WO2013132481A1

WO2013132481A1 - Aquaculture system

Info

Publication number: WO2013132481A1
Application number: PCT/IL2013/050190
Authority: WO
Inventors: Michael RACHIN
Original assignee: Aqua Green Ltd
Priority date: 2012-03-07
Filing date: 2013-03-04
Publication date: 2013-09-12
Also published as: IL218530A0; IL218530A

Abstract

A zero-discharge aquaculture system and process for treating water therewith, the system comprising an aquaculture tank (10) and a water treatment section comprising a foam fractionation unit (20) downstream the aquaculture tank (10); an anaerobic pond (30) downstream the aquaculture tank (10); an algae pool (40) downstream the foam fractionation unit (20) and the anaerobic pond (30); a common pool (50) downstream the algae pool (40); a bio-filtration reactor (60) downstream the common pool (50); and an oxygen injection unit (7 ) downstream the bio-filtration reactor (60) and upstream the aquaculture tank (10). The anaerobic pond (30) comprises an oxygen concentration reduction portion (36) upstream a pollution anaerobic treatment portion (38). The oxygen concentration reduction portion (36) comprises at least one oxygen concentration reduction member (31) positioned across a width of the oxygen concentration reduction portion (36). The oxygen concentration reduction member (31) comprises a network of oxygen removal agent support members (314) for providing a surface for an oxygen removing agent and defining spaces (316), wherethrough water can flow and come in contact with the oxygen removing agent.

Description

AQUACULTURE SYSTEM

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[001] This application claims priority from Israeli patent application 218530, entitled "Aquaculture System", filed on 07 March, 2012, the entire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

[002] The present invention relates to an aquaculture system, in particular, to a zero-discharge aquaculture system enabling the recycling of aquaculture water.

BACKGROUND OF THE INVENTION

[003] According to the Fisheries and Aquaculture Organization (FAO) of the United Nations, "aquaculture is understood to mean the farming of aquatic organisms including fish, molluscs, crustaceans and aquatic plants". World aquaculture has grown tremendously during the last 50 years from a production of less than a million tonnes in the early 1950s to 59.4 million tonnes by 2004, and this growth is expected to continue (Global Aquaculture Production Fishery Statistical Collections, FAO, Rome - http://www.fao.org/fishery/en). Thus, the aquaculture industry has been challenged to develop economically viable systems that produce aquatic species at high density. [004] A major concern in aquaculture systems is the accumulation of pollutants that can adversely impact the quality of the water used in the system. Theoretically, two options exist for maintaining an adequate water quality in aquaculture systems. Either the water in the system is continuously exchanged with clean water; or aquatic species culturing water is continuously treated in order to reduce the level of pollutants. However, the consumption of unlimited amounts of clean fresh water by aquaculture systems is a nonrealistic option, while the consumption of unlimited supply of treated seawater is limited to seashore areas.

[005] In light of these concerns, there is a propensity to prefer aquatic species culturing implementing closed water recycling systems. The closed systems are to be referred to hereinafter as "closed aquaculture systems" unless mentioned otherwise. Such aquaculture systems are closed-loop systems, including a culture tank and treatment units, that retain, treat and reuse the water of the system. The main objective in such systems is to preserve the water and maintain a high level of water quality. In closed aquaculture systems, the water flows in a full cycle fashion, serving as an environment for the treatment process before returning to the culturing tank. Thus, closed aquaculture systems effectively manage, collect and treat wastes that accumulate during the growth of aquatic species in the system and, under optimal conditions, do not require water replacement except to account for losses due to evaporation or incidental leakage and the like. Another reason for using closed aquaculture systems is that discharge of nitrate-rich effluent water is prohibited in many countries due to environmental and public health considerations. In addition, under certain conditions nitrate in the aquaculture system is converted to the toxic compound nitrite.

[006] Aquaculture systems in should provide a suitable environment to promote the growth of the aquatic species. Critical environmental parameters include the concentrations of dissolved oxygen, ammonia, nitrites and carbon dioxide in the water of the culture system. Nitrate concentration, pH, and alkalinity levels within the system are also important. During the water treatment process, closed aquaculture systems normally remove waste solids, oxidize ammonia and nitrites, remove carbon dioxide and aerate and/or oxygenate the water before returning it to the aquatic species culturing tank. More intensive systems or systems of culturing sensitive species may require one or more additional treatment processes such as fine solids removal, dissolved organics removal, some form of disinfection, and pH and alkalinity adjustment.

[007] The principal components of a closed aquaculture system include: a tank for aquatic species culture; a unit for water clarification; a unit for organic material removal; an aeration and carbon dioxide stripping unit; a unit for purified oxygen injection; a unit for managing pH and alkalinity of the water; and a unit for disinfection.

[008] Tanks for the intensive culture of aquatic species should be designed with considerations for production cost, space utilization, water quality maintenance, and aquatic species management. Geometry, water velocity, and flow patterns are particularly important factors to be considered in this respect. Aquatic species culturing tanks can be manufactured from many materials, for instance: fiberglass, concrete, enamel coated steel, and supported plastic liners; and be in various shapes, either, but not limited to, round, octagonal, rectangular, or D-ended. Regarding fish culturing tanks as an example, circular flow of the water in the tank is advantageous in the sense it provides a healthy and uniform culture environment. The circular velocity should be swift enough to carry solids and make the tank self cleaning, yet not faster than required to avoid over- exercising the fish. Water velocities of approximately 0.5-2.0 times fish body length per second are considered optimal to maintain fish health, muscle tone, and respiration. To generate centrifugal forces capable of driving settlable solids to the center drain of the tank, velocities should be greater than approximately 15- 30 centimeters per second. Another advantage of aquatic species culturing tanks with a circular flow is that the circular flow provides complete mixing, which maintains uniform water quality throughout the tank. Complete mixing means that the concentration of a constituent in the water flowing into the tank changes instantaneously to the concentration that exists throughout the tank. Complete mixing also means that the concentration of constituent in the tank will be the same as in the water leaving the tank through the drain(s). Thus, if good mixing can be achieved, all aquatic creatures within the tank are exposed to the same water quality.

[009] Units for water clarification are used for removing settlable and/or suspended organic and inorganic solids from the water. There are several types of water clarification units, a non limiting list of which includes, microscreen filters (e.g. drum, Triangel™ and disk); settling basins; tube/plate settlers; roughing filters (packed with random rock or plastic, and with structured plastic); swirl separators; pressurized filters (sand and plastic bead); gravity filters (high rate sand and slow sand); and foam fractionation units (also known as protein skimmers).

[010] Removal of dissolved organics and ammonia is achieved by using reactors for bio-filtration, a non limiting list of which includes, fluidized-media reactors (e.g. sand and plastic bead); rotating biological contractors; trickling biofilters; submerged large media reactors; and pressurized bead filters. Microbes attached to the surface of a bed material oxidize ammonia to nitrate and oxidize or metabolize and incorporate organic compounds. Attachment of the microbe population to the bed material keeps the microbes from being flushed out of the filter and provides the biosolids the retention time required for biological oxidation of ammonia to nitrates.

[01 1] Aeration and carbon dioxide stripping units reduce the concentration of dissolved carbon dioxide in the water. Accumulation of high levels of carbon dioxide can become a limiting toxicity factor with high aquatic species densities and inadequate water exchange. Aeration and carbon dioxide stripping are mass processes that occur together when water is contacted with air to bring the concentration of dissolved gases (such as nitrogen, carbon dioxide and oxygen) in the water into equilibrium with the partial pressures of these gases in the surrounding atmosphere. Aeration and carbon dioxide stripping units shift concentrations of dissolved carbon dioxide, nitrogen and oxygen towards equilibrium values, and include, but not limited to, mechanical-surface mixers; diffusers; packed-columns, tray-columns, or spray-columns (open to atmosphere, or enclosed with forced ventilation); shallow air-lifts; corrugated inclined planes; and stair-type drops. [012] Oxygenation is a process, in addition to aeration, that is used to maintain adequate levels of oxygen in the culturing water. In units for oxygen injection, pure oxygen gas is typically used instead of air to achieve oxygen levels in the water flow that are above standard saturation levels. Oxygen can be produced on site using pressure swing adsorption equipment or, alternatively, purchased from commercial sources as a bulk liquid or gas. The transfer of oxygen into the water must be efficient. When pure oxygen is transferred into water, the saturation concentration of oxygen in the water is increased nearly 5- fold over the saturation concentration obtained when air is used. The saturation concentration can also be increased by increasing the total pressure in which the transfer occurs (e.g., via a pump or hydrostatic head). Increasing the pressure during oxygen transfer from approximately 1 to 2 atmospheres nearly doubles the saturation concentration oxygen in the water. Additionally, mechanisms for stripping/venting nitrogen and argon gas released during oxygen absorption are optional both to reduce the total gas pressure of the water and to increase the efficiency of oxygen transfer. A non limiting list of oxygen injection units includes, U-tubes; packed columns (atmospheric pressure and pressurized); spray columns; pressurized columns; oxygenation cones; oxygen aspirators; bubble diffusers; multi-staged low head oxygenation units; and enclosed mechanical- surface mixers.

[013] A unit for managing pH and alkalinity of the water is of high importance in closed aquaculture systems. pH is a measure of hydrogen ion concentration and controls acid/base chemistry. Alkalinity, a measure of the acid neutralizing capacity of a solution, depends on the concentrations of bicarbonate, carbonate, hydroxide, and hydrogen ions. An ideal range of alkalinity for fish culturing, for instance, is approximately 20-300 milligram calcium carbonate per liter. The equilibrium of many of the chemical substances important in aquaculture is controlled by pH. Of great importance is the influence of pH on the equilibrium of ammonia and carbonic acid systems. Chemical treatment can be used to maintain a pH that will minimize the potentially toxic effects of ammonia and carbon dioxide in closed aquaculture systems. Therefore, the treatment process includes a unit for adding a supplemental source of alkalinity such as, but not limited to, lime (calcium oxide - CaO); caustic soda (sodium hydroxide - NaOH); soda ash (sodium carbonate - Na₂C03); or sodium bicarbonate (NaHCC ) to the water. Lime, caustic soda, and soda ash react with carbon dioxide to produce bicarbonate alkalinity. Adding sodium bicarbonate is a simple means for increasing alkalinity and thus to increase pH.

[014] Disinfection units are used for the elimination of pathogenic microorganisms from the aquaculture system water, and include, but not limited to, ozonation units, UV treatment units, or a combination of both. Ozone is a strong disinfectant and a powerful oxidizing agent that can be put to numerous beneficial uses within aquaculture systems. Ozone has a rapid reaction rate and few harmful reaction by-products. Another advantage of ozone is that the reaction end product of ozone is oxygen, which contributes to the concentration of dissolved oxygen in the water. Ozone can be used within closed aquaculture systems to reduce fish disease as well as to oxidize nitrites, dissolved nonbiodegradable organic material and organic particulate matter. Ammonia, however, is not readily oxidized by ozone except at pH values greater than 9. Oxidation of organic material can produce microflocculation and improve solids removal via sedimentation, foam fractionation, granular filtration, or microscreen filtration. Proper application of ozone requires consideration of four processes: ozone gas generation, gas to liquid absorption, contact time for reaction, and ozone residual removal. Ozone is relatively unstable in water. In a solution of pure water, the half-life of ozone is approximately 165 minutes at about 20°C. In systems containing organic carbon, the half-life of ozone may be less than a few minutes. In closed aquaculture systems where organic carbon level can be very high, ozone half-life can be less than 15 seconds. When ozone reacts with organic carbon, the reaction often takes place at many of the bonds between molecules that cannot be readily oxidized through biological metabolism and makes the partially oxidized organic compounds biologically degradable at a faster rate. The increased biodegradation rate is partially due to formation of smaller molecules and partially due to fewer higher order covalent bonds. Additionally, ozone oxidation can cause dissolved organic molecules to precipitate and colloidal organic solids to microflocculate. These reactions enhance the removal of organic matter from process flow streams. For disinfection, the required residual ozone concentration is usually between 0.1-1.0 milligram per liter and the hydraulic retention times are anywhere from 0.5 to 20 minutes. No ozone should remain in the water after treatment. However, ozone may be present in the water after the treatment, depending upon the applied ozone dose, the ozone demand of the water, and the contact time. There are several methods to eliminate dissolved ozone, for example: using greatly extended contact times; passing the flow through a bio-filter or bed of activated carbon; stripping the ozone into air with either a bubble chamber or a packed bed aeration column; or destroying the dissolved ozone with high intensity of ultraviolet light. Also, no ozone gas should escape to the atmosphere. All residual gases should be collected and vented to an ozone destruction process that will destroy the ozone before releasing the gas to the atmosphere. Ozone gas destruction can be catalyzed by heat, media (such as granular activated carbon or a manganese oxide or other coated media), or a combination of both. A downside of treating water with ozone is the formation of the bromate anion (Br03^~), which is a product of the reaction of ozone with Bromide (Br ). Bromate is toxic to aquatic creatures and suspected to be a human carcinogen. Therefore the formation of bromate in aquaculture systems should be avoided or at least minimized. Proposals to reduce bromated formation include lowering the water pH to the range of 5.9-6.3, and limiting the doses of ozone.

[015] A foam fractionation unit, also known as a protein skimmer, is a device used to remove organic compounds and particulate matter from water. Foam fractionation units inject a large number of air bubbles into a water column, thus generating a large air/water interface, i.e. a large bubble surface. Organic molecules that are present in the water, especially protein, collect on the air/water interface. In addition to protein, there are a number of other organic and inorganic molecules that collect on the surface of the air bubbles, for example: a variety of fats; fatty acids; carbohydrates; metals such as copper; and trace elements such as iodine; as well as particulates, and other detritus; along with phytoplankton and bacteria. As the bubbles accumulate at the top of the water column they create a foam that is separated from the water, thus carrying with it the materials and particulates collected on the air bubbles. [016] Another method that plays an increasingly growing role in water recycling, especially in aquaculture systems, is passing the water through an algae pool. Some of the benefits of using algae for the recycling of aquaculture water include: absorbance of ammonia, nitrate, carbon dioxide, sulfites and phosphates; stabilization of pH and alkalinity of the water; and an increased oxygen level in the water during day time. Another benefit of integrating an algae pool in closed aquaculture systems is the flexibility of the algae pool compared to the rigidity of the other components of the system. When needed, algae can be added or removed in order to meet changes in the amount of pollutants in the water. In addition, some species of algae suitable for the treatment of water have some economical values, like being used as feed additive to fish and for human consumption, as well as for various industrial uses. The algae grown in the algae pool can be either microalgae, or macroalgae also known as seaweed. Even though both types of algae are used in the treatment process of water in closed aquaculture systems, there is an advantage in using macroalgae over microalgae. As a result of the small size of the microalgae they can spread easily and contaminate the entire system. Therefore usage of microalgae requires investment in screening and other means in order to avoid escape of the microalgae from the algae pool and their establishment in other parts of the system. On the other hand, using macroalgae avoids this problem because their size does not allow them to escape from the algae pool to other parts of the system.

[017] There are aquaculture systems, known as aquaponic systems, that use plants, like herbs or vegetables, in the treatment process of the aquaculture water. The water is passed through a pond in which the plants are grown. The pollutants in the water have a nutritional value for the plants that absorb them. In addition, the plants, i.e. the vegetables themselves have an economical value for their growers. [018] Anaerobic digestion is the biological degradation of organic matter by microorganisms under conditions of very low concentration or lack of oxygen. During anaerobic digestion, organic matter is oxidized to carbon dioxide and methane. In addition, anaerobic digestion plays an important role in the elimination of ammonia from water. Ammonia is secreted by fish and other aquatic creatures. However, ammonia is highly toxic and therefore should be removed from the water. Natural elimination of ammonia can be achieved by nitrification, which is the microbial aerobic oxidation of ammonia in the presence of oxygen. However, nitrate is also toxic to fish and other aquatic species and should be also removed from the water. The elimination of nitrates is achieved under anaerobic conditions by microbial denitrification, a process in which nitrates are reduced to gaseous nitrogen. This process is strictly anaerobic, that is it can occur only in the absence of oxygen. Therefore, integration of an anaerobic treatment unit in closed aquaculture systems is beneficial since it plays an important role in the biological degradation of organic matter and the complete removal process of ammonia.

SUMMARY OF THE INVENTION

[019] The present invention relates to a zero-discharge aquaculture system enabling the recycling of aquaculture water. The system is suitable for culturing aquatic species, for example: fish, mollusks, crustaceans, and plants. The water used in the aquaculture system of the present invention can be fresh water or seawater, and at any temperature suitable for the species in culture.

[020] In accordance with embodiments of one aspect of the present invention there is provided a water recycling zero-discharge aquaculture system, including an aquaculture tank and a water treatment section for treating water. The water treatment section comprises: a foam fractionation unit disposed downstream of the aquaculture tank; an anaerobic pond downstream of the aquaculture tank and comprising an oxygen concentration reduction portion adapted to reduce the concentration of dissolved oxygen in the water, and a pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion for anaerobically treating the water; an algae pool downstream of the foam fractionation unit and the anaerobic pond; a common pool downstream of the algae pool; a bio-filtration reactor downstream of the common pool; and an oxygen injection unit downstream of the bio-filtration reactor and upstream of the aquaculture tank.

[021] In accordance with embodiments of another aspect of the present invention there is provided a process for treating water in a water recycling zero- discharge aquaculture system, comprising: transferring water from an aquaculture tank to a foam fractionation unit; treating the water in the foam fractionation unit with ozone; removing foam from the water surface in the foam fractionation unit, fluidizing the foam and collecting the fluidized foam in a fluidized foam tank; treating the fluidized foam in the fluidized foam tank with ozone; letting the ozone- treated fluidized foam stand until the ozone concentration is below a predetermined threshold; transferring the ozone-treated fluidized foam from the fluidized foam tank to an algae pool; transferring ozone-treated water from the foam fractionation unit to the algae pool; transferring water from the aquaculture tank to an oxygen concentration reduction portion of an anaerobic pond; lowering the concentration of dissolved oxygen in the water to a level below a predetermined threshold level by flowing the water through the oxygen concentration reduction portion; transferring the water having a concentration of dissolved oxygen below the predetermined threshold level, from the oxygen concentration reduction portion to a pollution anaerobic treatment portion of the anaerobic pond; treating the water anaerobically in the pollution anaerobic treatment portion; transferring water from the pollution anaerobic treatment portion to algae pool; treating the water in the algae pool; transferring water from the algae pool to a common pool; measuring the pH of the water in the common pool; adding an agent for raising the pH and alkalinity of the water to the water in the common pool; transferring the water from the common pool to a bio-filtration reactor; treating the water in the bio-filtration reactor; transferring the water from the bio-filtration reactor to an oxygen injection unit; increasing the dissolved oxygen concentration in the water in oxygen injection unit; and transferring the water from the oxygen injection unit to the aquaculture tank.

[022] In accordance with embodiments of yet another aspect of the present invention there is provided an anaerobic pond for anaerobically treating water, comprising: an oxygen concentration reduction portion adapted to reduce the concentration of dissolved oxygen in the water; a pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion for anaerobically treating the water, wherein the oxygen concentration reduction portion comprises at least one oxygen concentration reduction member positioned across a width of the oxygen concentration reduction portion, and the oxygen concentration reduction member comprises a network of oxygen removal agent support members for providing a surface for an oxygen removing agent, and the network of oxygen removal agent support members defines spaces, wherethrough water can flow and come in contact with the oxygen removing agent.

[023] In accordance with embodiments of yet another aspect of the present invention there is provided a method of treating water having pollutants and dissolved oxygen under anaerobic conditions, comprising: transferring the water into an oxygen concentration reduction portion of an anaerobic pond, which comprises a pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion, the oxygen concentration reduction portion comprising at least one oxygen concentration reduction member positioned across a width of the oxygen concentration reduction portion, and the oxygen concentration reduction member comprises a network of oxygen removal agent support members covered with an oxygen removing agent; lowering the concentration of dissolved oxygen in the water to a level lower than a threshold level by letting the water come in contact with the oxygen removing agent; transferring the water having dissolved oxygen at the level lower than the threshold level from the oxygen concentration reduction portion into the pollution anaerobic treatment portion; and anaerobically treating the water in the pollution anaerobic treatment portion. [024] In accordance with embodiments of yet another aspect of the present invention there is provided a method of maintaining a concentration of dissolved oxygen in water below a threshold level throughout all layers of the water, the method comprising: (a) flowing the water into an anaerobic pond, including an oxygen concentration reduction portion, having at least one oxygen concentration reduction member; and a pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion; (b) measuring oxygen concentration at or near the water surface in the pollution anaerobic treatment portion; (c) determining whether the measured oxygen concentration is below or above the threshold level; (d) lowering the flow rate of water in the oxygen concentration reduction portion if the measured oxygen concentration is above the threshold level; and (e) repeating from step (b).

BRIEF DESCRIPTION OF THE DRAWINGS [025] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which:

[026] Fig. 1 is a schematic presentation of a water recycling system in accordance with the invention; [027] Fig. 2 is a schematic top side view of an anaerobic pond according to an embodiment of the present invention;

[028] Fig. 3A is a front view of an oxygen concentration reduction member in accordance with embodiments of the present system; [029] Fig. 3B is a front view of an oxygen concentration reduction member, including a plurality of support substrate blocks in accordance with embodiments of the present system;

[030] Fig. 4 is a perspective view of a support substrate block in accordance with embodiments of the present system;

[031] Fig. 5 is a schematic top side view of an oxygen concentration reduction portion in accordance with embodiments of the present system, and

[032] Fig. 6 is a flow-chart illustrating a method of regulating a process of reducing concentrations of dissolved oxygen in water in an anaerobic pond in accordance with embodiments of the present invention.

[033] The following detailed description of embodiments of the invention refers to the accompanying drawings referred to above. Dimensions of components and features shown in the figures are chosen for convenience or clarity of presentation and are not necessarily shown to scale. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[034] Illustrative embodiments of the invention are described below. In the interest of clarity, not all features/components of an actual implementation are necessarily described. [035] In accordance with embodiments of one aspect of the present invention there is provided a water recycling zero-discharge aquaculture system in which water can be recycled.

[036] When describing the relative positions of the system components, the term "upstream" is used to describe a direction opposite to the direction of the water flow; and the term "downstream" is used to describe a direction similar to the direction of the water flow.

[037] Fig. 1 shows an aquaculture system, which includes an aquaculture tank 10 and a water treatment section 100. Water treatment section 100 incorporates a foam fractionation unit 20 downstream of aquaculture tank 10; and an anaerobic pond 30 downstream of aquaculture tank 10, adapted to rapidly lower the concentration of dissolved oxygen in the water before anaerobically treating the water. An algae pool 40 is downstream of both foam fractionation unit 20 and anaerobic pond 30 and is connected to a common pool 50, downstream of the algae pool. A bio-filtration reactor 60 is connected to and downstream of common pool 50 and to an oxygen injection unit 70, which is downstream of the bio-filtration reactor. Finally, to complete the circuit, oxygen injection unit 70 is connected to aquaculture tank 10, upstream thereof. The connections between the components are achieved by a plurality of conduits as will be described in more detail below.

Aquaculture tank

[038] Aquaculture tank 10 may be of any number, size, shape and material appropriate for growing the species in culture. The chemical and physical properties of water used in aquaculture tank 10 are not limited, and can be, for example, fresh water or seawater at any temperature suitable for the species in culture.

Foam fractionation unit

According to one embodiment of the present invention, aquaculture tank 10 has an outlet conduit 1 10, typically at an upper portion thereof, that leads to foam fractionation unit 20. In one embodiment, small air bubbles are passed through the water column in the foam fractionation unit 20. Such air bubbles attract organic waste, essentially protein, but also other organic and inorganic materials, notably particulate matter, typically most of which tends to accumulate on the water surface as foam. This foam is removed from the water surface, fluidized, and collected in a fluidized foam tank 24. The treated water that remains in the foam fractionation unit 20 exits into conduit 130. The fluidized foam tank 24 is also connected to conduit 130 by a passage 28.

Ozonation [039] In another embodiment of the present invention, foam fractionation unit 20 contains an ozone supplier 23, which allows mixing of ozone with the air bubbles passing upward through the water in foam fractionation unit 20. Ozone is a powerful oxidizing agent and a strong disinfectant; and enhances the flocculation of organic material. Therefore, the combination of ozone treatment and foam fractionation is beneficial, because the organic material particulate matter formed during ozonation is easily collected by the bubbles and removed with the foam generated on top of foam fractionation unit 20.

[040] In a quantitative example, the water in the foam fractionation unit 20 is treated with ozone from ozone supplier 23 to produce a concentration in the foam fractionation unit of about 0.07 milligram ozone per liter, continuously for 24 hours a day. This dosage of ozone is sufficient for treating the water, while keeping the ozone level below a detectable level in the water after treatment.

[041] In another example, the water in the foam fractionation unit 20 is treated with ozone only during the night, when the level of organic matter in the water is very low. During daytime, the aquatic species, such as fish, are fed and have high metabolic rates, thus a high amount of organic matter is accumulated in the water. However, during night time the fish are not fed and their metabolic rate is much lower. Therefore it is beneficial to treat the water with ozone during the night, when the demand of organic matter in the water for an oxidation agent is very low. As a result, lower amounts of ozone are used, thus lowering maintenance costs.

[042] In another embodiment of the present invention, fluidized foam tank

24 contains a foam treatment ozone supplier 25, which supplies ozone for treating the fluidized foam in fluidized foam tank 24. In one example, fluidized foam tank 24 is a tank in a volume of about 5 cubic meters, and the fluidized foam accumulated in fluidized foam tank 24 is treated with approximately 30 milligram ozone per liter using foam treatment ozone supplier 25. Then, the ozone-treated fluidized foam is let standing until the ozone concentration is below a predetermined threshold. For example the ozone-treated fluidized foam is let standing for at least 15 minutes. Then the ozone-treated fluidized foam is returned to the system via passage 28 that is connected to conduit 130.

Algae pool [043] Water exiting foam fractionation unit 20 flows into algae pool 40 via conduit 130. Algae pool 40 may be of any number, size, shape and material that is suitable for growing a particular species of algae, preferably, but not limited to, macroalgae. After treatment, the water exits algae pool 40 and enters common pool 50 via conduit 150.

[044] In one embodiment of the present invention, the shape of algae pool

40 is D-ended; the depth of algae pool 40 is about 70 centimeters; and the minimal volume of algae pool 40 is approximately 35% of aquaculture tank 10 for culturing fish. In another embodiment, the system contains a plurality of algae pools 40 and/or aquaculture tanks 10 for culturing fish. In this case, the minimal total volume of the algae pools 40 is approximately 35% of aquaculture tank 10 or of the total volume of the plurality of aquaculture tanks 10. In addition, algae pool 40 includes a mechanism, for example a propeller for producing turbulence in the water, for propelling the water by turbulence. The propelling of the water has two purposes: To aerate the water due to the creation of air bubbles, thus to bring the concentration of dissolved gases in the water to equilibrium with the surrounding atmosphere; and to mix the water with the algae in order to increase the exposure of the algae to sunlight.

[045] In one example, algae pool 40 is inhabited by the red macro-alga Gracilaria sp.. This species has benefits relevant to the process of the invention for several reasons: it has a high growth rate; high density per area; high rate of absorption of ammonia, nitrates, sulfides and carbon dioxide. As a result of photosynthesis, carbon dioxide is absorbed from and oxygen is released into the water, oxygen to the water. This process takes place under natural or artificial light. Another feature of Gracilaria sp. that renders it suitable for use in the system of the present invention is the high adaptability of Gracilaria sp. to various water treatment regimes. For example, Gracilaria sp. is resistant to chlorination in levels as may be encountered in such systems from time to time. Gracilaria sp. is also suitable as it has the ability to grow and be harvested all year around. Gracilaria sp. is an agricultural crop useful for the production of a variety of commercial products such as agar feed for shellfish and fish, and is a food additive; salt substitute. A strain of Gracilaria sp. that breeds only vegetatively (asexually) when grown in ponds is particularly suited for use in the present invention, as sexual reproduction in this species can induce a multitude of tiny propagules that can clutter the system and clog passageways.

Anaerobic pond

[046] According to one embodiment of the present invention, aquaculture tank 10 has an outlet conduit 120, typically at a lower portion thereof, that leads water through an inlet 32 into anaerobic pond 30, which includes an oxygen concentration reduction portion 36 and a pollution anaerobic treatment portion 38. The structure and function of anaerobic pond 30 is described in detail below, and illustrated in Figs. 2-6. Water exits the anaerobic pond through outlet 34 into conduit 140 that leads the water into algae pool 40. Bypass of algae pool

[047] In some embodiments, the system includes one or both of: a first algae pool bypass conduit 160 connecting between foam fractionation unit 20 and common pool 50; and a second algae pool bypass conduit 170 connecting between anaerobic pond 30 and common pool 50. Bypass conduit 160 allows algae pool 40 to be cut-off from the rest of the system, for instance when maintenance operations in the algae pool 40 are carried out. To cut-off algae pool 40 from the rest of the system, water from conduit 130, which transfers water from foam fractionation unit 20 to algae pool 40, is diverted to bypass conduit 160 that carries the water from foam fractionation unit 20 directly into common pool 50; and water from conduit 140, which transfers water from anaerobic pond 30 to algae pool 40, is diverted to bypass conduit 170 that leads the water into common pool 50. It is possible to direct water only from conduit 130 bypass conduit 160; or water only from conduit 140 to bypass conduit 170; optionally water is directed to both bypass conduits 160 and 170 simultaneously.

Common pool

[048] Common pool 50 collects the water that arrives from algae pool 40, and in times when algae pool 40 is cut-off, common pool 50 collects water that arrives from either foam fractionation unit 20, or anaerobic pond 30, or both. In addition, common pool 50 serves as a unit for managing pH and alkalinity of the water. The pH and alkalinity of the water that enters common pool 50 is normally low. The pH level of the water in common pool 50 is monitored, either periodically or continuously, manually or automatically. When the pH level of the water is below a recommended threshold specific to the species grown in aquaculture tank 10, the pH of the water is raised by adding a suitable agent. A variety of agents can be used in order to increase the pH level and alkalinity of the water, for example: lime (calcium oxide - CaO); caustic soda (sodium hydroxide - NaOH); soda ash (sodium carbonate - Na₂C03); or sodium bicarbonate (NaHCC ). In one embodiment of the invention, fish is grown in aquaculture tank 10, and the pH of the water in common pool 50 is adjusted to a level in the range of approximately 7.8-8.2 by adding caustic soda to the water.

Bio-filtration reactor

[049] Water exits common pool 50 into conduit 180 that leads the water into a bio-filtration reactor 60. The bio-filtration reactor 60 removes dissolved organic matter as well as ammonia and nitrate from the water. A non-limiting list of bio-filtration reactors that can be integrated in the system of the invention includes: fluidized-media reactor (sand and plastic bead); rotating biological contractor; trickling bio-filter; submerged large media reactor; and pressurized bead filter. [050] In one embodiment, bio-filtration reactor 60 contains a water heating apparatus 66 for heating the water, in order to increase the temperature of the water for the benefit of the species grown in aquaculture tank 10. Any type of apparatus 66 as known in the art may be used for heating the water, e.g., a heat- exchange system that includes a hot water pipe at the bottom of bio-filtration reactor 60. In one embodiment of the present invention, the water in bio-filtration reactor 60 is heated to a temperature of approximately 25±3°C, which is considered optimal for culturing certain fish in aquaculture tank 10.

Oxygen injection unit

[051] Water exits bio-filtration reactor 60 into a conduit 190 that carries the water into the top of an oxygen injection unit 70. A non-limiting list of oxygen injection units that can be integrated in the system of the invention includes: U- tube; packed column (atmospheric pressure and pressurized); spray column; pressurized column; oxygenation cone; oxygen aspirator; bubble diffuser; multi- staged low head oxygenation unit; and enclosed mechanical-surface mixer. In one embodiment of the present invention, the oxygen injection unit 70 includes a U- tube buried upright in the ground to a depth of about 20 meters, having an oxygen conduit 76 that transfers oxygen to the bottom of oxygen injection unit 70 where the oxygen is injected into the water as tiny bubbles. As a result of the depth that oxygen injection unit 70 is buried in the ground, the hydraulic pressure of the water in oxygen injection unit 70 is high, causing the oxygen to dissolve in the water to a concentration up to approximately 95% saturation. Following oxygen injection, the oxygen enriched water exits from the top of oxygen injection unit 70 into a conduit 200 that carries the water into aquaculture tank 10. Conduits

[052] In one embodiment of the present invention, the conduits connecting the components of the aquaculture system include open channels. Using open channels instead of pipes eliminates problems involved with pipes, like clogging. Normally pipes are clogged by algae and bacteria. Efforts to release these clogs result in a release of high amounts of sulfites that when arriving at aquaculture tank 10 may cause massive death of the aquaculture species.

[053] Fig. 2 is a schematic presentation of an anaerobic pond 30, containing an oxygen concentration reduction portion 36 upstream of a pollution anaerobic treatment portion 38 and includes an oxygen concentration reducing mechanism depicted by at least one oxygen concentration reduction member 31. Anaerobic pond 30 is bordered by a front wall 33, a rear wall 35, and side walls 37 and 39. The walls can be made of any suitable material, e.g., concrete. Water enters oxygen concentration reduction portion 36 through at least one inlet 32, located in any suitable location. According to one embodiment of the present invention, presented in Fig. 2, an inlet 32 is located at front wall 33. Water exits pollution anaerobic treatment portion 38 through at least one outlet 34, located in any suitable location. According to one embodiment of the present invention, presented in Fig. 2, an outlet 34 is located at rear wall 35. [054] Water with dissolved oxygen and pollutants enters through inlet 32 of anaerobic pond 30 into oxygen concentration reduction portion 36. As the water flows through oxygen concentration reduction member 31 the concentration of dissolved oxygen in the water is lowered gradually. A detailed illustration of the process of lowering the concentration of dissolved oxygen in the water within oxygen concentration reduction portion 36 is given in Fig. 5. The water that exits oxygen concentration reduction portion 36 and then enters pollution anaerobic treatment portion 38 has a low concentration of dissolved oxygen. In pollution anaerobic treatment portion 38, the water undergoes anaerobic treatment, including degradation of pollutants, by anaerobic microorganisms that reside at the bottom of pollution anaerobic treatment portion 38 and in the water body within pollution anaerobic treatment portion 38. The anaerobic treatment includes conversion of organic pollutants to carbon dioxide and methane, and a denitrification process of nitrate (NO3 ) to its final product - gaseous nitrogen (N₂). The treated water then exits pollution anaerobic treatment portion 38 through outlet 34.

[055] Fig. 3A depicts an embodiment of an oxygen concentration reduction member 31 in a one-piece form, positioned in between and attached to side walls 37 and 39. Oxygen concentration reduction member 31 contains a network of oxygen removal agent support members 314, for supporting one or more oxygen removal agents that remove oxygen from the water, and which is characterized by a high ratio of surface area to volume. Oxygen removal agent support members 314 define a plurality of spaces 316 through which the water flows. [056] Fig. 3B depicts another embodiment of oxygen concentration reduction member 31 , including a plurality of support substrate blocks 312, attached to each other, positioned between and attached to side walls 37 and 39. Each support substrate block 312 contains a network of oxygen removal agent support members 314, which is characterized by a high ratio of surface area to volume, and spaces 316 in between the network of oxygen removal agent support members 314.

[057] With reference to Figs. 2, 3A and 3B, in typical embodiments, as shown, wherein the entire width of oxygen concentration reduction portion 36, between side walls 37 and 39, is filled with an oxygen concentration reduction member 31 , with no gaps in between the ends of oxygen concentration reduction member 31 and side walls 37 and 39, the spaces 316 within the network of oxygen removal agent support members 314 are the only way for water to flow through oxygen concentration reduction portion 36.

[058] Fig. 4 depicts an embodiment of support substrate block 312, containing a network of oxygen removal agent support members 314, the structure of which is characterized by a high ratio of surface area to volume, and spaces 316 in between the network of oxygen removal agent support members 314. The surface of oxygen removal agent support member 314 is covered with an oxygen removing agent. In one embodiment, the oxygen removing agent includes aerobic microorganisms. The type of aerobic microorganisms that reside on oxygen removal agent support member 314 is determined, inter alia, by the environmental conditions that prevail in the water, especially the concentration of dissolved oxygen. In another embodiment, the oxygen removing agent includes a chemical oxygen adsorbent. A resin, or an anion exchange resin, coated with a reducing agent, for instance copper, copper substances, or silver, are examples of a chemical oxygen adsorbent. Anaerobic microorganisms and a chemical oxygen adsorbent are only examples of an oxygen removing agent that coats oxygen removal agent support member 314. [059] The structure of the network of oxygen removal agent support members 314, and respectively the structure of the spaces 316 in between the network of oxygen removal agent support members 314, can be of any type. In addition, the substance of which oxygen removal agent support member 314 is made of can be of any kind as well, e.g. polypropylene. In one embodiment of the present invention the network of oxygen removal agent support members 314 has an apiary structure that gives rise to a ratio of surface area to volume of about 300 square meters per cubic meter. Support substrate block 312 reduces the concentration of dissolved oxygen in the flowing water, for example, from 100% to 75% as depicted in Fig. 4. As the water passes through support substrate block 312 in spaces 316, the oxygen that is dissolved in the water comes in contact with an oxygen removing agent covering oxygen removal agent support member 314.

[060] A method of treating water having pollutants and dissolved oxygen, includes reducing the concentration of dissolved oxygen in the water to a level suitable for anaerobic treatment, and subsequently anaerobically degrading the pollutants in the water. This method will be best understood by describing an embodiment of the present invention as illustrated in Fig. 5.

[061] Fig. 5 shows a partial view of an example of a preferred embodiment of the present invention: an anaerobic pond 30, containing an oxygen concentration reduction portion 36 upstream of a pollution anaerobic treatment portion 38. Anaerobic pond 30 is bordered by front wall 33, rear wall 35 (not shown in Fig. 5) and side walls 37 and 39. The walls are made of concrete. The length of anaerobic pond 30 is about 36 meters, of which about 2.10 meters is the length of oxygen concentration reduction portion 36 and the rest is the length of pollution anaerobic treatment portion 38. Pollution anaerobic treatment portion 38 is shown partially in Fig. 5. The width of anaerobic pond 30 is approximately 9.60 meters, and the depth is about 2.50 meters. Oxygen concentration reduction portion 36 contains two inlets: 32A at one side corner of front wall 33 and inlet 32B at an opposite side corner of front wall 33. In addition, oxygen concentration reduction portion 36 contains four oxygen concentration reduction members 31 A, 31 B, 31 C and 31 D, each has a width of about 30 centimeters. Oxygen removal agent support member 314, of which oxygen concentration reduction members 31 are made, is made of polypropylene and has a surface area to volume ratio of approximately 300 square meter per cubic meter. The oxygen removing agent that covers oxygen removal agent support member 314 is aerobic microorganisms. The distance between oxygen concentration reduction members 31 A, 31 B, 31 C and 31 D, is about 30 centimeters. Arrows in Fig. 5 indicate the direction of the flow of the water. In this particular example, water originating from the bottom of aquaculture tank 10, having dissolved oxygen and organic material, enters oxygen concentration reduction portion 36 through inlets 32A and 32B. The flow rate of the water through oxygen concentration reduction portion 36 is about 7 centimeters per minute. The concentration of dissolved oxygen in the water that enters oxygen concentration reduction portion 36 is approximately 85% saturation at the bottom and about 100% saturation near the water surface. After passage of the water through the first oxygen concentration reduction member 31 A the concentration of dissolved oxygen in the water downstream of oxygen concentration reduction member 31A is reduced to approximately 60-75% saturation; downstream of the second oxygen concentration reduction member 31 B the concentration of dissolved oxygen in the water is reduced to approximately 35-50% saturation; downstream of the third oxygen concentration reduction member 31 C the concentration of dissolved oxygen in the water is reduced to approximately 15-25% saturation; and downstream of the fourth oxygen concentration reduction member 31 D the concentration of dissolved oxygen in the water is reduced to approximately 0% saturation at the bottom and 5% saturation near the water surface. Since the length of oxygen concentration reduction portion 36 is about 2.10 meters and the flow rate of the water is approximately 7 centimeters per minute, then the concentration of dissolved oxygen in the water is reduced from approximately 85-100% saturation to approximately 0-5% saturation within about 30 minutes. The water having a concentration of dissolved oxygen of about 0-5% saturation that exits oxygen concentration reduction portion 36 enters pollution anaerobic treatment portion 38, which is the site where pollutants in the water degrade anaerobically. It should be noted though, that the above mentioned levels to which the concentration of dissolved oxygen is lowered in the above description are only examples, demonstrating the process of lowering oxygen concentration in oxygen concentration reduction portion 36.

[062] Fig. 5 depicts another embodiment of the present invention, in which pollution anaerobic treatment portion 38 includes an oxygen sensor 400 adapted for measuring the concentration of dissolved oxygen at the surface level of the water in pollution anaerobic treatment portion 38. Oxygen sensor 400 can be of any type suitable for measuring reliably dissolved oxygen concentrations in water.

[063] Fig. 6 is a flowchart of a method of regulating the process of lowering the concentration of dissolved oxygen in water, by controlling the flow rate of water in oxygen concentration reduction portion 36. The method includes a step 202 of measurement on a regular basis of the concentration of dissolved oxygen near the water surface of pollution anaerobic treatment portion 38, using oxygen sensor 400, and a step 204 of determining whether the concentration of dissolved oxygen is above a threshold level, for instance about 5% saturation. If the concentration of dissolved oxygen is not above the threshold level, i.e., below the threshold level, then another measurement step 202 is scheduled. If the concentration of dissolved oxygen is above the threshold level, then the flow rate of water in the oxygen concentration reduction portion 36 is lowered 210, and another measurement step 202 is scheduled. Lowering the flow rate of the water in the oxygen concentration reduction portion 36 increases the contact time of the water with the oxygen removing agent covering oxygen removal agent support member 314, of which oxygen concentration reduction member 31 is made, thus giving the oxygen removing agent more time to remove higher amounts of dissolved oxygen from the water. The measurement of dissolved oxygen concentration and controlling the flow rate of the water in the oxygen concentration reduction portion 36 can be either manual or automatic, using any suitable means for performing these activities. [064] Using the method of regulating the process of lowering the concentration of dissolved oxygen in water, as shown in Fig. 6, results in continuously maintaining the concentration of dissolved oxygen throughout the entire water body in pollution anaerobic treatment portion 38 at a level lower than a threshold level, e.g. approximately 5% saturation, thus allowing a highly efficient anaerobic treatment of the water. Anaerobic treatment includes conversion of organic pollutants to carbon dioxide and methane, and denitrification of nitrate (NO3 ) to gaseous nitrogen (N₂). In the system of the present invention, the anaerobic treatment of the water is highly efficient, because it is performed in the entire water body in pollution anaerobic treatment portion 38, and starts the anaerobic treatment starts immediately upon entrance of the water into pollution anaerobic treatment portion 38. In one example, the water in pollution anaerobic treatment portion 38 flows at a rate of approximately 6-10 centimeters per minute, thus allowing sufficient time for the anaerobic treatment to bring the water to a desirable level of quality. [065] The present invention provides an anaerobic pond 30, including an oxygen concentration reduction portion 36 into which water to be treated enters. During the flow of the water through the oxygen concentration reduction portion 36 the concentration of dissolved oxygen in the water is rapidly lowered to a level lower than a threshold level, e.g. about 5% saturation. Then, the water enters a pollution anaerobic treatment portion 38 in which anaerobic treatment of the water is being held. Downstream of pollution anaerobic treatment portion 38 there is an outlet 34 through which the treated water exits. Even though the anaerobic pond 30 of the present invention is a portion of the aquaculture system, it should be understood by one of ordinary skill in the art that an anaerobic pond 30 as described in the present invention can be used in any system requiring anaerobic treatment of water, for example: municipal wastewater treatment systems, grey water treatment systems, livestock wastewater treatment systems, industrial wastewater treatment systems, hydroponics water recycling systems, and systems for anaerobic manufacturing of biogas.

[066] It should be understood that the above description is merely exemplary and that there are various embodiments of the present invention that may be devised, mutatis mutandis, and that the features described in the above- described embodiments, and those not described herein, may be used separately or in any suitable combination; and the invention can be devised in accordance with embodiments not necessarily described above.

Claims

1. An aquaculture system, of the water recycling zero-discharge type, the system comprising:

an aquaculture tank; and

a water treatment section for treating water and comprising:

^■ a foam fractionation unit disposed downstream of the aquaculture tank;

^■ an anaerobic pond downstream of the aquaculture tank and comprising an oxygen concentration reduction portion adapted to reduce the concentration of dissolved oxygen in the water; and a pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion for anaerobically treating the water;

^■ an algae pool downstream of the foam fractionation unit and the anaerobic pond;

^■ a common pool downstream of the algae pool;

^■ a bio-filtration reactor downstream of the common pool; and

^■ an oxygen injection unit downstream of the bio-filtration reactor and upstream of the aquaculture tank.

2. The system of claim 1 , wherein the foam fractionation unit is connected to the upper level of aquaculture tank.

3. The system of claim 1 , wherein the foam fractionation unit comprises an ozone supplier for allowing mixing of ozone with air bubbles that are passed through the water in the foam fractionation unit.

4. The system of claim 1 , wherein the foam fractionation unit comprises a fluidized foam tank.

5. The system of claim 4, wherein the fluidized foam tank comprises a foam treatment ozone supplier.

6. The system of claim 1 , further comprising a first bypass conduit to allow water to flow from the foam fractionation unit to common pool, and a second bypass conduit to allow water to flow from the anaerobic pond to the common pool, when the bypass conduits bypass algae pool.

7. The system of claim 1 , further comprising a conduit leading from the bottom of aquaculture tank to the anaerobic pond.

8. The system of claim 1 , wherein the oxygen concentration reduction portion comprises at least one oxygen concentration reduction member positioned across a width of the oxygen concentration reduction portion.

9. The system of claim 8, wherein the at least one oxygen concentration reduction member comprises a network of oxygen removal agent support members for providing a surface for an oxygen removing agent, and the network of oxygen removal agent support members defines spaces wherethrough water can flow and come in contact with the oxygen removing agent.

10. The system of claim 9, wherein the oxygen removing agent comprises aerobic microorganisms.

11. The system of claim 9, wherein the oxygen removing agent comprises a chemical oxygen adsorbent.

12. The system of claim 8, wherein a portion of the oxygen concentration reduction members is covered with aerobic microorganisms and another portion is covered with chemical oxygen adsorbents.

13. The system of claim 1 , wherein the pollution anaerobic treatment portion of anaerobic pond comprises an oxygen sensor adapted to measure the concentration of dissolved oxygen at a surface layer of the water in pollution anaerobic treatment portion.

14. A process for treating water in an aquaculture system of the water recycling zero-discharge type, the process comprising:

transferring water from an aquaculture tank to a foam fractionation unit; treating the water in the foam fractionation unit with ozone;

removing foam from the water surface in the foam fractionation unit, fluidizing the foam and collecting the fluidized foam in a fluidized foam tank; treating the fluidized foam in the fluidized foam tank with ozone;

letting the ozone-treated fluidized foam stand until the ozone concentration is below a predetermined threshold;

transferring the ozone-treated fluidized foam from the fluidized foam tank to an algae pool;

transferring ozone-treated water from the foam fractionation unit to the algae pool;

transferring water from the aquaculture tank to an oxygen concentration reduction portion of an anaerobic pond;

lowering the concentration of dissolved oxygen in the water to a level below a predetermined threshold level by flowing the water through the oxygen concentration reduction portion;

transferring the water having a concentration of dissolved oxygen below the predetermined threshold level, from the oxygen concentration reduction portion to a pollution anaerobic treatment portion of the anaerobic pond; treating the water anaerobically in the pollution anaerobic treatment portion;

transferring water from the pollution anaerobic treatment portion to algae pool;

treating the water in the algae pool;

transferring water from the algae pool to a common pool;

measuring the pH of the water in the common pool;

adding an agent for raising the pH and alkalinity of the water to the water in the common pool;

transferring the water from the common pool to a bio-filtration reactor; treating the water in the bio-filtration reactor;

transferring the water from the bio-filtration reactor to an oxygen injection unit;

increasing the dissolved oxygen concentration in the water in oxygen injection unit; and

transferring the water from the oxygen injection unit to the aquaculture tank.

15. The process of claim 14, comprising treating the water in the foam fractionation unit with a continuous flow of 0.01 -10 milligram ozone per liter water during night time.

16. The process of claim 14, comprising treating the fluidized foam in the fluidized foam tank with at least 0.01 milligram ozone per liter of water for at least 5 minutes.

17. The process of claim 14, comprising letting the ozone-treated fluidized foam stand for at least 15 minutes.

18. An anaerobic pond for anaerobically treating water, comprising:

an oxygen concentration reduction portion adapted to reduce the concentration of dissolved oxygen in the water;

a pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion for anaerobically treating the water,

wherein the oxygen concentration reduction portion comprises at least one oxygen concentration reduction member positioned across a width of the oxygen concentration reduction portion, and the oxygen concentration reduction member comprises a network of oxygen removal agent support members for providing a surface for an oxygen removing agent, and the network of oxygen removal agent support members defines spaces,

wherethrough water can flow and come in contact with the oxygen removing agent.

19. The anaerobic pond of claim 18, wherein the oxygen removing agent comprises aerobic microorganisms.

20. The anaerobic pond of claim 18, wherein the oxygen removing agent comprises a chemical oxygen adsorbent.

21. The anaerobic pond of claim 18, wherein a portion of the oxygen concentration reduction members is covered with aerobic microorganisms and another portion is covered with chemical oxygen adsorbents.

22. The anaerobic pond of claim 18, wherein the pond comprises an oxygen sensor adapted to measure the concentration of dissolved oxygen at a surface layer of the water in pollution anaerobic treatment portion.

23. A method of treating water having pollutants and dissolved oxygen under anaerobic conditions, comprising: transferring the water into an oxygen concentration reduction portion of an anaerobic pond, which comprises a pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion, the oxygen concentration reduction portion comprising at least one oxygen concentration reduction member positioned across a width of the oxygen concentration reduction portion, and the oxygen concentration reduction member comprises a network of oxygen removal agent support members covered with an oxygen removing agent;

lowering the concentration of dissolved oxygen in the water to a level lower than a threshold level by letting the water come in contact with the oxygen removing agent;

transferring the water having dissolved oxygen at the level lower than the threshold level from the oxygen concentration reduction portion into the pollution anaerobic treatment portion; and

anaerobically treating the water in the pollution anaerobic treatment portion.

24. A method of maintaining a concentration of dissolved oxygen in water below a threshold level throughout all layers of the water, the method comprising:

(a) flowing the water into an anaerobic pond, including an oxygen concentration reduction portion, having at least one oxygen concentration reduction member; and a pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion;

(b) measuring oxygen concentration at or near the water surface in the pollution anaerobic treatment portion; (c) determining whether the measured oxygen concentration is below or above the threshold level;

(d) lowering the flow rate of water in the oxygen concentration reduction portion if the measured oxygen concentration is above the threshold level; and (e) repeating from step (b).