GROUNDWATER |
AQUACULTURE
All of these issues lead to higher production costs and lower producer profits. The Oxygen Requirement for FishAll fish whether cultured or wild depend on a minimum level of ~ 6 mg/l oxygen in the water to survive. In the wild, fish are not generally crowded; therefore oxygen in a body of water does not get consumed to the point where the fish no longer have enough oxygen to survive, unless externally influenced, e.g. pollution of some form. However, under culture conditions where large numbers of fish are housed in tanks at high densities oxygen levels may become quickly depleted. If densities are kept low then pumping of new water from external sources is sufficient to supply the fish with enough oxygen. However, most hatcheries have to deal with situations that require some supplementation of the oxygen content of the water being pumped. These situations include: high densities of fish, elevated summer temperatures, loss of pumping capacity, and super-saturation with nitrogen. In order to be economically viable, fish farms often stock high densities of fish in their tanks and pumped water alone cannot hold enough oxygen to satisfy the fish’s respiratory requirements; therefore the growers use a combination of aeration of bubbled air and diffused oxygen. Recirculation grow operations, which typically use 90% less water than flow through, are even more reliant on pure oxygen to maintain sufficient oxygen levels in the water. Although warmer water up to a critical species-specific levels promotes faster growth among most fish species it cannot hold as much oxygen as colder water. It is therefore essential to add oxygen to the water at high temperatures to provide fish with a critical level of oxygen to perform their metabolic processes and continue to grow at optimum rates. Conventional Oxygen Levels and Methods of Addition Conventionally, oxygen levels in intensive fish culture facilities are kept at or slightly above saturation (100-110%). Levels of this nature provide enough oxygen for the fish to continue growing well even at high temperatures within the acceptable range for the species; and suppresses nitrogen levels to 100% or less, helping to avoid problems with nitrogen supersaturation. Higher oxygen concentrations are not normally used due to fears of gas bubble disease. This can occur when very high oxygen concentrations are used in a conventional manner. Although any supersaturated gas can cause gas bubble disease, in practice it is almost always caused by nitrogen supersaturation. This is compounded by the fact that the atmosphere has 4 X more nitrogen than oxygen. Unlike oxygen, which is used for respiration and carbon dioxide, which is used by blood and cells, nitrogen is an inert gas and stays supersaturated in the blood. Any decrease in pressure on the gas or localized increase in body temperature can bring such nitrogen out of solution to form bubbles (a process similar to “bends” in human divers), which can restrict blood flow to a point of asphyxiation. Gas bubble disease is generally chronic when water contains air between 105-140% and becomes acute to sub acute above 140%. Oxygen is typically added to water via a diffuser or vacuum degasser. Neither are particularly energy efficient. There is usually a mass of small bubbles, which reach the surface and are wasted. Controlling the Dissolved Gas Atmosphere using the PurGRO2® System At equilibrium with one atmosphere of air pressure, water will contain dissolved gases whose partial pressures will, quite naturally, sum to one atmosphere. Neglecting gases such as argon and CO2 for the moment, the one atmosphere of dissolved gas pressure will consist of 0.21 atmospheres of oxygen pressure and 0.79 atmospheres of nitrogen pressure. When it comes to the concentration of each of these molecules in the water, however, the apparent 4:1 ration of nitrogen to oxygen does not hold true. In actual fact, at 20’C, the water, at equilibrium, will contain 15.3 ppm of nitrogen and 9.3 ppm of oxygen-less than a 2:1 ratio! This difference is due to Henry’s Law and the ability of water to dissolve gases. The Henry’s Law constant for nitrogen at 20’C is approximately 79,000 (atm. per mol fraction) while that oxygen is approximately 44,000. Put another way, each ppm of dissolved nitrogen exerts a higher pressure than a dissolved ppm of oxygen. This difference, in areas such as aquaculture, can be exploited with the correct technology. PurGRO2® is such a technology. It is known that the level of dissolved oxygen in the water can influence the health and growth rate of fish. Generally speaking, the higher the DO level in the water, the better; a major problem is finding an economical method of introducing higher levels of oxygen into the water. It is also known that the overall dissolved gas pressure can play a significant role in fish health and growth rate, etc. Just like humans who can undergo the ‘bends’ as a result of exposure to high (principally inert) gas pressures, fish can also be adversely affected if the total dissolved gas pressure is too high. Conventionally, the limitation here is that an increase in dissolved oxygen content would result in an increase in total dissolved gas pressure. Conventional equipment or technology in the aquaculture industry seems focused only on achieving a minimum dissolved oxygen concentration in the water and not on what is really needed, reaching and maintaining an optimum dissolved gas ‘atmosphere’. There is an elegant, economical solution. PurGRO2® can increase dissolved oxygen to previously unimagined levels while simultaneously actual lowering the total gas pressure and do it all economically. How is the ‘Atmosphere’ Altered? The hydrophobic nature of microporous hollow fiber establishes a stable interface between an aqueous phase on one side of the fiber and a gas phase on the other. This interface remains stable by part of the PurGRO2® controls that governs the differential pressure between the phases. This stable interface can then be used to transfer mass from one phase to the other. This can be done with either the PurGRO2® HP System or the PurGRO2® LP System). It can also be used for two-way transport being the transfer of mass from the gas phase to liquid phase (addition of oxygen) and the simultaneous transfer of mass from liquid phase to gas phase (removal of nitrogen) using the PurGRO2® LP system. What can be achieved in an Altered Atmosphere? A five- month trial was conducted on 400 juvenile halibut (starting weight 4.5 grams). During that time frame, they were grown using a PurGRO2® system. The oxygen partial pressure ranged from 220- 280 mmHg. Conventional dissolved oxygen meters would have read between 140 and 180 %. Relative to industry norms, the growth rate of these fish over this time period was greater than twice as fast! A small quantity of store bought ornamental fish was raised for one month in an atmosphere up to and exceeding 600 mmHg dissolved oxygen partial pressure. On a molar basis this was 90-95% oxygen and 5-10% nitrogen. The fish not only survived but thrived, growing fast and fat! An independent study was completed in December, 2007, detailing the raising of cultured short-nose sturgeon (A. brevirostrum) in a PurGRO2® environment. A soon to be published report will relay significant increases in growth rate as well as substantial improvements in food conversion, solely as a result of being raised an atmosphere of 250% oxygen content. An inVentures prepared PowerPoint presentation is available. To recap, the PurGRO2® process consists of two components - low pressure and high pressure. When water and oxygen are in contact at low pressure, typically at the head pressure in a tank, oxygen will transfer into the water. Simultaneously, nitrogen gas (N2), which is already dissolved in the water, will transfer out of the water. This is crucial if an altered atmosphere is to be achieved. At higher pressures, much more oxygen will transfer into the water, while very much less nitrogen will transfer out. This is the reason for the two-part process of low pressure and high-pressure components. Benefits of the PurGRO2® System Simply stated, the PurGRO2® low-pressure and high-pressure systems offer some amazing benefits and options to the grower:
PurGRO2® HP and PurGRO2® LP systems of all capacities are available to demonstrate and purchase. For more information on the PurGRO2® click here |
