Tuesday, March 6, 2012

What is a protein Skimmer, and how does it work?

The heart of the modern reef external filter system is the protein skimmer. Make sure it is large enough to do the job. You cannot have too much skimming! Buy or make the largest one you can afford. Keep in mind: the larger your skimmer, the larger the skimmer pump will have to be. It should, however, be no larger than the main pump of the system.

I always cut the factory rating in half when planning for my skimmer. A protein skimmer is a clear plastic cylinder, usually 4-6” in diameter and 24-38” tall. Water is directed into the column by a water pump. Inside the column fine air bubbles are introduced and mixed with the water. This is usually done with wooden air blocks placed at the bottom inside the column, and fed with a strong air pump. This type of skimmer is called “counter-current” or “air-driven,” meaning that the water is pumped in at the top of the cylinder, and is directed downward at approximately a 45° angle. This makes the water swirl in the plastic column, while the fine air bubbles from the wooden air blocks rise to the top.

The bubbles drag water with them, rise to the top, and burst. Because there are dissolved proteins and wastes in the water, these too cling to the extremely small air bubbles, which rise and burst in the upper neck of the plastic column. A foam builds up in the upper neck of the plastic column, and overflows into a collection cup to be removed. This is also known as “foam fractioning,” and has been used in water treatment facilities. See Chapter 12 on the design of a protein skimmer.

A Venturi type of skimmer uses a Venturi air valve that is in line with the water being pumped into the skimmer. A Venturi eliminates the need for air pumps and blocks. It runs with a higher flow rate than counter-current systems, and therefore will need a stronger pump: one that is designed to operate under pressure, a “pressure pump.” This type of pump is designed to force water through the Venturi valve (see pump description). The Venturi is considered to be more efficient and overall less expensive to operate than air-driven counter-current skimmers.

The tower type:(Downdraft) There has been a revolutionary new design of skimmer on the market, which is neither Venturi nor counter-current. This new design is called E.T.S., for “Environmental Tower Scrubber.” Water is pumped through a tall, narrow “tower” (approximately 2” wide), filled with plastic media balls. The water is under considerable pressure, and it gets forced down the small neck through the plastic media, with air pulled in from the top of the skimmer creating fine air bubbles. Proteins and waste products cling to the fine air bubbles accumulating in a collection neck, rise, and overflow. This new dimension of skimming is extremely efficient and well worth considering, as the skimmer for your reef.

All skimmers operate on water being pumped in at the top and draining out by gravity at the bottom. The bottom of the skimmer should be above the top of the sump, thereby using gravity to full advantage. Most should have control valves in line before and after the skimmer to insure proper water control and fine tuning of the skimmer.

Also, you will hear talk of “efficient” skimming. This means that the skimmer is adjusted so that the accumulating foam has a relatively stiff consistency. If the bubbles are too close to the top of the neck, they will burst there (at the top), and you will not get the desired thick foam that accumulates until it reaches the top and overflows into the collection cup.

When the tank is first set up, a considerable amount of foam (waste) will be removed. Initially this is from die-off on the rock. The first couple of months are the most crucial from a monitoring standpoint. As time goes on and things balance out, the skimmer will be more predictable. This is good: when the skimmer is properly adjusted to produce a thick foam regularly, this indicates that the waste is being removed and the skimmer is operating correctly. The protein skimmer is the most important external filtering device of the enclosed reef, and will need sufficient monitoring for it to function properly, especially when it is first set up.

Here are some ideas on pumps.

1. Main pump, flow type:
You want to have a turnover of water at least six times the volume of the tank per hour with the main pump. For example, in a 55-gallon tank the turnover should be 55 x 6 = 330 GPH. This would require a main pump in the range of 330 GPH. Also, remember to take the vertical pumping action (head) into account when selecting the main pump. As a general rule, I would also add 20% to the figure as compensation for the head pressure. In this case, 20% of 330 = 66. Adding that to the original 330, we get 396, or about 400 GPH. By adding this 20%, I am assured of the pump’s capability. The main pump should be a flow pump, meaning it is designed to flow the water instead of having to force the water through something like a pressure pump. Also, I am not fond of submersible pumps, unless they are absolutely necessary in a particular situation. So, for a 55-gallon tank, we would use a 400 GPH main pump.

2. Skimmer pump:
For a Venturi skimmer, you will want a pressure pump. This is needed to force the water through the Venturi valve. For a counter-current skimmer, you will want a flow pump, because pressure is not necessary in this design. You will have to be very careful when selecting a pump for a factory-built skimmer. Closely follow the manufacturer’s recommendation of pump size! Skimmers are designed in such a way that the drain fitting is a certain size, and will only drain the water out so fast. I have found that valves are always needed on both sides of the skimmer. This gives you the advantage of being able to fine tune the skimmer, which is definitely necessary. If you make your own skimmer, the drain fitting can be larger than would be found on a factory-built skimmer. By having a large drain fitting controlled by a valve, you can increase the amount of water for the skimmer to process. Of course, this doesn’t mean you can have a small skimmer with a large drain hole and expect to remove waste properly. However, having a sizable drain fitting will give you more control over the rates of drainage and water flow.