Optimising RAS Feed Conversion Efficiencies With Aquamonitrix® - Aquamonitrix® - Nitrate and Nitrite Analyser

On the understanding that a stable ambient environment – without too many shocks to the system – supports healthy, hungry fish, an RAS facility on Ireland’s Atlantic coast is using real-time nitrite and nitrite monitoring to ensure the biofilter is operating correctly at all times and achieving complete conversion to nitrate.

The monitoring is being carried out using an Aquamonitrix® real-time nitrite and nitrate analyser, with the aim of achieving that ‘sweet spot’ where feed rates and water quality are optimised, so that the fish are feeding well and achieving maximum feed conversion efficiencies.

The RAS, located close to Clew Bay on the Atlantic coast, is operated by the Marine Institute – Ireland’s state agency for marine research and development. The system comprises eight x 5.5 cubic-meter tanks, with a total capacity to produce up to 12,000 Atlantic salmon pre-smolts.

Currently, the RAS environment is being used in a project aimed at producing larger and more robust pre-smolts than conventional flow-through systems while adhering to organic salmon production standards.

Benefits for RAS operation

One of the obvious boons of automated nitrite and nitrate monitoring is the potential to reduce the need for analytical chemists and laboratory time. Aquamonitrix® is easy to set up, operate and service, with minimal intervention requirements and long intervals between services.

However, the bigger game-changer is the higher temporal resolution of data and the availability of results in real-time. “We can observe the fluctuations in nitrite levels corresponding with feed input throughout the day,” Jamie says. “Feeding starts at 8.30 am every day, with 20.4 grams added to the system every 90 seconds for 12 hours. Once feeding starts in the morning, you see a build-up in nitrite throughout the day as the fish feed and metabolise the food. Then overnight, the bacteria break this down to nitrate.”

Graphs 1 & 2 (below): Nitrite levels follow a daily cycle, closely tracking the feeding regime

It is also interesting to see the close correlation between biomass gain and total feed input (See graphs 4 and 5 below). This indicates very effficient feed conversion, as would be typical in salmon species. But it also underscores the stress-free, high water-quality environment that the Marine Institute’s pre-smolts have enjoye

A new breed of analyser

The current standard within the RAS sector is to perform nitrite analysis by spectrophotometry – a technique that requires time-consuming sample preparation in the laboratory, using hazardous reagents to create a colorimetric reaction.

While some autonomous analysers are marketed for nitrite measurement, their applicability is limited in real-world aquaculture systems, because the turbid environment, presence of organics and high nitrate concentrations can cause interference and undermine instrument reliability.

Aquamonitrix® overcomes these challenges by combining the separation capabilities of ion chromatography with UV-LED detection. This allows for a highly specific and accurate reading of nitrite, even when nitrate concentrations are high. The column also acts as a ‘trap’ for colour, organics and air bubbles, which can adversely affect the performance of conventional analysers based on ISE, colorimetric or standard UV/UV-LED methods

User-friendly interface

“For us, the user interface is excellent,” Dr Downes adds. “I can log in remotely and look at the nitrate and nitrite levels any time, day or night. This informs our operation strategy – whether nitrite peaks at an acceptable level, or we have reached the limits of the biofilter, or we need to increase our water exchange rate.

The Datamonitrix user interface, accessible via PC, allows users to store & analyse data; assign users and email distribution lists; set up alerts and alarms, and receive analyser self-diagnosis warnings

In terms of the overall benefits of Aquamonitrix®, Dr Downes says: “It has allowed us to be really on-point with our water quality. By the end of the year, we expect the RAS salmon to have achieved double the weight gain of their counterparts outdoors.”