Biological Filtration

What is a Biological Filter?

A biological filter in aquaculture is a system or component used to maintain water quality by facilitating the removal of harmful waste products, primarily ammonia and nitrite, through biological processes. It is a crucial part of recirculating aquaculture systems (RAS) and other setups where water is reused or needs to be kept clean for the health of aquatic organisms like fish, shrimp, or other species.

How Biological Filters Work

Biological filters rely on the activity of beneficial bacteria that naturally break down toxic compounds produced by aquatic organisms and organic waste. The process is part of the nitrogen cycle, which converts harmful substances into less toxic forms. Here's a breakdown of the key steps:

  1. Ammonia Production:
    • Fish and other aquatic organisms excrete ammonia through their gills and waste. Uneaten food and decaying organic matter also contribute to ammonia levels.
    • Ammonia is highly toxic to aquatic life, even at low concentrations.
  2. Nitrification Process:
    Beneficial bacteria in the biological filter convert ammonia into less harmful compounds through a two-step process called nitrification: 
    • Ammonia to Nitrite: Ammonia is oxidized by bacteria such as Nitrosomonas into nitrite (NO₂⁻). Nitrite is still toxic but less so than ammonia.
    • Nitrite to Nitrate: Nitrite is further oxidized by bacteria like Nitrobacter or Nitrospira into nitrate (NO₃⁻). Nitrate is much less toxic and can be tolerated by most aquatic species at moderate levels.
    • The overall process can be summarized as: NH3/NH4+ → Nitrosomonas NO2 → Nitrobacter/Nitrospira NO3
  3. Nitrate Management:
    • While nitrate is less harmful, high levels can still be detrimental over time. In aquaculture systems, nitrate is typically managed through water changes, plant uptake (in aquaponics), or denitrification (conversion of nitrate to nitrogen gas by anaerobic bacteria in specialized systems).

Components of a Biological Filter

A biological filter typically consists of a medium or substrate that provides a large surface area for beneficial bacteria to colonize. Common materials include:

  • Bio-balls, ceramic rings, or plastic media: These provide surface area for bacterial growth.
  • Sponges or filter pads: Often used in smaller systems.
  • Sand or gravel: Sometimes used in under-gravel filters or fluidized bed filters.
The filter must be well-oxygenated because nitrifying bacteria require oxygen to carry out their metabolic processes. Water flow is also important to ensure that ammonia and nitrite are continuously delivered to the bacteria.

Maintenance

  • Monitoring: Regular monitoring of water parameters like ammonia, nitrite, and nitrate levels is essential to ensure the biological filter is functioning correctly.
  • Cleaning: Our moving bed biofilters are self-cleaning biological filters, which means they require little to no maintenance. What is important is to install a good pre-filter such as a drum filter. This system filters all small and coarse dirt particles from the water and prevents the biological filter from clogging.

Types of Biological Filters

There are several types of biological filters used in aquaculture, depending on the system design and requirements:

    • Water is trickled over a filter medium exposed to air, providing oxygen for bacteria.
    • Commonly used for (koi) ponds and RAS.
    • Also called moving bed bio filter.
    • Plastic filter media is kept in constant motion by water flow, increasing contact between bacteria and water.
    • Highly efficient and compact.
  1. Submerged Filters:
    • Media is fully submerged in water, and bacteria grow on the surface.
    • Simple and commonly used in smaller systems.
  2. Rotating Biological Contactors (RBCs):
    • A rotating drum or disc partially submerged in water provides a surface for bacterial growth.
    • Offers good oxygen transfer and is often used in larger systems.
  3. Bead Filters:
    • Small plastic beads act as both mechanical and biological filtration media.
    • Popular in smaller aquaculture setups.

Importance in Aquaculture

Biological filters are essential for maintaining a stable and healthy environment in aquaculture systems. Without them, ammonia and nitrite levels would quickly rise to toxic levels, leading to stress, disease, or death of the aquatic organisms. Key benefits include:
  • Improved Water Quality: Removes toxic compounds, ensuring a safe environment.
  • Sustainability: Allows for water reuse, reducing the need for frequent water changes.
  • Support for High Stocking Densities: Enables intensive farming by maintaining water quality even with high biomass.

Considerations for Biological Filters
  • Startup Time (Cycling): A new biological filter needs time to establish a colony of nitrifying bacteria. This process, known as "cycling," can take several weeks. During this period, ammonia and nitrite levels must be monitored closely.
  • Oxygen Supply: Nitrifying bacteria require oxygen, so the filter must be designed to ensure adequate aeration.
  • Maintenance: While biological filters require minimal maintenance, care must be taken not to disturb the bacterial colonies (e.g., avoid excessive cleaning with chlorine or other disinfectants).
  • System Balance: The size and capacity of the biological filter must match the waste output of the system to ensure efficient filtration.

In summary, a biological filter in aquaculture is a vital component that uses beneficial bacteria to convert toxic ammonia and nitrite into less harmful nitrate, maintaining water quality and supporting the health of aquatic organisms. Proper design, setup, and maintenance are key to its success in any aquaculture system.

What makes Aqua Exclusive™ special:

  • Own production facility in the Netherlands.
  • Production, development and designing is done in-house.
  • Handmade with the utmost care and precision
  • Our products meet the highest standard, professional quality.
  • Unique customer service, 24/7 available!

Ready to take filtration to
the next level?

View which products are immediately available in our webshop or contact our sales team.