Protein skimmer (foam fractionator)
A protein skimmer or foam fractionator has applications in saltwater systems to remove organic compounds from the water before they decompose (break down) into nitrogenous waste. Protein skimming physically removes organic compounds before they begin to decompose. This reduces the load on the biological filter and improves the water’s redox potential.
Protein skimming, or foam fractionation, is an effective filtration tool when used with closed recirculating systems. Saltwater systems can benefit from protein skimming especially in those carrying high fish loads.
Combining a small amount of ozone with protein skimming improves the performance by oxidizing the available organic contaminants. This leads to better flocculation and absorption of particles onto the bubble wall.
When considering the use of ozone, be certain that the air (preferably pure oxygen) used to feed the ozone generator is dry and clean. The feed gas to the ozone generator ultimately establishes the efficiency and operating life of the equipment. Managing ozone dosing can be achieved with an ORP (Oxygen Reduction Potential) Meter.
capacity(m3/h) detention time
capacity (m3/h) detention time
(1.5 min) HRT
HRT = hydraulic residence time
Different filtration equipment is suited to handling different particle sizes.
Suspended solids may be removed using foam fractionation, diatomaceous earth filtration or membrane filtration. Suspended solids are a technical problem in recirculating systems. As these solids accumulate they can cause slowed growth and reduced immune resistance (disease susceptibility). Solids removal (faeces, uneaten feed, bacteria flocks, and other solid particles) is key to successful production. Conventional mechanical bead filter separation and foam fractionation, are suitable to remove large particles and fine solid matter.
A protein skimmer is the most commonly used equipment by aquarium and aquaculture to remove suspended and dissolved waste from fish tanks by means of air floatation. Protein skimming is a relatively simple process which relies on the physical and chemical properties of coagulation. Protein skimming process requires a contact column in which air and water containing these surfactants can interact. The air and water interfaces are provided by supplying a constant injection of small bubbles to a proportional volume of water in the column and allowing them to mix and then migrate to the surface. During this migration, suspended and dissolved organic material (protein is a specific class of organic material) congregate at the air/water interfaces, with the bubbles forming a “skin” like character on their surface. At the upper levels of the column most of the water drains away leaving a concentrated residue of foam composed primarily of organic material. The skimming off of this foam waste removes it from the life support system. It is, therefore, no longer susceptible to undesirable microbiological degradation which will contribute to increased oxygen demand in an aquatic system. Protein skimming is essentially a perpetual backwash filter. In addition, skimming enhances the oxygenation of a system both biologically and physically. Biological oxygen demand is
reduced by the direct removal of biodegradable organic carbon (protein), as well as bacteria in the water column. Physically, the increased surface area provided by the
injection of large volumes of air (in the form of small bubbles) into a column of water creates a condition for gas exchange. The addition of a small dose of ozone to the process, with its coagulative and disinfectant properties, makes skimming possibly the best component one could add to a modern life support system.
Marine protein skimmer with no foam collection cup
Emphasis on Foam Fractionation
The presence of organic carbon (both dissolved and suspended) lowers the value of D.O., pH and redox potential. Sand filters are limited in their ability to eliminate or otherwise control organic carbon. By design, sand filters trap material to improve water clarity. Organic carbon present in this trapped material hydrolyzes and is released back into solution. Biofilms of heterotrophic bacteria develop, utilizing the hydrolyzed carbon and give off CO2, lowering system pH. This affect is only partially diminished by backwashing, since not all of the biofilm is purged during backwash. Depressed pH, D.O. and redox potential characterize traditional systems that rely primarily on sand filters.
Alternatively, foam fractionation (FF) provides continual removal of organic carbon. By design, it pulls long-chained organic molecules out of solution and flocculates this material into thick foam, which is continuously discharged. Trapped within the sticky foam are particles, which include a significant concentration of suspended organic carbon. Thus, the negative effects of organic biodegradation of sand filters are short-circuited by aggressive use of fractionation. Additionally, the loading on sand filters is reduced, allowing them to perform more efficiently.
It is recommended to use ozone to help increase the efficiency of the foam fractionation process in freshwater.
- Recent publication shows the used of ozone and foam fractionation together is very effective on reducing suspended small particles: https://www.sciencedirect.com/science/article/pii/S0144860921000510?via%3Dihub.
Ozone applied by itself significantly reduced the number of particles (83 %), bacterial activity (48 %) and particulate BOD5 (5-days biochemical oxygen demand; 54 %), and increased ultra violet transmittance (UVT; 43 %) compared to the untreated control group. Foam fractionation by itself lead to significant reductions in particle numbers and volume (58 and 62 %, respectively), turbidity (62 %), bacterial activity (54 %) and total BOD5 (51 %). A combination of both treatments resulted in a significant additional improvement of important water quality variables, including a 75 % reduction in total BOD5, 79 % reduction in turbidity, 89% reduction in particle numbers and 90 % reduction in bacterial activity compared to the control.
The removal efficiencies were within the same range as those observed in previous studies conducted with foam fractionators in saltwater systems (with or without ozone), corroborating that foam fractionation may become a useful tool for controlling organic matter build-up and bacterial loads in freshwater RAS.