Sludges of a recirculating aquaculture system: Characteristics and reutilization
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Aquaculture activity is responsible for the production mainly of fish, seaweed, crustaceans, mollusks, amphibians and reptiles both freshwater, brackish and saltwater of high commercial value and/or ecological importance. Its main target is the food industry, also having participation in the cosmetics industry, leather, pet, artificial feed manufacturing and restocking. Systems with high production rates, such as raceways, recirculating aquaculture systems (RAS) or bioflocs - zero discharge are projects planned where proper disposal of waste both liquid and solid resulting from production has priority because they have to comply with legislation on release and disposal of them. Another form of cultivation is floating cages, where a certain amount of unconsumed portion accumulates at the bottom of lakes or dams decreasing the depth of the water bodies. These factors cause great socio-economic and environmental impact for all those who benefit from aquaculture. Recirculating aquaculture systems are currently environmentally sustainable alternative for the production of high quality protein due to greater environmental control. A disadvantage is the production of sludge with high contents of nitrogen and phosphorus, leading to deterioration of receiving water bodies to be highly harmful and responsible for the loss of quality due to eutrophication processes. Exogenous feeding composed of proteins, carbohydrates, lipids, vitamins and minerals, different for each species and phase cultivated in order to meet the different volumes required by the market is necessary. The percentage of feed supplied biomass quantity to organisms in culture ranges from 1% to 15% depending on age and water temperature. Sludge will be generated from both extensive systems (<4 kg of biomass/m2) and relatively small areas, but with yields that exceed 100 kg of biomass / m3, called super-intensive systems. The food whether in pellet, extruded or flaked, is consumed and utilized by organisms in culture, and the part that is not utilized undergoes processes of disaggregation that will generate settable and suspense material, resulting in sludge with loads of nitrogen compounds, phosphorus, biological oxygen damne (BOD), and chemical oxygen demand (COD). It has been shown that the sludge of aquaculture can be used in farming as organic fertilizers, vermicomposting, but can also be an alternative source of energy. By knowing the characteristics and composition of the sludge when subjected to first stabilization, composting will not present health risk. Also, pyrolysis for biochar production accompanied by thermogravimetric and kinetic analysis could be conducted to verify the feasibility of using such wastes as an alternative fuel that previously could not be exploited and were wasted. Another option is to generate biogas by anaerobic decomposition.