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SantaMonica
01-12-2010, 07:17 PM
Scrubber Limitations: Light, flow, and roughness


(Note: The following is based on a scrubber growing Green Hair Algae (GHA); it does not apply to a scrubber that is growing dark, oily algae.)



After 1.5 years, this is just now starting to become clear after looking at the progress of hundreds of scrubbers. It's really simple if you think about it.

The scrubbers that we build today are fixed in size; they cannot get wider, or cover any more area. Algae in the wild (and in your display), can and do get wider and cover more area: They spread from rock to rock to rock, to sand, to back panels, etc. They do not really get any THICKER though, because they don't need to. They just spread out. Algae in our home built scrubbers, however, can't cover more area, and therefore they grow THICKER. Much thicker. Really, really thick. And this is where scrubber ratios come in.


As GHA gets thicker on the screen, several things occur:

1. It gets more three-dimensional, allowing water to flow throughout the algae strands.
2. It gets heavier.
3. It sticks out more and catches more of the water flow.
4. It blocks more light from reaching the bottom layers.

Number (1) is a great thing, since you get your strongest filtering when water is allowed to flow through, and not just over, the algae. This is best accomplished by having a bit of water "stacked up" on the bottom of the scrubber (via a slow drain), so the algae can float in it. This really opens up the strands and allows full 3D contact of the strands with the water, and it allow more light in the strands too.

Number (2) and (3) work together to detach algae off of the screen. Once algae let go, all filtering from this algae is lost.

Number (4) was originally the "killer" problem, but it is now understood to be only as important as the other factors.


Before understanding the limitations, you must first understand the components:

Lighting: As the total energy of lighting is increased (up to a "falloff" point), the outer layers of algae will grow faster, and the inner layers will stay alive longer, thus allowing algae to get thicker before the bottom layers die off. Total energy is wattage multiplied by the number of hours of light. Once the "falloff" energy point is reached, the algae does not get enough flow to deal with all the light, and thus the algae starts turning into a yellow rubber; filtering drops off quickly.

Flow: As flow is increased, more nutrients are delivered to the algae; more CO2 is removed from the algae; more of the boundary layer is reduced (all of which are good).

Roughness: As the screen gets rougher, algae can hold on longer before letting go.

Time: As you wait longer between cleanings, the top layers of algae get thicker which increases filtering, but the bottom layers start dying faster, which decreases filtering (and makes the water cloudy).



Here are the limitations:

Ratio of Flow to Roughness: Ideally you want lots of flow. But a limitation is screen roughness, because as the flow gets higher, it starts ripping more algae off the screen. So the rougher the screen is, the more flow you can use.

Ratio of Flow to Lighting: Ideally you want lots of lighting. But a limitation is flow, because as the lighting gets higher, it starts putting the algae into "photoinhibition", which means that the algae can't get nutrients into and out of it's cells fast enough to keep up with the lighting (thus is turns rubbery yellow). So the higher the flow, the more the nutrients are removed from the algae cells, and the more lighing you can use.

Ratio of Flow to Time: Ideally you want lots of flow. But a limitation is time, because as you let the algae grow for a longer time, it starts sticking out more and getting washed away by higher flow. So the longer you wait for cleaning, the less flow you can use.

Ratio of Lighting to Roughness: Ideally you want lots of lighting. But a limitation is screen roughness, because as lighting is increased, the GHA strands get longer quicker, and thus get washed away quicker. So the rougher your screen is, the more the GHA strands will hold on, and the more lighting you can use.

Ratio of Lighting to Time: Ideally you'd like lots of time between cleanings. But a limitation is that the bottom layers of algae starting dying after about three days, and the longer you wait for cleanings, the more of these dying layers start adding up. This is compensated for by the new growth on the outer layers, which continues to filter even though the bottom layers are dying. So the stronger the lighting, the more the new outer layers will compensate for the dying layers, and thus the longer you can go between cleanings, before there are so many dying layers that the new growth can't keep up.

Ratio of Roughness to Time: Ideally you'd like lots of time between cleanings. But a limitation is that the bottom layers of algae starting dying and letting go (taking the new growth with it). So the rougher the screen is, the longer the dead layers will hold on, and the longer you can go between cleanings.

Here is a typical example, Number 1: Dark stuff starts growing, but even after several weeks it has not turned green, and the nutrients have not dropped. Stronger light is the first thing to try, but if the flow is too low you will "burn" the algae (turn it into a rubbery yellow). So you try to increase flow, but now algae is detaching because the screen is too smooth. So, in a way, the entire future of the scrubber is based upon how much algae can stick to the screen.

Example 2: You have good GHA growing, and your tank looks good. Then you add more fish and start feeding much more. Your phosphate goes up, so you turn up the flow to deliver more nutrients to the screen. But then you start noticing bald patches on the screen that were not there yesterday. The screen has reached a point where it can't hold on to the long GHA anymore, so therefore you really can't use the higher flow. One solution is to clean more often.

Example 3: You've been cleaning every 7 days, but you want to try for 10 days. At 10 days, you start noticing bald spots, and when you clean, you notice that some of the GHA comes off easily and has a patch of light-brown "wheat" looking stuff under it. The "wheat" patches are dead algae from being without light and flow. They let go and caused the bald spots. A rougher screen would hold them a bit longer, but they are still dying and adding nutrients back into the water. A solution would be a rougher screen, combined with strong lighting. The lighting would cause more new algae to grow, which would compensate for the dying algae.