NEWSLETTER
Tradition of Excellence
ENGETEC
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By Lawrence Walker and Thomas Henessy
Rapid Sand Filtration
The most widely used filters to polish clarified water as described in Parts I through III, are rapid sand filters in which the filtration media is held in place by gravity and the flow direction is downward. The filter medium is supported on a gravel bed below, which is an underdrain system to collect the filtered water.
The gravel bed is built up from several layers ranging from coarse stone at the bottom to 0.1 inch diameter gravel at the top. This is designed to support the sand bed to prevent it from passing downward and out of the bed during filtration, and to spread the upward flow uniformly during backwashing. Filter bed media typically consists of silica sand, garnet sand or crushed anthracite.
Floc particle size and strength are important to the filtration process. The pore openings on the surface of the sand bed range from 0.1 to 0.5 mm diameter preventing large floc particles from passing through the filter by capturing the large floc on the surface of the sand filter. However, much of the smaller floc will pass into the bed and lodge within it. Unless the flocs are strong, those that lodge in the bed initially will break down as the hydraulic gradient increases. The floc passes into the filter bed through thousands of openings per square foot and by the end of a filter run, builds-up within the first one to four inches of the bed depth.When a cleaned filter bed is placed in service, most of the flocs pass into the bed. As the solids lodges between the sand grains below the surface, the free void area is reduced and the bed offers resistance to the flow of water. If floc is very strong and sand particle size is small, most of the solids may be captured at the surface of the filter. This leads to rapid flow loss through the filter and the length of a filter run is short. Longer filter runs are obtained when the flocs penetrate into the bed making greater use of the filter matrix.
As the accumulation of solids in the bed increases, the flow rate through the filter decreases due to the reduction of the available water channels. When flow diminishes to a low rate, the filter is taken out of service. It is then backwashed by forcing water upwards through the bed to rinse the solids out of the pore spaces, leaving cleaned sand behind. In practice, the factors limiting the rate of filtration through sand filters are:
- The concentration of solids to be removed by the filters. As the amount of solids increases, the time required filling up the void spaces decreases. Once the void spaces are full, the flow rate decreases and the filter must be backwashed.
- The size of the particles influences whether they will be trapped at the surface of the filter bed or pass on into the filter bed.
- The strength of the particles determines whether the flocs will break down to smaller particles as the f low gradient increases. If the floc is weak, it may not stay captured in the filter bed and will pass through the bed as "turbidity breakthrough."
- Particle charge is important as it affects the ability of particles to be adsorbed to the filter media. If the particles carry a high charge and are dispersed, they will have less of a tendency to be adsorbed to the surface of the sand particles.
- Water temperature affects both the water viscosity and the friction forces. Cold water will filter more slowly than warmer water
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