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

2. Introducrion to treatment of tannery effluent - Part 3 (of 6)
3. Sludge dewatering
Sludge thickeners
Sludge pumps
Filter press
Centrifuge
Belt filter press
Flow-chart of the physical-chemical (primary) treatment
Sludge drying beds
Inherently, the basic aim of wastewater treatment is to remove solids and some potentially hazardous substances from the wastewater whereas the biologically degradable organic substances are converted to bacterial cells, and the latter are removed from the wastewater. Therefore, effluent treatment results in two products, one desirable, i.e. the “purified” effluent and the other, causing a lot of headache, sludge. (To be more precise: as result of microorganisms breathing carbon dioxide and water are also produced !).
In terms of volume, crucial is the primary sludge, the amount of secondary (biological) sludge being very minor.
The sludge drawn from the bottom of the primary sedimentation tank is in the form of slurry with dry solids (DS) content of only 2-4 %.
The main purpose of sludge dewatering is not only to reduce the volume and weight of material to be transported but also to attain the dry matter content required for disposal at landfill. 
This is usually achieved by:
(i) thickening in sludge thickeners (very much like circular clarifiers),
(ii) mechanical dewatering in filter presses, belt filter presses or decanters – centrifuges.
Sludge drying beds are inadequate for larger capacities, outside arid zones or close to residential areas.
In addition to power and chemicals requirements, the key parameter for equipment selection is the achievable dry matter content in the dewatered sludge.
United Nations Industrial Development Organization (UNIDO)

3. Progressing cavity pump used for sludge transfer
Introducrion to treatment of tannery effluent - Part 3 (of 6)
Progressing cavity pump used for sludge transfer
Driving axe
INLET
Often colloquially called Mohno pump, this type of pump is considered very suitable for transport of tannery sludge.
Stator
Housing
Rotor
DISCHARGE
Cavity
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4. Introducrion to treatment of tannery effluent - Part 3 (of 6)
Gravity thickener
Construction of the gravity thickener is actually identical with the primary sedimentator except that the sludge at the bottom is considerably thicker.
The key parameter here is the surface solids rate. It is expressed in Kg/m2 indicating the quantity of SS crossing the surface area of the tank over per hour a certain time span (hour, day).
United Nations Industrial Development Organization (UNIDO)

5. Introducrion to treatment of tannery effluent - Part 3 (of 6)
Gravity thickener
A look inside the gravity thickener with a good view of scraper’s bridge and feeding well and the view from outside.
United Nations Industrial Development Organization (UNIDO)

6. Recessed plate filter press
Air injection
Clarified water
Air pump
Plate
Filtration is a batch process. Hydraulic ram closes the plates to stack, hydraulic pressure develops, slurry enters and fills the press. Filtration takes place as filtrate passes through the cloth and cake builds in chambers. Filtrate passes through discharge eyes and exits through outlets. The press is then opened, plates shifted one by one, the cakes drop into container below.
The drawing on the right hand side shows a press with cloth cleaning with compressed air.
Cloth
Compressor
Sludge cake layer
Emulsion layer
Slurry tank
Clarified water

7. Continuous counter-current solid bowl centrifuge
Introducrion to treatment of tannery effluent - Part 3 (of 6)
Continuous counter-current solid bowl centrifuge
With substantial increase in dry matter content, lower wear and tear centrifuge is gaining acceptance in the tanning industry. It works continuously.
United Nations Industrial Development Organization (UNIDO)

8. Introducrion to treatment of tannery effluent - Part 3 (of 6)
Belt filter press
Here we can se a rather complex rollers system.
United Nations Industrial Development Organization (UNIDO)

9. Introducrion to treatment of tannery effluent - Part 3 (of 6)
Belt filter press
Sludge
inlet
Drained water
Sludge
cake
Belt/cloth
Somewhat complex construction and high water consumption for belt washing, more often found in MWWW. It works continuously.
United Nations Industrial Development Organization (UNIDO)

10. Characteristics of sludge dewatering equipment
Introducrion to treatment of tannery effluent - Part 3 (of 6)
Characteristics of sludge dewatering equipment
Decanter
Belt press
Plate filter press
Way of operation
Continuous
Batch
Sludge conditioning
required
not required
Washing water
Labour
only supervision
required during cake discharge
Sensitive to sludge variability
very sensitive
less sensitive
Energy required (electricity)
high
medium
low
Maintenance
sophisticated
A comparative overview of advantages and disadvantages of various types of dewatering equipment.
United Nations Industrial Development Organization (UNIDO)

11. Introducrion to treatment of tannery effluent - Part 3 (of 6)
Dry matter content in sludge depending on process stage and/or type of dewatering equipment
Treatment stage, equipment
DS content %
Sludge after primary clarifier
3 - 5
Thickened mixed primary and secondary sludge (at specific load of about 60 kg DS/m2.d)
6 - 8
Chamber filter press with chemical conditioning
Belt filter press with use of PE
20 – 25
Centrifuge – decanter
Sludge after stabilization with CaO (mostly after belt filter press or centrifuge)
The figures given here vary a lot depending on operating conditions as well as type/model of equipment used.
United Nations Industrial Development Organization (UNIDO)

12. Simplified flow-chart of the physical-chemical (primary) treatment
Introducrion to treatment of tannery effluent - Part 3 (of 6)
Simplified flow-chart of the physical-chemical (primary) treatment
Submersible pump
Dosing pump
Motor
Gate valve
Agitator
SERVICE WATER
CATALYST
MnSO4
ALUM
LIME
POLYELECTROLYTE
Drain
Drain
Drain
Drain
MIXED EFFLUENT
EQUALIZATION TANK
WITH PUMPING STATION
(with ejectors
for mixing and aeration)
AUTOMATIC
COARSE SCREEN
FINE SCREEN
INLET PUMPING
STATION
PRIMARY SETTLING
TANK
COAGULATION
TANK
FLOCCULATION
FLOW MEASUREMENT
CHANNEL
END FLOW MEASURING
WITH FLOW SUMMARISING
As said earlier, the major part of chrome, SS and sulphide is eliminated in the course of pre-treatment and physical-chemical treatment. As a corollary, the major part of sludge is also produced at this stage.
It is important to emphasize the need to ensure smooth process by synchronizing the flow, volumes of all tanks, retention times and surface loadings at certain stages etc.
PRIMARY TREATED EFFLUENT
TO FURTHER
(biological) TREATMENT
e.g. at the municipal
Wastewater works G
SLUDGE
THICKENER
PRIMARY SLUDGE
PUMPING STATION
CHAMBER
FILTER PRESS
SLUDGE DRYING
BEDS (optional)
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13. Sludge drying beds Sludge inlet Drainage water outlet Drainage water
Easily constructed with locally available materials they were perceived as optimum solution for tanneries in hot climate developing countries. However, they require a lot of area, the output during rainy seasons drops considerably, there is the problem of malodour, they are not easy to clean and made ready for the next batch etc. For these reasons sludge drying beds are still used mainly by small tanneries not close to residential areas and/or as temporary fall back units during breakdown of mechanical dewatering equipment.
Drainage water
outlet
Drainage water
outlet
Sludge inlet
Sludge approx. 500mm
Sand 1-3 mm, approx. 150mm
Sand 3-6 mm, approx. 150mm
Broken stone or gravel 6-15 mm,
approx. 100mm
Broken stone or gravel mm,
approx. 100mm