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 this paper an overview is given of several alternative techniques for dolphinarium water purification, that may make water conditions more natural. Discussed are water composition, biological filtration, based on the trickling filter principle, and foam fractionation. Indications are given to their possible applications in dolphinarium water treatment systems.
In recent years a lot of attention has been paid, and rightfully so, to ways to improve as much as possible the circumstances under which marine mammals, especially cetaceans, in captivity live. Several attempts have been made to set up some standards for pool dimensions and housing conditions. Much progress has been made in the field of marine mammal medicine and husbandry. And also the importance of training for those animals, as occupational therapy or as a means towards a better understanding of their biology (Pryor, 1986) has been stressed. A logical extension of this development is the underlying attempt towards making the water conditions for inland dolphinaria more natural. When designing an artificial environment for the maintenance of dolphins, both physiology and ecology of the animals should be taken into account. Especially the ecological aspect is often overlooked. If you want to create optimal living conditions for these creatures, you have to take their natural habitat into account. Contrary to what Wallis (1973) stated, it is, at least now, very well possible to make use of the capability of self purification of natural waters for the purification of water in dolphinaria. Manton (1986) already mentioned that water treatment strategies for dolphinaria are based on processes for swimming pools and potable water, and ignored the effects of salt water. He stated: "Perhaps initially dolphins should have been kept by marine engineers and not by swimming pool designers." The strategies outlined in this paper are based on marine aquarium management techniques and on wastewater treatment techniques that make use of natural processes, that occur on both fresh and salt water.
It is remarkable that it is very often assumed, that dolphins don't anything more than a pure NaCl solution to live in. But this remains to be seen. Animals that are as well adapted to living in a marine environment as dolphins are, most likely make optimal use of that environment. Therefore the water they live in in dolphinaria should be as close to natural seawater as possible. Elements that are known not to be essential can be left out. But essential trace elements, such as copper, zinc and manganese, should be included, unless it has been proven that dolphins do not take them up from the water. It is difficult to determine to what extent they take up elements from the water, but it is likely that to some extent at least they do. Dudok van Heel (1975) quoted Dr Redlich, saying that human can take up enough copper by just rinsing the mouth once a day with water containing traces of copper. This happened through the mucosa of the mouth. If this is true for dolphins as well, then there should be ample opportunity for taking up elements from the water, since dolphins often swim with their mouths open. (It has also been demonstrated that dolphins do in fact drink sea water (Hui, 1981 (cited in Gaskin, 1986)) and are thus probably able to extract elements from the water in the gastro-intestinal tract. A strong indication for uptake of trace elements from the water is the observation that a raw dark patch around the blowhole of a dolphin (similar to lesions on the nostrils of zinc-deficient cattle) healed, when a mixture of trace elements was added to the water (Dudok van Heel, 1975). This raw patch reappeared when the addition of trace elements was discontinued. If the elements are taken up through the mucosa of the mouth, the gastro-intestinal tract or through the skin (or maybe to some extent through all of these routes) needs further investigation. (The often heard remark: "We don't add any trace elements to the water and our dolphins are doing fine" doesn't contradict this because it is indeed based on what is added. The fact that high levels of certain metals can be present, for instance from the fresh water supply or from contaminations in the salt (Wickins and Helm, 1981), is completely overlooked in that case. The only thing that is important is not what you add, but what is present in the water.) When you then also try to incorporate a biological water purification, in which bacteria, algae and microscopic invertebrates remove the materials the dolphin add to the water it seems even more logical to use (an artificial) seawater instead of a pure NaCl solution in order to satisfy the needs of the micro-organisms. Also for this reason the trace elements, like iron, copper, zinc and manganese are important.
Of the other elements especially calcium and magnesium are of interest. They play a major role in several processes, such as in the buffering system (Kester et al, 1975), floc formation (Johnson et al, 1986) and foam formation (Degens and Ittekkot, 1986), that are all of importance for the self purification of the water. (It is interesting to note that high concentrations of magnesium in the bottom inhibit the growth of the fungus Aspergillus sp. (de Bolster et al 1980). If magnesium has the same effect in water is not clear though.)
The salinity of ocean water is on average about 3.5% (Spotte, 1979b) and this should therefore also be the target value for artificial sea water for dolphins. However, we should keep in mind that this value is the total salinity. The NaCl salinity is approximately 3.0%. The rest is made up of, in order of importance, magnesium, calcium and potassium salts. Of the other elements only traces are present.
Natural sea water is buffered. This means that it resists sudden large changes in pH. This is necessary, because biological activities can alter the pH. Respiration and nitrification tend to lower the pH, while photosynthesis and denitrification tend to raise it. Sea water can cope with this through the carbon dioxide system.
Carbon dioxide (CO2) in water can form carbonic acid (H2CO3). This acid can dissociate to form the bicarbonate ion (HCO3-), which in turn can form the carbonate ion (CO32-). All these reactions are reversible and by shifting back and forth through this chain of reactions, sea water has a means of removing excess acid or hydroxide (see Spotte (1979b) for a more in depth treatment of the subject). At the normal pH of sea water, 8.2, the bicarbonate ion is the dominant form.
A frequently used measure for the buffering capacity is the carbonate alkalinity, which is defined as the sum of the concentrations of the carbonate and bicarbonate ions in meq/l. For details on the methods of determination of the alkalinity, the reader is referred to Almgren et al (1983).
Total alkalinity includes also boric acid, but its contribution to the alkalinity is so small that is frequently left out. The buffering system of sea water is very flexible and also several cations have an influence on it by forming complexes with carbonate and bicarbonate ions. In seawater the bicarbonate ion HCO3- is present as free ion for 63 - 81%, 11 - 20% is present as NaHCO3, 6 - 14% as MgHCO3 and 1.5 - 3% as CaHCO3. Of the carbonate ion CO32-, 6 - 8% is present as free ion, 3 - 16% as NaCO3-, 44 - 50% as MgCO3, 7% as Mg2CO32+, 21 - 38% as CaCO3 and 4% as MgCaCO32+ (Kester et al, 1975)
Up to now chlorination has been the most widely used technique for the removal of organic matter from marine mammal pools (Dudok van Heel, 1983). The method of chlorination has been described in considerable detail by Andersen (1973) and Manton (1986), both referring to White (1972), and will not be discussed further in this paper. The chlorination of marine mammal pools still gives rise to a lot of problems, despite a vast amount of literature on the subject of chlorination. As Geraci (1986) put it: "Chlorine as sodium hypochlorite is perhaps the most commonly misused chemical treatment". Although chlorine is used to remove organic matter from the system, it will not do that completely. In chlorinated marine mammal pools the TOC (total organic carbon) concentration will increase: about 7% of the carbon added to the system (as food) will remain (Spotte and Adams, 1979). Even occasional super-chlorination will not remove all the organic carbon from the system (Adams and Spotte, 1980). Among the more persistent organic molecules there could very well be potentially toxic or carcinogenic chlorinated carbon compounds, that could accumulate if a considerable amount of water is not replaced regularly. Duursma and Parsi (1976) pointed to the possible formation of persistent chlorophenols, which are hard to detect. These substances form in water in which phenols are present. Spotte (1979) noted that phenols are present in aquaria. These phenols probably come from algal pigments and since algae can grow in chlorinated pools, the presence of chlorophenols is likely. (The presence of algae is more likely in marginally chlorinated systems, since phytoplankton, which mainly consists of algae, has been shown to be more sensitive to free chlorine than to monochloramine (Duursma and Parsi, 1976). Monochloramine is the main disinfecting agent in marginally chlorinated systems, whereas system employing breakpoint chlorination rely more on free chlorine.)
Another disadvantage of chlorination is that it works better against bacteria than against fungi (Dudok van Heel, 1983). This could increase the chance of fungal infections. It seems more likely, that the renewed infection of a dolphin with Lobomycosis when it was placed in a so-called enriched medium (Dudok van Heel, 1975) was not the result of the enriched medium itself, as Manton (1986) suggested, but rather of the absence of competition by bacteria in that medium. Under normal biological growth conditions bacteria will always overgrow fungi: fungi cannot be cultured if the growth of bacteria is not inhibited by the use of antibiotics (McKinney, 1962). Other skin infections, encountered in dolphins kept in chlorinated (or ozonated) water can be the result of the destruction of beneficial microflora and of inactivation of antimicrobial substances excreted by the skin (Geraci et al, 1986).
The chlorination of (artificial) sea water is far more difficult than that of fresh water or of a pure NaCl solution. The presence of magnesium makes breakpoint chlorination impossible (Manton, 1986) and also iron and manganese interfere with proper chlorination (Dudok van Heel, 1983).
Chlorination may also affect a certain aspect of dolphin communication. We now know that dolphins have a sense and there is evidence that dolphins secrete signal substances, or pheromones, into the water, that are detectable by taste. These substances can play a role in social and sexual behaviour (Herman and Tavolga, 1980). Chlorine will almost certainly destroy these substances as soon as they are released. The continuous presence of chlorine and chloramines might also interfere in a more general sense with taste.
In recent years, ozone is used more and more for disinfection of water. One of the reasons for this could be that ozone can nowadays be produced much more safely and economically photochemically (McGregor, 1986). The principles of the photochemical generation of ozone (using mercury lamps) have been clearly outlined by Dohan and Masschelein (1986).
Ozone is mainly used for 2 purposes: disinfection and discolouration, but it can also have some effect in removing turbidity (Ramos and Ring, 1980). It has been claimed that ozone can reduce the TOC (total organic carbon) concentration in water by converting low molecular weight organic substances, such as methanol or ethanol to carbon dioxide (but also to acetic acid or aceton) (Elia et al, 1978). However, it doesn't have any effect on TOC levels in aquaria and marine mammal pools (Spotte (1979b), Adams and Spotte (1980)). When reacting with organic matter, ozone will first attack carbon double bonds (Kinne, 1976). It is probably this reaction that explains the ability of ozone of removing colour: pigments contain carbon double bonds and can also contain phenol rings (Spotte, 1979b). It means that ozone is active primarily against colour from organical sources.
As a disinfectant it is more active against E. coli, but also against plankton, insect larvae and possibly viruses, than chlorine. Its disinfecting powers decrease at higher densities of organisms (Farooq et al, 1977) and at higher turbidities (Ramos and Ring, 1980).
Its effect on turbidity can be explained by the conversion of POC (particulate organic carbon) into DOC (dissolved organic carbon). At higher DOC levels this conversion will not take place, because ozone will react with the DOC first (Spotte, 1979b). Ozonation of water containing dissolved organic matter can even increase turbidity. When ozone reacts with organic matter, polar, negatively charged groups (like carboxyl and hydroxyl groups) are formed. Complexing with polyvalent cations can then result in precipitation of the organic matter (microflocculation) (Rice, 1986). If turbidity increases or decreases following ozonation depends on the total composition of the water.
In sea water the reactions of ozone are more complicated, because all kinds of side reactions can occur. It can for instance react with the trace metals Fe2+ and Mn2+ and oxidize them to Fe(OH)3 and MnO2 respectively, both of which can precipitate in this form and may be removed from the solution (Rice, 1986). It can also react with the chloride ion Cl- and convert that to hypochlorite ClO- (Keenan and Hegemann, 1978).
Ozone does not leave any, possibly toxic, byproducts in the water and is therefore a convenient disinfectant for animal pools. On the other hand it doesn't leave a residual disinfecting agent either, so it doesn't have a long lasting effect. This makes the demands on the hydraulics of a water recirculation system higher.
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