Solid suspended particles adversely affect all aspects of land-based industrialized recirculating aquaculture systems. Therefore, the removal of solid suspended particles is the primary goal of recirculating aquaculture water treatment. In recirculating aquaculture systems, the majority of particles by mass are smaller than 100 micrometers. Due to water turbulence, biodegradation, and mechanical agitation, suspended particles not removed promptly can break down into finer suspended particles, typically smaller than 30 micrometers. Under these conditions, sedimentation treatment and mechanical filtration become ineffective.
Foam fractionation is a technique used for removing fine suspended particles. It involves introducing air into the water body, allowing surfactants in the water to be adsorbed onto the microbubbles. These bubbles, carrying the adsorbed material, rise to the surface by buoyancy to form foam, thereby removing dissolved and suspended substances from the water. Currently, foam fractionation is recognized as one of the primary processes for effectively removing fine particles in recirculating aquaculture systems and is a crucial component of these systems.
I. Principle of the Protein Skimmer
1. Foam Fractionation Principle
The protein skimmer operates primarily based on the principle of foam fractionation. In recirculating aquaculture systems, excretions from cultured organisms, uneaten feed, etc., contain large amounts of organic matter. Proteins and other surfactants within this organic matter possess an amphiphilic nature (both hydrophilic and lipophilic). When a large number of microbubbles are introduced into the water, these surfactant substances adsorb onto the bubble surfaces. As the bubbles rise, bubbles with adsorbed proteins and other substances form a foam layer on the water surface. The foam layer, having a lower density, separates easily from the water body, thus removing proteins and other organic matter from the water.
2. Physicochemical Process
From a microscopic perspective, surface tension exists between bubbles and water. As bubbles rise through the water, organic molecules in the water aggregate towards the bubble surface under the influence of surface tension. This is a physical adsorption process, accompanied by some chemical changes, such as interactions between organic molecules and chemical reactions with the bubble surface. For example, some protein molecules may undergo denaturation, making them more readily adsorbed onto the bubbles.
II. Role of Protein Skimmers in Recirculating Aquaculture
(I) Water Purification
1. Removal of Organic Matter
Protein skimmers effectively remove organic matter from water, including proteins, fats, carbohydrates, etc. If these organic substances accumulate in the water, they will be decomposed by microorganisms, consuming large amounts of dissolved oxygen and producing harmful substances such as ammonia nitrogen and nitrite. Removing these organics via the protein skimmer reduces the burden on subsequent biological filtration and lowers the production of harmful substances in the water. For instance, without a protein skimmer, the Chemical Oxygen Demand (COD) in the water can rapidly rise above 100 mg/L, whereas with a protein skimmer, COD can be controlled around 30 - 50 mg/L.
2. Reduction of Ammonia Nitrogen Production
By removing nitrogen-containing organic matter like proteins, protein skimmers reduce the potential sources of ammonia nitrogen in the water. This is crucial for maintaining low ammonia nitrogen concentrations in recirculating aquaculture systems, as ammonia nitrogen is highly toxic to cultured organisms.
(II) Improvement of Water Clarity
While removing organic matter, protein skimmers also remove some suspended particles smaller than 30 micrometers, thereby improving water clarity, which benefits the growth of cultured organisms.
(III) Reduction of Disease Transmission
Removal of Pathogen Vectors: Organic matter and suspended particles in the water often serve as carriers for pathogens, such as bacteria, viruses, and parasites. By removing these carriers, the protein skimmer reduces the opportunity for pathogen transmission within the water, lowering the risk of disease infection in cultured organisms.