【文章內(nèi)容簡介】 ed bed filters Mixed bed filters Trickling filters Rotating bio contactor Figure 1. Required unit processes and some typical ponents used in recirculating aquaculture production systems. gen is extremely toxic to most fish. The fraction of TAN in the unionized form is dependent upon the pH and temperature of the water. At a pH of , most of the TAN is in the ionized form, while at a pH of up to 30 percent of TAN is in the unionized form. While the lethal concentration of ammonianitrogen for many species has been established, the sublethal effects of ammonianitrogen have not been well defined. Reduction in growth rates may be the most important sublethal effect. In general, the concentration of unionized ammonianitrogen in tanks should not exceed mg/l. Nitritenitrogen (NO2 ) is a product of the oxidation of ammonianitrogen. Nitrifying bacteria (Nitrosomonas) in the production system utilize ammonianitrogen as an energy source for growth and produce nitritenitrogen as a byproduct. These bacteria are the basis for biological filtration. The nitrifying bacteria grow on the surface of the biofilter substrate although all tank production system ponents will have nitrifying bacteria present to some extent. While nitritenitrogen is not as toxic as ammonianitrogen, it is harmful to aquatic species and must be controlled within the tank. Nitritenitrogen binds with hemoglobin to produce methemoglobin. Methemoglobin is not capable of binding and transporting oxygen and the affected fish bee starved for oxygen. The toxicity of nitritenitrogen is species specific. However, a mon practice for reducing nitritenitrogen toxicity is to increase the chloride concentration of the culture water. Maintaining a chloride to nitritenitrogen ratio of 10:1 generally will protect against methemoglobin buildup and nitritenitrogen toxicity. Fortunately, Nitrobacter bacteria, which also are present in most biological filters, utilize nitritenitrogen as an energy source and produce nitrate as a byproduct. In a recirculating system with a mature biofilter, nitritenitrogen concentrations should not exceed 10 mg/l for long periods of time and in most cases should remain below 1 mg/l. Nitrates are not generally of great concern to the aquaculturist. Studies have shown that aquatic species can tolerate extremely high levels ( 200 mg/l) of nitratenitrogen in production systems. Nitratenitrogen concentrations do not generally reach such high levels in recirculating systems. Nitratenitrogen is either flushed from a system during system maintenance operations (such as settled solids removal or filter backwashing), or denitrification occurs within a treatment system ponent such as a settling tank. Denitrification occurs when anaerobic bacteria metabolize nitratenitrogen to produce nitrogen gas that is released to the atmosphere during the aeration process. For more information on the effects of water quality on fish production, see SRAC 452, Recirculating Aquaculture Tank Production Systems: Management of Recirculating Systems. Ammonia and nitritenitrogen control: Controlling the concentration of unionized ammonianitrogen (NH3) in the culture tank is a primary objective of recirculating treatment system design. Ammonianitrogen must be 210。removed211。 from the culture tank at a rate equal to the rate of production to maintain a safe concentration. While there are a number of technologies available for removing ammonianitrogen from water, biological filtration is the most widely used. In biological filtration (also referred to as biofiltration), there is a substrate with a large surface area where nitrifying bacteria can attach and grow. As previously noted, ammonia and nitritenitrogen in the recycle stream are oxidized to nitrite and nitratenitrogen by Nitrosomonas and Nitrobacter bacteria, respectively. Gravel, sand, plastic beads, plastic rings, plastic tubes, and plastic plates are mon biofiltration substrates. The configuration of the substrate and the man ner in which it es into contact with wastewater define the water treatment characteristics of the biological filtration unit. The most mon configurations for biological filters include rotating biological contactors (RBC), fixed film reactors, expandable media filters, and mixed bed reactors. For more information on biological filters and ponents see SRAC 453, Recirculating Aquaculture Tank Production Systems: A Review of Component Options. pH and alkalinity constraints The measure of the hydrogen ion (H+) concentration, or pH, in water indicates the degree to which water is either acidic or basic. The pH of water affects many other water quality parameters and the rates of many biological and chemical processes. Thus, pH is considered an important parameter to be monitored and controlled in recirculating aquaculture systems. Alkalinity is a measure of the water213。s capacity to neutralize acidity (hydrogen ions). Bicarbonate (HCO3) and carbonate(CO3) are the predominant bases or sources of alkalinity in most waters. Highly alkaline waters are more strongly buffered against pH change than less alkaline waters. Nitrification is an acidproducing process. As ammonianitrogen is transformed to nitratenitrogen by nitrifying bacteria, hydrogen ions are produced. As hydrogen ions bine with bases such as hydroxide (OH), carbonate and bicarbonate, alkalinity is consumed and the pH decreases. Levels of pH below are dangerous to fish。 a pH below will reduce the activity of nitrifying bacteria. If the source water for a recirculating system is low in alkalinity, then pH and alkalinity should be monitored and alkalinity must be maintained with additions of bases. Some bases monly used include hydrated lime [Ca(OH)2] quick lime (CaO), and sodium bicarbonate (NaHCO3). Dissolved gas constraints Althou