H2S – Hydrogen Sulfide

H2S – Hydrogen Sulfide

Mass mortality in RAS

The intention of this document is to try to explain why the industry have experienced a series of mass mortalities in RAS systems in the last years. The mortalities are linked to the release of hydrogen sulphide (H2S). This document explaines a simple, inexpensive and practical way to mitigate the release of hydrogen sulphide and mass mortality.

Pisco group have, with input from AquaCircle, the Danish Industry Cluster for Aquaculture development, elaborated this document. Further research will need be done to verify the details of the theory, and refine the recommendations. We feel that we need to bring this information to the table now, if there is a chance to save fish from H2S induced mortality in RAS systems.

The conclusion of the theory is: Always keep the Nitrate concentrations at a sufficient level!

During the last decade it has become common to use seawater in all steps of the salmon production. Salt is thus used from smolt production systems, where the aim is to control fungus, to postsmolt systems where higher salinity is required. Seawater contains more Sulphate (SO4-) than freshwater. Elevated Sulphate concentration increase the likelyhood of hydrogen sulphide generation in the systems.

Keep enough nitrate in the system

Until recently, the industry have only considered Nitrate (NO3-) as a by-product that needed to be deluded out of the production water in order not to reach toxic levels. Therefore, we have never been paying attention to the nitrate levels, when considering the mysterious mass mortalities that we have seen in RAS systems in recent years.

By studying data from various systems experiencing sudden mass mortality, we have found that many of these systems have seen a sudden drop in Nitrate 12-72 hours before the incident.

It is our genuine belief that we have all been inattentive to the importance of Nitrate in relation to Hydrogen sulphide generation and control. The stoichemtric formula below describes a process that are central to recirculation systems. We cannot quantify the extend, but this reaction is taking place all the time in all RAS systems. Our hypothesis is that Nitrate has been the “guardian angel” of RAS systems, by keeping the Hydrogen Sulfide generation in check and avoiding toxic levels of the substance to result in mass mortalities.

                                                                         5H2S + 8NO3- -> 5SO42- + 4N2 + 4H2O + 2H+

From the chemical equation above it can be seen, that if the concentration of Nitrate decreases, its scavening effect is reduced and toxic hydrogen sulphide will be released into the water and eventually kill the fish.

These practical recommendations will be further elaborated in the next explanatory section. The following considerations about biofilm and reactions are very simplified and serves only to illustrate the principle.


Why and where is toxic hydrogen sulphide produced?

Biofilm exists on almost all surfaces in a recirculated system (not just in the biofilter). Biofilm grows and becomes thicker with age. The biofilm can simplified be viewed as two layers: An aerobic top layer (oxygen present) of approximately 200 μm and an anerobic bottom layer (oxygen depleted).

The top layer plays an active role in RAS where it converts the toxic Ammonia (NH3) and Ammonium (NH4+) to harmless Nitrate through a bacterial process called nitrification. This process consumes oxygen.

The bottom layer contains bacterias that consumes organic material and converts sulphate to toxic hydrogen sulphide. This layer contains no oxygen and can in theory be very thick and contain large ammounts of hydrogen sulphide.

Again it must be emphazised that many other processes are taking place simultaniously in a biofilm. The illustration below serves to further illustrate this possible pathway.

When is hydrogen sulphide released into the water?

The following example explains a common production scenario where smolts or postsmolts are starved for a number of days before being collected by a wellboat and transported to sea.

Step 1

The system is in steady state. Ammonia and Ammonium is converted to Nitrate. The concentration of Nitrate in the water is high. Hydrogen sulphide “eats” Nitrate and coverts into harmless sulphate.

Step 2

Feeding has stopped. The excretion of Ammonia from the fish is reduced and the bacterias convert less Ammonia into Nitrate. The concentration of Nitrate in the water decreases because of water exchange and denitrification in the bottom layer of the biofilm. As the nitrification rate drops, Nitrate is depleted in the system. The Hydrogen sulphide is still consuming the Nitrate in the bottom part of the biofilm

Step 3

The toplayer of the biofilm is getting thinner, as the nitrifying bacterias are starved. The Hydrogen Sulphide is still “eating” the Nitrate in the bottom part of the biofilm.

Step 4

The system is now completely depleted of Nitrate and Hydrogen sulphide diffuses into the water, with the risk of reaching toxic levels.

How can you react in time? 

It is essential, always to have enough nitrate avalible in the water. Therefore you should:

  • Always have Calcium nitrate Ca(NO3)2 or another Nitrate source on stock to dose into the water if the Nitrate level drops to a critical level.

  • Make sure to measure Nitrate concentration at leats once a day.

  • Keep the nitrate level above 40 mg/l Nitrate-(N), in order to be on the safe side. 

  • Reduce water exchange rate with new make up water, if you experience a sudden drop in Nitrate concentration.

There are chritical situations, where you need to pay special attention because of the risk of a sudden drop in the nitrate concentration. In these situations you should measure at least twice a day.

These situations are:

  • Feeding has been stopped or reduced dramatically.
    (Remember to turn off the denitrification filter, in any, if you experience a sudden drop in Nitrate concentration)

  • Large amounts of water has been changed in the system

  • When restocking with fish, without having cleaned the system.
    (Remember to dose Ammonia into the biofilter during the empty period in order to keep the ammount of nitrifying bacteria alive and thereby keep a high Nitrate level.

Dosing Calcium nitrate

IF the Nitrate-(N) level drops below 40 mg/l, Calcium nitrate should be dosed into the water. Calcium nitrate is a relatively inexpensive compound, (approx. 10 – 15 NOK/kg). The compound can be bougt in granular- or liquid form.

For each 1,000 m3 of water in the system, dose 5.85 kg of Calcium nitrate to increase the concentration of Nitrate-(N) by 1 mg/l. The following equation can be used to determine the amount of Calcium nitrate to dose into your system:

  1.                                                        kg Ca(NO3)2 needed = Volume (m3) ∙ mg/l NO3 N increase ∙ 0.00585 


In a system with a total volume of 280 m3, a drop down to 30 mg/L Nitrate-(N) is observed. It is therefore recommended to increase the concentration by 20 mg/l to a final level of 50 mg/l Nitrate-(N). The necessary dosing of Calcium nitrate in granular form is thereby:

  1.                                                           kg Ca(NO3)2 needed = 280 m3 ∙ 20 mg/l ∙ 0.00585 = 32.8 kg

Leave a Reply

Email của bạn sẽ không được hiển thị công khai. Các trường bắt buộc được đánh dấu *