RAS Technical Analysis: MBBR Biofilm Reactors, Microscreen Drum Filters & Microporous Aeration Deliver 1,075 kg/mu Crab Yield

China's mitten crab farmers are caught between tightening effluent discharge standards, shrinking land and water resources, and consumer demand for consistent high-quality harvests. A 2024 recirculating aquaculture system (RAS) trial that paired three-dimensional crab shelters with an MBBR biofilm reactor, an 80–100 mesh microscreen drum filter, and a microporous aeration system points to a practical answer - one that pushed yield to 1,075 kg per mu while keeping ammonia below 0.3 mg/L.

The Three-Module RAS Backbone

As national effluent discharge standards for aquaculture become increasingly stringent and constraints on land and water resources continue to intensify, the Recirculating Aquaculture System (RAS) has moved from an option to an industrial necessity. The technical core of an efficient RAS can be reduced to three equipment modules: solid–liquid separation, biofilm reactors, and high-efficiency oxygenation. When these modules are correctly sized and integrated, they form a closed loop in which waste is physically removed, dissolved nitrogen is biologically converted, and dissolved oxygen is supplied with precision.

The design philosophy is "physical separation + biological purification + precision oxygenation." Each stage protects the next: the microscreen filter shields the biofilter from organic overload, the MBBR reactor stabilizes water chemistry, and microporous aeration sustains the oxygen demand of both the crop and the nitrifying bacteria. The result is a system that behaves more like a managed ecosystem than a sequence of treatment tanks.

Microscreen Drum Filter - Solid-Liquid Separation Frontline

In any recirculating system, uneaten feed and feces are the primary sources of water pollution. If large suspended particles are allowed to reach the biological treatment stage, they sharply increase the organic load on the biofilter and suppress the growth and metabolic activity of nitrifying bacteria - the very organisms the system depends on for ammonia removal. Interception must therefore happen early and reliably.

This case study used an 80–100 mesh microscreen drum filter as the core solid–liquid separation device. An 80-mesh screen retains particles ≥180 μm, while a 100-mesh screen retains particles ≥150 μm, capturing the bulk of feed waste and feces generated in crab farming. After filtration, large-particle impurities are separated out with suspended solids removal exceeding 85%, dramatically reducing the load handed to downstream biological treatment.

Selection guidance: For large-scale RAS projects, mesh choice should be based on stocking density, feeding rate, and water exchange rate. Stainless steel or nylon screens with automatic backwashing are recommended to reduce manual maintenance frequency. Finer mesh improves retention but is more prone to clogging, so there is a practical balance: 80–100 mesh delivers the best overall performance across retention efficiency and hydraulic capacity.

MBBR Biofilm Reactor - The Purification Heart

Biofiltration is the heart of any recirculating aquaculture system. Its job is to remove dissolved ammonia nitrogen (NH₃-N) and nitrite (NO₂^--N), which accumulate rapidly under high-density culture and become toxic to the stock. This system uses a three-stage biofiltration train - anaerobic, anoxic, then aerobic - that inherently embodies the Moving Bed Biofilm Reactor (MBBR) concept. In the aerobic tank, suspended or fixed biofilm carriers provide attachment surfaces for nitrifying and heterotrophic bacteria, forming dense, highly active biofilms.

Process principles:

• Anaerobic stage: denitrifying bacteria reduce nitrate to nitrogen gas (N₂), achieving nitrogen removal while degrading a portion of organic matter.

• Anoxic stage: combines aerobic and anaerobic metabolic capability, flexibly absorbing water quality fluctuations and strengthening shock-load resistance.

• Aerobic stage (MBBR core): under aeration, attached nitrifying bacteria oxidize ammonia to nitrite and then to nitrate, while heterotrophic bacteria degrade residual organic matter.

MBBR media deliver a high specific surface area - typically 500–1,200 m²/m³ - and support far higher volumetric loading rates than conventional activated sludge processes. Key selection parameters are summarized below.

Technical highlights: the MBBR process needs no backwashing, is simple to operate, and maintains balanced biofilm sloughing with low sludge production. In this crab-farming trial, the three-stage biofiltration system ran stably with ammonia nitrogen consistently below 0.3 mg/L and nitrite below 0.8 mg/L, fully validating the process for aquaculture water purification. The aeration grid must keep the media uniformly fluidized across the whole tank to avoid dead zones where biofilm activity collapses.

Microporous Aeration - Precision Oxygenation

Dissolved oxygen (DO) is the most sensitive and critical water quality parameter in recirculating aquaculture. Under high-density conditions, oxygen demand arrives from three directions at once - the respiration of the cultured animals, the degradation of organic matter, and the metabolism of nitrifying bacteria. Field experience shows that when DO is consistently maintained above 5 mg/L, feeding rates, feed conversion efficiency, and growth rates all improve noticeably.

This system uses a "liquid oxygen tank + microporous aeration disc" combined solution. A 1 kg-capacity liquid oxygen tank feeds high-pressure oxygen lines to microporous aeration pipe networks installed in each culture tank.

Core technical parameters:

• Two-stage pressure reduction: primary to 0.8–1.0 MPa, secondary to 0.1–0.2 MPa, delivering stable and controllable oxygen supply pressure;

• Microporous aeration discs produce uniform small bubbles (typically ≤3 mm), maximizing gas–liquid contact area and significantly raising oxygen mass transfer efficiency (KLa) versus conventional aeration;

• Integrated oxygen flow meters and pressure sensors enable real-time monitoring and automatic regulation of oxygen supply;

• DO in the culture zone is consistently maintained at ≥5 mg/L, with measured values above 5.2 mg/L in the core area.

Key advantages of microporous aerators: fine bubbles rise slowly through the water column with extended residence time, achieving oxygen dissolution efficiency of 30–50% - versus only 5–10% for conventional perforated pipe aeration.

Selection guidance: key factors include pore size (20–50 μm recommended), material (EPDM rubber or silicone for aging and chemical resistance), service area, and pressure loss. A liftable mounting frame simplifies maintenance and replacement. For large-scale RAS, a closed-loop DO controller can hold oxygenation within a set range, balancing energy efficiency and safety.

Circulation Piping & Hydrodynamic Design

Piping design directly controls energy consumption and water exchange efficiency. This case uses variable-frequency drive (VFD) pumps connected to PVC pipes with diameters of 50–80 cm (60 cm preferred) to create an efficient circulation channel between the culture zone and the purification zone.

• Hourly water renewal rate: 10–15%, precisely regulated by VFD pumps;

• Pipe installation: inclined at 45 degrees to reduce flow resistance;

• Flow monitoring devices at key nodes for real-time circulation control;

• Culture tank bottom sloped at 12 degrees with an annular collection trough, using gravity flow for efficient waste collection and discharge.

Design considerations: choosing the circulation flow rate means balancing purification needs against the animals' tolerance. The 10–15%/h renewal rate performed well throughout this trial, and VFD pumps let operators fine-tune circulation intensity by culture stage and season for refined operational control.

Equipment Integration & System Synergy

The greatest technical value of this case is the system-level synergy achieved through integration:

Microscreen Filter (Physical Separation) → MBBR Biofilm Reactor (Biological Degradation) → Microporous Aeration (Precision Oxygenation)

The microscreen filter removes suspended solids and lowers the organic load on the biofilter, allowing nitrifying bacteria on the MBBR media to thrive under low-organic-load conditions and maintain high nitrification activity. Residual organic matter and ammonia in the MBBR effluent are further degraded to ensure stable discharge quality. The microporous aeration system serves both the culture zone and the aerobic biofilter simultaneously, achieving multi-purpose energy optimization. Meanwhile, aquatic plants, snails, and microorganisms in the ecological purification zone form a bio-ecological barrier that complements the physical–biological treatment train.

Operational Results & Key Performance Data

After a full culture cycle from March to November 2024, the integrated system produced the following results.

The economics are as compelling as the water quality: revenue of 75,250 RMB/mu against costs of 45,880 RMB/mu leaves a net profit of 29,370 RMB/mu - roughly 440,550 RMB per hectare - demonstrating that the added capital cost of RAS equipment is recovered through yield and survival gains.

Conclusion & Outlook

This case demonstrates that an RAS technical route built on three core modules - microscreen filter, MBBR biofilm reactor, and microporous aeration - has reached the maturity needed for large-scale application. By combining equipment innovation (microscreen filter + MBBR media + microporous aerators) with process optimization (three-stage biofiltration + precision oxygenation + ecological regulation), the model systematically overcomes the three major bottlenecks of high-density aquaculture: water quality control, oxygen assurance, and waste discharge management.

For practitioners planning to build or upgrade RAS facilities, the following equipment selection and system design recommendations apply:

• Solid–liquid separation: 80–100 mesh microscreen drum filter with automatic backwashing for efficient suspended solids removal;

• Biological treatment: MBBR media (fill ratio 30–60%) in an anaerobic–anoxic–aerobic biofiltration train for effective ammonia and nitrite removal;

• Oxygenation: liquid oxygen tank + microporous aeration discs/tubes + two-stage pressure reduction, holding DO ≥5 mg/L with online monitoring and automated control;

• Circulation piping: VFD pumps + PVC pipes (45° inclined installation) with a 10–15%/h water renewal rate;

• Ecological polishing: aquatic plants and benthic organisms such as snails to create a "physical–biological–ecological" three-stage purification system.

Looking ahead, as MBBR media technology advances, microscreen filter automation improves, and microporous aeration energy efficiency is optimized, RAS will scale across more aquaculture species and push the industry toward more intensive, intelligent, and environmentally sustainable production.