I. What is a recirculating aquaculture system?

Many aquaculturists have heard of ‘factory fish farming’ and ‘indoor prawn farming’, but are not yet very familiar with recirculating aquaculture systems (RAS). Put simply, recirculating aquaculture involves keeping the aquaculture water ‘circulating’: rather than being discharged directly, the water undergoes a series of treatments — including physical filtration, biological purification, disinfection and aeration — before being returned to the rearing tanks for reuse, with a recycling rate of over 90%. In a nutshell: it allows you to rear the maximum number of fish using the minimum amount of water, without polluting the environment.

II. How does RAS work? Understanding the water cycle in four steps

Step 1: Physical filtration-removing uneaten feed and faeces

Water flowing out of the rearing tank first passes through equipment such as micro-filtration units (solid-liquid separators) to filter out solid impurities such as uneaten feed and faeces; this is the first stage of water treatment.

Step 2: Biological Purification — Degrading Ammonia and Nitrite Toxins

The excrement of fish and shrimp is converted into ammonia and nitrite; high concentrations of these can cause poisoning. Beneficial microorganisms in the biological filter bed break down these toxins and convert them into less toxic nitrate — this is the principle behind the saying: ‘To rear fish, first tend to the water; to tend to the water, first nurture the bacteria’.

Step 3: Disinfection and Aeration — Killing Pathogens and Supplementing Oxygen

The purified water is then disinfected using ultraviolet light or ozone to eliminate pathogens, and aerated using equipment such as oxygen cones to ensure that dissolved oxygen levels in the water remain consistently high.

Step 4: Intelligent Monitoring — 24-hour Water Quality Surveillance

Modern recirculating aquaculture systems are equipped with online monitoring devices that continuously monitor parameters such as dissolved oxygen, pH, water temperature and ammonia-nitrogen 24 hours a day. The system automatically triggers alarms in the event of abnormalities, transforming water quality management from a ‘rule-of-thumb’ approach to a ‘data-driven’ one.

III. What is the difference between recirculating aquaculture systems and traditional earthen ponds?

Table 1: Comparison of recirculating aquaculture systems with traditional earthen ponds

Three Common Misconceptions, Explained Once and for All

Misconception 1: “If the water is recycled repeatedly, won’t it become very dirty?” — Quite the opposite: the treatment standards for recirculated water are stricter than those for natural water bodies. Solid impurities, ammonia-nitrogen and pathogens are removed through a series of stages, resulting in water quality that is more stable than that of external water sources.

Misconception 2: ‘Do fish and prawns reared in recirculating aquaculture systems contain drug residues?’ — As there are fewer diseases in a closed environment, the need for medication is significantly reduced, making it easier to achieve antibiotic-free production and traceability.

Misconception 3: “RAS is only affordable for large enterprises; small-scale farmers don’t stand a chance.” — Modern recirculating aquaculture systems are now highly modularised, allowing operations to start at a pilot scale of just a few dozen cubic metres. Rearing shrimp from larvae to the grow-out stage, ‘suspended fish’ farming and the cultivation of high-value fish species are all excellent low-cost entry points.

IV. Which species and regions are suitable for recirculating aquaculture systems?

In terms of species, the white-leg shrimp, sea bass, tilapia, mandarin fish and trout are the most widely used, with highly matured technologies; in terms of regions, aside from coastal areas, inland water-scarce regions, deserts and areas on the outskirts of cities are all suitable—seawater species can be farmed by artificially preparing seawater, which is precisely why inland ‘land-based shrimp farming’ projects have been continuously implemented in recent years.

V. Summary

Recirculating aquaculture is not merely a concept, but a mature technology repeatedly validated by projects worldwide: it saves 90 per cent of water, increases stocking density by 5–10 times, enables year-round production, and is antibiotic-free and traceable. As equipment becomes increasingly modular and intelligent, the barriers to entry are falling year on year. If you are considering upgrading your aquaculture operations, it is well worth carefully evaluating a recirculating aquaculture solution.

VI. Contact Us

Would you like to learn more about recirculating aquaculture systems, or do you require a free project feasibility assessment? Please feel free to contact the WOLIZE technical team; we can provide professional advice tailored to your species, scale and site conditions.

Figure 1: Factory-based recirculating aquaculture facility