Water is, of course, the most important component of any aquaculture operation. An adequate water source is the primary prerequisite for siting traditional, extensive aquaculture, such as ponds or raceways. In recirculation aquaculture, water supply is somewhat less critical since water use is so much less. The best source of water is ground water because it is usually free of pollution and pathogens and is of consistent temperature. A spring or artesian well is ideal because little pumping is required, though a traditional well is also a good source. Surface water from a stream or lake usually has wild fish in it and will carry fish pathogens. It also is more likely to receive pollution and its temperature changes with the seasons. These factors make it much less desirable than ground water, but it is still usable. A third possibility is a municipal water supply. Unlike surface water, it will not contain any pathogens or pollutants and its temperature fluctuations will be somewhat moderated. It will, however, contain chlorine. The typical chlorine content of city water (about 1mg/L) will quickly kill fish. Most of it can be quickly and relatively inexpensively removed with activated carbon filters, allowing it to be used for aquaculture. There is some concern, however, that the trace amounts remaining after carbon filtration (a few thousandths of a mg/L) might have sublethal effects on the fish. No matter what water source is used, the big advantage of recirculation aquaculture is that a fairly large system may be operated on a water supply of only 10, or so, gallons (3.8 L) per minute, while the traditional aquaculture operation would require hundreds of gallons per minute.
Free Chlorine and Chloramine
Municipal water supplies are sometimes spiked with disinfectant to kill pathogenic microbes and reduce the buildup of nuisance bacteria in the delivery lines. Originally, free chlorine was used exclusively. Free chlorine (OCl- also termed hypochlorite) is formed by exposing water to chlorine gas and is not to be confused with harmless chloride (Cl-). The residual chlorine content of city water (about 1 mg/L) will quickly kill fish. About a decade ago many municipalities began changing to chloramine (NH2Cl), which would disinfect the water without the danger of also forming toxic trihalomethanes, which occurs under certain water quality conditions when free chlorine disinfection is used. Chloramines are made by adding ammonia to water containing free chlorine. Chloramine is slightly less toxic to bacteria than free chlorine and a residual of about 3 mg/L is maintained. Interestingly, municipalities that use chloramine may periodically switch to free chlorine disinfection for a couple of weeks to kill nitrifiers that may grow in the system using the ammonia for food, despite the chlorine toxicity. Chloramine is toxic to fish and the ammonia content is a concern, as well.
Chloramine poses more difficulties to the recirculation aquaculturist than free chlorine. For one thing, activated charcoal used for free chlorine removal is not nearly as effective in removing chloramine and regular activated charcoal filters are generally regarded as not effective for making chloraminated water safe for fish. Water softener companies offer catalytic activated carbon filters for removing the chlorine toxicity from chloramine. Chemical dechlorination will remove the chlorine toxicity from chloramine, as well. Even with the chlorine toxicity removed, a couple of mg/L of ammonia remain in chloraminated water, enough to cause ammonia toxicity problems. If only partial water changes (<10%) are conducted, the biofilter will handle the ammonia and a regular carbon filter or dechlor treatment can be used. If more than 10% of the water is changed out at one time, there are several possibilities. One is to use conventional dechlorination and carefully monitor ammonia. Another is to use conventional dechlorination and then remove ammonia with a powdered zeolite filter. Yet another option is to use sodium hydroxymethanesulfonate which is sold under the names AmQuel and ChlorAmX for treating chloraminated water. Like activated carbon and sodium thiosulfate, sodium hydroxymethanesulfonate reacts with free chlorine to form harmless chloride. However, it also binds the ammonia to form aminomethanesulfonate that, according to a company literature, is a non-toxic compound that is biodegradeable.
In recirculation aquaculture, water must be moved continuously. This requires a pump. Three types of pumps are used in aquaculture, the most common is the centrifugal (radial flow) pump. It works like a water pump on a car (or, for that matter, an "air pump" on a hair dryer), a spinning impeller forces water out of the pump housing at pressure. Smaller centrifugal pumps may have a pump body completely enclosed from the motor with the impeller driven through a magnetic connection. Larger pumps have a drive shaft that must be sealed. The centrifugal pumps typically used in recirculation aquaculture range from 1/2 to 5 hp. The centrifugal pumps sold for recirculation aquaculture are often the same pumps used to move water in swimming pools. Sometimes air lifts (think of a simple aquarium pump consisting of an airstone in a tube) are use to move water in recirculation systems. These lift water with a stream of rising bubbles. Air lifts work well enough in low head situations, but cannot generate much pressure or lift water great distances. There is a misconception that the rising bubbles in air lifts supply "free" pumping. Power is required to move the air that then moves the water and while airlifts supply some aeration as they pump, they are no more efficient at moving water than a centrifugal pump. A third type of pump that is less commonly seen is an axial flow pump (like a boat propeller in a tube). It excels in situations where large volumes need to be moved at low head.
The energy required to move the water in a system is one of the major costs in recirculation aquaculture. Thought should be given to keeping these energy costs as small as possible. The less head required to move water, the more profitable the aquaculture operation will be. Energy is needed to lift the water to the highest point in the system and this cannot be reduced, but there are substantial losses in the plumbing due to friction between the water and the pipe that can be minimized with thoughtful plumbing.
Pumps provide pressure to water, pressure moves water. Water pressure is measured relative to atmospheric pressure, so water pressure can be either positive or negative. Negative water pressure is commonly referred to as suction. Water pressure can be expressed in units like lbs/in2 or kPa, but in reference to pumps it is often referred to as head. Head is the vertical distance water can be pushed by the pressure. Flow rate and pipe diameter affect head (pressure). At a given flow rate, decreasing the pipe diameter will increase the head (for example, putting your thumb partially over the garden hose). At a given pipe diameter, increasing the flow will increase the head