Purpose
A major operating cost for water purification is the sludge disposal cost. The lower the dry matter content of this sludge, the more one is paying for the disposal of water. Sludge dewatering is often recommended in order to significantly reduce the costs for sludge disposal. Sludge dewatering implies free water between the solid materials is removed so that a semi-solid mass is obtained.
Concentration of solids, sludge dewatering
Separation of manure and digestate into a thick fraction and fine fraction
Dewatering biological or physicochemical sludge
Dewatering mineral slurries
Principle
A belt press is an advanced version of a dewatering table, in which the sludge ends up between two sieve belts. The sludge is compressed by pressure rollers and the water is pressed through the pores of the filter belt and separated from the sludge. A difference in speed between the two belts is applied so additionally to the pressure also shear forces assist in the drainage, obtaining a higher dry matter content. Like a decanter centrifuge a belt press is a non-stop device.
Typically a belt press is preceded by a gravity separation step, where already a large amount of free water can be separated from the sludge-water mixture. The sludge is then conditioned with polymer in order to increase the dewatering efficiency through the sieve belt press. The sludge water is then compressed under low pressure through the belt press in the first instance, possibly followed by compression under high pressure.
The performance of the filter belt press is determined by sludge quality, chemical conditioning, speed of and pressure exerted by the sludge on the sieve belt press and porosity of the sieve. For safety reasons the moving parts of the sieve belt press are to be screened from the environment.
Scheme
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Applications
A decanter centrifuge and a belt press require little supervision, which is an advantage with respect to a chamber filter press. A decanter centrifuge is easily adjusted for various types of sludge, though this is more difficult with a sieve belt press and the chamber filter press (other screen cloths). A chamber filter press has generally a higher dry matter content than a decanter centrifuge, which, in turn, performs better than a sieve belt press.
Approach Trevi
Trevi aims to initially minimize the sludge production by means of process oriented interventions. For this we look both to a reduction of the pollution load from the production processes as well as optimization of the water treatment plant (load, chemical use). We then investigate in function of the amount and type of sludge if there's an economic advantage to be gained by sludge dewatering and if so by what technique.With the degree of sludge dewatering the cost of dewatering increases. The balance between intensity of dewatering and sludge disposal costs have to be found.