Abstract. Just like every economic activity, aquaculture has a particular effect on environment, which manifests itself due to waste through declining quality of subterranean water and eutrophication of surrounding surface water. Less than half of the feed in aquaculture is digested and assimilated, the rest ending up as waste either solid or dissolved and the nutrients phosphorus (P) and nitrogen (N) that are derived from fish excretion, faeces, and uneaten feed. The advantage of recirculating aquaculture systems (RAS) is that their wastewater discharges are 10-100 times lower and pollutant concentration respectively 10-100 times higher and can reach the level of pollution of household waste. This makes the pollution much easier to control. Additionally, striving to ensure and improve the stability and longevity of RAS, additional technological solutions, warranting the reduction of the amount of used water, waste concentration increase and their secondary utilization as bio stock are added. This article presents the research information from the engineering perspective, summarizing RAS design sensitive data concerning aquacultural waste components excreted in RAS.

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In this article we analyse catfish rearing conditions in recirculating aquaculture systems (RAS). Based on research, performed by scientists from different countries, we evaluated optimal water parameter values for catfish rearing. African catfish growing process and with it associated factors were researched in JC ,,Šamas“. The amount of dissolved oxygen (on average 4,53 mg/l) in the rearing basin is adequate for the catfish to feel comfortable and for biomass growth. Water temperature only varies slightly – the average water temperature is 23,30C. The ideal pH for rearing fish in RAS is 7.0. Many aquaculture species continue to feel comfortable in pH boundaries of 6.5-8.5. In the samples, taken from catfish rearing system, the pH varied from 5.58 to 6.63. Such acidic water decreases the effectiveness of biological filter, but also decreases the toxic effect of excreted ammonia on the fish. Only a small amount of suspended solid is present in the samples, which means that they are being removed properly. The average amount of ammonia nitrogen in the water basins in analysed catfish rearing systems is 1,171 mg/l. The analysed water samples presented an average nitrite amount of 0,974 mg/l. The recommended amount of nitrites for catfish is less than 0.5 mg/l. The analysed samples presented relatively high nitrate values – from 412 to 495 mg/l. In order to decrease the concentration of accumulated nitrates it is required to change no less than 10% of total system water volume every day. The index of consumed biochemical oxygen corresponds to the requirements for water quality in RAS

Egg adhesiveness is one of the major problems in carp artificial breeding. The appropriate elimination of egg adhesiveness impacts the effectiveness of the breeding. This article discusses two used methods of carp egg adhesiveness removal: tannic acid+water and milk+water+salt solutions. Milk, water and salt solution was based on the following proportions: 1 litre of milk + 7 litres of water +50g of NaCl. Adhesiveness is removed from fertilized eggs in bowls by mixing them with geese feathers for no shorter than 60 minutes. The tannic acid solution was prepared by mixing 7 grams of tannic acid powder with 5 litres of water. 1 litre of solution is immediately poured into a bowl with fertilized eggs. Adhesiveness is removed from fertilized eggs in bowls by intensively mixing them for 10 minutes. After fertilization and removal of adhesiveness, the eggs were transferred into Weiss jars. The results showed that during the process of incubation, 3.7 million eggs were fertilized; out of them 1.6 million were rid of adhesiveness by milk solution, 2.1 – by tannic solution. It is plausible that unfavourable environmental conditions influenced the low egg vitality. 750 000 (47%) carp larvae hatched from milk solution treated eggs, while 800 000 (38%) of larvae hatched from the eggs treated with tannic solution.

This study was performed to evaluate use of zeolite for water quality management in cold water recirculating aquaculture system. The system was reared Arctic charr (Salvelinus alpinus) and Hybrid char (Salvelinus fontinalis × Salvelinus alpinus) fry. Fish biomass in the reservoirs was distributed as follows: Arctic charr accounted for 7.32 kg at a density of 23.74 kg•m -3 , and hybrid charr accounted for 7.78 kg at a density of 26.91 kg•m -3 . After increasing the feed rate and documenting water quality studies, a vast increase of ammonium and nitrite concentration was noticed in the rearing tanks. Natural zeolite, known as clinoptilolite with a cation exchange capacity (CEC) of 1.20-1.50 mol.•kg -1 , was used to reduce the amounts of these hazardous substances in the rearing tanks. Zeolite was used to decrease the increased concentration of ammonia and nitrites, which were reduced by 22.7% and 16.7%, respectively, during the study period. These results suggest that zeolite is suitable for water quality management in cold water recirculating aquaculture system with as little of an investment as possible.