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

Different kinds of natural and artificial filter media are able to retain phosphorus in the constructed wetlands. Due to the fact that the constructed wetland needs huge amounts of the filter media, it is very important to find locally available material which distinguishes itself by its ability to retain phosphorus. The materials found in Lithuania were considered and dolomite was chosen. Two dolomite fractions, dolomite powder (1–2 mm) and dolomite chippings (2–5 mm), and sand media were used in the laboratory- scale installed for the comparative experiments. The laboratory-scale with dolomite as the filter media was on average by 21% more efficient in total phosphorus removal in comparison with the sand media. Based on the laboratory research pilot–scale vertical flow constructed wetland of 160 m2 was installed and planted with reed Phragmites australis. The dolomite chippings as filter media were chosen in order to avoid the danger of the clogging of constructed wetland. Efficiency of total phosphorus removal in the pilot-scale vertical flow constructed wetland was on average 95.7%, phosphates removal – 94.8% within one year.

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.