Description
A biological reactor is essential in an intensive fish farming system. The fish excrete ammonia (NH3) through the gills and urea via the urine, both of which dissolve in the water. Urea also forms ammonia, which can be harmful to fish at low concentrations. In RAS, biological filters are used to convert ammonia in a two-step process into relatively harmless nitrate (NO3). In the first step of nitrification, ammonia-oxidizing bacteria convert the ammonia into nitrite (NO2-). In the second step, this NO2- is converted by nitrite oxidizing bacteria into nitrate the end product of nitrification reaction. The nitrifying bacteria in the bio-filter only perform well when the circumstances are right for them. When the organic load, in the water is too high, the nitrifying bacteria in the bio-filter will be dominated by the faster growing heterotrophic bacteria. This is one of the main reasons that the biological filter is installed after the mechanical filtration steps.
ACE utilizes several bio-filter types, depending on the type of fish produced, the climatic conditions and the dimensions of the farm building. Trickling filters are normally used when evaporative cooling is targeted or when the fish species has low tolerance for carbon dioxide. In other cases, moving bed bio-filters (MBBR’s) may be the bio-filter of choice.
Video Showcase
Quisque cursus, metus vitae pharetra auctor dui massa mattis sem interdum magna augue eget diam vestibulum ipsum.
Bio-Filter Process
A biofilter is a bed of media on which microorganisms attach and grow to form a biological layer called biofilm. Biofiltration is thus usually referred to as a fixed–film process. Generally, the biofilm is formed by a community of different microorganisms (bacteria, fungi, yeast, etc.), macro-organisms (protozoa, worms, insect’s larvae, etc.) and extracellular polymeric substances (EPS). The aspect of the biofilm is usually slimy and muddy.
Water to be treated can be applied intermittently or continuously over the media, via up-flow or downflow. Typically, a biofilter has two or three phases, depending on the feeding strategy (percolating or submerged biofilter):
- a solid phase (media);
- a liquid phase (water);
- a gaseous phase (air).
ACE utilizes several bio-filter types, depending on the type of fish produced, the climatic conditions and the dimensions of the farm building. Trickling filters are normally used when evaporative cooling is targeted or when the fish species has low tolerance for carbon dioxide. In other cases, moving bed bio-filters (MBBR’s) may be the bio-filter of choice.
Organic matter and other water components diffuse into the biofilm where the treatment occurs, mostly by biodegradation. Biofiltration processes are usually aerobic, which means that microorganisms require oxygen for their metabolism. Oxygen can be supplied to the biofilm, either concurrently or counter currently with water flow. Aeration occurs passively by the natural flow of air through the process (three phase biofilter) or by forced air supplied by blowers.
Microorganisms’ activity is a key-factor of the process performance. The main influencing factors are the water composition, the biofilter hydraulic loading, the type of media, the feeding strategy (percolation or submerged media), the age of the biofilm, temperature, aeration, etc.
Types of filtering media
Most biofilters use media such as sand, crushed rock, river gravel, or some form of plastic or ceramic material shaped as small beads and rings.
Advantages of Using a Bio-Filter
Although biological filters have simple superficial structures, their internal hydrodynamics and the microorganisms’ biology and ecology are complex and variable. These characteristics confer robustness to the process. In other words, the process has the capacity to maintain its performance or rapidly return to initial levels following a period of no flow, of intense use, toxic shocks, media backwash (high rate biofiltration processes), etc.
The structure of the biofilm protects microorganisms from difficult environmental conditions and retains the biomass inside the process, even when conditions are not optimal for its growth. Biofiltration processes offer the following advantages: (Rittmann et al., 1988):
- Because microorganisms are retained within the biofilm, biofiltration allows the development of microorganisms with relatively low specific growth rates.
- Biofilters are less subject to variable or intermittent loading and to hydraulic shock.
Operational costs are usually lower than for activated sludge.
Final treatment result is less influenced by biomass separation since the biomass concentration at the effluent is much lower than for suspended biomass processes.
Attached biomass becomes more specialized (higher concentration of relevant organisms) at a given point in the process train because there is no biomass return.
Get In Touch
Our mission is to improve sustainability and profitability for customers, and be a valuable partner for customers’ long-term business.
