Filtration

Filtration is the soul of any aquarium installation

Over the past 10 to 15 years there has been an enormous amount of investigations about development and application of different types of biological filters.  These investigations were not made for hobby aquarists, but for water treatments, aquaculture and – on a lesser amount – for public aquariums, often this study was focus on finding replacements for traditional plate, sand or small balls filters, that are use on recirculating systems.

A few results obtained from the investigation on water treatments or aquaculture were adapted to tropical fish hobbies and public aquariums. Unfortunately, a lot of hobbyist still trust on traditional plate filters, with their inherent problems, to do the job of controlling nitrogen biological contaminants.

Revolving biological filters, fluid beds, dripping filters, small balls filters, have been successfully adapted to aquaculture and tropical fish cares, and are use on fish hatcheries, on some central filtration systems, on mayor fish sales installations and domestic aquariums. In these aquariums, fish density and ammonia levels are usually higher, per volume unit, than public aquariums.
Numerous studies have proven the superiority of bio-filters over plate or sand filters. This article will briefly describe these diverse filters and explain the reasons to why they are more efficient.  We will also suggest different ways to adapt these filters to public aquariums.  Aquarists will find this very interesting.

Nitrification is defined as the oxidations of nitrogen from a reduced form (ammonia) to a more oxidize form (nitrate), by a microbial process (Killman, 1986). Within the intermediate products form and consumed during the process, there are hydroxylamine and nitrites.
Ammonia, the main excremental product of fish, is toxic for them even at very low concentrations such as 0.5 mg/l (milligrams per liter).
In exhibition aquatic systems, nitrification is use to eliminate ammonia from the water. This is done making water pass through or over filtrating materials, where eventually, the necessary bacteria for nitrification will grow.

The elimination of ammonia and nitrites from the aquarium water is called biological filtration, and the materials where a bacterium grows are called biological filters or bio-filters. The organisms that make the biological conversion of the nitrogen contaminating products are called nitrifying bacteria.

The mechanical and chemical are other types of aquatic filtration, necessary for a successful aquatic environment.

The mechanic filtration is the retention and consequent removal of particle materials, mainly of organic origin. This is a very important process use to maintain the clarity of the water and reduce the biodegradable organic matter (BOM) on the system.

The chemical filtration refers to a wide set that includes the use of activated carbon grains, the separation of proteins or ozonization to remove dissolved organic carbons of the water.
When a biological filter is design and/or selected, there are two characteristic of the nitrifying bacteria that needs to be consider. First, these bacteria are anaerobic. Even tough they can live in low oxygen environments; their efficiency grows as the dissolved oxygen increases towards the saturation of air (21% oxygen).

Saturated water contains from 8 to 9 parts per million (ppm) of oxygen at the usual temperatures use to maintain aquatic organisms, with a 15 ppm of oxygen all of the water can be maintained at low temperatures without getting to be saturated. On the other hand, air contains 210 ppm of oxygen. Even tough nitrifying bacteria can obtain the oxygen they require from any other source, the ones that are on a submerge bio-filter must acquire their oxygen from the one dissolve on water, while the nitrifying bacteria on a filtrating matter expose to the atmosphere can obtain their oxygen directly from air.

Second, the nitrifying bacteria grows very slowly, with reproductions times of about 24 hours or more, comparing to the heterotrophy bacteria , with reproduction  times of about 20 minutes. These characteristics mean that an efficient bio-filter will maximize the available amount of oxygen for bacteria.

The accumulation of BOM in the filter asphyxia nitrifying bacteria, and provides a surrounding that encourages the development of heterotrophy bacteria (Manem and Rittman, 1992). Then, the heterotrophy bacteria oxidize the organic matter and produce ammonia and consume oxygen. Then, the BOM must be eliminated from the system as soon as possible. BOM accumulation will also increase the biochemical demand of oxygen, which has proven to be a limitating factor of nitrification (Wu et al., 1980; Rittman and Dovantzis, 1983).

TYPES OF BIOLOGICAL FILTERS

There are a lot of alternative biological filters that can be use replacing sand filters (which includes the plate filters). These can be classified into two groups, divided whether if the filtration matter is or is not submerge. The group of submerge filters includes the reactive beds and the small balls filters. In the second group (the not submerge ones) there are the dripping filters and the revolving biological contacts. Although these filters have a lot of characteristics that contributes to their utility as biological filters, they all differ from the sand filters on the fact that they do not retain organic matter or do not require as much maintenance.

SAND FILTERS

The traditional filtrating matter use in biological filters has been sand. The sand is deposit on a porous grill that is cover with a sift, or that has holes small enough to let the sand pass. Water is pushed through the sand with a water pump or an air elevator and return back into the pond.
In aquariums as a hobby this kind of filter is called plate filter. The “sand” use is fine gravel. On public aquariums the sand is kept inside big concrete boxes, with water passing through one or more of these boxes before being return to the exhibition ponds.  A lot of public aquariums also use fast sand filters. Using water pumps of high resistance, the water is pumped through the sand at very high rates of flow. The obstruction of the filtration matter is inherent to the design and operation of sand and plate filters.  Big amounts of BOM material is accumulated, promoting the growth of heterotrophy bacteria. This damage the nitrifying bacteria, which grows slowly and can not compete against the heterotrophy ones. Even more, it has been proven that fluctuations on the amount of BOM that enters a biological filter impacts in a negative way over nitrification (Manem and Rittman, 1992). This scenario happens naturally on sand filters. BOM is accumulated until the matter must be washed to be reduced, but this way, nitrifying bacteria is loss. The sand filters are then, an environment inherently unstable for nitrification.

Another additional problem of the fast sand filters is their tendency to form channels as the BOM is accumulated. This gives place to a loss of oxygen in most of the materials, a consequent loss of nitrification and an increase of the costs of maintenance due to the work and necessary materials to make all of the cleaning operations and replacing the discharged water.

DRIPPING FILTERS

A dripping filter consists on a filtrating matter (usually made in plastic with some sort of a special design) with a big empty space (a measure of the opening of the filtrating matter) through which water pass. The matter it is not submerging under water and this is mechanically filtered before reaching the dripping filter. Dripping materials comes in a lot of different shapes and sizes, and it is very important to maintain and adapt material’s opening, in order to make the air flow that pass through it, easier. The advantages of a dripping filter are: that the material is not under water, so the air can circulate freely through it. Besides, the material is not easily obstructed and no after cleanings are required. Also, the material is very light and can be storage on simple and cheap plastic enclosures. Another advantage is that the bio-filter is transportable. To start a new bio-filter you can move some of the material parts to a new pond on a new container, and the normal period of 4 to 6 weeks during the one the nitrifying bacteria are stabilized, is shorten. A usual inconvenient of this filter is that the user has very little control over the road water takes once it enters the filter. It can happen during the operation, that not all of the material is wet, causing an unequal growth of the bacteria layer. This produces an efficiency reduction of nitrification in terms of quality of removal of ammonia per volume unit.

SMALL BALLS FILTERS

These types of filters are similar to the vertical sand ones, with the difference that they have a motor on the top. The material is thousands of pounds of polyethylene small balls that float. Investigations show that a cubic meter of these small balls are enough for about 24 to 32 kilograms of fish. The filter works with water that enters from the bottom, flows through the small balls and returns to the pond. Small balls act like a mechanical and biological filter similar to how the fast sand filter works, with very little differences. The small balls turn making whirls all the time, in a way that aerobic conditions are maintained above the whole surface and there is very little water canalization. So there are no dead points and the whole surface of the small balls is biologically active. With time, agglutination of bacteria can be formed, making the small balls stick to each other. This promotes the mechanical filtration of water. The dual nature of the filter, gives it the ability to adapt to the individual system requirements. An increase on the frequency to which we clean the filter, will increase the nitrification rates, though it will diminish the mechanical filtration capacity.  In terms of nitrification rates per volume, the small balls filter seems to overcome the reactive fluid beds, but it is inferior to the revolving biological contacts.

REACTIVE FLUID BEDS

A reactive fluid bed incorporates a big surface of sand or any other small material (such as glass small balls) with high rates of flow to maintain the material continuously in suspension in the water of the filter enclosure, and that way, eliminate material obstructions. The cultivation water enters from the bottom of the filter and flows through the material keeping it in suspension. The dirt is eliminated by self-cleaning of the material due to the fact that individual particles clean each other while they are shaken by the filter. This produces a very thin and alive layer, and so these filters need more surface than the other kind of bio-filters that generate a thicker layer. The first thing to keep in mind when designing a reactive fluid bed is to adapt the medium size of the particles to water flow rates. The grain’s size determines the surface for nitrification, while the amount of oxygen available for nitrification is a function of the water flow rates. The election of the size of the grain depends on the concentration of ammonia on the water that arrives, where the bigger grains are use in high concentrations of ammonia conditions. A fluid bed well designed virtually cleans itself up and should require very little maintenance. A disadvantage of these kind of filters is that the material is submerges, so that bacteria must consume their oxygen from water, not from air. So, the water flow rates must be high to ensure a high and constant level of oxygen so that the efficiency of bacteria is not compromised by oxygen. Fluid’s filter requires an adequate mechanic prefiltration to avoid material from getting obstructed or grouped.

REVOLVING BIOLOGICAL CONTACTS

These are the most efficient filters, biological speaking. They compress a big surface area into a small volume, which consists on multiple circular plastic discs supported on a central axis. The unit turns on the container with a 45% of air and the rest of the disc constantly submerge on water. This action alternatively exposes nitrifying bacteria that grows on the discs, to the water with ammonia and then to the air saturated with oxygen. This way, bacteria do not have limited oxygen. These filters do not have a mechanic filtration capacity, therefore, they are self-cleaning, and due that the rotation of the discs limits the thickness of the bacteria layer. As we can see, they have a great advantage having a minimum maintenance, and lamentably need an additional mechanic filtration.

CONCLUSIONS

Before choosing the adequate type of filtration for your aquarium, you must consider the relation between the nitrification efficiency and removal of solids. A more efficient system of nitrification will incorporate mechanic and biological filters separately, giving an easier maintenance for both and improving the nitrification without interrupting it for its maintenance. The adding of devises that destroy organic matter, as ozonizers, will be benefic because it reduces the need to use sand filters as clarifying and allows filters to work only for the nitrification.

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