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 II
Compiled by J. Buljan, I. Kral
United Nations Industrial Development Organization (UNIDO) 1

2. 2. Treatment within tannery compound
Introduction to treatment of tannery effluent - Part 2 (of 6)
2. Treatment within tannery compound
Pre-treatment for discharge into CETP collection network 
Screening
Grit & grease chamber
Physical-mechanical (primary) treatment for discharge into municipal sewage 
Lifting – lifting pumps
Equalization – homogenization, ejectors
pH adjustment, coagulation, flocculation
Primary clarification/flotation
Flow-chart of the physical-chemical (primary) treatment
Objectives:
 To eliminate the coarse material normally present in the raw wastewater that could clog/block pumps, pipes and, possibly, sewer lines
To well mix & balance different tannery streams and thus produce homogenised “raw material” that can be treated in a constant way
To adjust pH and eliminate toxic substances (sulphides) and/or shock loads that could negatively affect the rather sensitive biological treatment
To significantly decrease the BOD/COD load and thus reduce the cost of and simplify the biological treatment phase.
In short: to eliminate the coarse matter, nearly remove Cr and sulphide, remove the major part of Suspended Solids, considerably reduce the BOD and COD content.   
Basic steps:
1. Screening (bar, grease and sand trap, self-cleaning, rotary)
2. Pumping/lifting
3. Equalisation &sulphide oxidation
4. Chemical treatment (coagulation, flocculation)
5. Settling
6. Sludge dewatering
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3. Introduction to treatment of tannery effluent - Part 2 (of 6)
ROUGH BAR SCREEN
Introduction to treatment of tannery effluent - Part 2 (of 6)
rake
Collected
solids
bar screen
CHANNEL
The importance of good screening, including its role in protecting the treatment equipment cannot be overemphasised.
Raw effluent
From
TANNERY
Screened effluent To
TREATMENT
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4. Introduction to treatment of tannery effluent - Part 2 (of 6)
BAR SCREEN CLEANING
Manual
Rotary curved 3600
Rotary curved 900
Auto chain
Continuous chain raked
The level of mechanization – automation at this stage obviously mainly depends on the size – plant capacity and labour and electricity costs.
United Nations Industrial Development Organization (UNIDO)

5. NON-AERATED GRIT AND FLOATING MATTER REMOVAL CHAMBER
Introduction to treatment of tannery effluent - Part 2 (of 6)
NON-AERATED GRIT AND FLOATING MATTER REMOVAL CHAMBER
Periodically to the sludge disposal site
Screen
20 mm
Screen
10 mm
Screen
6 mm
SLUDGE
SCUM
Removal of sand, grit and oil is essential to avoid difficulties like clogging, excessive wear and tear of equipment, inadequate aeration etc. at subsequent stages of the process.
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6. SELF-CLEANING SCREEN PARKWOOD TYPE
Introduction to treatment of tannery effluent - Part 2 (of 6)
SELF-CLEANING SCREEN PARKWOOD TYPE
Effluent
inflow
Ready to install factory made Parkwood type screen. Many small tanneries have self-made units.
United Nations Industrial Development Organization (UNIDO)

7. SUBMERSIBLE PUMP FOR EFFLUENT LIFTING
Introduction to treatment of tannery effluent - Part 2 (of 6)
SUBMERSIBLE PUMP FOR EFFLUENT LIFTING
Pump maintenance
Lifting
chain
The pump can be brought to the ground inspection/maintenance/repair without emptying the tank/pit. For that purpose essential is good functioning of the special arrangement (not quite visible in this sketch) for easy decoupling and re-coupling of the pump.
Pump
Water inlet
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8. ROTARY DRUM SCREEN KONICA TYPE – outer flow
Introduction to treatment of tannery effluent - Part 2 (of 6)
ROTARY DRUM SCREEN KONICA TYPE – outer flow
WASTWATER INLET
WITH SUSPENDED SOLIDS
Filtering gap
Trapezoidal shaped bars
As a rule, fine screening is the next step after effluent lifting. Drum design should ensure avoidance of clogging and easy washing.
SOLIDS RECOVERY
FILTERED WATER
DISCHARGE
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9. Rotary fine screen (inner flow)
Introduction to treatment of tannery effluent - Part 2 (of 6)
Brush
Raw effluent
Solids
Internal screw
A different flow principle but again the drum design should ensure avoidance of clogging and easy washing.
Filtering panel
Filtered effluent
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10. ROTARY DRUM SCREEN
In some cases a different design with solids lifted/transported by the screw can be preferred.

11. Screw (Archimedes) type pump for effluent lifting
Effluent intake
The principle
Driving
mechanism
A different effluent lifting concept, robust and reliable, suitable for large volumes.

12. Screw (Archimedes) type pump for effluent lifting
Introduction to treatment of tannery effluent - Part 2 (of 6)
Screw (Archimedes) type pump for effluent lifting
A typical installation and modus operandi of Archimedes screw pump.
United Nations Industrial Development Organization (UNIDO)

13. Equalization – homogenization tank Schematic view
Introduction to treatment of tannery effluent - Part 2 (of 6)
Equalization – homogenization tank Schematic view
Continuous level
measuring
Pump dry
protection level
Bypass
TO SUCCESSIVE
TREATMENT
High level alarm
EFFLUENT INFLOW
Free board
Well homogenized, uniform effluent is a precondition for successful subsequent treatment. Its volume usually corresponds to one day effluent discharge. Here the robust, Venturi type ejectors keep the remaining particles in suspension and provide the necessary aeration to prevent bad smell – reversal to sulphide.
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14. Coagulation and flocculation (scheme)
Introduction to treatment of tannery effluent - Part 2 (of 6)
Service
water
POLY-
ELECTROLYTE
ALUM
LIME
Dosing
pump
Dosing
pump
Dosing
pump
DRAIN
WASTEWATER INLET
In order to improve and accelerate the settling of suspended solids, especially of the fine and colloidal matter coagulants such as industrial alum (Aluminium Sulphate Al2(SO4)3.18H2O) and flocculants - polyelectrolytes are added before sedimentators. The coagulants destabilize the colloids by neutralizing the forces that keep them apart resulting in creation of floc whereas flocculants bind them into large agglomerates or clumps. Usual dosage, industrial purity, in g/m3: manganese sulphate 30, aluminium sulphate (alum) 300, hydrated lime 300, anionic polyelectrolyte 1 g/m3 Proper dissolving essential. Note the difference in mixing speed in coagulation and flocculation tank, retention times differ as well.
WASTEWATER
OUTLET
COAGULATION
TANK
FLOCCULATION
TANK
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15. Ejector for mixing and aeration
Introduction to treatment of tannery effluent - Part 2 (of 6)
Air inlet
Wastewater
Submersible
pump
Here we can better see in operation the Venturi type ejector mentioned earlier – providing both mixing and aeration effect.
Air-wastewater
mixture outlet
Ejector
Wastewater inlet
United Nations Industrial Development Organization (UNIDO)

16. Type of sedimentation tanks
Rectangular horizontal flow
Sometimes, mainly for space and construction reasons rectangular shape for the sedimentator is preferred.

17. Type of sedimentation tanks
Circular, radial-flow
Circular, radial flow is possibly the preferred type of sedimentator in the tanning industry. Please note that here “feeding” is from the bottom to the surface and then radially, at calculated velocity to the edge and overflow. The slowly rotating arms scrape the sludge towards the collecting trough. Two main parameters are surface load in terms of effluent volume per surface area of the tank (m3/m2 of tank surface per hour or m/h, typically m3/m2 per hour) and retention time, i.e. the total time effluent spends in the tank.

18. Type of sedimentation tanks
Hopper-bottomed, upward-flow
The advantage of sloped bottom sedimentation tank is that a rotating sludge scraper is not required.

19. Typical sedimentation tank
Scraper bridge
Scraper
Here we can better see the overflow and the scraper bridge of the circular, up-flow sedimentator shown earlier.

20. Typical sedimentation tank
Introduction to treatment of tannery effluent - Part 2 (of 6)
Schematic cross-section view
Drive
Skimmer
Scum trough
Effluent
Influent well
Scum
draw
off
In this circular sedimentator the influent flows through the pipe into influent well, particles/sludge accumulating at the bottom and being drawn away; cleaner effluent leaves via overflow/pipe, the scum is collected by the skimmer drawn at the opposite side.
Scraper
Influent
Sludge
concentrator
Sludge draw-off
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21. Rectangular sedimentation tank with travelling bridge
Introduction to treatment of tannery effluent - Part 2 (of 6)
Rectangular sedimentation tank with travelling bridge
Scum
trough
TRAVELLING
BRIDGE
TRAVELLING
BRIDGE
Rail
INFLUENT
EFLUENT
SLUDGE
DRAWOFF
Screw cross
collector
Here we can see the rectangular type of tank in operation: influent enters sidewise, particles/sludge accumulate at the bottom and are being drawn away; cleaner effluent leaves at the right hand side, the bridge slowly travels to and fro and pushes the sludge at the bottom and the scum (removed at the top).
Sludge
hopper
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22. Dissolved air flotation unit (DAF)
Introduction to treatment of tannery effluent - Part 2 (of 6)
Floated
sludge
skimmer
TREATED EFFLUENT
RAW EFFLUENT
Pressure
release
valve
FLOTATED
SLUDGE
DRAWOFF
AIR
SETTLED
SLUDGE
PRESSURED
RECYLE
WTH
DISSOLVED
AIR
Air
saturation
tank
Due to characteristics of suspended solids and space reasons, instead of sedimentation the "opposite" process, flotation, is selected: air bubbles attached to particles lift them to surface to be subsequently skimmed, removed and dewatered.
RECYCLE FLOW
Recycle
pump
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23. Introduction to treatment of tannery effluent - Part 2 (of 6)
Purification efficiency of the treatment stages referred to raw effluent
Parameter
Physical-chemical
Biological
Reduction
Approx. value
Reduction* %
mg/l SS
80 – 90
300 – 600
95 – 98
20 – 50
BOD5
50 – 65
750 –1500
97 – 99
20 – 30
COD
1500 – 3000
87 – 93
250 – 400
TKN
40 – 50
250 – 300
60 – 70
100 – 200 Cr
92 – 97
5 – 10
99 – 99.5
< 1
S2-
10 – 20
TDS
mainly Cl- and (SO4)2
No reduction
It can be seen here that the best removal rates in the course of physical-chemical (primary) treatment are for Chromium, Suspended Solids and sulphide, i.e %, % and % respectively.
*Approximately at the load level of 0.30 kg BOD5/kg MLSS, oxygen requirement of
2.0 kg O2/kg BOD5 and MLSS 3300 g/m3.
United Nations Industrial Development Organization (UNIDO)