Recirculation Aquaculture: Buffering

4. Water Quality: Buffering

It Is All about the Carbon Cycle

The interplay between the carbon-based life on earth with the physical biosphere results in the profoundly important carbon cycle, which is responsible for everything from the earth's food supply to free oxygen in the atmosphere to global warming. One aspect of the carbon cycle that is of paramount importance to fish is the relationship between inorganic carbon and water. The vast majority of carbon on earth (six times more than the carbon in all the organic matter, living and dead, including fossil fuels) resides in the earth's water in the form of carbonates (carbon/oxygen compounds). These form the basis of the carbonate buffering system, which is the key to successful fish life.

As explained in an earlier chapter, (3. Water Quality: Gases) oxygen (and nitrogen for that matter) can dissolve in water and the amount of these gases in water is a matter of partial pressure and solubility. Carbon dioxide goes into simple solution, too (in fact, it is far more soluble than oxygen or nitrogen), but it also reacts chemically with the water molecule to form bicarbonate and carbonate. Once carbon dioxide turns into a carbonate it is no longer part of the gas pressure that governs the solubility and more carbon dioxide can dissolve.

The form that inorganic carbon in water takes is a function of pH. As pH changes, the predominant species (to use the word in chemical sense) changes. It is a yin/yang relationship because when the ratio between the species of inorganic carbon changes the pH must change, as well. This is what makes the bicarbonate (the middle form) a buffer.

What is buffering?

A buffer is a chemical compound in solution that acts to keep water within a certain pH range. Acidity is the presence of "naked protons", that is, positively charged hydrogen atoms or hydroniums (H+). They are very reactive because they "steal" electrons from other compounds, thus damaging those molecules. An acid burn is the cumulative effect of these reactions. At the other extreme, base is the presence of a strong electron donor, the hydroxyl ion (OH -). A strong base is as reactive as a strong acid. Since biological processes are very sensitive to pH, it needs to be controlled in a recirculation system.

Fortunately, nature provides a means through the carbonate buffering system. Carbonate buffering occurs to varying degrees in the waters of the world. In nature, it typically derives from limestone in the watershed. Carbonate buffering is measured in terms of alkalinity. This word may be misleading because, in this case, it doesn't mean the absence of acidity, but the presence of buffering. Buffering capacity is measured by titrating a water sample with a known concentration of acid in the presence of a pH indicator. As long as buffering capacity remains, the pH doesn't change. However, as soon as the buffering capacity is exhausted, the next drop of acid will cause a rapid drop in pH. Alkalinity is expressed as mg/L of CaCO3 (these units assume that limestone is the source of the buffering, though this is not always the case). In natural waters, alkalinity ranges from as low as 10 to several hundred. Hardness is not the same as alkalinity, but is often similar in a given sample. In essence, carbonate buffering is the presence of the bicarbonate ion (HCO3 -) which has the ability to react with either hydroniums or hydroxyls, neutralizing them.

pH Management

As you will learn in the next chapter, it is important to keep the pH of a recirculation system between 7and 8. The carbonate buffering system coupled with control of CO2 exchange allows the aquaculturist to do this relatively easily. Limestone is not a good source of alkalinity for recirculation aquaculture because the dissolution of limestone by carbonic acid, as shown in the game above, is slow. A much faster way to increase the bicarbonate content of water is to add sodium bicarbonate (NaHCO3), also known as baking soda!

Carbon dioxide, bicarbonate, and carbonate are all just different forms of inorganic carbon in water. Which species predominates is determined by pH. CO2 predominates at low pH, HCO3- at near neutral pH, and CO3-- at high pH. In fact, alkalinity, pH, and CO2 each may be calculated from the other two. In recirculation aquaculture, the tendency is for alkalinity to decline over time as it performs its buffering action. If buffering capacity falls too low, the CO2 produced by the fish and bacteria will cause a decrease in pH. If pH falls below 7, it can cause problems. The solution is for the aquaculturist to periodically to replace lost buffering capacity by adding sodium bicarbonate, thus keeping alkalinity up and pH in the desired range. Scoopfuls of sodium bicarbonate can safely be added directly to the fish tank to correct a pH/alkalinity imbalance. The other part of pH management involves CO2 stripping. When air is used as an oxygen source the aeration needed to increase the oxygen will also remove CO2, so CO2 stripping is essentially part of aeration. When liquid oxygen is employed, however, dedicated CO2 stripping is often required. This is typically done prior to oxygenation so that any O2 added from the air spares costly LOX. A typical CO2 stripper is a cylinder filled with inert substrate and a flow of air running counter to the trickle of water over the substrate. The substrate breaks the water flow into a large surface area and the opposite flow of water and air optimizes the gas exchange.

Measuring Alkalinity and pH

Arguably, alkalinity is the single most important water quality measurement that can be made in regards to fish, yet a lot of confusion surrounds it. First, this word may be misleading because, in this case, it doesn't mean the absence of acidity, but the presence of buffering. Quite simply, alkalinity is a measure of buffering capacity. To measure it, a sample is titrated with dilute acid until the buffering is exhausted and the pH falls (indicated by a color change of a pH indicator). The units of alkalinity are typically mg/L as CaCO3, though some kits yield results in antiquated units like grains per gallon. What mg/L as CaCO3 means is that a given sample has the buffering capacity equivalent to the carbonate buffering that would come from the dissolution of that quantity of limestone. Water hardness is similar to alkalinity but not exactly the same. Hardness is a measure of the ions Mg and Ca (and some relatively rare metallic ones like Al, Fe, Mn) in the water and is directly relevant to lathering of soap. In nature, most buffering comes from Ca compounds, so hardness and buffering are usually highly correlated, but they don't have to be. For example, in a later chapter the use of baking soda, sodium bicarbonate (NaHCO3), to achieve buffering without increasing hardness (no Mg or Ca) will be discussed.

Another point of confusion is so-called "carbonate hardness", sometimes abbreviated KH (from the German?). It is the portion of total hardness that is equivalent to total alkalinity. If hardness is greater than total alkalinity the amount of hardness in excess of total alkalinity is the "noncarbonate hardness". If the total hardness is less than total alkalinity, as it would be if sodium bicarbonate was used to boost buffering, all the hardness is "carbonate hardness" and there is no "non-carbonate hardness". The important point here is that total alkalinity can exceed "carbonate hardness" (though carbonate hardness cannot exceed total alkalinity). Best to ignore hardness altogether and measure total alkalinity.

Finally, total alkalinity can be further broken down into carbonate and bicarbonate alkalinity. Carbonate alkalinity only exists at pH's over 8 and is not generally a consideration in fish ponds. Consequently, the phenolphthalein endpoint in alkalinity titrations can be ignored. Bicarbonate alkalinity buffers our fish's water. Alkalinity is slowly lost over time as it reacts with acids or bases, but does not change according to season or time of day.

A tank's pH may be measured colorimetrically or with a meter. As discussed above, pH and alkalinity are linked, so measuring one will give information about the other. Also, pH must be known to evaluate the meaning of the total ammonia value.

Assignment 4