WO2022005297A1

WO2022005297A1 - Method of lowering concentrations of strong oxidants in wastewater from aquaculture

Info

Publication number: WO2022005297A1
Application number: PCT/NO2021/050154
Authority: WO
Inventors: Pernilla Marianne CARLSSON; Øyvind A. GARMO; Carlos ESCUDERO; Muhammad Umar
Original assignee: Niva
Priority date: 2020-07-01
Filing date: 2021-06-25
Publication date: 2022-01-06
Also published as: NO346187B1; NO20200773A1

Abstract

The present invention relates to a method of lowering high concentration of strong oxidants such as halogens in oxidation state 0 or higher, peroxides, organic peroxides, persulphates or ozone in wastewater from aquaculture using an agent selected from sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, in combination with UV light treatment. In one embodiment, the method of the invention relates to neutralizing high concentration of hydrogen peroxide (H₂O₂) in aquaculture delousing wastewater using an agent mentioned above in combination with UV light treatment.

Description

Field of the invention METHOD OF LOWERING CONCENTRATIONS OF STRONG OXIDANTS IN WASTEWATER FROM AQUACULTURE

The present invention relates to a method of lowering high concentrations of strong oxidants in wastewater from aquaculture before discharge to aquatic recipient. In one embodiment, the method relates to neutralizing hydrogen peroxide (H2O2) in aquaculture delousing wastewater.

Background of the invention Hydrogen peroxide, H2O2_, is a strong oxidant widely used in aquaculture as a delousing agent, i.e., for controlling sea lice and amoebic gill disease in fish. It causes release of large quantities of hydrogen peroxide into the environment after a delousing event, which is unfortunate to vulnerable species such as shrimps and zooplankton. Commercially made hydrogen peroxide solutions (e.g. Paramove® by Solvay and Nemona by AkzoNobel & Performance Chemicals AB, and Aperix Vet by Evonik Resource Efficiency GmbH) are not pure H2O2, but mixtures stabilized by other compounds. They are sold in a 49.5% concentration for veterinary treatments and distributed in large amounts. The pharmaceutical dose recommended by Norwegian Medicines Control Authority guideline is 1.3-1.7 g/L for 20 minutes. The annual usage in Norway varied between 4 000 - 43 000 tonnes treatment solution in 2015-2019 per year. Treatment of one single pen with a well boat may need 2-4 treatments which has been estimated to be around 1 900-3 800 m³ of medical hydrogen peroxide if a well boat is used. For treatments in the pen itself, the estimate is around 15 000 m³ for treatment of one pen (Refseth et ak, 2019).

At first, it was believed that commercial H2O2 would degrade into water and oxygen very quickly. However, experiments and modelling have proven the opposite and also showed that several non-target species are sensitive to very low concentrations (Bechmann et ak, 2019; Refseth et ak, 2019).

The predicted no effect concentration (PNEC) level in the marine environment is 11 000 times dilution of the treatment solution, i.e. a reduction from 1500 mg/L down to 0.14 mg/L to reach a level that can be considered safe for the marine ecosystem (Refseth et ak, 2019). Laboratory experiments have shown high mortality on shrimps at low concentrations, i.e. 500-1000 times dilution of the treatment concentration (Bechmann et ak, 2019; Frantzen et ak, 2019; Refseth et ak, 2019).

The delousing event takes place either straight into the fish pen that for the purpose has been sheltered by a tarpaulin during the treatment, or by moving the fish into a well boat where the fish is bathed with H2O2. Once the treatment in the pen is finished, the tarpaulin is removed and the H2O2 flushed out from the pen. When using a well boat, the water is flushed out from the boat at the end of the treatment time, while the fish is kept onboard until they can be transferred back to the pen. Today, delousing wastewater are released into the ocean without any treatment that removes the chemicals used. Treatment with bath chemicals such as hydrogen peroxide has to be performed more than 500 meters from known shrimp areas and cod spawning areas. If an aquaculture plant is situated within such area, any bath treatment should be done by well boat and discharge of the water has to be conducted outside shrimp and spawning areas. These areas are determined by the Directorate of Fisheries (The Aquaculture Act, Regulation relative to the operation of aquaculture facilities (Section 15)). Nevertheless, in some of the aquaculture areas, sites for discharge of bath delousing chemicals might be small and used by many companies within a short time range, which may add an unnecessary risk to the environment. Concentrations between 1-10 mg/L of hydrogen peroxide after a delousing event can occur kilometres away from the deloused pen, and put vulnerable species such as shrimps and zooplankton in risk. A well boat reduces the horizontal spreading but hydrogen peroxide can still sink quickly in the water mass after a well boat discharge and be present in concentrations well above 0.14 mg/L, which is the PNEC level for the marine environment. Which hydrogen peroxide concentrations that are actually present after a discharge will of course depend on amount released and oceanographic parameters such as depth and currents (Refseth et ah, 2019).

Considering the high concentrations of the chemical used and large water volumes involved, the removal of residual H_2Q2 after treatment is important for a sustainable development of aquaculture industry. As of today, there are no known solutions in place for treatment of delousing wastewater.

However, H₂O₂ is less persistent in the environment and resistance problems among sea lice are lower compared to other delousing chemicals. Therefore, IT₂O₂ is, despite its drawbacks, considered as one of the best treatments where fish health, sea lice resistance and environmental impact can be balanced.

Hence, a tool where H₂O₂ can continue to be used as a delousing agent, but at the same time protecting the surrounding environment is highly sought after from government, aquaculture, fishing industry and the general public.

Another related application of strong oxidants in aquaculture is disinfection of well boats and service vessels between operations. For this purpose, disinfection agents containing strong oxidants such as ozone, peracetic acid, hydrogen peroxide or reactive chlorine are in use, and the volumes can be large (> 1000 m³). After use, the disinfection wastewater is discharged to sea. Chlorine and reaction products from addition of strong oxidants to seawater (here called produced oxidants) are toxic to marine life even in low concentrations. Guideline values that should not be exceeded in order to be protective of 90-99 % of species, are in the range of 2.2-13 g/L (Batley & Simpson. 2020). Ozonation of seawater will produce several of the same oxidants as chlorination. Large volumes of disinfection water should ideally not be discharged without first minimising the concentration of strong oxidant.

Removal of large amounts of residual oxidants from disinfection water, and H₂O₂ at such high concentrations as present in delousing wastewater is challenging.

A reducing agent could be considered as a potential solution for removal of H₂O₂ and other strong oxidants. Ideally, the reducing agent should react selectively and quickly with H₂O₂ in a way that does not have a large effect on seawater pH or produce toxic biproducts. Sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur (including sulphites, metabisulphites and thiosulphates) will reduce H₂O₂ effectively.

For example, sodium sulphite (Na₂S0₃) can be used to remove H₂O₂, but has to be used in very high concentrations (1 : 1 molar ratio) to neutralize all the H₂O_2. However, high concentrations of Na₂SC>₃ or another reducing agent as mentioned above deplete the seawater from oxygen, which of course is undesirable.

UV light is in use against unwanted organisms, e.g. in cleaning of ballast water, sewage, etc. UV irradiation is a well-established technology, which is widely used in drinking water and wastewater treatment. While UV alone is widely used to inactivate pathogens including bacteria and viruses in drinking water disinfection, it has wide-ranging applications for the degradation of organics in different water matrices in the presence of oxidants such as H₂O_2. H₂O₂ is added in one of the best commercially applicable UV- based advanced oxidation process (AOP) as a part of the wastewater treatment.

UV irradiation photolyzes H₂O₂ leading to the generation of hydroxyl radicals (-OH) which can non-selectively oxidize a range of organic and inorganic compounds. Considerably lower levels of H₂O₂ compared to delousing treatments may be added to e.g. a sewage wastewater matrix where UV light can photolyze it into -OH radicals.

Photolysis of high concentrations of H₂O₂ would require longer treatment time as well as high doses of UV radiation, which would incur significant electrical energy demand for operating the UV system making the process prohibitively expensive and unsuitable for treatment of delousing wastewater.

Now, it has surprisingly been found that by adding an agent selected from sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates, in low concentration to the delousing wastewater, the time and dose of UV light needed to photolyze and thereby neutralize/remove the high concentrations of H₂O₂ therein are significantly reduced. Correspondingly, good results have also been found by treating water with high concentrations of the strong oxidant sodium hypochlorite in a similar way.

On this basis, the inventors have managed to develop a method for lowering or even removing strong oxidants, including H2O2, in wastewater from aquaculture and delousing wastewater by use of minimal amounts of chemical and UV demand.

Summary of the invention It is a main object of the present invention to provide a sustainable solution for delousing of salmon fish farms. That is, to provide an environmental friendly method where hydrogen peroxide in delousing wastewater is removed or greatly reduced, without being too expensive and chemical intensive, and without deoxygenation of the surrounding water.

Another object of the present invention is to provide a solution to the problem with release of large quantities of strong oxidants such as hydrogen peroxide, chlorine or other residual oxidants into the environment after a delousing or disinfection event in the aquaculture industry.

These and other objects are obtained by the method as defined in the accompanying claims.

Detailed description of the invention

The present invention provides a method of lowering concentration of strong oxidants comprising halogens in oxidation state 0 or higher, peroxides, organic peroxides, persulphates or ozone in wastewater from aquaculture, wherein the method comprises addition of at least one agent selected from the group consisting of sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates to wastewater and subsequent UV light treatment, wherein the amount of said agent is in the range from 0.1 to 0.9 stoichiometric equivalents of the strong oxidant. In a preferred embodiment of the invention, the agent is selected from sulphur dioxide (SO2); alkali salts of sulphite (alkali metakSCb), bisulphite (alkali metakHSCb), metabisulphite (alkali metabS205) and thiosulphate (alkali metabS203); and alkaline earth salts of sulphite (alkaline earth metalSCb), bisulphite (alkaline earth metalHSCb), metabisulphite (alkaline earth meta^Cb) and thiosulphate (alkaline earth meta^Cb); or any mixtures thereof. These agents may be distributed directly as powder, dissolved in a base such as e.g. potassium hydroxide (KOH) and/or bound by a chelator such as e.g. ethyl enediaminetetraacetic acid (EDTA). Use of distribution pathways where KOH and/or EDTA is added together with the agent may facilitate avoidance of sudden drops of pH and/or oxygen.

In a more preferred embodiment of the invention, the agent is selected from alkali salts of sulphite (alkali metabSO,), bisulphite (alkali metabHSO,), metabisulphite (alkali metabSiCb) and thiosulphate (alkali metabS₂0,).

In an even more preferred embodiment of the invention, the agent is selected from Na₂S0₃, K2SO3, Na₂HS0₃, K2HSO3, Na₂S₂0₅, K2S2O5, Na S C , and K2S2O3.

Most preferably, the agent is Na₂SO, or Na S O

The said agents enhance the neutralization reaction of H₂O₂ and are typically reducing agents.

In one embodiment, the method of the invention provides lowering concentration of hydrogen peroxide (H₂O₂) in aquaculture delousing wastewater, comprising addition of at least one agent selected from the group consisting of sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates to delousing wastewater and subsequent UV light treatment, wherein the amount of said agent is in the range from 0.1 to 0.9 stoichiometric equivalents of H₂O_2. Preferably, the concentration of H₂O₂ is virtually removed, i.e. neutralized.

The net reaction between H₂O₂ and e.g. Na₂S0 is as follows:

Na₂S0 (aq) + H2O2 (aq) H2O (1) + Na SCb (aq)

UV irradiation of H₂O₂ produces hydroxyl radical:

H₂O₂+ hv — _* 2HO

The hydroxyl radicals are very reactive, and react further to create water and oxygen as endpoint:

2H₂O₂+ hv — _* 4HO => 2 H₂O + O₂

By the present method, the time required for decomposition of H₂O₂ by a combination of the three reactions above is shortened by addition of less than stoichiometric equivalent mass of reducing agent prior to UV irradiation. This allows rapid removal of H202 while maintaining levels of dissolved oxygen. That is, H₂O₂ is neutralized into H₂O and O₂ according to this overall neutralisation reaction:

2 H2O2 2 H2O + 0₂

Chlorine or other strong oxidants added or produced after addition of strong oxidant used in aquaculture delousing, are reduced by the same reducing agents as H2Q2. Furthermore, both hypochlorite, inorganic chloramines, hypobromites and bromoamines are photolysed by UV light, speeding up their degradation (Li & Blatehley 2009, Watts & Linden, 2007). The method described for H2O2 can therefore also be used for the removal of high amounts of strong oxidants formed by chlorinating, ozonating or addition of other strong oxidants to water for disinfection.

In one embodiment of the invention, the amount of agent added is from 0.1 to 0.9 stoichiometric equivalents of the strong oxidant, preferably from 0.1 to 0.5 stoichiometric equivalents of the strong oxidant, more preferably from 0.1 to 0.35 stoichiometric equivalents of the strong oxidant the strong oxidant, and most preferably from 0.1 to 0.25 stoichiometric equivalents of the strong oxidant. In the embodiment of the invention where H2O2 is the strong oxidant, the amount of agent added is from 0.1 to 0.9 stoichiometric equivalents of H2O2, preferably from 0.1 to 0.5 stoichiometric equivalents of H2O2, more preferably from 0.1 to 0.35 stoichiometric equivalents of H2O2, and most preferably from 0.1 to 0.25 stoichiometric equivalents of H2O2.

In one embodiment of the invention, the UV treatment lasts/takes place in a range of 30 to 720 minutes, preferably from 60 to 480 minutes, and most preferably from 60 to 360 minutes. Traditionally, UV light techniques of mostly 254 nm (using mercury lamps) have been used to generate oxidative species. However, UV-LEDs provide much more flexibility and other wavelengths can be explored considering the absorption spectra of the strong oxidants, e.g. H2O2. UV-LEDs provide an opportunity to use other wavelengths including those in the low UVC range. Thus, UV light of wavelengths from low UVC (100-280 nm) to UVB (280-315 nm) range is included in the present invention since the rate of photolysis of strong oxidant such as hydrogen peroxide ([^Chj_o > 20 mM) follows zero order kinetics with regard to the quantum yield and intensity. Furthermore, the absorbance of UV by strong oxidant such as H2O2 differs depending on the water quality and also for that reason wavelengths other than 254 nm are possible within this invention. UV wavelength of 254 nm is preferred considering its practical applicability but other wavelengths and their combinations using UV-LEDs could also be used for enhanced photolysis performance. In one embodiment of the invention, UV light of wavelengths from low UVC (100-280 nm) to UVB (280-315 nm) is used. Preferably, UV light of wavelengths from 185 nm to 315 nm, and more preferably from 200 nm to 300 nm, is used. Examples of preferred wavelengths are 210 nm, 254 nm, 265nm and 280nm.

In one embodiment of the invention, the method is carried out in well boat. In this embodiment related to delousing, an amount of H2O2 is added and adjusted to the recommended delousing treatment dose. Oxygen is continuously added to the water wherein the fish is contained during H2O2 treatment since the water volume is very small compared to the amount of fish and their biological oxygen demand. Once the delousing treatment is finished (H2O2 during maximum 20 minutes), the water with H2O2 is removed from the fish and treated with at least one agent as defined above and UV light before the wastewater is flushed out from the well boat.

In another embodiment of the invention, the method takes place straight into the sea. In this embodiment, the fish pen is sheltered by a tarpaulin or optionally the fish is moved to a mobile treatment pen during the delousing treatment. Thereafter, the wastewater is released from the pen and filled into a moveable frame structure outside the pen where at least one agent as defined above is added followed by UV light treatment to neutralize/remove H2O2 before the wastewater is flushed out in the open sea.

The method of the present invention can be applied in environmental relevant conditions, i.e. sea temperatures <18°C, pH ranging 6.5-8.5 and saline water, i.e. 9-35 PSU.

The present invention also provides use of at least one agent selected from the group consisting of sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates in combination with UV light treatment in lowering concentration of strong oxidants comprising halogens in oxidation state 0 or higher, peroxides, organic peroxides, persulphates or ozone in wastewater from aquaculture.

In one embodiment, the present invention provides use of at least one agent selected from the group consisting of sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates in combination with UV light treatment in neutralization of hydrogen peroxide (H202) in aquaculture delousing wastewater.

The invention is explained in more detail in the examples below. The examples are only meant to be illustrative and shall not be considered as limiting. Example 1

Experiments carried out in lab-scale have shown that using NaiSCE is an effective way of reducing the UV doses required for the neutralization of H2O2. The experiments were performed at room temperature and H2O2 measured straight after adding the Na₂SC>_3. As shown in Fig. 1, the higher the molar concentration of Na₂SC>₃, the higher the concentration reduction of H2O2. However, since the purpose was to use minimum concentration of Na₂SC>₃, further tests using UV were carried out using 3 different molar ratios for the purpose of comparison (Fig. 2).

This experiment was carried out with a UV lamp of 15 W (output power: 3.5 W at 254 nm) in a UV collimated beam system. A stir bar of 25.4 x 7.6 mm was used to ensure sample mixing (330 rpm) during irradiation. The experiment was set up with continuous flow.

Fig. 2 shows the kinetics of decay in the concentration of H2O2 when using different H₂0₂:Na₂S0₃ molar ratios followed by UV treatment (254 nm) up to 360 min. Due to initial higher removal of H2O2 at the molar ratio of 1 :0.5, the greatest reduction was achieved after 300 min when compared with lower molar ratios (1 :0.25 and 1 :0.16). However, there was very little difference between the lower molar ratios tested throughout the UV treatment process with final reduction values being fairly similar. Although a higher UV irradiation time was needed when using lower concentration of Na₂SC>₃, a trade-off between the higher use of Na₂SC>₃ and resulting deoxygenation vs. UV treatment time had to be considered. The difference in the level of H2O2 removal was comparable for both 1 :0.25 and 1:0.16 molar ratios.

Example 2 Neutralisation of H2O2 with sodium metabisulphite (Na₂S₂0s) in a 1 :0.5 molar ratio, 45 L water tank was performed (Fig. 3). Two measurements (bottom; 2.13 g/L) and surface (1.44 g/L) were done before the experiment began. This difference was evened out due to constant stirring of the water and controlled measurements from both surface and bottom throughout the experiment. The pH sank quickly throughout the experiment. Oxygen concentrations were measured in a later experiment, where oxygen was quickly depleted. Another experiment where Na₂S₂0s was dissolved in seawater and potassium hydroxide (KOH, 10% w/w) was conducted. The pH was kept acceptable (decrease from pH=7.6 at to to pH=6.7 at t3o (min) but the oxygen was depleted from 8.6 mg/L to 0.73 mg/L within the same time period. H2O2 was reduced from initial concentration (1.5 g/L) to 0.8 g/L within 5 min. Example 3

Removal of total residual oxidant using UV irradiation with and without sodium sulphite were studied. The oxidant studied in this example is sodium hypochlorite.

Two molar ratios were investigated for determination of total residual oxidant (i.e. sodium hypochlorite) upon addition of sodium sulphite, i.e., 1:0.5 and 1:1.5 molar ratio of chlorine (initial concentration of 1000 mg/L) and sodium sulphite. Higher ratio of sodium sulfite (1:1.15 molar) in relation of chlorine (1 molar) gave -70% reduction of total oxidant whereas irradiation with UV gave a total reduction of 90% after 1 h irradiation. A reduction in oxygen was observed when sodium sulfite was added but it remained above 7 mg/L which is considered safe for fish welfare and growth. The oxidation reduction potential (ORP) reduced from initial value of 615 mV to 90 mV.

Further experiments were carried out using 1 :0.5 molar ratio of chlorine and sodium sulphite (Fig. 4). A significant reduction of total oxidants was observed when sodium sulphite was combined with UV irradiation such that 93% reduction was noted after 2 h of irradiation whereas it took 3.5 h of UV irradiation using UV alone. A synergy was noted when UV was combined with sodium sulphite during first 2 h of UV irradiation (Fig. 5). A reduction of ORP was noted from initial value of 677 mV to 110 mV after addition of sodium sulphite. No reduction in oxygen was noted whereas a reduction in ORP was noted from initial value of 677 mV to 110 mV after addition of sodium sulfite.

The present invention is based on a combination of two techniques (chemical agent and UV light) to minimise the chemical usage and at the same time speed up the process and limit the power usage of UV light. An agent selected from the group consisting of sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates used alone reduces oxygen in the sea. UV light alone would be inefficient in large commercial scale applications. The inventors have found that in combination, these two techniques emphasize and aid each other in a way that was not expect on forehand, particularly for the high H2O2 concentrations used in aquaculture delousing treatments.

The method of the present invention, comprising the combination of an aforementioned agent and UV in a synergistic way speeds up the lowering of concentration of strong oxidants and the neutralization process of H2O2 more than if only UV light is used. Hence, the chemical usage is kept as low as possible, providing environmental and economic benefits. The procedure developed here maximizes the animal welfare, since avoids an unnecessary reduction of dissolved oxygen provoked by sulphur dioxide and/or easily dissolvable salts of reduced oxyanions such as sulphur sodium sulphite. References

Bechmann, R.K., Arnberg, M., Gomiero, A., Westerlund, S., Lyng, E., Berry, M., Agustsson, T., Jager, T., Burridge, L.E., 2019. Gill damage and delayed mortality of Northern shrimp (Pandalus borealis) after short time exposure to anti-parasitic veterinary medicine containing hydrogen peroxide. Ecotoxicology and Environmental Safety 180, 473-482. https://doi.org/10.1016/1.ecoenv.2019.05.045

Frantzen, M., Evenset, A., Bytingsvik, J., Reinardy, EL, Tassara, L., Geraudie, P., Watts, E.J., Andrade, EL, Torske, L., Refseth, G.H., 2019. Effects of hydrogen peroxide, azamethiphos and deltamethrin on egg-carrying shrimp (Pandalus borealis) (FHF-report No. Akvaplan-niva report 8926-1). Akvaplan-niva, Tromso.

Refseth, G.H., Nost, O.A., Evenset, A., Tassara, L., Espenes, EL, Drivdal, M.,

Augustine, S., Samuelsen, O., Agnalt, A.-L., 2019. Risk assessment and risk reducing measures for discharges of hydrogen peroxide (H2O2). Ecotoxicological tests, modelling and SSD curve. Oceanographic modelling. (No. Akvaplan-niva report 8948-1). Akvaplan-niva.

Batley, G.E., Simpson, S.L., 2020. Short-Term Guideline Values for Chlorine in Marine Waters. Environmental Toxicology and Chemistry 39, 754-764.

Li, J., Blatchley III, E.R., 2009. UV Photodegradation of Inorganic Chloramines. Environ. Sci. Technol. 43, 60-65. Watts, M.J., Linden, K.G., 2007. Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water. Water Research 41, 2871-2878.

Claims

C l a i m s

1

A method of lowering concentration of strong oxidants comprising halogens in oxidation state 0 or higher, peroxides, organic peroxides, persulphates or ozone in wastewater from aquaculture, wherein the method comprises addition of at least one agent selected from the group consisting of sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates to wastewater and subsequent UV light treatment, wherein the amount of said agent is in the range from 0.1 to 0.9 stoichiometric equivalents of the strong oxidant.

2

The method of according to claim 1, wherein the strong antioxidant is hydrogen peroxide (H2O2) and the wastewater is delousing wastewater, wherein the method comprises addition of at least one agent selected from the group consisting of sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates to delousing wastewater and subsequent UV light treatment, wherein the amount of said agent is in the range from 0.1 to 0.9 stoichiometric equivalents of H2O2.

3.

The method according to claim 2, wherein hydrogen peroxide (H2O2) in aquaculture delousing wastewater is neutralized.

4.

The method according to any one of claims 1 to 3, wherein the agent is sulphur dioxide; an easily dissolvable salts of reduced oxyanions of sulphur selected from alkali salts of sulphite (alkali metabSCb), bisulphite (alkali metabHSCb), metabisulphite (alkali metabS₂0₅) and thiosulphate (alkali metabS₂0₃) and alkaline earth salts of sulphite (alkaline earth metalSCb), bisulphite (alkaline earth metalHSCb), metabisulphite (alkaline earth meta^Cb) and thiosulphate (alkaline earth metalS₂0_3); or a mixture thereof.

5. The method according to any one of claims 1 to 4, wherein the agent is Na2SCb, K2SO3, Na2HS03, K2HSO3, Na₂S205, K₂S205, Na2S2Cb, K2S2O3, or a mixture thereof.

6

The method according to any one of claims 1 to 5, wherein the amount of agent is in the range from 0.1 to 0.5 stoichiometric equivalents of the strong oxidant, preferably from 0.1 to 0.35 stoichiometric equivalents of the strong oxidant, and most preferably from 0.1 to 0.25 stoichiometric equivalents of the strong oxidant.

7.

The method according to any one of claims 1 to 6, wherein the UV treatment lasts/takes place in a range of 30 to 720 minutes, preferably from 60 to 480 minutes, and most preferably from 60 to 360 minutes.

8

The method according to any one of claims 1 to 7, wherein UV light of wavelengths from low UVC (100-280 nm) to UVB (280-315 nm) is used.

9.

The method according to any one of claims 1 to 8, wherein UV light of wavelengths from 185 nm to 315 nm, and preferably from 200 nm to 300 nm, is used.

10.

The method according to any one of claims 1 to 9, wherein UV light of 210 nm, 254 nm, 265 nm or 280 nm wavelength is used.

11 The method according to any one of claims 1 to 10, wherein it is carried out in wellboat or other infrastructure used for delousing fishes in aquaculture.

12

The method according to any one of claims 1 to 10, wherein it takes place straight into the sea in connection with the fish pen.

13.

Use of an agent selected from the group consisting of sulphur dioxide and easily dissolvable salts of reduced oxyanions of sulphur, including sulphites, metabisulphites and thiosulphates, in combination with UV light treatment in lowering concentration of strong oxidants comprising halogens in oxidation state 0 or higher, peroxides, organic peroxides, persulphates or ozone in wastewater from aquaculture.

14. Use according to claim 13, in neutralization of hydrogen peroxide (H2O2) in aquaculture delousing wastewater.