Water Quality Management in RAS: DO, pH, TAN, Nitrite, Nitrate and CO₂

Water Quality Management in RAS: DO, pH, TAN, Nitrite, Nitrate and CO₂ Water quality in RAS is a network of interacting processes. Temperature and pH control the toxic NH₃ fraction of TAN; alkalinity supports biofilter pH stability; CO₂ affects oxygen use; and solids increase microbial oxygen demand and disease pressure. Why is water quality central to RAS? In intensive production, fish, feed and the biofilter continuously consume oxygen and generate metabolites in the same water. A flow-through facility can export much of that load with new water; in a RAS, treatment must keep pace with production. Measurement is therefore a direct part of capacity and operating management—not merely a laboratory check. Read more about: RAS Design by Fish Species: Trout, Carp, Sturgeon and Tilapia Targets must be set for the fish species, life stage, temperature, salinity and operating policy. One generic table of ‘ideal values’ found online should not be applied to every RAS project. Read more about: How Does a RAS System Work? Water-Treatment Stages and Process Flow Dissolved oxygen (DO) DO is one of the fastest-changing critical parameters. Fish respiration, post-feeding metabolism, biofilter nitrification and organic degradation all consume oxygen. A high tank-inlet value is not sufficient; the minimum tank-outlet value and the system response during feeding peaks must be monitored. At high densities, pure-oxygen transfer may use an oxygen cone or NANOB. Selection must be based on water flow, inlet/outlet DO difference, pressure and verified transfer efficiency—not the target gas dose alone. pH and alkalinity pH affects fish physiology, biofilter activity and the NH₃/NH₄⁺ balance. Nitrification consumes alkalinity and tends to lower pH. Alkalinity trend should therefore be monitored alongside pH, and buffering chemicals should be dosed in a controlled and recorded manner. Critical relationship: At the same TAN concentration, increasing temperature and pH can increase the toxic unionised-ammonia (NH₃) fraction. TAN and ammonia TAN is the sum of ionised NH₄⁺ and unionised NH₃. Part of the nitrogen in feed enters the water as TAN. Its trend must be interpreted with feed input, biofilter capacity, biofilm maturity, temperature, pH, alkalinity and DO. A TAN increase does not automatically mean that the biofilter is undersized. A sudden feed increase, low alkalinity, low DO, chemical exposure, biofilm loss or reduced actual flow can create the same result. Read more about: How to Size a RAS Project: Feed Load, Biomass, TAN and Flow Calculations Nitrite and nitrate Nitrite is an intermediate product of nitrification and can impair oxygen transport in fish blood. New-system start-up, sudden loading or biofilter imbalance may raise nitrite. The chloride-to-nitrite relationship and species tolerance should be assessed by an aquaculture specialist. Nitrate is generally less acutely toxic than TAN or nitrite, but it accumulates in intensive RAS. Control may use make-up-water exchange, denitrification, production strategy or integrated plant systems. The operating limit must be species- and life-stage-specific. Carbon dioxide (CO₂) Fish, the biofilter and waterborne microorganisms produce CO₂ through respiration. High CO₂ can challenge oxygen transport and use and can affect pH. Fish gathering at the tank inlet or showing abnormal respiration despite apparently normal DO should trigger investigation of CO₂ and overall gas balance. A degasser must be sized for water-air contact area, air flow, water distribution and hydraulic loading. The presence of a blower alone does not demonstrate effective degassing. Solids, turbidity and organic load Faeces and uneaten feed should be removed before disintegration. A drum filter separates solids early, reducing heterotrophic bacterial pressure, oxygen consumption and fine-particle formation in the biofilter. In seawater systems, a protein skimmer can assist with dissolved and fine organic matter. Temperature, salinity and disinfection Temperature changes fish growth and oxygen demand, water oxygen-carrying capacity and bacterial reaction rate. Salinity affects oxygen solubility, nitrite toxicity and material selection. Freshwater and marine RAS should not automatically use the same equipment materials or chemical-control approach. Read more about: Recirculating Fish Farm Setup Cost: The Technical Factors Behind the Price UV disinfection is a barrier that reduces free-circulating microorganisms. Performance depends on water transmittance, turbidity, UV dose, actual flow and lamp/sleeve maintenance. UV does not replace quarantine, hygiene or veterinary programmes. How should a monitoring plan be structured? Monitoring level Example parameters and purpose Continuous / online DO, temperature, pH, water level and pump/blower status for rapid alarm and control Daily Portable verification of tank inlet/outlet DO, temperature, behaviour, flow and feed record Weekly or load-based TAN, nitrite, nitrate, alkalinity and TSS for biofilter and water-renewal trends Periodic Source water, microbiology, hardness, iron/manganese, chloride and instrument calibration Post-event Root-cause analysis after alarms, mortality, feed stoppage or chemical treatment   Read more about: Energy Efficiency, Redundancy and Emergency Design in RAS Monitoring principle: A sensor is not enough. Define its location, calibration interval, alarm threshold, responsible person and the action required after an alarm. Frequently asked questions What is the most important RAS water-quality parameter? DO is often the fastest-changing critical parameter, but it is not sufficient alone. CO₂, TAN, nitrite, pH, alkalinity, temperature and flow must be managed together. Why can fish be stressed when TAN is normal? Possible causes include low DO, high CO₂, nitrite, temperature, suspended solids, gas supersaturation or disease. Does falling pH prove that the biofilter is working? Nitrification consumes alkalinity and can lower pH, but source water, CO₂ and chemical dosing also affect it. Interpret pH with TAN, nitrite and alkalinity. Does UV prevent every disease? No. UV only reduces free microorganisms that pass through the unit at the required dose. Quarantine, hygiene, stock management and veterinary oversight remain necessary. FOR YOUR PROJECT: To design species-specific water-quality targets, sensor points, alarm thresholds and treatment capacity, Contact Atlas Aqua.

How Does a RAS System Work? Water-Treatment Stages and Process Flow

How Does a RAS System Work? Water-Treatment Stages and Process Flow A RAS is not simply a pumping loop that circulates the same water. It is an integrated life-support process that removes solids, ammonia and carbon dioxide, restores oxygen and hygiene conditions, and returns treated water to the production tanks. What is a RAS? A Recirculating Aquaculture System (RAS) treats and reuses most of the production water. Solids, dissolved metabolites, unwanted gases and microbial pressure are managed through controlled treatment stages. Make-up water is not eliminated: filter backwashing, sludge removal, evaporation, nitrate control and mineral balance still require a project-specific supply of new water. The principal advantage is the ability to control the fish environment around the production target. That control does not come from owning each item of equipment; it comes from making flow, treatment load, hydraulic level and automation work as one system. A sound RAS design is therefore a balanced process chain, not an equipment shopping list. The main stages of RAS water treatment Production tanks and hydraulic collection: Water movement carries faeces, uneaten feed and metabolic waste toward the tank outlet. Tank geometry and bottom drainage should remove solids quickly before they disintegrate. Mechanical filtration: A drum filter or another micron-rated mechanical filter removes suspended solids. Early separation reduces secondary oxygen demand and protects the biological stage. Biological filtration: Nitrifying bacteria in the biofilter convert total ammonia nitrogen first to nitrite and then to nitrate. Capacity must be based on maximum daily feed and TAN load, not simply on tank volume. Read more about: How to Size a RAS Project: Feed Load, Biomass, TAN and Flow Calculations Gas control: Fish and bacteria generate carbon dioxide. A correctly sized degasser helps remove CO₂ and other unwanted dissolved gases, supporting pH stability and efficient oxygen use. Oxygenation: Air or pure oxygen is added to the water. High-density projects may use an oxygen cone or NANOB. Oxygen capacity must cover feeding peaks and biological treatment demand as well as standing biomass. Disinfection and polishing: A UV filter forms a barrier against free-circulating microorganisms. In marine systems, a protein skimmer may support the removal of dissolved and fine organic matter. Monitoring and return: Temperature, pH, dissolved oxygen, water level, flow and equipment status are monitored. Treated water returns to the fish tanks at the required flow and oxygen concentration. Read more about: Water Quality Management in RAS: DO, pH, TAN, Nitrite, Nitrate and CO₂ Why must the components be sized together? The real capacity of a RAS line is limited by its weakest process stage. An adequate drum filter cannot prevent TAN from rising if the biofilter cannot process the feed load. A large biofilter cannot protect fish when CO₂ removal is insufficient. Even the correct pump model will underperform if pipe losses, fouling or hydraulic levels reduce actual flow. Read more about: Recirculating Fish Farm Setup Cost: The Technical Factors Behind the Price Engineering principle: Check every component against maximum feed day, actual hydraulic loading, remaining capacity during maintenance and the defined failure scenario. Where does RAS-BOX fit in the process? Atlas Aqua RAS-BOX is an integrated water-treatment centre that can combine mechanical filtration, biological treatment, circulation, gas management, UV and control components inside a compact PE100 body. Production tanks remain outside the RAS-BOX; the unit treats water received from those tanks and prepares it for reuse. This distinction is important for correct system positioning and capacity calculation. Operating indicators that require continuous attention Indicator Why it matters Actual recirculation flow Confirms whether filter loading and tank turnover remain at the design point. Dissolved oxygen The fastest-changing critical parameter for fish welfare, feeding and biofilter activity. pH and alkalinity Affect nitrification stability and the toxic NH₃ fraction of ammonia. TAN and nitrite Show whether biological treatment is keeping pace with feed input. Carbon dioxide Influences respiration, oxygen use and pH. Water level and pressure Provide early warning of blockage, pump loss or hydraulic imbalance. Common design and operating mistakes Selecting the system only from tank volume and ignoring maximum feed and waste load. Allowing solids to break down before removal and imposing avoidable organic load on the biofilter. Assuming that the presence of aeration automatically solves CO₂ removal. Treating the pump nameplate flow as actual site flow without measuring pipe, valve and elevation losses. Omitting standby pumps, blowers, emergency oxygen and generator scenarios. Applying a standard system to different source waters without a complete water analysis. Read more about: Energy Efficiency, Redundancy and Emergency Design in RAS Frequently asked questions Is a RAS completely waterless or zero-discharge? No. RAS can reduce water use substantially, but sludge discharge, backwashing, evaporation and dissolved-solids control still require make-up water. Zero liquid discharge is a separate advanced-treatment scope. Do all RAS projects use the same equipment sequence? The core logic is similar, but species, salinity, temperature, feed load, space and discharge conditions can change the order, bypasses and side-stream treatment. Read more about: RAS Design by Fish Species: Trout, Carp, Sturgeon and Tilapia   Is a drum filter alone sufficient for a RAS? No. It removes solids; ammonia conversion, gas control, oxygenation and biosecurity require additional processes. Can RAS capacity be determined from tank volume? Not by tank volume alone. Tank volume and stocking density describe biomass, while feed, TAN, oxygen and solids loads primarily determine treatment capacity. FOR YOUR PROJECT: To develop a balanced RAS process for your fish species, annual production target and source water, Contact Atlas Aqua.

How to Size a RAS Project: Feed Load, Biomass, TAN and Flow Calculations

How to Size a RAS Project: Feed Load, Biomass, TAN and Flow Calculations Reliable RAS sizing does not convert annual tonnage directly into an equipment model. It begins with a production schedule and peak simultaneous biomass, followed by separate mass balances for maximum daily feed, TAN, oxygen, solids and hydraulic flow. Read more about: How Does a RAS System Work? Water-Treatment Stages and Process Flow Start with the production plan A target of 100 tonnes per year does not mean that 100 tonnes of fish are present on any one day. Batch count, stocking weight, harvest weight, growth period, survival, grading and harvest intervals determine simultaneous biomass. The first sizing document should therefore be a production calendar.For each month or week, the plan should show fish count, average weight, total biomass and feed input. The design point is not the annual average; it is the highest simultaneous biomass and maximum sustainable daily feed load the facility will experience. 1. Maximum biomass and tank volume Core relationship: Required production-water volume (m³) = maximum biomass (kg) ÷ design stocking density (kg/m³). Stocking density is not merely the amount of fish that physically fits in a tank. Species oxygen demand, swimming behaviour, feeding method, minimum outlet DO, CO₂, solids transport, grading frequency and operator experience must be assessed together. A single universal kg/m³ value should not be applied across projects. Read more about: RAS Design by Fish Species: Trout, Carp, Sturgeon and Tilapia 2. Maximum daily feed load Core relationship: Daily feed (kg/day) = system biomass (kg) × daily feeding rate (% biomass/day). Feeding rate varies with species, live weight, water temperature, feed energy density and growth target. Treatment equipment must be checked against the highest sustainable daily feed load—not the average—because solids, TAN, oxygen demand and CO₂ production are all closely linked to feeding. 3. TAN load and biofilter capacity Part of the nitrogen in consumed protein becomes growth, while part is excreted into the water as total ammonia nitrogen (TAN). Preliminary sizing combines daily feed with protein and nitrogen content, digestibility and the selected nitrogen-excretion coefficient. The coefficient depends on feed formulation and species, so one fixed factor should not be used for every project. Biofilter selection must consider specific media area, validated nitrification rate, temperature, pH, alkalinity, dissolved oxygen, organic loading and start-up maturity—not only vessel volume. Atlas Aqua biofilter solutions can be configured around the project feed load. Read more about: Water Quality Management in RAS: DO, pH, TAN, Nitrite, Nitrate and CO₂ 4. System flow and tank turnover Preliminary check: Recirculation flow (m³/h) = system or tank volume (m³) × target turnovers per hour. Turnover is only a starting check. Final flow must be validated from the DO drop through the tank, CO₂ and TAN concentration difference, solids transport, filter surface loading and hydraulic elevations. The basis is measured site flow after pipe and valve losses—not the pump’s nameplate flow. Mechanical-filter capacity is selected from actual flow, maximum solids load, screen opening, allowable head difference and backwash demand. Atlas Aqua drum filters are available in project-specific flow and micron configurations. 5. Oxygen and carbon-dioxide balance Total oxygen demand includes fish respiration, the post-feeding metabolic peak, biofilter nitrification and microbial degradation of organic matter. A feed-based oxygen calculation is a strong primary method, but the result must also be checked against biomass, species, temperature and life stage. Gas capacity alone does not define oxygen-transfer performance. Water flow, inlet DO, target outlet DO, pressure and actual transfer efficiency must be assessed together. Depending on the project, an oxygen cone or NANOB may be used. CO₂ requires a separate degasser capacity calculation. 6. Make-up water requirement RAS achieves high water reuse, but make-up water is not zero. Demand depends on nitrate accumulation, salinity and mineral balance, drum-filter backwash, sludge purge, evaporation, fish-health policy and discharge strategy. Source water must also be analysed for temperature, pH, alkalinity, hardness, iron, manganese, ammonia, gas saturation and microbiology. Sizing control table Calculation stage Primary design data Production model Batch count, stocking/harvest weight, growth period and survival Tank system Peak biomass, species, stocking density and tank geometry Mechanical filtration Actual flow, solids load, screen micron and backwash Biofilter Maximum feed, TAN load, temperature, pH and alkalinity Oxygenation Feed peak, biomass check, inlet/outlet DO and transfer efficiency Degassing CO₂ production, water flow, gas-transfer area and air ratio Make-up water Nitrate, purge/backwash, evaporation and source-water quality Redundancy N+1 pump/blower, emergency oxygen, generator and alarms Why a blanket safety factor is not enough Oversizing every component indiscriminately can increase capital and energy costs and create hydraulic imbalance. Safety margins should be applied to known failure modes. Pump N+1 redundancy, biofilter allowance for peak feed and maturity, and oxygen capacity for feeding peaks and emergency supply are separate engineering decisions. Read more about: Recirculating Fish Farm Setup Cost: The Technical Factors Behind the Price At compact and modular capacities, RAS-BOX can provide standardised treatment blocks. Large projects or species with special requirements may be better served by centralised or independent project-specific modules. Read more about: Energy Efficiency, Redundancy and Emergency Design in RAS Frequently asked questions Can RAS capacity be selected from annual production tonnage? Annual tonnage is an initial input. Equipment cannot be selected reliably without peak simultaneous biomass, daily feed, production cycle and harvest plan. Should oxygen demand be calculated from feed or biomass? Feed represents the main process load well, while a biomass-based check captures basal respiration, species, temperature and life stage. Use both approaches together. Is tank-volume-based pump sizing sufficient? No. Turnover is a preliminary check. Final flow must also satisfy oxygen, CO₂, TAN, solids transport and actual pipe-loss requirements. Why does a biofilter not operate at full capacity on day one? Nitrifying bacteria need time to colonise media and establish a stable biofilm. Feed must be increased gradually while TAN and nitrite are monitored. FOR YOUR PROJECT: To convert your production schedule, peak feed load and site data into verified RAS equipment capacities, Contact Atlas Aqua.