Hi All
This thread is representative of innumerable threads on RC and other aquarium boards where interested parties attempt to assess skimmer performance and come to some conclusion about the merits of one skimmer (or one type of skimmer design) vs. another.
A critical question underlying these types of dialogs is “What criteria for skimming success is being cited?â€Â
Although this thread does not delve into some of the criteria raised in other skimmer threads, in general, aquarists seem to focus on criteria like (a) volume of skimmate removed per unit time, (b) color of skimmate, and/or (c) air flow rate (lph), presumably because all of these factors are readily measurable/observable. It then follows that if one of these criteria is used to judge skimmer performance, it is a relatively straightforward matter to collect data and arrive at a conclusion; i.e., skimmer A removes more skimmate per hour, or pulls more air into the skimmer body, than skimmer B, and so it is “betterâ€Â.
However, there are some concerns raised by this approach if these particular criteria are being used as surrogates for TOC (total organic carbon) removal.
Specifically, as an illustration of these concerns, when I lecture on skimmer performance, I use a slide that shows three skimmer cups full of skimmate. The first is black coffee colored and has lots of foam. The second is light tea colored and has little foam, and the third is in between the first two in terms of color and foam. I ask the audience which skimmate is the most desirable, and of course the deep, rich coffee colored example gets the most votes. I then go on to reveal that the coffee colored skimmate is, in fact, coffee with some dish soap thrown in to generate bubbles upon blowing with a straw. If the entire volume is concentrated to a dry solid, this "instant coffee" weighs 2.7 gm. The light tea colored skimmate is actually 10 gm of brown sugar in water with again dish soap/blowing to generate foam. That is, the weak, undesirable "skimmate" actually consists of 4x the amount of organic stuff. The third skimmer cup contains actual skimmate from my reef tank, and I go on to talk about its dry mass, with and without feeding the tank, etc. So, the take-home message that I hope that the audience absorbs is that color, volume, smell, etc. are not useful predictors of skimmate dry mass, which is the actual important metric if clearing TOC from aquarium water is the desired goal of the skimmer.
The tests described in the Advanced Aquarist article use different criteria than skimmate color or volume for measuring skimmer performance; the AA article tests use (a) the rate of TOC removal from authentic aquarium water, and (b) the amount of TOC removed from aquarium water, as metrics for successful skimming. The fundamental conclusions from these tests are described above by pjf.
The skimmers that we tested have been characterized as “relatively low tech†earlier in this thread. Over the summer 2009, we plan to test (by the same methodology) what we hope are viewed as “high-end†skimmers. At present, I am leaning towards a Bubble King mini and a Warner Marine recirculating skimmer (I forgot the model #) because I want to test further the relationship between water flow rate and skimmer performance, and the WM skimmer will allow me to set water flow rate and air flow rate independently. Both time and $$ constraints prevent us from pursuing more than two new skimmers. However, the methodology described in the article is robust enough for a skimmer manufacturer, for example, to pick up the gauntlet and run tests themselves.
FWIW, its highly unlikely that any significant amount of proteins survive long enough in aquarium water to be picked up by skimmers. That raises the question, "what exactly is TOC, anyway?" - an important question without an answer at present.
Are the low TOC pickup results (i.e., 20 â€"œ 30 % of the TOC present) extendable to other skimmers? The testing alluded to above with the new “high-end†skimmers will be useful in answering this question. If you read the text accompanying Fig. 1 of the referenced Advanced Aquarist article, you will find an explanation for that observation (only 20 - 30% TOC removal) that suggests the low pickup value is an intrinsic function of bubbles and TOC, and has nothing to do with the skimmer per se.
If those results extrapolate to all skimmers (or at least other skimmers), then it is clear that skimmers don't do much, and it then becomes questionable if it is really worth obsessing about all of the engineering modifications (bubble plates, cones, recirculating pumps, etc) that currently drive up the price of skimmers.
In terms of the larger topic of clearing TOC from aquarium water, two methods that far exceed skimmer capabilities are based on (1) GAC (granular activated carbon) and (2) bacterial consumption. On this latter point, I direct your attention to Fig. 6 in
http://www.advancedaquarist.com/2008/8/aafeature3/ . You will see that reef aquarium TOC levels drop from an after-feeding high of ~ 1.4 ppm of C down to a baseline value of ~ 0.9 ppm of C over 12 hours without any skimming or GAC. The TOC is likely consumed by the tank’s bacterial population, a hypothesis supported by the independent work of several other researchers (e.g., Ferrier-Pages, Means, Rohwer). On the topic of GAC-based removal of TOC, we have some unpublished results that demonstrate that a sufficient amount of GAC can strip an authentic reef tank’s TOC content from several ppm down to < 0.3 ppm (our detection limit) over several hours.
So, I think that what we are learning from all of these skimmer and GAC-based reef tank water purification studies is that bubbles (i.e., a skimmer) is a relatively poor methodology for TOC removal compared to GAC or bacterial action, but bubbles do have the advantage of being cheap! An entirely different question that should be at the core of these discussions (but is not at present) is, “What are appropriate TOC ranges for maintaining a healthy and thriving reef tank?†Some hints might be garnered from the data in Table 1 of the “2008/8/aafeature3†Advanced Aquarist article. From that perspective (i.e., mimicking the TOC levels on natural reefs), the ability of the various methodologies that modulate aquarium TOC levels can be judged as adequate or inadequate.
More to come, stay tuned….
Ken
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