The super-rate filter is defined by its high hydraulic and organic loadings, as
indicated in Table 3.5. The plastic media filters can be operated at the high/super-rate
range for secondary treatment.
Either plastic media filters or rock media filters can be used as the so-called
roughing filters. A roughing filter (see Fig. 3.18) is used to reduce the organic load in
which subsequent treatment may be applied to the effluent or where intermediate
treatment is required [112]. Therefore, the roughing filler is generally installed
advantageously ahead of activated sludge or any other secondary process. The filter
design differs from other biological filters mainly because the determining factor is
the high hydraulic loading as well as the high organic loading of certain wastewaters
that are to be handled. Although roughing filters may give a high weight per unit
volume of organic load removal, their settled effluent will still contain substantial
organics in terms of BOD 5 .
Most of the primary filters with plastic packing in a two-stage system may be
operated at high hydraulic and organic loadings because of the plastic media’s light
weight, requiring less support structure, less cost, and freedom from corrosion.
Plastic super-rate filters generally have deep beds with plastic media that have
large void spaces and are dosed continuously. The highest hydraulic loading found
in the literature approached 469.2 m
3
/m
2 /d (11,500 gal/ft
2 /d) which, however, was
applied to a shallow bed filter [103]. The depth of the super-rate filter depends on the
type of medium employed; however, the limitations in the medium available for use
in super-rate filters were overcome by considerations outlined by Pearson
[113]. Recirculation is normally practiced to maintain efficiency and keep the
slime film in a wetted condition.
6.3 Single- and Multi-stage Trickling Filter Plants
A single-stage plant is one in which wastewater is passed through a single trickling
filter; if there are two or more trickling filters, they would be operated in parallel.
Wastewater may be, and usually is, recirculated through single-stage filters. Useful
design charts were prepared by Baker and Graves [114] for single-stage filters using
various design formulas. Although the single-stage filter is generally adequate and
satisfactory for treating domestic wastewater, recent trends require an increase in
detention time for handling occasional or constant overloaded conditions. The
limitations of increasing depth are that the conventional rock filter media may not
be able to support a very deep bed and a significant foundation structure must be
provided. With the development of plastic media that are lighter in weight and
possess proper ventilation characteristics, single-stage trickling filter plants using
deep beds are being considered. Figures 3.16 and 3.17 show the flow diagrams of
single-stage treatment.
In a two-stage filtration plant (i.e., double filtration plant), the effluent from a
primary trickling filter, after the portion that is to be recirculated has been withdrawn
for return to it directly or through the primary clarifier, passes through a secondary
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L. K. Wang et al.
indicated in Table 3.5. The plastic media filters can be operated at the high/super-rate
range for secondary treatment.
Either plastic media filters or rock media filters can be used as the so-called
roughing filters. A roughing filter (see Fig. 3.18) is used to reduce the organic load in
which subsequent treatment may be applied to the effluent or where intermediate
treatment is required [112]. Therefore, the roughing filler is generally installed
advantageously ahead of activated sludge or any other secondary process. The filter
design differs from other biological filters mainly because the determining factor is
the high hydraulic loading as well as the high organic loading of certain wastewaters
that are to be handled. Although roughing filters may give a high weight per unit
volume of organic load removal, their settled effluent will still contain substantial
organics in terms of BOD 5 .
Most of the primary filters with plastic packing in a two-stage system may be
operated at high hydraulic and organic loadings because of the plastic media’s light
weight, requiring less support structure, less cost, and freedom from corrosion.
Plastic super-rate filters generally have deep beds with plastic media that have
large void spaces and are dosed continuously. The highest hydraulic loading found
in the literature approached 469.2 m
3
/m
2 /d (11,500 gal/ft
2 /d) which, however, was
applied to a shallow bed filter [103]. The depth of the super-rate filter depends on the
type of medium employed; however, the limitations in the medium available for use
in super-rate filters were overcome by considerations outlined by Pearson
[113]. Recirculation is normally practiced to maintain efficiency and keep the
slime film in a wetted condition.
6.3 Single- and Multi-stage Trickling Filter Plants
A single-stage plant is one in which wastewater is passed through a single trickling
filter; if there are two or more trickling filters, they would be operated in parallel.
Wastewater may be, and usually is, recirculated through single-stage filters. Useful
design charts were prepared by Baker and Graves [114] for single-stage filters using
various design formulas. Although the single-stage filter is generally adequate and
satisfactory for treating domestic wastewater, recent trends require an increase in
detention time for handling occasional or constant overloaded conditions. The
limitations of increasing depth are that the conventional rock filter media may not
be able to support a very deep bed and a significant foundation structure must be
provided. With the development of plastic media that are lighter in weight and
possess proper ventilation characteristics, single-stage trickling filter plants using
deep beds are being considered. Figures 3.16 and 3.17 show the flow diagrams of
single-stage treatment.
In a two-stage filtration plant (i.e., double filtration plant), the effluent from a
primary trickling filter, after the portion that is to be recirculated has been withdrawn
for return to it directly or through the primary clarifier, passes through a secondary
124
L. K. Wang et al.