A Recirculating Aquaculture System (RAS) is a closed-loop system in which culture water is continuously purified and reused. This system efficiently removes or converts waste products, such as solid waste, ammonium, and CO2, into non-toxic forms using various components. The purified water is reoxygenated and returned to the fish tanks, minimizing water and energy consumption. While RAS can reuse up to 90% or more of culture water, certain non-degradable waste products must be removed, and evaporated water replaced to maintain balance.
Advantages of RAS
- Fully controlled environment for fish.
- Minimal water usage.
- Efficient energy utilization.
- Optimal land use.
- Effective feeding strategies.
- Simplified fish grading and harvesting.
- Enhanced disease control.
Constraints of RAS
To ensure successful RAS operation, certain requirements must be met:
- Uninterrupted electricity supply (24/7).
- Access to a reliable water source, such as a borehole.
- High-quality fish feed with high protein and fat content.
- Technically skilled staff trained to handle medium-tech systems.
RAS is particularly suitable for urban or peri-urban locations with stable electricity. It is also ideal for farming tropical fish species indoors in temperate or cold climates.
Basic Components of RAS
- Fish Tank: Holds the fish being cultured.
- Mechanical Filter: Removes suspended solids.
- Biological Filter: Converts toxic ammonia and nitrite into less harmful nitrate.
- Pump Tank: Collects water for recirculation.
- Pump: Drives water through the system.
- Additional Components: UV-C light, oxygenation devices, aeration devices, feeders, and monitoring equipment.
Mechanical Filtration in RAS
Sedimentation Filter
- Removes non-soluble solid particles using gravity.
- The sedimentation tank includes polypropylene filter packs for trapping solids, which settle as silt.
- Up to 60% of nitrates are denitrified into nitrogen gas by bacteria in the silt.
- Regular cleaning is required, with waste drained into the farm’s wastewater system.
Drum Filter
- Continuously and automatically removes suspended solids.
- Works by filtering system water through a screen; clogged screens trigger an automatic flushing mechanism.
- Requires consistent electricity for operation.
Biological Filtration in RAS
Process of Ammonium Conversion
- Fish produce ammonium (NH4) as a byproduct of protein digestion, which is toxic at high levels.
- Specific bacteria convert ammonium into nitrite (NO2) and then into nitrate (NO3).
- Nitrate is relatively harmless but must still be managed for long-term water quality.
Types of Biological Filters
- Biotower (Trickling Tower/Degassing Tower):
- Contains polypropylene net filter blocks to support bacterial growth.
- Removes dissolved gases like CO2 when used as a degassing tower.
- Moving Bed Filter:
- Features filter beads with nitrifying bacteria biofilms.
- Provides high filtration capacity in a compact space.
Additional RAS Components
- UV-C Light:
- Emits electromagnetic radiation (200-280 nanometers) to kill bacteria, viruses, and fungi.
- Prevents excessive microbial growth and keeps system water clear.
- Aeration and Oxygenation Devices:
- Maintain optimal dissolved oxygen levels.
- Monitoring Systems:
- Ensure continuous tracking of water quality parameters.
- Feeders:
- Automate feeding schedules for efficiency and precision.
Flow Chart of RAS Process
Water flows from the fish tank to the mechanical filter.
Suspended solids are removed via sedimentation or drum filters.
Water passes through the biological filter to convert harmful substances.
Purified water is reoxygenated and pumped back into the fish tank.
Recirculating Aquaculture Systems represent a sustainable and efficient method of aquaculture, ensuring high productivity with minimal resource use. By understanding and optimizing each system component, operators can achieve superior fish health, water quality, and overall system efficiency.
