FR3114249A1 - Process for treating water for fish farming, and installation for implementing this process - Google Patents

Abstract

The invention relates to a method for treating salt water from an aquaculture farm, in which: a first filtration of the salt water (7) is carried out to extract therefrom pollutants selected from the group comprising organic matter, solids in suspension, phosphates, a second filtration of the salt water (21) which is finer is carried out that the first filtration (7), to extract therefrom sulphates and micropollutants selected from the group comprising heavy metals, pesticides, pollutants present in trace amounts, and the water thus treated is returned to the said basin (1 ). (Fig.1)

Description

Process for treating water for fish farming, and installation for implementing this process

The present invention relates to the field of aquaculture, and more particularly to the field of fish farming in salt water.

The invention relates in particular to salmon farming.

Classically, this breeding includes a first phase from the hatchery to the smolt, which takes place in fresh water.

Then, in a second phase, the smolts are placed in salt water until they reach adulthood.

This second phase most often takes place in cages placed at sea, or in basins located on land but close to the sea, supplied with seawater.

In the latter case, it is necessary to ensure a constant renewal of the salt water of the basins, to evacuate the various pollutants generated by the breeding: food, hormones, pesticides, droppings, etc.

Typically, the volume of salt water to be renewed each day in these basins located on land is around 5% of the total volume of these basins: for example, for a farm of 10,000 t of salmon annually, the volume basins is of the order of 200,000 m3 of salt water, which therefore implies that 10,000 m3 of water must be renewed per day.

There are technical solutions that make it possible to reduce this volume of water to be renewed: for example, patent application WO2011/152862 proposes an oxygenation and filtration device for aquaculture, making it possible to reduce the rate of renewal of water. basin water by a factor of approximately 10, i.e. a reduction of 5% to 0.5% of the total volume of water in the basins.

When used with salt water basins, this device of the prior art makes it possible to consume only 15 or 20 tons of salt per day for an installation of 200,000 m3.

However, 15 to 20 tonnes of salt must be brought and loaded into the basins every day, which represents considerable logistics and costs.

For this reason, breeding in salt water in onshore tanks located far from the sea was not possible until now, as the quantities of salt water to be renewed are so important.

The aim of the present invention is thus in particular to further reduce the rate of renewal of salt water in aquaculture ponds, in order to reduce - or even eliminate - the volumes of water and salt input, and thus to make economically and technically viable land-based operations.

1. on réalise une première filtration de l’eau salée pour en extraire des polluants sélectionnés dans le groupe comprenant des matières organiques, des solides en suspension, des phosphates,
2. on réalise une deuxième filtration de l’eau salée plus fine que la première filtration, pour en extraire des sulfates et des micropolluants sélectionnés dans le groupe comprenant des métaux lourds, des pesticides, des polluants présents à l’état de traces, et
3. on renvoie l’eau ainsi traitée dans ledit bassin.

This object of the invention is achieved with a process for treating salt water from an aquaculture farm, in which:

1. the salt water is first filtered to extract pollutants selected from the group comprising organic matter, suspended solids, phosphates,
2. a second filtration of the salt water is carried out, finer than the first filtration, in order to extract therefrom sulphates and micropollutants selected from the group comprising heavy metals, pesticides, pollutants present in trace amounts, and
3. the water thus treated is returned to said basin.

Thanks to these characteristics and these two levels of filtration, the process according to the invention makes it possible to clean the salt water of almost all of its pollutants, and thus to reinject it practically clean into the aquaculture breeding basins.

The rearing facility can thus operate practically in a closed circuit, with an extremely small quantity of salt water to be added to achieve the targeted quality objectives.

According to other optional characteristics of the process according to the invention, taken alone or in combination:

- the refusal of the second filtration of the salt water is subjected to a reverse osmosis treatment: this optional and complementary treatment makes it possible to desalinate this refusal, and to remove the last micro-pollutants, and to return the water very low salt resulting from this treatment towards the rearing pond, or else in the environment; in the case where the water is returned to the rearing pond, the recycling rate of salt water from the rearing ponds can be increased: in practice, a recycling rate of around 90% can be achieved water, and around 90% salt recycling rate;

- prior to step a), the salt water is subjected to a microbiological treatment: this microbiological treatment allows bacteria to carry out a degradation of the organic matter contained in the water coming from the rearing tanks;

- a phosphate precipitation agent is added to the salt water prior to step a): this agent facilitates the separation of the phosphates from the salt water during passage through the first filtration means;

- biocidal and anti-scaling agents are added to the salt water prior to step b): these agents make it possible to reduce the development of bacterial fauna as well as the clogging of the filters during passage through the second means of filtration;

- sulfuric acid and an anti-scaling agent are added to the salt water prior to the reverse osmosis treatment step: these agents make it possible to reduce the fouling of the membranes during passage through the treatment means of reverse osmosis.

The present invention also relates to an installation for the implementation of a method in accordance with the foregoing, comprising at least one aquaculture tank intended to receive a breeding in salt water, at least one ultrafiltration unit to carry out said step a), at least one nanofiltration unit for carrying out said step b), and suitable pipes and pumping means for circulating the salt water between these units, for returning the treated water to the basin, and for evacuating the refusal of filtration in landfill or in the environment: such an installation makes it possible to carry out aquaculture in salt water far from the sea, thanks to the high rate of renewal of the salt water obtained.

According to other optional characteristics of this installation, taken alone or in combination:

- This installation comprises at least one reverse osmosis treatment unit, located downstream of said nanofiltration unit;

- this installation comprises a bioreactor interposed between said basin and said ultrafiltration unit;

- this installation comprises means for recovering the sludge from said basin and from said ultrafiltration unit, and means for centrifuging this sludge: these means make it possible to separate the sludge from the salt water coming from the rearing basin, this sludge being made up of a mixture of organic matter and particles in suspension found in the salt water. At the outlet of the centrifugation means, the recovered sludge can be landfilled, incinerated, or used for biogas production purposes.

Other characteristics and advantages of the invention will become apparent on reading the following description, with reference to the appended figure which illustrates:

: a diagram of an installation according to the present invention.

In this diagram, the rectangles indicate the units forming the installation, and the arrows the fluid circulation ducts between these different units.

Reference is now made to the appended FIG. 1, in which there is shown schematically an installation making it possible to implement the method according to the invention.

As can be seen in this figure, this installation comprises at least one, and in general a plurality of basins 1 filled with salt water, allowing the breeding of fish, crustaceans or seawater shellfish.

This installation is particularly suitable for salmon farming, but of course other applications can be considered.

Basins 1 are located on land, and can be supplied as needed with city water 3, to which the amount of salt required for the farm concerned is added.

As a reminder, the salinity of seawater is typically around 37 g of salt per liter, but you can farm in salt water with a salt level down to 15 g of salt. per litre.

At the outlet of the basins 1, pumping means send the salt water to an aeration device 5, which can typically comprise at least one biological reactor at the bottom of which air is blown, in order to promote the activity of degradation of organic compounds.

Preferably, an adjuvant 9 is injected into the aeration device 5 allowing the phosphates present in the salt water to precipitate, in order to facilitate their separation from the water in the filtration step which will follow.

By way of example, this precipitation adjuvant may comprise iron chloride.

The salt water coming from the aeration device 5 is then sent to first filtration means, which can typically comprise one or more ultrafiltration units 7, making it possible to extract from the salt water pollutants selected from the group comprising organic matter, suspended particles, phosphates.

Without this being limiting, these ultrafiltration units 7 can be of the ZeeWeed 500 type offered by the company SUEZ: the filtration fibers of these devices typically comprise pores whose nominal dimensions are of the order of 0.04 μm.

A fraction 11 of the waste from the ultrafiltration units 7 is reinjected into the aeration device 5, and the sludge recovered at the outlet of these filtration units is sent to a sludge storage tank 13, then centrifuged and by means of a centrifuge 15: the liquid fraction 17 resulting from this centrifugation is returned to the aeration device 5, and the solid fraction is discharged 19.

Downstream of the 7 ultrafiltration units, the filtered salt water is pumped to 21 nanofiltration units.

Without this being limiting, these nanofiltration units 21 can be of the PROflex 72-3S type offered by the company SUEZ: typically, the nominal dimensions of the filtration micropores of these nanofiltration units are of the order of 0.004 μm.

These nanofiltration units 21 make it possible to rid the salt water of sulphates and micropollutants such as heavy metals, pesticides, pollutants present in trace amounts.

Preferably, the salt water intended to be treated by these nanofiltration units 21 is injected with biocidal and antiscalant adjuvants 23 making it possible to optimize the operation of these units.

The nanofiltered salt water 25 is returned to the rearing ponds 1, and the waste 27 of the nanofiltered salt water is reinjected into the ultrafiltration units 7.

Optionally, another part 28 of this refusal is sent to reverse osmosis treatment units 29.

Without this being limiting, the reverse osmosis treatment units 29 can be of the SeaPRO 8 type offered by the company SUEZ.

These reverse osmosis treatment units 29 make it possible to desalinate the refusal coming from the nanofiltration units 21, and to remove the last micropollutants.

Preferably, sulfuric acid and anti-scale agents 33 are injected into these reverse osmosis treatment units 29 to optimize the operation of these treatment units.

The slightly salty water recovered at the outlet of the reverse osmosis treatment units 29 is reinjected 35 into the rearing tanks 1, or else discharged into the environment 37 if the composition of this water is compatible with the regulations in force.

Waste from the reverse osmosis treatment units 29 is landfilled 39.

The installation which has just been described makes it possible to implement a process for cleaning the salt water coming from the rearing tanks 1 which is particularly effective: when the ultrafiltration 7, nanofiltration 21 , and reverse osmosis treatment 29, approximately 90% of the water can be reinjected into the basins 1, and 90% of the salt.

When, in addition, a filtration and oxygenation installation of the type described for example in patent application WO2011/152862 is used, almost all of the salt water can be recycled.

The quantities of water and salt to be brought to the installation are thus minimal, which makes it possible to envisage aquaculture farms in the ground, far from the sea.

By way of simple example, for rearing tanks 1 of 100,000 m3, the ultrafiltration units 7 can be sized so as to treat 2500 m3 of salt water per day, the nanofiltration units 21 so as to treat 2200 m3 of salt water per day, and 29 reverse osmosis treatment units to treat 170 m3 of salt water per day.

Incidentally, the centrifuge 15 can be adapted to treat around 720 m3 of sludge per day.

Naturally, the invention is described in the foregoing by way of example. It is understood that the person skilled in the art is able to carry out different variant embodiments of the invention without departing from the scope of the invention.

Claims (10)

Process for treating salt water from an aquaculture farm, in which:

1. a first filtration of the salt water (7) is carried out to extract pollutants selected from the group comprising organic matter, suspended solids, phosphates,
2. a second filtration of the salt water (21) finer than the first filtration (7) is carried out, in order to extract therefrom sulphates and micropollutants selected from the group comprising heavy metals, pesticides, pollutants present in the state of tracks, and
3. the water thus treated is returned to said basin (1).

Process according to Claim 1, in which the refusal of the second filtration of the salt water (21) is subjected to a reverse osmosis treatment (29). Method according to one of Claims 1 or 2, in which, prior to step a), the salt water is subjected to a microbiological treatment (5). Method according to any one of the preceding claims, in which a phosphate precipitation agent (9) is added to the salt water prior to step a). Method according to any one of the preceding claims, in which biocidal and anti-scaling agents (23) are added to the salt water prior to step b). A method according to claim 2, wherein sulfuric acid and an anti-scaling agent (33) are added to the salt water prior to the reverse osmosis treatment step (29). Installation for implementing a method in accordance with any one of the preceding claims, comprising at least one aquaculture tank (1) intended to receive salt water farming, at least one ultrafiltration unit (7) to produce said step a), at least one nanofiltration unit (21) for carrying out said step b), and appropriate pipes and pumping means for circulating the salt water between these units, to return the treated water to the basin (1), and to evacuate the waste from filtration in landfill (39) or in the environment (37). Installation according to claim 9 for implementing a method according to claim 2, comprising at least one reverse osmosis treatment unit (29), located downstream of said nanofiltration unit (21). Installation according to one of claims 7 or 8 for the implementation of a method according to claim 3, comprising a bioreactor (5) interposed between said basin (1) and said ultrafiltration unit (7). Installation according to any one of Claims 7 to 9, comprising means for recovering the sludge (13) coming from the said basin and from the said ultrafiltration unit (7), and means (15) for centrifuging this sludge.