Activated Carbon in Aquaculture - Tech Talk 42
We typically use activated carbon in three different facets of aquaculture: taking impurities out of water as it is brought into the facility; removing halogens such as ozone, chlorine and bromine; and removing color and metabolic by-products in recirculating systems. Activated carbon is the generic term used to describe the family of carbonaceous adsorbents with an extensively developed internal pore structure. A wide variety of activated carbon products are available, exhibiting markedly different characteristics. They are commonly made from wood, coal, lignite and coconut shell.
In activated carbon's manufacture, the material is first subjected to a heating process called carbonization, which forms a fixed carbon mass full of tiny pores. It is then activated by a second heat/steam treatment (200–1,600°C) while regulating oxygen level, which creates an even larger internal pore network and imparts surface chemistries that give carbon its unique filtering characteristics. Some carbons are activated with phosphoric acid, potassium hydroxide or zinc chloride, which makes them unsuitable for use in aquaculture. When selecting an activated carbon, consider the adsorptive characteristics of that carbon on the chemicals to be removed.
Activated carbon’s adsorptive characteristics are based on the principle that the greater the surface area, the higher the number of adsorptive sites available. The pore size and the pore size distribution are extremely important, as they affect the efficacy of the carbon. The macropores (larger than 25 nm) are used as the entrance to the carbon, the mesopores (1–25 nm) for transportation and the micropores (less than 1 nm) for adsorption. It is a generalization to say that the porosity of an activated carbon can be measured by adsorption of iodine from solution, but this measurement may not at all predict its ability to adsorb other chemicals.
The finer the particle size of an activated carbon, the better the access to the surface area and the faster the rate of adsorption. Small pore size must be weighed against pressure drop, as this will affect energy cost. Careful consideration of particle size can provide significant operating benefits.
Activated carbon will adsorb the following from water: chlorine and some chloramines, many dissolved organic contaminants, trihalomethanes (THM) and phenolics, total organic carbon (TOC), oil and hydrocarbon contamination, ozone, bromic acid and total organic halogens (TOX), adsorbable organic halogens (AOX) including chloroform, colors, pesticides, odors and more. Activated carbon will also reduce biological oxygen demand (BOD) and chemical oxygen demand (COD).
It is important to be able to measure the contaminant that the carbon needs to adsorb in order to know when the saturation capacity of the carbon is reached. Particle size, water flow rate, carbon bed depth and, in recirculating systems, the number of passes through the bed must be optimized for every system design. Typically, for a single pass system, a deep bed with very slow flow rates would be required, so that removal of dissolved organics can take place in the top portion of the bed. Change the carbon before it becomes saturated. If the carbon is not replaced, it could desorb what it has already removed. This can cause a nasty, toxic release. Always backwash the filter before use. In backwashing, a bed expansion of at least 25 percent should be used to remove any carbon dust.
If it is absolutely necessary to remove a contaminant from the water, use a series of activated carbon filters and do water sampling after the first filter. The second filter will act as guard bed. Carbon, like all surfaces in recirculating aquaculture, will support bacteria that consume some of the absorbed organics and, if left too long, can slime over the surfaces. Ozone and chloramines oxidize the carbon's surface, and they do not accumulate in the carbon structure.