Design criteria for the biological treatment
There are several factors to take into account when designing a biological treatment system.
Calculate the MBBR media surface
Amount of media to purchase:
For every bacterial support corresponds a protected surface, expressed in m²/m3 (ft²/ft3). This represents the surface area available for biofilm colonization, the size of the building so to speak.
The larger the area, the smaller is the media investment. In fact, for the same pollutant load, less media is required.
Example with a project requiring 52000 m² of surface:
- Chips 1h2o3 based on activated carbon: 10800 / 2600 = 20 m3
- Chips at 800 m²: 10800 / 800 = 65 m3
With a standard bacterial media, there is 3.25 times more media to purchase compared to MBBR 1H2O3 media.
What materials and biological agents to choose for salt water filtration
If you work with sea water, it is essential to choose equipment that is resistant to the corrosive power of chloride ions. Otherwise you may lose your installation very quickly. Therefore, it is recommended to use corrosion resistant installations such as PVC or polyethylene, etc.
Regarding the media and biological agents, they must also have this resistance to corrosive products.
Water temperature
Water temperature will have a strong influence on bacterial efficiency. As we know, bacteria tend to favour warm and humid environments and accelerate their metabolisms when the temperature increases (up to a certain value). In cooler waters, there will be a reduction in the effectiveness of the bacteria. The temperature is therefore a parameter to be taken into account when designing your installation.
Pollutant to be treated
Nitrogen
Nitrogen can be of different kinds. It can come from fish faeces in fish farms or directly from industrial or agricultural pollutants. It has the unfortunate tendency to cause anoxia in the environments in which it is found in excess. That is why it is essential to treat it in the water that is discharged into nature, so as not to destroy an ecosystem.
It will be treated by the action of bacteria whose metabolism requires this nitrogen, which they will transform into nitrite and nitrate. It is these molecules that under anaerobic conditions will be treated by bacteria of the same type and change nitrates into N2. Nitrogen is an inoffensive gas for the man and constitutes 80% of the air which we breathe
Carbon
As for nitrogen, carbon can have several natures. It is produced as a result of the decomposition of organic matter (plants, animals, etc.) and can come from municipal, agricultural and industrial effluents. Carbon removal is also essential. High concentrations can have harmful consequences in the water.
Dissolved carbon will reduce the availability of dissolved oxygen to aquatic organisms. In order to be biologically degraded, it will require oxygen and make bacterial populations grow, which will not only consume oxygen by breathing, causing a form of anoxia, but also present a risk for health.
Therefore, it is crucial to treat them in a MBBR system which will be regularly aerated to avoid any risk of anoxia.
