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Tannery process scheme, wastewater treatment, its impact to public health and effluent discharge

standards

BY

1. WILLIAMS NDIFREKE ETUK 201536942. MOSES FARRELL LUKA 201501683. SULAIMAN ISHAQ MUKTAR 201624184. OGBOGU TELMA 20154374

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Overview • Leather tanning is a wide common industry all over the world. • one of the most important industries in Mediterranean countries.• Due to complex waste characteristics leather tanneries are generally

located in so called organized industrial districts. • Fig a, some tanneries in Italy, a proportional distribution of the

leather tanneries in Italy and in EU countries is shown in Fig. b. The industrial processing from the raw hides to finished leather is shown in Fig. 2.

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Figure 1 Spatial distribution of leather tanneries in (a) Italy and (b) among the member states (UNIC. www.unic.itum, 2013).

5Figure 2 Overview of the tanning industry (Naturgerechte Technologien, 2002).

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Tannery processing and contaminant load• The production of fresh hides has been estimated at about 8-9 million

tonnes per year • During the processing of these hides a total of 1.4 million tonnes of

solid waste is produced• This means that in all likelihood ca 16% of the processed hides is

leather waste• puts the figures for trimmings and splittings (i.e. leather waste) at a

total of 225 kg/ton hide (i.e. ca 23%). • Tannery wastewater is slightly alkaline in nature ranging between pH

7.5-8.5. It is because of mixing of acidic liquor from pickling & chrome tanning effluent with alkaline liquor from beam house operation.

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Tannery processing and contaminant load

• The highest amount of the salt load occurs in the beam house area• Soaking is intended for the removal of the salt, impurities etc. and for

increasing the moisture content, which is lost during the curing operation. In soaking about 15% (w/w) to 40% (w/w) of common salt is used for preserving animal skins and it is removed during the soaking

• Liming and unhairing is accomplished for removal of epidermis layer and inter-fibre substances (mainly soluble protein and fats) providing the room for the tanning agent.

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Tannery processing and contaminant load• This fundamental process is conducted in paddle operation, which is followed by

another task called fleshing. Thereafter ‘Deliming & Bating’ is carried out in drum operation

• Deliming is practiced for the removal of the lime added during liming & unhairing and also for lowering the pH

• Bating, a special operation is also followed along with deliming for the production of good quality leather to be used in Bag making.

• During the liming and unhairing process, CaO and Na2S are normally used during liming-unhairing operation.

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Tannery processing and contaminant load• If a tannery is processing salted hides then the biggest salt

component in the wastewater is always the NaCl from hide and skin preservation

• Pickling & chrome tanning is accomplished after deliming and bating in the same drum. Pickling & chrome tanning is considered as the most polluting process in tanning operation due to presence of untreated Basic chrome sulfate (BCS) in the form of Cr(III).

• In tanning process about 25–30% of the total chrome used emanate through the wastewater.

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Tannery processing and contaminant load• The wet-end re-tanning, dyeing and fat liquoring processes have only a minor

impact on the total salt load dominantly originated from the hides in the initial pre-soak and main soak

• The tanning wastewater contains the highest concentration of total chromium (up to 4950 mgL−1)

• Coloring usually involves combining dyes with the tanned skin fibers to form an insoluble compound.

• Retanning and wet finishing streams are relatively low in BOD and TSS, high in COD and contain trivalent chromium, tannins, sulfonated oils and spent dyes

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Tannery processing and contaminant load

• reports the average composition of mixed effluents of leather tanneries. The average influent chromium (III) concentration varies in a wide range (30–260 mgL−1) depending on the tanning process applied.

• Generally tannery effluents are rich in nitrogen, especially organic nitrogen but very poor in phosphorous

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Sources and types of pollutants generated in leather processing(Naturgerechte Technologien, 2002)

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Sources and types of pollutants generated in leather processing(Naturgerechte Technologien, 2002)

Contd……….

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Treatment stage in Tannery industry

• Primary treatment• In order to carry out effluent treatment in the most effective manner,

flow segregation is useful to allow preliminary treatment of concentrated wastewater streams, in particular for sulphide- and chrome-containing liquors.

• Where segregation of flows is possible, thorough mixing of chrome-bearing effluents and other effluent streams improves the efficiency of the effluent treatment plant because the chromium tends to precipitate out with the protein during pretreatment the layout is shown in the Figure that follows

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Primary treatment

• This treatment stage is majorly to eliminate the coarse matter, remove almost completely Cr and sulphides, remove the major part of suspended solids, and considerably reduce the BOD and COD content which follows the following basic steps: screening (bar, self-cleaning), pumping/lifting, fine screening (Fig. 6).

• Equalization, and sulphide oxidation, chemical treatment (coagulation, flocculation), settling, sludge dewatering.

16 Flowchart of in-house segregation of streams, including chrome recycling, treatment of liming effluents and pre-treatment of mixed effluent

17Rough bar screen, operation principle (Hayelom and Adhena, 2014).

18 Layout of in-house segregation of streams, including chrome recycling and oxidation of sulphides in liming effluent (Aravindhan et al., 2004).

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Equalization – homogenization – sulphide oxidation

• The main aims here are • Homogenization of the effluent (quantity and quality); and• sulphide elimination, mostly by catalytic oxidation.• This is achieved by using mixing-cum-aeration devices such as

diffused-air systems (preferred), Venturi ejectors, and fixed or floating aerators (lately avoided due to lower efficiency and the problem of aerosols). In practice, to play it safe, the volume of the equalization tank corresponds to the total daily effluent discharge.

20Schematic view of an equalization, homogenization tank and Schematic view of the coagulation and flocculation system (Aravindhan et al., 2004).

21Simplified flowchart of the physical-chemical (primary) tannery effluent treatment (Naturgerechte Technologien, 2002).

Sludge dewatering

• The sludge drawn from the bottom of the tank is in the form of slurry with a dry-solid (DS) content of only 2-4%. For its evacuation, special pumps – usually of the Mohno type – are used.

• For further handling and disposal of sludge, it is necessary to reduce drastically the water content.

• This is usually achieved by: • (i) thickening in sludge thickeners (very much like circular clarifier); • (ii) mechanical dewatering in filter presses, belt-filter presses or decanters (centrifuges); • (iii) natural drying in sludge-drying beds.

• In addition to power and chemical requirements, the key parameter for equipment selection is the achievable dry matter content in the dewatered sludge.

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National and International Standards of Tannery Industry

• Solid and liquid wastes are the inevitable by-products of the leather industry process, which cause significant pollution unless treated prior to discharge.

• . In most cases, liquid waste is discharged into sewage systems (indirect discharge) where it undertakes full-scale treatment before being returned to the surface waters.

• . Effluent discharge standards limits are set by the authorities for compliance by industries in order to protect the environment (Table 1-2).

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Standard limits• (a) Based on standards which have been widely applied and found

generally acceptable. • (b) Set along the lines of mass-balance, whereby the quality of the

water upstream and the quality requirements of the water downstream (for industrial or drinking purposes) are determined. The difference between the two figures determines the tolerance levels at the point of discharge.

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Pollution limits for discharge of effluents into water bodies and sewersParameter Argentina

SurfaceArgentinaSewer

AustriaSurface

AustriaSewer

China Surface

ChinaSewer

NigeriaSurface

NigeriaSewer

pH 5.5 5.5-10 6.5-8.5 6.0-9.5 6.0-9.0 6.0-9.0 6-9 6-9

Temp 45 45 30 30 70-150 400 Less than 40 Less than 40

Sus Solids(mg/L)

- - 70 150 200 10 30 -

Sett.Solids (mg/L)

0.5 0.5 - - - 10 - -

BOD(mg/L) 50 200 25 - 20-100 600 50 500

COD(mg/L) 250 700 200 - 100-300 1000 - -

TDS (mg/L) - - - - - - 2000 2000

Shlphide (mg/L)

- 1000 0.1 2.0 1.0 10 0.2 -

Chlorides (mg/L)

- - - - - - 600 600

Sulhates (mg/L)

- 1000 - - - - 500 1000

Phosp (mg/L)

- - 2.0 - - - 5 10

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Contd

Parameter Argentina Surface

Argentina Sewer

AustriaSurface

AustriaSewer

China Surface

China Sewer

NigeriaSurface

NigeriaSewer

Chrome 3 (mg/L)

- - - - 1.5 2.0 less than 1 -

Chrome 6 (mg/L)

- - 0.1 0.1 0.5 0.5 Less than 1 -

Total Chrome (mg/L)

0.5 2 1.0 3.0 1.5 1.5 - -

Oil & grease (mg/L)

100 - 20 100 10-15 100 10 30

Phenol (mg/L)

0.5 - - 20 0.5 2.0 0.2 -

Detergent (mg/L)

- - - - - - 15 15

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Table 2 Comparative effluent waste from Tannery Industries from some countries

Parameter Lagos (Nigeria) Kano (Nigeria) Bangaladesh** FEPA* Permissible limit(WHO)

DO (mg/) - - - - 4.5

Sodioum (mg/L) 367.5 - 12006 - nm

Electric Conductivity

3487.5 - 42500 - -

TDS (mg/L) 3072.3 661.3 21300 2000 1200

pH 5.7 9.32 8.3 6.0-9.0 5.5-9.0

Cr (mg/L) - 1.33 10.35 (Cr3+,2) (Cr6+,0.2)

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Sulfate (mg/L) - 269.3 - - -

Nitrates (mg/L) - 171.6 - 20 -

Chlorides (mg/) - - 13.8 50 1000

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Bangladesh Tannery

• As shown in the Table 3, all the parameters analysed by the researchers highly exceeded the standard permissible limits prescribed by NEQS-2000, ISI-2000 and ISW-BDS-ECR -1997.

• For this reason, European commission considered banning leather imported from Hazaribag, because the tanneries discharge extremely hazardous chemicals in to the open environment.

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Table 3 Characteristics of Tannery Sludge in Bangladesh

Parameter Bangladesh National Standards (NEQS,2000), (ISI,2000),(ISW-BDS-ECR,1997)

pH 7.67 7.80

OM (%) 9.15 3.83

Sodium (mg/L) 6060 0.226-0.3

Calcium (mg/L) 233.50 4.51-6.0

Magnesium (mg/L) 66.15 1.126-1.5

Total Nitrogen (mg/L) 21.5 0.271-0.36

Phosphorus (mg/L) 50.15 18.1-24.0

Sulfur (mg/L) 169.5 27.1-36.0

Chromium (mg/L) 9800 0.05-0.1

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Table 4 Common pollutants, their sources and associated health effects on human

Pollutant Sources Health effectsSalt, Dust, Hair, Soot Fleshing, Dehairing, Soaking,

Washing process.Can cause cancer, aggravates respiratory and heart diseases, is toxic at high level causes coughing, irritates throat and causes chest discomfort.

Hydrogen Sulfide (H2S) Decomposition wastes, Liming, Pickling.

Has unpleasant odor, nausea, irritates and throat, if toxic at high level.

Chromium (Cr) Chrome tanning Mainly effect on skin and respiratory system, including mucous membrane irritation, sensitization, erosion, dermatitis and chrome ulceration, can also cause cancer.

Ammonia Mainly deliming, Bating. Inflame upper respiratory passages.

Sulfuric Acid Pickling process, Chrome tanning. Aggravates respiratory diseases, impairs breathing, and irritates eyes and respiratory tract.

Arsine (Arsenic) Tanning process. Breakdown red cells in blood, damage kidneys, causes jaundice

Nitrogen Oxides Tanning, Polishing, Finishing. Aggravates respiratory and heart diseases, irritates lungs, affects respiratory systems

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Alternative treatment

• The details of wastewater treatment alternatives with hierarchy are presented in the Table below.

• The economic criterion has three components which includes capital & O&M costs and land area requirement for the treatment alternative.

• The technical criterion is mostly used to judge the suitability of the treatment process.

• In the present project, factors such as BOD, COD, TDS, SS removal, permeate recovery & reject generation, total treatment time, sludge handling and operating flow capacity have been used.

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Various treatment options implemented at individual tanneries.

(Girish et al., 2011)

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Details of wastewater treatment alternatives along with hierarchies.

(Girish et al., 2011)

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References• UNIC. www.unic.itum, (2013).• Naturgerechte Technologien (2002). Treatment of Tannery Wastewater Bau- und

Wirtschaftsberatung (TBW) GmbH, Frankfurt (Germany). Infogate. 1-11 http://www.gtz.de/gate/gateid.afp

• Aravindhan, R., Madhan B., Rao, J. R., Nair, B. U., and Ramasam, T. I (2004). “Bioaccumulation of chromium from tannery wastewater: An approach for chrome recovery and reuse,” Environmental Science and Technology, American Chemical Society, 38(1):300–306

• Hayelom Dargo and Adhena Ayalew (2014). Tannery Waste Water Treatment: A Review International Journal of Emerging Trends in Science and Technology. 1(9):1488-1494

• Girish R. Pophali, Asha B. Chelani , Rita S. Dhodapkar (2011). Optimal selection of full scale tannery effluent treatment alternative using integrated AHP and GRA approach Expert Systems with Applications. 38: 10889–10895

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