XVI. Protein Skimmers - Dissolved Organic Nutrients
This article discusses Foam Fractionation, Protein Skimmers & CO2
In a closed-loop system containing a living organism it is critical to address the nutrient and energy needs of the inhabitants. Through proper lighting and the feeding of our coral various metabolic processes create organic compounds to be used as energy within the coral for biological functions. When properly addressed in a closed-loop system the outcome of these biological functions are various dissolved organic compounds (DOC). Carbon dioxide, proteins and waste matter are the primary compounds we will address with foam fractionation.
Foam Fractionation
As you walk along an ocean beach the waves will lap against the shore and quickly recede. Along the surface of the receding water pockets of seafoam float back into the ocean. As the air and the water are mixed by a wave, millions of tiny microbubbles are created. When the water is particularly choppy and larger volumes of foam is created, or along rocky coves and jetties, you will often notice that the foam accumulated is beige tinted, or even has visible sludge accumulated on the surface of the foam deposit.
Oxygen is a highly reactive element and along with gases it becomes trapped by various DOCs cling to the surface tension of the microbubble formed. These surfactants form a molecular film around the gas bubble. A large enough volume of microbubbles generated in a small enough space and these bubbles will begin to bind to each other as the ionic charges contained within the films attract microbubbles with opposing charges generated in the same manner. The microbubbles combine into a matrix with the thin films of surfactants binding them to one another. As conditions change the films of the matrix weaken. If an interior film weakens between two bubbles the two microbubbles may combine to form larger bubbles with the surfactants redistributing along the outer film.
When we have a large volume of microbubbles in a small enough volume of liquid this process continues to scale up. Microbubbles form matrices and condense into tiny bubbles which form matrixes and condense into progressively larger bubbles. The surfactants combine as the bubbles accumulate reducing the surface tension further as the film increases in volume allowing for larger and larger bubbles to form. As these larger bubbles form matrices foam results as the surfactants are capable of encapsulating larger quantities of gas within the bubbles of its matrix until a critical mass is accumulated and surface tension can no longer be maintained by the accumulated surfactants. When this happens on a large enough scale the accumulated surfactants deposited by the popping bubbles is observable with the human eye as a sludge forming along the surface of the foam.
Protein Skimmers
In an aquarium, we can harness foam fractionation as an extraction process to remove dissolved organic compounds in our aquarium and reduce the amount of waste in the water column. A protein skimmer is a filtration device that does just this. Comprised of four primary components, available in both internal and external mounting formats, and with a relatively small footprint, it is easy to incorporate into a reef tank system. The benefits of a protein skimmer are well sung by aquarists so much so that it is often viewed as a core filtration component in a reef tank.
The overall footprint of a protein skimmer is usually dictated by the diameter of its reaction chamber. The reaction chamber is a large tube in which the process of foam fractionation actually takes place. A venturi is used in tandem with a water pump to continuously inject a mixture of tank water and microbubbles into the reaction chamber. The overall water level within the reaction chamber as well a the ratio of air to liquid introduced to the reaction chamber are controlled through knobs or valves. Within the reaction chamber, microbubbles are allowed to accumulate away from conditions that would disperse a foam. As a foam builds in the reactor chamber it is pushed upward through a taper that causes the foam mass to condense and for the films within the foam matrices. Eventually, the force of upward escaping gases from the foam mass below will carry the viscous film developed into the collection cup.
The water within the reaction chamber is recirculated into the tank and leads to microbubbles being introduced where this return is plumbed. Due to the introduction of bubbles most aquarist prefer to plumb the skimmer to their sump. If you do not have a sump, you will need to pass the return water through a foam post filter to reduce the accumulation of bubbles in your tank. The waste in the collection cup will need to be drained periodically as part of normal tank maintenance. In removing the skimmate from the tank you are removing a large portion the dissolved organic compounds that would normally result in elevated nutrient levels, however, a protein skimmer offers a secondary benefit in our aquarium by heavily aerating the water.
CO2 scrubber
Employing a CO2 scrubber to feed your venturi increases the effectiveness of this process greatly. A CO2 scrubber is a small reaction chamber air passes through containing CO2 absorbing media. The media most often employed is simple soda lime which not only absorbs CO2 but also naturally changes color as the media is exhausted leading to the easy visual indication of maintenance. The CO2 introduced from the air around our tanks reacts readily with both Calcium and Magnesium which are critical to stony coral growth. For enthusiasts with a heavy population of stony corals available Calcium and Magnesium is so demanded a source such as Kalkwasser is dripped into the tank. To be spending money for additives or kalk for Calcium and Magnesium, the use of a CO2 scrubber increases the effectiveness of these additives by ensuring we are not encouraging these minerals to become bound and inaccessible to our coral. Overall a CO2 scrubber is a nice tool to help further control the environment within our tanks and ensure the efficiency of our systems.
Whether a CO2 scrubber is employed or not, the added aeration from the use of a protein skimmer also increases the Oxidizing Reduction Potential(ORP) of our tanks. The ORP measures the ability of a liquid to oxidize nutrients; a process that overall reduces nutrient buildups in our tanks. A protein skimmer should not be viewed as a method to extend the time needed between water changes. There is no substitution to a proper water change schedule however employing a protein skimmer greatly reduces the accumulation of nitrate and phosphate between water changes. Nitrate and phosphate both cause stress to coral and we want to maintain these levels as low as possible.