Bringing Aquatic Life Indoors
Designing large aquarium systems takes a unique skill set that combines mechanical engineering with the principles of water chemistry and marine biology
Replicating natural, pristine aquatic environments from around the world is the design challenge of an aquarium’s life-support engineer. Of course, bringing all of this indoors-in an economical and sustainable fashion-is the primary duty.
To achieve these goals, an engineer must combine the principles of hydraulic, mechanical and plumbing design with an extensive knowledge of water chemistry and aquatic biology. This takes a mastery of many disparate disciplines, just as the aquariums themselves bring together a variety of needs: life, entertainment and operability.
In the past, an engineer with this background was an extremely rare fish, and the impetus for a life-support design specialty was not created until a powerful wave of aquarium building began in the early 1980s (See ‘A Short History of the Aquarium Boom,’ p. 30). At the outset, mechanical engineers and marine biologists had limited water-quality data to develop designs, and the responsibility for gathering this data fell upon a young and inexperienced aquarium industry. But over the last decade, consulting engineers and seasoned aquarium professionals have begun to collaborate; and with nearly 10 years of collaboration behind them, the fundamental principles of life-support design have never been more clearly defined.
Setting a standard
A sustainable life-support technology must rely upon the principles of water filtration. The three methods of filtration are biological , chemical and mechanical . When it comes to creating a healthy environment for marine fish, mammals and invertebrates, biological filtration is by far the most critical consideration. While chemical and mechanical filtration play important roles, they are primarily supplemental to and supportive of the biological process.
The best designs should not only result in the crystal-clear water that every visitor and curator wants to see, but also the stable and sustainable water quality that closely parallels a natural aquatic environment. Indicators of a stable environment can be identified by, but are not limited to, common water-quality criteria, including water temperature, dissolved oxygen, pH and dissolved organic carbon (DOC). While these are common water-quality indicators, the life-support engineer knows how the design approach will change when creating a system that must sustain animals as diverse as a delicate moon jellyfish and a 1,500-pound manatee.
Regardless of the size or species of animal, biological respiration within their aquatic habitat requires that emphasis be placed upon the biological filtration design. The reasons stem from what occurs at the macroscopic and microscopic level in a mechanical sand filter and on all substrates in a marine aquarium.
The science and biochemistry of this process are extremely complex, but there are two microbiological activities that are essential for good aquarium water quality: Managing them dictates the proper cultivation and management of heterotrophic and autotrophic bacteria .
Autotrophic organisms are important for the biological oxidation of harmful ammonia and nitrite in an aquarium exhibit. Because the output of these substances by animals in an aquatic exhibit is fairly constant, these substances are processed easily by stable autotrophic populations within mechanical sand filters and biological filters.
Heterotrophic bacteria are responsible for a significant portion of the processing of DOC and suspended organic carbon (SOC) from animal waste and food. Carbon will accumulate to harmful levels if a life-support design is not effective. Therefore, the uptake and control of carbon is critical in managing the biological oxygen demand.
Mechanical management of organic carbon
The management of DOC and SOC is not accomplished solely through effective design. Proper system operation also plays a key role.
For example, mechanical sand filters serve as a primary component in the control of carbon, as they trap suspended organic material in the recirculating filtration loop and promote water clarity. But in order to manage organic carbon properly, life-support system operation requires regular backwashing of these filters to remove the trapped material.
However, the interval between backwashings can often be days apart. During that time, the organic material trapped in the depths of a sand-filter bed hydrolyzes and is quickly utilized by heterotrophic bacteria that make up the biofilms that coat every grain of sand in the filter. This process generates additional biofilm formation in the filter, thus increasing the biological oxygen demand of the entire habitat. This, in turn, lowers oxygen concentrations and increases carbon dioxide levels. This process is compounded by the animals on display through their normal biological respiration.
Depressed dissolved oxygen concentrations and low pH are signature water-quality problems for aquatic exhibits that use mechanical sand filters as the primary component. Current designs, while understanding the importance of mechanical filtration, put more emphasis on the role of biological and chemical filtration to manage organic carbon and maintain good water quality.
Using ozone for chemical filtration
Ozone gas is widely used in municipal water and wastewater treatment facilities throughout the world, and it has become essential in the development of a modern life-support design. When ozone gas is generated and applied effectively, it acts as the primary ingredient dictating the performance of the chemical filtration for a life-support system and, ultimately, the effectiveness of the biological filter. There are two integral processes to life-support design where ozone is of vital importance. First, ozone gas is a very strong oxidizer, and as any wastewater engineer knows, it has been used for decades to treat potable water and wastewater. But because ozone is an effective oxidizing agent, it also can be used to disinfect exhibit water in custom-designed ozone-contact vessels .
These vessels isolate the contact of gas with water on a side-stream process from the main filtration loop (See Figure 2, p. 30). Ozone gas is injected into the water at applied doses that can range between 0.3 to 1.5 mg/liter. Contact times of 3 to 5 minutes will disinfect the water of bacteria and even deactivate the parasitic organisms that could be harmful to fishes and aquatic mammals. Water treated with these high doses of ozone must be subject to a stripping process to remove any residual ozone from the solution and any disinfection byproducts that might have been formed in the contact chamber. Only then can this treated water be safely returned to the main filtration loop.
In addition, these contact vessels will oxidize organic material, breaking them down to substances of lower molecular weight that are more readily biodegradable. At the same time, this powerful oxidizing effect produces water that is extremely low in turbidity, generating water visibility that can exceed 100 feet.
However, while the ozone contact process provides health and aesthetic benefits, the increase in concentration of biodegradable organic carbon that the process generates acts to enhance the growth of biofilms in the sand and biofilters. An alternative ozone-enhanced process can reverse this negative affect.
The second use of ozone gas involves foam fractionation or protein skimming -the most effective air-flotation process in seawater applications.
This is a relatively simple process that relies on the physical and chemical properties of coagulation, micro-flocculation and adsorption. Depending upon the expected biological loading of an aquatic exhibit, 30% to 100% of the water to be treated is directed to the skimming chamber, or column, and exposed to fine bubble diffusion. Organic substances contain hydrophilic and hydrophobic components that align themselves across the air/water interface across the surface of the micro-bubble. While the process is effective with air alone, ozone gas enhances the micro-flocculation of organic substances. As the bubbles rise in the vessel, they bring with them a collection of organic material, concentrated into foam that is easily extracted and washed from the system.
With an applied ozone dose of as little as 0.03 mg/liter and a retention time in the vessel of 60 to 90 seconds, protein skimmers can effectively remove a considerable percentage of organic material. Removal of organic waste will immediately benefit an exhibit’s water quality because it is no longer subject to biodegradation in the life-support system. Protein skimmers can therefore be described as ‘perpetually backwashing’ filters. As fractionation will remove suspended and dissolved material, life-support systems that are designed with protein skimmers as primary components should see a dramatic impact on the loading of sand filters.
The final step: biological filtration
In a final step that closes the life-support recirculation loop, all the water that has been processed by the mechanical sand filtration, heat exchanger, UV units, protein skimming and ozone contact vessels is processed through an atmospheric biological filter.
These large custom-designed vessels must meet the biological requirements of the aquatic habitat, as well as the hydraulic demands that are placed on them to distribute water to the exhibit.
Biologically, they serve as the final filter to control ammonia and nitrite ions, as well as a final point of DOC reduction. The specialized media that is contained in the structure of the filter is designed to distribute processed water evenly over its complex surfaces.
This serves as a site to balance all the gases that are critical to animal health. It is in these vessels that dissolved oxygen is brought to the proper levels of saturation specified by the water-quality design criteria.
The biological filter also serves to discharge undesirable levels of carbon dioxide that have accumulated through the respiration of the aquatic animals, as well as the biofilms that are growing on all the exhibit and plumbing surfaces, specifically contributed by the sand filters.
Finally, because fish are susceptible to an affliction similar to what scuba divers sometimes experience-the bends-nitrogen gas from ambient air that could enter the water and become supersaturated within the pressurized filtration loop must be purged from the water. This process can also be accomplished in the final stage of the biofiltration vessel.
Integrating the systems
Integrating the biological, chemical and mechanical filters can be challenging, as the structures and vessels that make up these complex systems include both atmospheric and pressurized processes. Solid hydraulic calculations and computer-controlled distribution systems ensure that the water is delivered with precision to each process and returned to the aquatic habitat in a condition that will sustain a long-term healthy environment.
A Short History of the Aquarium Boom
As recently as the mid-1980s, a request for qualifications for a life-support design engineer would have produced a limited response. In fact, such an engineering specialty did not even possess a name or professional designation. The situation changed in the latter half of that decade when a municipal interest in waterfront development allied with growing public interest in the environmental condition of the oceans and the diversity of aquatic species that inhabit them. These interests led to numerous public/private alliances that produced an unprecedented period of growth in aquarium design and construction, lasting nearly two decades to date. And that momentum has now established a beachhead in the 21st century.
The popularity of aquarium attractions has not been limited to only the United States. During these last 20 years, more than 30 facilities have been constructed in North America, Asia and Europe. In that same period, an equal number of zoos and aquatic theme parks have added aquarium exhibits to encourage attendance.
Estimating some totals
A census of these facilities would likely reveal that more than 85 million gallons of marine and freshwater exhibits have been designed and built to display a diverse collection of fishes, mammals and invertebrates. While a detailed accounting has never been performed, the cost of designing and constructing the life-support systems for all of these aquatic habitats would approach $500 million-approximately 8% to 15% of the total cost of design and construction for aquariums.
These costs are not only a reflection of the investment in the architecture, but also the actual engineering requirements associated with the species of animal selected for display and the necessary water-quality environment.
Controlling Water Temperature
Water temperature may be the single most important water-quality parameter that life-support engineers are challenged with. It is not unusual that design tolerances of less than 1°F are required to ensure a healthy environment for aquatic animals from arctic or even desert biomes. Building chilled-water supply is routinely applied to titanium-plate heat exchangers to provide temperature control.
Thus, when building chillers are specified by the M/E/P engineer, it is important that the consulting life-support team works closely with him so that the heating and cooling requirements of the animals are met.
About the authors
Aquarium projects of the 21st century are strengthened by life-support design teams that bring not only solid engineering qualifications, but also include life support or environmental specialists to compliment the team with backgrounds in aquarium operation, water chemistry and aquatic biology. Such a collaborative association exists between authors Michael Rozenblum of Syska & Hennessy Engineers and David LaBonne of International Design for the Environment Assoc., Inc. (IDEA).
Currently, Syska & Hennessy and IDEA, Inc. are working on the designs for aquarium expansion projects for the New England Aquarium in Boston, the National Aquarium in Baltimore and the Virginia Beach Marine Science Museum.
New aquarium construction continues as well. The Wonders of Wildlife Aquarium in Springfield, Mo., is scheduled to open later this fall and, in addition, Syska & Hennessy and IDEA, Inc. are collaborating on new designs for the Long Island Aquarium in New York and The New Bedford Oceanarium and the National Marine Life Center, both in Massachusetts.
All told, these expansions and new projects represent more than $310 million in new construction.