In a Recirculating Aquaculture System (RAS), fish continuously release metabolic waste into the culture water. Among these waste products, ammonia is one of the most important compounds that must be controlled.
Unlike traditional pond systems, RAS farms continuously recycle the same water. This means dissolved waste can accumulate rapidly if the treatment system is not properly designed and operated.
This is where nitrification becomes essential.
Nitrification is a biological process in which microorganisms convert ammonia into nitrite and then nitrate:
Ammonia → Nitrite → Nitrate
A properly functioning biological filter provides the environment where these microorganisms can grow and perform this conversion.
For commercial RAS projects, understanding the nitrification process is essential for:
This guide explains how nitrification works in RAS, how biofilters remove ammonia, what affects nitrification performance, and how YUTANK MBBR biofilters can be integrated into commercial aquaculture systems.
Nitrification is an aerobic biological process carried out by specialized microorganisms.
The process occurs in two primary stages:
Ammonia → Nitrite
Ammonia-oxidizing microorganisms convert ammonia into nitrite.
Nitrite → Nitrate
Nitrite-oxidizing microorganisms convert nitrite into nitrate.
The overall process allows toxic nitrogen compounds to be transformed into a less immediately toxic form that can then be managed through the wider RAS.
The two stages are closely connected, but they do not necessarily develop at exactly the same rate during biofilter start-up.
Ammonia primarily enters the water through fish metabolism.
Major sources include:
The amount of ammonia produced is influenced by:
As biomass and feed input increase, the biological load placed on the RAS also increases.
This is why biofilter capacity should be evaluated according to the maximum expected production load, rather than only the initial number of fish.
Ammonia in aquaculture water is commonly discussed as Total Ammonia Nitrogen (TAN).
TAN consists primarily of two forms:
The proportion between these forms is affected by:
The unionized form, NH₃, is generally more toxic to fish than NH₄⁺.
This means that simply measuring TAN is not always enough to understand the actual ammonia risk.
Operators should interpret ammonia results together with:
The nitrification process can be understood as a biological treatment chain.
Fish metabolize proteins and excrete nitrogenous waste.
Ammonia enters the culture water.
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Water circulates from the fish tanks toward the treatment system.
Before biological filtration, mechanical filtration should normally remove suspended solids such as:
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Water enters the biological filter.
Microorganisms attached to the biofilter media interact with:
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Ammonia-oxidizing microorganisms convert ammonia into nitrite.
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Nitrite-oxidizing microorganisms convert nitrite into nitrate.
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Nitrate accumulates more slowly in terms of acute toxicity than ammonia and nitrite, but it still needs to be managed.
Depending on the system, nitrate management may involve:
A standard aerobic nitrification biofilter should therefore not be described as a complete nitrate-removal system.
Technically, a biofilter does not simply “capture” ammonia like a mechanical filter captures solid particles.
Instead, microorganisms transform ammonia biologically.
The microorganisms attach themselves to surfaces and form a layer known as a:
Biofilm
The biofilm contains different microbial populations distributed through the available surface.
In an MBBR system, these microorganisms grow on specially designed carrier media.
The moving media provide:
This allows the biological filter to process dissolved nitrogen compounds continuously.
MBBR stands for:
Moving Bed Biofilm Reactor
An MBBR uses floating carrier media that remain in motion inside a reactor.
Air introduced through diffusers provides:
The carriers continuously move through the water, allowing the biofilm to contact the surrounding water.
The actual performance of an MBBR depends on operating conditions rather than carrier surface area alone.
An important principle in RAS design is:
Remove solids before they become dissolved waste.
A drum filter can remove:
before they enter the biofilter.
This is important because excessive organic solids can:
Therefore, a complete RAS normally combines:
Mechanical Filtration + Biological Filtration
These two systems have different functions.
Primarily removes:
Physical particles
Primarily transforms:
Dissolved nitrogen compounds
They complement each other rather than replace each other.
Nitrification is an aerobic process and requires oxygen.
Oxygen in an RAS is consumed by:
As feed loading increases, total oxygen demand can also increase.
The biofilter therefore requires sufficient:
If oxygen becomes insufficient, nitrification performance may decline even when the biofilter contains enough media.
This is why oxygen-system design and biofilter design must be considered together.
Nitrification affects the water chemistry of the system.
During nitrification:
If alkalinity is not adequately maintained, pH can gradually decline.
Low pH can then reduce nitrification performance.
This can create a cycle:
Nitrification → Alkalinity Consumption → Lower pH → Reduced Nitrification
Therefore, professional RAS management should monitor:
together.
Simply correcting pH without understanding the underlying alkalinity balance may only provide a temporary solution.
Temperature affects both:
Different fish species operate at different temperature ranges, and nitrifying communities also respond to temperature.
A biofilter designed for a warm-water tilapia system should not automatically be expected to perform identically in a cold-water trout system.
When designing a RAS biofilter, consider:
The biological design should reflect the actual operating conditions of the project.
Marine and freshwater RAS systems have different biological conditions.
A change in salinity can affect:
This is particularly important when:
Abrupt environmental changes can temporarily reduce biological-filter performance.
For marine RAS projects, biofilter commissioning should therefore be conducted under the intended salinity conditions whenever practical.
Biofilter sizing should start with the biological load.
A simplified design workflow is:
Calculate the maximum expected fish biomass.
Estimate the highest daily feed input.
Use an appropriate nitrogen-loading model based on:
Select a conservative, validated nitrification rate for the intended operating conditions.
The required area depends on:
Consider:
The selected media fill fraction should allow:
Finally, confirm that the:
can operate together at maximum expected load.
A biofilter should be evaluated through trends rather than one test result.
Important indicators include:
Typically shows:
May show:
The first step should be identifying the cause rather than simply adding more media.
The biofilter may be receiving a higher nitrogen load than its established capacity.
Nitrifying microorganisms require oxygen.
Insufficient alkalinity can cause pH to decline.
New media need time to develop an active biofilm.
Rapid temperature changes can affect microbial activity.
Microbial communities may need time to adapt.
Poor mechanical filtration can increase the organic load entering the biofilter.
Some chemicals can damage beneficial microorganisms.
A newly installed biofilter needs a controlled start-up process.
A typical process includes:
Confirm:
Introduce an appropriate nitrogen source or compatible mature biofilm.
Track:
Do not immediately operate the new biofilter at maximum production load.
The biofilter should demonstrate repeatable ammonia and nitrite conversion before full production loading.
Possible causes:
Possible causes:
Possible causes:
Remove solids before they enter biological treatment.
Provide adequate oxygen to both fish and biofilter microorganisms.
Avoid increasing feed faster than biological capacity.
Monitor both parameters continuously.
Avoid unnecessary chemical exposure and sudden environmental changes.
Ensure sufficient water reaches all active media.
Use data to identify gradual changes before they become critical.
YUTANK provides customized MBBR biological filtration solutions for commercial RAS projects.
Our MBBR systems can be integrated with:
YUTANK's biological filtration solutions can be configured according to:
The objective is to create a balanced treatment chain in which mechanical filtration, biological filtration, oxygenation, and water circulation work together.
Learn more about YUTANK RAS solutions:
The RAS nitrification process is fundamental to maintaining stable water quality in intensive aquaculture.
The process can be summarized as:
Fish Waste → Ammonia → Nitrite → Nitrate
A properly designed biofilter provides the environment where nitrifying microorganisms can continuously perform this conversion.
However, successful nitrification depends on more than biofilter volume.
It requires a balance of:
For this reason, the biological filter should always be designed as part of the complete RAS rather than as an independent piece of equipment.
YUTANK provides customized MBBR biofilters and complete RAS engineering solutions to help global aquaculture projects achieve stable biological filtration and efficient fish production.
Nitrification is a biological process in which microorganisms convert ammonia into nitrite and then nitrate. It is one of the core biological treatment processes in a RAS.
A biofilter does not mechanically remove ammonia. Instead, microorganisms growing on biofilm surfaces biologically convert ammonia into nitrite and subsequently nitrate.
MBBR provides moving carrier media with surfaces for biofilm growth. Aeration keeps the media moving and supplies oxygen, creating a compact biological treatment environment.
A standard aerobic MBBR primarily performs nitrification. It converts ammonia and nitrite into nitrate but does not normally remove nitrate completely.
Possible causes include excessive feed loading, insufficient biofilter capacity, low dissolved oxygen, unsuitable pH or alkalinity, immature biofilm, temperature changes, salinity changes, or excessive organic loading.
In most RAS designs, mechanical filtration is placed before biological filtration to remove suspended solids and reduce the organic load entering the biofilter.
The maturation period varies according to temperature, salinity, loading, inoculation method, water chemistry, and biofilter design. A new biofilter should be loaded gradually and considered ready only after stable ammonia and nitrite conversion has been demonstrated.
