Instituí für Küsten- und Binnenfischerei der Bundesforschungsanstalt für Fischerei,
Hamburg, Aussenstelle Ahrensburg (Federal Republic o f Germany)
(Received 9 September 1976)
Naegel, L.C .A ., 1977. Com bined production o f fish and plants in recirculating water.
Aquaculture, 10: 17—24.
A pilot plant o f ca 2000 1 o f recirculating fresh water for intensive fish production was
constructed in a controlled-environment greenhouse. The feasibility was examined o f using
nutrients from fish waste-water, mainly oxidized nitrogenous com pounds, for plant production, com bined with an activated sludge system for water purification.
The reduction o f nitrates, form ed during the extended aeration process by nitrifying
bacteria, was not sufficient by higher plants and unicellular algae alone to reduce the nitrate
concentration in our system significantly. An additional microbial denitrification step had
to be included to effect maximal decrease in nitrogenous com pounds.
For fish culture in the pilot plant Tilapia mossambica and Cyprinus carpió were chosen
as experimental fishes. Both fish species showed significant weight increases during the
course o f the experiment.
Ice-lettuce and tom atoes were tested both in recirculating water and in batch culture.
The unicellular algae Scenedesmus spp. were grown in a non-sterile batch culture. All plants
grew well in the waste-water without additional nutrients.
Determination o f the physical and Chemical parameters for optim um water purification,
the most suitable ratio o f denitrification by plants and by microorganisms, and the most
favourable fish and plant species for com bined culture in recirculating water are important
and o f current interest in view o f the increasing demand for clean, fresh water, and the
pressing need to find new ways o f producing protein for human nutrition under prevailing
conditions o f an exponentially expanding world population.
The increased demand in water quantity and quality for intensive fish and
invertebrate culture has led to a growing interest in systems using recirculating
There are today various biological methods available to treat water for reuse
in intensified fish culture (Spotte, 1970; Liao and Mayo, 1974; Meade, 1974;
Kinne, 1976). For water purification in recirculating water systems biological
filters are mostly applied, which effect the removal of waste products, principally ammonia, through bacterial action. Initially all nitrogenous compounds
are oxidized to nitrate (nitrification) and then reduced under anaerobio conditions to free nitroge^i (denitrification).The same biochemical reactiQps ocour
in another water-purification process, where the different strains of bacteria
are free-floating as an activated sludge. Today there is a great deal of technical
data available on the construction of activated sludge systems, since most
communal waste-water treatment plants use activated sludge as a biological
step for the elimination of nitrogenous compounds. But only limited research
has been done in the field of rearing fish or invertebrates in recirculating water
that is permanently purified by activated sludge. It was Sengbusch et al. (1965)
who first tried to raise the common carp, Cyprinus carpió, in aquaria with
recirculating water using activated sludge for water treatment. A few years
later Scherb and Braun (1971) described in more detail studies with rainbow
trout, Salmo gairdneri. A constant increase in the total nitrogen concentration,
mainly of nitrate ions, could be observed during their experiments. A special
process of eliminating nitrogenous compounds in intensive fish culture with
recirculating water and water treatment with activated sludge by means of an
additional step for denitrification by microorganisms, higher plants and/or
algae seems to be an important improvement for pollution control in such
During recent years several attempts have been made using nutrients from
industrial and domestic wastes for plant and animal production. At the
Woods Hole Oceanographic Institution Goldman et al. (1974) and Ryther et
al. (1975) investigated the possibility of using waste-water effluent from a
communal secondary sewage treatment plant, mixed with sea water, to grow
lobsters and flounders at the end of a food chain. Continuous culture of algae,
which are grown in high rate oxidation ponds on wastes, and the simultaneous
treatment of waste-water are reaching industrial levels and could make a significant contribution to the provisión of proteins. These processes were described in detail by Oswald and Golueke (1968) and by Soeder (1972), and can
be employed with highest efficiency only in countries which have throughout
the whole year a continuous high light intensity. At the South Carolina Agricultural Experiment Station, Clemson, an attempt was made by Loyacano and
Grosvenor (1973) to remove excess nutrients from fish ponds stocked with
channel catfish Ictalurus punctatus by means of hydroponics of waterchestnut
Eleocharis dulcís. The decrease in nutrients of the water led to a higher production of channel catfish, compared with Controls, and an additional production
of about 3200 kg waterchestnuts per ha.
The aim of our experiments was to determine the possibility of using
nutrients from the fish waste-water, mainly oxidized nitrogenous compounds,
for plant production, combined with a sepárate microbial denitrification step
for maximal water purification for an intensive fish production under controlled conditions in recirculating water.
Since no previous results from similar experiments were available the
feasibility of such an attempt had to be tested. As has been shown at the
Environmental Research Laboratory, University of Arizona, Tucson
f an
’ et
ig3S1 can
► and
(Anonymous, 1973), plants grown in controlled microclimates require less
moistur%; and the known acceleration of weight increase^of carp with rising
temperatures, studied in detail by Meske (1973), confirmed our intention to
build the pilot plant in a controlled-environment greenhouse.
The closed water system had a water capacity of about 2000 1. Aquaria
with a water volume of ca. 250 1 were stocked at the beginning of the experiment with 5 kg of Tilapia mossambica and with 5 kg o f common carp,
Cyprinus carpió, which were fed 5% of their own weight daily with commercial trout feed. Water flow through the aerated aquaria was kept at about
1 1/kg fish per min. The nitrification and denitrification tanks each had a
capacity of ca. 400 1. The conical settling tank had a capacity of 500 1 and
was made out of transparent polyvinylchloride to facilitate convenient observation of the sludge level. The sludge concentration in the nitrification tank
was kept at a mixed liquid suspended solids (mlss) valué of 1 g/1. At a daily
load of about 3 g BOD5 /kg fish and a sludge amount of 400 g mlss in the
aeration tank it was always possible to achieve a daily sludge load in the nitrification tank of less than 0.2 g BODs /g mlss. This sludge load secures a complete oxidation of all nitrogenous compounds (Downing et al., 1964). Some
2—3% of water had to be added daily to the system, mainly due to loss of
water by evaporation, overspill and the disposal of excess sludge. The eight
plant tanks each had a volume of 30 1 and could be irradiated with artificial
light, if necessary. The purified water was maintained at a temperature of
20—23° C and aerated. It was pumped back to the aquaria and to the plant
tanks by an electric pump. The activated sludge in the nitrification tank was
aerated to a dissolved oxygen content of over 4 mg 0 2 /l. The sludge passed
from the settling tank to the denitrification tank by means of gravity flow.
The sludge in the denitrification step was stirred slowly with an electric stirrer
to prevent settling and was pumped back to the nitrification tank after about
1 h. Fig. 1 shows a diagram of the system.
The usual Chemical analyses for the examination of waste-water (pH, NH4 ,
N 02 , NO¡ , organic nitrogen, BOD5, mlss, mixed liquid volatile suspended
solids (mlvss), 0 2) were performed according to standard methods. The
fishes and the feed were weighed weekly and the food conversión ratio was
calculated. Several types of commercially available higher plants were tested
in hydroponic culture, and one preliminary experiment with the unicellular
algae Scenedesmus spp. was started.
During the first 7 weeks only the nitrification step was used in the system
to test the efficiency of the oxidation step. The denitrification step could be
F ig.l. Closed water system for com bined fish and plant production.
omitted during this time since the nitrate ions formed are not harmful to
Tilapia and carp at this concentration, and could serve in the following experiment as the substrate for denitrification by plants and microorganisms.
The nitrate concentration in the system increased during the first 7 weeks
to 1200 mg N 0 371, proving the effectiveness of the nitrification step. Nitrite
and ammonia could be detected only in trace amounts. Due to the reactions
of nitrification a significant decrease in the pH of the water could be observed
and several times the pH had to be adjusted with slaked lime. To prevent a
further increase in the nitrate concentration, eight 30-1 tanks for hydroponic
plant culture, and a microbial denitrification tank were included in the
system. For complete microbial denitrification the following requirements
have to be fulfilled: anaerobio conditions, an organic hydrogen donator, and
the presence of nitrate ions, which permit the bacteria to maintain an aerobic
metabolism in the absence of oxygen. Incomplete reduction of nitrates
produces nitrite ions, which are highly toxic to fish. During 10 weeks with the
denitrification step included in the system the nitrate concentration decreased
steadily to about 200 mg N 03″/1. Nitrite and ammonia could be found only
in trace amounts. This part of the experiment proved that a combined denitrification of nitrate rich waste-water by plants and microorganisms was possible.
Despite the changing nitrate concentrations in the system there was a constant
gain in body weight of Tilapia and carps, as shown in Fig. 2. Both fish species
hatched on the same day and after 2 weeks of feeding with Artemia salina they
were taken into the recirculating water system. The Tilapia could increase their
body weight during the first 3 months significantly faster than carp, but the
weight increase declined after reaching maturity and after about 4 months the
carps had the same average body weight. In recirculating water at 23°C carp
can gain an average body weight of over 0.6 kg in 6 months (Meske, 1973)
an$/in nature Tilapia can grow easily in 6 months 200 g.
An attempt was made to determine the amount of nitrates eliminated from
the recirculating system by higher plants. As plants which need high concentrations of nitrates, tomatoes and ice-lettuce were chosen. Both plant species
grew well in the used hydroponic raft culture system, without addition of
special plant nutrients. After 8 weeks 24 kg of ripe tomatoes could be gathered.
Ice-lettuce grew from a small plant to a harvestable size in 4 weeks. Due to
the short detention time of the recirculating water in the plant system no significant decrease in the nitrate concentration of the plant system could have
been observed. But since the plants grew well they must have extracted
nitrogenous compounds from the water. To determine the efficiency of
denitrification, ice-lettuce and tomatoes were therefore also grown in a
hydroponic batch culture in 30 1 of nitrate-rich fish waste-water and the nitrate
concentration was tested weekly. As can be seen from Fig. 3, a constant
Fig.2. Weight increase o f Tilapia and carp.
Fig.3. Denitrification by plants.
decline in the nitrate concentration could be observed. These two plant species
grew as well in batch culture as in the above-described continuous-flow
A preliminary mass cultivation experiment was started with the unicellular
algae Scenedesmus spp., to determine the possibility of utilizing algal production for both fish feed and removal of nutrients from the water. To facilitate
continuous observation the algae were grown in a non-sterile batch culture.
Inside the system Tilapia were kept in net cages. A slight increase in body
weight could be observed.
It seems most advantageous to extend and to improve these experiments
with algae and to establish techniques for non-sterile algal culture in a recirculating water system. The known technical problems of harvesting the algae
could be overeóme by growing Tilapia or other herbivorous fish species inside
the algal culture system. In this way the concentration of nitrogenous compounds could be reduced by growing algae, which could serve at the same
time as an additional feed for fish.
These preliminary experiments provide evidence for the feasibility of combining intensive fish and plant production in recirculating water systems.
During the extended aeration process carbón compounds are oxidized by
microorganisms to carbón dioxide and all nitrogenous compounds to nitrate
ions. The elimination of nitrates from the waste-water can be realized by incorporating nitrates into plant cells or through denitrification by microorganisms to free nitrogen. A combination of these two possibilities seems
advantageous, since the nitrates, which are very valuable plant fertilizers, can
be used at least in part for plant production. The excess of nitrates can be
reduced by microorganisms to free nitrogen, which is released into the atmosphere, thus leading to a decrease in nitrogenous compounds. Controlling
water quality for fish production and determination of the optimum parameters for the microbial nitrification and denitrification are of great importance in the system described. For combined fish and plant production in
recirculating water a modified dual sludge system (Johnson and Schroepfer,
1964; Mulbarger, 1971) for water purification seems to be more suitable than
the system described above. Fig. 4 shows a diagram of a dual sludge system for
combined fish and plant production in recirculating water. The nitrification
and denitrification step in this system are sepárate and each individual step
can be controlled, regulated and optimized, leading to a high efficiency of
water purification.
4- 4 4 Water I
Fig.4. Dual sludge system for com bined fish and plant production in recirculating water.
Determination of the physical, biological and Chemical parameters for
optimum water purification, the most suitable ration of denitrification by
plants, algae, or by means of microorganisms, and the most favourable fish
and plant species for a combined production in recirculating water are important and of current in teres t in view of the increasing demand for fresh water,
and the pressing need to find new ways of producing protein for human
nutrition under prevailing conditions of constantly expanding world population.
<él <$:
This study was supported by the BMFT (Project MFE 303).
Anonym ous, 1973. Annual Report 1972—1973. Environmental Research Laboratory o f
Arizona — Arid Lands Research Center Abu Dhabi.
Downing, A.L., Painter, H.A. and Knowleá, G., 1964. Nitrification in the activated sludge
process. J. Proc. Inst. Sewage Purif., (Pt.2): 130—158.
Goldman, J.C., Tenore, K.R., Ryther, J.H. and Corwin, N., 1974. Inorganic nitrogen removal in a com bined tertiary treatment—marine aquaculture system — I. Removal efficiencies. Water Res., 8: 45—54.
Johnson, W.K. and Schroepfer, G.J., 1964. Nitrogen removal by nitrification and denitrification. J. Water Pollut. Control Fed., 36 (8 ): 1015—1036.
Kinne, O., 1976. Cultivation o f marine organisms: Water quality management and technology. Mar. Ecol., 3 (1): 19—30.
Liao, P.B. and M ayo, R.D., 1974. Intensified fish culture combining water reconditioning
with pollution abatement. Aquaculture, 3: 61—85.
Loyacano, H.A. and Grosvenor, R.B., 1973. Effects o f Chinese waterchestnut in floating
rafts on production o f Channel catfish in plástic pools. Proc. Annu. Conf. Southeast.
Assoc. Game Fish Com m ., 27: 471—473.
Meade, T.L., 1974. The technology o f closed system culture o f salmonids. Animal Science/
N OAA Sea Grant Univ. Rhode Island. Mar. Tech. Rep., 30.
Meske, Ch., 1973. Aquakultur von Warmwasser-Nutzfischen. Ulmer, Stuttgart, 163 pp.
Mulbarger, M.C., 1971. Nitrification and denitrification in activated sludge systems. J. Water
Pollut. Control Fed., 43: 2 0 5 9 -2 0 7 0 .
Nagel, L., Meske, C. and Mudrack, K., 1976. Untersuchungen zur Intensivhaltung von
Fischen im Warmwasserkreislauf. Arch. Fischereiwiss., 27: 9—23.
Oswald, W.J. and Golueke, C.G., 1968. Large-scale production o f algae. In: R.I. Máteles and
S.R. Tannenbaum (Editors), Single-Cell Protein. MIT Press, Cambridge, pp. 271—305.
Ryther, J.H., Goldman, J.C., G ifford, C.E., Huguenin, J.E., Wing, A.S., Clarner, J.P.,
Williams, L.D. and Lapointe, B.E., 1975. Physical models o f integrated waste recycling—
marine polyculture systems. Aquaculture, 5: 163—177.
Scherb, K. and Braun, F., 1971. Erfahrungen mit der Intensivhaltung von Regenbogenforellen bei biologischer Reinigung mit belebtem Schlamm im Kreislaufsystem. Wass.-
Abwass. Forsch., 4: 118—124.
Soeder, C.J., 1972. M óglichkeiten zur Verwendung von Mikroalgen bei der Reinigung von
Abwassern. Wasser Abwasser, 113: 583—590.
Spotte, S.H., 1970. Fish and Invertebrate Culture. Water Management in Closed Systems.
Wiley-Interscience, New York, 145 pp.
Von Sengbusch, R., Meske, C. and Szablewski, W., 1965. Beschleunigtes Wachstum von
Karpfen in Aquarien mit Hilfe biologischer Wasserklarung. Experientia, 21: 614.