INTRODUCTION TO T HE T REATMENT OF TANNERY EFFLUENTS

1. INTRODUCTIONTOTHE TREATMENTOF TANNERY EFFLUENTS What every tanner should knowabout effluent treatment United NationsIndustrialDevelopmentOrganization Part IV Compiled byJ. Buljan, I. Kral

2. 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

3. Activated sludge process Air CLARIFIER Effluent Influent Sludge return AERATION TANK Q+R Sludge recirculation pump Q R Excess of sludge

4. 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]

5. Typical food to microorganisms (F/M) and oxygen to food ratios

6. Other operational parameters • Dissolved Oxygen, DO: The content of molecular oxygen in the aeration tank [mg O2/l]. Usually DO = 0.5-2.0 mg O2/l. • pH: The optimum pH range for aerobic process is 7.0-7.5 • 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.

7. 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, 20-60 rpm • Axial flow, high speed, 300-1200 rpm • Brush rotor (oxidation ditch)  • Submerged turbines  • Diffused air  • Bubblers – porous and nonporous diffusers • Tubular • Jets (developed from Venturi ejectors)

8. Classificationof mechanical aerators

9. Surface (floating) aerators

10. Surface (floating) aerators

11. Submersed aerator

12. Submersed aerator

13. Bubble bottom diffusers

14. Oxidation ditch

15. Oxidation ditch

16. Bottom diffusers in the oxidation ditch

17. Submersed turbine aeratorsin the oxidation ditch

18. Oxidation ditches

19. Purification efficiency of the treatment stages referred to raw effluent * 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 stageSimplified flow-chart AIR Submersible pump AIR AIR Pump PRIMARY TREATED EFFLUENT Motor BLOWER Gate valve TREATED EFFLUENT FOR DISCHARGE AERATION TANK 1 AERATION TANK 2 (DIFFUSED AERATION SYSTEM) END FLOW MEASURING WITH FLOW SUMMARISING SECONDARY SETTLING TANK FLOW MEASUREMENT CHANNEL SLUDGE THICKENER G CHAMBER FILTER PRESS SECONDARY SLUDGE PUMPING STATION SLUDGE DRYING BEDS (optional)

21. Submersible pump Dosing pump SERVICE WATER Pump POLYELECTROLYTE LIME ALUM CATALYST MnSO4 Motor Ball valve AIR Gate valve Agitator Drain Drain Drain Drain MIXED EFFLUENT BLOWER Fully-fledged treatment system AUTOMATIC COARSE SCREEN FINE SCREEN INLET PUMPING STATION EQUALIZATION TANKWITH PUMPING STATION (WITH EJECTORS FOR MIXING AND AERATION) AERATION TANK 1 AERATION TANK 2 (DIFFUSED AERATION SYSTEM) SECONDARY SETTLING TANK AIR AIR PRIMARY SETTLING TANK END FLOW MEASURING WITH FLOW SUMMARISING TREATED EFFLUENT FOR DISCHARGE FLOCCULATION TANK COAGULATION TANK FLOW MEASUREMENT CHANNEL G SLUDGE DRYING BEDS (optional) CHAMBER FILTER PRESS SECONDARY SLUDGE PUMPING STATION SLUDGE THICKENER PRIMARY SLUDGE PUMPING STATION