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
Although floating bead bioclarifiers can, and often, are used in conjunction with other biofilters, it is the opinion of the authors that the most cost-effective and
stable systems will be based upon simplified integrated design strategies that depend entirely on the FBF for clarification and biofiltration. Stability and transferability
within the context of an industry that will most likely be implemented by individu-
Fig. 1. Bead filter performance can vary dramatically with loading and management.
als with minimal formal training are major underpins to this strategy. This philosophy leads the authors to the conclusion that simple blown air or mechanical
aeration devices that inherently address carbon dioxide control best support the FBFs. When implemented with a straightforward alkalinity control program Loy-
less and Malone, 1997, this approach addresses pH collapse problems that continue to be a concern in the commercial sector despite the ongoing discussion in
the literature Grace and Piedrahita, 1994; Loyless and Malone, 1998.
The warmwater criteria Table 2 were developed in support of this strategic approach. Sizing variables are normalized to the feed application rates, W Fig. 2.
These criteria have provided a technically robust foundation for a number of experimental and commercial systems. They are currently used as a guide by the
principal floating bead filter manufacturer in the United States Drennan, 1999 for a wide variety of species, feeds, and filter configurations. This simple sizing chart
facilitates preliminary economic analysis and provides a sizing check for more detailed design work. It is presumed that common design practice is followed. In
recognition of the variety of conditions and devices encountered, the sizing criteria includes a 33 safety factor. The stated water quality objectives, mentioned earlier,
can be reasonably met with a device or rate at two-thirds of the stated value. Additional safety factors are realized from the conservative nature of the target
water quality levels.
The growout system volume criterion, 6
t
, is set at 1.67 m
3
kg
− 1
feed day to assure stability. For a 1 feedrate f , this results in fish density of 60 kg m
− 3
that has been proven to be stable under the rigors of commercial growout conditions
Beecher et al., 1997; DeLosReyes et al., 1997a; Sastry et al., 1999, when used with a 6
b
in the range of 0.062 m
3
kg
− 1
feed day. At this density, problems with low dissolved oxygen levels during feeding can be managed by feeding frequency, and
acute problems with nitrite or ammonia build-up can be detected with a once a day
Table 2 Interim guidelines for the design of recirculating systems employing floating bead bioclarifiers, water
pumps, and airstones Parameter
Ornamental and Growout applica-
Broodstock sys- tions
tems fingerlings
System characteristics 1.67
3.33 System volume m
3
water 6
t
6.66 kg
− 1
feed day 0.125
6
b
0.062 0.250
Bead volume m
3
beads kg
− 1
feed day 208
83 50
q
r
Circulation rate l min
− 1
kg
− 1
feed day 187
375 Air for airstones l min
− 1
g
a
375 kg
− 1
feed day b
a
242 242
NaHCO
3
dose g kg
− 1
242 feed
600 204
68 Water replacement l kg
− 1
q
f
feed Common system descriptors
10
f
1006
t
× f
60
e
15
e
Fish density kg fish m
− 3
water 10006
t
q
f
16 25
System hydraulic residence 11
time HRT in days 40
10006
t
q
r
Tank turnover min 32
33 4902
10
6
q
f
14 706 1667
Cumulative feed burden mg l
− 1
50 147
441 Nitrate accumulation mg N 30 000q
f
l
− 1
b,d
0.18 14.58q
r
0.29 0.07
Mixing constraint mg TAN l
− 1
a,b
Filter design parameters 5.76q
r
478 288
Oxygen delivery g O
2
kg
− 1
1198 feed
c
4792 3825
4645 5.76q
r
6
b
Oxygen delivery O
2
m
− 3
beads day
− 1
c
339 84
216
b
FBF TAN loading g m
− 3
168 beads day
− 1
a,b
10006
b
8 16
FBF feed loading kg feed 4
m
− 3
beads day
− 1
FBF hydraulic loading l 806
q
r
6
b
832 664
min
− 1
m
− 3
beads
a
Assumes I
s
= 0.3.
b
Assumes E
TAN
= 30 g TAN kg
− 1
feed.
c
Assumes 4 mg O
2
l
− 1
drop.
d
Neglects in-situ denitrification.
e
Assumes f = 1.
f
Assumes f = 3.
Fig. 2. The size of the major components for a recirculating bioclarifier system is determined by multiplying a system criterion from Table 2 by the feed application rate W i.e. Q
r
= q
r
· W.
inspection or monitoring. At 50 l min
− 1
kg
− 1
feed day, the recirculating rate constant, q
r
, assures oxygen delivery to the biofilter Manthe et al., 1988; Sastry et al., 1999. The air delivery rate to the airstones, g
a,
is set at 187 l min
− 1
kg
− 1
feed day, which balances the need for aeration with carbon dioxide stripping Loyless
and Malone, 1998. Ion balance is normally addressed by sodium bicarbonate addition at a rate b
a
= 242 g kg
− 1
feed that approximately replaces alkalinity lost due to the nitrification processes Loyless and Malone, 1997. A flushing rate of q
f
at 68 l kg
− 1
feed or HRT at about 25 days slowly replaces the recirculating waters, thus, avoiding problems with ion build-up nitrate.
The 6
t
criterion is increased to 3.33 m
3
kg
− 1
feed day
− 1
to provide additional stability for fingerlings and ornamentals whose quality maybe adversely impacted
by water quality fluctuations. The drop in substrate concentrations TAN and nitrite-N increases the importance of mixing constraints as opposed to oxygen
transport so the FBF hydraulic loading, q
r
6
b
, are held at a level similar to the growout criteria. The resulting q
r
of 83 l min
− 1
kg
− 1
feed day also facilitates nitrification within the biofilter by helping to maintain the interstitial TAN levels
within the bead bed. The air delivery rate, g
a
, is increased moderately to 375 l min
− 1
kg
− 1
feed day eliminating concerns about oxygen or carbon dioxide management. The flushing rate is raised moderately to 204 l kg
− 1
feed a HRT of about 16 days limiting steady state nitrate accumulations to about 150 mg N l
− 1
. The alkalinity addition rate remains at 242 g kg
− 1
feed although this value is reduced in areas of the country with source waters high in alkalinity.
The third design category, the broodstock category, is established for applica- tions that demand the utmost stability and pristine water quality. Both the bead
filter and tank volume criteria are increased, although in practice the latter is most often increased even further by previously defined breeding or maturation proto-
cols. The recirculation rate remains important as TAN transport clearly limits the filter’s nitrification capacity. The flushing rate is increased to 600 l kg
− 1
feed lowering the HRT to 11 days and the steady state nitrate level to 50 mg N l
− 1
. Aeration and sodium bicarbonate addition rates remain the same as mentioned
earlier in the fingerling category.
5. Discussion
