Sortkjær, O., Henriksen, N.H., Heinecke, R.D. & Pedersen, L-F. 2008: Optimering af behand-lingseffekten i akvakultur. Minimering af forbrug og udledning af hjælpestoffer. Danmarks Miljø-undersøgelser, Aarhus Universitet. 124 s. – Faglig rapport fra DMU nr. 659.
Summary
The aim of the project is to minimize the consumption and the discharge of chemicals used in aquaculture. Toxicity tests were made in the laboratory to show at which concentration it is possible to kill parasites and how the duration of the treatment influences mortality.
The chemicals formaldehyde and hydrogen peroxide were tested and subsequently the results were implemented in the fish farms. Gyrodactylus derjavinoides (former called G. derjavini) and theront stadium of the ciliate Ichthyophthirius multifiliis (white spot disease) were used as test organisms. Sodium perkarbonat was used as hydrogen peroxide compound. For both chemicals there was a positive relationship between the concentration and the mortality of the parasites, between the duration of treatment and mortality, and between the temperature and mortality of parasites.
I.multifiliis was more sensitive to treatment with formaldehyde at the same temperature than G. derjavinoides which needed a 5-7 times longer treatment period before the same effect was achieved, where half of the organisms died (LT50). For hydrogen peroxide, the difference was greater so I. multifiliis was 20 times more sensitive to treatment than G. derjavinoides. Corresponding differences can be expected among other parasites not tested in this investigation. At 21 oC a concentration of 8 mg/l formaldehyde was able to kill G. derjavinoides whereas the double concentration was needed at 12 oC to obtain the same effect. I. multifiliis was killed after 2.5 hours at 22 oC with a concentration of 2.6 mg/l whereas the double concentration was needed to kill the parasites at 12 oC with the same treatment period. Treatment with sodium perkarbonat killed G. derjavinoides after 8 hours with a concentration of 10.3 mg/l at 12 oC whereas it was sufficient to treat I. multifiliis for 2 hours with a concentration of 2.6 mg/l. To obtain the same effect within the same treatment period when the temperature is dropped by half, the concentration must be increased by 50 per cent.
When transferring the toxicity test to practical experiments in fish farms at a temperature of 12-14 oC and treatment duration of 4-6 hours, so the treatment could be done within a normal working day, a concentration of 15 mg formaldehyde/l and 8 mg hydrogen peroxide/l was used.
Treatment of fish ponds at low concentrations of chemicals combined with longer treatment periods makes it necessary to close the inlet and outlet of the ponds. By using an aerator, the oxygen level never falls with more than 1.5 mg/l. If the aerator is furthermore placed centrally in the pond, complete chemicals like salt, formaldehyde and sodium per carbonate can be totally mixed in the water in less than 15 minutes. By using a recirculation pump in stead of an aerator the mixing takes hours or at least double of the hydraulic retention time. It is necessary to know the volume of the water bodies in order to calculate the treatment dose. Different methods were applied, but the most precise method was to add salt to the pond, mix it and measure the increase in conductivity and compare the value with the conductivity for a known salt concentration, and then divide the added amount of salt with the concentration.
With a treatment period of 5-6 hours, a stable concentration of the chemical is required during the entire period. For formaldehyde the degradation is so slow in a traditional pond that the initially added amount of formaldehyde is sufficient for the treatment. Treatment with hydrogen peroxide is more problematic as an instantaneous decomposition takes place due to organic matter in the water and on the surface of the pond. Subsequently, a time proportional decomposition of the hydrogen peroxide left in the pond takes place. In the experiments, we used sodium per carbonate which is easily solved in water containing partly hydrogen peroxide and partly carbonate. Between one fifth and half of the added hydrogen peroxide could be lost to the atmosphere momentally, and the rest by a degradation rate reducing the concentration by 50 per cent in 40-80 minutes. In this case, it was necessary to add more sodium per carbonate regularly in order to maintain the required concentration for the treatment. Analytical test strips (Merck) were used to control the concentration on location, but normally it tested about 2 mg/l hydrogen peroxide lower than the real concentration.
In some fish ponds, the water level can even be lowered during treatment, and the total amount of chemical used for treatment can then be reduced further.
Three fish farms infected with different species of parasites were treated with formaldehyde. One farm (Mølbak) was slightly infected, another (Toudal) was moderate infected and a third (Silstrup) was heavily infected, especially with white spot disease. The ponds were treated with 12-18 mg/l formaldehyde, which is 1/5 of normal practice, and the concentration was kept for 4-6 hours. At Mølbak and Silstrup fish farms, the fish reacted by flickering after ½ -1 hour, but otherwise no negative reactions were observed during the treatment. As a result of the treatment, all parasites on the fish were killed after four hours in the slightly infected ponds at Mølbak. The two others fish farms had to be treated repeatedly up to 5-6 times because they were infected with white spot disease. For the heavily infected Silstrup fish farm, the mortality was lowered after each treatment. Longer treatment periods with reduced concentrations were as effective as the traditional treatment with high concentrations over a short period. The number of required treatments depends on the species and the level of infection in the fish and the ponds. The following parasites were affected by the treatment: Chilodonella sp., Ichthyobodo necator (former called Costia), Trichodina spp., Gyrodactulus sp. and Sessile ciliates (Apiosoma sp./Ambiphrya sp./Epistylis sp.). White spot disease (I. multifiliis ) was also affected if the treatment was repeated regularly.
Three fish farms were treated with hydrogen peroxide: Mølbak, which was slightly infected and Staulund and Toudal fish farms that were moderately infected with parasites. Hydrogen peroxide was added as sodium per carbonate, and with a high rate of decomposition it was necessary to control the concentration on site and supply with further sodium per carbonate more times. In this way, it was possible to keep the concentration between 5 and 10 mg/l hydrogen peroxide during the 5-8-hour treatment.
At Mølbak where the infection was very low, it was not possible to observe any effects on the parasites. The fish were not affected by the treatment even after 8 hours and a concentration during the final period of 8-15 mg/l. At Staulund fish farm, the hydrogen peroxide killed the following parasites: Chilodonella sp., Ichthyobodo necator (former called Costia), Trichodina spp. og Gyrodactulus sp., and the effect was already observed after two hours of treatment, although it had no effect on the white spot disease parasite on the fish. The treatment had a positive influence on the appetite of the fish and the gills were fine, but at the end of the treatment and on the following day, a tendency of bleeding from the gills was observed. At the Touldal fish farm the gills were covered with a lot of mucous and were infected with white spot disease. The treatment was repeated with an interval of a few days up to five times. After the third treatment, white spot disease parasites did no longer appear on the gills.
A single treatment with hydrogen peroxide at concentrations around 8 mg/l kills many different parasites, but further treatments are required to eliminate the white spot disease infection. In order to compare the effect of different treatments with different chemicals, six identical ponds at Toudal fish farms were treated in different ways. One pond was treated as usual with high formaldehyde concentrations over a short period of time. One pond was treated with 1/5 of the same concentration, but over a period of 4 hours. Two other ponds were treated with hydrogen peroxide like the ponds with formaldehyde and a further two ponds were treated with Virkon S. All treatments had a similar degree of effectiveness on parasites, which indicates that treatment with 1/5 of the usual concentration of formaldehyde or hydrogen peroxide is sufficient when the duration of the treatment is extended to 4-6 hours.
The hydrogen peroxide had an effect at a concentration of 8-15 mg/l with a treatment period of 4-6 hours on the following parasites: Chilodonella sp., Ichthyobodo necator (former Costia), Trichodina spp., Gyrodactulus sp. An infection with white spot disease (I. multifiliis) could be eliminated if the treatment was repeated several times.
Water recirculation aquaculture systems (RAS) are typically equipped with biofilters functioning as treatment units to reduce dissolved organic matter and nitrogenous compounds. Uses of therapeutants in RAS are often inevitable, with unintended risks of impairing biofilter functions. The project investigated use of formalin and hydrogen peroxide in terms of biodegradation and biofilter tolerance.
Experiments with formalin showed that the rate of formaldehyde biodegradation increased significantly when formalin was applied regularly. In pilot scale systems, daily addition of formaldehyde resulted in surface specific removal rates up to 25 mg?m-2?h-1. Prolonged treatment with formaldehyde concentrations of 10 and 20 ppm in closed systems did not affect the fish compared with untreated systems, neither did the biofilter performance. The results indicate that the prolonged treatments with formaldehyde at lower concentrations are an applicable alternative to existing treatment practices.
Experiments with sodium per carbonate showed that decomposition rates depended on the amount of dosage and on the water quality in terms of organic matter content. Use of sodium per carbonate subsequently resulted in a markedly increase in both pH and oxygen. The fish health or survival was not affected in the present treatment (dosage concentration from 10 to 100 g sodium-perkarbonat?m3). Some of the experiments caused temporarily impairment of the biofilter in the form of nitrite accumulation when compared with untreated systems. Sodium per carbonate degradation was found to be positively correlated to fish biomass, i.e. half of the amount of hydrogen peroxide had disappeared within five hours in systems with 75 kg biomass/m3.
PerAqua Plus (per acetic acid/hydrogen peroxide compound) was found to degrade at a similar first-order kinetic rate when applied to closed pilot scale RAS. Experiments in closed setups showed that PerAqua concentrations at 25 ml/m3 had negative effects on the nitrification of the biofilter in terms of nitrite accumulation. Applications with PerAqua in pilot scale RAS with fish caused relatively rapid decomposition of hydrogen peroxide with moderate oxygen liberation, and treatment with either 5 or 15 ml Peraqua/m3 did not impair biofilter stability. Six simulated bath treatments with PerAqua of one-hour duration (C0= 10, 20, 30, 40, 50 & 60 ml/m3) showed that the fish were negatively affected at concentrations from 30 ml/m3, whereas mortality was observed at the higher concentrations.
Experience with the use of hydrogen peroxide in RAS is limited, and due to potential associated risks it is still not used routinely. Prolonged formalin treatment at lower concentrations seems a possible alternative treatment practice to be used in RAS.
The intention was to follow the concentration in two connected raceways, and the water flow was driven by airlifts. The experiment was performed at Kongeåen fish farm. The degree of recirculation was 90 % and the water ran from the raceways to a lagoon before running into the river. 150 l of 37 % formaldehyde was added for 10 minutes, hereby creating a pulse of formaldehyde with an initial concentration of 130 mg/l. If the added formaldehyde had been mixed up in the total water body instantaneously, the theoretical concentration should be 22 mg/l. Mixing was not completed until after seven hours. The rate of decomposition was in the same magnitude as for the experimental biofilters and the formaldehyde was zero after 17 hours. To keep the same concentrations as used for the pond experiments of 15 mg/l over 4-6 hours, the treatment strategy for recirculated raceways could be to add the formaldehyde over the duration required for one circulation to achieve a better mix with the water body. A subsequent supply of formaldehyde should be added concurrently and at the same rate as the decomposition of formaldehyde happens in the biofilter, so the concentration can be kept as long as necessary to obtain an effective treatment.
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