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

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)

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 United Nations Industrial Development Organization (UNIDO)

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)

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)

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

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)

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)

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)

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)

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 35 - 45 Belt filter press with use of PE 20 – 25 Centrifuge – decanter 20 - 30 Sludge after stabilization with CaO (mostly after belt filter press or centrifuge) 60 - 90 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)

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) United Nations Industrial Development Organization (UNIDO)

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 15-50 mm, approx. 100mm