CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/DK2017/050248, having a filing date of Jul. 24, 2017, which is based on DK Application No. PA 2016 70584, having a filing date of Aug. 5, 2016, the entire contents both of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to a centralized water monitoring station that provides rapid analysis of water quality for fish farms, for example land-based fish farms. In particular, it relates to a method and system for monitoring water quality in a plurality of fish tanks, especially for centralized water monitoring in a fish farm.

BACKGROUND

Fish today are increasingly cultured in land-based recirculation aquaculture systems (RAS). To ensure optimal fish performance in terms of growth rate, feed conversion and fish welfare, it is critical to ensure a high quality and stable water quality.

Water quality today is managed by a combination of sensors hanging directly in fish farm water and by manual measurements. This solution typically implies that a single fish farm has many decentralized sensors. However, systems with decentralized sensors imply a number of disadvantages. For example, decentralized sensors are difficult and time consuming to calibrate and must be cleaned, typically, by time-consuming manual means. Decentralized sensors also require an expensive electrical installation and result in difficult error detection. Some sensors cannot generate stable and correct results when hanging directly inside a fish tank or water treatment system. For example, some sensors require a stable flow velocity past a measurement membrane to provide a stable measurement. Other sensors can be damaged when they are submerged during a power failure or damaged directly due to mechanical forces.

Furthermore, the sensors used in fish farming today are very susceptible to corruption of data due to bio fouling. Bio fouling on sensing components interferes with readings and often causes false positives, increased noise and sensor failure. When using multiple, decentralized sensors, there are different calibration accuracies for each sensor, varying levels of bio fouling for each sensor, and different physical conditions for each sensor. These differences combine to ensure that the data generated by decentralized sensors are nearly impossible to compare against each other.

There are often multiple fish farms within a single organization. Learning on one farm could be beneficial to the other fish farms within the organization. However, when using multiple, decentralized sensors across multiple fish farms, the number of variables increase to an extent that influences data quality such that data cannot be applied for useful comparison or learning.

In contrast to the above-described decentralized sensors, U.S. Pat. No. 8,534,228 discloses an apparatus with a central measuring device for measuring the water quality for a plurality of fish tanks. Peripheral water conduits from the various fish tanks are connected to a switchable multiplexer, the outlet of which is connected to the central measuring device to selectively feed sample water from each one of the peripheral conduits to the central measurement conduit to test the water in the specific fish tank. Each of two parallel measurement arms of the central measurement conduit comprises multiple sensors connected in series for measurements on water samples from a single fish tank at a time. The water from the fish tanks flows only by gravity to the multiplexer.

The system in U.S. Pat. No. 8,534,228 is a simplification relative to the above mentioned decentralized system. However, it still has some disadvantages. One of the disadvantages is a relatively long measuring time and inferior utilization of the overall measurement capacity, as each water sample has to undergo various measurements, some of which take substantially longer than others, which results in those detectors that finish the measurements quickly to be in an idle state for rather long time while the longer lasting measurements are being done. This is especially so because the system is configured to measure water samples only from a single fish tank at a time. Only once the water samples from a single fish tank are finished, the manifold switches to provide water from another fish tank to the central measurement device. Accordingly, the system is not optimized with respect to speed. To this adds that the gravity flow from the tanks to the measurement device is slow. This is also why the system in the disclosure in U.S. Pat. No. 8,534,228 appears more useful for small-scale fish tank systems than large-scale systems of fish farming where measurement speed has to be optimized. Large-scale systems also require great flexibility in transferring water samples to a centralized water monitoring unit. For example, industrial fish farms have often tanks submerged below the floor surface or in some other way impede the creation of a gravity flow back to a centralized water monitoring unit. Furthermore, a powerful system to evacuate used water samples, remove contaminates from the water sample, and to minimize the effect of small contamination upon the final measurement result are essential when completing centralized measurement on an industrial fish farm.

It would be desirable to provide a system and method that has the advantages of a centralized measurement system but not the disadvantages of the prior art.

SUMMARY

An aspect relates to an improved system for monitoring water quality in fish farms, for example land-based fish farms. This aspect is achieved by a method and system for monitoring water quality in a plurality of fish tanks, especially for centralized water monitoring of a fish farm, as described in more detail in the following.

The following relates to a centralized water monitoring station that is designed for fish farms, for example land-based fish farms, fish farms in ships, or fish farms in open water.

Advantageously, it measures various relevant water quality parameters. The system comprises a first sensor system and a second sensor system for measuring a different parameter, or various different parameters, with the first sensor system as compared to the second sensor system. The inclusion in the design of a dual measurement system maximises the number of water samples that can be measured over the course of a day. This allows the water monitoring station to develop comparable water quality profiles across an entire fish farm with enough detail to help management staff to make significantly improved decisions. Optionally, the first and second sensor systems are contained in a single building or facility, making a water monitoring station a compact entity.

The method comprises the following step. By the first sensor system, a first sample of water is received from a first of the plurality of fish tanks, and a first parameter of the first sample is measured with the first sensor system. By the second sensor system, a second sample of water is received from a second of the plurality of fish tanks, and a second parameter is measured with the second sensor system. The term first parameter should also be understood as a single first parameter or a set of first parameters. For example, the first sensor system comprises a first set of a plurality of sensors, optionally connected serially or in parallel, by which a first set of parameters is measured in the first sensor system. Correspondingly for the second parameter or parameters.

Advantageously, the second parameter, or second set of parameters, is measured of the second sample with the second sensor system while simultaneously measuring the first parameter, or first set of parameters, of the first sample with the first sensor system. The second parameter is different from the first parameter, thus implying different types of sensors in the first sensor system as compared to the second sensor system.

If the water monitoring station uses two sets of sensors, one set in each sensor system, time is saved by simultaneous measurements of two samples from two different fish tanks by the two sets of sensors.

Once, the two samples are measured, the measurements of the respective sensor systems for the two tanks are reversed, such that, subsequently, another water sample from the second fish tank is received by the first sensor system and the first parameter measured and a further water sample from the first fish tank is received by the second sensor system and the corresponding second parameter measured.

In cases where the measurements for the first and the second sample are done simultaneously, simultaneous measurements are possible but not necessary for the other sample in the first sensor system and the further sample in the second sensor system. This is, especially, so because the time it takes to measure the first parameter with the first sensor system may be substantially longer, for example multiple times longer, than the time it takes to measure the second parameter with the second sensor system. This is the case if the first sensor system comprises a slower sensor or a set of slower sensors, for example for measuring a parameter of at least one of unionized ammonia, carbon dioxide, and total carbonate, and the second sensor system comprises a faster sensor or a set of faster sensors for measuring at least one parameter of oxygen concentration, temperature, redox, turbidity, conductivity, or pH. In such case, the second sensor system can measure several samples during the time it takes for the first sensor system to measure only a single sample.

For example, the water monitoring station uses only one sensor of each type, however, potentially multiple types of sensors in each sensor system. This implies that a higher quality grade of sensor can be afforded and be applied for less cost as for similar systems with multiple sensors of the same type. Applying centralized sensors also makes calibration easier with fewer sensors in one location, especially a central location. Furthermore, using only a single sensor of each type of sensors, especially high-quality sensors, for all measurements means that all results from all measurement locations are directly comparable to each other. Only by accurately benchmarking each measurement location, is it possible to measure the change in water quality across a fish tank. The change in water quality across the fish tank is key to fish performance, with fish performing markedly better when the water environment is stable. By standardizing the quality of data from each water monitoring station, an organization has the ability to benchmark all fish farms against each other.

For example, the system comprises a plurality of fish tanks divided into a first and a second subgroup. A first sample conduit only connects to the first of the subgroups through a first set of pipe connections, and a second sample conduit connects only to the second of the subgroups through a second set of pipe connections. Each one of the pipe connections is connected to only one fish tank. The pipe connections are configured for water sampling from a selected fish tank of the first subgroup into the first sample conduit and from a selected fish tank from the second subgroup into the second sample conduit.

By this system, water samples are received by a first and second sensor system from the first and second sample conduits, respectively, for simultaneous measurements of the two samples by the two-sensor system. In more detail, the system is configured for providing a first water sample selectively from one of the first and second sample conduit to the first sensor system and a second water sample from the other one of the first and second sample conduit to the second sensor system. Measurements of the first parameter of the first sample and the second parameter of the second sample can then be done simultaneously by the first and second sensor system, respectively.

For example, after measuring the second parameter of the second sample, the second sample conduit is emptied, for example by release of pressurized air into the second sample conduit, and a third sample is received by the second sensor system from a third of the plurality of tanks through the second sample conduit. Thus, the second as well as the third sample from two different tanks, and potentially further samples from further tanks, are measured with respect to the second parameter, or second set of parameters, while the first parameter measurement of the first sample is still in progress.

In some embodiments, each one of the first pipe connections comprises a sample point pump for pumping water from the respective fish tank of the first subgroup into the first sample conduit, and each one of the second pipe connections comprises a sample point pump for pumping water from the respective fish tank in the second subgroup into the second sample conduit.

Optionally, each of the pipe connections comprises a water filter and is connected through a valve and filter clean conduit to a source for pressurized air for cleaning the water filter with the pressurized air.

In some embodiments, the samples from either of the two subgroups of fish tanks are provided into two corresponding sample collection chambers, which function as intermediate buffers, one for each subgroup, however, with the freedom to select the sample collection chamber for one or the other subgroup and with the possibility to switch from one subgroup to the other. The sample collection chambers are advantageously larger, for example at least an order of magnitude larger, than the samples taken from them, in order to minimize the variations in samples taken from a single fish tank.

In more detail for a concrete embodiment, a first sample collection chamber is connected to only the first sample conduit for receiving a first water volume from anyone of the fish tanks in the first subgroup into the first sample collection chamber through the first sample conduit. Correspondingly, a second sample collection chamber is connected to only the second sample conduit for receiving a second water volume from anyone of the fish tanks in the second subgroup into the second sample collection chamber through the second sample conduit. The first and second sensor system are selectively connectable to the first and the second sample collection chamber, respectively, for connection of one sensor system to one sample collection chamber while the other sensor system is connected to the other sample collection chamber. This opens the possibility for simultaneous measurements of a first sample from the first sample collection chamber by one sensor system and a second sample from the second sample collection chamber by the other sensor system.

In practice, a first volume of water is collected from the first fish tank via a first sample conduit into a first sample collection chamber and the first sample of water provided to the first sensor from the first sample collection chamber. Further, a second volume of water from the second fish tank is collected via a second sample conduit into a second sample collection chamber, and the second sample of water is provided to the second sensor from the second sample collection chamber. Typically, the amounts of water of the first and second samples are at least an order of magnitude smaller than the first volume and the second volume.

This results in a large, continuous sample flow from the sample delivery system versus a relatively smaller flow of sample water across the sensors. This ensures that a contaminate originating from the tank or sample delivery system, impacts least upon the analysis result.

After, the second sample of water from the second sample collection chamber has been provided to the second sensor, and the second parameter has been measured, the second sample collection chamber and the second sample conduit are emptied, for example by using pressurized air. Subsequently, a third volume of water is collected in the second sample collection chamber via the second sample conduit from a third of the plurality of fish tanks and provided to the second detector system, with a subsequent emptying of the second sample collection chamber from the third volume of water.

In those embodiments, where the time for measuring the first parameter with the first sensor system is multiple times longer than the time for measuring the second parameter with the second sensor system, this measurement of the second and the third sample, optionally including the emptying after measuring the second parameter of the third sample, are done before the measurement of the first parameter of the first sample with the first sensor system is finished.

Advantageously, the cleaning process comprises emptying the second sample collection chamber by releasing pressurized air into the second sample conduit and causing air-pressurized flow of water from the second sample conduit into the second sample collection chamber, followed by emptying the second sample collection chamber through a dump valve. Similarly, the first sample collection chamber and the first sample conduit are emptied with pressurized air, once the measurement of the first parameter in the first water sample has been finalized. Advantageously, the cleaning process comprises flushing the first and second sample conduit with a cleaning agent, for example a sodium hydroxide solution, followed by fresh water flush, and then releasing pressurized air into the first and second sample conduit for pressing the fresh water out of the first and second sample conduit.

A centralized water monitoring system and station, as described, has a number of advantages. It provides the correct water flow velocities for each sensor, and engineers each sensor armature to ensure the correct physical environment under all situations to protect the sensor. For example, the centralized water monitoring station applies automatic cleaning for all sensors and tubes, optionally a combination of compressed air, mechanical washing, ultrasonic cleaning, and chemical cleaning. The sample delivery system for the water monitoring station is configured in such a way as to minimize the distance which the water sample shall travel. This also maximizes the speed and efficiency at which the sample delivery system can be evacuated from used sample water and maximizes the speed at which the sample delivery system can be cleaned. In some embodiments the system provides a full automated cleaning procedure to remove biofilms from every tube, armature and sensor within the entire system.

In some embodiments, the first sensor system comprises a carbon dioxide sensor, CO2 sensor, for indirectly measuring a parameter of total carbonate. The method comprises addition of acid, for example citric acid, to the water sample in order to set carbon dioxide free from the water sample prior to measuring the free carbon dioxide, which is then a measure for the concentration of total carbonates in the water sample.

In some embodiments, the first sensor system comprises an ammonia sensor for measuring a parameter of unionized ammonia, and the method comprises base addition to the water sample prior to measuring the parameter of unionized ammonia.

In some embodiments, the first sensor system comprises a CO2 sensor, for example a standard carbon dioxide probe, for measuring carbon dioxide. Optionally, a breather valve is provided for the CO2 sensor, and opening the breather valve to atmosphere between measurements by the CO2 sensor protects the probe from condensation, and also allows the sensor to drop to a near zero level between measurements. A near zero level between measurements is important to ensure an earlier measurement does not impact upon a future measurement.

In some embodiments, the system comprises a wash manifold that is pipe-connected to sources of cleaning fluids, air, and fresh water and aggregates all the cleaning gases and fluids in a central block of valves. From this central block, any mix of various cleaning fluids, air, and fresh water can be charged at pressure to any selected part of the system, for example selectively to the sample conduits, the sample collection chambers, and the sensor systems.

In some embodiments the system provides the use of pressure relief valves to accelerate the emptying of used sample water from tubes, armatures and sensor heads of the sample delivery system and inside the water monitoring system itself.

For example, each pipe connection is communicating with the water in the fish tank only through a sampling unit that has a water entrance and a water exit, optionally the water exit being at the bottom of the sampling unit. The water entrance has a total entrance area for entrance of water into the sampling unit through the entrance area. The water exit has a total water exit area for exit of water from the sampling unit through the exit area. Advantageous, the entrance area is multiple times larger than the exit area, as this results in a smooth flow into the sampling unit and thereby minimizes the risk for air entering the system. In some embodiment, each pipe connection is communicating with the water in the fish tank only through a sample cup inside the corresponding fish tank, the cup is communicating with the water in the fish tank only through an upward opening below the surface of the water in the fish tank. The upward directed opening prevents air to enter the cup and releases possible air from the cup. Water from the fish tank is sampled into the pipe connection only through the upward opening of the cup. The combination of sample cup and filter is used to exclude gas bubbles and contaminates from water samples collected by the sample delivery system.

Special Remarks to Carbonate Measurements

It should be emphasized that only a small percentage of the free carbon dioxide that is excreted from the fish remains as gaseous free carbon dioxide in the water. The largest portion binds into the water to form carbonate. The percentage that binds is pH dependent—higher the pH the more free carbon dioxide binds into the water. Typically, in fish farm water of pH 7.0, around 90% of the carbon dioxide binds into the water. For example, fish farm water may consist of 100 mg/L total carbonates, of which 10 mg/L remains as free gaseous carbon dioxide and 90 mg/L is bound carbonate. Carbonate is an important chemical to measure as it is responsible for pH stability in the fish tanks, and carbonate is the dominating component of alkalinity in fish farms. Environmental pH is directly important to the physiology of the fish—more stable pH means more stable culture conditions for the fish. The pH also affects the toxicity of a range of chemicals in fish farm water—notably the concentration of gaseous carbon dioxide and the concentration of unionized ammonia.

A problem is the measurement of carbonate for fish farming. As an improvement for measuring carbonate, acid is used, for example citric acid, to lower the pH of the respective water sample. This means that the carbonates in the water sample gets unbound, forming free gaseous carbon dioxide. The free gaseous carbon dioxide is then measured using the carbon dioxide probe.

For this reason also, the above system also provides an embodiment, namely, a method for measuring the total carbonate in water, for example water in a fish tank. The method comprises providing a sample of the water and adding acid, for example citric acid, to the sample to lower the pH of the sample, thereby unbinding carbonate, forming free gaseous carbon dioxide, and measuring the amount of free carbon dioxide with a CO2 sensor. The measured amount of carbon dioxide is then related to the volume of the water sample in order to determine the total carbonate concentration of the water. Optionally, the free carbon dioxide amount for the water is measured prior to the addition of the acid in order to differentiate between free carbon dioxide and bound carbonate in the water.

For a water sample of 100 ml, typically less than 0.1 ml of citric acid is added to free the carbon dioxide from the bound carbonate, where the concentration of the citric acid is 30%, for example in order to lower the pH from 7 to less than pH 4 Alternatively, other acids can be used, for example hydrochloric acid. The concentration and amount of the alternative acid is adjusted to lower the pH from an initial value, for example 7 to less than pH 4.

This embodiment is useful for a fish tank system and advantageously combined with the features of the method and system above.

CO2 Sensor

Common carbon dioxide probes have a CO2 sensor that becomes very hot during operation. This implies that moisture that normally is generated at the air-to-water interface of the probe evaporates into the air. However, when the CO2 sensor after a measurement cools down, when powered off, moisture condenses from the air to form large droplets. When the probe is powered up again subsequently, this moisture enters to the sensor head, causing damage. Especially, the process of stabilizing the carbon dioxide probe between measurements is commonly a very slow process. This means that an earlier measurement can affect the following measurement if an insufficient stabilizing period is not given.

As an example, if a tank1had a high free CO2 content of 20 mg/L and tank2had a free CO2 content of 10 mg/L, one has to expect about one hour for measuring two tanks, because it would take around 15 minutes to read 20 mg/L, then a stabilizing period of about 30 minutes before the sensor is a sufficiently low level to measure a significantly lower level, and finally around 15 minutes to read 20 mg/L of the second sample.

This is a very slow procedure, why there is a desire for improvements

An improvement can be achieved with a free carbon dioxide probe if the CO2 sensor is combined with a breather valve that provides the CO2 sensor with atmospheric air in order to drain the humidity from the sensor, especially during powering-up. Using such a “breather valve”, the process of stabilizing the carbon dioxide probe between measurements is very fast. If a tank1had a high free CO2 content of 20 mg/L and tank2had a free CO2 content of 10 mg/L, one has to expect only about 30 minutes to measure2tanks, which is only half of the time as compared to the foregoing example. The time is distributed to around 15 minutes to read 20 mg/L from tank1, then a stabilizing period of about 3 seconds, to get the probe back to a zero level, and around 15 minutes to read 20 mg/L. Thus, instead of the prior art 30 minutes stabilizing period, the intermediate period is reduced to three seconds, which is almost instantly.

For this reason, also, the above system also provides an embodiment, namely, a method for measuring free carbon dioxide in water, for example water in a fish tank. In this case a CO2 sensor is provide in combination with a breather valve which in a closed state separates the CO2 sensor from atmospheric air and which in an open state provides a passage from the CO2 sensor to atmospheric air. The method comprises providing a sample of the water and measuring the amount of free carbon dioxide with a CO2 sensor, then opening a breather valve to open a path between the CO2 sensor and atmospheric air and thereby evaporating possible moisture from the CO2 sensor into air, then closing the breather valve again and providing a new sample of water for a subsequent carbon dioxide measurement with the CO2 sensor. For example, the time in which the breather valve is open is less than 1 minute. Thus, the time between measurements can be correspondingly reduced to this time of less than 1 minute.

This embodiment is useful for a fish tank system and advantageously combined with the features of the method and system above. It is also useful when combined with the above method of total carbonate measurement.

