EAZA News 57-12 - European Association of Zoos and Aquaria

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eaza news 57 2007 14 sustainability Re-using water at Burgers’ Ocean Max Janse, Burgers’ Zoo, Arnhem, the Netherlands One of the largest challenges in displaying aquatic organisms in aquaria is to control the third dimension; water. When managing an aquarium, water expenditure can be very high. Burgers’ Ocean, an Indo-Pacific seawater aquarium at Burgers’ Zoo, has a total volume of eight million litres. Being an inland aquarium, there are logistical limitations to using seawater, thus artificial seawater is used instead. Through different techniques, Burgers’ Zoo tries to use its water in a sustainable way. Re-using fresh water High-rate sand filters are used as mechanical water filters in most aquarium systems. These sand filters are cleaned by backwashing them on a regular basis (one to four times a week). At Burgers’ Ocean, freshwater is used to backwash these filters, which prevents the loss of large quantities of sea water. Within the zoo, three types of fresh water are used; rainwater, well water and drinking water. Rainwater is mostly used to fill ponds and to create artificial rain showers in the large tropical rainforest enclosure Burgers’ Bush. Well water is pumped into large buffers and can be used as ‘grey’ water in, for example, the aquarium. It is pushed through a large reversed osmosis installation to remove all nutrients and elements, like silicate and calcium, to ensure these will not cause problems within the sensitive aquarium systems. The end result is clear water. A disadvantage of a reversed osmosis installation is the production of large quantities of ‘brine’ water, containing the removed nutrients and elements. However, this brine water can also be re-used in the grey water circuit in other parts of the zoo, like for filling up ponds, as drinking water for the animals or to clean cages. Artificial seawater The clean water from the reversed osmosis treatment is used to top up the aquarium, which is necessary due to evaporation, and to make artificial seawater with special aquarium salt (Reefmix by Tropic Marin®). Within Burgers’ Ocean, 12 different aquarium systems with different water qualities are used. A large coral reef tank contains the best quality water, and water from this tank is used to refresh water in, for example, the shark tank. This type of cascading is used with the many different aquarium systems and is a successful way to re-use water. Preventing nitrate increase Aquatic organisms produce many nutrients such as ammonia, that need to be removed from the water. Ammonia can be removed by biological filters. Within these filters, nitrifying bacteria are commonly used to change the toxic ammonia into another toxic component, nitrite, followed by the formation of the less toxic nitrate. The toxicity of the Filterunits photo burgers’ zoo nutrients depends on the species kept in the aquarium. The toxic nutrients may not kill an animal, but can cause a chemical stress that could affect its immune system or reproductive cycle. Normally, nitrate levels are controlled by changing 10 to 30% of the water volume per month. However, at an inland aquarium like Burgers’ Ocean, large quantities of fresh seawater are not available, so only 3 to 5% of the total aquarium volume is changed per year. Nitrification is reduced via mechanical filtration (e.g. high-rate sand filters) and via the usage of protein skimming. Different filter techniques Two different filter techniques are being used at Burgers’ Ocean to learn more about their advantages and disadvantages. Both filter types use denitrifying bacteria that consume nitrate and produce nitrogen gas. The first filter type is based on autotrophic bacteria, the second type uses heterotrophic bacteria. Besides nitrate, these bacteria need a separate energy or carbon source such as sugar, lactate, methanol or ethanol. The past three years Burgers’ Ocean used methanol filters in the shark tank (water volume of 3000 m3 ) and in the tunnel display (water volume of 1600 m3 ). Since methanol is toxic for both the aquatic organisms and the people that work with it, it was decided to switch to the non-toxic ethanol instead. An ethanol filter consists of an 800 l tank filled with plastic bio rings. Ethanol is injected in the incoming water, which is pushed upwards through the bio rings. In the first centimetres of the filter bed all oxygen is used by bacterial activity.
The disadvantage of heterotrophic bacteria is the production of large amounts of extra-cellular biomass. The filter easily clogs if it is not cleaned on a regular basis. Secondly, part of the produced biomass may exit the filter and foul the aquarium water. Extra filtration with a protein skimmer and a mechanical filter such as a sand filter may be necessary. The positive side of this type of filter is the easy regulation of its activity via the amount of incoming ethanol. Though care must be taken when no nitrate is left and ethanol is still dosed, toxic substances like hydrogen sulphur or sulphite can be produced. The flow through the filter is very slow (around 150 l/h), making it a system which only has a long-term effect. The fish aquaria at Burgers’ Ocean are not heavily stocked, to give a realistic impression of a coral reef. Thus food for the fish added to the aquarium is limited, which in turn limits production of nitrate. The tunnel display for example, is fed with 25 kg wet weight per week. Nitrate levels have been kept constant with the denitrification filter for the last three years at 35 mg NO3--N/l, with a 3% water change per year. Denitrification with elemental sulphur During the European Union of Aquarium Curators (EUAC) Conference in 1997, Michel Hignette of Aquarium de la Porte Dorée in Paris, presented a new de-nitrification technique (Hignette et al., 1997). This technique is based on drinking water filtration, in which the bacteria convert nitrate and elemental sulfur into nitrogen gas and sulfate. A disadvantage of this technique is that it also produces sulfuric acid, which will deplete the buffer capacity and decrease the pH. This technique is also used at Burgers’ Ocean to deplete nitrate in fish aquaria. Sulfur-based denitrification units photo burgers’ zoo sustainability Methanol-based denitrification unit photo burgers’ zoo The effluent of the systems has a pH between 6.2 and 6.5. To prevent depletion of the pH in the aquaria, this water is first run through a calcium carbonate or a coral sand bed. This increases the pH to between 7.3 and 7.6. Burgers’ Ocean uses three parallel columns of 300 l each (including 250 l sulfur) for an aquarium with a volume of 600 m3 . The volume of the serial coral sand buffers is the same as the sulfur columns. The flow through the sulfur columns is approximately 150 l/h. During the three years this filter is used, the nitrate level is kept constant at 40 mg NO3--N/l, with a 4% water change per year and a feeding regime of approximately 15 kg wet weight per week. Phosphate removal When nitrate levels are controlled by the techniques described earlier, a second important nutrient like phosphate may become a problem. It is commonly known that, in low concentrations, phosphates are a problem to corals (> 0.05 mg PO4 3--P/l). While the effect on fish is unknown it may also be negative. Therefore, it was decided to look for techniques to remove phosphates as so little water change takes place. Granulates work perfectly for small systems, but are extraordinarily expensive to use in large volumes of water. Aluminium, iron or lanthanum chloride may be better alternatives. These elements will draw on the phosphates to precipitate into aluminium-, iron- or lanthanum phosphates. The first pilot experiments with the non-toxic iron chloride addition look promising. An iron phosphate and iron hydroxide complex is formed, which settles down in a settlement tank. Residuals are filtered out with a high-rate sand filter. A disadvantage of this is the usage of hydroxide and thus a decreasing pH. It may therefore be necessary to increase the pH with a sodium hydroxide or sodium carbonate addition. It seems that the techniques described above make it possible to re-use water in fish aquaria in nearly closed systems. Cascading techniques and backwashing sand filters with freshwater help keep the seawater usage to a minimum. It may be necessary to apply other techniques at Burgers’ Ocean in the future, if other problems relating to the small amount of water change appear. • eaza news 57 2007 15
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