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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.
1.5.2.1 Effect of Therapeutants and Disinfectants on Biofilters
Certain therapeutants and disinfectants, such as iodine and ozonation can have a detrimental effect on the functioning of active biofilters. Always consult product information before using these products in an aquasystem.
1.5.3 Environmental Toxins and Pollutants
1.5.3.1 Heavy Metals
Heavy metals occur naturally in the environment due to geologic weathering but can also occur from human use. It is important to survey the immediate environment as well as the areas upstream to the water source used in an aquasystem. Atmospheric pollution from coal combustion is a major cause of many heavy metals being found in source water. Residential and agricultural use of pesticides and fertilizer may contribute to several types of heavy metals in groundwater, well water, lakes, ponds and estuaries. Other types of industrial waste to consider are leather tanneries, textile industry, battery storage or production, plastic production, metal finishing, mining, and pigment production.
There are essential and non‐essential heavy metals. Essential metals include copper, zinc, chromium, nickel, cobalt, molybdenum and iron. Nonessential metals include cadmium, mercury, tin and lead (Sfakianiakis et al., 2015).
Essential metals can cause disease if they are deficient and if the levels are too concentrated. The most bioavailable form of these metals is the dissolved ionic form. These metals help during larval development of the neurologic and gastrointestinal systems (Sfakianakis et al., 2015; Authman et al., 2015).
Essential and nonessential heavy metals can cause damage at high concentrations. Knowing that there is potential for heavy metal in the water source is an important part of diagnosing decreased reproductive health, slow growth, and acute mortalities. The periods during which fish are most susceptible to heavy metal toxicity are the embryonic and larval stages. They can cause lordosis, increased mortality, hatching delays and other anomalies. According to Sfankianakis et al. (2015), morphological deformities are commonly used as a biomarker to study the effects of contamination.
Fish are susceptible to heavy metals through their gills, gastrointestinal tract and through skin absorption. These metals will accumulate in the liver, kidney, and gills. Arsenic has also been found to accumulate in the retina and cadmium has been found in the heart muscle (Authman, 2015).
Although there are differences in the mechanism of action, most heavy metals cause damage to the liver, gills, reproductive, immune, and neurologic systems. They cause damage to the gills through hyperplasia and necrosis. Aluminum causes lamellar fusion which leads to necrosis. Iron accumulates in the gills as a precipitate causing vascular occlusion. At chronic doses, many heavy metals negatively affect leukocytes, erythrocytes, and antibodies. They can also affect the endocrine system, decreasing reproductive health. Iron can precipitate on eggs causing a reduction of oxygen reaching the larvae. Other heavy metals decrease growth through increased biological stress, disruption of osmotic balance, and developmental changes. Many heavy metals cause neurologic damage, including mercury, which causes behavior changes, cognitive changes, ataxia, and convulsions (Kennedy, 2002).
Studies on cadmium show that it is primarily absorbed via the gastrointestinal system and the gills. It interrupts the absorption of calcium, which can cause problems with reproduction, growth, and development. It can also cause acute death from hypocalcemia. Cadmium levels as low as 0.001 ppm affect the hatching and larval survival of many species of fish.
Studies have been performed showing that excessive copper can affect the gills, gastrointestinal system, and the sensory system. These studies showed that zebra fish larvae had difficulty orienting in the water column, reduced hatching and impairment of growth (Authman, 2015).
1.5.3.2 Insecticides
Insecticides may be used in the local environment for the control of agricultural pests or insects that cause vector borne diseases. These compounds can affect fish through runoff and atmospheric accumulation on pond surfaces. There are many types of insecticides on the market. Most insecticides affect the neurologic system using different mechanisms of action. They may affect acetylcholinesterase activity, block sodium channels, bind nicotinic receptors, affect the peripheral nervous system and directly degrade neurotransmitters in several other ways. They can also cause chromosomal aberrations, decrease protein levels, affect the immune system, and mimic or block reproductive hormones (Sabra and Mehana, 2015). Compared with insects, they differ in their specificity for vertebrates, their toxicity, their bioaccumulation, and their environmental persistence.
Organochlorines