BASIC PRIMER:

Nitrite & nitrate in RAS aquculture are key variables for the control and optimisation of the biofilter to avoid toxic build-up.

Designed to remove solids and recycle treated water back to the fish culture tanks, closed containment or recirculated aquaculture systems (RASs) offer many advantages.

With reduced water requirements, the cultivation of marine species no longer requires a coastal location. In fact, warm-water species can be produced in more temperate climates, and fish farms can be sited almost anywhere, even in urban spaces. Compared with traditional pond and floating cage methods, the more controlled closed environment also reduces ecological risks such as the transmission of diseases to wild stock or dilution of genetic diversity as a result of interbreeding with escaped farm species.

However, accurate control to prevent the build-up of nitrite and other potential toxins such as hydrogen sulphide is essential for maintaining healthy aquatic environments.

 

Sources of nitrite & nitrate in RAS aquaculture

  • In aquaculture, nitrogen enters the system as protein in the feed.
  • Depending on the feed conversion efficiency, about 20 to 25% of this accumulates in the animal tissues. The remainder is excreted as ammonium and organic nitrogen by fish/aquatic species through their gills or as urine.
  • Other sources of ammonia in the system include decaying organic substances such as dead animals or uneaten food
  • Depending on the source, water may also contain nitrate pollution, typically as a result of sewage or agricultural sources

The risk to species survival and health

  • Depending on temperature and pH, ammonium (NH4) may become unionized. Un-ionized ammonia (NH3) is toxic and can cause gill and organ damage to aquatic species and may cause death in concentrations as low as two parts-per-million (2 ppm/mg/l).
  • In biological filter systems, the biofilter converts toxic ammonia to less harmful nitrate (NO3).
  • This nitrification process occurs in two steps process. First, a community of micro-organisms known as the ammonia oxidising bacteria (or AOB oxidise the ammonia is  to nitrite.
  • This nitrite intermediate is poisonous to fish at all pH levels. Nitrite poisoning follows closely on the heels of ammonia as a major killer of aquaculture species.
  • In turn, a second set of bacteria, known as the nitrite oxidising bacteria or NOBs oxidise the nitrite is  to less harmful nitrate . 
  • Nitrate is much less toxic than either ammonia or nitrite. However, high concentrations and long exposure times can reduce animal growth and decrease survival. As explained below, nitrate accumulation imust be dealt with in closed-loop systems. On the other hand, when hydrogen sulphide formation is a problem, maintaining higher nitrate levels can be beneficial.

 When nitrite becomes a problem

  • Nitrite problems can arise in closed culture systems due to insufficient, inefficient management and malfunctioning filtration systems.
  • If nitrite levels exceed that which resident bacteria can rapidly convert to nitrate, a buildup of nitrite occurs, creating the risk of brown blood disease.
  • Nitrite enters the bloodstream through the gills and turns the blood to a chocolate-brown colour. Haemoglobin, which transports oxygen in the blood, combines with nitrite to form methemoglobin, which is incapable of oxygen transport. Brown blood cannot carry sufficient amounts of oxygen, and affected fish can suffocate despite adequate oxygen concentration in the water.
  • Fish that are exposed to even low levels of nitrite for long periods of time suffer damage to their immune system and are prone to secondary diseases.
  • Mitigation actions such as saline flushes or water exchange can be used to proect fish when safe nitrite levels are breached. However,  by disrupting ambient conditions, this can affect the both the fish population, temporarily halting feeding, and the bacteria communities that power the biofilter.

Dealing with nitrate accumulation

  • Water exchange is one of the most common strategies for dealing with nitrate accumulation. However, the discharge of effluent containing high nitrate levels may not always be permitted at inland sites, nitrate vulnerable zones, and other environmentally senstive areas. e
  • Therefore, some RAS units may incorporate a denitrification system. This involves the anaerobic conversion of nitrate to nitrogen gas. The most common wastewater denitrification systems are based on denitrification with heterotrophic bacteria and the addition of a carbon source – typically methanol. The nitrate is converted to nitrogen gas and the dissolved organic carbon is converted to CO2 and water.
  • Phytoremediation, e.g. aquaponics or hydroponics, is another strategy for dealing with nitrate
  • In aquaponics, nitrate is removed by crops. According to the Institute for Systems Biology, the recommended level of nitrate in a stable aquaponics system is between 10 and 150 ppm. If the nitrate levels are much lower, the plants won’t be getting the nutrients they need. On the other hand, if the nitrate level is above 150 parts per million ppm, there is a risk of ‘nutrient burn’ of the plant roots.

 

Benefits of operating at higher nitrate levels

The biofilter can also be a hotspot for hydrogen sulphide – another toxic gas that can form in RAS environments. The upper safe limit is  2 ppm  for freshwater species and 5 ppm for marine species. Long-term exposure to  H2S can increase stress, reduce feeding and growth and affect  swimming performance.

Sulphate is present naturally in both freshwater  at levels of ~5–50 ppm) and at high levels of up to 2,700 ppm in seawater, so that the problem can be more accute in marine RAS setups.

In anaerobic zones, which can occur in the biofilter and pipes, sulphate reducing bacteria  (SRB) can convert water, sulphate and volatile fatty acids to hydrogen sulphide.

But sulphate reducing bacteria (SRB) compete with nitrate reducing bacteria for volatile fatty acids. As a result, at a higher nitrate levels, nitrate reducing bacteria (NRB) can outcompete the SRB for finite volatile fatty acids. This means that the presence of nitrate reduces the risk of hdyrogen sulphide formation.

Understanding and controlling nitrite & nitrate in RAS aquaculture

Aquamonitrix® is the only nitrite & nitrate analyser for RAS aquaculture that enables users to obtain highly specific and accurate readings for both the nitrate and nitrite, autonomously in real-time, at relevant limits of detection for RAS species – in fresh or fully saline water environments.

This availability of accurate, high-frequency data allows users to better understand, control and optimise biofilter performance for a better understanding of risk factors, healthier fish, better feed conversion efficiency and higher profits.