Chapter 24 The Mechanics of Aquarium Water Conditioning

Beyond the fundamental water condition practices such as mechanical and bacterial filtration, there are many techniques used in modern aquarium water systems to ensure clean and safe environments for aquatic life. The art and science of water conditioning is the ultimate preventive medicine challenge for the veterinarian working with aquatic systems. It is imperative that the veterinarian understand the components and chemical, biologic, and physical properties of water conditioning.

The artificial filtration methods used in zoos and aquariums are similar to the ones used in swimming pool, municipal waste water, and drinking water treatment. Better understanding and consideration of natural stabilizing processes may provide more efficient and cost-effective life support for aquatic animals. First, understanding aquatic animal housing water flow systems is critical. Open systems have direct circulation with a natural body of water and do not usually require filtration. Semiclosed systems have basic filtration systems but rely on partial exchanges with an external water body. The third and most complex type of system is the closed system. Because this includes all types of filtration, this chapter will concentrate on this system. Basic marine mammal or bird pools are the simplest closed systems. These afford much more freedom from a water conditioning standpoint because the animals are not dependent on ultraclean water for respiratory needs, as in fish systems (Fig. 24-1). The most complex is the mixed exhibit, in which the fastidious needs of fish and the federal regulations for marine mammals must be addressed. To complicate issues, many exhibits now have people swimming in these systems and thus must meet public health regulations.

Figure 24-1 Filtration continuum. The left portion of the spectrum is the most basic filtration, and least expensive. Progression to the right indicates more sophisticated conditioning methods, which allow more taxa to be displayed.

The simplest is the basic marine mammal system, which consists of dump and fill. This is based on human swimming pool rules of chlorination and shock chlorination when the parameters are out of acceptable levels. Brine (NaCl) may be used, although many in the industry do not believe that NaCl alone fulfills marine mammal needs. Therefore, a number of institutions use a saline marine mammal mix—MgSO4, MgCl, NaCl. The next level consists of the use of chlorine and chloramines with filtration to keep the particulate and bacterial loads down to acceptable levels for longer periods, saving on water and salt expenses. The next generation of marine filtration systems includes ozone with chlorine (or) bromine to keep environmental oxidant levels down. From there, foam fractionation and ultraviolet (UV) sterilization may be used. Once biologic filtration is added, the addition of chlorine becomes complicated; it usually is discouraged because chlorine interferes with the establishment and maintenance of a biofilter.

Once fish are added to systems, general oxidant sterilization cannot be used. These systems operate without chlorine and bromine and residual ozone oxidants should maintain an oxidation-reduction potential (ORP) below 350 mV. Systems in which water exchanges are nonexistent or limited need to consider denitrification systems to convert nitrates to nitrogen gas, which is off-gassed. With fish exhibits or mixed taxa exhibits, gas exchange partial pressures need to be considered. Once invertebrates are added, detailed chemistry control is needed but is beyond the scope of this chapter. It is important to recognize that conflicting information is based on varying philosophies between older traditional thoughts verses new ideas of ecologic approaches to water conditioning. For further reading, see studies by Carlson,³ Overby,⁵ Spotte,⁶ Van der Toorn,⁷ and Watson and Hill,⁸ and the website of the Aquatic Animal Life Support Operators (http://aalso.org).

Filtration Methods

Organic and Bacterial Loads

We believe that there is a fine balance between organic load, bacterial load, color of the water, and health of the animals. Thus, this chapter is organized around how to address organic loads in the context of water management. Mechanical filtration will be discussed, followed by biologic filtration, chemical filtration, sterilization, and gas exchange.

Origin of Filtration Methods

Most of the artificial filtration methods used in aquariums and zoos were adopted from the municipal waste water, drinking water, and swimming pool industries. There is very little in the regard to filtration design and methods that have been developed specifically for aquatic life support on a large scale. Closed recirculating systems are those that are completely dependent on human manipulations to condition the water and sustain life. The heart of most of our filtration systems is natural biologic (bacterial) filtration.

Because there are missing links in artificial systems compared with natural ones, some supplemental mechanical and chemical filtration is also necessary. At our institution, some but not all natural filtration processes in our systems are used, and an incomplete cycling of nutrients results. This causes the accumulation of byproducts from these processes, such as nitrates, phosphates, and some insoluble organics. It is our goal to find ways to complete these natural cycles as much as possible to recycle or eliminate these accumulates from our systems.

Mechanical Filtration

Mechanical filtration is divided into large, medium, fine, and dissolved materials. Large particulates are usually removed via a prefilter, which involves screens such as basket screens in skimmers during surface skimming or large rotating drum screens for the most challenging systems. Medium-sized material is removed by pressure sand filtration or canister filters. Sand filters may use various grades (decreasing in size) of gravel and sand are referred to as mixed bed filters. Water flows through the filter, with large to smaller particulates being removed. Water pressure builds as material gathers in the filters. When the filters reach a predetermined saturation, determined by water pressure, they are back-flushed and the filtrate is flushed to sanitary sewer outlets. Canister filters use the same principle but use pleated filters.

Flocculation is a method of using chemicals to bind particulates to make them bigger and easier to remove. It focuses on dissolved organic carbon (DOC). Common flocculants used in freshwater include aluminum sulfate (alum) and natural and synthetic polymers (cationic polyelectrolytes). These act by reducing surface charges on dissolved or suspended particles, thus allowing them to collide and coagulate. They reach their peak effectiveness at pH < 7.5. The true effects on fish and invertebrate species is not fully known and should be used with caution especially when other techniques are available.

Dissolved organics may also be removed using activated carbon, which has specialized binding sites. As with mechanical filtration, carbon may become saturated and bacteria may use it as a growth surface, which will decrease its efficiency.

Foam Fractionation

A safer alternative is foam fractionation, in which particles are brought together mechanically by causing flow conditions that promote particle collisions. Mucopolysaccharides produced by bacteria and algae stick together, along with other substances, as they collide. Foam fractionation may be accomplished by slowly pushing water (salinity ≥15 ppt) down a column. As this is done, air is injected at the bottom of the column, creating a steady upward stream of very small bubbles. Organic molecules (including the mucopolysaccharides) often have hydrophilic and hydrophobic parts, causing them to collect at air-water interfaces. As they come into contact with the air bubbles, these molecules will collect and form an organic film on the surface of the bubbles. When the bubbles reach the surface, the film stays intact for a period of time, which creates foam. The resulting foam is then skimmed and flushed to a drain, thus removing the organic waste from the system.

Biologic Filtration

In natural systems, it is the role of plants to acquire nutrients continually from the water to build their tissues. It is the role of bacteria to break down organics rapidly enough to supply plants with these needed nutrients. As a result, without human interference, nutrients are typically not accumulated in the water of natural systems.

In artificial systems, plant growth is discouraged. Unnaturally large populations of relatively few species of bacteria (a fraction of the natural biodiversity in nature) are encouraged to convert organic waste products and nitrogenous toxins (ammonia and nitrite) to less toxic waste products, such as nitrate, phosphate, carbon dioxide, and biologically inert organic compounds, which cause the yellowing of aquarium water. These bacteria also compete with the aquarium’s fish and invertebrate population for oxygen.

This makes biologic filters very sensitive and fickle to environmental changes. Filter bacteria are simple cells that are not capable of carrying out the complex internal digestive and excretory processes of higher organisms. In the presence of much accumulated organic matter, the bacterial environment becomes acidic, high in CO₂, methane, and ammonia and low in dissolved oxygen (DO), with many bacterial enzymes for organic breakdown. Bacteria use complex organic foods, but have simple nutritional needs (e.g., simple sugars; small amounts of vitamins, amino acids, and micronutrients). Only a small portion of the nitrogen, phosphorus, and sulfur in these foods is used. (The majority of) these become organic wastes and tend to accumulate in the aquarium and become toxic or stressful to other aquatic organisms.¹

The whole organic cycle is usually represented to the veterinarian in only partial form. It consists of the aerobic conversion of ammonia to nitrate and usually does not consider other organic processes, such as denitrification, which is usually incorporated in plants in natural systems. Organic biodegradation is an aerobic process in which reactions catalyzed by heterotrophic bacteria convert organic waste to inorganic chemical byproducts and reduced organics. Nitrification is an aerobic two-step process in which catalyzed reactions by highly specific genera of nitrifying bacteria convert NH₄⁺ to NO₃⁻². Denitrification is an anaerobic two-step process whereby reactions catalyzed by specific bacteria reduce NO₃⁻² and release N₂ as gas from the system. For further details of the denitrification process, see Chapter 25.

Biologic Treatment Trouble Shooting

A large mass of bacteria is needed for biologic treatment of closed recirculated systems. They are living cells containing enzymes that catalyze a wide range of reactions needed for cell maintenance and reproduction. An environment is needed that is conducive to accumulating the cells that can perform the desired reactions. The actual species that establish a system and primarily carry out the desired biologic reactions will vary, based on the environmental conditions provided by the system and the species’ competitive edge (e.g., presence of light usually inhibits nitrifying bacteria growth).

Bacterial species have sensitivities to their environment (e.g., sensitivities to extremes in pH, temperature, and/or toxic substances). The space provided for their growth will have to cater to their specific needs, with some degree of stability and protection. This filtration component, which houses the primary bacterial population, is usually known as a bioreactor or biofilter. Inoculating a new system with nitrifying bacteria off the shelf will probably have mixed results because different species will inhabit different systems, depending on the environment, and therefore will not initially result in the complete cycling of the filter. Drastic environmental changes in a system, like switching from freshwater to salt water, will result in the complete reestablishment of the biologic filter. Cells need three types of basic materials—an electron donor, an electron acceptor, and nutrients. The most common limiting factor to biologic filtration is the DO (electron acceptor) concentration in the water. The high rate of biologic filtration used in these systems reduces the amount of dissolved oxygen available to the animals and should not be underestimated. Electron donors such as NH₄⁺, Fe²⁺, Mn²⁺, NO²⁻, H₂, H₂S, and HS⁻ are found in many organic compounds and may limit microbial physiology when restricted. The electron transfer from donor to acceptor is called the energy reaction and is the mechanism whereby cells gain energy. If limitations of electron acceptors or electron donors are present, better biologic filtration will not be accomplished by adding another biologic filter to a system, and a new system will not complete the establishment of adequate bacteria.

Because our waste compounds serve as critical electron donors and/or electron acceptors for cells, the cells will become dependent for life on a consistent supply. Irregular supply or a change in supply feed rate may have major implications on cell population size in the bioreactor. Other nutrients, along with O and H, provide the building blocks for the bacterial cells; the major nutrients are C, N, P, and S. Other critical elements include Ca, Fe, Mn, and Mo. Some of the common limiting nutrients that provide many of these elements are complexes of bicarbonate and phosphate, which need to be monitored closely.

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