SLUDGE DEWATERING. The costs for trucking sludge to the ultimate disposal site become substantially lower when sludge volume is reduced by dewatering.

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1 SLUDGE DEWATERING Sludge dewatering is a physical (or) mechanical unit operation used to reduce to the moisture content of sludge. It is carried out for one or more of the following reasons. The costs for trucking sludge to the ultimate disposal site become substantially lower when sludge volume is reduced by dewatering. Dewatered sludge is generally easier to handle than thickened or liquid sludge. Dewatering is normally required prior to the incineration of the sludge to

2 Methods of Sludge Dewatering:- Air drying on open sludge Drying Beds Mechanical Methods Vacuum filtration Centrifugation Pressure diltration 3. Heat Drying 4. Incineration

3 Sludge Drying Beds:- This method of dewatering of sludge can be used in all places where adequate land is available and dried sludge can be used for soil conditioning. The method consists of applying the sludge in 20 to 30cm thick layer on specially prepared open beds of sand and gravel and allowed to dry. Open jointed tile under drains are laid in the gravel of uniform size over which is laid a bed of the sand, to provide positive drainage as the liquid passes through the sand and gravel.

4 The sludge inlets to the drying bed are so arranged to easily drain and have minimum dia of 20cm terminating at least 30cm above the sand surface. Dried sludge cake can be removed by shovel or forks, when the moisture content is less than 70% when the moisture content reaches 40% the cake becomes lighter and suitable for grinding.

5 Mechanical Methods Vacuum Filtration:- Vacuum filter consists of a cylindrical drum over which is laid a filtering medium of wool, cloth or felt, synthetic fibre or plastic or stainless steel mesh or coil springs. The drum is suspended horizontally so that one quarter of its diameter submerged in a tank containing sludge. Valves and pipings are arranged to apply a vacuum on the inner side of the filter medium as the drum rotated slowly in the sludge. The vacuum holds the sludge against the drum as it continuous to be applied as the drum rotates out of the sludge tank. This pulls water away from the sludge leaving a moist cake mat on the outer surface.

6 Centrifugation:- Here, the sludge dewatering may be accomplished by solid bowl and basket centrifuges. In a solid bowl centrifugal sludge is fed at a constant flow rate into the rotating bowl. The dilute stream contains fine, low density solids and its returned to the untreated sludge. It can remove 40% to 10% of moisture content.

7 Pressure Filters:- In this method dewatering is achieved by forcing the water from the sludge under high pressure. Various types of pressure filters have been used for dewatering of sludges.

8 High concentration of cake solids Filterate clarity Solid capture Chemical consumption Advantages:- Disadvantages:- High labour costs Life of filter cloth being limited needs frequent replacement involving a high cost. Prior to filtration chemical conditioning of sludge is necessary resulting in higher operational cost.

9 The purpose of heat drying is to reduce further the moisture content and volume as well as weight of the dewatered sludge, so that it can be used after drying without causing offensive odurs o risk to public health, or it can be incinerated efficiently. Heat Drying:- Incineration:- The purpose of incineration is to destroy the organic material the residual ash being generally used as land fill. During the process all the gases released from the sludge are bunt off and all the organisms are destroyed.

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