DEWATERING

1. DEWATERING PREPARED BY: MD.ALAM (492/08)

2. INTRODUCTION: The separation of solids from fluids (dewatering) is one of the important subjects in the field of mineral dressing • Water purification • In mining and smelting operations

3. WHAT IS DEWATERING? • A simple answer would be the process of removing water from sludge (suspension of solid matter in liquid). • Dewatering is the process of natural, chemical, or mechanical removal of water from sludge, thereby reducing it to a damp solid with the lowest level of moisture attainable .

4. Mechanical separation of solids from liquids is accomplished by one of two general procedures: • Thickening. • filtration. (Ultimate moisture removal is obtained by drying.) • Separation of solids from gases: • settling. • Filtration.

5. THICKENING: • A suspension of solids in a liquid is allowed to settle until a clear liquid layer tops a mud layer. • A volume reduction of approximately 30 – 80 % can be reached with sludge thickening before a further treatment.

6. IMPORTANT TERMS: • Flocculation: it is a process of contact and adhesion whereby the particles of a dispersion form large size clusters. • Floccules or flocs: these are rounded aggregates of solid particles. • Pulp: any soft or soggy(soft and watery) mass. • Slime: any thick viscous matter. • Flocculant: Particles finer than 0.1 µm (10-7m) in water remain continuously in motion due to electrostatic charge (often negative) which causes them to repel each other

7. THICKENING: •  At smaller wastewater treatment plants, where the sludge is driven off regularly, • thickening usually takes place directly in the sludge storage tank. The sludge is compressed at the tank bottom only by the force of gravity, while above the sludge a cloudy water layer is formed, which is taken off and led back into the inlet.

8. STEPS IN PROCESS OF THICKENING: • Flocculation of the minute solid particles so as to form aggregatesor flocs of many particles. • Sedimentation of the liquid-laden flocs,leaving clear liquid. • Compaction of the sedimented flocs. • Elimination of the clear fluid and of the thickened mud.

9. PROCESSES INVOLVED IN DEWATERING FLOCCULATED PULP: • Free settling of flocs. • Hindered settling of flocs. • Exudation of water from the settled flocs,under the influence of pulp pressure. • Exudation of water from the settled and naturally exuded flocs by application of outside pressure or suction.

11. SLIME SETTLING ZONE IN DORR THICKENER.

12. DORR THICKNER: • Although less expensive per square foot of settling area they are • Less effective and more difficult to operate. • More power consumption.

14. WORKING: • The revolutionary EIMCO® E-CAT® Clarifier-Thickener combines optimized flocculation, high-rate clarification and high-density thickening in a single compact unit. It streamlines liquid-solid separation flowsheets by optimizing chemical settling aids and providing a dedicated escape route for displaced free liquid. Even more impressive, the E-CAT Clarifier Thickener system accomplishes all of this with no moving parts. 

16. FEATURES AND BENEFITS: • No moving parts- Lower capital cost - Lower energy consumption - Less maintenance • Smaller surface area than conventional thickeners- Lower capital cost - Smaller footprint • Self diluting feed- Optimal flocculant utilization - No external dilution pumps required • Good overflow clarity- Effluent re-use • Dense underflow- Lower energy requirement for drying - Less ponding area required - Reduced handling volume

18. FILTRATION: • Filtration is that separation of finely divided solid particles from a fluid which is accomplished by driving the pulp to a membrane or septum, porous to the fluid but impervious to the solid, through which the fluid passes, and by the removal of the solid cake from the septum.

19. MECHANISM OF FILTRATION: • The simplest type of filter consists of a tube of small bore through which the fluid is sucked while the solid particles accumulate at the entrance. As the device is operated, solids at first pass through the tube, but they quickly arch or bridge across the opening, allowing only clear liquid to pass

20. RATE OF FILTRATION: • The filtering area. • The difference in pressure between two sides of the filter. • The average cross section of the pores within the filter cake. • The number of pores per unit area of the septum. • The thickness of the filter cake.

21. TYPES OF FILTERS: • Pressure type: pressure is applied to push the liquid through the filter bed. (for hydrometallurgical work in which interest centers on the filtrate, as in cyanidation.) eg. Hardinge filter thickener. • Suction type: suction is applied to pull the liquid through the filter bed. (used in chemical laboratories, for mineral dressing in which interest centers on the cake.) eg. Genter thickener.

22. PRESSURE AND SUCTION FILTERS: Adv: • higher fluid capacity per square foot of filtering surface, • better control of cake formation, • possibility of better washing of cake. Disadv: • A great disadvantage is that they are intermittent (stopping and starting at regular intervals) instead of continuous.

23. BENEFITS FROM DEWATERING: A good reason would be cost savings and environmental awareness . • Lower Transport and Disposal costs . • Dewatered solids are cheaper and easier to incinerate. • Ease of handling with dewatered solids. • Less offensive smells. • Reduced volume in landfills . • Reduced leachate production in landfills . • Lower amount and cost for bulking agents for composting

24. CONCLUSION: • Keeping in mind the optimum sludge removal and dewatering technique a site and sludge specific evaluation process should be performed with the objective of developing the safest, most economical, and environmentally responsible approach that meets or exceeds the objectives and expectations of the client.

25. THANK YOU…

26. REFERENCES: • PRINCIPLES OF MINERAL DRESSING BY:A.M. GAUDIN. • www.FLSmidthMinerals.com • www.lenntech.com