By John M. Overby
Summary: Unlike most water treatment applications, aquariums need to preserve the many living organisms existing in the water. Over the years, ozone has become a viable alternative to producing desired results. Some of the reasons are given here.
Ozone has been widely used for drinking water and wastewater applications for many years and has gained acceptance in the water industry as an alternative to chlorine. For some drinking water facilities, an ozone system can be produced with minimal problems. A wastewater ozone project can be designed as well with careful testing and planning. Aquarium water treatment requires both aspects of water treatment—drinking water and wastewater.
Water circulating around an aquarium facility is recycled through a series of treatment steps. These not only treat the influent wastewater but disinfect and clean the water prior to sending it back to the facility. The typical aquarium facility doesn’t have the luxury of using a flow-through system of municipal clean water. The typical aquarium facility can’t fill and dump the water based on its quality. If the waste was removed continuously via a flow to sewer, fresh water would have to be always flowing, causing great expense. Given that water in a typical aquarium facility is turned over every one-to-four hours, the amount of wastewater dumped down the drain would be astronomical. The aquarium facility tries to recycle the water through a life support system (LSS) with a 100 percent recovery rate. This total reuse of water requires careful attention to the water treatment equipment.
The life support system is a crucial element of all animal habitats. The LSS controls and balances the total water chemistry of a system and maintains its viability to sustain life for the fish and marine mammals. To do this, the LSS is designed to receive water (salt or fresh) from the aquarium facility through a series of surface skimmers and bottom drains, and deliver it to the LSS to transform the wastewater into pure water. The LSS for a multi-species marine system has to be designed to perform a number of functions. These functions are water flow dynamics, particulate waste removal (filtration—mechanical and chemical/biological), disinfection and temperature control.
Water flow dynamics
The water turnover rate of a typical aquarium facility varies depending on treatment objectives (primarily water quality). At 1-4 hours, this means that in a 100,000 gallon facility the water would get pumped at a rate of 1,666 gallons per minute (gpm). This is a constant rate that needs to be running 24 hours a day, 365 days a year. Energy cost to transport this volume of water is a critical concern and is factored into the design of a system.
The introduction of filtered system water into the exhibit(s) area is done in a manner that will ensure proper flow dynamics and is a very important part of the architectural design of the aquarium. It’s crucial that the marine life’s waste products are swept away from the system into the skimmers and bottom drains, and the return water is properly distributed back to the exhibit.
Mechanical filtration in the LSS is designed to primarily remove particulate matter; sand pressure filters are most commonly used. These consist of closed vessel(s) containing a quantity of sand held above a supporting underdrain assembly. Water is normally pumped into the vessel passing through the sand, which strains and retains particulate matter (to about 20 microns). As the sand becomes clogged, an increase in pressure across the filter bed can be noted due to the build-up of particles. When the manufacturer’s prescribed level (pressure drop) for cleaning the filter bed has been reached, the filter flow is reversed and the dirt and water used for the cleaning operation are diverted to waste or collection for backwash reclaim. Once the water washing the filter bed appears to run clean, the system flow is returned to normal and continues to filter the pool’s water.
In addition to removing particles, other water constituents need to be treated. Some of the main contaminants are ammonia, dissolved organic compounds and dissolved gases (CO2, O2, etc). Biological filtration takes care of ammonia (NH3) or nitrogen dioxide (NO2) and converts it into less toxic nitrogenous compounds such as nitrate (NO3) or nitrogen gas (N2). This is accomplished through the chemical action of bacteria living on a suitable substrate such as a gravel bed or plastic packing media. Dissolved organic compounds are typically removed with a carbon filter, ozone injection or a foam fractionator. The foam fractionator is a method in which foam is created by gas injection into the water while the foam carries away the waste.
Disinfection of LSS is generally found at the end of a filtration system to improve the quality of water. Ozone is typically introduced into a 10-15 percent side stream of the filtration water. The side stream percentage is determined based on the ozone-desired dosage and equipment of choice. The disinfected ozone side stream is carefully monitored and returned back to the filtration flow.
Ozone is utilized as the primary disinfectant in multi-species marine and freshwater systems. Ozone, with its high oxidation-reduction potential, is the most effective and efficient disinfectant that can be employed. Dissolved ozone inactivates cell membranes of a wide range of bacteria, viruses and protozoa. Its highly reactive nature guarantees that it’s short-lived and consumed within the treatment process. Deaeration equipment ensures that no harmful residual ozone is sent back to the systems and controls saturation of the gas in the water. This is how aquatic systems can allow the coexistence of mammals and fish in the same water. Without an active residual in the water, the health of the fish is maintained while the bacteria (from animal waste) is controlled.
Lightning storm in a can
Flowing clean, dry air or oxygen through high voltage corona discharge chambers produces ozone. Most high output ozone generators are equipped with enriched oxygen source gas equipment to increase the concentration of the ozone gas. Ozone gas is injected into the side stream through a venturi, which operates under negative pressure and dissolves the ozone in the water stream via the high surface area bubbles that are created in the venturi’s orifice. The gas and liquid mixing is done under pressure to ensure the high ozone dissolution.
In salt water systems there will be a significant amount of bromide ion present. Bromide reacts very quickly with ozone to form hypobromous acid and bromine by-products. This residual, if it’s high enough, could have a significant toxic effect on the fish. Careful design needs to be implemented to ensure against this. System design can prevent mortality by carefully monitoring ozone addition and by providing carbon filtration for removal of the potentially harmful bromine by-products. Water quality monitoring determines when the activated carbon has become exhausted and needs to be replaced.
The temperature of the water may need to be regulated and controlled according to season. Chillers keep the water from becoming too warm during the summer months, while heaters can warm the water in the winter. Aquarists and life support operators control the temperature settings according to a yearly schedule that closely replicates temperatures in the natural environment.
The treatment of a water system in an aquarium environment is no easy task. The LSS needs to not only treat incoming wastewater but needs to produce a high quality disinfected water for return to the facility (24 hours a day). Careful system design and monitoring is therefore required.
Ozone evolution in aquarium water treatment
Ozone for aquarium water treatment use was very limited in the ’60s and early ’70s. In the early ’70s, consulting engineers remained skeptical about its use—although based on European success—the attitude changed and ozone began gaining popularity. The main stumbling block was in limited knowledge of ozone applications and equipment durability. The time was right for market growth and the big problems were in equipment, materials of construction, and the general knowledge in design.
During the mid and late ’70s, there were many advances made in air preparation equipment, construction materials as well as understanding reactor kinetics of ozone for water treatment. This was a big breakthrough for ozonator manufacturers in that they could offer an ozone system rather than just an ozonator, and leaving the rest up to the design of the consulting engineers.
In the early ’80s, ozone came to fruition as the reagent of choice for maintaining healthy fish communities and vastly improving the water quality. Ozone systems required large footprint space and height requirements (typically 20 feet) while utilizing much of the limited valuable land.
In the early ’90s, the general population became conscious of the sharp decline of species and prompted the zoological parks and aquariums to move ahead. They had a good funding base and people were willing to pay the bill via gate receipts. Ozone manufacturers improved the design of their equipment and produced ozone through a lower cost per unit ($/lb) production. Ozone contacting was refined and allowed much higher ozone utilization for a given water treatment objective.
The late ’90s produced ozone equipment refinement and improvement in production capacity, cost and reliability. The ozone contacting kinetics was fine-tuned and provided 95 percent ozone utilization. The system space and height requirements were minimized and more valuable land was available to be used more effectively. The compactness and effectiveness of the conventional ozone system requires much higher safety monitoring than was previously required.
The knowledge of highly trained LSS design engineers must be utilized in any facility design or modification. Organizations such as the American Zoological Society offer accreditation for facility LSS operation and organizations such as the Aquatic Animal Life Support Organization (AALSO) offer an excellent exchange of information. Knowledge and communication are imperative to secure the future for ozone in the aquarium market.
Today, equipment is designed to produce ozone with concentrations of six-to-10 times greater ozone capacity than 10 years ago with ensured reliability. The use of ozone equipment in the aquarium market has advanced markedly in performance, reliability and capability for successful gas/liquid contacting. Still, the next five years will be a continued learning curve for the aquarium users of ozone systems. The more potent ozone gas production combined with oxygen feed gas requires important design reviews and constant monitoring of new and retrofitted systems. The ozone generation system design of years past is no longer valid and new criteria need to be established and closely monitored.
About the author
John M. Overby, of Phoenix-based Ozone Water Systems Inc., has been involved in the ozone industry for the past 10 years. He holds a bachelor’s degree in chemical engineering from Arizona State University. Overby can be reached at (480) 421-2400 or email: email@example.com