Biological Filtration Types and Techniques
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 Here, we will cover definitions and simple explanations
of the most commonly-used methods and types of biological filtration. As you will soon see, there are many methods and techniques that may be used to accomplish the same thing. All have one thing in common: They all serve to convert toxic
ammonia and ammonium fish waste into less harmful nitrates or nitrogen gases through natural, biological processes.
The Balanced Aquarium: Once very common, aquariums like this are rare these days. Before modern filtration devices were invented, this was the only means of keeping an aquarium. Usually equipped only with aeration or a simple power filter, a balanced aquarium has many plants (or lots of photosynthetic organisms in a marine version), and very very few fish, so that all of the nitrogen produced by the fish is
used by the plants (or algae and corals in saltwater). An example of a balanced freshwater aquarium would be a 55 gallon tank, heavily planted with only about twelve small to medium fish. This does not satisfy most aquarium owners who usually want a lot more fish in their aquariums.
Sponge Filters: These are simple air or water pump operated filters that drive water through a porous cellulose sponge which is a great media for cultivating beneficial bacteria. These filters are typically used in smaller aquaria, or in breeding tanks where they do not pose a danger to tiny baby fish that might get sucked into larger filters.
Large Power Filters: Before the invention of the undergravel filter, people used large, bulky power filters to cultivate beneficial bacteria. These original power filters were gigantic, compared to the modern hang-on power filter. Some were as large as twenty four inches across, and held up to a gallon or two of water and filter media. They used glass filter wool as the media to cultivate bacteria, which
was usually layered with charcoal (We didn't have activated carbon back then.). Most of the larger power filters are now obsolete. The Supreme AquaMaster series of filters are one of the few existing mammoth power filters available today. Ironically, the Supreme PowerKing and SuperKing, no longer manufactured, were the original U.S. made, gargantuan power filters used in the 1950s, and we wish that Danner manufacturing would bring those bad boys back! (Hint, Hint Mr. Danner!) Please do
not confuse old-fashioned hang-on filters with the smaller modern power filters in use today. Modern power filters with their small cartridges, foam pads, and spinning or whirling gimmick devices, will never be adequate for proper biological filtration, in spite of the misleading and exaggerated claims made by some manufacturers.
Under-Gravel Filters: This was the first true biological filter invented for the aquarium. The gravel in an aquarium is suspended above one or more plastic plates, which are equipped with "lift tubes" that extend vertically from the bottom to the water surface. Air is pumped into the bottom of the lift tubes and rises up the tubes, carrying water with it. This causes water to flow down through the
gravel and up the lift tubes back into the aquarium. Instead of air, power heads may be used to drive the filter. Power heads are submersible pumps that are mounted at the top of the lift tubes and pump water directly up the tubes and out into the aquarium. Power heads are a much better means of operating an under gravel filter, increasing flow by several hundred percent.
Many folks mistakenly think that undergravel filters are obsolete. However, when it comes to freshwater aquaria, nothing could be further from the truth. We solve many people's water quality problems by advising them to install an undergravel system, when the only filter they have is a small power filter on their freshwater tanks. Undergravel filters are inexpensive, and though they require monthly vacuuming of the gravel, most aquaria need this
anyway. They do a great job in freshwater tanks. In saltwater aquaria, which has much lower levels of dissolved oxygen, undergravel filters should not be used, as the submerged aerobic bacteria in the gravel are competing directly with the fish for oxygen. It was this fact, that led to the invention of the wet-dry filter in the 1980s. The use of aerating power heads helps eliminate this problem to some extent.
Canister Filters: Although canister filters are designed primarily for mechanical and chemical filtration, many people are using them these days as biological filters. They will hold enough media to support a good population of beneficial bacteria. However, they have the same disadvantage as an undergravel filter for marine aquarium use, in that the bacteria are fully-submerged, and so are competing for
oxygen with fish in the aquarium. Many models come with a spray bar return, which if placed horizontally just above the water surface in the tank, will help to re-oxygenate the water.
Notes on Prefilters Used for Stand-Alone Wet-Dry & Berlin Filter Systems:
Water is fed into wet-dry filters and Berlin reservoirs from a separate "overflow prefilter," which may be built-into the aquarium (called a "reef-ready" aquarium) or purchased as a stand-alone unit which hangs on the back of the aquarium.
Built-in prefilters are the best type, as there are no siphon tubes involved which can sometimes collect air, stopping the siphon action, which will cause the aquarium to over-fill. This is called "breaking" the siphon. With a built-in design, water overflows into the prefilter from the surface of the aquarium, is pre-filtered through sponge elements, and feeds via gravity, directly into the filter below the aquarium.
Hang-on prefilters usually consist of two parts, the actual prefilter box that hangs on the back of the aquarium, and a "waterfall" collection cup, which goes inside the aquarium. The two parts are usually connected using nylon bolts and nuts. On most models, water overflows into the waterfall cup from the water surface in the aquarium. A siphon tube delivers water from the waterfall cup inside the tank, over the side of the
aquarium, and into the prefilter box. The water then passes through a sponge prefilter element, and feeds via gravity, into the filter below the aquarium. Some hang-on prefilters are of the "continuous-siphon" type. These prefilters actually have a built-in channel which allows the water to travel over the side of the tank without using a removable siphon tube. This prefilter design greatly reduces the chances of the siphon action breaking, either from collected air or when the filter
pump stops.
Wet-Dry Filters: Wet-dry filters are typically rectangular acrylic or glass reservoirs that sit below the aquarium. Most are divided into two basic sections, the "wet-dry" chamber and the "sump." Some wet-dry filter models may also have additional sections or chambers for in-sump protein skimmers, de-nitrification systems, canister filters or for pre-filtration of the water.
The wet-dry chamber houses the biological media on which bacteria cultivate. Biological media used in wet-dry filters can vary widely. The types of media used include plastic ball media, DLS (Double Layer Spiral) material, sintered (porous) glass, and many others. Although some media are better than others, all have one thing in common, and that is high surface area for bacteria cultivation.
The water entering the wet-dry filter usually feeds into an area above the wet-dry chamber, equipped with a trickle plate. The trickle plate is a piece of plastic or acrylic, with many uniform small holes drilled into it, through which the water passes. This is why wet-dry filters are sometimes referred to as "trickle" filters. The trickle plate forces the water to be delivered uniformly to the wet-dry chamber, so that it passes through
all areas of the biological filter media. Water collects under the wet-dry chamber, and then passes through a baffle which usually has some form of chamber for chemical or sponge media. The baffle also serves to eliminate air bubbles. After passing through the baffle, the water flows into the sump, where it is then delivered back to the aquarium by the filter pump.
Hang-On & Integrated Wet-Dry Filters: Some wet-dry filters are designed to hang on the back of the aquarium. These are useful only for smaller aquaria, or in situations where there is no space below the aquarium for a true wet-dry filter. In most designs, more than half the media is submerged, which is a major drawback. They also do not have the capacity of a normal wet-dry, and usually include built-in
protein skimmers that do not work very well. Some aquarium manufacturers build tanks with a built-in wet-dry filter inside the tank against the back panel of the aquarium. These integrated filtration systems (IFS) work identically to the hang-on wet-dry filters, but have the advantage of having no siphon tubes or waterfall cups, so the look is cleaner.
Berlin Systems: A simple definition of a Berlin system is a wet-dry filter without the wet-dry chamber. These systems are designed without an area for holding biological media, so they are more or less a large open sump or reservoir. Designs vary widely from manufacturer to manufacturer. Some include a built-in prefilter. Many also include built-in protein skimmers, canister filters, chemical media chambers,
sensor probe holders and other features. As the main purpose of a Berlin Reservoir is to supply a means of using an efficient protein skimmer, the other features are simply added bonuses. A Berlin reservoir may also be as simple as a small aquarium tank placed below the main aquarium.
Fluidized-Bed Filters: Fluidized-bed filters are a very unique biological filter. If properly designed and built tall enough, they have the ability to cultivate not only aerobic nitrifying bacteria, but also facultative anaerobic denitrifying bacteria. This means they may have the capability to remove not only ammonia and nitrite, but also nitrates. They usually consist of some type of column chamber which
houses several cups or more of coarse sand or similar media. Water enters at the bottom of the filter and exits at the top. There is usually a control valve for regulating water flow. A check valve is usually placed on the filter intake to prevent the sand from packing-down when the filter is turned off.
Because water flows upward through the filter, the sand in the filter becomes suspended or "fluidized" in the water column, forming a fluidized bed of sand. If the flow of water is controlled properly, the sand does not flow out of the filter, but remains suspended. This happens because the flow of water is just fast enough to keep the sand in suspension. The weight of the sand prevents it from escaping the filter. Because the sand is
suspended in water, fluidized-bed filters are self cleaning, and require little or no maintenance.
The water at the bottom of the filter is fresh and high in dissolved oxygen, so aerobic bacteria cultivate in the bottom half of the sand bed, and remove ammonia and nitrite, using up oxygen in the process. In taller fluidized-bed filters, enough aerobic bacteria cultivate in the bottom half so that as water flows past them, they remove most of the oxygen from the water, so facultative anaerobic bacteria cultivate in the top half of the sand bed
where they remove nitrates. Not all fluidized-bed filters are tall enough to promote anaerobic denitrifying bacteria, but most are very efficient at cultivating beneficial aerobic bacteria.
Some fluidized-bed filter designs are stand-alone units that are too tall to be placed under an aquarium in a cabinet stand. These are actually the best designs, but are not practical for most aquariums. Other fluidized-bed filters are designed to hang on the back of an aquarium or sit in a reservoir, and may be driven by a small pump or the return line of a canister filter. These types of fluidized-bed filters are an excellent way to provide
biological filtration on an aquarium equipped only with a canister filter.
Plenum Systems: Describing a plenum requires far more than just a single paragraph. The plenum method was discovered by Dr. Jean Jaubert and is often referred to as the Jaubert System. "Plenum" refers to an un-oxygenated layer of water trapped beneath a deep layer of gravel or sand at the bottom of an aquarium. This layer of water becomes anaerobic. Bacteria cultivate in this oxygen-free environment
and remove nitrates from the aquarium. There are several variations of the plenum, depending on what literature you read.
The basic structure of the plenum system is essentially a deep (3" to 4") layer of gravel, suspended 1/2" to 1" above the bottom of an aquarium. It is usually recommended that very little or no water flow should be directed across the gravel surface. A basic plenum may be created by suspending egg crate material or under gravel filter plates above the aquarium bottom, using inert supports such as PVC pipe. If egg crate is
used, it is covered with nylon screening over which the deep gravel bed is placed. The actual materials, including what grade of gravel or sand is used, vary depending on what literature you read, but usually coarse live sand, fine coral gravel, or a combination of both are used. One technique is to use 2" of live sand on the bottom, over which nylon screening is placed. The screen is then covered by 2" of fine coral gravel. This allows the aquarist to turn over the top inch of gravel
occasionally to clean it, without disturbing the anaerobic layer below.
Anaerobic bacteria form in the bottom depths of the substrate and in the water below. As anaerobic bacteria cultivate, they remove nitrates. Anaerobic action produces a fair amount of heat. The heat warms the water layer below the gravel. The warmer water flows upwards, displacing cooler water above the gravel. This action moves water through the plenum at very slow rates. The slow movement of water through the gravel helps to prevent dangerous
hydrogen-sulfide gases from forming in the plenum. The deep gravel bed also provides a home for burrowing motile invertebrates which feed on solid organic mulm and detritus. These burrowing animals, which are either purchased and/or cultivate on their own from live rock, serve to keep the plenum porous and aid in the slow movement of water through the system. "Burrowing" sea cucumbers that serve this function well may be purchased from a good aquarium store.
The aquarium is stocked with live rock, but not as much as is typically used. The live rock construct should be suspended above the gravel bed, allowing water to move freely beneath the reef. This may be accomplished in a number of ways. One way is to place a couple of "anchor" pieces on the gravel bed, and glue others to the back of the tank using aquarium epoxy putty (not silicone!). The rest of the live rock may then be bridged
across these supports to build the reef, but not too high. The construct should be loose, not tightly packed together, to allow good flow through the rock. Care should be taken not to direct flow from pump returns across the gravel. The emphasis in aquariums using a plenum should be on the reef, with a minimal number of fish to balance the ecology.
The Jaubert Plenum System, if constructed properly, can work well, provided the aquarium is only lightly-stocked with fish. While we are sure some hobbyists would disagree with us here, we feel that more research is necessary on this fascinating subject. The original plenum method used aeration as the only means of water movement in the tank. We highly recommend that a plenum be used in conjunction with a Berlin system. The protein skimmer will
remove other dissolved and solid organics, not removed by the plenum. The use of a protein skimmer combined with the plenum method may be the key to keeping more fish while using this type of biological filtration. If you're setting up a new aquarium, installing a plenum will be relatively easy. If you have an existing reef tank, installing a plenum will involve a fair to major amount of work. Definitely read up on the subject before you go to work.
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