1. INTRODUCTION TO THE TREATMENT OF TANNERY EFFLUENTS
Introduction to treatment of tannery effluent - Part 1 (of 6)
United Nations Industrial Development Organization
INTRODUCTION TO THE TREATMENT OF TANNERY EFFLUENTS
What every tanner should know about effluent treatment
Part I
Compiled by J. Buljan, I. Kral
United Nations Industrial Development Organization (UNIDO) 1

2. 1. Load, norms, in-house treatment
Introduction to treatment of tannery effluent - Part 1 (of 6)
1. Load, norms, in-house treatment
Pollution load
Discharge norms
General overview of treatment of tannery effluents
Segregation of streams
Treatment of spent liming floats
Treatment of chrome-bearing floats
Linking of individual tanneries to a CETP or MWWW
In view of ever increasing legal and social pressures, no tanner can afford the luxury of not being familiar with main issues and principles of environmental protection pertaining to tannery operations.
Pollution prevention, persistent promotion of cleaner leather processing, ultimately leading to lower treatment costs, remains the supreme priority.
Yet, despite all preventive measures, there is still a considerable amount of pollution load to be dealt with by the end-of-pipe methods.
The purpose of this presentation(s) is to help a tanner or a tannery manager, possibly a well-trained leather technologist, to familiarise with basic principles and methods of treatment of tannery effluents. This knowledge should make him/her better equipped for a dialogue with the factory's own environmental unit, environmental authorities and NGOs.
With the view of keeping it short and concise there are many simplifications and omissions of details; thus, for in-depth understanding of the complexities of treatment of effluents and solid wastes (sludge) it is recommended to consult extensive literature on this subject.
All data and information here refer to combined chrome tanning because it is still by far the most prevailing leather tanning method.
Due to varieties in raw material, process, chemicals, water consumption etc. it is small wonder that figures about pollution load in the literature vary a lot and should be interpreted very cautiously. Charts and tables presented here below may give the general idea, the usual reference being a tonne of wet salted hides.
United Nations Industrial Development Organization (UNIDO)

3. Introduction to treatment of tannery effluent - Part 1 (of 6)
LEATHER PROCESSING
PRODUCTS
Wet-blue
Raw hides
TANNERY Site
Crust
TANNERY Plant/Building(s)
Chemicals
Finished
leather
Effluent
for
treatment
Water
Air pollution
Fresh air
Here we have a general overview of all material inputs and outputs in leather processing.
Solid waste
Processing
Disposal
United Nations Industrial Development Organization (UNIDO)

4. Introduction to treatment of tannery effluent - Part 1 (of 6)
WATER
pollutants
SOLID
pollutants
RAW HIDES
AIR
pollutants
Soaking
BOD,COD, SS, TDS, organic N
Green fleshing
Fleshings
(fat containing organic matter)
BOD,COD, SS, TDS, alkalinity, sulphides, organic N, ammonia N
Unhairing
Liming
Hair, lime, organic matter containing sludge
H2S
Lime fleshing
BOD,COD, SS, TDS, alkalinity, sulphides
Lime fleshings
(fat containing org. matter)
Lime splitting
Trimming
Lime split and trimmings
(limed organic matter)
Deliming
Bating
BOD,COD, TDS, ammonia N
NH3
Degreasing
(sheep/pigskins)
BOD,COD, TDS, ammonia N
Pickling
Tanning
BOD,COD, SS, TDS, acidity, salts, chrome, vegetable tans, syntans
At macro (strategic) level, it should be borne in mind that wet blue production is capital and NOT labour intensive and it brings rather low added value. However, possibly even more important is the fact that the major part of pollution load emanates from this stage of process !!! Please note a significant amount of solid and air discharges/pollutants.
Sammying
BOD,COD, SS, TDS, acidity, salts, chrome, vegetable tans, syntans
Chrome splitting
Chrome split
(chrome containing org. matter)
Shaving
LEGEND:
BOD – Biochemical Oxygen Demand
COD – Chemical Oxygen Demand
TDS – Total Dissolved Solids
SS – Suspended Solids
Shavings
(chrome containing org. matter)
WET-BLUE
United Nations Industrial Development Organization (UNIDO)

5. Introduction to treatment of tannery effluent - Part 1 (of 6)
WATER
pollutants
SOLID
pollutants
AIR
pollutants
WET-BLUE
Retanning, dyeing
Fat liquoring
BOD,COD, TDS, chrome tans, dyes, fats
Drying
Buffing
Trimming
Chrome trimmings
(chrome and prefinishing chemicals containing org. matter)
CRUST
Finishing
Reminder of finishing agents
Solvents, CH2O
While the pollution load from this stage is comparatively much lower, the COD emanating from certain retanning agents can be of recalcitrant character i.e. very hard to degrade and making the additional, tertiary treatment unavoidable.
LEATHER
LEGEND:
BOD – Biochemical Oxygen Demand
COD – Chemical Oxygen Demand
TDS – Total Dissolved Solids
SS – Suspended Solids
United Nations Industrial Development Organization (UNIDO)

6. Introduction to treatment of tannery effluent - Part 1 (of 6)
WATER
pollutants
RAW HIDES
SOLID
pollutants
AIR
pollutants
Soaking
BOD,COD, SS, TDS, organic N
Green fleshing
Fleshings
(fat containing organic matter)
Unhairing
Liming
BOD,COD, SS, TDS, alkalinity, sulphides, organic N, ammonia N
Hair, lime, organic matter containing sludge
H2S
Lime fleshing
BOD,COD, SS, TDS, alkalinity, sulphides
Lime fleshings
(fat containing org. matter)
Lime splitting
Trimming
Lime split and trimmings
(limed organic matter)
Deliming
Bating
BOD,COD, TDS, ammonia N
NH3
Degreasing
(sheep/pigskins)
BOD,COD, TDS, ammonia N
Pickling
Tanning
BOD,COD, SS, TDS, acidity, salts, chrome, vegetable tans, syntans
Sammying
BOD,COD, SS, TDS, acidity, salts, chrome, vegetable tans, syntans
Chrome splitting
Chrome split
(chrome containing org. matter)
Shaving
Shavings
(chrome containing org. matter)
WET-BLUE
Too often our attention is focused only on pollutants contained in the floats/effluent; however please note the substantial amount of solid waste – by-products and air emissions.
Retanning, dyeing
Fat liquoring
BOD,COD, TDS, chrome tans, dyes, fats
Drying
LEGEND:
BOD – Biochemical Oxygen Demand
COD – Chemical Oxygen Demand
TDS – Total Dissolved Solids
SS – Suspended Solids
Buffing
Trimming
Chrome trimmings
(chrome and prefinishing chemicals containing org. matter)
CRUST
Finishing
Solvents, CH2O
Reminder of finishing agents
LEATHER
United Nations Industrial Development Organization (UNIDO)

7. EXAMPLE OF MASS BALANCE OF CHEMICALS IN LEATHER PROCESSING
Introduction to treatment of tannery effluent - Part 1 (of 6)
EXAMPLE OF MASS BALANCE OF CHEMICALS IN LEATHER PROCESSING
kg/t of wet salted hide
Chemicals
Added in process
In/on leather & splits
Wasted
Chrome oxide (Cr2O3) 25 12 13
Organic tannins 20 5
Fatliquors 22 17
Dyestuffs 4 1
Acids, bases, salts
191 -
Tensides 3
Enzymes
Finishing products
100 88
Total
452 72
380
Even many practical tanners are surprised by the amount/percentage of chemicals not absorbed by leather: typically more than 80 % of the total amount offered.
United Nations Industrial Development Organization (UNIDO)

8. TYPICAL TOTAL POLLUTION LOAD
Introduction to treatment of tannery effluent - Part 1 (of 6)
TYPICAL TOTAL POLLUTION LOAD
Conventional process, expressed as concentration in combined raw effluent
Parameter
Unit
Biochemical oxygen demand (BOD5)
mg O2/l
Chemical oxygen demand (COD)
Suspended solids (SS)
mg/l
Trivalent chromium (Cr3+)
mg Cr/l
Sulphides, S2-
mg S/l
Total Kjeldahl Nitrogen (TKN)
mg N/l
Chlorides, (Cl-)
mg Cl/l
Sulphates (SO42-)
mg SO4/l
Oil and grease
Total Dissolved Solids (TDS)* pH
Average total pollution load
2000
4000
150
160
5000
1400
130
10000
6 - 9
Typical limits, surface waters
100
Locally specific
It is given here for a conventional process and expressed in terms of concentration in effluent at water consumption level of 45 m3/tonne of w.s. hide. Nowadays, with better water management and lower water consumption the concentration is much higher and, thus the purification task in order to reach the norms shown in the last column is more difficult.
*The main constituents of the TDS are chlorides (Cl-) and sulphates (SO42-) listed earlier.
United Nations Industrial Development Organization (UNIDO)

9. TYPICAL TOTAL POLLUTION LOAD
Introduction to treatment of tannery effluent - Part 1 (of 6)
Conventional process, kg/t of wet salted hide
Tannery with 5 t/day wet-salted input
Parameter
Biochemical oxygen demand (BOD5)
Chemical oxygen demand (COD)
Suspended solids (SS)
Trivalent chromium (Cr3+)
Sulphides, S2-
Total Kjeldahl Nitrogen (TKN)
Chlorides, (Cl-)
Sulphates (SO42-)
Oil and grease
Total Dissolved Solids (TDS)* pH
kg/t 90
180 7
7.5
225 63 6
450
6 - 9
kg/day
450
900 35
37.5
1125
315 30
2250
It is given here for a conventional process and expressed in terms of load per tonne of w.s. hide, i.e. regardless of water consumption and that is why parameters expressed in this manner are safer indicators of pollution load.
*The main constituents of the TDS are chlorides (Cl-) and sulphates (SO42-) listed earlier.
United Nations Industrial Development Organization (UNIDO)

10. IN-HOUSE SEGREGATION OF STREAMS Linked to a CETP
Introduction to treatment of tannery effluent - Part 1 (of 6)
IN-HOUSE SEGREGATION OF STREAMS Linked to a CETP
Screens
To the treat-
ment plant
Grit floating matter chamber
SULPHIDE OXIDATION UNIT
Post-tanning effluent
Soaking and other effluent
Septic tank (optional)
Sanitary effluent
Tanning liquor
Screens
Screens
Liming effluent
TANNERY
S h o w e r s, t o i l e t s
CR-RECOVERY UNIT
Strict segregation of streams, i.e. of soaking, liming, tanning and post-tanning streams is essential for efficient treatment of effluents, possible recycling and staff health and safety. In case of retanning with Cr-containing agents it is desirable to remove the residual Cr from that spent float too.
Recovered Cr-liquor
United Nations Industrial Development Organization (UNIDO)

11. EFFLUENT HANDLING WITHIN TANNERIES
SOAKING EFFLUENT
20 mm
10 mm
SULPHIDE OXIDATION
(optional)
MnSO4(H2O) dosing
20 mm
10 mm
LIMING EFFLUENT
TRANSPORT TO SLUDGE LANDFILL
Sludge drying beds
(optional)
CROME RECOVERY PLANT
(optional)
NaOH dosing
Steam
20 mm
10 mm
TANNING LIQUOR
Recovered Cr-liquor
RECYCLED
Grit and floating matter
(grease and oil) chamber
6 mm
Cr-sludge
Diatomaceous
earth
H2SO4
Sulphide oxidation with manganese sulphate as catalyst is a kind of standard.
Chrome recycling using NaOH for precipitation and H2SO4 for re-dissolving is most common method but some prefer precipitation with MgO or even “endless” direct recycling of chrome float.
Good Cr management, resulting – inter alia – in low Cr content in the common sludge is essential in improving the scope of options for ultimate chrome reutilization and/or safe disposal.
Impurities
TO TANNERY EFFLUENT COLLECTION AND CONVEYANCE SYSTEM
POST-TANNING & OTHER EFFLUENT
Septic tank
(optional)
SANITARY EFFLUENT

12. TYPICAL EFFLUENT TREATMENT SETUP
VARIANT: CETP for a tannery cluster
River/channel
Reservoir/lake/sea
TANNERY
SITE
CETP
Common Effluent
Treatment Plant
TANNERY PT
Pre-treatment
In a well organized typical tannery cluster, there is mandatory sulphide oxidation and Cr recovery and simple pre-treatment in each individual tannery followed by full treatment in the common effluent treatment plant (CETP) and final discharge into a water recipient.
Other tanneries S
S2-
oxidation Cr
recovery

13. TYPICAL EFFLUENT TREATMENT SETUP
VARIANT: Final treatment together with urban wastewaters at the municipal plant
River/channel
Reservoir/lake/sea
Communal and other effluent
MWWW
Municipal Wastewater Works
TANNERY
SITE
PhCh
Physical-
Chemical
treatment
TANNERY PT
Pre-treatment
In industrialized countries each tannery has to have full physical-chemical treatment within the tannery compound before discharge into sewage channel for joint treatment at the municipal wastewater works, MWWW. However, nowadays it is often insisted that both primary and secondary (biological) treatment have to be carried out before discharge into municipal sewer. S
S2-
oxidation Cr
recovery