The method of complex biological water treatment in aquaponic recirculation systems

. The article discusses the results of experimental research work on the development of technical solutions aimed at creating an integrated innovative biotechnology for recycled water treatment in recirculating aquaculture systems. Schemes and descriptions of experimental units using the multitrophic approach when creating water treatment systems are given. The system is completed according to the principle of integration into a single multitrophic system of spatially delimited biomodules with various types of hydrobionts, where particular types of organisms serve as a food base for subsequent trophic levels. As a result of trophic chains, phytoplankton and zooplankton are cultivated and used as starter live food when rearing juvenile fish. Natural food base contributes to the production of high-quality viable fish stock, which is essential for commercial fish farming. The use of tiered units can significantly reduce the need for production space. Polyculture cultivation of various species of fish, crustaceans, and mollusks together with hydroponic plants allows to diversify production for obtaining additional profit and reducing the risks. Cultivation of high-value fish species (e.g. sturgeon) or crustaceans (Australian red claw) together with less valuable cyprinids, tilapia or African catfish allows to promote products in different market segments.


Introduction. The condition and trends of aquaculture development
In recent decades high rates of aquaculture development are caused by a combination of the following two factors: the ever-increasing demand for seafood and the reduction of fish stocks in the world's oceans.According to the Food and Agriculture Organization of the United Nations (FAO), 75 % of the world's fish stocks have reached the limit of possible exploitation or are subject to overfishing [1].An alternative to extensive fishing is aquaculture, which today provides almost half of the fish consumed in the world, a significant part of which is freshwater aquaculture [2].Fish farming in recirculating aquaculture systems (RAS) is of particular interest.In conditions of fresh water deficiency, this method allows fish farming using a controlled microclimate in almost all climatic zones.
The sustainable development of basin aquaculture depends on a number of factors, among them are efficient use of the nutrients that make up the feed in order to save costs for their production; improvement of methods of recycled water treatment to create optimal conditions for growing fish; production of high quality viable fish seed; environmentally friendly, including organic, products; the use of RAS in aquaponic vegetable production; cultivation of fish in polyculture, together with crustaceans and mollusks; cultivation of fodder hydrobionts [3].
An example of the diversification of various aquaculture and plant products is aquabiocomplex developed by the Federal State-Funded Educational Institution of Science "Federal Research Center of Southern Scientific Center of the Russian Academy of Sciences" [4].The complex includes a basin with sturgeons, from where water enters the basin with catfish, and then the water is fed into a horizontal flow sedimentation tank for sedimentation of organic suspensions.The sludge from the sedimentation tank is removed for processing into a vermicomposter with vermiculture.The clarified water after sedimentation tank enters the hydroponic growing module, which includes slotted, lay-flat pipes.After the hydroponic module, water is supplied to crustacean farming containers, then to the shellfish growing container.The cycle ends with sequential water treatment in mechanical and biological filters, after which the purified circulating water again enters the basin with sturgeon fish.In aquabiocomplex, sturgeons and African catfish, Ampullaria gastropods and Australian red claws are grown all together.There also such plants as lettuce, parsley, pepper, dill, coriander, and basil are grown.Additional products are compost worms and biohumus (a natural organic fertilizer) as the result of bioconversion of sludge sediment by worms.
Numerous descriptions of other models of recirculating aquaculture systems and the methods of growing aquaculture objects are also known [5][6][7].The most significant among them are the following: the Swiss recirculating aquaculture system for the integration of crustaceans into the production of tilapia and fish food from tropical plants [8]; the Dutch experience in growing tilapia in RAS [9], an aquaponic device [10; 11], and a method for growing plants and farming fish and shellfish in aquaponic system [12].One of the most important directions in the development of modern fish farming in conditions of intensive production is the production of high-quality viable fish seed.Starter feeds are the limiting factor for growing larvae and juvenile fish.Long-term practice of artificial fish farming in aquaculture has shown that live feeds are the most effective in the early stages of fish rearing.As a result, the demand for research focused on integration in a single space, technological complexes for the joint rearing of juvenile fish and fodder hydrobionts is very high.Their production under controlled RAS conditions will provide commercial fish farms with highquality fish stock.Juvenile fish reared in RAS is also used for stocking water bodies in order to preserve the biodiversity of the ichthyofauna and restore fish stocks.
To obtain high-quality fish seed material, the authors have introduced a special unit.Its principle of operation is based on the combined cultivation of fodder hydrobionts (microalgae, rotifers, moinas, daphnia) and juvenile fish (larvae, fry) [13].The unit works as follows: water from a two-section basin, divided by a vertical longitudinal partition for growing juveniles of different fish species, is supplied to a mechanical filter using a circulation pump, then it undergoes biological treatment, aeration and thermoregulation and returns to the basin with fish.Part of the recycled water enters the bioreactor for the cultivation of microalgae.An aqueous suspension of microalgae serves as the initial food for hydrobionts.Consequently, from the bioreactor, with the help of an electric pump, the algal suspension is dosed into two basins for growing fodder hydrobionts.As needed, water with the biomass of food organisms is dosed into the basin with fish.The species grown in RAS are in close biological connection with each other, forming a trophic chain: microalgaezooplankton -juvenile fish.
The improvement of aquaculture technologies, product diversification, the transition to cultivation in polyculture are inextricably linked with the development of circulating water treatment systems, especially in terms of removing organic substances dissolved in it (metabolic products of cultivated aquatic organisms).Currently, the approach of complex biological treatment is used, in which the waste products of fish, primarily ammonium nitrogen, phosphates and carbon dioxide dissolved in water, serve as food for nitrifying bacteria and green unicellular algae, which form the first link in the RAS trophic chain.These phytoplankton organisms are food for ciliates and rotifers, which, along with phytoplankton, are the food base of zooplankton.The simplest crustaceans (moina, daphnia, streptocephalus, shieldfish, etc.) are an excellent food base for the development of larvae and juvenile fish.The bivalve zebra mussel is used as a water filter in some RASs.Integration of biomodules with higher aquatic plants into RAS allows additional recycled water treatment by phytoremediation.Thus, the creation of integrated multitrophic modular biological treatment systems will make it possible to comprehensively use the potential of RAS, diversify and improve the quality of products, and reduce operating costs.

Experimental research aqua complex
In 2020, the researchers from the Chair of Ecology and Life Safety of the Federal State-Funded Educational Institution of Higher Education "Donbass State Technical University" (Russian Federation) started the search in the field of the development of recommendations for fish farming in recirculating aquaculture system in the context of regional conditions.The research was carried out within state budget theme No. 212 GB "Development of recommendations for technological processes optimization in recirculating aquaculture systems to reduce the cost of production".To complete the set tasks a laboratory of hydroecology and hydrobiology was organized at the above-mentioned Chair.During this time the researchers have developed six variants of experimental recirculating aquaculture systems (RAS), two of which, functionally independent, are combined into one four-tiered aquaponic structure [14][15].
Since 2023, the research work has been carried out in cooperation with the Autonomous Non-Profit Organization "Donbass Research Institute of Ecotechnologies", created with the support and on the basis of the university.In order to conduct comprehensive studies of artificial aquatic ecosystems (which are recirculating aquaculture systems), functionally independent RAS laboratories were united into a single network.Such integration made it possible to create a scientific and technical cluster for complex biological water treatment in recirculating aquaculture systems.
Figure 1 shows a layout of a laboratory aqua complex for studying the processes of biological water treatment in recirculating aquaculture systems.The aqua complex includes fish-breeding basins (1); bioplateau for complex biological water treatment (2), three-tiered plant with zoo filters (3); system of drain (4) and water pressure (5) pipes.Aquaponic systems of the complex are equipped with the following hydroponic modules and devices: two five-tiered benches with slotted lay-flat pipes (6); containers for growing plants by the media bed method (7); nets stretched over the basins (8) and floating platforms (9) for hydroponic crops.Plants grown in hydroponic modules and basins, being, in addition, commercial products, also perform the function of phytoremediators.Recirculating aquaculture systems are equipped with vortex electric pumps for water circulation (10), air compressors for water oxygen enrichment and bactericidal irradiators for water disinfection (not shown in the figure).Stop valves are installed on the pipelines connecting the recirculating aquaculture systems into a single network.Daily water quality control is carried out according to the following parameters: temperature, pH, oxygen, and ammonium nitrogen content.
Lighting is typically natural, and additional lighting for plants is provided by LED phytolamps.Lighting mode is 16h per day: 8h per night.The water supply is centralized (from the public water supply system).Waste water is discharged into the sewerage facilities, then -into the public sewer.Heating in winter is centralized from the municipal heating network, and UFO heaters are also used.Air conditioners are used to cool the air in summer.Air circulation indoors is forced, shop ventilation is applied.

RAS basins
The aqua complex includes four round plastic basins, three of which are of 3000 mm in diameter and one is of 2000 mm in diameter.The height of the basins is 750 mm.Every basin has similar design.They are installed on a 250 mm high foundation, on top of which 9 mm thick USB plates are put.The walls of the basins are wire mesh fenced (350 mm high) to prevent fish from jumping out.In the center of the bottom of each basin there is a drain hole with a diameter of 110 mm.The hole is barred to prevent fish from swimming into.An elbow is soldered into the drain hole outside the basin.From there a sewer pipe with a diameter of 110 mm passes at 90 0 along the bottom of the basin.Directly at the outlet of the basin, a side outlet elbow is mounted into the pipe.The side outlet elbow is connected to a sewer pipe to discharge water from the basin into the sewerage system.Stop valves are installed directly behind the side outlet elbow.A plastic pipe of 110 mm in diameter is led out from the upper socket of the side outlet elbow parallel to the basin wall to a height of 700 mm.A plastic pipe serves to drain water from the basin.In the upper part of the outlet pipe there is side outlet elbow, its side socket maintains a water level at 700 mm in the basin due to overflow.

Hydroponic modules
In addition to fish farming basins, recirculating aquaculture and aquaponic systems are equipped with containers for hydroponic modules, bioplateaus or other purposes.The containers used in RAS aqua complex are made of intermediate bulk containers (IBC), which are containers of medium capacity with dimensions of 1200 × 1000 × 1000 mm (V = 1.0 м 3 ).The containers are inside aluminum frames and are mounted on plastic, metal or wooden trays.In the laboratory, the basins with dimensions of 1200 × 1000 × 500 mm (V = 0.50 м 3 ); 1200 × 1000 × 250 mm (V = 0.25 м 3 ), and 1200 × 1000 × 750 mm (V = 0.75 м 3 ) were made from IBC using various cutting methods.
In aquaponic systems of the aqua complex, hydroponic methods of deep water culture (DWC); nutrient film or NFT (nutrient film technique) and the media bed are applied for growing plants.

Deep water culture, DWC
Plants were grown directly in fish basins on floating foam sheet platforms of 50 mm thick and measuring 1000 × 500 mm with planting holes of 50 mm in diameter, staggered at a distance of 100 mm from each other and 50 mm between rows.When using the DWC method on floating platforms, the water surface of the basin is used.To grow plants by the DWC method, a wire mesh stretched over the water surface was also used.Slotted cups with plants or plants wrapped in foam rubber were placed into a wire mesh.Figure 2 shows photographs of plants grown by the deep water culture method.

Fig. 2. Photo of hydroponic modules with plants grown by deep water culture (DWC)
This method involves the air surface above the basin mirror.The DWC method is used for growing plants with a developed root system (lettuce, basil, etc.).When growing plants on a wire mesh, their roots hang freely into the water.Hydroponic plant cultivation by DWC method does not require such fillers as expanded clay, pebbles, etc., for microflora to develop.

Nutrient film technique, NFT
It is used to grow shallow rooted plants (pepper, onion, garlic, radish, etc.).Under this method of plant cultivation, they are planted into slotted lay-flat pipes, where their roots are irrigated with a thin surface film of water with nutrients.The use of pipes for growing plants allows them to be placed in several tiers and used for mounting tiered hydroponic modules.Such modules are mostly demanded in urban areas with their limited space.The tiered arrangement of hydroponic modules allows using vertical space efficiently, and due to the design features of the device, minimal moisture evaporation is guaranteed in hydroponic modules, which is vital for areas with fresh water shortage.The aquaponic systems of the aquacomplex include two five-tiered hydroponic modules for growing plants using the NFT method.The modules are equipped with the pipes of 110 mm in diameter and of 2000 mm length.To maximize natural light for plants, hydroponic modules are fitted in window apertures.The pipes of the upper four tiers are slotted with holes of 50 mm in diameter at intervals of 150 mm.A bactericidal irradiator for water disinfection is fitted in the slot of the lower pipe.After the fish-breeding basin water is supplied through the pipeline to the pipes of the upper tier, from where it flows by gravity through every tier and returns to the fish-breeding basin.
Figure 3 shows photographs of a five-tiered module with chlorophytum, used as a phytoremediator of RAS water.The left photo shows plants immediately after planting, and the right one shows plants after nine months of cultivation.When growing plants using the nutrient film technique, and in the deep water culture method as well, soil or fillers that serve as a substrate for bacterial flora growth are not used.As a result, aquaponic systems that use deep water culture or nutrient film technique methods to grow plants are equipped with additional devices for mechanical and biological water purification.

The media bed method
It is used for growing large vegetables and fruit crops (cabbage, tomatoes, cucumbers, etc.) that require a substrate in which the root space and bed are formed to hold the crops of plants.The substrate, which is used in the hydroponic module, must have a porous structure for water and air, and surface area for plant spacing.The most optimal for these purposes is a layer of volcanic gravel.The water layer in hydroponic system covers only the surface of substrate layer.Water is supplied by periodic flooding using mechanical siphons to discharge water.Hydroponic modules operating by the media bed method do not require additional devices for mechanical and biological water purification.
In the RAS aqua complex, plastic IBC are used as hydroponic modules for growing plants by the media bed method.Module size is 1200 × 1000 × 250 mm.Made from the top quarter of IBC, the hydroponic module in the center of the bottom has a lid with a built-in siphon for periodic water discharge.The siphon scheme is shown in Figure 4.The siphon includes an external slotted plastic pipe ( 1), open at the top for air intake, and preventing the substrate leakage.The perforation helps water to enter the siphon.Inside the siphon there is a drain drop pipe (2) -a pipe, the upper end of which is located at the height of the substrate covering the bottom of the hydroponic module.The lower end of the siphon is brought out through the bottom of the module and has a knee-shaped bend at the exit point.The drain drop pipe is enclosed in a larger diameter pipe (3) tightly closed from above.Its base includes the slots for air intake.Between the top of the drop pipe and the pipe covering it there is a space to create an air lock.The mechanism of the siphon operation is based on the Bernoulli principle: it is initiated by a strong initial water flow and is disrupted by the creation of an air lock at the inlet of the drain drop pipe.
The use of hydroponics in aquaculture to obtain additional crop production at the expense of fish waste allows the authors to consider hydroponics as a method of phytoremediation of recycled water in RAS, since plants intensively absorb ammonium and nitrate nitrogen, phosphates, carbon dioxide which are the main metabolites excreted by fish and having a negative effect on them.The media bed method is of particular interest, when soil is used to grow plants, which assumes the functions of a biofilter.If we shift the emphasis of the use of hydroponic modules towards phytoremediation, then with the appropriate approach, these modules can be used as bioplateau.
Hydroponic modules for obtaining plant products were redesigned as bioplateau for bioremediation of recycled water from RAS. IBC with a volume of 0.25 and 0.50 м 3 were used.Gravel of 8-12 mm fraction and 50 mm layer with river pebbles of 3-5 mm fraction and 50 mm layer at the top of it was placed at the bottom of the containers.Instead of agricultural crops, aquatic plants were planted into bioplateau (floating (Riccia, duckweed) and underwater (Elodea, Vallisneria) plants).

Development of scientific and technical cluster for multitrophic water treatment in recirculating aquaculture systems
The Autonomous Non-Profit Organization "Donbass Research Institute of Ecotechnologies" in cooperation with the Chair of Ecology and Life Safety of the Federal State-Funded Educational Institution of Higher Education "Donbass State Technical University", has developed a project of scientific and technical cluster of integrated multitrophic water https://doi.org/10.1051/bioconf/2024840504384 treatment system, which provides the synergistic cultivation of aquatic organisms of various trophic levels in a single recirculating aquaculture system.
The system is completed according to the principle of integration into a single multitrophic system of spatially delimited biomodules with various types of hydrobionts, where particular types of organisms serve as a food base for subsequent trophic levels.
At the top of the artificial food pyramid are commercially significant aquaculture objects -various types of fish, mollusks and crustaceans.Plants that are not trophic and topical competitors are grown in polyculture.
Water enriched with aquaculture waste products serves as a medium for microalgae growth, and a source for bacteria nutrition.Control over the microflora growth is carried out in photobioreactors and biofilters used in the RAS system for biological water purification from metabolites.
Suspension of microalgae and nitrifying bacteria of biofilters serve as food for unicellular and protozoan multicellular organisms.Together they are a food base for bivalve filterfeeding mollusks and cultivated crustaceans used as starter and grower feeds in the cultivation of agriculturally important fish species.
The resulting detrital sludge is partially decomposed by decomposers (bacteria and fungi), saturating water with mineral and simple organic compounds; partially absorbed by detritivorous of zoobenthos.Sedimentary sludge is used for growing insect larvae, such as bloodworm, black soldier fly, as well as for cultivating compost worms and obtaining biocompost.
Additional biological water purification from metabolites is carried out by aquatic higher plants floating and planted in the ground, which serve as a refuge and food for invertebrate hydrobionts.Aquaponics, as a kind of integrated multitrophic aquaculture, allows, in addition to fish farming, to obtain plant products and participate in water purification processes.
Figure 5 shows a scheme of a tiered integrated multitrophic unit.Four-tiered unit includes containers made of the parts of IBC; hydroponic module of four lay-flat slotted pipes; four modules of the bubbling-type photobioreactor.The first (lower) tier (1) includes 10 containers with a volume of 0.75 м 3 for rearing larvae and juvenile fish.The next three tiers are equipped with 0.25 м 3 containers.Each tier consists of 10 containers.The distance between the first tier and the second is 0.50 m.The distance between following tiers is 0.25 m.The second tier (2) is used for growing crustaceans.The third (3) and fourth (4) tiers are intended to breed feeding hydrobionts and keep aquatic plants as phytoremediants.Air compressor hoses are connected to the containers with hydrobionts for water aeration.
Above the fourth tier, at a distance of 0.25 m, alternately connected pipes of the hydroponic module (5) for growing plants are laid.LED phytolamps are fitted above the hydroponic module at a distance of 0.3 m.
The unit includes a vertical settling tank consisting of two containers with a volume of 0.5 м 3 , which are equipped with pumps for water circulation.At the bottom of the settling tanks, a drain is equipped to remove sediment.
In the window apertures there are bubbling columns of the photobioreactor (7) for microalgae cultivation.Hoses of air compressor and carbon dioxide cylinders are connected to the bubbling columns.The oxygen produced in the photobioreactor is fed through the disposal hoses to the fish containers.LED phytolamps are fitted for additional lighting.
Functionally, the unit is divided into two recirculation systems.The first one includes two lower tiers, which contain fish and crustaceans, a settling tank and a hydroponic module.The second one consists of containerss with feeding hydrobionts and a photobioreactor.Both systems are interconnected by pipelines with stop valves.
The multitrophic approach will make it possible to diversify aquaculture in order to expand the range of products, improve their quality, and switch to organic production.An integrated multitrophic approach to water treatment system optimizes the use of water resources, reduces the amount of waste, decreases expenses on water purification, and food chains provide live fodder.

Fig. 1 .
Fig. 1.Scheme of a laboratory aqua complex for studying the processes of biological water treatment in recirculating aquaculture systems /doi.org/10.1051/bioconf/2024840504384

Fig. 3 .
Fig. 3. Photo of Chlorophytum grown by nutrient film method (NFT) immediately after planting (left) and nine months later

,Fig. 4 .
Fig. 4. The scheme of discharge water siphon in hydroponic module by the media bed method