Recirculating aquaculture systems (RAS) are an environmentally friendly alternative to conventional fish production methods and enable fish breeders to significantly reduce the nutrient loads on adjacent water bodies due to production. Therefore, RAS can be seen as an important technology for improving water quality in inland and coastal waters (Bregnballe 2015). A further step in the evolution of land-based fish farming systems are aquaponic-systems, which combine RAS technology with the production of plants in hydroponics by using nutrients directly from fish production to fertilize the plants. The most modern of these systems are decoupled aquaponic systems (DAPS: Decoupled aquaponic systems) of various types (Goddek et. 2019). DAPS, which were developed in Germany, are characterized by the separation of the RAS compartment from the hydroponic compartment via a one-way valve (Kloas et al. 2015, Monsees et al. 2017). Since DAPS make it possible to optimize the water quality in the two different compartments of the aquaponic system (RAS / hydroponics) separately, considerably higher yields can be realized in the hydroponic system (Monsees et al. 2017).
This guide is intended to provide a compact overview of various water quality parameters that are usually measured in RAS and aquaponic systems. In addition, some recommendations on the monitoring and control architecture as well as measures to improve production safety and control are presented.
The optimal temperature range in RAS depends on the requirements of the species produced. From less than 14 ° C for species such as Arctic char (Salvelinus alpinus) to 30 ° C for whiteleg shrimp (Penaeus vannamei). The temperature also influences other parameters in RAS (e.g. dissolved oxygen, biofilter function) and should be measured continuously.
Due to evaporation, biological deposits in pipes, changing the valve settings, clogging of the filters, and variability of the cleaning interval or leakage in the pipes, the water level in RAS can change significantly. In order to ensure that your RAS functions optimally and to avoid stressing the fish, the tank water level should be continuously monitored electronically.
System flow rate:
Calculating the required flow from the breeding tank to the water treatment components to control the accumulation of nutrients, suspended solids, and carbon dioxide and to maintain an adequate concentration of dissolved oxygen is an important step in the design process of an aquaculture system. Maintaining the determined flow rates is also a mandatory prerequisite for achieving the necessary water quality in the breeding tank. The simplest method for determining the flow rate is the so-called calibration (the collection of water from the inlet of the breeding tank over a specified period and the calculation of the volume per hour). Another option is hydraulic flow meters or electrical flow meters. Since the flow rate does not normally change suddenly, except in the event of a malfunction of important system components (e.g. pump failure), it is usually sufficient to use hydraulic flow meters or even manual gauges to determine the flow rate. However, since the flow rates can gradually deteriorate over time depending on the microbiological growth, the flow rate must be checked regularly in order to take the necessary maintenance measures due to the potential clogging of the filter components.
In intensive aquaculture systems, oxygen is often crucial for optimizing the growth, feed conversion rate and well-being of a fish species. Therefore, ventilation and oxygen injection must be of paramount importance in order to optimize the production performance in your aquaculture system. Depending on the type of oxygen input and the degree of intensity of the respective aquaculture system, the oxygen saturation in different sections of your RAS should be between a minimum of ~ 70% and a maximum of 250% saturation. Because of the inverse relationship between oxygen solubility and temperature, more oxygen is needed at higher temperatures. Since the individual oxygen content in the breeding basin can change suddenly depending on the physiological condition of the fish (e.g. immediately after feeding), we recommend that each tank be individually oxygenated in addition to a centralized oxygen injection (e.g. Low Head Oxygenation: LHO) supply. Since certain solenoid valves for oxygen are automatically opened in the event of a power failure, the supply systems of individual tanks can simultaneously serve as a life-supporting emergency system (see Figure 1).
Nitrogen gas (N2) can be problematic for fish and cause various disturbances, such as B. a gas bladder disease. Nitrogen gas saturation can be estimated by measuring the total gas pressure (TGP) and should be kept below ~ 101%. This value is normally only reached when air is injected under pressure somewhere in the system. If the TGP is more than 101%, the RAS should be checked to determine the reason for the supersaturation of the gas.
Fish produce 1.38 kg of carbon dioxide (CO2) per kg of oxygen (O2) consumed. Because of the interrelation between CO2, pH and alkalinity, it is possible to calculate the CO2 values in RAS using these two variables without directly measuring CO2. Certain control units in combination with pH sensors automatically calculate the CO2 values in your RAS even more conveniently if they enter information on alkalinity. Since CO2 should always be kept below 25 mg L-1 in order to reduce the physiological stress for fish, it is necessary to monitor the carbon dioxide content regularly and to adapt the CO2 stripping capabilities of your RAS appropriately to the growing fish biomass.
Ammonia / ammonium:
Fish excrete a mixture of ammonia (NH3) and ammonium (NH4 +), which are usually measured as TAN (TAN = (NH4 +) + (NH3)). In general, ammonia is toxic to fish at concentrations above 0.025 mg / l. The amount of non-ionized ammonium can vary and increases with the pH. Lower pH values therefore minimize the risk of exceeding this toxic ammonia limit of 0.025 mg / l. However, it is not advisable to reach a pH below 7 to ensure adequate biofilter performance. The ammonia values of your RAS can be calculated manually using TAN, pH and temperature (see recommended literature).
Nitrite (NO2 -) is poisonous to fish from 0.10 mg / l. If the biofilter is not fully retracted or impaired, the addition of chloride in the form of salt (sodium chloride) or calcium chloride until the salt content reaches 0.3 ‰ (corresponding to ~ 1.1 mS/cm conductivity) reduces the toxicity of nitrite on fishing.
Nitrate (NO3 -) is the end product of the nitrogen cycle, and although it is considered harmless, high levels (over 100 mg / l) can negatively affect growth and feed conversion. One way to avoid accumulation is to increase the exchange of new water, which dilutes the high concentration to a lower level. To save water, nitrate concentrations can also be reduced by denitrification. Under normal conditions, water consumption of more than 300 liters per kg of feed is sufficient to dilute the nitrate concentration. The use of less water than 300 liters per kg of feed makes the use of denitrification worth considering.
pH & alkalinity:
The pH value plays an adverse role in RAS, as lower pH values have a negative impact on fish health and can change the toxicity of TAN. In addition, the nitrification rate of the biofilter begins to decrease at pH values of ~ 6.8 and lower. It is therefore an important priority at RAS to continuously monitor and control the pH value. Alkalinity and pH are closely related and the most important control mechanism for stabilizing the pH is to increase the alkalinity to values above 100 mg / l (~ 1 mmol / l). During the biofiltration process, the oxidation of ammonium constantly produces hydrogen ions. These ions consume alkalinity and lower the pH. Likewise, the production of CO2 shifts the carbonate system towards a lower pH. To ensure adequate biofilter function, the pH must be kept above a minimum of ~ 7.0 while ammonium toxicity is minimized by stabilizing the pH at or below 7.5. Various chemicals are available for adding alkalinity to an aquaculture system. Hydroxides and carbonates are most commonly used. Commonly used carbonates include baking soda (sodium bicarbonate; NaHCO 3) and lime (calcium carbonate; CaCO 3). The most commonly used hydroxides include hydrated lime (calcium hydroxide; Ca (OH) 2) and caustic soda (sodium hydroxide; NaOH). The most convenient way to stabilize the pH is with an automatic dosing system consisting of a control unit connected to a pH meter and a dosing pump or solenoid valve that drips aqueous hydroxide solutions into the pump sump. When adding alkalinity to the system, it is important not to overdose, especially with hydroxides that can quickly change pH. Therefore, set sufficient time intervals in your control system between each dosing event to enable an adequate mixture. Since hydroxides can burn eyes and skin with difficulty, safety precautions must be taken and glasses and gloves must be worn when handling these substances. Just as it is possible in the RAS compartment to raise the pH value, the use of different acids (nitric acid, hydrochloric acid, and phosphoric acid) can shift the pH value to acid in order to optimize the plant production (Monsees et al. 2017).
In contrast to ammonia, nitrite, and nitrate, phosphate is an inert substance with no toxic effects. Nitrogen-based compounds are mainly excreted in the urine and found in aqueous solution, while phosphate is mainly bound in the feces and is therefore separated off by mechanical filtration in RAS. While phosphate is a waste product in RAS, it is an essential nutrient for plants in the hydroponic compartment of aquaponic systems. Phosphate can either be obtained from the waste stream of the mechanical filter unit by certain reactors or must be replaced with nutrient solutions. Because of its importance for plant growth, it is particularly important in aquaponic systems.
Total Suspended Solids: TSS (suspended particulate solids):
TSS in RAS mainly consists of feces and leftover feed pellets. TSS is measured by taking a water sample of known volume and passing it through a paper filter. Solids remaining on the filter paper are then weighed on a dry weight basis and the solids are reported in milligrams of solids per liter of water (mg / l). Popular filtration methods for solids include bead filters, sand filters, drum filters or a settling tank, and each of these filter solutions can be useful for a specific application. Target values for TSS are between 10 and 25 mg / l. It is very important to choose a suitable solids filtration method with sufficient dimensions to cope with the resulting solids in the system, as failure to meet these target values could often require a complete redesign of the RAS.
The oxidation-reduction potential (ORP), also called redox potential, measures the ratio of oxidized substances to reduced substances in a specific system. Since oxidation is the loss of electrons and reduction is the gain of electrons, ORP is measured in mV and gives an indication of the amount of organic substance in the process water. Monitoring the redox potential is particularly important with regard to the use of ozone (O3) in RAS, and ozone injection is normally controlled with regard to the redox potential. The redox potential should be kept between 200 and 400 mV since ozone above 500 mV becomes toxic to fish and can oxidize fish's gill surfaces.
Salinity is the number of salts dissolved in the water, most often sodium and chloride. The salinity is typically expressed in percent (%), parts per thousand (ppt), salt per kg water (g / kg) and can easily be determined electronically via the conductivity (S / cm2). Because it is possible to breed a wide range of species in RAS, including freshwater and marine organisms, the salinity is species-specific.
Measuring conductivity in the hydroponic part of aquaponics may be a useful way to estimate the total nutrient concentration of the solution, but nevertheless continuous monitoring of the chemical composition of individual nutrients is necessary to provide the water in the hydroponic compartment to appropriate levels depending on the cultured plants. Specific nutrient concentrations that fish produce in aquaponics are significantly lower for most nutrients compared to hydroponic systems. Nevertheless, plants grow in solutions that have a lower nutrient content than "standard" hydroponic solutions.
Sources and reading recommendations:
Bregnballe J. (2015) A Guide to Recirculation Aquaculture: An introduction to the new environmentally friendly and highly productive closed fish farming systems.
Goddek, S., A. Joyce, B. Kotzen, GM Burnell. 2019. Aquaponics food production systems. combined aquaculture and hydroponic production technologies for the future. Springer. UNITED STATES. 630 pp.
Kloas, W., Groß, R., Baganz, D., Graupner, J., Monsees, H., Schmidt, U., et al. (2015). A new concept for aquaponic systems to improve sustainability, increase productivity, and reduce environmental impacts. Aquaculture Environment Interactions, 7 (2), 179-192.
Monsees H, Kloas W, Wuertz S (2017) Decoupled systems on trial: Eliminating bottlenecks to improve aquaponic processes. PLoS ONE 12 (9): e0183056.
Neil, DM, Thompson, J., and Albalat, A. (2013) Freshwater Culture Of Salmonids In Recirculating Aquaculture Systems (RAS) With Emphasis On The Monitoring And Control Of Key Environmental Parameters. Technical report. University of Glasgow, Glasgow, UK. 85116
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optical oxygen sensor
powerless open solenoid valve
high pressure ceramic oxygen diffuser
Figure 1: Aquaculture rearing tank with individually regulated oxygen supply and simultaneous emergency supply in the event of a power failure or pump failure. Feeding can be regulated depending on the oxygen content as well as the temperature.
control tasks unit 2:
O2-regulation in rearing tank
control tasks unit 1:
O2- regulation RAS
dosing pump: pH
monitoring of the water level and oxygen saturation
monitoring of the water level
ORP, EC, pH, O2, T
EC, T, pH
solenoid valve: hydroponics dosing pump: pH
Figure 2: Possible configuration of a decoupled aquaponic system (DAPS). This is divided into a RAS and a hydroponic compartment. In addition, water filtration steps and suggestions for measurement and control architecture are displayed.
A short guide to monitoring and control of water quality in recirculating aquaculture systems (RAS) and aquaponic systems