In 2003, at my first aquaculture site, I was faced with a CO2 problem. pH in a seawater system was being kept at 7.0 by addition of soda ash. Cascade degassers consisted of 15 trays in a vertical column through which the water was exposed to air. Water entered the top at 20ppm CO2 and left the degasser at 12ppm CO2.
Were the degassers too small? Measuring between each tray revealed a strange phenomenon. Tray #1: 20ppm, Tray #2: 16ppm, Tray #5: 12ppm, Tray #6: through 15: 12ppm. The degassers were actually 3x larger than necessary, so why couldn’t they lower the CO2 below 12ppm?
Carbon dioxide (CO2) occurs in culture water from two main sources. The water supply can often contain C)2 especially if using well water. And, the most common source is, of course, fish respiration. Just like humans, a by-product of consuming oxygen is the release of CO2. Because of molar ratios, the consumption of 1 gram of oxygen results in the release of 1.37 grams of CO2.
CO2 has some well known and some yet-unknown effects on fish. The simplest way to describe the effect is that the higher the CO2, the more difficult it is for a fish to consume oxygen. From a layman’s perspective, the biological effects of CO2 on a fish may be very similar to oxygen deprivation.
The actual level that causes adverse effects varies with each species. The levels that cause short term behavioural changes such as inhibiting feeding (and therefore production) are relatively well known for commonly cultured species. However, newer research has been inquiring into chronic effects and finding evidence of slower growth occurring in the presence of much lower levels of CO2 than previously considered to be problematic.
Discussing CO2 in culture water is not complete without mentioning that when CO2 is present in the water, it equilibrates with several other forms of carbon starting with carbonic acid which then dissociates further. Its’ dissociates can then form new compounds with Mg and Ca for example.
The tendency of CO2 to form carbonic acid in water directly affects the pH of the water making it more acidic. This is another way in which CO2 may adversely affect the cultured species. This explains why the degassers in the marine system I worked on couldn’t lower the CO2 below 12ppm, even though they had plenty of degassing capacity. The CO2 simply wasn’t free to leave.
When aquaculture operator measures CO2, he measures it as aqueous CO2 or CO2(1). Measurement can be done using a chemical reagent test kit or (recently made feasible for most operations) a digital meter and probe.
TWO TREATMENT METHODS
Treatment methods generally take two different approaches:
The first is to attempt to remove CO2(1) from the water using aeration or degassing. Aeration is generally more suitable where longer treatment times and lower CO2 levels are needed. Degassing may be more suitable where the species is not overly susceptible to the presence of some CO2. In a recirc system, it may make sense to use both approaches in various stages of the system. A foam fractionator or MBBR, for example, may both be sized to also degas some CO2, while a degasser further downstream may be used to bring the CO2 to the desired level.
Packed columns (or other cascade style degassers) are commonly employed prior to water entering a culture tank both to guard against nitrogen supersaturation, but can also be useful in removing some CO2. Forcing air though this type of degasser can increase its effectiveness and is often seen in more intensive applications. As a rule of thumb 10-20x air to water flow ratio is employed. However, this ratio is dependent on water temperature, salinity, water surface area in the degasser, biomass, pH, alkalinity. If energy is an issue here, a feedback loop could be used to control the quality of air used. In one such application, a 3:1 air:water ratio was needed.
This removal of CO2 via an air-water interface causes the carbonic acid to react and reform CO2(1). The reaction between CO2(1) and HCO3- is not instantaneous, but “kinetically slow”meaning it takes time. If you need to remove CO2 from your system, factor this time into your water treatment approach.
The second treatment method is to raise the pH by adding CaOH or Na2CO3 which forces more CO2 to be “free” and therefore easily aerated or degassed from the water. Predicting how much dosage is needed is possible, but a burdensome mathematical problem. pH, alkalinity, temperature, salinity all play roles and need to be accounted for. This is a job for software which some are working on. Meanwhile, the cost to run a test on an existing system or water supply is not prohibitive and may be quicker than proving the calculations. Alternatively, there are tables available that can provide you with some answers if you know your inputs.