History of Protein Skimming
The first documented examples of using foam generation to remove organic compounds from water can be traced to Ostwald, and independently, Schutz in 1937 (Ostwald, 1937; Schutz, 1937). Thiel claims that water purification via skimming was introduced into the aquarium hobby in the 1960's by Huckstedt (Thiel, 1997; Huckstedt, 1972), but the practice did not gain much traction until a resurgence of interest in keeping corals brought it to the fore again in the 1990's. Another early notable advance in using foam flotation technology for saltwater purification was described by Wallace (Wallace, 1969). The early developments in water purification then led to advances in two disparate venues; wastewater remediation, and protein purification (Lemlich, 1972; Okamoto, 1979; Clark, 1983; Caballero, 1990). The application of skimming in aquarium husbandry was an outgrowth of successful implementation of foam fractionation techniques in these areas, and the development of modern skimmers owes much to these pioneering efforts. Foam fractionation in particular proved to be a valuable asset in enabling the isolation/recovery of desirable proteins from dilute solutions in many areas of food and pharmaceutical science. In this context, the goal was just the opposite of protein skimming in aquaria; recovery of valuable proteins in the foam with discharge of the depleted water phase. In contrast, of course, protein skimming in aquaria is used to remove undesirable organics from the (valuable) tank water. Nevertheless, the processes are identical, a conceptual convergence that becomes important in assessing the influence of various input parameters on skimmer performance. Specifically, the pivotal role of foam fractionation-based purification in protein recovery has prompted many research groups to conduct studies designed to optimize protein purification by tweaking input variables. It is possible that these studies can inform the aquarium skimming area as well. Much effort has been directed to measuring how changes in (a) gas flow rate, (b) liquid flow rate, and (c) bubble size influence two important figures-of-merit in the protein purification (and by inference, aquarium skimming) area; enrichment (E) and recovery (R). Enrichment (E) is defined slightly differently by different authors. Some authors describe E as the ratio of the protein concentration in the (collapsed and removed) foam head relative to the protein concentration in the skimmer feed solution (E = Crecov/Cin in Fig. 1) (Uraizee, 1996; Brown, 1990), whereas other authors define this quantity as the ratio of protein concentration in the foam head compared to the protein concentration in the output solution of the skimmer (E = Crecov/Cout in Fig. 1) (Ahmed, 1975; Schnepf, 1959). The numbers obtained by these two definitions do not differ greatly, and so this distinction is not critical. A second figure-of-merit often cited in these skimmer performance studies is recovery (R), which is defined as the amount of protein removed from the solution by the skimmer relative to the amount of protein fed into the skimmer. The recovery R can be expressed as a percent of protein removed after a specified time: i.e., 50% of the protein has been recovered after 90 minutes. These two measurable quantities typically run in opposite directions; that is, those changes that increase the enrichment typically decrease the recovery, and vice versa.
Both enrichment and recovery have counterparts in the aquarium skimming area. Dry skimming implies very little water hold-up in the foam, and this scenario is more closely aligned with enrichment. Thus, maximizing foam enrichment while dry skimming should maximize impurity removal from aquarium water. In contrast, wet skimming, with its proportionally larger liquid hold-up in the foam, falls more under the aegis of the "recovery" manifold of skimmer operation. That is, the removal of organic-rich foam and entrained aquarium water that contains organics (= wet skimming) should lead to a greater overall removal of the organic impurities in the aquarium water. In this case, maximizing recovery R should lead to maximizing water purification. To the extent that an aquarist aligns their skimming technique with one or the other extreme, then the lessons learned about optimizing either enrichment or recovery might prove insightful. Wet skimming bears the added burden of introducing possible salinity fluctuations, as the aquarium water removed in the foam phase must be replaced by water of equivalent salinity in order to maintain the overall tank's salinity. To the extent that this match is not maintained, the tank's overall salinity may vary. Thus, a compromise between wet and dry skimming often is sought.
Feldman, Maers, Vernese, Huber, Dept. of Chemistry, Penn State University. Citing Sources: (http://www.advancedaquarist.com/2009/1/aafeature2#section-5]: para. 4: [January 2009]