J.D. van der Toorn (1987)
A biological approach to water purification: I. Theoretical aspects
From: Aquatic Mammals 13(3): 83-92
Table of contents
The biology of water treatment
Organisms in a trickling filter
Organic matter and its conversion
Nitrification and denitrification
In many dolphinaria aluminium compounds are used as flocculants (Andersen (1973), de Block (1983), Manton (1986)). The aluminium ion, when released into the water, will form aluminium hydroxide, Al(OH)3, flocs, which will partly block the pores of a sand filter and thus enhance filtering action.
A flocculant should in principle also be able to destabilize colloid solutions. If aluminium compounds are able to do that in sea water is doubtful. A colloid in water containing a low concentration on some ions, will be surrounded by charged particles, that create an electrical double layer around it. The electrical potential of that layer is called the zeta-potential. The higher this zeta potential is, the stronger the repulsion between the particles and thus the smaller the chance that 2 particles will collide and then coagulate. If to such a solution aluminium ions are added, the zeta-potential of the colloid will be strongly reduced, the colloid solution becomes unstable and flocculation will occur. This process is called salt flocculation.
In sea water however the zeta-potential of colloids is already very low due to the presence of high concentrations of ions, including many bivalent ions (the higher the charge of an ion, the stronger its effect on the zeta-potential), such as calcium and magnesium (Liss et al, 1975). In sea water colloids are usually stabilised sterically by organic matter. In this case adding aluminium ions will have no effect. In sea water salt flocculation does not, or at least hardly, occur (Eisma, 1986).
In sea water flocculation occurs when particles are brought together mechanically, either by flow conditions causing particle collisions (Eisma (1986), Kranck and Milligan (1980)) or by organisms that collect particles in faeces or pseudo-faeces (Eisma,1986). Sticky organic matter makes that particles, once brought together, will stay together. Especially the mucopolysaccharides, produced by bacteria and algae are responsible for this glueing process.
There is also another way to create flocs and that involves air bubbles. Because the processes involved are very similar to those in foam fractionation, this kind of flocculation will be dealt with in the next section.
Certain organic molecules tend to collect at the air-water interface. These molecules usually have a hydrophilic and a hydrophobic part. At the interface these molecules orientate themselves in such a fashion that the hydrophilic part is in water, while the hydrophobic part is in air. This situation is the most favourable position energetically for those molecules. Therefore they tend to collect at water surfaces and are often called surfactants. When air bubbles are led through water containing surfactants, these materials will also collect of the surface of the bubbles (because the bubble surface is in fact another air-water interface). Thus a film of organic material is formed around the bubble. If it then reaches the surface, the film will stay intact for a while. If many air bubbles reach the surface, many films will stay there and a foam will be built up. (A foam can only be formed in the presence of surfactants; a pure fluid cannot foam (Aubert et al, 1986)).
When an air bubble rises through water it will not rise as a motionless sphere, but the surface of the bubble will be in constant motion. A rising bubble can in that way collect material at its surface, also materials that are not specifically surface active. The motion of the surface will compress the film around it and a matrix of strongly adsorbed organic matter is formed (Johnson et al, 1986). This explains why a lot of different materials can be trapped in foam.
In nature this process causes for instance the formation of foam on beaches. The surf will cause a lot of air to be forced through the water. If the water is rich in organic matter, a combination of this matter with, among others, calcium will produce surface active substances (Degens and Ittekkot, 1986). These are responsible for the foam formation.
This process has already been widely applicated in waste water treatment (see for instance Fair et al (1968) or Conway and Ross (1980)) and in aquarium water treatment (see Spotte (1979b) or Kinne (1976)) in the form of foam fractionation, sometimes also called protein skimming. In a foam fractionators air is thoroughly mixed with water and the foam that is formed is removed in one way or another and with it the materials adsorbed onto the foam film. How well a certain fractionator works depends on a combination of the following factors: organic matter concentration of the water, air bubble size, amount of air with respect to the amount of water and the efficiency of foam removal. If the foam is not removed properly the foam will collapse and the skins will appear as flocs in the water (Johnson et al, 1986). These skins can act as substrates for further aggregation of suspended matter. This explains the increase in filterable material that is sometimes observed in systems that use foam fractionation (Wickins and Helm, 1981).
The materials that can be removed by foam fractionation include of course many organic substances such as proteins, fatty acids, polysaccharides and phospholipids (Kinne, 1976; Conway and Ross, 1980). Also larger biological material can be removed that way: flocs, bacteria and algae (Conway and Ross, 1980). Although the paper does not deal with foam fractionation as such, but with the behaviour of air bubbles in water, it can be derived from Johnson et al (1986) that also viruses can be removed with foam, since they can be trapped in the film, surrounding air bubbles. Also inorganic material can be removed by foam fractionation, if it forms some kind of bond with organic matter. This can happen in 2 different ways. Calcium carbonate (Degens and Ittekkot, 1986) and calcium phosphate complexes (Tri, 1975) can collect organic material around them, while also other materials can do so. So-called micro-flocs are often a combination of organic and mineral matter (Eisma, 1986). The flocs thus formed can be removed as already mentioned above. The other way inorganic matter can be removed in by the formation of ligands of organic molecules with metal ions. Certain surfactants, especially glycoproteins, have a high affinity for trace metals (Liss et al, 1975) so that several metal ion species can be removed by foam fractionation. (See Eichhorn (1975) for an overview of the organic molecules that can act as ligands for trace metals)
All this indicates that foam fractionation can have a marked effect on both dissolved and particulate organic matter concentrations. The fact that flocs and colloids can be removed indicates that foam fractionation can help reduce turbidity as well. This is confirmed by Spotte (1979b), who stated that foam fractionation helps reduce DOC (dissolved organic carbon) and POC (particulate organic carbon) levels and that also turbidity can be lowered.
In a design for a facility for the keeping of dolphins, as many aspects of their biology as possible should be taken into account. Already in the design of the pools this should be the case. Bottlenosed dolphins usually live in coastal waters, which vary in depth from less than 2 meters to over 10 meters (Wells et al, 1980). It could therefore very well be that having access to areas of different depths is better for them than living in a pool of uniform depth. Some dolphins actually seek shallow areas (less than 1.5 meters) in their pool system to rest or sleep (J. D. van der Toorn, unpublished data). The amount of variation in pool design is however limited by hydraulic demands.
The composition of the water should be as close to natural seawater as possible. Chlorination of seawater is very difficult however (Dudok van Heel (1983), Manton (1986)). Also persistent toxic substances are formed when seawater is chlorinated (Duursma and Parsi, 1976). Therefore large amounts of water have to be replaced regularly. This may make the costs of maintaining an artificial seawater in a chlorinated system prohibitive. It is therefore necessary to look into alternative ways of water purification. As indicated in this paper, biological water treatment can be an alternative.
In a biological water treatment system, the need for water replacement is considerably less. The only substances accumulating in such a system will be nitrate and to some extend phosphates (Spotte, 1979b), both of which are not known to have any toxic properties. One of the main advantages of a biological filter is its flexibility. When the load is high, there is food available for many organisms and consequently more organisms will grow in the filter, thus increasing its efficiency. When the load decreases, part of the microbial population will starve and will be broken down by the surviving organisms. So a biological filter is in fact a self regulating system: you do not have to change the dosage of chemicals when the load changes, nor do you need to have expensive equipment to do that for you.
As already indicated a trickling filter seems to be the best choice for a dolphinarium. In a trickling filter, large amounts of flocs are formed (Mudrack and Kunst, 1986). Therefore the trickling filter should be followed by a sedimentation (or settling) tank, in which the water flows slowly enough to allow the flocs to settle. The clarified water will leave such a tank through an overflow. The sludge collected at the bottom should be removed. This can be done continuously by pumping water with sludge from the bottom and leading it over sand filters. It can also be done intermittently in several ways. An in-depth discussion of all the possible ways of sludge removal is outside the scope of this paper.
Since a biological filter acts primarily on dissolved organic matter, there should be a mechanical treatment parallel to it. Filtration is probably not very effective without the help of flocculants. As indicated already above, the most commonly used flocculants, based on aluminium, might not be as effective in seawater as they are in fresh water and in pure NaCl solutions. And, unless you have a fairly open system in which considerable amounts of water are replaced regularly, there is a chance of accumulation of aluminium, which is potentially toxic (Andersen, 1973). However, foam fractionation can be very effective in removing dissolved, colloidal and particulate matter, both organic and inorganic (Conway and Ross, 1980). Foam fractionation works well without any need for addition of chemicals. As already indicated, the need for disinfection in a biological system is very small (Wickins and Helm, 1981). It is however desirable to have at least the possibility for disinfection available. Chlorine is not suitable because it leaves a residual disinfectant in the water which may harm the biological filter. A good disinfectant for a biological system reacts quickly, does not form any toxic byproducts and leaves no residual disinfecting agent in the water. Ozone meets those demands. If it used on clean water, coming from the biological and the mechanical treatment systems, the amount of ozone needed will be small (Farooq et al (1977), Ramos and Ring (1980)). Apart from disinfection, ozone can be active in colour removal and it reacts with dissolved organic matter (Spotte, 1979b).
In short a possible biological water treatment system includes a trickling filter followed by a settling tank. Next to that there should be some kind of mechanical treatment and foam fractionation is a good candidate for that. Final treatment of the water with low concentrations of ozone can help keeping the amount of micro-organisms in the pools low, although there is no real need for that. If the system is closed, provisions should be made for denitrification to prevent the nitrate levels from increasing continuously. Alternatively regular partial replacements of the water could keep the nitrate levels down.
The inspiration for this paper comes from lengthy discussions about water treatment with Dr. W. H. Dudok van Heel, who stimulated me to dig deeper into the theory of water purification. Dr. P. Vuoriranta of the Technical University of Tampere helped me find the relevant literature on microbiology and Dr. P. Keskitalo of Air-Ix, Tampere, supplied me with literature on ozonation. Their help is gratefully acknowledged. And finally I would like to thank my wife Jolanda for her help in improving the manuscript.
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