W.H. Dudok van Heel and J.D. van der Toorn (1988)
A biological approach to water purification: II.
A practical application: The Delfinaario in Tampere, Finland

From: Aquatic Mammals 14(3): 92-106

Table of contents

Original design
System hydraulics
The biofilter
Foam fractionators
Water composition
Nutrient level in the Delfinaario water
Recent developments


Since the management was a bit disappointed with the underwater visibility in the pools, some attention has been paid to factors that may have an influence on this. In May and June of 1986 several water samples were taken and analysed for the concentration of suspended solids (larger than 10 µm.) in mg/l and the turbidity in FTU. During this period the concentration of suspended solids ranged from 1.5 to 6 mg/l and the turbidity ranged from 0.65 to 1.3 FTU. Both were highly variable and had no significant influence on the visibility in the pools. There was also no correlation between the suspended solids concentration and the turbidity (correlation coefficient r = 0.018).

In this period the bio-plates were still in place and it was suspected that they were releasing particles into the water that increased the turbidity. After the bi-oplates were removed several samples have been analysed for turbidity. Now the turbidity was far less variable and also lower: 0.355 ± 0.09 FTU. There are two reasons for this decrease in turbidity. First of all, as already indicated, it was suspected, that the bio-plates were releasing small particles into the water. This influx of particles was stopped by the removal of the bio-plates. At the same time the remainder of the brushes was installed in the trickling filter, partly submerged. It was noted that especially in this submerged part of the trickling zone extensive flocculation occurred. This means that the capacity for particle and colloid removal of the system was greatly increased, due to this change. After the new filter was started this did not change significantly (turbidity: 0.342 ± 0.07 FTU).

There was however a change in visibility, due to the reduction of the intensity of the green colour. This was caused by a sudden drop in the concentrations on calcium and magnesium. The calcium level dropped from 440 to 239 mg/l and the magnesium level fell from 800 to 600 mg/l. Apparently the calcium and magnesium absorbed a lot of light, thereby reducing the visibility. None of the trace metal concentrations changed. The water is still greenish, but now a little less dark.

How one filter could be responsible for this dramatic change in calcium and magnesium levels is still unclear. The drop constitutes a loss of about 550 kg of calcium and 1000 kg of magnesium in only two days without a significant increase in the headloss over the filter. Possibly these elements reacted with the filter bed and created a coating around the sand grains. No samples of the filter medium have been taken to test this, however.

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Recent developments

In January 1986, the Aluko filters were taken out of operation. They were replaced by a multi-layered deep bed sand filter. This filter was installed in early 1987 and was put into operation on June 13, 1987. From January 1986 until June 1987 the system had no mechanical filtration. All the results mentioned in this paper are from the period that there was no mechanical filtration. None of the parameters were influenced by the start of the filter in June 1987, except for the ones mentioned under the heading "Clarity".

Shortly after the filter was put into operation the Photozone units were taken away from the fractionators and the Photozone gas produced by all four of them is now lead into the pipe leading from the filter to the place in the settling tank where the bio-plates used to be. The ozonated water mixes with a large volume of not ozonated water to remove possible residual ozone from the water before it splashes into the mixing channel. This is done mainly to prevent releasing ozone into the engine room.

This type of filter has to be taken out of operation for backwashing. Backwashing has to be done at least once a week. The backwash is started manually, but runs automatically on a timing device that takes care of opening and closing of the automatic (pneumatic) valves, and the starting and stopping of the pump. The filter uses fresh water for the backwash, which has to be stored in collecting vessels, since the pressure of the town water supply is insufficient for proper backwashing.

Note: some time after publication of this paper, the Photozone units have been replaced by an oxygen-fed electrical ozone generator, with a higher ozone output. A contact chamber has been added to allow a longer reaction time. This has resulted in a further reduction of the water colour and a further improvement of water clarity.

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The quality of the water is reflected in the health and appearance of the dolphins. In chlorinated systems the formation of chloramines and other chlorinated organic compounds often leads to skin and eye disorders (Geraci, 1986). Up to now no such disorders have been found in the dolphins of the Delfinaario. Their skin and eyes are in perfect condition, as has been noted by veterinarians and trainers from other oceanaria. Also the skin has so far retained its natural dark grey dorsal and pink ventral colouration. Very often dolphins in captivity become paler, but in the Delfinaario this has not happened. This might be due to the presence of trace elements in the water, since especially copper plays an important role in skin colouration (Robbins, 1983).

This system was designed to consist of two separate sections: a biological treatment section and a physico-chemical treatment section. Those sections serve different purposes. The biological treatment section acts primarily on organic matter. It is self-regulating, because the amount of micro-organisms that grows in the biological filter fluctuates along with the amount of food offered to the filter, i.e. the concentration of organic matter in the water. The reaction of the filter is always slightly delayed, since it takes some time for a population of micro-organisms to build up. For this reason there is a physico-chemical treatment section parallel to the biological section. This section consists in this case of an array of foam fractionators. These fractionators immediately start producing more foam when the concentration of organic matter in the water increases. Therefore they lessen the fluctuation in the food supply for the bio-filter, thereby creating a more stable environment for the filter. In addition, foam fractionators can also remove matter that cannot be converted by a biological filter. This makes the combination of a biological filter with foam fractionators a very attractive setup, since these sections both support and complement each other.

The original design called for a 50-50 distribution of the water flow between the biological section and the foam fractionators. This probably is indeed the best distribution. In the system as it was built, the temporary loss of one fractionator, due to maintenance, results in a temporary decrease in water clarity. This means that the 35% of the total flow that the fractionators get now is just about sufficient. By reducing the importance of the foam fractionators in the total system, compared to the original design, the built-in safety margin was lost. Despite this the system has been working well and has provided the dolphins with clean and clear water, without the use of potentially harmful additives. Recently a new sand filter has been installed, and the Photozone units moved. The Photozone gas is mixed with the filter effluent. At this point, the Photozone units have no measurable effect on water quality. The filter was responsible for making the water appear lighter by removing large amounts of calcium and magnesium, reducing their concentrations to far below the desired levels. Since both calcium and magnesium play a role in the buffering system, this change might affect the alkalinity.

One of the great advantages of the system described in this paper, of course apart from the good and natural water conditions it creates and maintains, is the fact that it is self-regulating. Foam fractionators start producing foam as soon as the concentration of organic substances rises above a certain threshold level and the size of the population of micro-organisms in the bio-filter increases and decreases along with the concentration of organic matter. Since no chemicals are added to the system to convert organic matter it is not necessary to monitor the concentration of such chemicals and their products, as is the case in chlorinated systems. In this system only the pH and the alkalinity are measured once a day. Samples were taken to an independent water laboratory for measurement of the concentrations of nutrients and the amounts of coliform and total bacteria twice weekly. This was done mainly to get a good idea of how the system was performing, because after all it was an experiment. Now that we know more about the system, several of those analyses could be done less frequently. We suggest that measurement of the concentrations of ammonia and nitrite and the bacterial levels are performed twice weekly. Measurement of the concentrations of nitrite and phosphate could be done once every two weeks or even less frequently, since their rate of change is very low. Also salinity can be measured just every now and then, since it doesn't change much in this closed system. The pH and alkalinity should be measured at least once a day. These two parameters are also good indicators of the performance of the biological filter, since the water is slowly acidified by the conversion of ammonia to nitrate. A sudden change in the rate of change of these parameter could indicate a change in the performance of the biological filter and indicates the need for additional tests for nutrient levels. As already indicated, the pH and alkalinity are the only parameters that need regular adjustments. In this system it was necessary to add sodium bicarbonate to this end roughly once every two weeks.

The only nutrient element that is not removed by the system is nitrogen. It accumulates in the system in the form of nitrate. Nitrate is not known to have any toxic effects even at high concentrations (Spotte, 1979b). Nevertheless, from a management point of view it is better to remove any substance that accumulates. In the case of nitrate this can be done with the help of a small anaerobic biological filter. Preliminary calculations have indicated that an anaerobic filter with a capacity of about 2 m3/hr is able to reduce the nitrate concentration to about 20 mg/l and maintain it at that level (20 mg/l is considered to be safe even for fish and invertebrates (Spotte, 1979a)). The larger the capacity of the denitrification unit, the lower the final concentration will be. Without such a unit, the only way to remove nitrate from the system will be the replacement of large amounts of water.

Click here to load the system design drawing, with suggested modifications, including a denitrification unit. Click here to load this drawing in a new window.

In conclusion we can say that the system described in this paper has proven to be a reliable and good water treatment system, that helps to maintain natural and healthy water conditions for the dolphins of the Delfinaario. The system is in many respects self-regulating, which makes it fairly easy to operate. Since it is designed as a closed system, water and salt losses are minimal, which reduces the operating costs. The combination of a trickling filter with foam fractionators turned out to be a good choice. Sand filters are not essential in a system like this. The use of Photozone, either in combination with the foam fractionators or after the new sand filter had no measurable effect on water quality. This indicates that the system is able to keep the concentrations of organic substances at a very low level and that the Photozone did not provide additional oxidation power.

Note: The water treatment system of the Delfinaario is still operational and providing good water quality. As mentioned, an electrical discharge oxygen-fed ozonator has been added later, replacing the Photozone units. This ozonator has a higher ozone output. It is fed water coming from the sand filter and a prolonged contact time in a special contact chamber is allowed. This setup has helped reduce water colour and improve water clarity. A substantial amount of water has been dumped and replaced once to reduce the level of nitrate. Also, an amount of water has been dumped for pool maintenance reasons.

A denitrification unit has been added fairly recently. The design and operation has been outlined in:

Kettunen, R.H. and P. Keskitalo (1998)
Continuous denitrification for removal of nitrate in dolphinarium life support systems.
Paper presented at the 26th Annual Symposium of the European Association for Aquatic Mammals, March 13th-16th, 1998, in Tampere, Finland.

Foam fractionation has been used as an effective water treatment device in another biological water purification system in the dolphinarium of the Zoo in Duisburg, Germany. In this system, a different design fractionator was used and sand filters were used as substrate for the nitrifying bacteria. In this system, the fractionators removed most of the particulate matter, since hardly any increase in headloss over the filters was noted. The filters were backwashed only once in six months, basically as a precaution (M. Garcia Hartmann, pers. comm.). This system was described in:

Overath, H., M. Garcia Hartmann and K. Pelzer (1996)
First experiences with the protein skimmer Aquaflotor® in a large dolphinarium setting.
Paper presented at the 24th Annual Symposium of the European Association for Aquatic Mammals, March 15th-18th, 1996, in Albufeira, Portugal

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The board of Tampereen Särkänniemi OY, the owner of the Delfinaario in Tampere, should be praised for its decision to install the described experimental biological water treatment system. The engineering firm Erkki Leskinen KY of Tampere carried out a thorough study of the possibilities of the biological system as well as of a more traditional chemical treatment system. Their study was vital for the decision of the board of Särkänniemi. The technical discussions with Mr. B. Kanne of Van Reekum Materials of Apeldoorn, the Netherlands and Mr. N. Verduijn of Aluko B.V. of Amersfoort, the Netherlands have contributed to the success of this project and are gratefully acknowledged. The help of Seppo Näsi of Kokemäenjoen Vesistön Vesiensuojeluyhdistys OY of Tampere and of Dr. Pertti Vuoriranta of the Water Sanitation Department of the Technical University of Tampere with the examination of the water samples and the evaluation of the results is gratefully acknowledged. Jolanda van der Toorn should be thanked for her help in correcting the manuscript and Alex Hoeksma for his help in typing and editing the manuscript.

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