1995; DeLosReyes and Lawson, 1996. For this category some deterioration in aesthetics is permitted, but water quality is held below safe levels TAN and
nitrite B 1.0 mg l
− 1
to avoid growth inhibition and disease problems. Both organic and nitrogen loading levels are permitted to rise to a level where rapid solids
accumulation rate and rapid net biofilm growth dictate careful attention to manage- ment factors.
A fourth growout category, undoubtedly, exists for the most tolerant species [e.g. carp Cyprinus carpio Arbiv and Van Rijn, 1995; snakehead Channa striatus, Qin
et al., 1997; Kemp’s ridley turtles Lepidochelys kempi Malone et al., 1990; alligators Alligator mississippiensis DeLosReyes et al., 1997b] that show vigorous
growth under deteriorated water quality conditions. Here substrate levels may be allowed to rise to the level where heterotrophic domination limits nitrification
conversion rates. Thick biofilms can induce oxygen rather than TAN diffusional limitations Harremoes, 1982; Rogers and Klemetson, 1985; Zhang et al., 1995;
Henze et al., 1997. A submerged biofiltration format may be arguably inappropri- ate in this category with the floating bead filter operating in a supporting clarifica-
tion role. This category is not supported here, as the authors are not convinced that the category is ethically appropriate or economically justified since the rise in
substrate concentrations induces little treatment advantage. These applications are best handled under the growout category where the tolerance of the species can
contribute to the safety factor for the operation.
3. Sizing rationale
The primary method for the sizing bead volume, V
b
of floating bead filters is based on a volumetric organic loading rate. This approach assumes: 1 the filter is
being employed as a bioclarifier, 2 organic loading is the principal factor con- trolling nitrification conditions within the bioclarifier, 3 the organicnitrogen ratio
is relatively consistent across a wide spectrum of feeds, and 4 the filter is well managed to sustain nitrification. The ultimate source of organics in a recirculating
system is the feed; therefore the FBF sizing criterion, 6
b
, is based upon feed loading. The volume of bead media required for any application, V
b,
can then be determined by the rates of the peak feeding rate, W, and the FBF sizing criterion, 6
b
: V
b
= [L f 100]6
b
V
b
= W6
b
6 where L = maximum weight of fish in the system kg; f = feedrate percent of body
wt. fed day
− 1
; 6
b
= FBF sizing criterion m
3
kg
− 1
feed day; W = peak feed application rate kg day
− 1
. Eq. 6 was developed in an environment where the average protein content of
feeds was typically 35. Variations in feed protein content are normally absorbed in the criterion’s safety factor 6
b
values set at 67 of readily achievable peak performance. Furthermore, increasing the protein content of a feed effectively
lowers the organicnitrogen-loading ratio to benefit the nitrification process. How- ever, in practice when the protein content is known to be very high, Eq. 6 is
modified:
V
b
= L f 6
b
P35100 V
b
= W6
b
P35 7
where P is the protein content of the feed . Peak carrying capacities for the various bead filter models discussed in this paper
occur at values from 24 to 32 kg m
− 3
day
− 1
when filled with standard spherical beads. The criterion of 16 kg m
− 3
day
− 1
Table 1 has been tested and has proven to be stable in the local commercial sector Beecher et al., 1997; DeLosReyes et al.,
1997a; Sastry et al., 1999. At this feeding level the filters can reliably provide solids capture, BOD reduction, and nitrification, while sustaining water quality conditions
suitable for the growout of most food fish species. TAN and nitrite levels can be expected to remain well below 1 mg N l
− 1
. Reduction of the criterion to 8 kg m
− 3
day
− 1
allows the reliable maintenance of water quality conditions demanded by the fingerling category. Finally, a loading guideline of 4 kg m
− 3
day
− 1
is used for
Table 1 Typical values for the performance parameters under conditions derived from operational filters
a
Units Management parame-
Typical operational values observed in practice ters
Broodstock Ornamental
Growout kg feed m
− 3
media Feed loading
5 4
5 8
5 16
day
− 1
g TAN m
− 3
media Design TAN
0.3 0.5
1.0 day
− 1
B 0.3
B 0.1
Typical TAN B
0.5 g TAN m
− 3
media day
− 1
VTR g TAN m
− 3
media 35–105
70–180 140–350
day
− 1
g N m
− 3
media day
− 1
Design nitrite 0.3
0.5 1.0
Typical nitrite B
0.5 B
0.3 B
0.1 g N m
− 3
media day
− 1
35–105 g N m
− 3
media day
− 1
70–180 VNR
140–350 OCF
2.5–3.0 1.4–2.5
0.7–2.5 kg O
2
m
− 3
media day
− 1
OCNOCF 25–35
25–35 45–55
65–75 45–55
OCHOCF 65–75
Temperature °C
20–30 20–30
20–30 FBF effluent O
2
\ 3.0
\ 3.0
mg l
− 1
\ 3.0
mg CaCO
3
l
− 1
\ 100
\ 80
Alkalinity \
50 pH range
6.5–8.0 6.8–7.0
7.0–8.0 Backwash inter6al
1–7 Aggressive wash
1–3 Days
1–2 1–3
Gentle wash 1–2
Days 0.5–1
a
Values derived principally from Wimberly 1990 and Sastry et al. 1999.
breeding and broodstock maintenance programs where pristine conditions are justified by the value of the stock.
An alternate approach to sizing bead filters is in terms of volumetric nitrification capacity Malone et al., 1993. This criterion is based on a wide spectrum of
floating bead and other filters that are found to display areal conversion rates with a magnitude of about 300 mg TAN m
− 2
day
− 1
in recirculating systems with TAN and nitrite levels between 0.5 and 1.0 mg N l
− 1
. The authors suspect that this plateau of performance reflects TAN diffusion constraints as the biofilm thickens in
response to increased loading Harremoes, 1982; Henze et al., 1997. Below a TAN concentration of about 1.0 mg N l
− 1
, laboratory evidence and empirical observa- tions indicate that conversion rates decline with TAN concentration Chitta, 1993.
Thus, observed VTR values tend to increase with the increasing TAN tolerances 0.3, 0.5 and 1.0 associated with the categories Table 1. These VTR values can
then be used in conjunction with Eq. 2 to estimate the size of the bead filter:
V
b
= 1 − I
s
E
TAN
WVTR 8
where I
s
= in situ nitrification fraction unitless; E
TAN
= TAN excretion rate in g
TAN kg
− 1
feed. The in situ nitrification fraction recognizes the effect of nitrification occurring on
the sidewalls of tanks, and in particular, the systems piping configuration Mia, 1996. A value of I
s
= 0.3 is conservatively estimated, although values in excess of
50 are frequently observed. The TAN excretion rate is normally assumed to be around 30 g kg
− 1
for a 35 protein feed used to support warmwater fish Malone et al., 1990; Wimberly, 1990. If E
TAN
is not known for a feed with substantially different protein content, then the value of E
TAN
can be proportionally adjusted from a known excretion rate for a similar species fed a feed with a known protein
content. The design values given are conservative with the indicated values easily achiev-
able when the filters are managed to sustain nitrification. The values can be expected to hold for fresh and saltwater applications where the temperature is
maintained between 20 and 30°C. However, the bead filter nitrification performance can vary widely Fig. 1, and peak conversion rates are almost always associated
with careful management Wimberly, 1990; Chitta, 1993; Sastry et al., 1999. Bead filters primarily operated for clarification display nitrification performance that are
largely supplemental Mississippi Power and Light, 1991; DeLosReyes, 1995; DeLosReyes and Lawson, 1996.
4. Bioclarifier integration
