The granular-medium filters may be classified according to: 1. The Direction of Flow 2. Types of Filter Beds 3. The Driving Force 4. The Method of Flowrate Control.
1. Direction of Flow:
Filters used for the filtration of the sewage effluent may be classified according to the direction of flow as downflow, upflow or biflow filters. Schematics of these three types of filters are shown in Fig. 18.2.
The downflow filter is the most common type used. In upflow filters the flow passes up through the granular medium. The medium stratifies after backwashing with coarse material on the bottom and fine material on the top. It is claimed that such filters are more efficient because the passage of the liquid to be filtered in the upward direction allows greater penetration of the suspended solids into the bed.
During the filtering operation the granular medium is retained by a metal grid located at the top of the filter. The filtering medium forms an inverted arch with the evenly spaced bars. If the arch structure is disrupted by increasing the flowrate through the filter or by injecting air into the filter, the medium moves up past the retaining bars. This principle is used in backwashing the filter. The same liquid that is being filtered is also used for backwashing which is another advantage claimed for these filters.
In biflow filters the features of both the downflow and upflow filters are combined. As shown in Fig. 18.2, the effluent from the filter is collected through a strainer placed within the filter bed. Backwashing is accomplished simply by increasing the flowrate to the bottom of the filter.
2. Types of Filter Beds:
The filter beds used for sewage filtration may be classified according to the number of filtering media used, as single-medium, dual-medium, or tri-medium (or multi-medium) beds.
A further classification may be made according to stratification, as follows:
(i) Single-medium stratified,
ADVERTISEMENTS:
(ii) Single-medium or mixed-medium un-stratified,
(iii) Dual-medium stratified, and
(iv) Multi-medium stratified
Single-Medium Stratified Filter Bed:
ADVERTISEMENTS:
Although single-medium stratified beds of conventional design have been used for sewage filtration, but they are not used ordinarily, because of their unfavourable head- loss-buildup characteristics.
Single-Medium Un-Stratified Filter Bed:
Two types of single-medium un-stratified filter beds are now in use. In the first type, a single, uniform, coarse medium (2 to 3 mm) is used in beds with depths upto 2 m. It has been found that these large-medium, deep filters offer longer filter runs.
The principal disadvantages of these filter beds are:
ADVERTISEMENTS:
(1) The need for a uniform size medium,
(2) The high backwash velocities required to fluidize the bed for effective cleaning, and
(3) The added cost for the backwashing facilities and the structure needed to contain the deep bed.
In the second type, a single medium of varying sizes is used with a combined air-water backwash. This type has proved to be an effective alternative to the filter with a single medium of uniform size. The combined air-water backwash scours the accumulated material from the filtering medium without the need for fluidizing the entire bed.
This backwash system also eliminates the normal stratification that occurs in single-medium and multimedium beds when only a water backwash or an air-followed-by-water backwash is used. Thus it is possible to obtain a filter bed with a more or less uniform pore-size distribution throughout its depth.
The effective size of the medium used in unstratified filters is in the range of 0.8 to 2.0 mm with a typical value of 1.2 mm. The depth of such filters is 0.9 m.
Dual-Medium and Multimedium Filter Beds:
Some dual-medium filter beds that have been used are composed of:
(1) Anthracite and sand;
(2) Activated carbon and sand;
(3) Resin beads and sand; and
(4) Resin beads and anthracite.
Multimedium filter beds that have been used are composed of:
(1) Anthracite, sand and garnet or ilmenite;
(2) Activated carbon, anthracite and sand;
(3) Weighted spherical resin beads (charged and uncharged), anthracite and sand; and
(4) Activated carbon, sand and garnet or ilmenite.
Typical data on the depth and characteristics of the filtering materials most commonly used in dual- medium and multimedium filters are presented in Table 18.1.
Dual-medium and multimedium filter beds were developed to allow the suspended solids in the liquid to be filtered to penetrate farther into the filter bed and thus use more of the solids—storage capacity available within the filter. The penetration of the solids farther into the bed also permits longer filter runs because the buildup of head loss is reduced.
3. Filtration Driving Force:
Either the force of gravity or an applied pressure force can be used to overcome the factional resistance to flow offered by the filter bed. Gravity filters of the type shown in Fig. 18.1 are most commonly used for the filtration of treated effluent at large plants. Pressure filters of the type shown in Fig. 18.3 operate in the same manner as gravity filters and are used at smaller plants.
The only difference is that, in pressure filters, the filtration operation is carried out in a closed vessel under pressurized conditions achieved by pumping. Pressure filters are normally operated at higher terminal head losses. This generally results in longer filter runs and reduced backwash requirements.
4. Method of Flowrate Control:
The methods used to control the rate of flow through gravity filters may be classified as:
(1) Constant-rate filtration; and
(2) Variable-declining-rate filtration.
In constant-rate filtration, the flow through the filter is maintained at a constant rate by means of an effluent-flow control valve that can be operated manually or mechanically. At the beginning of the filter run since the filter bed and the underdrain system are clean the resistance to the flow is less and hence the valve is almost closed.
During the filter run as solids start to accumulate within the filter more resistance is offered to the flow by the clogged filter bed and underdrain system and hence the valve is opened. Since the required control valves are expensive and also they have malfunctioned on a number of occasions, alternative methods of flowrate control have been developed and are being used.
In variable-declining-rate filtration, the rate of flow through the filter declines and the level of liquid above the filter bed rises throughout the length of the filter run. When the allowable head loss is reached, the filter is removed from service and backwashed.