8.4.1.1 Pressure Sand Filters

Pressure sand filters (PSF) are used in many industrial applications including a DM plant and often are popularly termed rapid sand bed filters. APSF consists of a pressure vessel that is normally vertical or horizontal, in rare occasions, depending on the layout of the plant. The filter vessels are generally of welded mild steel construction lined with rubber/epoxy. A minimum of 50% freeboard is provided over the filtering bed depth to enable efficient backwash.

Graded silica quartz sand and anthracite supported by layers of graded underbed, consisting of pebbles and gravels, are provided with a water inlet at the top. Incoming water is distributed uniformly throughout the cross-section of the filter to ensure that there are no preferred fluid paths where the sand may be washed away and jeopardize filter action. The bottom drainage system is kept to collect filtered water.

The selection of the sand’s grain size is important because smaller sand grains provide an increased surface area and, consequently, more decontamination at the water outlet that, on the other hand, demands extra pumping energy to drive the fluid through the bed. In an attempt at a compromise, grain sizes are generally selected in the range 0.5 to 1.50 mm. A sand bed depth of ∼0.5 to 2.0 m is recommended regardless of the application of which the ratio of quartz sand and anthracite is ∼7 to 50.

Raw water flows downward through the filter bed and the suspended matter is retained on the sand’s surface and between the sand grains immediately below the surface. Rapid-pressure sand bed filters are typically operated with a feed pressure of 1 to 4 kg/cm2. The differential pressure (DP) across a clean sand bed usually is insignificantly low. The DP gradually builds up for a given flow rate as particulate solids are captured in the bed; this may not be uniform with depth. For obvious reasons, buildup would be more at the higher level with the concentration gradient decaying rapidly.

This type of filter captures particle sizes down to very small ones. In fact there is no true cutoff size below which particles would not be arrested. Interestingly, the shape of the characteristic curve of efficiency versus filter particle size is a U one with the highest rate of particle capture for the smallest and largest particles, with a plunge in between for mid-sized particles. When the pressure loss, or flow, is unacceptable, it is sensed by a pressure drop across the PSF of ∼0.5 kg/cm2. The filter is then taken out of service and cleaning of the filter is effected by flow reversal or the bed is backwashed or pressure-washed to remove the accumulated particles. Backwashing of pressure filters normally is done once every 24 hours while the system is online.

During backwash, the sand becomes fluidized and the expansion in volume may go up to about 30%, which allows the sand grains to mix, and the particulate solids are driven off as they start rubbing together. The smaller particulate solids are then forced out with the backwash fluid. The fluidizing flow requirement is typically 5 to 30 m3/hr/m2 of filter bed area, depending on the depth of the bed, for a short period (i.e., for a few minutes only). The filter backwash fluid is taken to a common inlet chamber of raw water pumps. The backwashing process would cause sand loss though not significantly noticeable, thus requiring periodic top up of sand in the bed.

To assist in cleaning the bed, the backwash operation is often preceded by air agitation through the under drain system. The process of air scouring agitates the sand with a scrubbing action, loosening the intercepted particles. After backwashing, the filter is ready to be put back into service. For a 500 MW TPS, the typical backwashing flow rate would be between 25 to 30 m3/hr/m2 of bed area and the air-flow rate would be 50 m3/hr/m2 of filter bed area.