WO2002082895A1

WO2002082895A1 - Method of shrimp farming in seawater or brackish water ponds

Info

Publication number: WO2002082895A1
Application number: PCT/IB2001/001306
Authority: WO
Inventors: Placido Spaziante; Krisada Kampanatsanyakorn; Andrea Zocchi
Original assignee: Squirrel Technologies Pte Ltd
Priority date: 2001-04-16
Filing date: 2001-04-16
Publication date: 2002-10-24
Also published as: CN1499925A

Abstract

Accumulation of ammonia and urea in salt water filled ponds of a shrimp farm is effectively prevented by continuously converting them into non-toxic substances (nitrogen, nitrates). The process consists in circulating the aquaculture water of a shrimp rearing (1, 1A, 1B) pond through an auxiliary reconditioning pond (2, 2C, 2D, 2A, 2B) and through an electrolyzer (4) for generating active chlorine by way of elctrolysis of chlorides contained in the water; flowing the electrolyzed portion of water into the bulk of water contained in said reconditioning pond (2, 2C, 2D, 2A, 2B) and recycling the water from said reconditioning pond (2, 2C, 2D, 2A, 2B) back into said shrimp rearing pond (1, 1A, 1B), after settling and stabilization, with a residual active chlorine ranging grom 0 to 0.1 ppm. Release of pollutants and risk of introducing lethal microorganism together with make-up water is practically eliminated, control of algae overgrowth and turbidity are greatly enhanced and the conditions of shrimp rearing are markedly improved.

Description

METHOD OF SHRIMP FARMING IN SEAWATER OR BRACKISH WATER PONDS

Shrimp farming in brackish water or seawater filled ponds is an important and fast growing industry in many tropical nations. In Thailand shrimp farming in coastal ponds filled with seawater has had an extraordinary growth along miles and miles of costs.

Methods developed for fresh water aquaculture that are applicable to a large extent also to brackish water and seawater aquaculture activities such as shrimp farming are generally known and reported in an abundant scientific literature.

The document: "Water quality management and aeration in shrimp farming", by Claude E. Boyd, printed and distributed by American Soibeans Association and US Wheat Associates, provides for a punctual and precise information on the factors affecting the rearing according to an intensive culture mode of shrimps in ponds, their specific effects on shrimp development and health and on the available techniques and countermeasures to resort to for controlling the various factors.

In Thailand, traditional shrimp farming is based on the use of isolated ponds, the typical dimensions of which are of about 100m x 100m with a depth of 1 to 1.3m, of a rectangular or square shape.

Post larval shrimps (shrimps of about 0.2g) are introduced in the pond, generally in a quantity comprised between 50 and 70 shrimp/m², and they are fed with pellettized food until they have grown to the size of about 20-25g of weight.

This rearing period of shrimps lasts approximately from 2.5 to 4 months.

As any other aquatic animal, shrimps consume the oxygen dissolved in the water and the concentration of dissolved oxygen (DO) in the pond water must be prevented to drop below a safe level of about lmg/ , and generally it is maintained at a level comprised between 4mg/l and saturation. Aeration with mechanical means, paddle wheels, air bubbler systems and the like are normally used for preserving a correct oxygen budget in the pond water. The shrimp metabolism produces ammonia that accumulates in the pond water in addition to an input coming from the metabolic activity of other organisms that are normally present in the pond. Shrimps also produce as a metabolic by-product urea.

Ammonia, in particular un-ionized ammonia (NH₃), and urea are toxic for the shrimps. The mechanism of ammonia accumulation in the pond limits production at high feed rates and becomes a main limiting factor of productivity even if dissolved oxygen would be adequate to support a more intensive rearing.

If concentrations of ammonia and urea are not kept below a few tenth of a g/1, shrimps start to get stressed, they stop eating and become susceptible to diseases and eventually die.

At present, the accumulation of ammonia and urea in the pond is contrasted by natural conversion to nitrites and thence to innocuous nitrates. Several microorganisms that are normally present in the pond water, transform ammonia and urea in nitrites that are as poisons as ammonia for the shrimps, however other organisms naturally present in the water of the pond transform eventually the nitrites in nitrates that are not poisons for the shrimps.

Unfortunately, such a natural biological process of ammonia and urea reduction is rather slow and grossly insufficient to keep their concentration below the safe limit to keep the shrimps healthy, specially in an intensive culture mode of conducting shrimp farming.

The only countermeasure adopted for controlling the concentration of ammonia and urea is to exchange pond water with fresh seawater directly taken from the sea. This practice has dramatically undesirable drawbacks: it carries the risk of introducing into the pond lethal viruses together with the fresh seawater taken from the sea causing epidemic phenomena that may often kill all the shrimps; the sea receives an important input of pollutants that manifest itself especially along the coastline of areas densely populated by shrimp farming ponds and notwithstanding the recurrent water exchanges with the sea, the pond soil becomes progressively laden with ammonia and urea to the point of becoming irremediably unsuitable to support further shrimp farming activity. Long stretches of shoreland, formerly densely occupied by shrimp farms, have been completely abandoned because of these polluting effects that have progressively but irremediably affected productivity to the point of driving shrimp growers away to yet uncontaminated coasts.

On the average, shrimp farms in Thailand are abandoned after about 5 to 7 years of production. Nowadays, more than half the shrimp farm sites in Thailand are abandoned.

Of course newer and newer sites are subject to this substantially irreversible (certainly so from an economic point of view) process of degradation.

It has now been found and is the object of the present invention, an effective method and an efficient shrimps rearing plant that overcome the until now unresolved problem of progressive degradation of a newly installed shrimp farming pond leading to the an economically unreclamable poisoning of the pond soil and to its eventual abandonment.

It has been found that accumulation of ammonia and urea as well as other noxious byproducts such as sulfides in the shrimps rearing pond water may be effectively prevented by continuously converting them into substantially nontoxic compounds locally in an economical way.

The ability of preventing the accumulation of ammonia and urea in the pond's soil, permits a prolonged ability of the installation to support production at economically rewarding level of productivity.

Sea pollution and wasting of shoreland are greatly reduced.

It has been found that notwithstanding the well known aggressive biocide characteristics of active chlorine, which could qualify it as a micidial poison for a living organism as a shrimp, it can be effectively and safely used for continuously converting by-product ammonia, urea, sulfides and other amino compounds into nontoxic and substantially nonpolluting compounds. As will become evident from the following description, the use of active chlorine, generated in situ by electrolyzing a portion of the seawater or brackish water of the aquaculture in situ, by passing it through an electrolyzer, has other positive effects such as that of providing an easily modulable source of a sterilizing agent for controlling excessive growth and/or a blooming of algae, turbidity, and excess bacteria among the most significant.

According to this invention, the method of shrimp farming in seawater or brackish water filled ponds comprises the steps of circulating the aquaculture water of a shrimp rearing pond through an auxiliary reconditioning pond either by pumping reconditioned water from said auxiliary pond into the shrimp rearing pond while collecting water overflowing from the shrimp rearing pond into said water reconditioning auxiliary pond or viceversa; passing a strained fractional portion of the water of the reconditioning pond through an electrolyzer for generating active chlorine dissolved therein by way of electrolysis of chlorides contained in the water; letting the electrolyzed portion of water settle in a reservoir before returning it to the bulk of water contained in the reconditioning pond and eventually recycling the water of said reconditioning pond into the shrimp rearing pond after stabilization with a substantially null residual active chlorine content or in any case not above 0.5 ppm.

An essential feature of this invention is the use of a secondary (or auxiliary) reconditioning pond along with one or more shrimp rearing ponds.

The investment will therefore include the cost of providing for such an auxiliary reconditioning pond of sufficient water capacity, generally of the same order of magnitude of that of the shrimp rearing pond or ponds, beside the hardware and instrumentation costs, however, it has been demonstrated that the investment is quickly recoupled through an enhanced productivity and in the longer run by a greatly diminished occurrences of sudden losses of the shrimps due to accidental development of deadly conditions in the rearing pond and by a prolonged operating life of the farm installation.

There are important differences between commercially available microcide treatment compounds, such as chlorine (Cl ) and hypochlorite (CIO^'), and active chlorine generated by way of direct seawater or brackish water electrolysis and it is deemed helpful for a full appreciation of this invention to recall some pertinent concepts. Terminology

The word chlorine, active chlorine, free chlorine, hypochlorite, hypochlorous acid are scientifically incorrect but commonly used to describe the "oxidating power" of a given seawater sample. Gaseous chlorine, as well as the commercially available hypochlorites, when added to sea water, react immediately with any oxidable compound or element producing other compounds. Even if chlorine and hypochlorite as such completely disappear producing other oxidizing compounds these oxidizing compounds are still conventionally referred to and accounted as the "active chlorine" content of the water sample.

Electrochemical reactions occurring when electrolyzing seawater or brackish water

When direct electric current flows between two electrodes immersed in seawater or brackish water the following electrochemical reactions occur:

At the positive electrode (anode)

Chlorine forms at the electrode surface according to the main reaction:

2Cl- = Cl₂ + 2e- [1]

Chlorine is a soluble gas and remains completely dissolved in the water.

An important (parasite) reaction that occurs at the anode is the evolution of oxygen:

H₂0 = '/_ 0₂ + 2i + 2s [2a]

or

2OH- = '/_ 0₂ + H₂0 + 2e^" [2b]

In seawater, with a salt concentration of 15 to 35 ppt (=g l) of sodium chloride and pH 8, this parasite reaction supports approximately 5 to 15% of the total current. The more diluted is seawater the more this parasite reaction becomes competitive and at a salinity of a few ppt (=g/l) supports half of the anodic reaction, which means that the Faraday efficiency of chlorine production drops to approximately 50%. When brackish water of particularly low salinity is used, titanium anodes with an iridium- based coating must be used instead of the more common and less expensive ruthenium oxide coated titanium anodes. Electrodes last more than two years of continuous operation at full rated capacity. Of course, discontinuous use or lower operation rate prolong their life proportionally.

At the negative electrode (cathode)

Water reduction generates hydrogen gas and hydroxyl ions according to the reaction:

2 H₂0 + 2e = H₂ + OH^" [3]

Hydrogen is insoluble and is generally separated and vented in a safe location. Minor amounts may remain in the streaming seawater without causing problems because they will escape freely in the air. It is advisable not to smoke or carry open flames in the vicinity of the chlorinated water outlet.

A relevant parasite reaction that occurs at the cathode is the reduction of hypochlorous acid, (often referred to as hypochlorite)

2HC10 + 2e^" = 2C1^" + 7_ H₂ + H₂0

This reaction will reduce the efficiency of the process. The rate of this reaction is directly proportional to the concentration of hypochlorious acid HCIO (active chlorine or commonly hypochlorite). For concentrations of up to 2'000 ppm (2 g/1), the rate of reduction of hypochlorious acid (hypochlorite) is very limited, it may account to 5% of the total electric current. For concentration of hypochlorite in the range of 5 to 8 g/1 any further produced hypochlorite will be reduced at the cathode. It is important to pass through the electrolytic cell seawater at a rate sufficient to maintain the concentration of active chlorine always below 2 g/1 at the outlet of the cell.

Chemical reactions occurring in the electrolyte

The following chemical reaction readily occurs in the water streaming between the electrodes of the cell, between the chlorine generated at the anode and the hydroxyl ions generated at the cathode,

Cl₂ + 2HO- = 2HC10 The hypochlorous acid (HCIO) represents the real active chlorine and is responsible for the oxidizing power of the electrolyzed water. HCIO dissociates according the following reaction:

HCIO = H* + CIO^"

HCIO is a powerful oxidant while CIO^* is a mild oxidant (approximately 10 times less powerful). In alkaline pH conditions, HCIO is completely dissociated. In the condition of operation of seawater or brackish water cell, it is only slightly dissociated.

This is what distinguishes the properties of a chlorinated water sample by direct electrolysis from a water sample to which a commercially available hypochlorite has been added. Commercially available hypochlorite is completely dissociated and stabilized to permit transportation and storage without decomposition. The most common stabilizer is caustic soda, which will react with hypochlorite forming sodium hypochlorite [NaOH + HCIO === NaClO + H₂0] and calcium hydroxide, which will form calcium hypochlorite [Ca(OH)₂ + 2HC10 = Ca(C10)₂ + 2H₂0]. The oxidizing effect of commercial hypochlorites on organic material, ammonia and amines is very mild. The intermediate chlorinated compounds that are formed with ammonia and amines are relatively stable and long lasting, as hereafter described.

On the contrary, the active chlorine [HCIO] generated "in situ" by directly electrolyzing seawater or brackish water is very unstable, it readily decomposes into chloride ions and active oxygen radical (active oxygen):

HCIO = H + C^" + 0» (active oxygen)

The decomposition behavior of active chlorine generated in the water by direct electrolysis resembles that of ozone:

0₃ = 0₂ + O

The oxygen radical becomes the active agent of the oxidation process. It will readily oxidize and decompose any organic matter to carbon dioxide and water and in particular it effectively reacts with ammonia, urea as well as with the resulting amines, eventually decomposing them to nitrogen and nitrates according to the following reactions: 1.5 HCIO + NH₃ = 0.5 N₂ + 1.5 H₂0 + 1.5 HC1

4 HCIO + NH₃ = NO.- + 4 CY + 5 i + H₂0

In turn, the nitrates so formed are readily reduced to nitrogen at the cathode of the electrolytic cell.

Similar reactions occur with urea and amines and the end products will be nitrogen and various N-containing compounds that are substantially inert and not toxic to shrimps.

By contrast, such an effective decomposition of ammonia does not occur with stabilized commercial hypochlorites, relatively, stable and long lasting chlorinated amines are produced in the water.

It has been found that, by employing an auxiliary pond of sufficient capacity, normally of the same order to magnitude of the capacity of the shrimp rearing pond or ponds, into which stabilize and control the amount of residual active chlorine in the water to be recycled back into the shrimp rearing pond after having been freed of its load of by-product ammonia and urea by electrolyzing a portion of water according to the above described process, conditions of safe and highly productive shrimp rearing can be ensured for exceptionally long periods of time without requiring expensive reclaiming interventions on a producing pond.

Preferably, in the auxiliary pond a reservoir of a fractional capacity (generally of about one tenth) is realized by an appropriate partition from this reservoir the portion of water to be flown into the electrolyzer is drawn and into this reservoir the electrolyzed water is returned.

The presence of such a settling reservoir greatly facilitates the control of the overall reconditioning treatment as will be better described later.

Most preferably the functions of the settling reservoir and of the auxiliary water reconditioning pond may be surrogately fulfilled by an efficient layout of the auxiliary pond itself which is conveniently realized in the form of a relatively narrow E-shaped channel serving two shrimp rearing ponds, as will be better described later. In any case the auxiliary water reconditioning pond provides a buffer reservoir for eliminating the risks associated with any direct intake of water from the sea (without any biocide treatment) into the shrimp rearing pond.

The different aspects and advantages procured by this invention will become even clear through the following description of preferred embodiments and by referring to a full size experimental shrimp farm made according to the present invention and illustrated in the attached drawings, wherein:

Figure 1 is a functional layout of a shrimp farming plant made according to the present invention;

Figure 2 is a functional diagram of the chlorinating plant;

Figure 3 shows a preferred arrangement of the air lift pumps recycling water back into the shrimp pond;

Figure 4 shows the structure of an air lift pump.

Figure 5 shows a preferred E-shaped modular layout of a shrimp farming plant unit;

Figure 6 shows a possible expanded layout composed of an array of modular plant units.

With reference to Fig. 1, the experimental full size plant used for demonstrating feasibility and effectiveness of the invention included a shrimp rearing pond 1 having a volume of approximately 2500 m³. The auxiliary reconditioning pond 2 had the same dimensions of the shrimp pond. Of course, this is relatively ininfluent, indeed the auxiliary pond 2 may be smaller or larger than the shrimp rearing pond.

According to the experimental set-up, the exchange of water between the two ponds was arranged by pumping water from the auxiliary pond 2 to the shrimp pond 1 by way of a battery of air lift pumps 9 while collecting water outflowing from the shrimp pond into the auxiliary pond, by way of a siphon tube 10 that was simply held submerged into the water and buried in the soil of the banks of the two ponds to release the overflowing water at the nearest corner of the auxiliary pond 3. The inlet mouth 1 1 of the siphon tube 10, suitably equipped with a strainer, was held just off the bottom of the pond and about at the center of the shrimp pond 1 and could be easily shifted manually about the center of the shrimp pond 2. This arrangement is preferable to adopting a reverse scheme of exchange of water between the two ponds, because the level of water in the shrimp pond 1 is raised by pumping back water into it from the auxiliary pond 2 and the solids that are induced to settle preferably about the pond's center (as will be described later) are to a large extent dragged together with outflowing water stream into the auxiliary pond.

According to a preferred embodiment of the invention, the collected outflowing water passes first through a buffer reservoir 3 of fractional capacity before reaching the bulk of water contained in the rest of the auxiliary pond 2. The reservoir 3 may be realized by installing a partition wall 3a in the auxiliary pond 2.

At the farthest point from the inlet of the collected water of the reservoir 3, there may be an adjustable overflow device 3b, through which the water eventually flows merging with the much larger bulk of water contained in the rest of the auxiliary pond 2.

The functional diagram of the chlorination plant EC is depicted in Fig. 2. The chlorinator cell or electrolyzer 4 consisted of six cells in series, each cell made of six electrode blades of 100 mm x 300 mm, intermeshed with seven electrode blades of the same dimensions. The surface area of each electrode plate was 0.03 m² (considering both sides of the plate), accounting to a total electrodic surface of 0.18 m².

By forcing through the electrolyzer a DC current of 20 A, 120 g/h of active chlorine were produced; at 30 A the yield was of about 180 g/h and at 100 A it was in the vicinity of 600 g/h. Correspondingly, the concentration of active chlorine, measured at the outlet of the electrolyzer, was 0.15, 0.22 and 0.75 g/1, respectively.

Commonly, the chlorination plant EC includes, as depicted in Fig. 2, a step-down electrical transformer, a rectifying bank and control circuitry contained in the electrical cabinet 7 and common hydraulic devices to monitor and control the flow of water through the electrolyzer 4. Of course, the plant may include a separate hydraulic circuit (not shown) for periodically cleaning the electrodes (typically the cathodes) from deposited oxides and for removing concretions with diluted HC1 or equivalent cleaning agents. Complete self-contained chlorination plants provided with full instrumentation and control devices are commercially available for any desired rated output from various sources. In the experimental plant a chlorinator plant, with a rated capacity of 500 g/h of chlorine, was satisfactorily employed.

The stream of electrolyzed water was released in the volume of water contained in the confined portion 3 of the pond 2, defined by placing a partitioning wall 3 a along with one side of the perimeter of the auxiliary pond 2, to form a settling or buffer reservoir 3 into which the submergible pumps 5 and 6 were installed for pumping a stream of water through the electrolyzer 4.

The reservoir 3 through which the water coming from the shrimp pond 1 first passes is not strictly necessary, but highly preferable to provide for a temporarily isolated volume (buffer) of aquaculture water in which what is left of killed floating algae and other suspended matter, may settle to the bottom of this reservoir from when it will eventually be removed from time to time and for facilitating overall the control of the level of active chlorine.

The presence of a flow through reservoir 3 greatly facilitates the control of algae by killing completely the amount of algae contained in the isolated volume of water present in the settling reservoir during a chlorination phase.

Indeed, though the whole process may be carried out in a continuous mode, it may be often easier to carry out a chlorination phase at intervals. During these chlorination phases of finite duration (for example lasting few hours), the pumping of water from the auxiliary pond 2 back into the shrimp pond 1 may even be stopped, while chlorinating plant EC is activated, pumping the water from the relatively isolated body of water contained in the reservoir 3 through the electrolyzer and thus injecting a programmed dose of active chlorine into the isolated body of water contained in the reservoir 3.

Once a new chlorination phase has terminated, a certain time (generally from 6 to 24 hours) may be given for the suspended matter (killed algae, etc.) to settle on the bottom of the reservoir 3. Thereafter pumping of water into the shrimp pond may be resumed and the restarted flow of water will cause the chlorinated water of the reservoir 3 to flow into the bulk of water contained in the auxiliary pond 2. Occasionally, a substantial sterilization to eliminate excess bacteria may also be safely carried out at need in the isolated settling reservoir 3, even without stopping the pumping of water back into the shrimp rearing pond by simply temporarily lifting the overflow device 3b to temporarily isolate completely the body of water contained in the reservoir 3.

The active chlorine that is contained in a relatively high concentration in the body of water contained in the reservoir 3, when it reaches the bulk of water contained in the water reconditioning pond 2, disperses in the much larger volume of water and continue to react with by-product ammonia and urea that may still be present in the water according to the following reactions:

3 C12 + 2 NH3 — N2 + 6 CI- + 6H+

4 C12 + NH3 + 3H20 — N03- + 8 CI- 9H+

and to convert the intermediate compounds to nitrogen and nitrates, as well as reacting with any other oxidable compound such as sulfides and organic matter.

The volume of the auxiliary water reconditioning pond 2 is such to imply an average residence time of water in the order of one or more days. The input active chlorine is eventually consumed through the above-discussed oxidation reactions that are also stimulated by the exposition to sun light during day time and the residual concentration of active chlorine in the suction zone of the battery of water recirculation pumps 9 should stabilize itself in the range of 0.05 to 0.1 ppm, a level that has been proven to be perfectly compatible for the shrimps living in the first pond 1.

Monitoring of the level of residual active chlorine in the water near the suction point of the water recirculating pumps 9 is effected by appropriate instruments, as schematically indicated in Fig. 1 with RCC.

In the experimental plant, a portable chlorine photometer made by WPA Catalogue No. HC 6000, was used to control the amount of active chlorine remaining in the water near the suction of the recirculating pump 9.

Each air lift pump may have a structure as depicted in Fig. 4. Air is forced by motor driven blowers 13 through distribution pipes 14 (shown in Fig. 3) and through dip tubes 15 to the bubblers 16 fitted inside a Venturi shaped bottom suction inlet 17 of a vertical conduit 18 having an outlet elbow 19, the outlet mouth of which partly emerges from the water level of the shrimp rearing pond.

In order to provide for an adequate input of atmospheric oxygen in the water of the shrimp rearing pond 2 such to maintain a sufficiently high concentration of dissolved oxygen, instead of using common air bubblers, distinct arrays of air lift pumps 12 are used with a primary function of air bubblers to favor dissolution of atmospheric oxygen in the water.

The distinct arrays of air lift pumps, suitably disposed along the perimeter of the shrimp rearing pond 1 may advantageously be exploited to promote a slow circular movement of the water in the pond, as schematically depicted in the partial plan view of Fig. 3. This favors the settling of suspended particles preferentially about the center of the pond and thus enhances a constant subtraction of settling matter from the shrimp pond through the outflow mouth 11 of the siphon pipe 10.

EXAMPLE

Evaluation conditions in the experimental shrimp rearing plant is were the following.

The volume of the shrimp pond was about 2500 m³.

The water exchange with the auxiliary pond 2 occurred at a rate of about 500 m³/day. In practice, water was completely exchanged in a period of about 5 days.

The electrolyzer 4 of the electrochlorination plant had the structure as already described above. The rate of flow of water, strained and pumped through the electrolyzer was of approximately 800 1/h.

Depending on the current that was actually forced through the electrolyzer, the concentration of active chlorine at the outlet of the electrolyzer 4 was generally in the range of 0.15 to 0.75 εt/1. On the average to the 500 mVday rate of water overflowing from the shrimp rearing pond 1 into the close end of the reservoir 3, 3 to 4 kg/day of active chlorine was added. The concentration of active chlorine in the water, monitored at the farthest end of the reservoir 3, near and upstream of the overflow 3b, ranged between approximately 1 to 3 ppm.

On a volumetric basis, the content of active chlorine should have been comprised between 5 and 8 ppm, however active chlorine reacts rapidly with ammonia, urea, sulfites and other organic matter and the residual amount of active chlorine per sample of water progressively decreases along the length of the reservoir 3.

The blending of the water outflowing from the reservoir 3 into the bulk of water contained in the remaining portion of the auxiliary water reconditioning pond 2, the continuing reaction of active chlorine with residual ammonia, urea and other organic material in the water of the pond 2 and prevailing conditions of sunlight determined a steady state residual active chlorine concentration in water samples taken near the suction mouth of the air lift pumps 9 in the auxiliary pond 2 from 0.00 to 0.1 ppm.

After a period of operation of one month of operation of the experimental plant of the invention, the concentration of ammonia was of 0.005 ppm and the shrimps appeared m perfect health.

Comparative parallel test runs have been made on the experimental plant of the invention and on a traditional plant under comparable conditions.

The traditional plant consisted of only one pond (50 x 55 xl.3 m), employed two peddle wheels with five blades for oxygenating the water, each driven by a 5 HP motor maintained constantly in operation (except when feeding the shrimps). The total rated installed power was 1 1 HP and the actual total absorbed power was 7 HP.

The experimental plant of the invention used two ponds, each of identical dimensions of the single pond of the traditional system. One air blower, driven by a 3 HP motor was continuously in operation (except when feeding the shrimps). The total absorbed power was of 2.8 HP. The power consumption by the electrochlorinator plant was of 600 W, corresponding to approximately 0.8 HP. The total power absorbed was of 3.6 HP.

Yields: in the traditionally operated pond 4 kilograms of shrimps were produced in a period of 4 months while absorbing 7 HP;

in the experimental plant of the invention about 800 Kg of shrimps were produced in the same period of 4 months with a power absorption of only 3.6 HP.

In the traditionally operated pond, the concentration of ammonia, ranged between 0.1 and 0.68 ppm, the concentration of nitrate ranged between 0.5 and 1.7 ppm, suspended solids between 220 and 300 ppm.

In the experimental plant of the invention, concentration of ammonia in the shrimp rearing pond ranged between 0.0 and 0.01 ppm, the concentration of nitrate ranged between 0.01 and 0.03 ppm and concentration of suspended solids ranged between 100 and 150 ppm.

Indeed, the coordinated functions of the auxiliary water reconditioning pond and of the settling reservoir eventually defined therein may be most preferably fulfilled by an auxiliary water reconditioning pond 2 (of Fig.5 and 6) having a functionally efficient layout, reproducing a relatively narrow "E"-shaped channel, 2C, 2D, 2A and 2B, serving two distinct shrimp rearing ponds 1A and IB, juxtaposed in the spaces between the three a ms 2A, 2C, 2B of the E-shaped reconditioning pond. This outstandingly effective layout is depicted in Fig.5.

According to this preferred embodiment of the invention, a basic (modular) plant unit may conveniently comprise two distinct shrimp rearing ponds 1A and IB and a single auxiliary water reconditioning pond 2 having a layout in the form of a "E", the three arms of which, 2A, 2B, 2C, respectively, and the interconnecting main portion 2D practically enhancing the two substantially square shrimp rearing ponds 1A and IB.

Flow arrows a and b describe the circulation of water from and to the shrimp pond 1A from and to the shrimp pond IB, respectively. As may be observed, water is drawn through the sunk flexible hoses or conduits 10 from about the center of respective shrimp rearing ponds 1A and IB, and flows into the auxiliary recondition pond 2 in the extreme portion of the central arm 2C of the E-shaped auxiliary water reconditioning pond 2, from where a portion of filtered water is electrolyzes in the electrolyzer 4 and is returned thereto with a certain concentration of active chlorine dissolved therein.

Relative volumes and flow rates are such that the central channel or arm 2C of the E-shaped auxiliary pond 2 practically acts as the settling reservoir 3 of the previously described embodiment.

The residual active chlorine concentration in the water steadily decreases along the relatively long recirculation paths, identified by the arrows a and b that the water under reconditioning follows before finally reaching the extreme portions of the two side arms 2A and 2B of the of auxiliary pond 2, from where the reconditioned water finally returns into the respective shrimp pond 1A and IB.

The total residence time of water in the reconditioning pond 2 may be of two or more days.

According to an even preferred embodiment, the so-defined shrimp rearing plant unit of the invention may include, as depicted in Fig.5, a Service Reservoir of a larger capacity than that of each single shrimp rearing pond 1A or IB.

Such: service reservoir may have several purposes.

For example, during the growth of the shrimps in the two ponds 1A and IB, water losses due to evaporation and seepage through the soil in the shrimp rearing ponds 1A and IB as well as in the E-shaped auxiliary water reconditioning pond 2, may be made-up by pumping water from the service reservoir through the electrolyzer 4 into the central arm 2C of the auxiliary water reconditioning pond 2 to reconstitute the correct level of water in the operating ponds 1 A, IB and 2, without risking to introduce viruses and other lethal organisms.

Another useful function of the service reservoir is that of holding the water pumped out of a shrimp rearing pond at the time of harvesting the grown up shrimps therefrom. Instead of discharging the water in the environment, the water can be pumped into the service reservoir and reused for the next crop. Even this practice will serve to greatly reduce the release of pollutants into the free waters.

Another important feature of the peculiar layout of the plant of the invention of Fig.5, is its "modularity".

Indeed, the plant unit depicted in Fig.5, represents a module that may be combined with another identical module and so forth to increase production capability.

Fig.6 schematically shows the overall layout of a plant composed of eight units as the one depicted in Fig.5 arranged into four identical sub-portions, each made up of two unitary modules of Fig.5. As clearly observable from the layout of Fig.6, each electrolyzer 4 may conveniently serve two juxtaposed modules and also used to disinfect, time by time when necessary, the water of the reservoir.

Claims

C L A I M S

1. A method of shrimp farming in seawater or brackish water filled ponds, characterized in that it comprises the steps of circulating the aquaculture water of a shrimp rearing (1, 1A, IB) pond through an auxiliary reconditioning pond (2, 2C, 2D,.2A, 2B) either by pumping reconditioned water from said auxiliary pond (2, 2C, 2D, 2A, 2B) into the shrimp rearing pond (1, LA, IB) while collecting water outflowing from the shrimp rearing pond (1, 1A, IB)) into said water reconditioning auxiliary pond (2, 2C, 2D, 2A, 2B) or viceversa; passing a strained fractional portion of the water flown out of the shrimp rearing pond (1, 1A, IB) through an electrolyzer (4) for generating active chlorine dissolved therein by way of electrolysis of chlorides contained in the water; flowing said electrolyzed portion of water into the bulk of water contained in said reconditioning pond (2, 2C, 2D, 2A, 2B); recycling the water from said reconditioning pond back into said shrimp rearing pond, after stabilization, with a residual active chlorine content ranging from 0 to a limit concentration of 0.1 ppm.

2. The method of claim 1, wherein said electrolyzed portion of water is let to settle in a buffer reservoir (3, 2C) before flowing it into the bulk of water contained in said second reconditioning pond (2, 2D, 2A, 2B).

3. The method of claim 1 or 2, wherein said steps are conducted in a continuous mode.

4. The method of claim 1 or 2, wherein said steps are conducted periodically.

5. The method of claim 2, further characterized by the fact that comprises inducing a motion of the water contained in said shrimp rearing pond (1 , 1A, IB), promoting accumulation of sedimenting suspended solids at a certain position in the pond where the outlet of said outflowing water is located.

6. The method according to any one of the preceding claims 1 to 5, characterized in that a plurality of shrimps rearing ponds (1, 1A, IB circulate the water contained therein through a common reconditioning pond (2, 2C, 2D, 2A, 2B).

7. The method according to claim 6, wherein a common reservoir (3, 2C) is defined in said common reconditioning pond (2, 2D, 2A, 2B).

8. A shrimp rearing plant using a seawater or brackish water filled pond (1, 1A, IB), including means for promoting dissolution of atmospheric oxygen in the water driven by electric motors, characterized in that it comprises

at least a second water reconditioning auxiliary pond (2, 2C, 2D, 2A, 2B) of a capacity comparable to that of the shrimp rearing pond (1. 1A, IB); pumps (9) for recirculating the aquaculture water through said ponds either by pumping reconditioned water from said auxiliary pond (2, 2C, 2D, 2A, 2B) back into said shrimp rearing pond (1, 1A, IB) while collecting water outflowing from the shrimp rearing pond (1, 1A, IB) into the water reconditioning auxiliary pond (2, 2C, 2D, 2A, 2B) or vice versa; an electrochlorinator plant (EC) including means for passing a strained fractional portion of the water outflowing from the shrimp rearing pond (1, 1A, .B) through an electrolyzer (4) for generating active chlorine dissolved therein by electrolysis of chlorides; means for returning said electrolyzed portion of water containing active chlorine dissolved therein to the bulk of water contained in said water reconditioning auxiliary pond 2, 2C, 2D, 2A, 2B; monitoring means (RCC) of the residual active chlorine content in samples of water of said auxiliary pond (2, 2C, 2D, 2A, 2B) to be recycled into said shrimp rearing pond (1, 1A, IB).

9. The plant according to claim 8, characterized in that an isolable body of water

(3) of a fractional volume is defined in said auxiliary pond (2) by a dividing wall (3a) disposed along one side of said auxiliary pond (2); siphon means (10) conveying water outflowing from said shrimp rearing pond ( 1) into said buffer flow reservoir (3) defined in said auxiliary pond (2) at one end thereof and overflow means (3b) at the other end thereof releasing water containing active chlorine into the bulk of water contained in the remaining portion of said auxiliary pond (2).

10. The shrimp rearing plant according to claim 8, characterized in that it includes

one or more units, each unit comprising a first (1A) and a second (IB) shrimp rearing ponds;

a unified auxiliary water reconditioning pond (2C, 2D, 2A, 2B) having the shape of an "E", the three arms (2C, 2A, 2B) of which juxtaposely embrace said shrimp rearing ponds (1A, IB);

siphon means 10 conveying water outflowing from said shrimp rearing ponds (1A, B) into the extreme portion of the central arm (2C) of said unified auxiliary water reconditioning pond;

reconditioned water returning into the respective shrimp rearing pond (1A, IB) flowing out of the extreme portions of the respective side arms (2A, 2B) of said unified auxiliary water reconditioning pond;

means for drawing water from said extreme portion of the central arm (2C), passing it through said electrolyzer (4) and returning it into said extreme portion of the central arm (C) of the unified auxiliary water reconditioning pond.

11. The plant according to claim 10, wherein each unit includes a service water reservoir of a capacity larger than the capacity of each of said two shrimp rearing ponds.