Use of floating bead filters to recondition recirculating waters

Aquacultural Engineering 22 (2000) 57 – 73
www.elsevier.nl/locate/aqua-online

Use of floating bead filters to recondition
recirculating waters in warmwater aquaculture
production systems
Ronald F. Malone *, Lance E. Beecher
Department of Ci6il and En6ironmental Engineering, Louisiana State Uni6ersity, Baton Rouge,
LA 70803 -6405, USA

Abstract
Floating bead filters (FBFs) are expandable granular filters that display a bioclarification
behavior similar to sand filters. They function as a physical filtration device (or clarifier) by
removing solids, while simultaneously encouraging the growth of bacteria that remove
dissolved wastes from the water through biofiltration processes. Presently, there are two
classes of FBFs that exist. Hydraulic and air washed units fall into the ‘gently washed’
category, which display reduced biofilm abrasion during backwashing and must be washed
at a high frequency. Conversely, propeller-washed and paddle-washed filters inflict damage
to a relatively heavy biofilm during backwashing, and are considered ‘aggressively washed’.
FBFs capture solids through four identifiable mechanisms: straining, settling, interception,
and adsorption. In the biofiltration mode, bead filters are classified as fixed film reactors,
where each bead becomes coated with a thin film of bacteria that extracts nutrients from the
recirculating water as it passes through the bed. In this paper the authors first establish
application categories and parameters for recirculating system use, then give criterion for the
sizing of recirculating system components in tabular form. Sizing variables for FBFs are
normalized to the feed application rates, and the primary method for the sizing is based on
a volumetric organic loading rate. Evaluation parameter equations are also given for
comparison of bioclarifier performance. These equations include volumetric TAN conversion
rate (VTR), the volumetric nitrite conversion rate (VNR), and the volumetric oxygen
consumption rate of the bioclarifier (OCF). © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Aquaculture recirculating systems; Floating bead filters; Nitrification

* Corresponding author. Tel.: + 1-225-3888666; fax: + 1-225-3888652.
E-mail address: (R.F. Malone)
0144-8609/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 4 4 - 8 6 0 9 ( 0 0 ) 0 0 0 3 2 - 7


58

R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

1. Introduction
Advocates of the production of commodity aquaculture products in recirculating
systems are confronted with severe economic realities. In many cases, alternate
modes of production, including harvesting of natural fisheries, depress prices. The
animals not only have to be sustained for extended periods, but also the growout
process must be successfully repeated to maintain income. The five major processes
(aeration, degasification, clarification, biofiltration, and circulation) must be implemented with components that are affordable, yet, highly reliable.
The aquaculture industry is faced with the serious technical challenge of identifying or developing a cost-effective production format that is capable of consistently
producing a high quality product. This must be accomplished in the face of
increasing societal concerns about water use, environmental impact, exotic species
introductions, disease impacts on natural populations, migratory bird issues and
coastal land use; to name a few (Malone, 1994). Traditionally, constructed recirculating systems cannot compete directly with pond production on an economic basis
(Losordo and Westerman, 1994). Commercially viable recirculating systems typically exist only where exotic species regulations, weather, or market expectations
are incompatible with pond production (Lutz, 1997). The cost of recirculating
water-reconditioning systems must be reduced if recirculating systems are to
compete directly with other production formats (ponds and net pens). This cost
reduction should be made within the context of a system capable of producing good
water quality reliably over extended periods of time.
Over the last decade, the unit processes required for treatment of recirculating

systems have been clearly defined (Lucchetti and Gray, 1988; Huguenin and Colt,
1989; Rosenthal and Black, 1993). The four most critical treatment processes are
aeration (oxygen), clarification (solids, Biochemical Oxygen Demand (BOD)), biofiltration (BOD, ammonia, nitrite), and degasification (carbon dioxide), which all
are linked by means of circulation. Systems with extended hydraulic retention times
must generally have an alkalinity replenishment regime to compensate for the
alkalinity-consuming nitrification reaction. Additional treatment processes are denitrification (nitrate, alkalinity), ozonation (BOD, color), disinfection (pathogens),
and foam fractionation (surfactants), which can be required to comply with specific
production needs. The rationale for implementation, including both device selection
and sizing criteria, vary widely (Parker, 1981; Kaiser and Schmitz, 1988; Losordo
and Westerman, 1994; Arbiv and Van Rijn, 1995; Summerfelt, 1995; Twarowska et
al., 1995; DeLosReyes and Lawson, 1996; Heinen et al., 1996). However, the ‘unit
process’ approach that has been significant to the training foundation of both the
environmental and aquacultural engineers, underpins the design strategy in almost
every case. This strategy calls for the assembly of a treatment train consisting of a
series of focused and optimized operations to meet a stated water treatment need.
The unit process concept was developed in an economic reality substantially
different from that confronting the recirculating systems engineer. This strategy
works well in meeting the technical objectives, but has thus far failed to overcome
the severe economic challenge confronting recirculating productions systems.


R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

59

In this paper, the authors urge the virtual abandonment of this strategy, arguing
instead that the multiple treatment objectives normally associated with a complex
treatment train must be assigned to one or two treatment components. These are
optimized within the context of the broader systems objectives. Failure to meet the
high plateau of performance that can be achieved when a treatment train component is optimized with respect to a single objective, may be more than offset

economically in both capital and operational costs by the reduction in the number
of the components. Additionally, the increased stability or reliability that may
result from simplification will contribute possible significant economic benefits to an
industry that must maintain active systems continuously for months on end to be
profitable.

2. Background
The floating bead filters (FBFs) are expandable granular filters that display a
bioclarification behavior similar to sand filters. However, they are specifically
designed and managed to enhance their biofiltration capabilities while avoiding
the caking problems that plague traditional down flow sand filters when faced
with high organic loadings. They function as a physical filtration device (or clarifier)
by removing solids, while simultaneously encouraging the growth of desirable
bacteria that remove dissolved wastes from the water through the biofiltration process (Malone et al., 1993). FBFs are resistant to biofouling and generally
require little water to backwash. Specific surface area of the small (2–3 mm),
spherical polyethylene beads typically used for the filtration bed is moderate
(1150–1475 m2 m − 3) (Malone et al., 1993). Hull shapes of the floating bead filters
vary widely.
The FBFs are operated in the filtration mode most of the time. As the
recirculating water passes through the packed bed, suspended solids are captured
and the biofiltration processes are active. Periodic cleaning of the bead bed is
accomplished by mechanical (Malone, 1992, 1995; Malone et al., 1993, 1998;
DeLosReyes et al., 1997b), hydraulic (Wimberly, 1990), or pneumatic (Cooley,
1979; Malone, 1993) means. The objective of the backwashing step is to release
solids and excessive biofloc from the beads, thus, restoring hydraulic conductivity.
Sludge is removed with or without the benefit of settling, allowing re-initiation of
another bioclarification cycle. Floating bead filters should not be confused with
dynamic bed filters (Junius and Junius, 1996; Greiner and Timmons, 1998) that
may also utilize plastic media and display some bioclarifier attributes. FBFs for the
purpose of this paper are assumed to display two distinct modes of functional

operation; a packed mode and an expanded or backwashing mode.

2.1. Clarification performance
FBFs capture solids through four identifiable mechanisms, which include strain-


60

R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

ing, settling, interception, and adsorption. With the exception of adsorption, the
solids capture mechanisms are physical in nature and are common to all types of
granular media filters. On a single pass basis, they provide almost complete removal
of suspended solids above 50 mm and 40–50% removal of fine solids (B 10 mm). In
a multi-pass recirculating mode, bead filters provide complete clarification (Ahmed,
1996). They are fully capable of maintaining almost display quality turbidities (B1
NTU) even in the presence of high loading. Clarification efficiency is rarely an issue
for a recirculating bioclarifier application, allowing focus on biofiltration performance.

2.2. Biofiltration performance
In the biofiltration mode, bead filters are classified as fixed film reactors. Each
bead becomes coated with a thin film of bacteria that extract nutrients from the
recirculating water as it passes through the bed. Heterotrophic and nitrifying
bacteria co-exist in the filter. The heterotrophic bacteria oxidize organic carbon,
and most species have a higher specific growth rate and higher yield coefficients
than the autotrophic nitrifiers do (Metcalf and Eddy, 1991; Henze et al., 1997). In
almost every case, the heterotrophic bacteria dominate the biofilm defining the
conditions the autotrophic nitrifiers must encounter (Bovendeur et al., 1990;
Manem and Rittmann, 1992).
Organic enrichment encourages heterotrophic bacteria growth which compete

with the nitrifiers for space and potentially limiting nutrients such as oxygen
(Matsuda et al., 1988; Zhang et al., 1995). This phenomenon is not unique to bead
filters. Studies by Bovendeur et al. (1990) with trickling filters and Figueroa and
Silverstein (1992) with rotating biological contactors both associate increasing BOD
levels with declining rates of nitrification. The FBFs recirculating clarification
function leads to the interstitial accumulation of organically-rich solids. This
condition is controlled in a FBF by backwashing, which removes solids and
abrades excessive biofilm.
A FBF’s optimum nitrification performance occurs when sludge accumulation
concerns are properly balanced with Mean Cell Residence Time (MCRT) considerations. This is accomplished by adjusting the backwashing intensity and frequency.
Extensive experimental studies have been conducted on this aspect of bead filter
management, and evaluated by a mathematical model (Golz, 1997). These studies
indicate that two broad classes of FBFs exist. Hydraulic and air washed units fall
into the ‘gently washed’ category. This type of washing displays reduced biofilm
abrasion during backwashing and must be washed at a high frequency. Optimum
performance for heavily loaded filters occurs when the filters are washed several
times daily (Wimberly, 1990; Sastry et al., 1999). Conversely, propeller-washed and
paddle-washed filters inflict damage to a relatively heavy biofilm during backwashing, and are considered ‘aggressively washed.’ They must be backwashed infrequently, usually every other day, to allow growth of biofilm, thus, avoiding MCRT
problems (Chitta, 1993; Malone et al., 1993).


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61

2.3. Performance indicators
The volumetric TAN conversion rate (VTR), the volumetric nitrite conversion
rate (VNR), and the volumetric oxygen consumption rate of the bioclarifier (OCF)
can be used as principal parameters for evaluation and comparison of bioclarifier
performance. The volumetric TAN conversion rate can be obtained by using Eq.

(1):
VTR =Kc(TANI −TANE)Qr/Vb

(1)

where VTR= volumetric TAN conversion rate (g TAN m − 3 day − 1); Qr = flow rate
through the filter (l min − 1); Kc =conversion factor of 1.44; TANI = influent
ammonia concentration (mg N l − 1); TANE = effluent ammonia concentration (mg
N l − 1); Vb =total volume of bead media (m3).
The actual level of nitrification occurring in the filter may be higher because TAN
is a by-product of heterotrophic breakdown of nitrogen-rich organic compounds
and biofloc. Despite its limitations, VTR allows the relationship between design and
management parameters to be more closely examined.
The volumetric nitrite conversion rate (VNR in g NO2-N m − 3 day − 1) is defined
by Eq. (2):
VNR=VTR +Kc(NO2I −NO2E)Qr/Vb

(2)
−1

where NO2I =influent nitrite concentration (mg N l ) and NO2E = effluent nitrite
concentration (mg N l − 1).
As this equation illustrates, the readings of influent and effluent nitrite must be
combined with the volumetric ammonia conversion rate to determine the level of
nitrite conversion activity, since nitrite is being produced as the ammonia is
converted within the bead bed. Because of this phenomenon, the apparent nitrite
removal efficiency may be near zero (i.e. influent and effluent values are nearly
identical), although the filter may be vigorously processing nitrite to nitrate.
The volumetric oxygen consumption rate (OCF in g O2 m − 3 day − 1) is very
helpful in the management of bead filters (Manthe et al., 1988). It indicates the total

amount of bacterial activity within the filter, and can be obtained using Eq. (3):
OCF = Kc(DOI −DOE)Qr/Vb

(3)

where DOI =influent dissolved oxygen concentration (mg O2 l − 1) and DOE =
effluent dissolved oxygen concentration (mg O2 l − 1).
OCF measures the combined respiration of the nitrifying bacteria, the heterotrophic bacteria extracting soluble BOD from the water column, and the heterotrophic bacteria responsible for the breakdown of solids (sludge) held in the filter.
The apparent oxygen consumption rate of the nitrifying bacteria (OCN in g O2
m − 3 day − 1) can be computed directly from the volumetric conversion rates for
nitrification using Eq. (4) since we can estimate the amount of oxygen required for
nitrification from chemical equations (Golz et al., 1999):
OCN =(3.47VTR +1.09VNR) × 0.92

(4)


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R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

The factor 0.92 (unitless) corrects for oxygen assimilation during bacterial
growth. The volumetric oxygen consumption rate that can be attributed to heterotrophic activity (OCH in g O2 m − 3 day − 1) can then be calculated by Eq. (5):
OCH =OCF − OCN

(5)

The ratio of OCN to OCF expressed as a percentage is a valuable indicator of the
efficiency of a backwashing protocol. A high OCN percentage (\ 50%) indicates
that the nitrifying population is relatively high, i.e. the heterotrophic bacterial

population has been successfully controlled without excessive loss of the nitrifying
population. The OCN percentage tends to drop under lightly loaded regimes as the
backwashing interval is extended allowing for more complete digestion of accumulated sludges. The nitrification capacity, however, is not adversely impacted as
substrate (TAN) availability, not biofilm diffusion characteristics, limit the conversion process under these conditions.

2.4. Application categories
The sizing of a FBF is facilitated by identification of three categories of
application that are identified here according to a common production objective.
The first is the broodstock category. Valuable broodstock are often held at low
densities with oversized treatment units to assure a stress-free environment displaying excellent water quality (Watanabe et al., 1998). This category calls for levels of
total ammonia nitrogen and nitrite nitrogen less than 0.3 mg N l − 1. Low total
ammonia nitrogen (TAN) and nitrite concentrations dictate operation of FBFs in a
thin film mode of operation, a condition induced by low organic and nitrogen
loading levels. For example, the best water quality is often demanded for shrimp
maturation (Lawrence and Hunter, 1987). Eggs and fry of many species are very
sensitive to water pollution (Holt and Arnold, 1983; Russo and Thurston, 1991).
Similarly, display aquaria are often kept in an oligotrophic state to assure that the
highest aesthetic standards are encountered.
The second, fingerling category, reflects the need for very good, but not pristine
water quality demanded by many species during early stages of life. This category
is also compatible with ornamental fish production where concerns about quality
dictate improved water quality conditions (Ng et al., 1992; Kaiser et al., 1998).
Substrate levels can be allowed to increase to a level (0.5 mg N l − 1) encouraging
the development of a healthy biofilm that can be maintained within broad management parameters. Recirculating soft-crab (Callinectes sapidus) shedding systems
(Malone and Burden, 1988b) and soft-crawfish (Procambarus clarkii and P. zonangulus) shedding systems (Malone and Burden, 1988a; Malone et al., 1996) are
designed to display mesotrophic conditions to protect the animals as they pass
through vulnerable molting processes.
The third, or growout category, describes the bulk of high-density production
systems where risk and economics must be carefully balanced to achieve profitability (e.g. trout [Salmo gairdneri ] — Kaiser and Schmitz, 1988; Heinen et al., 1996);
tilapia [Oreochromis niloticus] — Losordo and Westerman, 1994; Twarowska et al.,

R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

63

1995; DeLosReyes and Lawson, 1996). For this category some deterioration in
aesthetics is permitted, but water quality is held below safe levels (TAN and
nitriteB1.0 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 management 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 inappropriate in this category with the floating bead filter operating in a supporting clarification 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, Vb) 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 controlling nitrification conditions within the bioclarifier, (3) the organic/nitrogen 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, 6b, is based upon feed loading.
The volume of bead media required for any application, Vb, can then be determined
by the rates of the peak feeding rate, W, and the FBF sizing criterion, 6b:
Vb =[L( f )/100]6b
Vb =W6b

(6)

where L= maximum weight of fish in the system (kg); f= feedrate (percent of body
wt. fed day − 1); 6b =FBF sizing criterion (m3 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 (6b values set at 67% of readily achievable peak
performance). Furthermore, increasing the protein content of a feed effectively


R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

64

lowers the organic/nitrogen-loading ratio to benefit the nitrification process. However, in practice when the protein content is known to be very high, Eq. (6) is
modified:
Vb =L( f )6b(P/35)/100
Vb =W6b(P/35)

(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 filtersa
Management parameters

Units

Typical operational values observed in practice

Broodstock
Feed loading
Design TAN
Typical TAN
VTR
Design nitrite
Typical nitrite
VNR
OCF
OCN/OCF
OCH/OCF
Temperature

FBF effluent O2
Alkalinity
pH range
Backwash inter6al
Aggressive wash
Gentle wash
a

kg feed m−3 media
day−1
g TAN m−3 media
day−1
g TAN m−3 media
day−1
g TAN m−3 media
day−1
g N m−3 media day−1
g N m−3 media day−1
g N m−3 media day−1
kg O2 m−3 media day−1
%
%
°C
mg l−1
mg CaCO3 l−1

Days
Days

54

Ornamental
58

Growout
516

0.3

0.5

1.0

B0.1

B0.3

B0.5

35–105

70–180

140–350

0.3
B0.1
35–105
0.7–2.5
25–35

65–75
20–30
\3.0
\50
6.5–8.0

0.5
B0.3
70–180
1.4–2.5
25–35
65–75
20–30
\3.0
\80
6.8–7.0

1.0
B0.5
140–350
2.5–3.0
45–55
45–55
20–30
\3.0
\100
7.0–8.0

1–7
1–3

1–3
1–2

1–2
0.5–1

Values derived principally from Wimberly (1990) and Sastry et al. (1999).


R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

65

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 observations 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:
Vb =(1 −Is)(ETAN)W/VTR

(8)

where Is =in situ nitrification fraction (unitless); ETAN = 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 Is =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 ETAN is not known for a feed with substantially
different protein content, then the value of ETAN 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 achievable 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-


66

R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

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 (Loyless 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, 6t, is set at 1.67 m3 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 6b in the range of 0.062 m3 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


R.F. Malone, L.E. Beecher / Aquacultural Engineering 22 (2000) 57–73

67

Table 2
Interim guidelines for the design of recirculating systems employing floating bead bioclarifiers, water
pumps, and airstones
Parameter

System characteristics
System volume (m3 water
kg−1 feed day)
Bead volume (m3 beads
kg−1 feed day)
Circulation rate (l min−1
kg−1 feed day)
Air for airstones (l min−1
kg−1 feed day)
NaHCO3 dose (g kg−1
feed)
Water replacement (l kg−1
feed)
Common system descriptors
Fish density (kg fish m−3
water)
System hydraulic residence
time (HRT in days)

Tank turnover (min)
Cumulative feed burden
(mg l−1)
Nitrate accumulation (mg N
l−1)b,d
Mixing constraint (mg TAN
l−1)a,b
Filter design parameters
Oxygen delivery (g O2 kg−1
feed)c
Oxygen delivery (O2 m−3
beads day−1)c
FBF TAN loading (g m−3
beads day−1)a,b
FBF feed loading (kg feed
m−3 beads day−1)
FBF hydraulic loading (l
min−1 m−3 beads)
a

Ornamental and
fingerlings

Growout applications

6t

6.66

3.33

1.67

6b

0.250

0.125

0.062

qr

208

83

50

ga

375

375

187

ba

242

242

242

qf

600

204

68

100/(6t×f )

15e

10f

60e

10006t/qf

11

16

25

10006t/qr

106/qf

32
1667

40
4902

33
14 706

30 000/qf

50

147

441

14.58/qr

0.07

0.18

0.29

5.76qr

1198

478

288

5.76qr/6b

4792

3825

4645

84

168

339

4

8

16

832

664

806

21/6b
1000/6b
qr/6b

Assumes Is = 0.3.
Assumes ETAN = 30 g TAN kg−1 feed.
c
Assumes 4 mg O2 l−1 drop.
d
Neglects in-situ denitrification.
e
Assumes f= 1%.
f
Assumes f= 3%.
b

Broodstock systems


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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. Qr =qr · W).

inspection or monitoring. At 50 l min − 1 kg − 1 feed day, the recirculating rate
constant, qr, assures oxygen delivery to the biofilter (Manthe et al., 1988; Sastry et
al., 1999). The air delivery rate to the airstones, ga, 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 (ba =242 g kg − 1 feed) that approximately replaces alkalinity lost
due to the nitrification processes (Loyless and Malone, 1997). A flushing rate of qf
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 6t criterion is increased to 3.33 m3 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, qr/6b, are held at a level similar to the
growout criteria. The resulting qr 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, ga, 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 applications that demand the utmost stability and pristine water quality. Both the bead


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69

filter and tank volume criteria are increased, although in practice the latter is most
often increased even further by previously defined breeding or maturation protocols. 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
The warmwater bioclarifier system criteria are functionally robust and fully
suitable for application in the commercial environment that exists today in the
United States. Although competitive with other technological approaches, the
criteria described in the design categories above have not been fully and economically optimized. Three areas for further cost reductions include (1) reduction in the
capital cost of biofilter units, (2) reduction in pumping costs associated with water
recirculating, and (3) refinement of alkalinity adjustment strategies. These issues are
actively being addressed through ongoing research efforts.
The utilization of alternate floating media that display a degree of biofilm
protection and increase bed porosity is showing promise in addressing the first two
concerns cited above. These new media facilitate operation under a high frequency
washing regime (Sastry et al., 1999) that improves carrying capacities. Operational
headloss that occurs through the bead bed are also dramatically reduced permitting
the use of low head airlift pumps and cost-effective, non-pressurized hulls. The use
of airlift pumps as an energy saving tool is well documented (Wheaton, 1977;
Spotte, 1979; Castro and Zielinski, 1980; Bronikowski and McCormick, 1983;
Reinemann and Timmons, 1989; Turk and Lee, 1991). An airlift criteria is currently
undergoing evaluation (DeLosReyes et al., 1997a) and with commercial feedback it
should evolve into an alternate and more cost-effective criteria for some
applications.
The alkalinity adjustment approach has proven to be safe and easy to manage,
but the requirements for sodium bicarbonate addition have proven to be substantial
and contribute about 6% to the cost of operating services in the authors’ economic
projections for commodity fish such as tilapia. The use of a less expensive, more
caustic additive such as lime or sodium hydroxide (Weaver, 1999) may warrant
consideration in future criteria.
Finally, increasing interest in marine applications has raised concerns about the

flushing rate associated with the criteria. Inland marine systems can face significant
costs obtaining salt, and particularly in arid regions, face environmental obstacles
for disposal of saline waters. Nitrate accumulations are expected to limit the degree
of water reuse in many marine applications (Whitson et al., 1993). The use of a
denitrification unit in future marine recirculating criteria may be needed. The cost
of the denitrification units would be partially offset by the elimination of the
alkalinity replenishment requirement (Kaiser and Schmitz, 1988; Nijhof and Bovendeur, 1990; Van Rijn and Rivera, 1990; Van Rijn, 1996).


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Definition of criteria for coldwater applications (10–20°C) continues to be an
issue with the authors, reflecting their commercial experience in the southern United
States. Current commercial sizing practice is to simply reduce 6b by 50% for the
selected category of application in recognition of the general reduction in bacterial
kinetics associated with cooler waters (Knowles et al., 1965; Srna and Baggaley,
1975; Sharma and Alhert, 1977; Wortman and Wheaton, 1991). This approach
works, but is probably overly conservative since the bacterial consortium and the
nitrifying bacteria density in the biofilm undoubtedly change. Further research is
needed in this area.
The authors continue to advocate a stepwise simplification of supporting processes to assure stability and economic viability of production systems. In the near
future, recirculating systems can be expected to serve a growing role in support of
extensive (pond) production as they contribute to the production of disease-free
fingerlings, over wintering of warmwater broodstock, and purging of off-flavor
(Malone, 1994). Eventually, if the systems can be adequately simplified and
economically optimized, recirculating systems may compete directly as an economically viable alternative to extensive grow out systems.

Acknowledgements

This work was funded, in part, by the Louisiana Sea Grant College Program, an
element of the National Sea Grant College Program, under the direction of NOAA,
U.S. Department of Commerce. This unit has sustained support of this research
effort for over a decade. Douglas Drennan, managing member of Aquaculture
Systems Technologies, L.L.C. of New Orleans, LA has been particularly helpful in
providing feedback from the commercial sector on the criteria. Steve Abernathy of
Tiltech, Inc. in Robert, LA has worked closely with the research team and allowed
his facility to be used as a testing base.

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