Nature has, for thousands of years, demonstrated to perfection the perpetual cycle of production, consumption and destruction. Things come and go, they can be broken down into their basic building blocks and become the foundation for new life again. Although this is rarely achieved so completely and to such perfection in technical systems, for as long as we have known the basic activities and connections that these processes entail, we have made increasing use of them, for example to treat polluted water or to improve water quality. A very simple but effective biofilter can be seen, for example, in wetlands that clean waste water relatively inexpensively in a natural way. During this process a large number of chemical and biochemical reactions, as well as physical processes, particularly sorption, take place. The first “real” biofilter for treating water is said to have been built in England in 1893. It was based on the trickling principle.
With the advent of engineered production systems in aquaculture, particularly in recirculating aquaculture systems (RAS), a completely new field of biofilter applications came into being. In these systems the water in which the fishes grow circulates perpetually. It is cleaned regularly and reprocessed so that it meets the fishes’ requirements. Essentially, the systems consist of three components: the fish tank, a mechanical cleaning unit, and the biofilter. As with a three-legged stool, it is impossible to say which leg is most important, and all three elements of a recirculating aquaculture system are equally important and indispensable. Nevertheless, it would probably not be wrong to view the biofilter as the “heart” of the system. The biofilter’s performance is the limiting factor for the fish biomass, it is the most sensitive link in the chain, and the most likely to break down. Biofilters form the habitat for innumerable bacteria and microorganisms that are mainly responsible for biological water treatment. Two biological processes can be distinguished here. On the one hand, the activated sludge process in which the microorganisms are suspended in the water body. And on the other hand, the biofilm process in which the microorganisms are settled on the surface of a substrate (filter bed, fixed bed). All the biofilters that are used in recirculating aquaculture systems are of the biofilm type.
Fish farming and recirculating systems offer optimal conditions for a biofilter. The impurities that have to be removed from the water are always of the same quality and, apart from during feeding or at harvesting times, when the tanks are empty, they occur in more or less consistent quantities. They mainly consist of metabolic products from the fishes that enter the water partly in dissolved form (via the gills) and partly in solid form in the faeces. The fraction of solid particulate substances also includes a small amount of feed residues that have either been left uneaten by the fish or are suspended as fine abrasion products in the water. So as not to overload the biofilter these solid particles should be removed from the fish tank and the water as quickly as possible before they decompose, dissolve or are deposited as sludge in still water areas of the system, use oxygen unnecessarily or form anaerobic zones. Because solid impurities are usually removed using mechanical methods this means that the bigger the particles are the easier, the more effective and less expensive it is to remove them.
Remove particulate impurities as early as possible
Mechanical water treatment methods are based on two different physical principles. The first is sedimentation in which it is gravity that brings about the desired effect. In sedimentation tanks (e.g. lamellar separators) the water current is slowed down so that the particles can settle on the bottom and be removed. However, a disadvantage here is that the impurities remain in the water for a certain time, nutrients are washed out, and germs can accumulate. Vortex separators and centrifuges are also based on the principle of gravity. In these devices the water rotates, which causes the solid, relatively heavy particles to drift either outwards (centrifugal force) or inwards (centripetal force) depending on the design of the device. They can then be removed before they decompose and dissolve.
The second method of mechanical water treatment is the removal of particulate impurities using fine mesh screens. Such screen filters are technically complicated, they require a lot of energy, are relatively expensive and demand a certain degree of maintenance, but they are preferred in practice because they do not take up much room and apart from that they are very effective and reliable. With tight-meshed screens with meshes measuring between 20 and 100 microns a considerable share of the particulate impurities can be removed. As a rule, the water flows through the rotating drums and disc filters from the inside to the outside. Sensors detect when the sieve meshes become clotted and then trigger a high-pressure rinsing programme to clean the filter fabric. The impurities are caught in a special collection pan from which they flow out of the system independently.
Protein skimmers and flotation plants are also suitable for separating particulate impurities. Even very small particles will attach themselves to the produced air bubbles and then be carried to the surface with the foam.
When the coarse particulate impurities have been removed, the water still contains very fine solid impurities and dissolved substances that cannot be removed using mechanical methods, or only with difficulty. And this is where biofilters come in: the metabolic activity of microorganisms transforms the impurities into harmless and mainly low-molecular substances. In the wild, the community found living in a biofilm is usually very rich in species. The slimy, musty-smelling layer contains both microorganisms such as bacteria, fungi and yeasts, and slightly higher organisms such as protozoa, insect larvae and worms. In contrast, in aquaculture, particularly in recirculating systems, the biofilm is less rich in species because the range of tasks the microorganisms have to fulfil is much more limited and geared to just a few substances.
Fishes mainly excrete ammonia (NH3) through the gills. Ammonia is highly toxic even in low concentrations. Small amounts of ammonia also result from the breaking down of protein-rich ingredients contained in feed remains and fish faeces. Although once in the water ammonia is immediately converted into the safer ammonium (NH4+) this reaction depends on the pH value and the temperature of the water. The warmer the water and the higher and more alkaline the pH value, the more strongly will the scales be tipped from ammonium to ammonia. To keep the risk of poisoning the fish population low, these substances thus have to be oxidized as quickly as possible to the less toxic compound nitrite (NO2-), and then to nitrate (NO3-). This process, usually called “nitrification” for short, is handled in the biofilter by the “nitrifying” bacteria species nitrosomonas and nitrobacter. Because nitrification is aerobic (requires oxygen) the efficiency of a biofilter largely depends on its oxygen supply. In general, it is believed that the oxygen concentration should not fall below 2 mg/l water. During the course of the nitrification process, hydrogen ions (H+) are also released. These act like an acid and gradually reduce the pH value. To counteract this effect the water in the system has to have an appropriate buffer, usually lime solution. Optimal conditions for nitrification are a good oxygen supply, pH values between 7 and 8, temperatures around 25°C, and not too much light, because microbial processes prefer the dark. In addition, ammonium should of course be constantly available so that the nitrosomonas and nitrobacter have sufficient “food” to give them energy.
Nitrification processes in the biofilm require a lot of oxygen
With that, the main requirements that have to be met when constructing a biofilter have been named. Because the effectiveness of nitrification also depends on the number of microorganisms, the settlement area should be of a reasonable size. Basically, many materials are suitable for this but as a rule the choice falls on specially designed blocks or variously shaped plastic media (for example looking rather like hair curlers) which combine low volume with a large interior surface. The interior cavities should not be too small or narrow, however, because the biofilm grows constantly, forming new biomass that can block the openings. If the biofilm is too thick the lower layers will no longer be supplied with sufficient oxygen. To remove the excess sludge some types of biofilters have to be “backwashed” regularly against the usual direction of water flow. In general, a biofilm should if possible not be much thicker than 0.1 mm. The problem of oxygen supply can be solved in different ways. Probably the oldest method is to use trickling filters in which the polluted water is trickled over the biofilm substrate so that every single drop comes into direct contact with atmospheric oxygen. Another possibility is that the biofilm is located on slowly rotating discs that protrude half out of the water so that with each turn the bacteria are alternately in the air and in the water. Supplying oxygen to biofilms that are constantly submerged is somewhat more difficult because here the required oxygen has to be added directly to the water. In the case of fixed solid settlement substrates, for example bioblocks in a fixed bed filter, this can be achieved using aeration from below via a diffuser. In the case of loose substrates in moving beds the relatively small media which have a low specific weight are constantly in motion through the agitation and circulation of the water, making them rise sporadically to the surface where they will then have contact with the atmosphere.
Each of these techniques has its advantages and disadvantages. Keeping the media in motion in moving bed filters requires strong pumps and consumes energy, but the biofilm is permanently abraded, kept thin and efficient, because the media are rotated like in a washing machine drum and rub against one another. Trickling filters have the advantage of an intensive gas exchange. Microorganisms are well supplied with oxygen and can pass harmful gases such as carbon dioxide or nitrogen that are generated in the course of their metabolism to the outside. If the biofilm grows too thick the excess biomass can be sheared off in small pieces and carried away with the water flow. Trickling filters are relatively large, however, and so take up a lot of space so that they cannot be installed everywhere.
To keep the investment and operating cost of biofilters low, manufacturers and operators of aquaculture systems are constantly looking for new substrates that offer a large surface area for the settlement of the biofilm, are easy to clean or replace when necessary, and are not an overly expensive investment. In practice, one thus finds in trickle filters and submerged fixed and moving bed filters various media that can generally be divided into organic and inorganic substances depending on their origin. Organic media such as bundles of sticks, wood chips, coconut shells or peat blocks are restricted in their use for biofilters because they release tanning agents, dyes and other substances and can influence the chemical properties of the water, but they are still used regionally because they are very cheap and easy to obtain. However, they are seldom used in recirculating systems but sooner for the treatment of polluted water that is to be fed back into natural waters.
Balance between volume, surface area and oxygen supply
Some inorganic materials can also release undesirable substances, even if only to a limited extent. The materials used include coarse gravel, stones or sand, broken glass and sea shells, particularly however materials made of various synthetic materials that have a low specific weight and often even float freely in the water. This can save a lot of energy in moving bed filters. Plastic media are mostly relatively small and have complex interior and exterior surface structures that offer large settlement areas for microorganisms. A rough measure for this settlement area is the specific surface area (SSA), also called packing density. The SSA of modern plastic media is mostly between 100 and 1,000 square metres per cubic metre. If the specific surface area exceeds this upper value significantly there is a great risk that the biofilters will become clogged quickly and thus reduce water flow and nitrification efficiency… particularly since there is always a difference between the theoretically available SSA and the actual effective biofilm surface. This is because nitrification really only takes place in the upper layers of the biofilm where the microorganisms have direct access to oxygen, ammonium or nitrite. Depending on the construction and functioning of a biofilter, nitrification lies between about 0.1 and 1.8 g ammonium nitrogen per square metre of effective filter surface and day. When planning the dimensions of a biofilter a safety factor should thus be taken into account. In general the SSA is set at double the amount that would be needed for the nitrification of the theoretically expected ammonium released from the fishes after feeding.
Although conditions in the deep layers of the biofilm into which no oxygen penetrates are not anaerobic (anaerobic is a condition in which neither free nor bound oxygen is available) they are anoxic (oxygen is available in the bound form only). For three oxygen atoms are bound in the nitrate that is formed in the upper layer of the biofilm through nitrification. Specialized bacteria (anaerobic heterotrophs) are capable of removing the oxygen and using it for their own metabolism. This process in which nitrate is reduced and converted to elementary nitrogen (N2) that is released from the water and can thus leave the system is called denitrification. Denitrification can take place both in zones that are not well supplied with oxygen in a “conventional” biofilter (denitrifying bacterial strains, so-called “denitrifiers” are mostly present in the species-rich microflora of a biofilter) and in denitrification chambers specially equipped for this, called deni filters for short. At least three prerequisites are necessary for a functioning denitrification:
• Denitrifiers that settle in a sufficiently large number under constant environmental conditions (no free oxygen),
• Anoxic conditions (oxygen not in free but in molecularly bound form, e.g. in the nitrate)
• Adequate supply of biologically degradable organic substrates (e.g. methanol or glucose) that serve the denitrifiers as a “food” and energy provider and often have to be supplied from the outside.
The effort and cost of a functioning denitrification are relatively high which is why many recirculating systems do without it. Especially since this effect, the reduction of the nitrogen load, can also be achieved by regularly changing part of the water within the system.
Or one goes further back to basics and tries to imitate conditions in the wild and close the material and energy cycles. The spectrum of possibilities for this ranges from the use of nitrate-rich process water from fish farming systems for the downstream production of crops to aquaponic techniques that directly link the fertilising effect of the runoff water with nutrient elimination, or in other words, with water treatment.