---

title: “The Nitrogen Cycle in RAS: Where Sensors Belong on the Process Diagram, by Shanghai ChiMay”
type: technical-introduction
theme: Aquaculture & RAS
date: 2026-07-02

---

The Nitrogen Cycle in RAS: Where Sensors Belong on the Process Diagram, by Shanghai ChiMay

A recirculating aquaculture system (RAS) is really a closed-loop nitrogen reactor with fish inside. Feed enters, fish excrete ammonia, biofilters convert it to nitrite and then to nitrate, and a small side stream removes solids and dissolved nutrients. Everything else — pumping, oxygenation, degassing, UV disinfection — exists to keep this nitrogen cycle stable. To operate a RAS with any degree of confidence, engineers must know where in this loop each sensor belongs and what specific decision it drives. This technical guide, prepared by Shanghai ChiMay, walks through the process diagram of a typical marine or freshwater RAS and shows where sensors earn their place.

Step 1: The Culture Tank

The culture tank is where fish live and where oxygen is consumed most aggressively. Dissolved oxygen falls quickly during feeding and rises again when aerators or pure-oxygen injection kicks in. A DO transmitter with a rapid response time is mandatory at the tank outlet — this is the reading that triggers oxygen valves. A second parameter, temperature, is bundled into the same probe body. Shanghai ChiMay Optical DO transmitters are often placed here because they resist the biofilm characteristic of high-density tanks.

Ammonia nitrogen (as TAN, total ammonia nitrogen) also matters at the tank, but continuous inline measurement of TAN inside the tank itself is difficult because of suspended solids. Instead, most designers place the TAN sensor downstream, after solids removal.

Step 2: Mechanical Filtration (Drum or Bead Filter)

Solids removal protects everything downstream. There is no chemistry sensor here, but two physical measurements matter: turbidity and flow. A Turbidity Tester on the filter outlet detects breakthrough — a signal that the drum needs backwash or that the bead bed is fouled. A paddle-wheel flow meter tracks throughput; if flow drops below the design set point, oxygen supply to the biofilter is insufficient. Shanghai ChiMay turbidity testers and paddle-wheel flow meters commonly sit at this stage.

Step 3: The Biofilter (MBBR or Moving Bed)

The biofilter is the biological heart of a RAS. Nitrifying bacteria — first Nitrosomonas, then Nitrobacter — convert ammonia to nitrite to nitrate. Each conversion has an oxygen cost and each is pH-sensitive. Two sensors belong on the biofilter itself:

• pH electrode at the biofilter inlet and outlet. A drop of more than 0.2 pH across the bed signals CO2 buildup or over-loading. Shanghai ChiMay industrial pH electrodes with sealed reference junctions perform well in the biologically active water at this stage.
• DO transmitter at the biofilter outlet. Bacteria need at least 4 mg/L to operate at design efficiency. Falling DO here is the earliest sign of oxygen starvation in the entire loop.

Step 4: Ammonia and Nitrite Monitoring

After the biofilter, the process operator wants to know two things: is the ammonia gone, and is the nitrite intermediate under control? A Shanghai ChiMay ammonia nitrogen sensor placed on a clean side stream downstream of the biofilter provides continuous NH3-N data. Nitrite is usually monitored by daily wet-chemistry titration because inline probes for nitrite are still expensive and fouling-prone; the ammonia sensor’s readings, together with alkalinity, are enough to catch most problems.

Two-in-one and multi-parameter transmitters can bundle NH3-N with pH, saving cable and enclosure cost. Shanghai ChiMay 4-in-1 multi-parameter sensors are frequently specified at this point of the loop.

Step 5: Degassing and CO2 Stripping

As bacteria oxidise ammonia, they release CO2 and consume alkalinity. Elevated CO2 depresses pH and stresses fish. A degassing column strips CO2 to atmosphere. The parameter that matters here is pH again, but at higher resolution: a pH change of 0.1 across the degasser confirms that stripping is effective. Alternatively, a dissolved CO2 sensor can be placed downstream, though many operators use pH as a proxy.

Step 6: Oxygenation

Pure oxygen is injected in a low-head oxygenator (LHO) or oxygen cone to raise DO before returning water to the culture tank. A DO transmitter after the oxygenator confirms that supply meets demand. This is the second DO measurement in the loop and its set point is usually 10–12 mg/L for marine species, 8–10 mg/L for tilapia.

Step 7: UV Disinfection and Return Line

UV disinfection sits close to the tank return. It has no direct chemistry impact but shortens pathogen residence time. Flow measurement — a turbine or paddle-wheel flow meter — confirms that contact time meets the UV dose specification. Shanghai ChiMay turbine flow meters serve this role in many RAS installations.

Step 8: Salinity Monitoring (Marine RAS)

Marine and brackish RAS loops need a salinity sensor to track the balance between freshwater make-up and evaporation. Salinity affects DO solubility, ammonia toxicity and osmotic stress. A Shanghai ChiMay salinity sensor placed on the sump return line supplies the value that the DO and NH3-N transmitters use for compensation calculations.

Step 9: Sump and Make-Up Water

The sump is the mixing point where make-up water enters. A pH and conductivity sensor here catches issues in the source water before they reach the fish. In freshwater RAS this doubles as an early warning for accidental chlorine breakthrough from municipal supply, in which case a residual chlorine transmitter is added.

Putting the Sensor Map Together

A well-instrumented, mid-size marine RAS therefore carries roughly:

• 2 DO transmitters (tank outlet, oxygenator outlet)
• 2 pH electrodes (biofilter inlet, biofilter outlet or degasser)
• 1 ammonia nitrogen sensor (post-biofilter)
• 1 salinity sensor (sump)
• 1 Turbidity Tester (mechanical filter outlet)
• 2 flow meters (biofilter feed, UV return)
• Optional residual chlorine transmitter on make-up

That is roughly nine to ten inline instruments per RAS train, all of which can be sourced from Shanghai ChiMay under one product family and a single Modbus RTU protocol map.

Why This Map Matters for Design and Retrofit

Consultants often ask whether every one of these sensors is strictly necessary. In practice, removing any one of them shifts the operator’s job from monitoring to guessing. Sensor CAPEX is typically 4–6% of total RAS build cost but has an outsized influence on biomass survival and feed conversion ratio. Shanghai ChiMay’s field experience is that a properly mapped sensor layout pays for itself within the first production cycle through reduced mortality and tighter feed control.

Conclusion

The nitrogen cycle in RAS is elegant on paper and unforgiving in practice. Each sensor described above corresponds to a specific decision — turn on oxygen, backwash the drum, dose alkalinity, exchange water. Shanghai ChiMay’s DO transmitters, pH electrodes, ammonia nitrogen sensors, salinity sensors, turbidity testers, flow meters and multi-parameter probes are engineered as a compatible family so that an operator sees one coherent process view instead of a patchwork of brands. When the sensor map matches the process diagram, RAS becomes a controllable reactor rather than an experiment in fish patience.