Process Technology: An Introduction - Haan A.B. 2015

10 Solid-liquid separation
10.8 Deep-bed filtration

Deep-bed filtration is typically used for the removal of very low concentrations of suspended submicrometer solids that do not settle or filter readily. Their relatively high efficiency in removing fine particles present in low concentration is used extensively for drinking water filtration and the final polishing of effluents before discharge, or process liquids prior to further processing. The high capacity available in modern deep-bed filters can be used in cases where conventional equipment cannot produce an economical separation.

Commercial deep-bed filters are of simple construction, consisting of a holding tank for the granular bed of solids, through which the liquid to be filtered is passed. Flow may be directed downwards or upwards, with typical liquid velocities of 40 and 15 m/h. The granular bed is usually made up of sand, gravel, anthracite, or a variety of other materials having a particle size of 0.5-5 mm. Although the diameter of the pores is 100-10 000 times larger than the diameter of the suspended particles, a mixture of mechanisms are responsible for their deposition in the bed. In addition to mechanical interception, particle deposition due to electrostatic and London/van der Waals attractive forces takes place. The deposited particles cause blockage and an increased pressure drop of the fluid flowing through the bed. A common design to improve the effectiveness of the deep-bed filter is to use multiple layers of solids on top of each other. This is illustrated in Fig. 10.38, where a dual layer of anthracite/sand is contained in a vessel. The anthracite particles, being coarser than the sand, serve to prevent the formation of surface deposits on the sand surface. During filtration the bed is contaminated with the particles from the filtered liquid. At fixed times the filter is backwashed with water in counterflow to rinse the bed clean. The filter bed is fluidized, quickly releasing the particles deposited during the filtration stage.

Fig.10.38: Dual-layer deep-bed filter.

Nomenclature