7 Waste water characteristics of electroplating and tannery industries

J.S. Laura

epgp books

 

 

Electroplating Industry

 

8.1 Introduction

 

Electroplating is the process to change the surface properties of an object to make it- abrasion & wear resistance, corrosion proof, glossy and aesthetically better. In electroplating a thin protective metallic layer is deposited on to a prepared metallic surface by electrochemical process. In India, there are thousands of independently operating units both in organized and unorganized sector, which are of tiny to small scale. These units are widely spread out across the country. A large number of units are concentrated in some towns / cities, such as Hyderabad, Delhi, Ahmedabad, Faridabad, Bangalore, Mumbai, Pune, Nashik, Ludhiana, Chennai, Madurai and Noida. It is estimated that Indian electroplating is now worth Rs 2000 crores and this sector employs approximately 130,000 people in the industry in 12000 organized sectors.

 

The toxic material discharge is of main concern in electroplating industry and therefore it has been included among 17 major polluting industries in India by Central Pollution Control Board, Govt. of India.

 

 

Environmental concern of electroplating industry

 

Electroplating industry releases toxic materials and heavy metals through its wastewater effluents, air emissions and solid wastes in environment. Electroplating waste water contains high levels of heavy metals, such as Cd, Cr, Cu, Ni, As, Pb, and Zn. The solvents and vapors from hot plating baths result in elevated levels of volatile organic compounds (VOCs) and, in some cases, volatile metal compounds, which may contain chromates.Certain factors are largely responsible for extensive pollution from electroplating units, which include

  • – their sporadic distribution
  • – small-scale operation
  • – poor housekeeping
  • – lack of space for installing waste treatment facility in view of their being located mostly in areas of high commercial activity or in a composite industrial complex and
  • – the high costs of treatment of the waste water particularly for the tiny & small-scale units

Process and chemical used in electroplating

 

The various operations in electroplating industry are:

 

  1. Cleaning

 

Cleaning is important part of electroplating process. In this surface of material is prepared and cleaned for electroplating. This step determines the appearance, adhesion of electroplated deposit and anti-corrosion properties of material. In cleaning process first of all smoothening of object’s surface is done and then degreasing is carried out. Later is done using organic solvents. These operations contribute to alkali waste containing sodium hydroxide, carbonates, silicates, wetting agents and emulsifiers. Whenever cleaning is performed with organic solvents, the effluent from this operation consists of the solvents such as benzene, toluene, petrol etc. as well as emulsifiers.

 

  1. Stripping or Pickling

 

Pickling is next step; it means removal of impurities such as scales. In case of steel rust from the surface is removed in this step. This is done by mineral acids such as hydrochloric acid and sulfuric acid etc. The effluents from this step contain unused acids (e.g., HCl, H2SO4 and HNO3), impurities and ferrous sulfate.

  1. Electroplating

 

It is process of deposition of thin metallic layer on substrate. All the constituents of the plating baths contribute to the wastewater stream either through part drag-out, batch dump or floor spill. Electroplating baths may contain Cu, Ni, Ag, Zn, Cd, Cr, Sn, Pb, Fe, ammonia, etc. The anionic components likely to be present include borate, cyanide, fluoride, tartrate, phosphate, chloride, sulfide, sulfate, sulfamate, nitrate, etc. Further, many other additives to induce grain refining, deposit brightening, surface leveling, etc. are also added to the plating baths. These include Mo, Se, As, Co, saccharin, aldehydes etc., all of which contribute to the waste streams. Apart from these, contaminants like oil, grease, biodegradable mass, suspended solids etc may also be present in the wastewaters.

  1. Post –treatment

 

Usually post treatment is done to enhance the physical appearance, improve corrosion resistance of items and for decorative purposes. Different types of post treatments includes-

  1. Conversion coating: It prevents corrosion and limits growth of salts. Many heavy metals have ability to prevent corrosion and used in conversion coating like chromium, vanadium, molybdenum, tungsten etc. But hexavalent chromium is most commonly used, using its soluble salts.
  2. Phosphating: In this process metal surface is coated with layer of insoluble phosphate of metals like zinc, iron, manganese etc. It acts as excellent base for paints.
  3. Sealing: Sealing is done to improve the durability and resistance to staining and corrosion.
  4. Metal coloring: This is old process to color the metal surface, presently not used much.

 

Pollutants in waste water

 

Any or all of the substances used in electroplating can be found in the wastewater. They enter the effluents via rinsing of the product, from spillage and by dumping of process baths. The waste water from electroplating units contain acid residues, alkali residues, rinse water from pre-treatment and plating operations, spent bath containing sulphides, cyanide and toxic metals, sludge from bath containing organic solvents, phosphate sludge, etching residues, plating metal sludge, chemical sludge from wastewater treatment. All metal finishing processes tend to create pollution problems and generate wastes of varying degrees. Some processes are of high concern which uses highly toxic or carcinogenic ingredients. Pollutants released by such processes are difficult to destroy, stabilize or dispose of in an environmentally sound manner. Some of these processes are:

 

  • Cyanide based plating (especially zinc, copper, brass, bronze and silver plating)
  • Chromium plating and conversion coatings based on hexavalent chromium compounds
  • Cadmium plating
  • Lead and lead-tin plating

 

In comparison with other industries, the electroplating industry uses much less water; hence the volume of the waste water produced by this sector is also comparatively much smaller. However, the wastewater is highly toxic in nature.

 

Cyanide: The cyanide is used in plating and stripping solutions because of its ability to form weak complexes with metals typically used in plating. Cyanide plating solutions produce metal deposits of finer grain. It is advantageous over other plating from an acidic solution. Also the cyanide-based plating solutions are more tolerant to impurities than the other solutions, which offer better finishes over a wide range of conditions. Cyanide-based electroplating solutions usually operate at basic pH levels to avoid decomposition of the cyanide complex and the formation of highly toxic hydrogen cyanide gas.

 

Chromium: Chromium coatings provide excellent wear resistance and corrosion protection, as well as a bright, highly reflective surface. Two main types of chromium coatings are used-decorative and hard chrome. Decorative coatings are applied primarily for appearance purposes, while hard chrome coatings are thicker layers of chromium used to give a part extra wear resistance. Decorative coatings is almost always applied over a bright nickel plated deposit, which in turn can be easily deposited on steel, aluminum, plastic, copper alloys and zinc die castings. Hard chrome plating is normally not applied over bright nickel plating, although in some cases, nickel or other deposits are applied first to enhance corrosion resistance.

 

Traditionally, chromium deposits are produced from an electrolyte containing hexavalent chromium ions. Decorative chromium plating is produced using lower toxicity aqueous solution that contains trivalent chromium compounds. The trivalent chromium process is considered less toxic and more environmentally friendly and has lower content of chromium in plating solution.

 

 

Due to the discharge of large amounts of metals and other harmful chemicals, the electroplating industry is among the most hazardous chemical-intensive industries. Heavy metals, because of their high solubility in the aquatic environments, can be absorbed by living organisms. Once they enter the food chain, large concentrations of heavy metals may accumulate in the human body. If the metals are ingested beyond the permitted concentration, they can cause serious health disorder. Cyanides are very toxic. In order to control such pollutants various techniques of treatment of wastewater are applied. To bring the levels of pollutants to the specified limits, it is necessary to treat the effluents. Indian standards for discharge of wastewater from electroplating industry are given in the table (8.2).

 

 

Treatment of contaminants

 

There are several types of treatment methods developed for each specific contaminant. Table summarizes some common methods of treatment of the three types of contaminants.

 

  1. Treatment of Cyanide

 

  1. Chlorination: It is most widely practiced treatment method and is considered to be most suitable method for small electroplating industries. In this method sodium hypochlorite is used to oxidize the cyanide to less toxic products. The destruction of cyanide by sodium hypochlorite is accomplished in the following stages:

 

Ist stage: Cyanide oxidize to cyanogens chloride and cyanogens chloride hydrolyses to cyanate.

 

  • (a) NaCN+NaOCl+H2O → CNCl+2NaOH
  • (b) CNC1+2NaOH →NaCNO+NaCl+H20

2nd stage: Cyanate breaks down to nitrogen and carbon. 2NaCNO +2NaOH+3NaOCl→2Na2CO+3NaCl+N2+H2O Overall reaction:

 

2NaCN +5NaOCI +2NaOH → 2Na2CO+ 5NaCl+ N2+ H2O

 

The rate of destruction of cyanide is pH dependent (high pH around 11.5) and may be affected by the presence of metals. In case of large plants chlorine gas is used for degradation of cyanides.

  1. Ozonation: Similar to chlorination, the destruction of cyanide by ozone occurs in two stages as given below in reactions. In it cyanate is formed as an intermediate. NaCN+O3→NaCNO+O2

 

2NaCNO+ 3O3 + H2O → NaOH +2CO2 +N2+ 3O2 Overall reaction:

 

2NaCN+5O3+H2O → 2NaOH+2CO2+N2+5O2

 

In contrast to chlorination, strict pH control is unnecessary and consumption of chemicals can be reduced when the wastewater contains a substantial amount of ammonia. However, the initial capital costs and operating costs of this method are higher than those of chlorination.

 

  1. Ion exchange: Cyanide in wastewater can also be removed by adsorption on anionic exchange resins. The adsorbed cyanide may later be eluted off by passing regenerants (such as sodium hydroxide or sodium chloride), through the resins. The regenerated cyanide may be reused as process chemicals or treated before discharge.
  • Treatment of Chromium

 

  • Reduction of hexavalent chromium and precipitation: The commonly used method of treating hexavalent chromium is to reduce it chemically to a trivalent state and subsequently precipitate it out of solution as hydroxides. The reducing agents commonly used to carry out the above process are sodium sulphite (Na2SO3), sodium bisulphite (NaHSO), sodium metabisulphite (Na2S2O5) and ferrous sulphate (FeSO4.7H2O).
  • Cementation: In this method hexavalent chromium is reduced electrochemically by using scrap metals. When the hexavalent chromium containing electrolplating waste comes in contact with scraps of reactive metals such as iron, zinc or aluminium etc., the hexavalent chromium is reduced to trivalent chromium.
  • Precipitation as insoluble barium salt: Hexavalent chromium can be precipitated by forming insoluble barium chromate
  • CrO42- + Ba2+ → BaCrO4

 

The reaction can be carried out at pH 8-9.

 

  1. Treatment of Heavy Metal

 

For most of the heavy metals in the electroplating wastewater, precipitation as insoluble hydroxide or sulphide is most widely adopted treatment method. It is achieved by addition of simple reagents followed by pH adjustment and sedimentation.

 

  1. Removal as hydroxides: For precipitation of metals as hydroxide, each metal species has a particular optimum pH range for its maximum removal. The commonly used chemicals for pH adjustment are caustic soda and lime. Caustic soda is more expensive, but it contributes to a much smaller amount of sludge in comparison to the use of lime. Lime is cheaper to purchase but its reaction rate is slower and a considerable excess will be required if encapsulation occurs. Sedimentation of the resulting hydroxide sludge tends to be more rapid and may not require the use of a flocculant.
  1. Removal as sulphide: Precipitation of metals as hydroxide may not reduce the metal contents to a very low concentration (such as to 0.1 ppm), because of solubility of metal hydroxides. In such cases, metals may be precipitated as insoluble sulphides. The principle of sulphide precipitation is that the solubilities of metal sulphides are generally much lower than the hydroxides. Hence, by forming metal sulphides, the metal concentrations in a wastewater can be reduced to very low levels. The sulphide reagent may be added in form of soluble sodium sulphide or insoluble ferrous sulphide. However this method also suffers from some operational difficulties such as less settleable sludge and the need for control of addition of reaction chemicals.
  1. Destruction of complexes and precipitation: In some of the metal finishing operations complexing agents are used. Therefore the wastewater discharged from these operations may contain complexed metal species. The later are difficult to treat with conventional treatment methods such as hydroxide precipitation. So the complexes would have to be broken to affect the metal removal, but a ready treatment method to destroy the complexes does not exist. Depending upon the type of complex ions the measures that may be applicable for metal complex destruction are dilution, oxidation, etc.

Tanning Industry

 

8.2 Introduction

 

Tanning is the processing of animal skins and hides to produce leather. Almost 90% of the tanning capacity in India is concentrated only in four states. The major leather producing units are clustered in Tamil Nadu, West Bengal, Uttar Pradesh and Punjab. India has a large cattle population and holds 10% of the global raw material (hide and skin) accounting for 2% of the global trade. Leather industry is one of the top eight export revenue generating industries in India. India is also among world’s four major leather producing countries with Italy, China and Republic of Korea. Although this industry has been in existence for long time but the problem of environmental pollution is receiving serious consideration in recent years.

 

Tanning word comes from tannin, an acidic compound which is used in tanning. Tanning is the chemical process that converts animal hides and skin into leather and related products. This transformation is usually done by means of tanning agents. Along with tanning agents, many other pollutants are released from tanneries, which pollute the various water resources. Before coming directly to the characteristics of tannery waste water, it is necessary to understand the process of leather making, for better understanding of the concept.

 

Leather manufacturing process and inputs

 

The manufacturing of leather can be divided into two parts; beam house operations and tanning process. The steps between preserving hide and tanning are known as beam house operations. Brief explanation of steps and chemicals used in leather manufacturing is as follows.

 

Soaking: In soaking process dirt and blood is removed by washing the hides. They are then soaked in clean water for softening and removal of salts (sodium chloride) and bactericides, which were used for preservation of skin and hide.

 

Liming: Liming swells the hides for the better penetration of tanning agents. This step is to remove the keratinous material like hair and epidermis, soluble proteins and fats. Liming is done with sulphides (NaHS, Na2S) and lime. Another way of liming includes using organic compounds like mercaptans and sodium thioglycolate, with alkali and amino compounds. In this step high concentration of sodium sulphide, lime and organic matter is released to the wastewater.

 

Deliming and bating: Now to lower the pH and to reduce swelling, weak acids are used. Hides are neutralized with acid ammonium salts. This causes the impurities to flush out of skin and conditions are created suitable for the bating process.

Bating is process of removal of hair roots and pigments. It is carried out using the enzymes, which partially degrade non-collagenic proteins such as hair remnants etc. Deliming and bating results in a major part of the ammonium load in the effluent.

 

Pickling: Pickling is usually done to prepare the hides for tanning. The pH value of hides is adjusted by addition of acids (mainly sulphuric acid). Salts are added to prevent the hides from swelling.

 

Tanning: Tanning is the reaction of the collagen fibres in the hides with tannins, chromium, alum or other chemical agents. Tanning agents stabilize the skin matrix against biodegradation. Commonly used tanning agents are alums, syntans, formaldehyde, glutaraldehyde and heavy oils. During the tanning process, about 300 kg chemicals are used per ton of hides. Based on the tanning agents, tanning operations are further divided into two, vegetable tanning and chrome tanning.

 

  • Vegetable tanning is usually done in series of tanks by using natural organic substances. It is classical, traditional and eco friendly method of tanning. Natural tannins used in this process are obtained from plants. Commonly used natural tannins are obtained from Quebracho wood, Tara pod, Chinese gallut, Mimosa bark and Oak wood etc.
  • Chrome tanning is done at a higher pH using chromium salts. Majority of leather industries use this process. It makes hides resistant to bacteria and high temperature. In chrome tanning retaining, dyeing and fat liquoring are the additional steps as compared to the vegetable tanning. Fat liquoring is the process of introducing oil into the skin before the leather is dried to replace the natural oils lost in beam house and tan yard processes. After drying, a number of finishing operations like buffing, plating and embossing are carried out to make the leather softer and aesthetic. After tanning, tanned leather is piled down, wrung and graded for thickness and quality. This leather is split into flesh and grain layers and shaved to desired thickness.

 

Environmental concern

 

Tanning industry uses huge amount of water. Water consumption in various unit operations varies from one tannery to another. Large amount of waste water is generated from tanneries. The average volume of wastewater generated from different operations per ton of raw hides processed is given in table (7.1).

For every kilogram of hides processed about 30 liters of effluent is generated. Total quantity of effluent discharged by Indian industries is about 50,000 m3/day. Tanneries generate highly turbid, colored and foul smelling wastewater. The major components of the effluent include sulphide, chromium, volatile organic compounds, large quantities of solid waste, suspended solids like animal hair and trimmings etc.

 

Major water pollutants from tanneries

  1. i) Salt (NaCl): Salt is used as the curing agent. It is an important inorganic chemical that has been identified as a pollutant due to its inertness and disruption of the soil biological activities.
  2. ii) Organic Matter: Organic matter associated with tannery waste includes biodegradable organic matter (proteins, fats and carbohydrate). They cause depression in the dissolved oxygen content of stream waters caused by microbial decomposition.
  • iii) Sulphide: Main sulfides found in tannery effluent include sodium sulfide, sodium hydrosulfides and hydrogen sulfide. Sulfides are corrosive in nature and harmful for skin. Hydrogen sulfide (H2S) is a toxic gas with noxious odour. In tanneries it is mainly produced in liming yard and the anaerobic lagoons. The exposure to H2S can leads to inhibition of the cytochrome oxidase and other oxidative enzymes, resulting in anoxia or cellular hypoxia. In humans it causes keratoconjunctivitis, respiratory tract irritation and olfactory fatigue.
  1. iv) Chromium: Most widely used tanning substance is basic chromium sulphate, which is employed by 90% of the tanning industry. Chromium is a micronutrient and Chromium salts such as chromium polynicotine, chromium chloride and chromium picolinate (CrP) have been demonstrated to exhibit a significant number of health benefits in animals and humans. But difficulty arises with the toxic compounds of chromium, due to the oxidation state of chromium e.g. hexavalent chromium. About 80-90% of the world-wide tanneries use Cr (III) salts in their tanning processes. In some parts of the world, the Cr (III) is obtained from Cr (VI) species, which are a hundred times more toxic, but generally tannery effluents are unlikely to contain this form. Under certain conditions Cr (III) can change into Cr (VI). Ingestion of hexavalent chromium can cause stomach problems (like ulcers) and can damage the functions of kidney and liver. It also causes skin problems like rashes, sores and ulcers on coming in contact with skin. Conventional chrome tanning results in wastewater containing as high as 1500−3000 ppm of chromium. But, the discharge limits for trivalent chromium vary broadly ranging from 1 to 5 ppm in the case of direct discharge into water bodies and 1 to 20 mg/l in the case of discharge into the public sewer system.

 

One problem with chrome tanning is that all the chemicals dissolved in the water are not absorbed by the hide, so effluent from the tanning process contains a lot of chrome and other fixing chemicals. The mass balance of chemicals used in leather processing of hide is given in table (7.2):

 

 

(Where SS= Suspended solid, COD= Chemical oxygen demand, BOD= Biological oxygen demand-5 day, Cr= Chromium, NH3N= Ammonical nitrogen, TKN= Total kjeldah nitrogen, Cl-= Chloride, SO42-= Sulphate.)

 

 

The environmental preparedness of tanning sector varies widely in India, adopting either common or individual effluent treatment facility. The non-compliance of environmental regulations has resulted in closure of tanneries in some parts of the country.

 

Treatment of waste water generated from tanneries

 

The design of an effluent treatment plant (ETP) should be according to the requirements of a specific site and type of processes. The goal is to reduce or remove organic matter, solids, nutrients, Cr and other pollutants since each receiving body of water can only receive certain amounts of pollutants without suffering from degradation. Pollutants contained in effluent cannot disappear; they are only converted into something which is environmentally more acceptable or easier to dispose of (sludge). Therefore, each effluent treatment plant must adhere to the limits usually given by the relevant environmental authority to discharge the levels of pollutants, expressed as BOD, COD, suspended solids (SS), Cr, total dissolved solids (TDS) and others. Indian standards of discharge of effluent by tannery industry are given below in table (7.4).

Table 4: CPCB standards of discharge of effluent of tannery (in mg/l)

 

The treatment of tannery wastewater include following steps:

  1. Mechanical treatment
  2. Primary Effluent treatment
  3. Secondary treatment
  4. Post-purification, sedimentation and sludge handling Mechanical treatment

 

In this step, coarse materials are removed from raw effluent mechanically using grits, screens etc. Up to 30-40% of gross suspended solids in the raw waste stream can be removed by properly designed screens. Fats, grease and oils are removed by skimming and gravity settling. After mechanical treatment, physico-chemical treatment is usually carried out, which involves the chrome precipitation and sulphide treatment. Coagulation and flocculation are also part of this treatment to remove a substantial percentage of the COD and SS.

 

Primary effluent treatment

 

It involves the physical and chemical means to remove the settleable organic and inorganic solids by sedimentation and floating solids by skimming. For carrying out the effective effluent treatment, wastewater effluents from different steps are kept separated (especially sulphide and chrome containing liquors), because concentrated effluents are often easier and more efficient to treat. Until treatment sulphide-containing effluent from the beam house operation is kept separated at a higher pH to prevent the formation of toxic hydrogen sulphide (H2S) gas. The sulphides in the deliming and pickle liquors is oxidised in the drum by adding hydrogen peroxide, sodium metabisulphite or sodium bisulphite. But when segregation of sulphide-bearing liquors is not possible, the sulphides are generally removed by means of precipitation with iron

 

  • (II) salts and aeration. During primary treatment about 25-50% BOD, 65% of oil and grease, and 50-70% of TSS can be removed from tannery effluent.

 

Secondary treatment

 

After primary treatment the secondary treatment is done to remove the biodegradable dissolved and colloidal organic matter. For this, aerobic biological treatment processes is carried in which aerobic bacteria metabolize the organic matter in presence of oxygen in the wastewater. This way more micro-organisms are produced which further help in degradation of organic matter. This process mainly produces inorganic end products (principally CO2 and H2O). There are many different processes used for secondary treatment using aerobic microorganisms, they differ primarily the way in which oxygen is supplied to the microbes.

 

Post-purification, sedimentation and sludge handling

 

In this last step, sludge in the wastewater treatment plant is separated from the water by gravity settlement. After dewatering this sludge by means of filter presses, sludge cake with up to 40% dry solids can be achieved. An effective method of massive disposal of tannery sludge by incorporating in the clay mixture of building bricks has been tried and found successful. The tannery sludge in the range of 5-15% by weight has been added to the mixture of clays by blending and grinding. Such bricks are fired in specially modified kilns under oxidation-reduction condition; with maximum temperature in the range of 850-950ºC where black to grey coloured bricks having acceptable properties are obtained.

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References

 

  1. World Bank Group, 1998. Electroplating. Pollution Prevention and Abatement Handbook. https://www.ifc.org/wps/wcm/connect/fe6dca8048855399afbcff6a6515bb18/electroplating_PPAH.pdf?MOD=AJPERES
  2. Central Pollution Control Board, 2013. Guide to cleaner production in electroplating sector. http://cpcb.nic.in/divisionsofheadoffice/pci-ssi/Report_on_cleanerprod_electroplating_sector.pdf
  3. Central Pollution Control Board, 2007. Comprehensive industry document on electroplating industries. Comprehensive Industry Document Series (COINDS). http://cpcbenvis.nic.in/scanned%20reports/COMPREHENSIVE%20INDUSTRY%20DOCUMENT%20ON%20EL ECTROPLATING%20INDUSTRIES.pdf
  4. Environment (Protection) (Second Amendment) Rules, 2012. The Gazette of India: Extraordinary, Ministry of Environment and Forests.. http://www.cpcb.nic.in/Industry-Specific-Standards/Effluent/Rev_Stand_ElectPlate_30032012.PDF
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  8. Dargo, H., and Ayalew, A., 2014. Tannery wastewater treatment: A Review. IJETST, 1(9):1488-1494.
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