believed to be related to excessive amounts of bacterial growth that clog the trickling
filters and can be controlled by higher flow rates to keep surfaces flushed. However,
if both high organic and high hydraulic loadings are applied to a trickling filter, then
the filter can no longer to be classified as a low-rate filter.
High-rate filters are normally designed for substantially higher organic and
hydraulic loadings than low-rate filters. There are three principal types of high-rate
filters depending on rate of feeding, recirculation, or loading. The three types are the
biofilter, the accelo-filter, and the aero-filter, as indicated in Fig. 3.17.
The biofilter is a relatively shallow filter, generally 1.2–1.5 m (4–5 ft) in depth,
which utilizes recirculation of a portion of the filter effluent to the primary sedimentation basin for a second passage through the filter. Organic loadings of biofilters are
in the order of 0.88–1.11 kg BOD/m
3 /d (55–69 lb BOD/1000 ft
3 /d) based on the
strength of the primary tank effluent, with hydraulic loadings ranging from 9.38 to
28.15 m
3
/m
2 /d (230–690 gal/ft
2 /d). If additional treatment is necessary to lower the
BOD content in the effluent, a second-stage filter may be provided. By appropriate
selection of flow patterns and recirculation ratios, it is possible to satisfy the desired
degree of treatment.
The aero-filter, which has a relatively deep media bed of 1.83–2.74 m (6–9 ft),
utilizes a low momentary rate of wastewater application to the filter by means of a
special type of distributor designed for frequent “raindrop” applications over a
maximum area of the filter at one time. Recommended organic loadings are from
1.11 to 1.19 kg BOD/m
3 /d (69–74 lb BOD/1000 ft
3 /d), and the hydraulic loading
rate is more than 9.38 m
3 /m
2 /d (230 gal/ft
2 /d) with recirculation if necessary to
maintain this rate. It should be noted that recirculation is used only during periods of
low wastewater flow or only in amounts necessary to ensure proper operation of the
distributor. BOD removal of single-stage treatment ranges from 63% to 78%. If
additional treatment is desired, a second-stage filter may be provided, and with very
strong organic wastewater, intermediate clarification may be used. When two-stage
treatment is used, the organic loading of first-stage filter is 1.65–1.80 kg BOD/m
3 /d
(2.75–3.0 lb BOD/yd
3 /d), and the loading of second-stage filter is about 60% of the
first-stage loading. Some of the arrangements used in the aero-filter plants are shown
in Fig. 3.17.
The accelo-filter, which is normally 1.83–2.44 m (6–8 ft) deep, utilizes direct
recirculation of unsettled filter effluent back to the inlet of the distributor. The
recommended organic loadings are in the same range as the biofilter, in the order
of 0.88–1.11 kg BOD/m
3 /d (55–69 lb BOD/1000 ft
3 /d) based on the strength of the
primary clarifier effluent. Recirculation is used as in the biofilter to affect the desired
degree of treatment. As with the biofilter, a large variety of flow patterns is possible,
including use of a primary high-rate filter and a secondary low-rate filter.
Early investigators considered an applied hydraulic loading of 9.38 m
3 /m
2 /d
(230 gal/ft
2 /d) or more is necessary to flush organic solids from the filter media
and prevent clogging. It was found later that serious clogging did not occur when
dosing rates ranged from 4.08 to 9.38 m
3 /m
2 /d (100–230 gal/ft
2 /d). Many trickling
filters were then designed to operate in this so-called intermediate-rate range.
3 Biological Processes
123
filters and can be controlled by higher flow rates to keep surfaces flushed. However,
if both high organic and high hydraulic loadings are applied to a trickling filter, then
the filter can no longer to be classified as a low-rate filter.
High-rate filters are normally designed for substantially higher organic and
hydraulic loadings than low-rate filters. There are three principal types of high-rate
filters depending on rate of feeding, recirculation, or loading. The three types are the
biofilter, the accelo-filter, and the aero-filter, as indicated in Fig. 3.17.
The biofilter is a relatively shallow filter, generally 1.2–1.5 m (4–5 ft) in depth,
which utilizes recirculation of a portion of the filter effluent to the primary sedimentation basin for a second passage through the filter. Organic loadings of biofilters are
in the order of 0.88–1.11 kg BOD/m
3 /d (55–69 lb BOD/1000 ft
3 /d) based on the
strength of the primary tank effluent, with hydraulic loadings ranging from 9.38 to
28.15 m
3
/m
2 /d (230–690 gal/ft
2 /d). If additional treatment is necessary to lower the
BOD content in the effluent, a second-stage filter may be provided. By appropriate
selection of flow patterns and recirculation ratios, it is possible to satisfy the desired
degree of treatment.
The aero-filter, which has a relatively deep media bed of 1.83–2.74 m (6–9 ft),
utilizes a low momentary rate of wastewater application to the filter by means of a
special type of distributor designed for frequent “raindrop” applications over a
maximum area of the filter at one time. Recommended organic loadings are from
1.11 to 1.19 kg BOD/m
3 /d (69–74 lb BOD/1000 ft
3 /d), and the hydraulic loading
rate is more than 9.38 m
3 /m
2 /d (230 gal/ft
2 /d) with recirculation if necessary to
maintain this rate. It should be noted that recirculation is used only during periods of
low wastewater flow or only in amounts necessary to ensure proper operation of the
distributor. BOD removal of single-stage treatment ranges from 63% to 78%. If
additional treatment is desired, a second-stage filter may be provided, and with very
strong organic wastewater, intermediate clarification may be used. When two-stage
treatment is used, the organic loading of first-stage filter is 1.65–1.80 kg BOD/m
3 /d
(2.75–3.0 lb BOD/yd
3 /d), and the loading of second-stage filter is about 60% of the
first-stage loading. Some of the arrangements used in the aero-filter plants are shown
in Fig. 3.17.
The accelo-filter, which is normally 1.83–2.44 m (6–8 ft) deep, utilizes direct
recirculation of unsettled filter effluent back to the inlet of the distributor. The
recommended organic loadings are in the same range as the biofilter, in the order
of 0.88–1.11 kg BOD/m
3 /d (55–69 lb BOD/1000 ft
3 /d) based on the strength of the
primary clarifier effluent. Recirculation is used as in the biofilter to affect the desired
degree of treatment. As with the biofilter, a large variety of flow patterns is possible,
including use of a primary high-rate filter and a secondary low-rate filter.
Early investigators considered an applied hydraulic loading of 9.38 m
3 /m
2 /d
(230 gal/ft
2 /d) or more is necessary to flush organic solids from the filter media
and prevent clogging. It was found later that serious clogging did not occur when
dosing rates ranged from 4.08 to 9.38 m
3 /m
2 /d (100–230 gal/ft
2 /d). Many trickling
filters were then designed to operate in this so-called intermediate-rate range.
3 Biological Processes
123