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Vol.
6:
295-298, 1981
MARINE ECOLOGY
-
PROGRESS
SERIES
Mar. Ecol. Prog. Ser.
Published November
15
Pathogen Reduction
in
Closed Aquaculture
Systems
by
UV
Radiation: Fact
or
Artifact?*
Stephen spottel
and
Gary
~darns~
'
Sea Research Foundation, Inc., Mystic Marinelife Aquarium, Mystic, Connecticut
06355.
USA
Department of Mathematics and Physics. Thames Valley State Technical College, Norwich, Connecticut
06360,
USA
ABSTRACT:
Differential
equations were used to set a theoretical upper limit for the efficacy of UV
radiation in
3
hypothetical aquaculture systems: (a) a plug-flow system, (b) an idealized closed system
with no influx of pathogens, and (c) a conventional closed system in which the influx of pathogens is
continuous. The equations demonstrate that, in a conventional closed system, the mass of pathogens
never reaches zero even if the UV sterilizer is 100
%
effective. This suggests that agents such as UV
radiation, which do not form persistent residuals, may be incapable of preventing the spread of water-
borne pathogens in systems that are recirculated. Use of
UV
radiation in aquaculture is most effective
in sterilization of raw water supplies and discharges lnto receiving waters, both of which are single-
pass applications.
INTRODUCTION
Sterilizing agents commonly are used in aquaculture
installations, fish hatcheries, and public aquariums, in
which crowded conditions often hasten the spread of
transmissible diseases. Diseases produced by single-
celled organisms, or multicellular organisms that do
not require vectors, are particularly troublesome in
closed systems, because all water is reused, rather than
diluted cont~nuously with clean influent. Free-floating
infectious organisms (referred to here simply as patho-
gens) accumulate, unless the sterilization rate equals
or exceeds the rate of contamination.
METHOD
We
used differential equations to model the perform-
ance of
UV
sterilization in
3
hypothetical water sys-
tems illustrated in Fig.
1.
The curves in Fig.
2
show the
mass of pathogens remaining in the systems after
sterilization. The effective kill rate on contact in the
sterilizer was presumed to be
100
%
for each of the
3
schemes. Scheme
A
of Fig.
1
and Curve
A
of Fig.
2
show the total mass of pathogens remaining if water
flows through the sterilizer in a single pass into a
second tank with no replacement water (plug-flow
system). Scheme
B
of Fig.
1
and Curve
B
of Fig.
2
illustrate a system in which sterile water is returned
from the
UV
unit to the culture facility with complete
mixing, but without the influx of additional pathogens
(idealized closed system). Scheme
C
of Fig.
1
and
Curve
C
of Fig.
2
represent a conventional closed
system; that is, water returned from the sterilizer is
disease-free, but the influx of pathogens from within
the culture facility itself is constant. The percent kill of
pathogens in Scheme
C
is limited by an equilibrium
level at which the kill rate equals the rate of influx.
This equilibrium equals the rate of influx divided by
the flow rate through the sterilizer. The efficacy of the
sterilizer depends on only
2
factors: the rate at which
pathogens are introduced and the effectiveness of the
treatment device, as shown in the equations below.
The same equations can be used to calculate the rate at
which chemical contaminants are removed by physical
adsorption contactors, such as foam fractionators and
activated carbon beds.
Symbols:
V
=
total volume of the system (constant);
C,
=
initial mass of pathogens (constant); to
=
initial
time;
F
=
volume flow rate through the sterilizer (con-
FC
stant);
C
=
mass of pathogens;
=
kill rate (mass
per unit time);
R
=
influx of pathogens (mass per unit
'
Contribution No.
22,
Sea Research Foundation, Inc.
time)
O
Inter-ResearchIPrinted in
F.
R. Germany
0171-8630/81/0006/0295/$
02.00
Mar Ecol. Prog. Ser. 6:
295-298.
1981
Scheme
A
Scheme
B
Scheme
C
1st tank 2nd tank
Sterllhzer
Pathogen removal
Culture
facllity
removal
Pathogen
,Adit;%
/
\
I
\
Culture
facility
removal
Fig.
1.
Plug-flow system (Scheme
A):
water flows through the
sterilizer
in
a single pass, and pathogens are removed at a
constant rate. Idealized closed system (Scheme
B):
all water is
recycled, there is no new addition of pathogens, and organ-
isms are removed at a rate proportional to concentration.
Conventi.ona1 closed system (Scheme
C):
all water
is
recycled,
pathogens are added at a constant rate, and removed at a rate
proportional to concentration. The rate of sterilization at all
3
contact sites is assumed to be
100
%
Tlme units
Fig.
2.
Mass of pathogens remaining
in
Schemes
A
through
C
(Fig.
1)
versus tlme.
In
closed systems (Scheme
C),
complete
sterilization is unattainable
Scheme
A
In Scheme
A,
water is pumped through the sterilizer
on a single pass and discharged into a second tank. No
water is recycled.
If
F
=
0.1
V
(10
%
of the system
volume h-'), it would take
9
h,
to kill 90% of the
pathogens in the system.
Scheme
B
In Scheme
B,
all water is recycled through the
sterilizer and returned continuously to the main por-
tion
of
the system. There is no new influx of pathogens.
Thus
I
herefore
and
C
=
Coe
=@
If
F
=
0.1
V
per unit time
C =Ce-Ol'
and
C
=
0.1 CO (90% kill)
e-O."
=
0.1
then
t
=
23
time units
For a flow through the sterilizer equal to
10
%
of the
volume of the system per hour,
23
h are required to kill
90
%
of the pathogens.
Scheme
C
In Scheme C, all water is recycled through the
sterilizer and returned continuously to the original
source, as in Scheme
B.
However, there also is a con-
tinuous influx of pathogens at rate
R,
and
F
dC
=
-~(~)dt
+
Rdt
Spotte and Adarns: Pathogc
?n reduction by radiation 297
Integration yields
If
F
=
0.1 V and
R
=
0.01 for a 90% kill.
C
=
0.1
C,,
then because
rt
RV
RV
C
=
e-by
(C.-~)-L~
in this case
By substituting different values for
t
it can be shown
that a 90% kill can be attained only after inf~nite time.
In fact, the mass of pathogens remaining cannot be less
RV
than
-,
which in this case is 10% of
C,.
F
DISCUSSION
As noted by Herald et al. (1970), the effectiveness of
UV
radiation
is limited to
in
vitro
situations; in other
words, to the destruction of pathogens that are free-
floating in the water. Organisms that are systemic, or
attached to exterior surfaces of their hosts, are unaf-
fected and can only be controlled chemotherapeuti-
cally. Organisms shed into the water, or which detach
from their hosts, thus represent a reservoir of contami-
nation that cannot be eliminated from closed systems,
if sterilization occurs at a single contact site (Scheme
C).
Spotte (1979) reviewed the use of UV radiation in
aquatic animal culture. Some pathogens always sur-
vive, despite kill rates that sometimes approach 100
%
at the contact site. Animals maintained in closed sys-
tems thus are subject to possible reinfection from water
returning from the sterilizer, the degree of reinfection
depending on the virulence and concentration of the
pathogen and the immune status of the host. Bullock
and Stuckey (1977) studied the effect of UV radiation
on bacterial counts of salmonid hatchery water. In
some instances, bacteria at the contact site were
reduced 99.99
%,
but the authors cautioned against
placing undue emphasis on the results, because the
number of bacteria necessary to transmit disease is
difficult to predict. They pointed out that, in their
experiments, a 99.99
%
kill of a pathogen at a cell
density of 104
ml-'
would leave only 1.0 ml-'. They
concluded that even this low concentration might be
adequate to transmit disease during intensive culture
if the pathogen is virulent, considering the growth
potential of bacteria. Buck (1980) recorded low cell
densities of a variety of potentially pathogenic fungi in
a UV-treated marine mammal pool operated as a
closed system. Spotte and Buck (1981) studied the
efficacy of UV sterilization on pathogen reduction in
the same pool. They observed that cutaneous and sys-
temic infections in the captive dolphins and whales,
caused by the yeast Candida albicans, continued to
flourish even though cell counts never exceeded 100
I-'
anywhere in the water system.
Disease-causing organisms can also be transmitted
among cultured animals directly, short-circuiting the
sterilizer completely (Scheme C). The result may be a
high percent kill at the contact site that is not accom-
panied by a concomitant decrease in the incidence of
disease or mortality. For example, Herald et al. (1970)
reported that mortality rates among exhibit fishes at a
public aquarium were unchanged by installation of a
UV sterilizer that lowered the total bacteria at the
contact site by 98
%.
Spanier (1978) noted that mortal-
ity rates of bream
(Sparus
aurata) larvae were unaf-
fected by
UV
sterilization of the recycled water,
despite a substantial decrease in bacterial counts at the
sterilizer effluent.
The application of UV radiation in the treatment of
raw influent water is effective in lowering the numbers
of pathogens and thus reducting the chances of disease
organisms entering from external sources (Stickney
and White, 1974; Hoffmann, 1975; Kimura et al., 1976;
Blogoslawski et al., 1978; Brown and Russo, 1979).
Results derived from this application can be consi-
dered fa c t.
If
the water is recycled, however, any
apparent efficacy of a UV sterilizer, based on kill rate,
is an a r t i
f
a c
t
,
because the mass of pathogens in the
immediate vicinity of the cultured animals is always
greater than the mass in the sterilizer effluent. Equilib-
rium is never attained, and the entire system cannot be
rendered disease-free, even when the sterilization pro-
cess is 100
%
effective.
In conclusion, UV sterilization should perhaps be
limited to the treatment of influent water supplies, or to
the final effluent from culture installations before it is
discharged into receiving waters. Both are single-pass
applications, for which the effectiveness of UV treat-
ment is well documented.
Acknowledgements. We wish to thank James W. Atz, Carol
E.
Bower. Walter
.I.
Blogoslawski, John
B.
Gratzek, and Gary
D.
Pruder for their reviews of the manuscript.
LITERATURE
CITED
Blog~sla~ski,
W.
J.. Stewart,
M.
E.,
Rhodes,
E.
W
(1978).
Bacterial disinfection in shellfish hatchery disease control.
Proc. World Maricult. Soc. 9: 589-602
Brown,
C.,
Russo,
D.
J
(1979). Ultraviolet light disinfection of
shellfish hatchery sea water.
I.
Elimination of five
pathogenic bacteria. Aquaculture 17: 17-23
Buck, J.
D.
(1980). Occurrence of human-associated yeasts in
the feces and pool waters of captlve bottlenosed dolphins
(Turs~ops
lruncatus)
J.
Wildl. Dis. 16: 141-149
Bullock.
G.
L..
Stuckey,
H.
M.
(1977). Ultraviolet treatment of
water for destruction of five gram-negative bacteria
pathogenic to fishes.
J.
Fish. Res. Bd Can. 34: 1244-1249
298
Mar Ecol. Prog. Ser. 6: 295-298, 1981
Herald, E.
S.,
Dempster,
R.
P,, Hunt.
M.
(1970). Ultraviolet
ster~l~zat~on of aquarium water. In: Hayen, W. (ed.)
Aquarium deslgn criter~a (Spec ed.), Drum and Croaker,
U
S Department of the Interior, Washihgton,
DC,
pp.
57-7
1
Hoffman.
G.
L.
(1975). Whirling disease
(Myxosomd
cere-
brdlis):
Control with ultraviolet irradiation and effect
on
fish. J. Wildl. Dis. 11: 505-507
Kimura. T., Yoshimizu.
M.,
Tajima.
K..
Ezura.
Y.,
Sakai,
M.
(1976). Disinfection
of
hatchery water supply by
ultraviolet
(U.
V.)
irradiation.
I.
Susceptibility of some
fish-pathogenic bacter~um and m~croorganisms Inhabit-
ing pond waters. Bull. Jap Soc. SCI F~sh 42. 207-211
Spanier,
E.
(1978). Prel~rninary trlals w~th an
ultraviolet
11quid
sterilizer. Aquaculture 14:
75-84
Spotte, S. (1979). Seawater aquariums The captlve envlron-
ment, Wiley, New York
Spotte, S., Buck,
J.
D. (1981). The eff~cacy of UVlrradiation in
the microbial disinfection
of
marine mammal water
J.
Wildl. Dis. 17: 11-16
Stickney,
R.
R.,
White, D.
B.
(1974). Lymphocystis in tank-
cultured flounder. Aquaculture 4: 307-308
This paper was submitted to the editor; it was accepted for printing on July 23. 1981
