WO2017138818A1

WO2017138818A1 - Method and apparatus for purification of water from aquaculture plants

Info

Publication number: WO2017138818A1
Application number: PCT/NO2017/050036
Authority: WO
Inventors: Stig Johansen; Bjørn MONSEN
Original assignee: Normex As
Priority date: 2016-02-12
Filing date: 2017-02-13
Publication date: 2017-08-17
Also published as: NO20160241A1

Abstract

Method for the purification of water from aquaculture system plants, comprising the steps of: a) adding ozone (12) to at least a part of a flow of water (11) from an aquaculture plant; b) allowing ozone and water to be (thoroughly) mixed in a mixing zone (14); c) allowing the mix of water and ozone a contact time sufficient to create microflocks; d) charging the mix of water and ozone (15) into a flotation tank (16); e) creating air bubbles at the lower end of the flotation tank (16); f) controlling a flow rate through the system which allows a desired retention time in the flotation tank (16); g) removing floated particles (18) from the top surface of the liquid in the flotation tank (16); and h) discharging purified water (21) from the flotation tank through a low end discharge opening (164). An apparatus for conducting the method is also contemplated.

Description

Method and apparatus for purification of water from aquaculture plants

The present invention relates to a method for the purification of water in an aquaculture plant as indicated by the preamble of claim 1. According to another embodiment the present invention provides an apparatus for the purification of water in an aquaculture plant as indicated by the preamble of claim 12.

Background

Aquaculture plants have become a major global primary industry the last couple of decades. In 2013 the production of salmon in Norway alone amounted to 1.18 million tons in according to official (governmental) figures. While salmon is the decidedly largest specie produced in aquaculture plants, the number of species produced in such plants is increasing and the increase is expected to continue. One particular area of growth is plants for cultivation of algae of different kinds, useful in production of components varying from fuels to pharmaceutical products.

The demand for circulating, fresh or saline, water is very high in such plants and there is a corresponding need for purification of the water before discharging it to the environment or before recirculating it to the plant for further use. Typically, water from aquaculture plants is contaminated with a high number of particles and micro-organisms that should not reach the surrounding environment for obvious reasons and which would lead to disease among the species produced if not removed from the recirculation flows of water. CN 2042 81505U teaches a system for marine aquaculture wastewater treatment system which mainly comprises a water inlet pipe, a primary settling tank, an adjusting pool, an air floatation tank, an S8 pool and a water outlet pipe, wherein the water inlet pipe is connected to the primary settling tank, the primary settling tank is correspondingly connected with the adjusting pool, the adjusting pool is connected with a water pipe by a lift pump, and then connected with the air floatation tank, the air floatation tank is connected to the SB pool by the water pipe, and the SBR pool is connected with the water outlet pipe. According to this publication, an 'air flotation + SBR' process is adopted, so that the treatment effect is better; water fed in the primary settling tank is fed from a central guide tube, so that the water is fed more uniformly; the air floatation tank is divided into a flocculation area, an air floating area and a water outlet area, the flocculation area is provided with a stirrer, water in the air floatation tank enters a dissolved air vessel through an air dissolving pump, compressed air is blown in the dissolved air vessel by an air compressor, the compressed air and the water are fully mixed in the dissolved air vessel, and then air-water mixed liquid flows back to the air floating area of the air floatation tank by a dissolved air pipe; and an aeration pipe is arranged in the SB pool, air is provided by an air biower, and water is discharged by a water decanter.

Quite sophisticated devices, apparatuses and plants have been developed to ensure that recirculation water and discharge water do not contain particles or microorganisms in amounts or types harmful for living species within or outside the aquaculture plants, but still there is a need for improvements in this biotic area of globally increasing volume and importance.

Objectives

Thus the objective of the present invention is to provide a method and/ or an apparatus well adapted for use as an element of purification of water in aquaculture plants, that being water for discharge to the environment or for recirculation and further use within the plant.

The present invention

The above objective is fulfilled by the present invention which in a first embodiment concerns a method as defined by claim 1.

According to another embodiment the present invention provides an apparatus as defined by claim 12, suited for use in the method constituting the first aspect of the invention.

Preferred embodiments of the invention are disclosed by the dependent claims.

A vital aspect of the present invention is that a sufficient mixing is obtained of the water and the ozone and sufficient time allowed for this mixed phase when flowing towards the flotation tank, to initiate micro-coagulation or micro-flocking of particles in the water already at the time the water enters the flotation tank(s) or even before.

It will be understood by a person skilled in the art that the different steps of the method generally, with a possible exception at start-up, may all be performed simultaneously.

Further details of the present invention

We will now give a more detailed description of the generic principle of the present invention but also of certain preferred embodiments thereof, with reference to the accompanying drawings, in which:

Figure 1 is a schematic drawing of the principles of a plant arranged in accordance with the present invention.

Figure 2 is a schematic drawing of a plant according to a preferred embodiment of the present invention. Figure 3 is an enlarged side sectional view of a flotation tank being part of the plant illustrated by Figure 2.

Figure 4 is a bottom view of the flotation tank shown in Figure 3.

Figure 1 shows schematically a plant 10 designed in accordance with the present invention. A discharge flow from an aquaculture plant represents a feed flow 11 to the plant of the present invention. Ozone 12 is added to the feed flow 11 to produce a combined flow 13 of impure water and ozone which is directed to a mixing zone 14 in which the ozone is allowed to be thoroughly mixed with the impure water to thereby induce formation of micro-coagulation or micro-flocking of particles in the combined flow to increase the particle size of the impurities to a size allowing the impurities to be removed in a flotation tank. In figure 1 the mixing zone is illustrated as a tank provided with vertical flow diverting plates 141 arranged alternatingly from the bottom wall of the tank and the top wall of the tank to increase the flow distance for the combined flow therethrough and to narrow the flow path, thereby effectively enhancing mixing of the components. The thoroughly mixed combined flow 15 leaving the mixing zone is directed to a flotation tank 16 and charged to the vertically arranged, cylindrical flotation tank at a point near the bottom of said flotation tank and near the centre axis of the tank. From the bottom of the flotation tank a cylindrical partition wall 162 extends up to an elevation lower than the top wall of the tank and lower than the vertical level at which foam and particles 18 are arranged to be discharged from the flotation tank. The charge of thoroughly mixed impure water and ozone comprising flocked particles is made within the open cylinder defined by said partition wall 162. Through the bottom wall of said open cylinder is added, in addition to the thoroughly mixed combined flow of impure water and ozone, air or gas bubbles 17 to provide a plurality of gas bubbles rising through the water with the ability to catch and transport to the water surface any fine, solid particles in the water, hereunder the particles flocked by the action of the ozone added to the feed flow.

A discharge flow 21 for purified water is arranged at the bottom of the flotation tank in the annular area surrounding the partition wall 161. An annular grid 163, typically a perforated plate, is arranged, preferably horizontally, above the outlet or outlets for the discharge flow 21 to ensure stable and even flow conditions in the annular area independent of the position. Generally there will be an upwards flow of water - as well as of air bubbles - within the partition wall 162, and a downwards flow of water in the annular area outside the partition wall 162. The air bubbles rising through the flotation tank will gather as impure foam on top of the water. Thus, the water descending to the outlet will therefore be significantly purer than the water entering the flotation tank. This is the general function of a flotation tank, the novel concept being its use for aquaculture discharge or recycle flows, the addition of ozone and the use of a mixing zone prior to feeding the thoroughly mixed combined flow of impure water and ozone to the flotation tank. A window may be arranged in the flotation tank to allow visual inspection of the conditions therein during operation.

The discharge flow 21 from the plant may be discharged to the environment, but more typically it is recycled to the aquaculture plant.

Figure 2 shows schematically a preferred embodiment of the plant according to the present invention. Here, the mixing zone is comprised by two towers 142, 143 through which the combined flow of impure water and ozone is brought to flow in an upwards direction. These simple towers have a simpler structure than the tank with alternating flow diverting plates and it is quite simple for the designer of a plant to choose the most convenient number of towers to use for achieving the desired mixing of the flows. Thus, the number of towers for the mixing zone can vary in dependence of the actual need and the dimensions of the towers.

Also shown in Figure 2 is a point of drainage Tl for testing of the liquid in flow 15 and further equipment T2 for liquid testing of flow 15. A point of drainage T4 is also shown on flow 21 from the flotation tank and further test equipment T3 is shown within the flotation tank 16. Such drainage or test equipment (T1-T4) for testing of the liquid flows do not constitute a mandatory part of the present invention and is not described in any detail here.

In Fig. 2, like also in Fig. 1, a dotted line is indicated between feed flow 11 and discharge flow 21, indicating that at least part of the feed flow 11 may be by-passed the apparatus of the present invention at least part of the time.

Figure 3 is an enlarged view of the flotation tank 16, comprising an outer cylindrical wall 161, a vertical cylindrical partition wall 162 and a horizontal grid or perforated plate 163, discharge opening 164, means for adding air or gas bubbles 17, and discharge flow 18. The partition wall 162 divides the lower part of the flotation tank 16 into a cylindrical central region 16c and an annular, peripheral region 16p.

The means for adding air or gas bubbles may be a multiphase pump or any other convenient means. Also shown is an inspection window 165. For the purpose of inspection, the partition wall 162 may be made of glass or transparent polymer material. As indicated by Fig. 3 the addition of air bubbles may be introduced to a small "internal" recycle flow of water from the annular region of the flotation tank, but it may also be introduced with a part of the thoroughly mixed combined flow of impure water and ozone 15, a variant not depicted in the drawings.

Curved arrows show the general movement of the liquid in the flotation tank 16 while the lighter bubbles of air and entrained particles are shown as rising to the top of the tank for temporary accumulation and removal, continuously or periodically.

Figure 4 shows schematically a bottom view of the flotation tank. Conveniently the air or gas bubbles 17 are distributed to plural inlet openings 171-174 to obtain a good distribution of air bubbles over the cross-section within the partition wall. The discharge flow 21 is typically a flow combined from a plurality of outlets 211-214 in order to ensure stable and even flow conditions over the cross section of the flotation tank. A combination of plural outlet openings with a perforated plate 163 provides the optimal flow conditions in the flotation tank. It should be noted that the drawings in general, and Figure 4 in particular, only shows principles of the design. Thus, the actual designs of the details shown as well as their mutual sizes may be significantly distorted compared to actual designs and actual sizes. The perforated plate gives an even distributed water flow over the entire bottom, which ensures maintaining an even drag for maximum overflow rate. Even during normal operation the function of the plate is important in this regard and in case of variations in the inflow of water to the flotation tank, the presence of the perforated plate is even more crucial for maintaining the desired steady state conditions in the flotation tank. It is particularly to be noted that the present method is suited as well for discharge flows of water as for recirculation flows of water from aquaculture plants.

In a preferred embodiment the mixing zone comprises at least one tank with a substantially vertically oriented liquid flow therethrough. In another preferred embodiment the mixing zone comprises at least two tanks with a substantially vertically oriented liquid flow therethrough. Bubbles are created in the flotation tank by means of a plurality of static mixers, at least two and more typically three or four, arranged at the inlet openings 171-174. Also the charge of the mix of water and ozone to the flotation tank may be performed through static mixers at the bottom wall of the flotation tank, to a central region thereof.

The creation of air bubbles may also or alternatively be made by means of static mixers arranged in a separate recirculation loop at the bottom of the flotation tank, which allows a regulation of the density of air bubbles independently from the mass flow into the flotation tank. Arranging the static mixers directly to the bottom wall of the flotation tank gives the best effect.

The flow rate through the apparatus is preferably adjusted until steady state conditions are obtained in the flotation tank, as decided by visual inspection.

The perforated plate is typically arranged over the at least one discharge opening and more preferably over all discharge openings from the flotation tank.

The means for adding ozone to the flow of water typically is comprised by a pressurized gas bottle provided with an adjustable reduction valve.

A particular beneficial effect of the use of ozone in aquaculture systems is the initiation of precipitation of dissolved organic molecules and micro flocculation of colloidal organic matter, allowing particles formed in this way to be effectively removed by flotation in industrial scale plants.

Another effect is the breaking up of relatively non-biodegradable refractory organic compounds into smaller more biodegradable compounds.

Still further effects are disinfection and direct oxidation of nitrite to nitrate.

Claims

1. Method for the purification of water from aquaculture system plants, characterized in comprising the following process steps:

a) adding ozone (12) to at least a part of a flow of water (11) from an aquaculture plant, b) allowing ozone and water to be (thoroughly) mixed in a mixing zone (14)

c) allowing the mix of water and ozone a contact time sufficient to create microflocks, d) charging the mix of water and ozone (15) into a flotation tank (16),

e) creating air bubbles at the lower end of the flotation tank (16),

f) controlling a flow rate through the system which allows a desired retention time in the flotation tank (16),

g) removing floated particles (18) from the top surface of the liquid in the flotation tank (16), and

h) discharging purified water (21) from the flotation tank through a low end discharge

opening (164).

2. Method as claimed in claim 1, wherein the flow of water (21) is selected among a discharge flow of water and a recirculation flow of water.

3. Method as claimed in claim 1, wherein the mixing zone (14) comprises at least one tank with a substantially vertically oriented liquid flow therethrough.

4. Method as claimed in claim 3, wherein the mixing zone (14) comprises at least two tanks (142, 143) with a substantially vertically oriented liquid flow therethrough.

5. Method as claimed in any one of the preceding claims, wherein the creation of bubbles is made by means of static mixers at inlet openings (171-174).

6. Method as claimed in any one of the preceding claims, wherein the charge of the mix of water and ozone (15) to the flotation tank (16) is made through static mixers at the bottom wall of the flotation tank (16) to a central region (16c) thereof.

7. Method as claimed in any one of claims 1-5, wherein the creation of air bubbles is made by means of static mixers arranged in a separate recirculation loop (17) at the bottom of the flotation tank (16).

8. Method as claimed in any one of the preceding claims, wherein the flow rate is adjusted until steady state conditions, as decided by visual inspection, are achieved in the flotation tank (16).

9. Method as claimed in any one of the preceding claims, using a flotation tank (16) with a perforated plate (163) arranged over the at least one discharge opening (164).

10. Method as claimed in any one of the preceding claims, wherein the flow of purified water (21) discharged from the flotation tank (16) is recycled for further use in an aquaculture plant.

11. Method as claimed in any one of the preceding claims, using a flotation tank (16) with a central region (16c) into which the mix of water, ozone and air is introduced in an upwards flow direction and a peripheral region (16p) separated therefrom by a partition wall (162) extending from the bottom wall to a level below the top wall, in which the purified water flows in a downwards direction towards at least one discharge opening (164) located at the bottom wall in the peripheral region (16p) of the flotation tank (16).

12. Apparatus for the purification of water from an aquaculture plant, comprising:

a conduit for receiving a flow of water (11) from an aquaculture plant,

means for adding ozone (12) to water received,

a mixing zone (14) for allowing added ozone to mix thoroughly with the water,

a flotation tank (16),

a pump () for adding air bubbles to the mix (15) of water and ozone,

means for charging the mix of water, ozone and air bubbles to the flotation tank,

means for removing impurities from the top liquid surface in the flotation tank,

conduits for discharging purified water (21) from the flotation tank (16).

13. Apparatus as claimed in claim 12, wherein the means for adding ozone to the flow of water comprises a pressurized gas bottle provided with an adjustable reduction valve.

14. Apparatus as claimed in claim 12 or 13, wherein the pump for adding air bubbles to the mix of water and ozone is a multiphase pumps.

15. Apparatus as claimed in any one of claims 12-14, wherein the means for creating air bubbles at the lower end of the flotation tank (16) is comprised by static mixers at the inlet openings (171-174).

16. Apparatus as claimed in any one of claims 12-15, wherein the static mixers are present in a number from two to four.

17. Apparatus as claimed in claim 15 or 16, wherein the static mixers are also used for charging the mix of water and ozone (15) to the flotation tank (16).

18. Apparatus as claimed in any one of claims 12-14, the flotation tank furthermore comprising a cylindrical partition wall (162) extending vertically from the inner bottom wall of the flotation tank, dividing the lower part of the flotation tank (16) into a central region (16c) and a peripheral region (16p).

19. Apparatus as claimed in claims 18, the flotation tank (16) furthermore comprising a perforated plate (163) in the lower part of the peripheral region (16p).