In the field of factory-based recirculating water aquaculture, the biological filter can be regarded as the "heart" of the entire system, undertaking the crucial mission of purifying water quality and maintaining the ecological balance of water bodies. Today, let's delve into the components and basic principles of biological filters together!
The core components of a biological filter: the tank body, filter media, water distribution and aeration
1. Pool
The tank body serves as the external framework of the biological filter, providing support and protection for the internal components. The shape of the pool is usually square or circular. It is necessary to ensure that the water flow is evenly distributed and avoid dead corners. The materials include reinforced concrete, fiberglass reinforced plastic or PP material, etc. Among them, PP material has a high cost performance and is easy to install, suitable for various types of recirculating water aquaculture systems.
2. Biological filter media
Filter media is the core component of a biological filter, providing an attachment surface for microorganisms. When choosing biological filter media, factors such as specific surface area and pore structure, mechanical strength, and procurement cost should be mainly considered. At present, the commonly used filter media mainly include K5 biological filter media and QA polyurethane biological filter media.
3. Aeration system
Nitrifying bacteria are aerobic bacteria and require sufficient oxygen to survive. The aeration system supplies oxygen to the filter through blowers or jet injectors to maintain the dissolved oxygen concentration. Meanwhile, aeration can also drive the filter material to move, preventing the biofilm from being too thick and clogging.
4. Inlet and outlet water systems
The water inflow method should ensure that the sewage can be evenly distributed in the biological filter to avoid excessive local hydraulic shock or dead zones. Common water inlet methods include perforated pipe water inlet and nozzle water inlet, etc. The water discharge method should prevent the loss of filter media while ensuring stable water discharge.
5. Sewage discharge system
The sediment in the biological filter needs to be removed regularly to prevent the filter material from clogging. Common designs of the sewage system include: bottom inclined + sewage trough, which is convenient for cleaning.
Ii. Working Principle of Biological Filters: How Do Microorganisms Purify Water Quality?
The working principle of a biological filter is essentially a "biochemical battle" of microorganisms. The following is its core purification process:
Microbial attachment and biofilm formation
After the wastewater enters the filter tank, microorganisms (such as nitrifying bacteria and heterotrophic bacteria) gradually adhere to the surface of the filter material, forming a viscous "biofilm". This film is not only the "home" of microorganisms, but also the "secret weapon" for purifying water quality.
2. Nitrification
Ammonia nitrogen in aquaculture wastewater is highly toxic to fish. The biological filter converts it into harmless nitrate through a two-step nitrification reaction:
Nitrification: Nitrifying bacteria oxidize ammonia nitrogen to nitrite.
Nitrification: Nitrifying bacteria further oxidize nitrite to nitrate.
This process requires sufficient oxygen, an appropriate temperature (20-30℃) and a pH value (7.5-8.5).
3. Organic matter degradation
Heterotrophic bacteria are responsible for decomposing organic matter in water (such as leftover food and feces), converting it into carbon dioxide and water. This process not only reduces water pollution but also provides a carbon source for nitrifying bacteria.
4. Suspended solids filtration
The biofilm and filter cake layer formed on the surface of the filter material can retain suspended particles in water and reduce water turbidity.
5. Denitrification
Under anaerobic conditions, denitrifying bacteria can reduce nitrate to nitrogen gas, achieving complete removal of nitrogen. The moving bed biological filter can simultaneously achieve nitrification and denitrification by adjusting the aeration volume.