by DAF is a widely used process in the treatment of wastewater from the tanning industry. This solution has demonstrated high performance and cost efficiency, both operational, chemical and energy consumption.

In a DAF system, compressed air is introduced into a recirculation stream of the clarified product. This air dissolves in the liquid medium and subsequently generates 30 to 50 µm bubbles as it is released through a dispersion head in the flotation chamber. The coagulated and flocculated particles adhere to the bubbles and float to the top of the DAF unit, where they are removed mechanically.

The settleable matter descends to the sediment compartment at the bottom of the DAF unit and is discharged through a sludge extraction system, usually a screw conveyor.

Sigmadaf also offers equipment for the treatment of collected sludge.

The clarified water leaves the DAF unit through an adjustable supernatant system. Part of this clarified water stream will be redirected by the recirculation pump to enter the compression and air saturation system.

Sigmadaf offers a wide range of DAF flotation equipment, which can be customized according to the flow rate to be treated and space requirements. We manufacture equipment that can treat flows from 5 m3/h to 1,000 m3/h. The treatment capacity of Sigmadaf equipment covers contaminant load ranges up to 40 kg of solids per flotation surface. Additionally, we manufacture compact "plug&play" equipment.

These are our main models:

  • SIGMA DAF FPAC: flow rates between 5 and 160 m3/h with very high solids load. It is a high performance cross flow system. 
Figure 1. SIGMA DAF FPAC equipment.

SIGMA DAF FPBC: flow rates between 10 and 250 m3/h containing low to medium solids loads. The equipment applies countercurrent flow and offers high throughput.

Figure 2. SIGMA DAF FPBC equipment.

SIGMA DAF FPHF: flow rates between 200 and 1000 m3/h with high suspended solids content. A combination of counterflow and crossflow is used to provide optimum performance.

Figure 3. SIGMA DAF FPHF equipment.

COMPACT DAF: compact equipment installed in skid, including the coagulation-flocculation system and the DAF, saving space and energy consumption.

Figure 4. SIGMA COMPACT DAF equipment.

The advantages of SIGMA DAF systems include:

  •  High quality of treated water.
  • Fast start-up.
  • High-speed operation.
  • Thicker sludge(lower sludge volume production).
  • Footprint reduced.
  • Easy to operate.
  • Well-known and robust technology.
  • Simple, adaptable and efficient control systems.

The analysis of the wastewater, applying Jar-Test tests, is fundamental to decide which isthe best combination of chemicals for the coagulation-flocculation and pH adjustment stages.

At Sigmadaf we carry out these analyses, as a previous phase to the design of the coagulation-flocculation processes, pH adjustment and the construction of the DAF equipment.

3.2. Biological treatment

Currently, most of the plants are based on BIOLOGICAL TREATMENT, which is carried out after homogenization and physical-chemical pre-treatment. Biological treatment is highly efficient, since almost all the organic matter present in the wastewater is biodegradable.

Wastewater from tanning processes presents very easily biodegradable organic matter, so biological treatment will be highly effective as long as the pretreatment has been properly designed for the elimination of the inhibiting agents described above.

Aerobic or anaerobic treatments can be used.

3.2.1. Aerobic Systems

The most widely applied is a simple activated sludge system, which consists of a sequence of anoxic (absence of oxygen) and aerobic (introduction of oxygen) reactors that carry out, through biological reactions performed by microorganisms, the removal of COD and nitrogen. This treatment can be designed continuously in one or several reactors, or sequentially as an SBR (sequencing batch reactor) system.

Figure 6. Operating stages of an SBR biological reactor for COD and nitrogen removal.

3.2.2. Anaerobic systems

Anaerobic systems can withstand very high concentrations of COD and BOD5 and also allow the generation of biogas, which is a high-energy gas resulting from the biological process of eliminating biodegradable organic matter in the absence of oxygen.

The most commonly used reactors in tannery wastewater treatment are the UASB ('Upflow anaerobic sludge blanket' orupflow anaerobicsludge blanket reactor). These reactors are compact, high performance units where COD removal, biogas collection and solids separation are carried out in a single unit.

The main disadvantages of anaerobic systems are their high sensitivity to toxic substances and that COD reduction does not exceed 60%, requiring a very well designed pretreatment and subsequent aerobic biological treatment.

The following table shows the overall removal rates resulting from the application of different sets of technologies for the treatment of wastewater from the tanning industry:

Table 3. Estimated yields for tanning industry wastewater according to the treatment applied. Adapted from European IPPC Bureau.

Fat flotation20-40-----
Sulfide oxidation10---90-
Chromium precipitation---90-99--
Pre-treatment combinations
Homogenization + sedimentation25-3525-3550-7020-30-25-35
Homog. + physical-chemical + sedimentation50-6550-6580-9020-502-1040-50
Homog. + physical-chemical + flotation*.55-7555-7580-9520-502-540-50
Biological treatment (pre-treatment + ...)
... aeration85-9590-9790-98--50
... anoxia - aeration85-9590-9790-98--80-90
... anaerobic UASB65-7560-7050-80--20-30

3.3. Sludge management and treatment

Sludge from tanning industry wastewater contains 60-70% organic matter and 3-5% nitrogen, with a negligible potassium content.

Prior to treatment, the sludge must be subjected to dewatering to reduce its volume and facilitate its transport. Equipment such as:

  • Belt filters
  • Filter presses
  • Centrifugal decanters

Where salts and/or polyelectrolytes are applied to condition the sludge.

Several processes are used for the final treatment of sludge:

  • Anaerobic digestion: it also allows obtaining biogas together with the digested sludge.
  • Agricultural application: direct application is allowed as long as the legal restrictions on pesticides, pathogens, heavy metals and other contaminants are complied with.
  • Aerobic composting: generation of organic compost, which must also meet legal requirements.
  • Thermal treatment: incineration, gasification or pyrolysis can be applied. These methods allow energy recovery.

3.4. Advanced treatments with membranes

Typical pretreatment and biological treatment processes are effective in removing sulfides, chromium, organic load, nitrogen and suspended solids, but the effluent still contains large amounts of salts and dissolved solids (sodium Na+, chlorine Cl-, sulfate SO42-, calcium Ca2+, magnesium Mg2+) and recalcitrant impurities, so it is not possible to reuse the treated water if these contaminants are not removed.

In industries that consume high volumes of water, it is increasingly necessary to implement a CIRCULAR MODEL, which allows the reuse of treated wastewater in the process. The technologies that allow the elimination of these pollutants and the reuse of water are FILTRATION MEMBRANES.

Membrane technology can be included in the treatment of wastewater from the tanning industry in two different ways:

  • Applying filtration membranes as post-treatment.
  • Applying MBR ('membrane bio-reactor') membrane bioreactors

3.4.1. Filter membranes as post-treatment

There are four types of membranes which, ordered according to their pore size, are: Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF) and Reverse Osmosis (RO). Figure 17 shows this classification according to pore size (in nm) and according to the contaminants that each type is capable of retaining.

  • MF microfiltration: MF membranes remove colloids, suspended solids, bacteria and viruses. They are generally applied as pretreatment for other UF, NF or RO membranes. They are usually not applied alone, as their effectiveness is low for tannery wastewater requirements.
  • UF Ultrafiltration: a selective filtration process by applying pressure of up to approximately 10 bar. These membranes remove high molecular weight organic macromolecules and particles, allowing the fractionation of organic matter. The effectiveness of a UF membrane is highly dependent on the type of material of which the membrane is composed. UF membranes are generally applied as a pre-filtration to optimize RO processes and prevent clogging. They are the most commonly installed membranes in MBR systems.
  • NF nanofiltration: NF membranes are applied for the removal of recalcitrant organic matter and heavy metals. These membranes generate very little volume of concentrate. NF membranes require careful control to avoid clogging.
  • Reverse Osmosis RO: RO is a highly efficient technique that applies pressure for wastewater purification. A RO process allows concentrating all dissolved solids, trace organic compounds, heavy metals and monovalent ions.

Reverse osmosis membranes, in combination with ultrafiltration or nanofiltration, achieve removal efficiencies of about 95% of dissolved solids, 94% of Na+ and Cl ions, 98% of sulfates, 65% of Mg2+ and 55% of Ca2+.

The main disadvantages of a reverse osmosis system are its high tendency to clog, which makes pretreatment necessary, and the generation of a large volume of concentrate that has to be properly treated.

The biggest challenge in the application of membrane technologies is to achieve a good efficiency/cost balance and to overcome clogging problems. The selection of products and cleaning methods is very important when designing and installing a membrane system that optimizes both operational cost and performance.

3.4.2. MBR Membrane Bioreactors

This technology combines in a single system ultrafiltration membranes with a biological treatment, which acts as a biomass retention medium.

The membranes act as a separator of the sludge generated in the reactor, which means that it is not necessary to install a subsequent clarifier, and also prevent the loss of biomass (loss of nitrifying microorganisms and microorganisms capable of degrading slowly biodegradable organic matter), achieving very high biomass concentrations inside the reactor.

These reactors are equipment that achieve high biomass concentration, very low sludge production and high pollutant removal performance. In addition, they allow for flexible design and operation. After an MBR reactor, it is very common to install reverse osmosis membrane units that act as post-treatment.

The main disadvantage of these systems is the fouling of the membranes, so a carefully designed cleaning system is necessary.

MBR configurations: a) external membrane module and b) internal or submerged membrane module. Adapted from Artiga 2005.

Membrane technology offered by SIGMA.

SIGMA offers its membrane technology for the design and installation of Ultrafiltration, Reverse Osmosis and MBR reactors.

Photo of the MBR (Membrane Biological Reactors) membranes installed by SIGMA for the biological treatment of industrial wastewater.
Figure 10. MBR membranes installed by SIGMA.

SIGMA DAF SMBR reactors offer the following advantages:

  • They are a PLUG&LAY solution.
  • They offer maximum reliability and durability.
  • They allow to obtain a constant effluent quality.
  • It is a compact plant that allows the modular addition of ultrafiltration membranes.
  • Operation and control are simple and robust.
  • They are highly resistant to oxidizing agents.

SIGMA DAF SMBR plants are specifically designed for each wastewater stream to be treated. Flow rates range from 20 to 100 m3/day.

Figure 11. SIGMA SMBR compact equipment.

SIGMA also offers equipment for the treatment of sludge generated in the reactor.

4. SIGMA success stories in tannery water treatment.

The following is a case of success in the installation of one of our plants for the pretreatment of wastewater from the tanning industry.

Water originMixed wastewater from footwear tanning process
TargetCompliance with the legal limits for discharge to the sewage network, which will be directed to a WWTP and the possibility of planning for reuse of the effluent. Integral sludge treatment.
Flow rate200 m3/day
Influent characteristics- pH = 3
- TSS = 1000 mg/L
- COD = 3000 mg/L
- BOD5 = 1500 mg/L
- Total nitrogen = 100 mg/L
- Chromium Cr3+ = 140 mg/L
Installed equipment- Roughing screens
- Homogenization tank
- Preparation and dosing system for coagulant (FeCl3), flocculant (polyelectrolyte) and pH control (NaOH)
- Coagulation, flocculation and pH control tank system
- SIGMA DAF FPAC-40 Clarifier
- Integral sludge treatment system: adaptation and dewatering by filter press, preparation and dosing of lime slurry (Ca(OH)2)
Performance> 90% in SST
> 67% in COD
> 67% in BOD5
> 50% in total nitrogen
> 98% Cr3+ chromium
Sludge dry matter = 35%.

5. References

Alvarez S.G., Maldonado M., Gerth A., Kuschk P. Characterization of Tannery Wastewater and Study of Aquatic Lily in Chromium Recovery. Cite Journals.

Artiga P. 2005. Contribution to the improvement of the biological treatment of wastewater from the tanning industry. Doctoral Thesis Report University of Santiago de Compostela, Department of Chemical Engineering.

Bernardino S. 2019. Production of biogas/bioSNG from anaerobic pretreatment of milk-processing wastewater. Chapter in 'Substitute Natural Gas from Waste. 397 - 424.

Córdova H.M., Vargas R., Cesare M.F., Flores L., Visitación L. 2014. Treatment of wastewater from traditional and alternative tanning process using chromium complexing agents. Journal of the Chemical Society of Peru. 80(3), 183-191.

Minimization Study. Sector: Tanneries. Ministry of Public Works, Transport and Environment. General Subdirectorate of Waste.

European IPPC Bureau.

Song Z., Williams C.J., Edyvean R.G.J. 2003. Treatment of tannery wastewater by chemical coagulation. Desalination. 164, 249-259.

Suárez A.F., Agudelo R.N. 2014. Treatment of wastewater from the tannery industry by subsurface wetlands using Zantadeschia Aethiopica. Advances Engineering Research. 11(1), 121-126.

Suthanthararajan R., Ravindranath E., Chitra K., Umamaheswari B., Ramesh T., Rajamani S. 2003. Membrane application for recovery and reuse of water from treated tannery wastewater. Desalination. 164, 151-156.

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