but there are some disadvantages. The main disadvantage of an UGF is the trapping of detritus in the gravel bed or below the filter plate, which leads to a declining water quality and reduced oxygen level in 1 le tank. Some UGFs are designed to prevent this accumulation though, and passing water through a gravel bed can allow the formation of beneficial denitrifying zones (see denitrification, chapters 2, 7, and 8). We wish to emphasize, however, that no further biological filtration devices are needed or desired in addition to what is already provided by the live rock or other substrates in a reef aquarium (see maintenance of gravel/sand bed, chapter 7).
Wilkens and Birkholz (1986) described the disadvantages of wet/dry biological filtration for reef aquariums. Our experience has shown that trickle filters are not only unnecessary for coral reef aquariums with adequate amounts of live rock, in fact, they can be detrimental in hard coral aquariums, as Wilkens and Birkholz indicated. However, we have included a discussion of trickle filters for the sake of completeness, since they are popular now in aquarium keeping in North America, and they do work as biological filters.
The concept of the trickle filter is not really new to the aquarium hobby. Early aquarium designs were outlined in the popular aquarium literature in the late 1960s and 1970s (deGraaf, 1968; Siddall, 1977). However, it was not until articles by Dutch hobbyist George Smit were published in Freshwater and Marine Aquarium magazine in January 1986, that they became widely used in North America. Similar filters had already proved their efficiency in Europe 10 years earlier. They had also been widely used in aquaculture and public aquariums. The National Aquarium in Baltimore uses trickle filters on the majority of its exhibits, including the 1 000 000 L (250,000 gal ) Atlantic reef display.
The main advantage of a trickle filter is its ability to fully oxygenate water in the biological filter chamber, which makes nitrification proceed very rapidly. A trickle filter consists of a chamber through which water is allowed to fall over some type of medium. The medium causes the water to spread out and cascade slowly downwards. It finally collects in another chamber (the sump) below the trickle filter. From there it is pumped back to the aquarium. The medium of the trickle filter is not submerged in
Typical installation of a "wet/dry" trickle filter. The mechanical pre-filter prevents clogging of media in the "dry" portion of the filter. A perforated standpipe vents air as water drains down. In the wet portion or sump additional mechanical, biological, and/or chemical filter media may be incorporated.
Optional Mechanical Filter or
Activated Carbon r X T T t
Figure 5.2b Trickle filter
This modification of the trickle filter has the media rotating in the sump. A portion of the media is thoroughly wetted as it plunges into the water, and it drains on the upward cycle.
Sump m as
water. The water is merely allowed to fall through it. The term dry
filter has also been used to describe this chamber, mainly to highlight the fact that the media is not submerged. As the water flows through the trickle filter, a thin film of water covers the media, resulting in a very thin barrier across which oxygen can easily diffuse. This allows a more thorough gas exchange for the oxygen hungry nitrifying bacteria living on the surfaces of the media. The result of this gas exchange is a very large population of aerobic nitrifying bacteria with no possibility of the formation of anaerobic areas. As long as the prefilter is performing its function, a properly designed trickle filter should never have to be cleaned.
Water is delivered to the trickle filter either by a rotating spray bar or through a plate in which numerous small holes have been drilled, commonly known as a drip plate. There seems to be some debate about the merits of each, but both systems work fine biologically. Spraybars spread the water more thoroughly over the media, but are prone to stopping, and are more maintenance intensive. Drip plates are more trouble-free in the long run. Another kind of wet/dry filter uses a rotating bio contactor (RBC) mounted over the sump. In this system, bacteria grow on a wheel of biological filter media (also called a "bio nitro filter wheel "). The stream of water fed over the contactor makes it rotate like a mill or paddlewheel. As the wheel turns, the media is immersed in the sump and lifted out, allowing the water, mixed with air, to drain back through the media.
The media used in trickle filters has undergone an amazing explosion of variety. When they first appeared on the Nonh American market in 1986, shallow trays with gravel were used. Then commercial units appeared that utilized a double layered spiral coil of floss and plastic wire matting material (DLS). Although DLS is still available (and has been used successfully by many aquarists), there are also various balls, rings, blocks and other media previously used only in the water treatment industry. It seems that almost every month a new media appears along with various claims about its efficiency and superiority over existing media. There are really only two main factors that are important: usable surface area and void space (Moe, 1989). The amount of surface area available to bacteria can sometimes be much less than that claimed by advertisements. Obviously the greater the available surface area, the more bacteria that can be grown, and the greater the nitri l ying ability of the filter. Manufacturers decrease the size of the media or increase the number of protrusions such as ridges, bars, rods and bumps, to provide greater surface area for growing bacteria.
This leads us to void space, which is basically the amount of empty-space available in the dry chamber of a trickle filter after the media has been added. Void space is important for gas exchange, and to prevent clogging of the media by detritus or bacterial slime. If the media is too densely packed, then there will be a low void space and a large surface area. This impedes gas exchange, and clogging of the media can also become a problem. If the media is too loosely packed you will have a large void space and a low surface area. Although this may be ideal for gas exchange, it does not offer the most surface for nitrifying bacteria. The best media offers the greatest amount of surface area without significantly decreasing
In reef tanks using sufficient live rock, the issue of whose media has the greatest surface area is unimportant. These aquariums run quite successfully without a trickle filter. They rely instead on the bacterial population in the rock and other substrates, and the associated algal growth and photosynthetic organisms that consume nitrogenous wastes. With regard to gas exchange, the water skimming over the overflow picks up oxygen and gives off carbon dioxide as it tumbles down to the sump below. A protein skimmer further assists with gas exchange, and removes o o compounds that contribute to biological and chemical oxygen demand (BOD and COD).
Trickle filters are among the most efficient means of biological nitrification available to aquarists, but one does not need a trickle filter, or external biological filter of any kind, in order to maintain a reef aquarium. In fact, the trend of relying on protein skimming combined with natural biological processes within the aquarium for the maintenance of water quality has finally achieved wide acceptance, but there will always be a spectrum of opinion about what pieces of equipment are really useful, and we wish to present all options.
The topic of chemical filtration is described in just about every textbook written on marine aquarium keeping, yet few aquarists really seem to understand it's capabilities, limitations and applications. There are numerous forms of filtration that fall under the category of chemical filtration, depending on their mode of operation. For our purposes we will limit our discussion to the common forms of chemical filtration used in reef systems, namely activated carbon, protein skimming, molecular adsorption media and ozone.
Due to the various biological processes that occur in an aquarium, a build-up of organic substances takes place. They are referred to
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