ESCAP - ENVIRONMENT AND DEVELOPMENT SERIES

INDUSTRIAL POLLUTION CONTROL GUIDE- LINES

VI. ELECTROPLATING INDUSTRY

. UNITED NATIONS

ECONOMIC AND SOCIAL COMMISSION FOR ASIA AND THE PACIFIC

Bangkok, 1982

First edition 1982

The designation employed and the presentation of the material in this document do not imply the expression of any opinion on the part of the Economic and Social Commission for Asia and the Pacific concerning the legal status of any country, territory, city or area of its authority, or concerning the delimitation of its frontiers or boundaries. The views expressed do not necessarily represent the decisions or the stated policy of ESCAP, mention of trade names or commercial processes does not imply the endorsement of ESCAP.

FOREWORD

In conformity with the recommendations of the Committee on Industry, Technology, Human Settlements and the Environment at its fourth session, the ESCAP secretariat initiated a study on the methods and costs of industrial pollution control from agro-allied, agro-based and small-scale industries of the ESCAP region. The industries selected for the study were feitilizer industry as agro-allied, sugar, brewery and distillery, palm oil and tapioca industries as agro- based and tanning, fish processing and electroplating industries as small-scale industries.

While the technology for treatment of the liquid and gaseous emissions of the aforesaid industries are well established and are now implementable at the plant level, sophistication of some of the technologies for pollution control and their high costs have tended to retard the progress of effective industrial pollution control in the ESCAP region. Keeping this point in view, this industrial pollution control guide-line on electroplating industry has been prepared emphasizing on the appropriate technologies which are cost-effective and suitable to the con- ditions prevailing in the region.

.

August 1982 /S.A.M.S. Kibria

Executive Secretary

CONTENTS

Page

FOREWORD

I. INTRODUCTION.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

11. MANUFACTURING PROCESSES AND SOURCES AND CHA- RACTERISTICS OF LIQUID- AND AIR-BORNE POLLUTANTS AND SOLID WASTES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

111. POTENTIAL ENVIRONMENTAL PROBLEMS FROM ELEC- TROPLATING PROCESSES . . . . . . . . . . . . . . . . . . . . . . . . . . 10

IV. CURRENT POLLUTION ABATEMENT TECHNOLOGY AND COSTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

V. FUTURE POSSIBILITIES OF ALTERNATIVE AND IM- PROVED TECHNOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

VI. SUMMARY AND RECOMMENDATIONS.. . . . . . . . . . . . . . . . 32

VII. BIBLIOGRAPHY.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

ACKNOWLEDGEMENTS

FIGURES

Page

Figure 1 : Batch treatment of cyanide wastes . . . . . . . . . . . . . . . . . . 16

Figure 2 : Continuous treatment of cyanide wastes . . . . . . . . . . . . . . 17

Figure 3 : Alternative flow sheet for the continuous treatment of cyanide wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 4 : Batch treatment of hexavalent chromium wastes . . . . . . . . . 21

Figure 5 : Continuous treatment of hexavalent chromium wastes . . . . . 22

Figure 6 : Batch treatment of metal wastes not containing hexavalent chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 7 : Continuous treatment of metal wastes not containing hexava- lent chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 8 : Overall flow sheet for treatment of various wastes from elec- troplating works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 9 : Ion exchange method of recovering swill water and treatment of metal plating wastes . . . . . . . . . . . . . . . . . . . . . . . . . . 27

.9 :

m . w e , .

3 . 3 : cu

0

L u . 0 .

cd cu 0

Q :

9 . (;I.

a .

3 :

x . z - o o : m . a . w m *

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k . 0 .

.. .. .. Y Y Y Y u

Y

I. INTRODUCTION

The electroplating or metal finishing industry has been playing a momentous role in the development and growth of numerous metal manufacturing and other engineering industries since the early part of this century. While electroplating operations have, in the course of time, become an essential and integral part of many engineering industries throughout the world, there has also been a steady growth of independent and small- to medium-scale electroplating industries, especially in the developing countries including those of South East Asia after World War 11.

The growth of these independent small-scale electroplating industries in the developing countries may be attributable to the growth of light and medium engineering industries which found it more convenient and economical to have their products metal plated by independent electroplaters. A case in point is Hong Kong, where there are presently about 770 registered electroplating in- dustries, out of which about 450 units are located in Kowloon area (1). Accord- ing to the Environmental Protection Agency, Hong Kong, there are also a large number of unregistered electroplating units in the area, but their numbers are not known. The same may be true also with other countries of South East Asia.

In comparison with other industries, the electroplating industry uses much less water, hence the volume of the waste waters produced by this industry are also comparatively much smaller. Nevertheless, the waste waters are highly toxic in nature because of the presence of metals such as copper, zinc, nickel, cadmium and chromium, and of acids and the highly dangerous cyanides. The tolerable limits of such metals and other constituents in water and land environment are in general considerably lower than that of organic type of pollution. Although the methods of removal of these toxic constituents from waste waters are well established now, the problem of pollution of water courses, and particularly of disposal of untreated electroplating waste waters into municipal sewers, in many countries of South East Asia is yet to be solved.

Among the factors seemingly responsible for the slow progress of controlling pollution from this particular industry in many countries of South East Asia the major ones would appear to be the very large number of such industries in a particular area, their sporadic distribution, small-scale operation, 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 in multistoried buildings (as in Hong Kong) and the high recurring costs of treatment of the waste waters particularly for the small-scale units.

While disposal of untreated waste waters from electroplating industries into municipal sewers has in many cases prevented direct contamination of a water course, it has, on the other hand, posed a problem of disposal of sludge from municipal sewage treatment plants owing to the accumulation of toxic metals in

I '

2

the sludge. For example, it has been estimated (2) that if the existing number of metal finishing industries in the town of Ludhiana, Punjab, India, do not adequately remove the toxic metals in their waste waters before disposal into municipal sewers, the sludge from the proposed sewage treatment plant of the town would contain 1,850 mg nickel/kg of sludge and 415 mg chromium/kg of sludge respectively, and be totally unfit for use as manure on agricultural land.

It would appear, therefore, that the problem of treatment and disposal of waste waters from a large number of small-scale electroplating industries in South East Asian countries is unique in nature and is different from that of large engineering industries using electroplating as an integral part of their production processes. Nevertheless, the information which has been compiled on the methods and costs of pollution control in electroplating industries and which is incorporated in this report may assist the industries and the concerned reguiatory agencies in the affected regions in organizing appropriate pollution control programmes for this important industry.

11. MANUFACTURING PROCESSES AND SOURCES AND

POLLUTANTS AND SOLID WASTES CHARACTERISTICS OF LIQUID- AND AIR-BORNE

The types of plating generally employed in an electroplating industry are copper, chromium, nickel, cadmium, zinc, silver and gold. The plating operations are always wet and are preceded by cleaning to remove grease, rust and scale from the metal surface.

Cleaning Grease on the metals usually comes from machining, stamping, polishing

and preservation stages. If the grease is of organic nature it is removed by saponification with warm alkali. Petroleum and mineral oil greases cannot be removed by this method and trichloroethylene, benzene, gasoline, and carbon tetrachoride are employed. But the most commonly employed method of degreasing is emulsification with alkalis available on the market as metal cleaners. These are usually mixtures of sodium carbonate, caustic soda, trisodium phos- phate, sodium silicate, sodium cyanide and borax. The spent cleaning solutions are discharged as waste water.

stripping Removal of rust and scale is usually carried out by what is called pickling

with sulphuric acid or hydrochloric acid in case of articles made of iron. Lately, the electrolytic method of stripping has been increasingly used because of its rapid action. In this method the material to be plated is made the anode. The end products in both the processes are essentially the same. Fine sand particles remaining on the surface due to sand blasting can be removed separately by hydrofluoric acid. The spent acid solutions and rinse waters from this opera- tion constitute the waste water.

Plating The pickled articles are placed in wooden or mild steel vats with special

lining wherever necessary or in tanks made entirely of polymer material. The metal to be plated is made the cathode in an electrolytic cell. Plating baths are acidic in nature and generally contain sulphuric, hydrochloric or nitric acids. Alkaline baths containing sulphide, carbonate, cyanide and hydroxide are also used. The concentration of chemicals normally used in some of the common plating baths is shown below (3).

Rinsing After plating has been done, the plated objects are rinsed with water. They

are first dipped in stationary water baths, which are allowed to drain and are then dipped in running water baths to remove the adhering plating solution.

4

Bath formulae Rinse

Bath concentration concentration (average 5 5 0 l/h)

(drag-out 3.4 l/h)

Cadmium Cadmium cyanide,

Sodium cyanide,

Copper (cyanide) Cuprous cyanide, 22.5 g/l Sodium cyanide, 30 g/l Sodium carbonate, 10 g/1

Copper (acid) Copper sulphate, Copper sulphate, 120 g/l Sulphuric acid 80 g/1

Chromium Chromic acid, 300 g/1 Sulphuric acid, 3 g/l

Nickel Nickel sulplate 210 g/l Nickel chloride 60 g/l Nickel citrate, 30 g/l

Silver Silver cyanide, 25 g/l Sodium cyanide, 25 g/1

Zinc Zinc cyanide, 50 g/l Sodium cyanide 60 g/l Sodium hydroxide

25 g/l

30 g/l

Cd - 19000 mg/l CN - 22000 mg/l Cd - 1.2 mg/l pH 9.2 pH - 8.3

CN - 20 5000 mg/l Cu - 10 4000 mg/l

CN - 8.6 mg/l Cu - 1.34 mg/l

pH - 10.9 pH - 8.2

Cu - 49 000 mg/l Cu - 2.3 mg/l pH - 0.2 pH - 6.3

Cr - 140 000 mg/l Cr - 30 mg/l pH - 1.2 pH - 7.3

Ni - 93 000 mg/l pH - 5.5 pH 7.8

Ni - 6.3 mg/l

Ag - 24 600 mg/l CN - 21 800 mg/l

Ag - 51 mg/l CN - 45 mg/l

Zn - 38 000 mg/l CN - 59 000 mg/l pH- 11

Zn - 6.2 mg/l CN - 7.0 mg/l pH - 9.4 mg/l

Stationary baths are utilized to make solution for the plating operations while the running water baths are discharged into the drains. The quantity of drag-out depends upon the nature of the solution, its temperature, shape of the material being plated and the time allowed for draining. Manual plants are known to have higher drag-out losses than automatic ones.

Volumes and characteristics of waste waters The sources of waste waters in plating operations are generally two, namely

batch solutions and rinse waters, and they are distinctly different in volume and

5

characteristics. Batch solutions from vats are highly concentrated and are seldom discharged. Rinse waters are comparatively much more dilute but form the bulk of the waste waters of an electroplating industry.

Cleaning solution They include cleaners of various types and are generally prepared according

to manufacturers’ specifications. They are spilled out during the drag-out opera- tion. The rinse vat discharges are generally continuous in flow and have low solids concentration. The pH of rinse waters is usually in the neutral range.

Spent alkaline and rinse waters They include all the spent alkaline solutions containing suspended solids,

soaps, grease, and globules of oil. The frequency of discharge of these waste waters varies from plant to plant and depends, to a large extent, on the grease protection given to the metallic surfaces before they are received in the plating shop. The pH of these waste waters when discharged intermittently is usual!y very high (around 12.0). They are generally held in steel tanks for controlled discharge or for blending with acid wastes for lowering the pH. The continuous flow of alkaline waste waters, however, occurs from the dip and rinse operations, the pH of which varies from 8.8 to 9.8.

Acid pickling and rinse waters Strong spent acid solution originate from stripping operations in metal

cleaning vats. They contain mostly ferrous sulphate and residual acids usually with pH below 2.5. The frequency of discharge of pickling wastes is much higher than cyanide concentrates. Pickled and acid-dipped articles are washed in vats provided with a continuous flow of water. This rinse water is acidic and has a pH in the range of 4.5 to 5.6. The operation is not normally continuous and the rinse waters are stored in acid-proof tanks for controlled discharge.

Cyanide concentrates They arise from cyanide vats, dips, drip from the articles and rinse operation

and contain fairly large quantities of cyanide. The discharge is usually controlled by holding it in a tank, which gives an opportunity for its pretreatment before disposal or further treatment, Rinse waters do not pick up significant amounts of cyanide during the washing operation. They have continuous flow as compared to those from vats and dips. The vats are emptied once in every 6 to 8 weeks and the contents are very rich in cyanide.

Chromate wastes The bulk of the chromium bearing waste waters originate from chromium

plating, anodizing, electroplating solutions and dip solutions like passivating dips, bright dips, etc., and small portions arise from rinsing of metals treated with chromate solutions. The plating vats are normally not discharged. The strength

6

of plating solutions is made up by the addition of required quantum of chromic acid and sulphuric acid. The main sources of chromium in the waste waters are the drag-out and washing operations.

Chromium concentration in the waste water vanes from 3 to 30 mg/l depending upon the care with which the plating operations are carried out. The pH of the rinse waters is generally in the neutral range and rarely goes below 5.5. The waste water is normally stored for a regulated discharge after proper dilution or pretreatment to reduce hexavalent chromium to trivalent state.

Metal wastes from plating baths They include rinse waters from copper, zinc, nickel, cadmium and lead vats.

All these metals seldom occur together because the baths are never used all at a time and the operations are staggered to meet the demand. However, a com- bination of these baths is practised occasionally resulting in the appearance of several of these metals at a time in the mixed rinses. The metals are present in soluble ionic form and most of them are extremely toxic.

Floor washes Occasionally the plating shop is washed by running water to clean the floor.

During plating operations, some spilling and splashing may occur from the baths and the wash water thus possibly contains cyanide and many of the metals that are used for plating. The concentrations of these toxicants in the floor washings depend on the plating operations and vary widely.

Regeneration wastes from ion exchange columns Most of the plating operations need demineralised water to prepare vat

solutions. After exhaustion, the strong acid cation exchange resins are regenerated with 2 to 5 per cent sulphuric acid or 5 to 8 per cent hydrochloric acid solution. The weak base anion exchange resins are regenerated with 4 to 6 per cent sodium hydroxide or 6 to 8 per cent sodium carbonate solution. During regeneration and rinse operation, the excess regenerants drain out as a waste. The regeneration frequency depends on the size of the columns and the quantity of demineralised water required. The alkaline waste is normally mixed with cyanide concentrates and the acidic waste with chromate wastes.

Other waste waters Other forms of waste waters from a plating shop are from the cooling and

process operations. The volume and characteristics of waste waters produced in these operations do not change substantially to warrant treatment, and hence they are either recirculated or discharged without treatment.

Composite waste waters The volume and characteristics of various waste-water streams vary conside-

rably from one plating shop to the other and within the same shop from day to

7

day. Information collected by the Indian Standards Institution (3) from eight electroplating industries in various parts of India is summarized in Table I. The flows are only approximate and are mostly based on the information supplied by the factories.

The composite plating wastes may be acidic or alkaline depending on the type of baths used. Normally all the drains are interconnected inside the factory, mainly due to ignorance of the consequences, and partly to faulty layout of the factory. Typical analysis of composite waste waters of a few electroplating industries in India (3) is shown in Table 11.

Sources of air pollution The important sources of air pollution in an electroplating industry are the

acid pickling baths and plating baths. Acid mists and vapours are usually evolved from these baths and may not only pollute the atmosphere inside the plating shop but also can be a cause of air pollution when vented to the atmosphere through stack. While in most of the modern and well designed electroplating industries these acid mists and other gases generated at various points are trapped at the sources by using hoods with suction devices and are scrubbed with water before venting the offgases into the atmosphere through stack, in most of the older type of industries adequate arrangements for removal of the acid vapours and other gases are not exjsting.

Sources of solid wastes Solid wastes from electroplating industries are of two types, one being in

the form of precipitates and sludges accumulated over a period of time in the acid pickling baths and plating baths, and the other in the form of sludges from waste- water treatment plants. While the quantum of sludge accumulated in the baths is quite small and is normally discharged along with the waste waters, the waste treatment plant sludge contains all the precipitated toxic metals and lime and other chemicals. Safe disposal of this sludge has presented a problem in many situations.

Table I

Typical characteristics of industrial wastes resulting in plating operations (3)

Perman- Other items

Ether ',$:: Total Suspended extrac- solids tables Waste Flow Basicity Cyanides COD solids

Cleaning solution (rinse waters)

Cyanide concentrates (rinse water)

Acid pickling rinse waters

Spent alkali rinse waters

Chromate rinse waters

Copper (cyanide) rinse waters

Copper (acid) rinse waters

Nickel rinse waters

Cadmium rinse waters

zinc rinse waters

Floor wash waters

450-680

-do-

900

2700-2650

1360-2270

450- 680

-do-

-do-

-do-

-do-

-do-

7.8-8.4

9.2-9.9

4.5-5.5

8.8-9.8

5.5-6.8

-

6.1

7.4-8.3

8.0-8.8

8.9-9.8

7.6-8.0

300-650

800- 1700

28.43

180-410

31-56

2 10-340

97-1 10

187-299

191-320

210-405

170-230

0

0.3-21.2

-

-

-

7.3-11.6

-

-

3.2-4.6

5.4-9.0

0.1-0.3

290-350

25- 42

-

300-550

-

-

-

-

-

-

-

14-22 960-1120

4- 7 430-600

- 450-590

24-44 800-1350

- 460-750

610-720

23-35

76-141

-

-

-

-

-

-

-

65-79

73-120 Fe: 2.3-3.1

15-21 CU: 2.8-9.7 Ca: 0.3-0.8 Zn: 1.8-2.4

- Fe: 80-110

520-910

- Cr(VI)3-30 W

Pb: 0.1-0.7

- Cu: 1.1-1.4

- CU: 1.8-3.1

- Ni: 48-8.7

- Cd: 1.8-3.1

- Zn: 5.4-7.3

25-27 CU: 0.05-01 Zn: 0.3-0.5 Cd: 0.1-0.2 Ni: 0.1

~ ~~ ~

Note: The values shown in the table ave the rangesgenerally obtained. These may vary with the process.

E'Z ? - - - S Z - E6 S 02 SI -

- 2'1 P'O S'O O'E 08 os2 I 06 O L L - -

- 2-1 1.0 1.0 6.1 OS'Z 02'1 06 0061 09 I 08'9 9

- P ' E 1.0 2'0 P'E OE'Z 06'1 OL I 009 I 06 I OP'9 9

- 0'9 09'0 06'2 OLZ OS81 08 I OE'9 9 - S I 2 -

- L'9I SL'O 08'9 01 6 OE92 02 I OS9 s - - L'9

OS€ OSPP 082 SI 'L P

- PE'I SZ'I 29'0 08 09E I 08 2 06'L E

- - 02.6 - - - -

0 . 3

- - IO0

- 29'0 S8'1 092 OOP I 062 OO'L 2 - - - -

OS2 0012 SIP OS'L I - - IE 'O - - - -

008 002s OLOI OL'8 I - - 08'E - - - -

10

III. POTENTIAL ENVIRONMENTAL PROBLEMS FROM ELECTROPLATING PROCESSES

Waste waters from electroplating operations are low in volume, contain relatively little organic matter but are highly toxic. Pollutional effects of elec- troplating effluents can be broadly divided into four groups:

(a) Toxicity to fish and other aquatic life (b) Effects on sewers (c) Effects on biological sewage treatment processes (d) Pollution on ground water and surface waters

Toxicity to fish and other aquatic life Toxicity of any one of the constituents present in the waste waters (acids,

alkalis, cyanide and toxic metals) has been found to depend on the size, age and species of the fish or fish food. Toxicity depends also on other factors such as pH, temperature, hardness, alkalinity, oxygen content and other dissolved sub- stances in the receiving water. Further, the type and quantity of biota in the watersource, the degree and nature of other pollutional sources, the extent of stratification, the amount of aeration and the presence of synergistic or anta- gonistic compounds in water has also to be taken into consideration. The toxicity of various chemicals such as are found in plating wastes (4) is shown in Table 111.

Effects on sewers Electroplating waste waters are highly corrosive due to the presence of

acids. They attack metal and concrete structures, especially the concrete sewers, in which the waste waters are discharged. Further, the acids hydrolyse the soaps present in sewage, liberating fatty acids, which may form floating scum, cause the floating objects to stick together and clog the sewers. Alkaline wastes are also corrosive but they are not as aggressive as acid wastes to most of the construction materials.

Effects on sewage treatment Electroplating wastes have a deleterious effect on biological sewage treat-

ment processes due to the presence of acids, alkalis and toxic metallic ions such as Cr (VI), Cu (II), Zn (II), etc. These compounds inhibit or kill the micro-organisms that take part in the purification of sewage. Although presence of small concentra- tions of the heavy metals in sewage may not affect its biological purification, most of these metals are precipitated during the process and get accumulated in sludge, rendering it unfit for use as manure.

Although cyanides exert direct toxicity to fish and other higher forms of life, they can be oxidized by acclimatized bacterial flora in a sewage treatment

11

Table III

Toxicity of plating wastes constituents to fish and fish food (3)

Substance Concentration msP

Test Organism Effect

Chromic acid

Hydrochloric acid

Nitric acid

Sulphuric acid

Strong acids

Cadmium chloride

Cadmium sulphate

Copper sulphate

Copper sulphate

Sodium chromate

Potassium dichromate

Potassium dichromate

Chromate (ion)

Chromate (ion)

0.3 (as Cr)

60 (as HC1)

107 (as HNO,)

83 (as H,SO,)

To pH 5.0

0.01 (as Cd)

5 13 (as Cd)

0.04 (as Cu)

0.8 (as Cu)

0.1 (as Cr)

36 (as Cr)

180 (as Cr)

20 (as Cr)

50 (as Cr)

Daphnia magna*

Daphnia magna*

Daphnia magna*

Daphnia magna

Fish

Goldfish

Minnows

Daphnia magna*

Goldfish

Daphnia magna*

Goldfish

Goldfish

Trout and minnows

Sunfish, bluegills

Toxic

Toxic

Toxic

Toxic

Toxic

Kills in 8-18 hrs.

Kills in 3 hrs.

Toxic

Kills in 24 hrs.

Toxic

No effect in 108 hrs.

Kills in 3 days

Kills in 8 days

Not toxic in a month

Chromate (ion) 0.01 (as Cr) Micro-flora Toxic

Ferric chloride 24 (as Fe) Goldfish Kills in 1-15 hrs.

Ferrous sulphate 37 (as Fe) Goldfish No effect in 100 hrs.

Ferrous sulphate 368 (as Fe) Goldfish Kills in 2-10 hrs.

* Daphnia magna is a representative fish food organism commonly found in streams.

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Table Ill

Toxicity of plating wastes constituents to fish and fish food - contd.

Substance Concentration Test organism Effect mgll

Nickel chloride

Lead nitrate

Stannous chloride

Zinc sulphate

Zinc (ion)

Sodium cyanide

Potassium cyanide

Cyanogen chloride

Potassium ferrocyanide

Ammonia

Ammonia

Hydrogen sulphide

Sulphide (ion)

Sulphide (ion)

Potassium cyanate

Sodium hydroxide

Trisodium phosphate

Chlorine

4.5 (as Ni)

63 (as Pb)

626 (as Sn)

25 (as Zn)

0.3 (as Zn)

0.3 (as CN)

0.04 - 0.12 (as CN) 0.08 (as CNC1)

948 (as CN)

2.5 (as NH,)

2.7 (asNH,)

10 (as H, S)

3 (as S)

0.5 - 1.0 (as S)

265 (as KCNO)

156 (as NaOH)

52 (as Na,PO,) 0.05 - 1.0 (as CI)

Goldfish

Goldfish

Goldfish

Trout

Fish

Minnows, fatfish carp Goldfish

Fish

Minnows, goldfish Goldfish

Fish

Goldfish

Trout

Fish

Trout, fingerings, adult minnows

Daphnia magna*

Daphnia magna*

Fish

~ ~

Kills in 200 hrs.

Kills in 80 hrs.

Kills in 45 hrs.

Kills in 133 minutes Kills some fresh- water fish

No affect in 24 hrs. Kills in 3-4 days

Critical Not lethal

Kills 1-4 days

Lethal

Kills in 96 hrs.

Kills in 5 minutes

Critical

No effect in 24 hrs.

Toxic

Toxic

Critical

* Daphnia magna is a representative fish food organism commonly found in speams.

13

plant. Therefore, the presence of cyanides in sewage within certain limits may not inhibit the sewage purification process once the system is acclimatized to cyanides. It has been found that hexavalent chromium and nickel ions even at small con- centrations (1 to 10 mg/l) affect nitrification and the general performance of the sewage treatment plant. Sodium and potassium cyanides also inhibit nitrification initially but have no effect after a few days. Carbon oxidation is also affected by sudden discharges of cyanide but is not affected if the sludge is acclimatized. Higher concentrations of copper, iron, nickel and cyanide affect gas production in anaerobic sludge digestion. Cyanides could be tolerated much easier than the metals after a period of acclimatization.

Pollution of ground and surface waters Discharge of untreated electroplating waste waters on land may pose a

problem of ground-water contamination with toxic metals and may render it unfit for drinking purposes. While some of the metallic ions may be retained in the soil during the passage of the waste water through the soil and as a result the ultimate quantum of the toxic metals reaching the sub-soil water table may be much lower than those present in the raw waste water, the concentration of the toxic metals in a surface water receiving the waste waters would be dependent solely on the dilution available in the surface water.

The U.S. Public Health Service Drinking Water Standard (USPHS 1962) states (5) that the presence of the hexavalent chromium in excess of 0.05 mg/l shall constitute grounds for rejection of the supply. However, nickel is con- sidered to be relatively non-toxic to man and a limit for nickel is not included in the U.S. EPA National Interim Primary Drinking Water Regulations, 40 FR 59566, December 24,1975.

14

IV. CURRENT POLLUTION ABATEMENT TECHNOLOGY AND COSTS

A. WATER POLLUTION ABATEMENT

Before treatment of the waste waters from a plating shop, the various means for reduction of the volume and strength of the waste waters or their prevention, if at all possible, should be explored. Waste reduction/prevention not only reduces the cost of treatment but also saves a lot of costly chemicals. Another objective of treatment is to recover the metals, if it is economically feasible.

Waste prevention The waste-water volume and the size and operating costs of a treatment

plant can be considerably reduced by observing the principles of good shop- keeping. The following guide-lines (4, 6 and 7) for modifications in design and operation to reduce the plating wastes are worth considering. They are:

(a) installing a gravity-fed, non-overflowing emergency holding tank for

(b) eliminating breakable containers for concentrated materials, (c) provision for special drip pans, spraying rinses, and shaking mechanism, (d) reducing spillage, drag-out and leakage to the floor, or other losses by

curbing the area and discharging these losses to a holding tank, (e) using high pressure rinses rather than high volume water washes,

( f ) recirculating valuable materials from concentrated plating bath wastes, (8) evaporating reclaimed wastes to desired volume and returning to

plating bath at rate equal to the loss from bath, (h) recirculating wet washer wastes from fume scrubbers,

(i) use of counter-flow rinses to reduce the concentration of contaminants in the final rinse water and

toxic materiab,

0) recovery of metals from the wastes.

Cyanide treatment Although not less than ten methods of treating cyanides are known (8), the

most popular method is alkaline chlorination, using either gaseous chlorine or chlorine dioxide in presence of caustic soda, or hypochlorite of sodium or calcium, or bleaching powder. Ozonation of cyanide wastes, conversion of cyanides to less toxic ferrocyanides, electrolytic oxidation, acidification followed by venting the fumes through high stack and lime sulphur treatment are the other methods.

15

Alkaline chlorination Of the several methods available. for eliminating cyanides from plating

wastes, this is the most widely used one. In practice this is referred to as ‘alkaline chlorination’ process. When chlorine is added to a waste containing free cyanide (soluble cyanide) and sufficient alkali is added to raise the pH to about 10.5, the free cyanide is oxidized to cyanate with cyanogen chloride as an intermediate product. This reaction is normally instantaneous or takes not more than 10 minutes. Theoretically, 3.08 parts of alkali (NaOH) and 2.73 parts of chlorine are required for each part of CN oxidized. With excess chlorine, however, cyanate of the first stage is further slowly oxidized to carbon dioxide and nitrogen. This second stage of reaction takes from one half to one hour. This needs an additional 3.08 parts of alkali and 4.09 parts of chlorine for each part of CN. The probable overall reaction with excess chlorine in presence of NaOH for complete conver- sion of cyanide to carbon dioxide and nitrogen gas is:-

2 NaCN t 5 C1, t 12 NaOH _+ N, t 2 Na,CO, t 10 NaCl t 6 H,O The chlorine required in practice is somewhat higher than 6.82 parts and the

alkali lower than 6.16 parts per part of CN. The chlorination is always done with thorough agitation to provide proper mixing of cyanide with chlorine to avoid formation of solid cyanide precipitates which resist action of chlorine in solution. It is important to carry the reaction all the way to carbon dioxide and nitrogen, as otherwise there is a chance of cyanates being reduced back to cyanides.

Water Pollution Control Federation, U.S.A., has suggested (9) two methods:

(a) a batch type (Fig. l), and (b) a flow through continuous type (Figs. 2 and 3) for alkaline chlorination

of cyanide bearing waste waters. The batch method is generally preferred by plating shops with small

volumes of effluent, while the continuous type is generally used by shops with large effluent volumes. Two holding tanks for the raw waste are provided as a minimum necessity in either cases to ensure dependable treatment. In both types, a solution of chlorine in the waste water itself is preferable as it obviates the necessity of water as a carrier. This is achieved by feeding chlorine to the suction side of the pump, drawing the waste from a reservoir to a mixing tank in the flow through type treatment. Alkali is added to the tank and the contents rapidly agitated by a fast mixing device. If necessary, air spargers are also employed. To recirculate the contents in the mixing tank, a sump well pump is used and sufficient chlorine is injected into the suction side of the pump to maintain a constant free residual chlorine in the waste. The appearance of residual chlorine generally indicates the destruction of all the cyanide. However, metallic complex cyanide resists chlorination and in such cases the presence of residual chlorine does not indicate the completeness of the treatment. Under these conditions chlorination is continued until no cyanide is found by standard methods of cyanide estimation.

OR '

BLEACHING POWDER STORAGE

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REACTION TOWER CHLORINE GAS rc7

19

Alternatively, in the continuous type treatment, chlorination is achieved by injecting chlorine in an interacting tower with arrangement to spray the effluent from the top of the tower with perforated compartments packed with inert mate- rials. The excess chlorine, if any, in the treated effluent is returned back to the holding tank from where the solution is taken for recirculation (Fig. 3).

In the batch type treatment (Fig. l), two or more holding tanks are used so that when the waste is being chlorinated in one, the others can be used for filling up with the raw waste water. Intimate mechanical mixing is achieved by suitable means.

Chlorine gas, chlorine dioxide, sodium hypochlorite or bleaching powder are generally employed in chlorination. With chlorine gas the destruction of cyanide is 100 per cent complete with sufficient reaction time. Presence of heavy metals prevents 100 per cent oxidation when chlorine dioxide is used. No addi- tional alkali is needed with sodium hypochlorite. A small quantity of alkali is, however, required with calcium hypochlorite. Bleaching powder (35 per cent available chlorine) produces greater sludge volume than calcium hypochlorite (70 per cent chlorine). Sodium hypochlorite produces least amount of sludge. Hypo- chlorites can be added more rapidly than chlorine or chlorine dioxide, the solubi- lity of which is limited. Bleaching powder and calcium hypochlorite require to be strained before feeding, since choking of lines is an associated problem with these reagents.

Ferrous sulphate treatment Conversion of simple cyanides to complex ferrocyanides had earlier been

a fairly common procedure in England and Europe for disposal of mixed plating wastes from small plating shops. Basically, the process involves formation of various soluble and insoluble ferrocyanides and ferricyanides that are less toxic. Treatment with ferrous sulphate usually takes place in alkaline solution (pH 7.5 to 9.0). This method is always associated with the formation of dark blue dudge. This treatment practice has not been widely recommended because of question- able efficiency of the process and permanency of result, large volume of sludge produced and pronounced odour of the sludge and the effluent. In presence of intense sunlight, ferrocyanides get converted to cyanides.

As a partial treatment for removal of cyanides, such as in the case of pre- treatment before discharge to sewer, addition of ferrous sulphate solution and adjustment to a pH value between 7.5 and 9.0 with slaked lime, is used by some plating shops in India (3) for precipitating the insoluble complex cyanides of iron. The theoretical requirements of ferrous sulphate may be worked out from the equation:-

6NaCN + FeSO, - Na2S04 + Na,Fe(CN), but practically it has been found that for each part of cyanide (as HCN) removed, 15.4, 20.6 and 196 parts of ferrous sulphate (FeS0,.7H20) are required to treat potassium cyanide, potassium zinc cyanide and sodium cuprocyanide respectively.

20

Alkali cyanides or unstable complex cyanides such as sodium zinc cyanide respond best to the treatment of ferrous sulphate; cuprocyanide requires much larger amounts. If nickel is present in the cyanide solution, the ferrous sulphate requirements are still higher. Cyanide removals are better in strong solutions. It is reported (8) that while under laboratory conditions cyanide contents under this process can be reduced to 5 mg/l, in full-scale practice it is not possible to reduce it to less than 10 mg/l, and, if nickel is present, to less than 20 mg/l as HCN. As the reagents are cheap and non-toxic, the process is suitable for continuous operation. However, apait from the incompleteness of removal of cyanides, the chief disadvantage is the formation of considerable quantities of sludge.

Chromium treatment Chromates, dichromates and chromic acid occur in the waste waters

generated from chromium plating, anodising, and other metal finishing operations. The hexavalent chromium ions in these wastes are highly toxic even in very low concentrations and need to be almost completely removed from the waste waters before they are discharged into a stream, sewer or on land. The most effective and economical way of treatment involves reduction to trivalent state, Cr (III), and subsequent precipitation with an alkali (Figs. 4 and 5). Reduction to trivalent state takes place most effectively in acid solution. Ferrous ion is used to reduce chromate quantitatively in acid solution. Ferrous sulphate along with sulphuric acid is commonly used for this purpose. This reduction takes about one hour. Other reducing agents are sulphur dioxide and sodium bisulphite. Maximum reduction occurs in the pH range 2.0 to 2.5. Fresh mineral acids or stored spent pickle acids are used for this purpose. Sodium metabisulphite is equivalent to 66 per cent sulphur dioxide by mass and is more expensive than sulphur dioxide. In small installations the convenience and the safety of handling sodium salts in powder form make them preferable to sulphur dioxide gas. In large installations the gas is generally more economical.

The reduced trivalent chromium is precipitated individually or in combina- tion with other metal wastes by the addition of an alkali, lime or caustic soda. Lime is commonly used, since it is cheaper than caustic soda. The volume of the sludge depends on the sulphuric acid concentration in the waste when the neutraliza- tion alkali is lime. The concentration of the metal is another factor which in- fluences the sludge volume. The step-wise reactions for precipitation of chromium (hexavalent) with ferrous sulphate and lime are:

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23

Treatment of other metal-bearing wastes A common holding tank is normally used for all the metal wastes. The

previously oxidized cyanide and reduced chromium wastes are also allowed to flow into the same holding tank along with those containing solids, greases, alkalis and soaps. Any waste water likely to contain even traces of cyanide or hexavalent chromium is pre-treated as outlined above before it is allowed to mix with other metallic wastes in the holding tank.

The pH of the mixture in the holding tank depends on the relative quantities of alkaline and acidic wastes let into it. Precipitation occurs in the holding tank if the pH is on the alkaline side. To prevent settling of the precipitate and to ensure proper mixing of the contents in this tank, agitation is provided either mechanically or by compressed air. In either batch or continuous operation (Figs. 6 and 7) the flow from the holding tank is led into a pH adjustment tank provided with alkali and acid feeders and mixing facilities. The pH is controlled manually or automatically. Almost all the metals precipitate completely in the pH range of 9.5 to 10.5. The pH corrected wastes flow into slow mixing tank for completion of the reaction and for building up the size of flocs.

The wide variation in the concentration of metal bearing waste waters necessitates prolonged agitation of the more concentrated waste waters to ensure complete reaction. The slow mixed effluent flows into a hopper-bottom sedimentation tank with 4 to 5 hours of holding time. The sludge drawn out periodically from the sedimentation tanks is collected in a sump well from where it is pumped to sludge drying beds or lagoons (Fig. 8). Normally, two such lagoons, each with a holding capacity for a year, are provided and are operated in rotation.

The sludge from the sedimentation tanks usually contains 4 to 8 per cent suspended solids when lime is used as a neutralizing chemical. It is 1.5 to 2.5 per cent when caustic soda is used. The characteristics of the sludge vary from plant to plant and should be determined experimentally for each case. The lagooned sludge is not free flowing and is difficult to handle or load into trucks.

The holding tank is provided with baffles at the inlet and outlet ends and the floating oil is scraped from time to time. Emulsified oils pass through the system and hence such waste waters are pretreated with alum and caustic soda before entry into the holding tanks.

Recovery of metals and swill water by ion exchange The bulk of the waste water in an electroplating industry generally arises

from plating swills. The rinse water which is considerably less contaminated as compared to the strong wastes, normally contains less than 500 mg/l total dissolved solids and as such is eminently suitable for reuse. The rinse water is treated in many cases by ion exchange process, (Fig. 9), whereby all the cationic and anionic impurities are removed and a continuous closed circuit demineralised water supply is available for plating rinse. Use of demineralised water for plating

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FIG.9 ION EXCHANGE METHOD OF RECOVERING SWILL WATER AND TREATMENT OF METAL PLATIN6 WASTES (3)

28

rinse is most desirable to obtain better finished products. Some metals are also recovered from the concentrated regenerated solutions provided their recovery is found economical and feasible.

Neutralization is the most essential treatment for the strong waste waters. However, the treatment is complicated in view of the presence of both cyanides and chromates. The regenerant wastes from the cation and anion exchange units of swill water demineralisation are utilised in some cases for achieving the necessary pH value of the treated waste water.

As mentioned earlier, cyanides are treated under alkaline conditions and hexavalent chromium is converted to trivalent chromium under acidic conditions before precipitation with alkali. Where these two waste waters get mixed before entering the treatment plant, the treatment process becomes complicated and less effective. Segregation of these waste waters and their separate treatment are now practised in many electroplating industries. The acidic waste waters including the acid regenerant stream from the cation exchange unit are sent to a chrome reduc- tion tank where the hexavalent chromium is reduced to trivalent form by means of a strong reducing agent. The caustic regenerant from the anion exchange unit and also other alkaline discharges are sent to a tank containing other cyanide bearing wastes and chlorinated for the oxidation of the cyanides. The strong acid and alkali originating from regenerating the beds are utilized for maintaining optimum pH values for the chrome reduction and cyanide oxidation respectively.

Metal recovery by solution concentration In large electroplating industries, chrome, nickel and copper in acid type

plating solutions are reclaimed from the rinse tank by evaporation in glass-lined equipment or other suitable evaporators and the concentrated solution returned to the plating system. This process appears to be economically feasible (8) where chemical quantities involved are relatively large and would justify recovery.

Costs of waste-water treatment Little information is available on the costs of installation and operation of

waste-water treatment plants of small- or medium-size and independent elec- troplating industries existing particularly in the countries of South-East Asia. One of the important reasons would appear to be that these industries are mostly located in areas of high commercial activity of a town or in industrial estates or multistoried buildings and have the facility of discharging their waste waters into sewers after giving minimum pretreatment in the form of neutralization of acids/alkalis and settlement of suspended solids which may include some preci- pitated metals. Waste-water treatment plants of other engineering industries which have electroplating sections are designed to handle the entire waste waters including those from electroplating processes. As a result, the costs of installation and operation of such treatment plants are not truly representative of that for independent electroplating industries. As mentioned earlier, the wide variation in the flow and characteristics of the waste waters, which depend upon the operating

29

practices in the electroplating industry, presents another difficulty in estimating the costs of treatment of such waste waters.

However, the major items of costs of a plant for treatment of waste waters from electroplating industry are the holding tanks and reaction tanks which must be lined with acid-proof materials, stainless steel or polymer lined agitators and pumps and the chemical solution preparation and dosing equipment. Depending on the consumption of chemicals, storage space for the the chemicals may also be an important item of civil cost.

The major item of operating costs of an electroplating waste-water treat- ment plant is the chemicals for conversion and/or precipitation of metals and destruction of cyanides. An idea of the requirements of the various chemicals, and for that matter their costs, may be obtained from the following theoretical quantities of chemicals used and sludge produced in conventional treatment of cyanide and hexavalent chromium bearing waste waters.

Cyanide destruction

To convert 1.0 kg of CN to N, and CO, :

Chlorine (C4) required = 6.83 kg Caustic soda (NaOH) required = 6.16 kg

or Bleaching powder (CaOCl,) required = 12.22 kg

Sludge (from bleaching powder treatment) 1 .O kg CN will theoretically produce 2 kg calcium carbonate (CaC03).

Chromium removal Using ferrous sulphate:

Chemicals 1 kg Cr requires 16.03 kg copperas (FeS04, 7H2O) 1 kg Cr requires 6.01 kg sulphuric acid (66’Be’) 1 kg Cr requires 9.48 kg lime (90%) for precipitation of metallic

sulphates. Sludge

1 kg Cr produces 6.09 kg ferric hydroxide, Fe(OH)3. 1 kg Cr produces 17.443 kg calcium sulphate, CaS04 (not all sludge) 1 kg Cr produces 1.98 kg chromium hydroxide, Cr (OH),.

30

Using sodium metabisulphite ma2 S2 05 ) : Chemicals

1 kg Cr requires 2.81 kg sodium metabisulphite (97.5%) 1 kg Cr requires 1.52 kg sulphuric acid (66OBe’) 1 kg Cr requires 2.38 kg lime (90%).

1 kg lime, as Ca(OH)2, produces 1.84 kg CaS04 (not all sludge) 1 kg chromium, as Cr, produces 1.98 kg Cr(OH)3.

Sludge

B. AIR POLLUTION CONTROL

Good housekeeping and maintenance and operating practices can signi- ficantly reduce the problem of air pollution from an electroplating shop. The principal air pollutants from this industry are the acid mists and gases which emanate from the various acid holding tanks and plating baths. These baths are provided at their top with hoods to trap the mists and gases which are blown to a wet scrubber before the noncondensable and inert gases are vented to the atmosphere. Since the problem of air pollution from an electroplating shop located in a multistoried building may be more acute than in a shop located other- wise, most of the electroplating industries in Hong Kong have good arrangements for air pollution control.

C. SOLID WASTES HANDLING

As mentioned earlier, the solid wastes which are of any consequence in an electroplating industry are the sludge produced from treatment of the waste waters. As the waste treatment plant sludge may contain all the toxic metals removed from waste waters, final disposal of the sludge should be done with utmost care.

Lagoons are usually provided for the ultimate detention and de-watering of the sludge, although sand de-watering beds are also used in many cases, especially where land is scarce and land values are high. Where lagoon is used, a duplicate unit is usually provided so that while one lagoon is being filled, the sludge settled in the other lagoon can be excavated and trucked to the final disposal site. Each lagoon should have capacity for a minimum of one year of sludge production.

The de-watered sludge from sand drying beds is easier to handle than the wet sludge from a lagoon. Land filling areas using the waste treatment plant sludge are usually protected from the entry of animals and human beings.

31

V. FUTURE POSSIBILITIES OF ALTERNATIVE AND IMPROVED TECHNOLOGY

The technology of electroplating has advanced over the years so much so that today maximum utilization of the metals and conservation of water are practised in the electroplating industries. However, this is particularly noticeable in large organized engineering industries having electroplating sections. Many small-scale and independent electroplating industries, especially in South East Asian countries, have still to improve these measures, on the other hand.

In respect of the technology of treatment of the waste waters from elec- troplating industries, the methods described in the previous chapter are well proven and are being practised by most of these industries with satisfactory results. However, in certain cases where the permissible limits of the heavy metals like chromium, etc., have been made more stringent by the local regulatory agencies, further treatment of the pretreated chrome waste waters (by ferrous sulphate followed by lime) with iron sulphide, FeS, or insoluble starch xanthate has been found satisfactory (9 and lo), since the heavy metal sulphides and xanthates are least soluble in water.

32

VI. SUMMARY AND RECOMMENDATIONS

Summary Electroplating or metal finishing industries have increased rapidly with the

growth of numerous metal manufacturing and other engineering industries, especially in South East Asian countries after World War 11. In comparison with other industries, however, the electroplating industry uses much less water, hence the volume of the waste waters produced by this industry is also compara- tively much smaller. Nevertheless, the waste waters are highly toxic in nature due to the presence of metals such as copper, zinc, nickel, chromium, etc., and of acids and the highly dangerous cyanides. Although the methods of removal of these toxic substances from waste waters are well established now, the problem of pollution of water courses, and particularly of disposal of untreated waste waters into municipal sewers, in many countries of South East Asia is yet to be solved.

The plating operations are preceded by cleaning and stripping of metal surfaces by alkali and/or other solvents and acids respectively. After plating is completed, the plated objects are rinsed with water. The cleaning, stripping and rinsing are the main sources of waste waters in an electroplating industry. The volume and characteristics of electroplating waste waters vary widely from industry to industry, depending on the operating practices and water conservation measures adopted in the industry.

Most of the chemicals and metals, especially the hexavalent chromium and cyanides, present in the waste waters above certain concentrations are highly toxic to fish and other aquatic life. Sewage treatment processes may also be affected by these constituents if they are admitted in municipal sewer. The heavy metals may also get accumulated in sewage treatment plant sludge and may render it unfit for use as manure. Discharge of untreated waste waters on land may also contaminate ground waters.

Pollution abatement programme in the electroplating industry starts from the in-house measures for the control and prevention of leakages and spillages, water recycle and reclamation of costly metals from spent concentrated solutions. These measures reduce the concentrations of heavy metals and cyanides conside- rably in the final waste water which is further treated before disposal.

Among the various methods used for the destruction of cyanides, the most common is known as alkaline chlorination. Under this process, when chlorine is added in the waste water in presence of alkali to maintain a pH of about 10.5 and allowed to react for the desired period of time, the cyanides are converted into gaseous nitrogen and carbon dioxide. While metals other than hexavalent chromium are present in a waste water, they are usually precipitated by treatment with alkali (primarily with lime). For treatment of chromium bearing waste waters, the hexavalent chromium is first reduced to trivalent chromium by the addition of ferrous sulphate or sodium metabisulphite in acidic pH range. The trivalent chromium is then precipitated with lime.

33

The aforesaid methods of treatment of electroplating waste waters are usually satisfactory for production of an effluent fit for discharge into a water course or in public sewers. However, where stricter limits in respect of heavy metals are in vogue, the waste waters are further treated with ferrous sulphide or starch xanthate to reduce the concentrations of the heavy metals further.

The major items of costs of a waste-water treatment plant for an electro- plating industry are the holding tanks and reactions tanks, which must be lined with acid-proof materials, acid-proof agitators and pumps and the chemical solution preparation and dosing equipment. The major item of the operating costs is the chemicals for conversion and/or precipitation of metals and destruc- tion of cyanides.

Recommendations Reviewing the foregoing, the following recommendations may be made:- 1. It would be desirable to segregate the cyanide bearing waste waters from

other acidic and metal bearing waste waters and combine them with other alkaline discharges of an electroplating shop.

2. Cyanide may be removed by alkaline chlorination and the treated effluent may be used for precipitation of metals including trivalent chromium.

3. Chrome-bearing waste waters should also be segregated from other waste waters, and the hexavalent chromium be first reduced to trivalent chromium in the pH range of 2.0 to 2.5 by the addition of ferrous sulphate and sulphuric acid in a separate tank.

4. The effluent from the chrome reduction tank may be mixed with other metal bearing waste waters and acidic discharges in another tank, where the mixture may be treated with alkali for precipitation of the metals and neutraliza- tion of the acids.

5 . The effluent from the metal precipitation tank should be settled in a settling tank before disposal. The settled sludge may be de-watered in lagoons or sand de-watering beds before disposal.

6 . In order to make the aforesaid system of waste-water treatment simple and workable, it would be desirable for the electroplating industries to lay or rearrange the factory sewers so as to segregate the respective waste water streams and provide them the sequence of treatment as suggested above.

34

VII. BIBLIOGRAPHY

1. Personal Communication with Dr. Michael T.L. Chiu, Sr. Env. Protec- tion Officer, Environmental Protection Agency, Hong Kong, November 1981.

2. Second Report on the Design of Sewage Treatment Plants for Eight Towns of Punjab (unpublished), prepared by Universal Enviroscience Pvt. Ltd, New Delhi, for the World Bank funded project of Punjab Water Supply and Sewerage Board, Chandigarh, India, November 1980.

3. Guide for Treatment of Effluents of Electroplating Industry, IS: 7453- 1974, Indian Standards Institution, New Delhi, 1975.

4. Burford, M.G. and Masselli, J.W., “Plating Wastes” in Industrial Wastes - Their Disposal and Treatment, Ed. W. Rudolfs, Reinhold Publishing Corp., New York, 1953.

5. Public Health Service Drinking Water Standards (rev. 1962), USPHS Publication 956, Washington, D.C.

6. “Plating-Room Controls for Pollution Abatement”, Ohio River Valley Water Sanitation Commission Report, Cincinnati, U.S.A., 1953.

7. Nemerow, N.L., Liquid Wastes of Industry - Theories, Practices and Treatment, Addison-Wesley Publishing Co. Inc., Massachusetts, 197 1.

8. “Methods for Treating Metal - Finishing Wastes”, Ohio River Valley Water Sanitation Commission Report, Cincinnati, U.S.A., 1965.

9. Scoot, M.C., “SULFEX - A New Process Technology for Removal of Heavy Metals from Waste Streams”, Proc. 32nd Industrial Waste Conference, Purdue University, West Lafayette, Indiana, U.S.A., 1977.

10. Wing, R.E. and Rayford, W.E., “Heavy Metal Removal Processes for Plating Rinse Waters”, Proc. 32nd Industrial Waste Conference, Purdue University, West Lafayette, Indiana, U.S.A., 1977.

ACKNOWLEDGEMENTS

The Industrial Pollution Control Guide-lines were initially drafted by the ESCAP secretariat with the assistance of Dr. R.N. Chakrabarty, Managing Director, Universal Enviroscience Ltd., 32-33 Nehru Place, New Delhi, as consultant, and were subsequently reviewed and endorsed by the Expert Group Meeting on Methods and Costs of Industrial Pollution Control, organized by ESCAP from 15 to 19 June 1982.

The Guide-lines, in their draft form, were also circulated to other United Nations agencies, in particular, UNEP, UNIDO and WHO. The contributions made by these organizations towards finalizing the draft Guide-lines are greatly appreciated.

Finally, the extrabudgetary funds provided by the Government of Japan for organizing the study and the Expert Group Meeting are gratefully acknow- ledged.

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