Centra l Env i ronmenta l Authori ty , Parisara Mavvatha

Maligawat ta New Town *• Co lombo 10 SRI LANKA. '

T e l e p h o n e No: 449455/6, 437487/8/9 Fax No: 01-446749

ISBN 955-9012-03-7 r s 100.00

PfWXTEDBV UNITED MERCIANS LTD COLOMBO-13

GOVERNMENT OF THE DEMOCRATIC SOCIALIST REPUBLIC OF SRI LANKA

I N D U S T R I A L P O L L U T I O N C O N T R O L G U I D E L I N E S

No. 4 — Leather Industry

C E N T R A L E N V I R O N M E N T A L A U T H O R I T Y Ministry of Environment and Parliamentary Affairs

INDUSTRIAL POLLUTION CONTROL GUIDELINES

No 4 Leather Industry

CEA Library

04428

Prepared by the Central Environmental Authority With Technical Assistance from The Government of the Netherlands

1992/93

First edition 1992

0044-: G.I

Published by the Central Environmental Authority Parisara Mawatha Maligawatta New Town Colombo 10 SRI LANKA

Telephone No:449455/6, 437487/8/9, 439073/4/5/6 Fax No: 01-446749

This document may be reproduced in full or in part with due acknowledgement to the Central Environmental Authority

ISBN 955-9012-03-7

Front Cover Design & concept by W A D D Wijesooriya Artwork by Somasiri Herath

024/waddw/guide4

PREFACE

The Government of Sri Lanka is promoting rapid industrialization in order to create better employment opportunities for the growing work force of the country, and to increase the income level of the people. At the same time the Government is conscious of the fact that some of the existing industries significantly contribute to the deterioration of the quality of the environment in the country, especially in the urbanised and industrialized areas. Ill-planned industrialization will no doubt accelerate the process of environmental degradation.

The Government has, therefore, introduced environmental legislation to enhance environmental protection and pollution control. The Central Environmental Authority (CEA) is the lead agency in the implementation and enforcement of the environmental legislation. It has initiated various programmes for the protection of the environment, with special attention on industrial pollution control.

The CEA has requested technical and financial assistance from the Government of The Netherlands for a number of projects in this field.

As a result technical assistance for a programme which consist of the following projects was provided by the Government of The Netherlands:-

1. Development of environmental quality standards on the basis of designated uses .

2 . Development and updating of emission/discharge standards and pollution control guidelines for selected priority industries

3 . Feasibility studies on pollution control for priority industries or industrial sectors

4 . Study tour to The Netherlands for Sri Lankan officers involved in compliance procedures in the Environmental Protection Licensing Scheme (enforcement)

Under project No 2 above, industrial pollution control guidelines, were prepared for the following eight(8) industrial sectors, considered as major polluters in Sri Lanka:-

1. Natural Rubber Industry 2 . Concentrated Latex Industry 3 . Desiccated Coconut Industry 4 . Leather Industry 5. Dairy Industry 6. Textile Processing Industry 7. Pesticide Formulating Industry 8. Metal Finishing Industry

The main objective of the preparation of these guidelines w a s to assist the Central Environmental Authority in industrial pollution control with special reference to the introduction of the Environmental Protection Licensing Scheme.

In the preparation of these guidelines attention was focused on the generation of liquid, gaseous and solid wastes and their impacts on the environment. In the process aspects of industrial counselling, including in-plant measures to prevent and reduce waste generation and measures to improve occupational safety and health were also considered. Alternative methods were discussed for end-of-pipe treatment of liquid . gaseous and solid wastes .

Existing wastewater discharge quality standards were considered and intermediate standards ( with respect to the phased installation of treatment systems) were proposed in these guidelines.

The guidelines weretnainly prepared on the basis of data available on industrial pollution and its abatement in sri Lanka, from studies and reviews carried out in the past and from missions to Sri Lanka specifically carried out for preparation of these guidelines.

The project was directed by Mr K G D Bandaratilake. Director of the Environmental Protection Division of the CEA, and coordinated by Mr W A D D Wijesooriya, Senior Environmental Scientist of the CEA. The CEA project team consisted of Mr C K Amaratunga and Mr S Seneviratne, Environmental Officers.

Technical assistance was given by a team of BKH Consulting Engineers, Comprises Dr I van der Putte (team leader), Mr J G Bruins and Mr H J F Creemers.

This document contains pollution control guidelines for Leather Industry.

G K Amaratunga Chairman Central Environmental Authority

Contents Page

1. INTRODUCTION 1

2. PRODUCTION DATA 2

2.1 Production volumes 2 2.2 Production processes 2

3. WASTE PRODUCTION AND ENVIRONMENTAL IMPACTS 8

3.1 Introduction 8 3.2 Wastewater production 8 3.3 Solid wastes 9 3.4 Air pollution 10 3.5 Environmental impacts 10

4. INDUSTRIAL COUNSELLING 11

4.1 Introduction 11 4.2 In plant waste reduction 11 4.3 Waste utilization 12 4.4 Improvement of occupational safety and health 12

5. POLLUTION ABATEMENT METHODS 13

5.1 Wastewater treatment alternatives 13 5.2 Wastewater treatment system selection 17 5.3 Existing tannery wastewater treatment plant 17 5.4 Air pollution control 18 5.5 Solid waste disposal 18

6. DISCHARGE AND EMISSION STANDARDS 20

6.1 Existing discharge standards 20 6.2 Proposed discharge standards 20

7. REFERENCES 22

INTRODUCTION

In Sri Lanka 13 tanneries are manufacturing leather from raw hides. All tanneries are located in the Colombo and Gampaha districts. The production capacity of the tanneries varies between 300 and 2,000 tonnes of raw hides per year.

Three of the tanneries are relatively large plants (production capacity higher than 6 tons of raw hides per day), equipped with modern manufacturing equipment. They mainly use the chromium process, but the vegetable tanning process is used for the tanning of buffalo hides on a small scale.

A few of the small tanneries in the Colombo area (Rajagiriya) are being closed down. A new large tannery will be set up near Chilaw.

The tanning processes may cause serious environmental pollution due to the discharge of heavily polluted wastewater, the generation of solid wastes and malodorous emissions from raw hides storage, production processes and waste disposal facilities. Occupational hearth and safety in tanneries are affected by the generation of large quantities of polluted water and solid wastes, the emission of bad odours and dust, and by the handling of toxic chemicals and potentially dangerous machinery.

The environmental problems related to the tanneries in Sri Lanka consist mainly of river water pollution and uncontrolled dumping of solid wastes, especially in the Colombo area.

This document describes a number of general guidelines for pollution control in the tanning industries, including in-plant pollution control measures, improvement of occupational safety and health, prevention of malodorous emissions, wastewater treatment and solid waste disposal methods.

1

2. PRODUCTION DATA

2.1 Production volumes

Thirteen tanneries are presently in operation in Sri Lanka. It is expected that this number will change, since some smaller tanneries are being closed and the construction of a new large tannery (production capacity 1 5 - 2 0 tonnes of hide per day) is underway.

The largest tannery is the Ceylon Leather Products Corporation plant with a production capacity of 10 tonnes hide per day. The second largest tannery is the Tan Lanka plant (production capacity 6 tonnes hide per day).

No accurate data on production quantities of leather in Sri Lanka are available.

2.2 Production processes

Both chromium and vegetable tanning processes are used in the Sri Lankan tanneries. It is estimated that more than 80% of the leather produced, is manufactured by the chromium tanning process.

Both types of tanning processes are described below using data from the Ceylon Leather Products Corporation (CLPC) tannery.

In the CLPC tannery 85% of the hides (cow and goat) is processed by the chromium tanning process, and 15% (buffalo hides) by the vegetable tanning process.

2.2.1 Chromium tanning process

The tannery obtains salted hides and converts these into leather in the following steps. The hides are first weighed (green weight).

a) Soaking Water is added to the hides (2 m 3/t) in a rotating drum, to rehydrate the hides and to remove sand, dirt, blood and salt. After two hours the excess water is drained off.

b) Liming Chemicals are added to loosen wool, hair and flesh, so that these are easier to remove. 2 m 3 water, 60 kg lime (Ca(OH)2), 12 kg sodiumsulphide (Neu,S) and 5 kg sodiumbisulphide (NaHS) are added per tonne of hides. After 24 hours the excess water is drained off and the hides washed.

c) Fleshing The hides are taken out of the drum and any flesh is removed by a machine.

d) Trimming The hides are checked for blemishes and bad spots are trimmed off manually.

2

e) Splitting

The hides are split mechanically into two layers one of which, the corium, is processed into leather. The corium layers are weighed (pelt weight). Usually the pelt weight is equal to 75% of the green weight. After splitting, the hides are inspected. Good quality skins are chromium tanned, poorer quality skins are vegetable tanned.

f) Deliming

The hides are placed into a rotating drum containing water, and the lime is washed out. Ammonium sulphate ((NH 4) 2S0 4) is then added to promote lime removal. 20 kg (NH 4) 2S0 4 and 1 m 3 water is added per tonne of hide (pelt weight). Excess water is drained off after half an hour.

g) Bating

Proteins in the hide are degraded by adding an enzyme. 2.5 kg of enzyme (Pancreol 2 AS) is added per tonne of hide. After half an hour the hides are washed and excess water is drained off.

h) Pickling

Salt and acid are added in order to halt the bating process, sterilize the hides and make them receptive to fixation of the tanning agents. 0.5 m 3 water, 60 kg salt (NaCI), 4 kg formic acid (H 2C0 2) and 5 kg sulphuric acid (H 2S0 4) are added per tonne of hide.

i) Chromium tanning

The tanning agent is added after one hour of pickling. 60 kg Baychrom F is used per tonne hides. This is a self-basifying chromium powder, consisting of 30% chromiumoxide (Cr 2OJ. After 9 hours the "wet blue" leather is taken out of the drum.

k) Sammying/cutting/sorting

The leather is then passed through pressurized rollers to remove excess moisture and cut into regular sizes and sorted into various types of leather according to grain quality.

I) Shaving

The leather is shaved to a uniform thickness and weighed (shave weight),

m) Neutralizing

The leather is placed in a rotating drum and water is added, at a rate of 2 m 3 / t of leather (shave weight). The leather is washed for 10 minutes and the water drained off. Then sodium formiate (NaHCOJ is added (15 kg/t leather) and water (1 m 3 / t leather). After 40 minutes excess water is drained off.

n) Retanning

The chemicals used for retanning vary and depend on the quality of leather required. An example of a combination of chemicals for retanning is, per tonne of leather: 30 kg retanning resin, 50 kg mimosa (vegetable tanning agent) and 20 kg aldehyde resin.

3

o) Dyeing After one hour of retanning, dyes are added (10 kg anionic dyes per tonne leather).

p) Fat liquoring

35 kg sulphonated natural oil or synthetic oil are added per tonne leather,

q) Drying The leather is taken out of the drum and hang-dried for one night. Then the leather is sammied and stretched. The leather is further dried by vacuum drying or by toggle drying.

r) Trimming, staking, buffing The edges of the leather are trimmed manually. The leather is staked mechanically to soften the leather. Then the leather is buffed and undusted mechanically.

s) Impregnation The leather is impregnated by manual patting with a mixture consisting of 100 parts acrylic resin, 100 parts isopropylalcohol and 300 parts water. 100 g of this mixture is applied per m 2. The leather is then hang-dried overnight.

t) Hot plating

The leather is pressed by a hot plate.

u) Rebuffing and undusting

The leather is buffed and undusted.

v) Colouring A colouring mixture consisting of 100 parts pigments, 125 parts acrylic dispersion, 25 parts casein and 100-150 parts water is applied in manual padding. 70 g per m* is applied in a first coat. 30 g per m 2 is applied in a second coat. A coat of 100 g per m 2 is then applied by machine spraying. Finally a top coat of nitrocellulose emulsion is applied at a rate of 70 g per m 2 .

w) Drying, hot plating The leather is dried by hot air and pressed on a hot plate.

x) Measuring and grading

The surface of the leather is measured and graded into different qualities, and stored.

A schematic lay-out of the chromium tanning process in the CLPC tannery is shown in Figure 2.1.

2.2.2 Vegetable tanning

Processes a) to e) are similar to Chromium Tanning.

f) Deliming

The hides are washed with water using 1 m 3 / t hides. Chemicals are then added, 15 kg (NH 4) 2S0 4 and 5 kg NaHS0 3 per tonne of hide.

g) Bating

5 kg Pancreol 2 AS is added per tonne of hide. After half an hour the hides are washed and excess water is drained off.

h) Pickling

0.5 m 3 water, 60 kg NaCI, 5 kg H 2 C0 2 and 50 kg phenol condensated pretanning agent are added per tonne of hide. After 2 hours excess water is drained off.

i) Vegetable tanning

300 kg mimosa vegetable tanning powder, 50 kg disperging synthetic tanning agent, 5 kg antiseptic and 1 m 3 water are added per t hides. The hides are allowed to rotate for 8-9 hours. The tanning liquors are collected and recycled.

j) Washing

The hides are washed in 20 kg bleaching syntan and 10 kg fat liquors per tonne of hide. After 2 hours the hides are taken out of the drum.

k) Sammying, spreading, drying

The leather is sammied, spread out and hand dried.

I) Dewrinkling and rolling

Wrinkles are removed from the leather and the leather is pressure rolled.

m) Weighing and storing

The leather is weighed and stored.

A schematic lay-out of the vegatable tanning process in the CLPC tannery is shown in Figure 2.2.

5

Finished leather

Raw hides

Bufflng-

Staklng-

Crading

Measuring

Hotplating

Drying

Colouring

Rebuffing

Hotplating

-Trimming-

Soaking

Fleshing

Trimming 1 Liming

Splitting

i Sammying

Sorting

-Drying-

-Deliming

Bating

Pickling

-Chromium tanning

Shaving —Neutralizing

Retanning

Dying

Fat liquoring

Figure 2.1 Schematic lay-out of chromium tanning processes in the CLPC tannery (Scale 1 : 400 for building)

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Chromium tanning of goatskins

finishing (in different building)

hides (from chromium tannery after splitting)

drying -

deliming I

bating i

pickling J

vegetable tanning-

sammylng

Figure 2.2 Schematic lay-out of vegetable tanning processes in the CLPC tannery (Scale 1 : 250 for building)

3. WASTE PRODUCTION AND ENVIRONMENTAL IMPACTS

3.1 Introduction

The tanning processes generate large quantities of liquid and solid wastes. The direct discharge of tannery wastewater may cause serious water pollution, since it contains high concentrations of organic pollutants, nitrogen compounds, solids and potentially toxic compounds such as chromium and organic micropollutants. The disposal of the solid wastes from tanneries generally creates a problem because of the quantity and the composition (non biodegradable and toxic compounds).

Air pollution in the surroundings of a tannery is mainly caused by the emission of malodorous substances and dust.

The characteristics of the various wastes and their environmental impacts are described in the following sections. Data from the CLPC tannery have been used for this.

3.2 Wastewater production

All quantities of wastewater given below are based on a production of 10 tonnes of hide per day.

a) Soaking and liming:

Wastewater from these processes contains salt, dirt, blood, hair, flesh, lime and sulphide and has a high pH. The daily discharge of wastewater from these processes including washing water, is 180 m 3.

b) Deliming, bating, pickling and chromium tanning:

Wastewater from these processes contains ammonium, nitrogen, chromium and the residues of other process chemicals. This wastewater has a low pH. The daily discharge from these processes, including washing water, is 40 m 3.

c) Neutralizing, retanning, dyeing and fat liquoring:

The daily discharge from these processes is 50 m 3 , which mainly consists of water used for washing and neutralizing. The wastewater contains residues of the chemicals used in these processes.

d) Vegetable and chromium tanning of goatskins:

The daily discharge from these processes is 80 m 3 . This wastewater contains salt, dirt, hair, lime and residues of all chemicals used for vegetable and chromium tanning. Part of the vegetable tanning liquids can be recycled.

e) Combined wastewater:

The wastewater from the various sources is combined into one flow.

The average characteristics of the combined CLPC tannery wastewater (data from an analysis programme) are given in Table 3.1. Data published by ESCAP and WHO are also given in Table 3.1.

8

Table 3.1 Comparison of CLPC tannery wastewater characteristics with data from ESCAP and WHO

CLPC tannery

ESCAP WHO

Wastewater flow (m 3/t of hides) BOD (mg/l) COD (mg/l) total solids (mg/l) suspended solids (mg/l) total nitrogen (mg/l) total chromium (mg/l) 61 PH

615 (60)* 9.5

2,860 13,500

35 1,920

960 510

n.a. 10,000-25,000 1,250-6,000

30-35 2,000-3,000

n.a. n.a.

7.5-10.0

1,710 4,960 9,400 2,650

290 67

1-13

52

* The chromium concentration has decreased greatly after modifying the chromium tanning agent

Source: ESCAP - Tanning industry. Industrial pollution guidelines; ESCAP Series No. VII; August 1982.

WHO - Rapid assessment of sources of air. water and land pollution. WHO Offset Publication No. 62, WHO, Geneva, 1982.

From these data it can be concluded that wastewater production at the CLPC tannery is typical of the figure quoted by ESCAP. The BOD of the wastewater is average, COD and suspended solids concentrations are relatively low, and the concentrations of nitrogen and chromium are high. However a new tanning agent now used in the chromium process has reduced the chromium concentration of the wastewater to about 60 mg/l.

3.3 Solid wastes

In leather production solid wastes are generated in the following processes.

a) Fleming, trimming and splitting.

About 250 kg of flesh, fat, hair, outer skin (epidermis) and damaged parts of the hide are removed in these processes per tonne of hides. At present this waste is dumped at a municipal garbage disposal site. The splittings are hung outside the tannery to dry and then vegetable tanned. Sometimes splittings are sold and exported for gelatine manufacture.

b) Cutting and shaving:

Wastes from cutting and shaving the wet blue leather are also dumped at a municipal garbage disposal site. The quantity of wastes is about 90 kg per t of raw hides.

n.a. not available

9

c) Trimming and buffing: Wastes from trimming and dust from buffing the dried, retanned leather are also dumped at a municipal garbage disposal site. The quantity of wastes is about 35 kg per t of raw hides.

3.4 Air pollution

Air pollution in the tannery area is mainly caused by the emission of foul .odours from putrefying raw hides and solid wastes, such as fleshings. Foul odours are also emitted from open drainage channels, sedimentation pits and wastes dumped inside the tannery premises.

3.5 Environmental impacts

a) Wastewater discharge

With the exception of one tannery (Tan Lanka, see Chapter 5) all tanneries discharge their wastewater without treatment into the nearest river, stream or marshy lands.

Tannery wastewater has high concentrations of organics, nitrogen compounds, sulphides, suspended solids and chromium.

The water quality of the receiving surface waters may be seriously affected by the discharge of organics. Oxygen depletion can occur by the aerobic biodegradation of organics, resulting in the killing of fish and other water organisms. Oxygen depletion is also caused by the oxidation of sulphides and ammonia. Anaerobic putrefaction may result in the emission of foul smelling gases, such as H 2S and NH3. Tannery wastewater may have a high chromium concentration. Most of the chromium is in the trivalent form, but also low concentrations of hexavalent chromium, which is extremely toxic for aquatic life, may occur. Transformation of Cr3* into Cr6* does not usually take place in the aquatic environment.

b) Solid waste disposal

Solid wastes from tanneries contain biodegradable organics, inert materials and potentially toxic compounds. Uncontrolled dumping of the solid wastes may cause serious environmental problems, since it may result in pollution of air, surface water, soil and groundwater. It may also cause health hazards for people living near the dumping site.

c) Air pollution

Air pollution is mainly caused by emission of foul odours and dust from storage areas, production processes, wastewater discharge channels and dumping of solid wastes. In several areas where tanneries are located complaints about odour nuisance have been registered from residents.

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4. INDUSTRIAL COUNSELLING

4.1 Introduction

The industrial counselling procedures are directed towards the introduction of environmentally sound technology ("clean technology"). Clean technologies contribute to more efficient production methods by saving -energy" and raw materials and reducing emissions to air, water and soil. They include good housekeeping measures, modification of production processes and raw materials use, as well as recycling of waste and process waters.

Industrial counselling aims at environmentally sustainable industrial development by promoting a combination of in-plant pollution control and end-of-pipe treatment in order to protect the environment and to optimize the conservation of energy and raw materials.

Additional advantages of the application of cleaner production processes are the reduction of safety hazards and the improvement of occupational health. Therefore, initial investments aimed at pollution control become more cost effective.

In this chapter in-plant pollution control measures and methods to improve occupational safety and health are proposed.

4.2 In-plant waste reduction

Various measures can be taken in tanneries to reduce the production of liquid, solid and gaseous wastes. Such measures include:

- Reduction of water use If the process water use is in accordance with internationally accepted processing standards, the wastewater discharge amounts to approximately 35 m 3 / t of hide. If the wastewater volume is larger, it could be reduced by standardization of the water use, by improving the production process and equipment, and by applying good housekeeping practices. Recycling of process waters is usually not practised.

- Reduction of process chemicals

Internationally accepted standards for the use of process chemicals in tanneries should be applied. Tanneries should use these standards in order to avoid the excessive use of process chemicals. If necessary the production equipment should be modified in order to make the standardized use of process chemicals possible.

In some cases process chemicals could be replaced by less polluting substances. However, most of the possible methods are still at an experimental stage. The discharge of chromium is reduced significantly by the use of self-basifying chromium tanning agents (e.g. Baychrom F), since these chemicals are much better fixed by the hides. The discharge of chromium can also be reduced by recovering the chromium from the chromium tanning wastes. The recovered chromium can be reused for tanning.

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A chromium recovery system consists of a collection tank, in which the pH of the chromium tanning wastes is raised to about 9, resulting. in the formation of chromium hydroxide (Cr(OH)J. The liquors are subsequently led into a sedimentation tank where the chromium hydroxide settles into a sludge. After separation the sludge is re-dissolved in a sulphuric acid solution, resulting in the formation of chromium sulphate, which is used as a tanning agent.

4.3 Waste utilization

There are several possibilities to manufacture by-products from solid wastes generated by the tanning processes. Examples of such by-products are:

- production of gelatine and glue from untanned skin and hide trimmings and splittings;

- manufacture of leather board from tanned shavings and trimmings; - extraction of tallow and grease from fleshings in a rendering plant; - recovery of proteins from fleshings and untanned trimmings for conversion into

animal or fish feeds.

The application of one of the methods for solid waste reuse will depend mainly on its economic viability. No reuse of solid waste is practised in Sri Lanka at present.

4.4 Improvement of occupational safety and health

Some general guidelines for measures which would improve working conditions and remove or alleviate occupational health risks in the tannery are given below.

- avoidance of physical contact with chemicals and liquors, by wearing aprons, gloves and glasses;

- proper ventilation to prevent the inhalation of vapours, gases (H2S, solvents and NHJ and dust (from shaving and buffing);

- avoidance of excessive physical exertion, e.g. lifting heavy weights;

- avoidance of contact with moving machinery, electricity and steam by installation of protective shields or insulating materials on machines and appurtenances;

- protection against excessive noise levels.

The U.S Department of Health, Education and Welfare has published guidelines for improving working conditions in two brochures: "Good work practices for tannery workers* and 'Health and safety guide for the tanning industry*. A counselling programme should review these guidelines and consider which measures could be applied in the tannery industry in Sri Lanka to improve the working conditions of the labourers.

An educational programme should be formulated and extended to all workers at regular time intervals, so that workers may develop a good housekeeping awareness and have knowledge of the safety instructions.

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5. POLLUTION ABATEMENT METHODS

5.1 Wastewater treatment alternatives

Tannery wastewater can be treated efficiently by mechanical or physical/chemical pretreatment methods in combination with a biological treatment method. Pretreatment of tannery wastewater before biological treatment is required because tannery wastewater generally has high concentrations of coarse materials, suspended solids and chromium and also a high pH. In the following the outline designs of three different treatment systems a.#e described:

1. Pretreatment, anaerobic/facultative aerated/maturation ponds

2. Pretreatment, oxidation ditch system

3. Physical/chemical treatment

The dimensions of system components are given on the basis of a preliminary design made for treatment of tannery wastewater with the following characteristics:

Flow 35.: m 3 /d COD 2,SO: mg/l BOD 1.9CC mg/l pH 9.5 Total solids 13.5CC mg/l Suspended solids 9€C mg/l Total Nitrogen 5'C mg/l Chromium * 6C mg/l

The wastewater has a chromium conce- -.ration of 60 mg/l if Baychrom F is used as a tanning agent. When other c^-omium tanning agents are used, the chromium concentration may be 10 times ligher.

5.1.1 Ponds system

A ponds system may consist of pre-trea:ment followed by respectively an anaerobic pond, a facultative aerated pond a.-d a maturation pond.

a) Pretreatment system

The pretreatment system consists c' screens, chromium recovery and chemicals addition.

Screens

Coarse materials are removed by pass - c the wastewater through a series of three screens with decreasing apertures

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Chromium recovery plant

The chromium tanning liquors are collected in a tank where the liquor is mixed with lime to achieve a pH of 9. After one day the liquor is pumped into another tank in which sedimentation of the sludge containing Cr(OH)3, takes place. This sludge is drawn off at the bottom of the tank and dried in a drying bed. The supernatant is combined with the other wastewaters. The dried chromium sludge is dissolved in sulphuric acid, whereby chromium sulphate is formed, which may be used as a tanning agent.

Dimensions and characteristics:

Chromium liquor flow 6 m 3 /d Collection pit volume 7 m 3

Process pH 9.0 Settling tank volume 7 m 3

Sludge production 0.25 m 3 /d Sludge drying time 20 d Drying bed area 25 m 2

Chromium removal efficiency 98 %

Chemicals addition

The effluent of the chromium recovery system is mixed with the other wastewaters. Chemicals are added to the other wastewater to reduce the pH and to add the necessary nutrients (phosphorus) before biological treatment. Subsequently the combined wastewater is led into the anaerobic pond.

b) Anaerobic pond

Volume 2,200 m 3

Depths 4 m Area (including embankments) 1,200 m 2

Effluent BOD 600 mg/l

Settled solids have to be removed from the anaerobic pond at regular intervals.

c) Facultative aerated pond

Volume 3,000 m 3

Depth 4 m Area (including embankments) 1,700 m 2

Power input 6 kW

d) Maturation pond

Volume 1,050 m 3

Depth 1.5 m Area (including embankments) 840 m 2

Effluent BOD 60' mg/l

5.1.2 Oxidation ditch system

The oxidation ditch system consists of a pretreatment system, an oxidation ditch, a secondary sedimentation tank and sludge drying beds.

The process of the oxidation ditch system is shown in Figure 5.1.

a) Pretreatment system

The pretreatment system consists of screens, chromium recovery, equalization, primary sedimentation and chemicals addition.

Screens

As for the ponds system (5.1.1.)

Chromium recovery plant

As for the ponds system (5.1.1.)

Equalization tank

The effluent from the chromium recovery plant and the other wastewater flows from the tannery are mixed in the equalization tank. From this tank the wastewater is pumped with a constant flow of 15 m 3 /h to the primary sedimentation tank.

Dimensions:

Volume 210 m 3

Depth 4.2 m Area 50 m 2

Power input for mixing 1.25 kW Pump capacity 15 m 3 /h

Primary sedimentation tank

Volume 27.2 m 3

Area 15 m 2

Sludge production - Primary sludge 350 kg/d - Excess sludge from secondary 95 kg/d

sedimentation tank Total 445 kg/d Sludge volume 127 m 3 /d Effluent BOD 700 mg/l

Chemicals addition

Identical as for the ponds system (5.2.1).

b) Oxidation ditch

Volume 408 m 3

Depth 1.5 m

Area Power input

272 16

15

Secondary sedimentation tank

Volume 75 m 3

Depth 5.3 m Area 30 m 2

Sludge recirculation pump capacity 10 m 3 /h Effluent BOD 70 mg/l

Sludge drying beds

The design of the sludge drying beds is based on the average sludge production per tonne raw hides processed.

Sludge production per tonne hides 1.27 m 3

Hides processed 1,800 t/yr Sludge production 2,300 m z/yr Average daily sludge production 6.3 m 3 /d Drying time 20 d Drying bed depth 0.2 m Drying bed area 630 m 2

5.1.3 Physical-chemical treatment system

A physical-chemical treatment system is actually a pretreatment system, in which the treatment efficiency is improved by addition of chemicals, such as aluminium or iron salts.

A physical-chemical treatment system consists of screens, chromium recovery, equalization, sedimentation and sludge drying beds.

Screens

As the pond system.

Chromium recovery

As the pond system.

Equalization

As for the oxidation ditch system, but chemicals are added to it.

Sedimentation tank

Volume 75 m 3

Area 30 m 2

Depth 5.3 m Effluent BOD 500 mg/l

Sludge drying beds

Sludge production per tonne hides processed 1.0 m 3

Annual sludge production 1,800 m 3

Average daily sludge production 5 m 3

Sludge drying beds area 500 m 2

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5.2 Wastewater treatment system selection

Before selecting a tannery wastewater treatment system it has to be emphasized that the physical-chemical system should be considered as a pretreatment system. One tannery in Sri Lanka is presently operating a physical-chemical wastewater treatment plant (see Section 5.3). The ponds system can only be used if sufficient area is available. The ponds system should also be situated at an adequate distance from housing because of the possible emission of bad odours.

The oxidation ditch system is a very reliable and efficient system for treatment of tannery wastewater. It is already widely applied for treatment of tannery wastewater. Its costs are however, relatively high.

A good approach to the installation of tannery wastewater treatment facilities might be to implement the wastewater treatment plant in two phases, the first phase being a physical-chemical treatment system, and the second phase an oxidation ditch system.

5.3 Existing tannery wastewater treatment plant

One tannery in Sri Lanka is presently operating a wastewater treatment plant. The tannery is located on the north bank of the Kelani River near the river mouth. The production capacity is 6 tonnes hide per day. The wastewater flow is 150 m 3 /d . The wastewater treatment plant is a physical-chemical system. It consists of an equalization tank and a sedimentation tank (Dortmund type). The sedimentation tank is completely covered.

Lime is added to the wastewater in the equalization tank (volume = 300 m 3). Aluminium sulphate and poly-electrolyte are added in the sedimentation tank. The

- settled sludge from the sedimentation tank is dried in drying beds by evaporation. The dried sludge is transported in bags to a disposal site, where it is used for making bricks.

No reliable data on the treatment efficiency of the plant are available at the moment.

The investment costs of the wastewater treatment plant were Rs. 5,000,000 of which Rs. 1,200,000 were for electrical/mechanica! equipment. The operation and maintenance costs are:

Chemicals Rs. 480,000/yr

Sludge removal Rs. 240,0Q0/yr

Electricity Rs. 60,000/yr

Maintenance Rs. 100,000/yr

The maintenance costs are relatively high, due to frequent breakdown of equipment, particularly the pumps.

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5.4 Air pollution control

Air pollution from tanneries is mainly caused by the emission of foul odours. Sources of foul odour are uncontrolled storage of putrefying waste, stagnant open wastewater channels, uncovered sedimentation pits and uncontrolled waste disposal in the tannery premises.

These problems can be solved by:

Storage of putrefying waste in closed containers and regular removal to the waste disposal site;

Covering of wastewater channels and regular cleaning to promote a proper flow and discharge;

Covering and regular emptying of sedimentation pits or manholes;

Regular cleaning of the tannery and the premises to remove all dirt.

5.5 Solid waste disposal

All solid wastes including wastewater sludge should be removed frequently and transported to a controlled waste disposal site. Since tannery waste contains putrefying waste, which may cause odour nuisance, the diposal site should be well away from residential areas. No water should seep from the waste disposal site into the ground or into nearby surface waters, since this water may be heavily polluted with organics, nitrogen compounds and chromium (mostly trivalent).

In order to limit the quantity of seepage water, the waste disposal site should be covered with a water proof layer.

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Vegetable tannery

waste water

chrome liquors

Mixing tank E

Sedimentation tank

super-natans

s ludge

Sludge drying bed

recycling or controlled dlsposa 1

Screens

surplus sludge Equalization tank

chemicals dosing

sludge recycle

Primary sedimentation tank

Oxidation ditch

Chromium tannery

waste water

sludge

Sludge drying beds

sludge disposal

Clarlfier

T effluent

Kelani Canga

Figure 5.1 Process diagram of the oxidation ditch system

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6. DISCHARGE AND EMISSION STANDARDS

Since the leather industry causes no atmospheric pollution, specific emission standards are not relevant for this type of industry. In the following the existing wastewater quality discharge standards for tanneries are discussed and some modifications of these standards are proposed.

6.1 Existing discharge standards

The Government of Sri Lanka has established specific quality standards for the discharge of tannery wastewater into inland surface waters and marine coastal waters (see Table 6.1).

Table 6.1 Wastewater quality discharge standards for tanneries (maximum allowable concentrations)

Component Unit Inland Marine Coastal Waters Waters

pH 5.5 - 9 5.5 - 9 Suspended solids mg/l 100 150 BOD mg/l 60 100 COD mg/l 250 300 Alkalinity (CaCO,) mg/l 750 not applicable Chloride mg/l 1000 not applicable Chromium (hexavalent) mg/l 0.5 0.5 Chromium (total) mg/l 2 2 Oil, grease mg/i' 10 20 Phenolic compounds mg/l 1 5 Sulphide mg/l 2 5

The standards for inland waters can only be achieved by extensive biological treatment, e.g. ponds or oxidation ditch.sytem. Presently no tannery in Sri Lanka, discharges effluent which meets the standards of Table 6.1. The standards do not specify a maximum allowable Nitrogen concentration.

6.2 Proposed discharge standards

Since no complete wastewater treatment plants have been installed yet at tanneries in Sri Lanka, and since the tanners have indicated that installation of wastewater treatment facilities will be a time consuming and costly process, it is proposed that intermediate standards are developed on the basis of a pretreatment system, e.g. physical-chemical treatment.

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The proposed intermediate standards and the final standards for discharge of tannery effluent into inland surface waters are given in Table 6.2. These standards should be complied with in a phased programme, the first stage consisting of physical-chemical pre-treatment and the second, successive stage consisting of a ponds system or an oxidation ditch.

Table 6.2 Proposed intermediate and final effluent standards for tanneries (maximum allowable concentrations)

Component Unit Intermediate Final

PH 5.5 - 9 5.5 - 9 BOD mg/l 500 60 COD mg/l 900 300 Chromium mg/l 2 2 Total nitrogen mg/l 400 60

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REFERENCES

Wastewater treatment (or the Ceylon Leather Products Corporation BKH Consulting Engineers Ceylon Leather Products Corporation 1989

UNEP/IEO - Technical Report Series No. 4 Tanneries and the Environment A Technical Guide UNIDO/UNEP Paris, 1991

Review of Industrial Pollution Regulations in Sri Lanka BKH Consulting Engineers Asian Development Bank/Central Environmental Authority Colombo, 1991 ;

no 44?8

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