John D Dineley
International Zoo News, Volume 37/7, Number
224.
INTRODUCTION
More and more zoos, wildlife parks and aquaria that display aquatic mammals are now using some form of water treatment system to produce a more healthy environment for their animals. Also, with new U.K. and E.C. legislative control over doiphinaria, it has become even more important to understand the fundamental principles of good water quality management for this ever popular group of animals. This paper outlines the basic require-ments for such management.
There are three ways in which water in the captive environment may be maintained in an unpolluted state:
(1) Open System - Water is pumped or changed by tidal action from a natural pure source, i.e. the sea.
(2) Semi-closed System - Water is filtered and perhaps chemically treated, while 10% or more of water from a natural source is added into the pools, allowing the displaced water to run to waste.
(3) Closed System - Pool water is continually recirculated through filters and chemically or biologically treated to maintain it in a pure and fit state for the animals in the exhibits. However, it should always be remembered that even the best closed water systems will not prolong the life of a body of water indefinitely, and that some water replacement must take place at periodic intervals.
The open system is simple and straightforward to apply if a suitable coastal location is available. Various systems can be used for drawing water from the sea. Sub-abstraction from beneath the beach sand can yield water of a very high quality and clarity. Penning off areas of a bay or inlet is another method; for example, one oceanarium in North America housed its marine mammals in a floating dock in a bay, using inverted cages. In most systems, though possibly not the last two, some kind of mechanical or physical method is used to render the water clear of suspended matter.
PHYSICAL TREATMENT OF WATER
Filtration
This is a physical method for the removal of solid suspended particulate matter. Zoos and oceanaria, in general, use filter systems designed for public swimming pools or municipal water treatment. These can be classified into two groups: sand filters and diatomaceous or pre-coat filters.
(1) Sand Filtration.
These filters all pass water down under pressure through a body of sand to collector pipes, to be returned filtered to the pool. The bed of sand collects particles of matter from the water. These filters may be subdivided into four groups:
(a) Rapid Gravity Filters. This was the first type of filter to be used in public swimming pools and water treatment works at the turn of the century. They are generally box-shaped, but can also be round, and rely for pressure through the sand on the head of water maintained above it.
They work at flow rates of 2.5-5.0 m3/m2 (approx. 50-100 gallons/sq. ft per hour).
(b) Standard Rate Pressure Filters. Here the sand is held in a sealed unit, and the water is pushed through under pressure by a pump at 10-12.5 m3/m2 (approx. 200-500 gallons/sq. ft) per hour.
(c) Medium Rate Pressure Filters. These run at a higher flow rate of 20-25 m3/m2 (approx. 400-500 gallons/sq. ft) per hour. They therefore need careful designing to ensure correct flow through the filter bed.
(d) High Rate Pressure Filters. These work at very high rates of flow, up to 50 m3/m2 (approx. 1,000 gallons/sq. ft) per hour. They work on a filtration-in-depth principle, with suspended solids penetrating the sand bed for a depth of 8 inches (20 cm), unlike the other pressure filters, which use only the top 3-4 inches (7.5-10 cm) of the filter bed.
If well designed, all types of sand filter give good service. However, a word of caution is needed regarding some high rate systems. It has been found that when some of these systems have to clean a pool with a high animal-to-water ratio, the sand in the units tends to cake into lumps after just a few weeks of operation. This should not happen where there is a reasonable animal-to-water ratio (see Chlorination), but caking has also been found in high rate systems when used in public swimming pools, and with this in mind it is advisable to backwash these systems before they become too dirty. However, these- systems can operate very well; they tend to be made of glass fibre and have resistance to the corrosion of salt water, and there are many exhibits containing aquatic mammals which have used them with great success.
(2) Diatomaceous Earth or Pre-coat Filters.
Diatomaceous earth (D.E.) consists of the fossilized remains of certain unicellular plants and can be obtained in various grades. Alternatively, a specially treated volcanic ore called 'perlite' can be used. Many designs of pre-coat filters are available; however, they all work on the same principle, that is, laying down a coating of the D.E. on rigid porous supports called 'candles' or 'septa' to form a filter bed. These systems yield water of very high quality: they are, however, expensive to maintain, since the D.E. can only be used for one filter cycle, whereas sand may give good service without being replaced for many years.
Turn-over Rate
Whatever the filter system, it is very important to ensure that the water can be cleaned at a rate quicker than it can be fouled. The turn-over rate of a pool is the amount of time it will take for its total volume to pass through the filter system once; e.g. a pool containing 100,000 gallons of water, which could be processed at a rate of 50,000 gallons an hour, would be said to have a turn-over rate of two hours. It has been demonstrated by experiment that it takes seven complete turn-overs to remove 99% of dirt from a pool, discounting the continuous addition of pollution that normally occurs. With an animal-to-water ratio similar to that recommended for chlorination purposes (see below), a two to four hour turn-over rate should prove adequate. The European Association for Aquatic Mammals recommends a turn-over of at least four hours or less.
CHEMICAL TREATMENT OF WATER
pH and Alkalinity
Put simply, pH is the measurement of acidity and alkalinity, measured from 0 to 14, with 7 being neutral (above 7 is alkaline, and below 7 acid). Sea water is alkaline, naturally ranging from 7.6 to 8.2. The use of chemicals such as chlorine and aluminium sulphate requires certain pH levels for optimum performance: so levels in an aquatic mammal pool have to be a compromise, animal comfort being the most important consideration. It is generally considered that a pH level of between 7.6 and 8.0 is acceptable for animal welfare and water treatment efficiency.
The level of bicarbonate alkalinity is also important in maintaining a stable pH in a pool. Alkalinity acts as a 'buffer' in the pool, compensating for the addition of acid and alkali from faeces and urine. Levels of 100 to 200 ppm are the recommended range.
Coagulation
Filtration alone, especially with a closed pool system, will generally not give good results unless accompanied by some form of precipitation of the colloidal materials in the water. Coagulation is a process whereby chemicals such as aluminium sulphate or ferric chloride are added to the filter system to precipitate soluble impurities so that they are large enough to be removed by the sand filters.
Aluminium sulphate is the most commonly used chemical for coagulation, and is safe provided its level is monitored. Levels of up to 0.1 ppm seem to cause little problem, but research has shown that the level should always be kept below 0.3 ppm, as above this concentration it can be an irritant. As with any water treatment chemicals, the amounts used should be kept to the lowest level that can achieve good results. Aluminium sulphate is pH dependent in its reaction, working best at the lower pH level of 7.6-7.8.
Chlorination
Because aquatic mammals breathe air, chlorine can be used to disinfect and break down organic waste in their pools. The chemistry of chlorination is a very complex affair. Chlorine when added to pure water produces free available chlorine, which is non-toxic at high levels and inactivates pathogenic agents within a short space of time. However, when added to water containing organic matter, particularly ammonia, it forms chloramines. Some of these compounds are the ones that can cause eye irritation to swimmers in public swimming pools, and in high concentration can even burn and cause damage to skin. Chloramines do also kill pathogens, but at a much slower rate.
In the early days of chlorination only chloramines were used to kill pathogens, because it was found that to add too much chlorine would cause irritation and burning. However, research showed that although when more chlorine is added there may be a rise in the chloramines or combined residual chlorine, as the group of compounds is sometimes called, when the dosage of chlorine is increased a second reaction takes place and the free chlorine begins to break down the chloramines and become the main form of chlorine present. Maintaining a free chlorine level in water sufficient to suppress other chlorine compounds is known as 'break-point' chlorination.
The level at which this point is reached depends on the amount of organic matter present in the water being treated.
In this context, the 'animal-to-water ratio' is important. In the case of dolphins, for example, the break-point system is only effective where there is at least 20,000 gallons (approx. 90 cubic metres of water per animal or putting it another way, for every 6 kg of fish fed dally). It is very important to exceed this animal-to-water ratio, as otherwise chloramine levels will become too high for the animals' comfort and could damage their health.
As can be seen, it is necessary to discriminate between the different types of chlorine compounds in order to ensure that free chlorine predominates.
Chlorine levels can be measured by using an indicator chemical, diethyl-p- phenylenediamine, in a test known as the Palin DPD test.
Chlorine is a gas, but it is safer to introduce it to the water in a solution of sodium hypochlorite (which contains approximately 14% available chlorine); alternatively, in salt water pools, it can be produced in situ from the salt water by using an electrolytic cell. One interesting situation that arises when natural seawater is chlorinated is that - due to the presence of large quantities of another halogen, bromide - the chlorine reacts to release bromine. The chemistry of bromine is similar to that of chlorine in that it forms both free and combined bromine; it is also a powerful disinfectant and can destroy nitrogenous matter such as ammonia. It may be that many seawater pools are, in fact, using bromination rather than chlorination.
Certainly, standard testing with Patintest DPD tablets does not discriminate between these two halogens, though by using Palintest Glycine tablets it is possible to distinguish them if necessary.
Ozone
Ozone is a form of oxygen (03) which is a powerful oxidising agent. It has been used in general water treatment for the last 80 years, although not to the same extent as chlorine. Its use in marine aquaria is widespread, and it is being used more and more in large marine mammal pools. Ozone is produced on site; as it is a gas with low solubility in water, it needs to be mixed well in the filter system before returning the water to the pool Because of its poor solubility, it has the advantage of not producing residues in the pool water. Ozone generators have to be professionally made and installed, as the gas is toxic and, hence, should not be allowed to leak in bubbles into the pool or vent into the air of the filter house.
A problem with using ozone is that it is unable to oxidize nitrogenous organic compounds beyond the ammonia stage, which can result in high levels of ammonia remaining in the pool. The use of biological filtration (see below) could help to resolve the problem, as it does in fish aquarium systems. Breakpoint chlorination methods may also work, though this could not be used in conjunction with a biological system, as the chlorine would kill the bacteria. Ozone has the added ability to aid the precipitation of the colloidal particles.
Other Forms of Sterilization Water may also be sterilized by ultra-violet radiation, which kills bacteria, or by introducing low ionic concentrations of silver and copper to kill bacteria and inhibit algae growth.
Activated Carbon
Activated carbon, when used in a filter bed or in its own chamber in the recirculating water system, has the property of being able to absorb and retain many toxic agents, including some of the troublesome stable chlorocompounds which seem to cause problems in some closed water systems using chlorination.
Biological Filtration
At present, very few marine mammal exhibits use a biological filter system: two which do are Disney World's Epcot Center 'Living Seas' exhibit in Florida, U.S.A., and Sarkanniemi Dolphinarium in Finland. The use of such systems is regarded by many marine mammal experts as a possible way forward in future exhibit design. The biological system still relies on the removal of suspended matter by standard sand filters; but the water then passes to a second treatment, in which detoxifying bacteria are used to break down organic matter, using aerobic decomposition from ammonia (NH3) to nitrite (NO2) to nitrate (NO3), and anaerobic decomposition from nitrate to nitrogen. The bacteria are encouraged to grow in various ways, and there are many different variants of this system; but all have in common a large surface area for bacterial growth. The bacteria can also be grown in the substrate of the sand filters used for mechanical filtration, as is the case at the oceanarium tank containing fish at Ocean Park in Hong Kong: research by the designers of this system showed that it was effective at rates of up to 7.5 m3/m2 (approx. 150 gallons/sq. ft) per hour, in a unit containing a volume of 180 m3 (approx. 40,000 gallons) and a fish load of 204 kg.
Biological filter systems may include protein summers (also called foam fractionaters). These pass a strong flow of bubbles up a column with a through-flow of pool water; this produces a foam containing protein colloids, organic dyes and other substances, which are removed to waste from the top of the column. The water may be pre-treated with either ozone or ultra- violet to destroy pathogenic agents.
Ammonia
The measurement of ammonia in pool water, along with nitrites and nitrates, determines the true efficiency of your treatment system's oxidization of harmful waste products from captive animals. A simple peel- side test can measure these chemicals approximately, complemented by more detailed periodic laboratory examination of the pool water.
Water Replacement
Whatever the system of water treatment, in a closed system it is important to discard and replace some water at regular intervals. This is because no system yet developed can purify a body of water indefinitely. There is a gradual build-up of compounds which, if left unchecked, will damage the health of the animals. It has been suggested that the replacement of 5% to 10% daily with new water will check undesirable organic and inorganic chemical residues.
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| Table 1: Suggested Water Quality for Marine Mammals in Closed Systems* |
* Source: R. C. Squires, Oceanaria water for mammals and fish, Journal of Water Engineers and Scientists, 1978.
Acknowledgments
I wish to thank the following for their help in preparing this article: Peter Bloom of Flamingo Land Dolphinarinm, North Yorkshire, U.K.; Lee Chanona of Brighton Aquarium, East Sussex, U.K.; Kai Mattsson of Sarkanniemi Delfinarium Finland; and F.W. Crowley of Binnie and partners, Redhill, Surrey, U.K.
References
Krajniak, E., el al. (ed.): Water quality in marine mammal exhibits. Proceedings of the 1981 Conferenceof the International Marine Animal Trainers Association..
Squires, R.C.: op. cit.
