Читать книгу Fundamentals of Aquatic Veterinary Medicine - Группа авторов - Страница 52

As urban populations grow and the science surrounding water quality improves, regulatory changes in discharge limits for large‐scale aquaculture operations have led to a need for more intense handling of solids. One sophisticated and large‐scale method of processing biological solid wastes uses the theory of biological flocculation or “bioflocculation”. While the veterinarian may not typically be called upon to manage such systems, it is important to understand that these systems might be in use, how they operate at a basic level, and how they may impact animal health. Perhaps the most common concern for aquatic veterinarians is how chemicals used for disease treatment in a system may impact the function of these solids treatment systems.

Flocculation involves the clumping of suspended particles together into a settleable “floc”. Chemical flocculation is used in wastewater treatment plants to remove solids from water on a large scale, where surfactant chemicals such as alum or ferric chloride are used. More recently, polyacrylamide has also been used.

Large‐scale aquaculture systems with large biological loads (and thus a high volume of fecal wastes) may use this chemical flocculation method. Home aquaria are also often treated with chemical flocculants to clear cloudy water.

Another method that may be used in small‐scale aquaculture systems is bioflocculation, where microorganisms cause particles to clump together when natural chemicals such as polysaccharides are secreted as a result of their normal biological functions. This naturally occurring process is important for the proper functioning of all filter systems. Small‐scale aquaculture facilities may employ an enhanced system of bioflocculation. While fine suspended organic particles from animal wastes will settle out on their own in a stagnant tank or pond over time, organic particles such as phosphorus never quite settle out. Microorganism activity speeds up the process and leads to a more effective solids removal by clumping particles together into a floc which can be removed from the water. This type of system can be used to treat wastewater and reduces the total dissolved solids and phosphorus following prefiltration to remove larger particles. The intensity of the wastewater treatment required will depend on many factors, and the discharge of such chemicals as nitrogen and phosphorus may be limited by a water jurisdiction agency to reduce eutrophication impacts on the environment. Flocculation is commonly used in combination with coagulation, which creates microflocs by decreasing the negative charges that repel particles and cause them to remain in suspension.

The budget, amount of land area available for this application, biological load in the system, and the regulatory limits of the discharge permit will determine the type and size of system used. Typically these systems require a long time to “turn over” and are limited in volume. These methods are well suited to a settling basin or belt filter.

As with a biological filter, the type and amount of chemicals or microorganisms present in the system will impact the speed and effectiveness of this water treatment method. Treatment chemical residues in the wastewater may interact with chemical flocculants. High concentrations of toxic chemicals containing formaldehyde or chlorine used to treat diseases in fish may negatively impact the microorganism population of the system and treatment may need to be altered to a lower concentration over a longer period to reduce the lethal dose in the water treatment system. In some cases, bypassing the settling basin or belt filter may be considered.

The principle of bioflocculation has other applications and is currently a growing area of research for aquatic animal feeds, pathogen reduction, and biofuel production.