1. Introduction and Summary

Traditional pond aquaculture is facing multiple practical constraints, including limited water and land resources, strict environmental controls on effluent discharge, frequent extreme weather events and the proliferation of exogenous pathogens. Land-based recirculating aquaculture systems (RAS) transfer aquaculture from open natural water bodies to controlled, standardised land-based facilities. By establishing a closed-loop process comprising ‘hydraulic sedimentation-physical and mechanical filtration-in-depth microbial degradation-high-efficiency oxygen dissolution-biological and spectral disinfection’, they have completely broken the century-old cycle of traditional aquaculture being ‘at the mercy of the weather’.

Within a compact and intensive land-use footprint, this system boosts the water recycling rate to over 95 per cent, requiring only 1-3 per cent of fresh water replenishment on average per day. The overall survival rate has surged from 75-80 per cent in traditional open ponds to over 95 per cent, with the aquaculture carrying capacity per cubic metre of water increased by 10 to 30 times, truly realising a win-win scenario of industrialised intensive aquaculture, full traceability and control of all water quality parameters, and year-round, off-peak marketing at premium prices.

2. Tackling the four major weaknesses of traditional aquaculture: how does RAS overcome them one by one?

With water and land resources becoming increasingly scarce, environmental protection and discharge regulations becoming the norm, stricter controls on ‘non-food crop’ use of agricultural land, frequent extreme weather events, and the difficulty of controlling exogenous diseases… traditional pond and open-channel aquaculture are entering a period of transition characterised by meagre profits and even high risks. Whilst the traditional fisheries sector remains preoccupied with concerns such as ‘algal blooms leading to pond flushing, concentrated market releases driving down prices, and cross-infection of diseases’, a group of pioneering enterprises has already moved aquaculture into modern production facilities-Land-based Recirculating Aquaculture Systems (RAS) are comprehensively reshaping the profit model of the modern aquaculture industry through industrialised standards characterised by ‘controllability, high stocking density, water conservation and year-round production’.

3. Industrial-grade closed-loop water treatment: Creating value from every drop of water–an explanation of the core process closed-loop architecture

A land-based recirculating aquaculture system (RAS) is not merely a matter of installing filtration equipment; rather, it relies on precise fluid dynamics, microbiology, and biochemical environmental control to establish a highly self-purifying, dynamically balanced closed-loop water circulation system:

[Schematic of the process flow] Integrated process architecture for closed-loop water treatment in a land-based recirculating aquaculture system (RAS)

STEP 01-Land-based dual-outlet aquaculture tanks

The design utilises circular, octagonal or rectangular tanks with rounded corners, combined with tangential water inlets at the base to create a stable swirling current. Utilising rotational hydrodynamics, fish faeces and uneaten feed are gathered within a few minutes to the central discharge outlet at the bottom of the tank; highly concentrated sludge at the bottom is rapidly discharged by gravity (removing approximately 10-15 per cent of the bottom water), whilst clean surface water overflows into the main circulation pipeline (approximately 85-90 per cent of the surface water), thereby achieving solid-liquid separation at source.

STEP 02-Precision solid-liquid separation using a drum microfiltration unit

The overflow water and the concentrated effluent from the bottom drain are combined and fed into the fully automatic drum microfiltration unit. Utilising high-quality 30-75 μm 316 L stainless steel or polyester precision filter screens, in conjunction with an intelligent level and differential pressure-sensing backwash mechanism, the system rapidly retains and removes more than 85 per cent of total suspended solids (TSS) before suspended organic debris can degrade, dissolve and release ammonia nitrogen. This significantly reduces the organic carbon load on the subsequent biological filtration tanks at source.

STEP 03-Composite moving-bed bioreactor

The water flows by gravity into the MBBR (Moving Bed Bio-Reactor). The reactor is packed with highly hydrophilic, modified suspended biological media with a specific surface area exceeding 1,200 m²/m³; microporous aeration pipes are arranged at the bottom to drive the media into thorough tumbling and fluidisation. Highly active nitrosomonas and nitrifying bacteria, concentrated on the surface of the media, form a robust biofilm. In an environment with ample dissolved oxygen, this biofilm rapidly oxidises highly toxic total ammonia nitrogen (TAN) into nitrite, which is subsequently converted into less toxic nitrate, thereby stabilising the ammonia nitrogen concentration throughout the system at below 0.4 mg/L under normal operating conditions.

STEP 04-Protein separator and advanced ozone oxidation

Fine colloidal particles (<30 μm), surfactants and dissolved organic macromolecules that cannot be retained by the microfiltration unit enter the counter-current protein separator. A Venturi jet injects micron-sized ultrafine bubbles, which, in combination with a microdose of ozone (O₃, 0.02-0.05 mg/L) for synergistic oxidation, utilises the surface tension of the air flotation process to convert proteins and lipids into foam for discharge. The ozone simultaneously exerts powerful decolourising and deodorising effects, whilst reducing COD and BOD levels.

STEP 05-Medium-pressure UV sterilisation and disinfection

The water, following air flotation purification, flows through a full-flow medium-pressure UV reactor. Medium-pressure UV lamps emit high-intensity UV radiation across a broad spectrum of 200-300 nm, thoroughly destroying the DNA/RNA replication structures of bacteria, water moulds and parasitic ciliates, achieving an inactivation rate of over 99.9 per cent. At the same time, they effectively photolyse residual free ozone in the water, completely preventing the invasion of exogenous pathogens and cross-contamination between aquaculture ponds.

STEP 06-Degassing Tower and Pressurised Pure Oxygen Dissolution Cone

Free carbon dioxide exhaled by the farmed fish is rapidly stripped and dissipated in the stripping degassing tower by a high-powered counter-flow fan (reducing CO₂ concentration to below 10 mg/L to prevent acidosis in the fish stock); subsequently, the main water flow enters the high-pressure pure oxygen dissolution cone, where, under operating pressures of 0.1-0.2 MPa, pure oxygen undergoes vigorous shear mixing with the water flow, achieving a mass transfer dissolution efficiency of over 92 per cent for pure oxygen, ensuring dissolved oxygen levels in the effluent reach 12-15 mg/L and maintaining a constant dissolved oxygen level of 7.5-10 mg/L in the fish ponds.

STEP 07-Air-source heat pump temperature control and closed-loop recirculation

Treated water that meets standards passes through a high-efficiency titanium plate heat exchanger, where an air-source or water-source inverter heat pump precisely regulates the water temperature (with a control accuracy of ±0.5°C). Finally, the water is returned to the aquaculture facility via gravity flow or a low-head inverter circulation pump, forming a 24-hour closed-loop circulation system. The daily volume of fresh water added accounts for only 1-3 per cent of the total circulating water volume.

Table 1: Key technical design parameters for each core treatment unit in industrial-scale land-based recirculating aquaculture systems

4. Doing the maths: Why is this a sound business venture with a high degree of certainty?

Investing in a modern, industrial-grade recirculating aquaculture system is not merely a technological upgrade; it is a business investment model characterised by robust risk resilience and highly predictable returns. Taking a standardised commercial demonstration facility for California bass (largemouth bass) with 1,000 m³ of effective water volume (calculated on the basis of two harvests per year and a total annual output of 120 tonnes) as an example, the key economic benefits are as follows:

(1) . Exceptionally high spatial production capacity:

The system achieves a stocking density of 50-70 kg/m³ per unit volume of water (compared to just 3-5 kg/m³ in traditional earthen ponds). Requiring only a few mu of land for a standard workshop, its output scale rivals that of hundreds of mu of traditional contiguous fish ponds.

(2) . Extreme optimisation of the feed conversion ratio (FCR):

In an ideal environment characterised by constant temperature, constant oxygen levels and the absence of water quality-related stress, fish exhibit vigorous metabolism and extremely high feed conversion efficiency. The FCR for California bass is as low as 1.05-1.15 (compared to 1.35-1.55 in traditional open ponds), directly reducing feed costs by more than 15 per cent.

(3) . Zero antibiotics and premium pricing: A fully enclosed biosecurity system prevents the indiscriminate use of drugs.

The resulting fish have a glossy, injury-free skin and firm flesh without a muddy odour, meeting the direct procurement standards of high-end supermarkets and branded catering supply chains, with a sales premium of 15-30 per cent.

Table 2: Comparison of traditional aquaculture methods with land-based industrialised recirculating aquaculture systems

5. Diverse application scenarios and flexible implementation

Land-based recirculating aquaculture systems break free from the geographical constraints of having to rely on large natural bodies of water and high-quality natural rivers and streams, and can be tailored to suit various existing resources:

(1) . Green transition in areas subject to ecological farming bans or phase-outs:

For fishermen affected by fishing and aquaculture phase-outs in the Yangtze River basin, key water source protection zones and nearshore waters, factory-style facilities are constructed on compliant building land to achieve ‘restoring wetlands from fish ponds, vertical fish farming, and income generation through industrial diversification’.

(2) . Revitalisation and conversion of idle inland industrial and mining facilities:

By directly utilising idle factory buildings, logistics warehouses or agricultural machinery depots within industrial estates, the system eliminates the need for costly large-scale civil engineering and infrastructure investment. It requires only ground impermeabilisation and the laying of pipework to be up and running within 2-3 months.

(3) . Resource utilisation of barren saline-alkali land and slightly brackish water:

Targeting the vast areas of saline-alkali wasteland in the north-west, north-east and along the coast, this approach combines desalination of slightly brackish groundwater with the adjustment of trace elements to enable large-scale farming of California bass, red seabream, grouper or white-leg prawns, transforming ‘waste alkaline water’ into a ‘treasure trove’.

6. Partnering with professional engineering teams to pioneer intelligent manufacturing in the aquaculture industry

Setting up a factory-based recirculating aquaculture system is by no means as simple as purchasing a few plastic tanks and assembling a few filter pumps. The calculation of frictional head loss in piping, the matching of biochemical nitrification loads, the balance of gas-liquid dynamics, and emergency redundancy for extreme failures are all interlinked and indispensable.

Drawing on the technical expertise of our team of senior specialists in fluid mechanics, water supply and drainage engineering, and aquatic microbiology, we provide aquaculture enterprises and investors with a comprehensive ‘turnkey (EPC)’ service covering the entire process-from on-site surveying and system design to the delivery of complete equipment packages, as well as biofilm colonisation commissioning and on-site operational guidance:

[Stringent Standard Configuration: Industrial-grade Quadruple Biosafety Redundancy Mechanism]

(1). Millisecond-level Automatic Start-up and Grid-Connection System for Dual-Circuit Solar Power Generation and Diesel Generators.

(2). High-pressure self-contained pure oxygen emergency bypass; in the event of a complete power failure, relies on tank pressure to autonomously stabilise oxygen supply for over 12 hours.

(3). Seamless hot-swap ‘one active, one standby’ configuration for core circulation pumps, with a gravity-fed bypass pressure relief line.

(4). PLC-based cloud central control system with 24-hour, millisecond-level audible and visual water quality alerts pushed to multiple mobile devices.

7. Call to Action and Collaboration Enquiries

Leave the uncertainties of aquaculture to the certainty of industrial engineering! Modern land-based recirculating aquaculture systems not only serve as a robust moat for the aquaculture industry against cyclical fluctuations, but also represent a prime sector currently receiving strong support from policies on facility-based agriculture and rural revitalisation. Please feel free to telephone us to book a site visit to our modern, digital land-based recirculating aquaculture demonstration centre, where you can obtain a bespoke process design and investment feasibility study tailored to your needs.

Figure 1: Photograph of a land-based recirculating aquaculture facility