Water Quality: Gases
Understanding Oxygen in Water
To fully understand how oxygen moves into a recirculation system it needs to be considered form a number of perspectives. Gas pressure can be expressed in several ways. The pressure of the atmosphere on the ocean's surface (or your lungs if you are standing on the beach) is about 15 lbs/in2 (100 kPa) which is termed one atmosphere (A). If you fill a glass tube having one closed end with a fluid and immerse the open end into an open container of the fluid, the fluid will fall in the tube and pull a vacuum until the atmospheric pressure pushing it back up equals the weight of the column of fluid. This is a barometer. The level of fluid will vary slightly as the total air pressure varies. Mercury is commonly used as the fluid. Standard air pressure at sea level will support a 760 mm (29.9 inch) column of mercury (in physiology, mmHg is the common unit of gas pressure, meteorologists use inches). Sometimes a mmHg is referred to as a "torr" honoring Torricelli, the inventor of the barometer. 760 mmHg is also termed one atmosphere (A), referring to the pressure of the atmosphere pressing down. What is commonly termed suction is really the pressure of the atmosphere acting in the opposite direction.
Air is a mixture of gases (about 80% nitrogen, 20% oxygen, and 0.035% of carbon dioxide). These have a combined total pressure of 760 mmHg. Each gas has a partial pressure proportional to its fraction of the total. Total gas pressure of the atmosphere averages 760 mmHg and oxygen makes up 20%, therefore the partial pressure of oxygen (PO2 ) is 0.20 x 760 = 150 mmHg. The difference in partial pressure will tell you which way oxygen (or any gas) will diffuse. Oxygen will flow from a region of high pressure to a region of lower pressure until the PO2 is the same in both regions. When air and water have the same partial pressure (in equilibrium) the water is said to be saturated. Partial pressures increase and decrease in nature. For example, a change in the atmospheric pressure changes the PO2 proportionally thus water at high elevations will have slightly less PO2 than water at sea level, but both of these are relatively minor. More substantial changes occur as the result of biological activity. Algae blooms can increase the PO2 by 75 mmHg or more. On the other hand, if the water has a high biological oxygen demand from bacteria and fish, the PO2 can fall. When the PO2 falls far below 150 mmHg the water is hypoxic. When this occurs, oxygen isn't being "forced" into the fish as hard and the fish begin to suffocate. If the PO2 of water falls to 0, it is termed anoxic and all aerobic aquatic life will cease. The partial pressure gradient between air and water drives gas exchange. The movement of gas into water is dependent on the solubility of gas (which varies slightly with temperature and dissolved solids content), the partial pressure gradient across the gas/water interface, the surface area of the gas/water interface, and the inverse of the thickness of the stagnant boundary layer (explained below).
What Aeration Does
Atmospheric aeration increases the rate at which oxygen will flow into water, but it does not change the partial pressure gradient of oxygen between air and water. Aeration speeds the movement of O2 into the water if the water is below saturation (PO2 < 150 mm Hg), but cannot raise the concentration above air saturation. Aeration affects two of the four factors upon which gas exchange is dependent. One factor is surface area. The greater the surface area available for the diffusion of oxygen, the faster the PO2 of the water will increase. The other factor is the thickness of the stagnant boundary layer.
The stagnant boundary layer model of gas exchange holds that the rate of gas movement in and out of water is limited by diffusion across a stagnant boundary layer at the air/water interface. Gas molecules are carried to the surface of the water rapidly by air movement, but when they dissolve into the water they must move slowly by diffusion across the stagnant boundary layer until they reach the area of water movement on the other side of this layer. They then can be carried away rapidly by water movement. The rate of diffusion across the stagnant boundary layer is dependent on the mass transfer coefficient of the gas (the speed of diffusion, a constant) divided by the thickness of the stagnant boundary layer. The thickness of the stagnant boundary layer changes with the degree of water movement. The thickness of the stagnant boundary layer in a still tank approaches 1 mm, but if the water is moving the stagnant boundary layer thins out and diffusion can occur more quickly. Aeration, whether by bubbling airstones, squirting fountains, or whatever method, speeds gas exchange by: 1) increasing the surface area for diffusion (the surface of the bubbles, etc.), and 2) stirring the water thus thinning the stagnant boundary layer.
Definition – Saturation...
If you have a glass of water at air saturation the PO2 is approximately 150 mm Hg or 0.2 A, the same as in the air above it. Stirring or bubbling the water will not change this. But if you cover the glass and enrich the O2 concentration over it until the air in the glass is say, 40% oxygen instead of the normal 20% , the PO2 will also double as O2 diffuses into the water and reaches equilibrium resulting in a PO2 in the water of 300 mm Hg instead of the normal 150 mm Hg. The water is still saturated, but under different atmospheric conditions. Alternatively, you may increase the PO2 of water by increasing the total gas pressure, say by sealing the top of the glass and pressurizing the normal air mixture to 2A (1500 mm Hg), this would also lead to a PO2 in the water of 300 mm Hg. If the cover was removed from the pressurized glass, the water would be supersaturated until the excess gas escaped to the atmosphere.
Definition – Supersaturation...
If a bubble is driven down, say, 30 m, then the partial pressures of oxygen and nitrogen are multiplied by 3 . Their solubilities remain the same so the water now contains 3 times the amount (mg/L) of the gases. At that depth, and under that pressure, the water is saturated, but when that pressure is released as the water circulates to the surface the amount of gas the water can hold in solution decreases and the water becomes supersaturated. The gases then effervesce, like opening a carbonated drink. At surface, Solubility x 150 mm Hg = 9.2 mg/L so at 30 m Solubility x 450 mm Hg = 27.6 mg/L that means an additional 18.4 mg/L may go into solution at 30 m and that same 18.4 mg/L will be released at the surface when the pressure is released. Gas that dissolves in water at the surface can circulate to great depths and the gas pressure remains the same as it was at the surface, it is the pressure at which it goes into solution that determines the gas pressure of water.
What about the Use of Pure Oxygen?
The use of pure oxygen, purchased in liquid form (LOX) or generated from air, for oxygenation is widespread in recirculation aquaculture. It can dramatically increase the transfer of oxygen into the water by increasing the partial pressure gradient across the gas/water interface. For a given surface area and stagnant boundary layer thickness, pure oxygen aeration will speed up the rate of oxygen movement five times because the partial pressure is no longer 150 mm Hg, but one atmosphere, 760 mm Hg, (760/150 = 5.06).
For every 10 m down into the water column that a gas bubble is driven, there is an increase of 1 A pressure (the weight of 10 m of water is about the same as the entire atmosphere). Consequently, as gas dissolves into the water from that gas bubble it enters at high pressure and this increases the quantity and pressure of the dissolved gases in the water. This is termed supersaturation. If a fish is swimming at the same depth (pressure) as that at which the dissolution occurred the gas pressure in the fish's blood is high, as well. This is not a problem as long as the fish stays at that depth; however, if the fish swims upward the hydrostatic pressure decreases and, since the solubility remains constant, the amount of gas the water can hold decreases and the excess gas is released as bubbles. Bubbles of gas in a fish's blood can quickly lead to fatal embolisms. This condition is called "gas bubble disease". Human divers can suffer from the same disease, but it is called "the bends". The deadly bubbles are usually not oxygen, but nitrogen. Gas supersaturation can occur below dams where the bubbles are driven deep into a plunge pool or are entrained under pressure in the turbines or passageways. It can also occur in the plumbing of aquaculture systems and in ground water.
Note: supersaturation can only occur when gas goes into solution at pressure greater than 1 A. If gas goes into solution at the surface (1 A), then it can flow down to great depths without a change in gas pressure.
Dissolved oxygen must be measured frequently by the recirculation aquaculturist because of its life or death importance. The "gold standard" of oxygen measurement is the Winkler titration. In skilled hands, it can determine DO with precision down to 0.02 mg/L. Fish farmers do not need this level of precision, and therefore use simpler methods. DO meters use an electrode to give digital readouts of oxygen content. They can be set up to run continuously and be a part of an alarm and emergency life support system. They are also expensive and must be calibrated and maintained to be accurate. Nevertheless, they are present in all large aquaculture ventures. Smaller fish farms use DO kits with simplified "count the drops" modifications of the Winkler method.
Oxidation reduction (or redox) potential is the proportion of oxidized substances to reduced substances in a particular system. The potential part of the name means that it is expressed as the ability to oxidize or reduce another system. That is, a system with a given redox potential can undergo a reduction and oxidize a system of lower redox potential or be oxidized by reducing a system with a higher potential. Remember, oxidation is the loss of electrons and reduction is the gain of electrons. Redox potential is therefore the ability of something to give or receive electrons relative to something else. Not surprisingly, then, the units are volts. Meters that measure pH do it by measuring electrons (hydronium has them, hydroxyl does not) and so many of those can also be set to read out in ORP (which is affected by pH, so ORP readings are always corrected to a pH of 7). This has been technical, now let's consider ORP in nature and aquaculture. Redox potential can measure how reduced and anaerobic sediments are relative to the water over them. At the surface of the mud exposed to oxygenated water the redox potential would be in the range of 250 to 500 mV. This oxidized layer would extend a fraction of an inch into the sediment where it would change in color from brown to black as the redox potential fell into the negative range, once it reached -400 mV, the sediment is strictly anaerobic. Because hypoxic conditions give a negative reading, the ORP index has been referred to as a "pollution index", since water receiving organic pollution (sewage) tends to be more hypoxic. This an over simplification, because hypoxic and anaerobic conditions are part of pristine, natural ecosystems and are not necessarily caused by pollution, but certainly low ORP readings from the water column (as opposed to the sediments or biofilter) indicate a level of reducing substances that can have negative affects on fish. Increasing oxidized substances, either through increasing aeration or by the addition of an oxidizing agent (potassium permanganate) will increase ORP.