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Water quality is the driver of recirculating aquaculture system (RAS) success. Parameters like dissolved oxygen, temperature, pH, and nitrogen compounds critically impact fish growth and health. Even minor changes can be catastrophic. With advanced monitoring tools, aquaculture operations can proactively maintain optimal conditions, ensuring lower fish stress, high yields, and sustainable, profitable production.
Did you know that 25% of fish feed immediately becomes waste in an aquaculture system? Recirculating aquaculture systems (RAS) water quality management is the most critical factor to determine success or failure in aquaculture systems.
Water quality is key. It directly affects production yields, profit margins, and most importantly, fish health. Even small fluctuations in water parameters can have severe consequences. An RAS water quality monitoring system must track different variables simultaneously. The concentration of toxic compounds such as ammonia increases tenfold when the pH in water increases by one unit. Also, proper alkalinity is essential for promoting strong biofilter performance and processing nitrogenous wastes.
With modern technology such as IoT-enabled sensors, we can now monitor these critical water parameters in real-time, preventing issues before they affect livestock.
In this article, you’ll learn how to select appropriate water sources, maintain optimal parameters, manage nitrogen compounds effectively, and use reliable monitoring systems. All of these crucial elements are what determine whether an RAS operation works or fails.
The foundation to any successful RAS begins with selecting an appropriate water source. The initial water quality directly determines system stability and influences treatment requirements during operation.
Well water is the most popular choice for RAS operations because of its consistent quality and freedom from pollution and pathogens. Groundwater drawn from springs or wells requires minimal pumping, which reduces operation costs. It’s important to note that well water often contains low dissolved oxygen (DO) levels, which can be problematic with dissolved gases like carbon dioxide (CO2), iron, and manganese. Therefore, proper aeration or degassing treatments are required before use in RAS.
On the other hand, as surface water comes from streams or lakes, it carries a higher risk of contamination from pathogens, pollutants, and seasonal temperature fluctuations. Aside from this, surface water contains more DO, making it suitable when proper water purification/filtration methods are used.
Municipal water offers consistent supplies; however, it contains chlorine or chloramine, which are highly toxic to fish. Activated carbon removes chlorine from water, but chloramines require both carbon filtration and biological treatment for complete elimination.
In smaller RAS operations, chemical dechlorination using sodium thiosulfate can be used as an alternative.
Rainwater harvesting (which is commonly used in stormwater management) is another viable source, particularly in water-scarce areas like Africa and the UAE. Effective collection involves pre-storage treatment via diversion and screening methods. If you use rainwater, it will require disinfection and filtration before using it in aquaculture systems.
Reverse osmosis (RO) technology has enabled extraordinary purification capabilities for RAS operations. RO systems effectively remove impurities, bacteria, and dissolved solids, creating ideal water conditions that promote optimal fish growth rates and reduce stress. RO-treated water is particularly beneficial in hatcheries and nurseries where precise water quality control is essential.
Additionally, RO systems allow precise salinity management. This is particularly important when treating brackish water for freshwater species or when maintaining specific ionic compositions for sensitive life stages. When paired with technologies like biofilm reactors, RO systems contribute significantly to creating closed aquaculture production with minimal waste discharge.
Water quality parameters tell us a lot about an RAS system. Monitoring and maintaining water quality parameters within optimal ranges are key for sustainable aquaculture operations.
Temperature affects metabolic rates in fish. For example, with every 10°C increase, the metabolic rate of cold-blooded aquatic organisms doubles, which impacts their food consumption and growth rates.
With the increase of water temperatures due to climate change, models have predicted that up to 77% of current cold-water aquaculture facilities could face suboptimal conditions by 2100, making temperature control increasingly important for future RAS operations. To measure temperature, use a reliable temperature sensor.
Dissolved oxygen is the most limiting water parameter in RAS operations. A minimal requirement of 5 mg/L of DO is needed in RAS to ensure optimal fish growth.
Affects of low DO levels in RAS include:
Typically, oxygen consumption in well-designed systems equals approximately 50% of the feed rate. Microporous aeration offers high efficiency with lower energy consumption, while precision aeration methods can reduce energy usage by up to 26.3% compared to manual aeration. To measure DO levels in RAS operations, a dissolved oxygen meter is the most reliable method.
In RAS operations, alkalinity must be maintained between 100 and 200 mg/L to provide optimal biological filtration, reduce ammonia concentrations, and increase nitrite removal rates. When maintained within this range, significantly lower CO2 concentrations are achieved after water treatment. Alkalinity is important as it acts as a buffer against pH fluctuations. Most aquatic species thrive in pH ranges between 7.0 and 8.4.
To consistently measure pH in RAS operations, a high-quality pH sensor is required.
Fish maintain osmoregulation within 280 and 360 mOsm kg-1 (one third of seawater). When salinity changes, fast-acting cellular responses operate within a few minutes to hours, followed by slower endocrine-mediated responses taking a few days.
Studies have found that at 11 PSU salinity, some fish species show lower oxygen consumption and increased growth rates. This demonstrates the complex relationship between salinity and metabolism. By maintaining salinity levels in RAS operations, you can help reduce stress related to osmoregulation.
One of the most difficult challenges in maintaining RAS water quality is managing nitrogen compounds. It is estimated that 50% of all aquaculture feed ends up as waste solids in culture water. Therefore, effective management of nitrogen compounds will directly determine fish health, growth rates, and RAS sustainability.
Ammonia comes in two forms:
The balance between these forms depends on the pH level of the water. When the pH level is high, it has a higher toxicity. For example:
Understanding the relationship between pH and ammonia toxicity is crucial in RAS. Concentrations of NH3 above 2.0 mg/L are very lethal to sensitive fish.
Nitrite is produced during the first stage of nitrification and can cause brown blood disease in fish, a condition that causes suffocation, even in adequately oxygenated water.
The most effective way to prevent brown blood disease involves maintaining chloride levels at a ratio of at least 10:1 (chloride:nitrite); however, some species (like pike-perch) require ratios as high as 24:1 for complete protection. This is why most RAS operations maintain 50 – 100 ppm chloride in their systems.
What we thought about nitrate being relatively non-toxic has changed in recent studies. One study found that nitrate levels as low as 80-100 mg/L can cause significant health issues to rainbow trout. Studies have also found that at 1000 mg/L of nitrate, juvenile tilapia had 62.5% survival rates, a 29% decrease in growth, and 56% reduced feed conversion.
The primary management strategy for nitrate is water exchange, similar to how water changes are used in aquariums. However, environmental restrictions can limit discharge options in RAS operations. Alternatively, denitrifying systems using heterotrophic bacteria with carbon sources can convert nitrate to nitrogen gas.
Solid management is fundamental for RAS operations. It distinguishes between settleable solids and suspended solids. Species have different tolerances to solids in water; for example, shrimp are pretty hardy and can tolerate up to 400 mg/L of suspended solids, while trout can only tolerate around 20 mg/L.
Effective solid removal in water includes:
A biofilter is a type of filtration system that uses beneficial bacteria to break down and remove harmful products in RAS operations. Their effectiveness depends on bacterial populations that convert ammonia to nitrate.
First, ammonia-oxidizing bacteria convert ammonia to nitrite, and then the nitrite-oxidizing bacteria convert nitrite into nitrate.
Biofilter media selection can affect the performance, so you should always look at what option is best for you. Many RAS operations use coconut shells, as they are more efficient at removing ammonia than commercial plastic media.
Effective monitoring technology serves as the eyes and ears of your RAS operation, enabling precise control of water quality parameters that directly impact fish health and system efficiency.
Traditional manual monitoring involves sampling at fixed intervals, often missing critical water quality fluctuations that follow daily rhythmic patterns. Although manual methods offer high accuracy when conducted by trained technicians, they remain labor-intensive and provide only snapshots of system conditions.
Automated systems, meanwhile, collect data continuously, revealing important patterns like CO2 concentration peaks that manual sampling frequently misses. While automated systems require higher initial investment, they eliminate human error in data entry and transcription that can compromise system integrity.
Modern RAS operations increasingly rely on real-time monitoring of dissolved oxygen, pH, and CO2. Optical oxygen sensors offer distinct advantages over traditional probes, including insensitivity to air bubbles, minimal maintenance requirements, and direct integration with control systems for oxygen generators.
These systems can support up to four calibration-free sensors simultaneously, reducing operational costs. CO2 monitoring proves equally critical—when levels rise too high, fish may suffocate during transport despite adequate oxygen levels.
Oxidation-Reduction Potential (ORP) serves as a cost-effective proxy for monitoring ozone levels in RAS systems. The upward inflection point of ORP effectively marks the end of the rapid ozonation process.
For actual ozone measurement, the indigo trisulfonate method offers accuracy across wide concentration ranges with minimal interference from other compounds. This colorimetric technique works effectively even in challenging environments like biologically treated wastewater.
Wireless Sensor Networks eliminate the disadvantages of cable-based systems while providing real-time data transmission. Tree topology networks demonstrate superior reliability with packet loss rates as low as 3.2%. Energy-saving strategies significantly extend sensor node life cycles, particularly important when nodes are deployed on water surfaces where battery replacement proves challenging.
IoT integration enables not only monitoring but predictive maintenance through data analytics, with M5 model tree algorithms showing the highest accuracy for tracking water parameter changes.
Water quality management directly affects fish health, growth rates, and profitability in RAS operations. From water source selection to nitrogen compound management, your attention to water quality parameters will determine whether your RAS operation will merely survive or thrive.
If you would like to learn more about how you can monitor your RAS operation or what monitoring technology we have that can transform reactive management into proactive control, contact the world-class team at Atlas Scientific.
Water quality is the driver of recirculating aquaculture system (RAS) success. Parameters like dissolved oxygen, temperature, pH, and nitrogen compounds critically impact fish growth and
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