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

2. 4. Biological (secondary) treatment
Introduction to treatment of tannery effluents - Part 4 (of 6)
4. Biological (secondary) treatment
Activated sludge process – principle, parameters
Aeration devices
Oxidation ditch
Flow-chart of the biological (secondary) treatment
Flow-chart of the fully-fledged treatment
Objective and basic principles:
The main objective is to further reduce the amount of organic and other substances (their content “measured”- expressed as BOD and COD) still present in the effluent after the primary treatment stage and thereby satisfy the standards/limits for discharge into surface waters (rivers, lakes).
The biological treatment stage duplicates the processes that take place in nature but under controlled conditions and, especially, at highly accelerated pace; however, the efficiency of this treatment stage largely depends on the biodegradability of the polluting substrate i.e. its inherent capacity to decompose by the biological process. The remaining suspended and colloidal solids are removed by flocculation and adsorption.
While the biological treatment may be aerobic, facultative or anaerobic (or some combination thereof) in practice, almost only aerobic systems are used; exceptionally, in countries with hot climate and where a lot of land is available, facultative (or preferably aerated/facultative) lagoons are also used.
Due to inherent characteristics of tannery effluents, primarily sulphide/sulphate content, in practice anaerobic treatment is to some extent used only in sludge digestion.
Among many variations of the aerobic processes, the most widely used method is (complete mix) activated sludge treatment with extended aeration: it uses the metabolism of microorganisms to remove substances causing oxygen demand.
Simply said, we stimulate the microorganisms to convert (eat & digest) harmful, oxygen demanding organic compounds into environmentally more acceptable form (microorganisms) and low energy, stable compounds like water and carbon dioxide.
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3. Activated sludge process
Introduction to treatment of tannery effluents - Part 4 (of 6)
Activated sludge process
Air
CLARIFIER
Influent
AERATION TANK
Effluent
Q+R Q
Sludge return
Excess of sludge
Sludge
recirculation
pump
Note the main feature of this process: circulation of active sludge – floc from clarifier to aeration tank and back. R
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4. Activated sludge process
Introduction to treatment of tannery effluents - Part 4 (of 6)
Activated sludge process
Total Influent Volume, Q: Volume of the treated effluent [m3/day]
Tank Volume, V: Aeration tank volume [m3]
Organic Loading, F: The total BOD5 applied [kg/day]
F = (f * Q)/1000
where f – the BOD5 of the influent [mg/l]
Mixed Liquor Suspended Solids, M:
M = (MLSS * V)/1000
where MLSS - is the concentration of SS in Mixed Liquor in the aeration tank [mg/l]
Loading factor, F/M: BOD5 kg per day per kg of Mixed Liquor Suspended
Solids (MLSS) in the aeration tank [mg/l].
Hydraulic retention time, T:
T = (V/Q) * 24
where V – aeration tank volume [m3]
The key parameter here is the Loading Factor, F/M: kg BOD per day per kg of Mixed Liquor Suspended Solids (MLSS) in the aeration tank, i.e. food to microorganisms ratio, because pollutants (expressed as BOD) are food to microorganisms contained in floc (sludge).
United Nations Industrial Development Organization (UNIDO)

5. Typical food to microorganisms (F/M) and oxygen to food ratios
Introduction to treatment of tannery effluents - Part 4 (of 6)
Typical food to microorganisms (F/M) and oxygen to food ratios
Process
Extended aeration
Conventional
High rate
Food to microorganisms
F/M
0.1
0.3
1.0
Oxygen to food
kg O2/kg BOD
2.0
1.2
0.8
According to the F/M ratio we speak about extended aeration, conventional and high load process with corresponding supply of oxygen.
United Nations Industrial Development Organization (UNIDO)

6. Other operational parameters
Introduction to treatment of tannery effluents - Part 4 (of 6)
Other operational parameters
Dissolved Oxygen, DO:
The content of molecular oxygen in the aeration tank [mg O2/l].
Usually DO = mg O2/l.
pH:
The optimum pH range for aerobic process is
Temperature:
Affects the metabolic and growth rates of the organisms.
Optimum: 10-30oC.
Nutrients, desirable ratio:
BOD : N : P = 100 : 5 : 1
Sludge Volume Index, SVI:
Volume of 1 g of activated sludge after settling the aerated mixed liquor in a 1000 ml graduated cylinder or Imhoff cone for 30 min.
Desired value < 100 ml/g.
Other important parameters of the biological process. They are monitored on- and off-line, Dissolved Oxygen (DO) and Sludge Volume Index (SVI) possibly most frequently checked in small plants.
United Nations Industrial Development Organization (UNIDO)

7. Introduction to treatment of tannery effluents - Part 4 (of 6)
Aeration devices
There are many types and subtypes of aerators, models being continuously improved. Here is an arbitrary classification of types frequently installed in tannery ETPs.
Surface aerators
Radial flow, low speed, rpm
Axial flow, high speed, rpm
Brush rotor (oxidation ditch) 
Submerged turbines 
Diffused air 
Bubblers – porous and nonporous diffusers
Tubular
Jets (developed from Venturi ejectors)
This is just one among many possible classifications of aeration devices.
United Nations Industrial Development Organization (UNIDO)

8. Classification of mechanical aerators
Water (effluent) aeration is important business with wide range of models, designs. In addition to cost, reliability etc. the key criterion is the amount of air (oxygen) transfer per KW installed.

9. Surface (floating) aerators
Introduction to treatment of tannery effluents - Part 4 (of 6)
The schemes show the work principle of floating aerators that used to be popular due to ease of access and maintenance.
United Nations Industrial Development Organization (UNIDO)

10. Surface (floating) aerators
Introduction to treatment of tannery effluents - Part 4 (of 6)
Due to aerosol problem floating aerators are nowadays avoided, especially in vicinity of residential areas.
United Nations Industrial Development Organization (UNIDO)

11. Introduction to treatment of tannery effluents - Part 4 (of 6)
Submersed aerator
Introduction to treatment of tannery effluents - Part 4 (of 6)
One among many types of turbine aerators; this model is installed at the tank bottom. The effluent enters the rotor from the upper side whereas the air, supplied by the blower enters it underneath resulting in strong mixing and aeration effect.
United Nations Industrial Development Organization (UNIDO)

12. Introduction to treatment of tannery effluents - Part 4 (of 6)
Submersed aerator
Another model of submersed aerator fixed to the tank bottom.
United Nations Industrial Development Organization (UNIDO)

13. Bubble bottom diffusers
Introduction to treatment of tannery effluents - Part 4 (of 6)
If their design and material are “clog resistant”, owing to their high oxygen transfer rate, fine bubble bottom diffusers are certainly the optimum choice for aeration at the biological step in most ETPs.
United Nations Industrial Development Organization (UNIDO)

14. Introduction to treatment of tannery effluents - Part 4 (of 6)
Oxidation ditch
Introduction to treatment of tannery effluents - Part 4 (of 6)
Oxidation ditches are considered shock resistant, comparably simple to operate and easily expandable. Also suitable for upgrading/retrofitting to include nitrogen removal - nitrification and denitrification. In the scheme below please note the anoxic zone needed for denitrification, i.e. the last stage of nitrogen removal.
United Nations Industrial Development Organization (UNIDO)

15. Introduction to treatment of tannery effluents - Part 4 (of 6)
Oxidation ditch
Introduction to treatment of tannery effluents - Part 4 (of 6)
External view of oxidation ditches.
United Nations Industrial Development Organization (UNIDO)

16. Bottom diffusers in the oxidation ditch
Introduction to treatment of tannery effluents - Part 4 (of 6)
Often in addition to rotary brushes at the surface, bottom diffusers are installed to improve the aeration – oxygen transfer.
United Nations Industrial Development Organization (UNIDO)

17. Submersed turbine aerators in the oxidation ditch
Introduction to treatment of tannery effluents - Part 4 (of 6)
Submersed turbine aerators in the oxidation ditch
The same effect can be achieved by installing submersed turbine aerators.
United Nations Industrial Development Organization (UNIDO)

18. Introduction to treatment of tannery effluents - Part 4 (of 6)
Oxidation ditches
Introduction to treatment of tannery effluents - Part 4 (of 6)
And here we can see several examples of large scale oxidation ditches in operation.
United Nations Industrial Development Organization (UNIDO)

19. Purification efficiency of the treatment stages referred to raw effluent
Parameter
Physical-chemical
Biological
Reduction
Approx. value
Reduction*
Approx. value %
mg/l SS
BOD5
97- 99
COD
TKN Cr
5 - 10
< 1
S2-
TDS, mainly Cl- and (SO4)2-
No reduction
The figures here indicate only the typical orders of magnitude.
* 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.

20. Biological treatment stage Simplified flow-chart
Introduction to treatment of tannery effluents - Part 4 (of 6)
AIR
BLOWER
Submersible pump
Pump
Motor
Gate valve
AIR
PRIMARY TREATED
EFFLUENT
AERATION TANK 1
AERATION TANK 2
(DIFFUSED AERATION SYSTEM)
SECONDARY SETTLING
TANK
FLOW MEASUREMENT
CHANNEL
END FLOW MEASURING
WITH FLOW SUMMARISING
TREATED
EFFLUENT
FOR DISCHARGE
SECONDARY SLUDGE
PUMPING STATION
The process shown in the flow-chart looks quite simple. However, in reality, it takes a lot of experience to optimize the balance of many complex hydraulic and biological aspects of the treatment in order to create suitable conditions for varying microorganisms population (floc) to feed on not necessarily always appetizing ingredients (pollutants) and converting them environmentally acceptable organic mass – sludge. In terms of cost, due to energy consumption, sludge dewatering and biology are the most expensive parts of the overall treatment. G
SLUDGE
THICKENER
CHAMBER
FILTER PRESS
SLUDGE DRYING
BEDS (optional)
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21. Fully-fledged treatment system
Introduction to treatment of tannery effluents - Part 4 (of 6)
Submersible pump
Dosing pump
Motor
Ball valve
Gate valve
Agitator
Pump
SERVICE WATER
CATALYST
MnSO4
ALUM
LIME
POLYELECTROLYTE
Drain
Drain
Drain
Drain
MIXED EFFLUENT
AUTOMATIC
COARSE SCREEN
FINE SCREEN
PRIMARY
SETTLING TANK
EQUALIZATION TANK WITH PUMPING STATION
(WITH EJECTORS FOR MIXING AND AERATION)
COAGULATION
TANK
FLOCCULATION
INLET PUMPING STATION
BLOWER
AIR
SECONDARY
SETTLING TANK
FLOW MEASUREMENT
CHANNEL
END FLOW MEASURING
WITH FLOW SUMMARISING
AIR
For the sake of clarity no on-line monitoring points/instruments are indicated. Similarly, this presentation set does not cover the complex issue of utilization and/or safe disposal of sludge produced as result of effluent treatment.
AERATION TANK 1
AERATION TANK 2
(DIFFUSED AERATION SYSTEM)
TREATED
EFFLUENT
FOR DISCHARGE G
SLUDGE
THICKENER
SECONDARY SLUDGE
PUMPING STATION
SLUDGE DRYING
BEDS (optional)
PRIMARY SLUDGE
PUMPING STATION
CHAMBER
FILTER PRESS
United Nations Industrial Development Organization (UNIDO)