Calcium reactor

Calcium Reactor

In marine and reef aquariums, a calcium reactor is a device used to create a balance of alkalinity in the system. An acidic solution is produced by injecting carbon dioxide into a chamber with salt water and calcium rich media. The carbon dioxide lowers the pH by producing a solution high in carbonic acid, and dissolving the calcium media. The effluent is then returned to the reef aquarium where the calcium is consumed by organisms, primarily corals when building skeletons. A calcium reactor is one of the most efficient methods of supplying calcium to a reef aquarium. Although more commonly used with marine reef aquariums, calcium reactors are beginning to find use in elaborate freshwater and brackish aquariums where some species of freshwater clams, corals, and other invertebrates may need a constant supply of calcium.

Reaction

The objective is to dissolve the calcium laden media to provide both bicarbonates HCO₃^- (alkalinity) and calcium (Ca⁺⁺) ions at the sames rate as consumed during calcification. Effectively dissolving the media requires an acidic pH. Saltwater typically has a pH of at least 7.8, so to reduce the pH carbon dioxide (CO₂) is used. The general reaction formula is:

CaCO₃ + H₂O + CO₂ <——> Ca²⁺ + 2HCO₃^-

Design and operation

CO₂ Chamber

Inside the reaction chamber, a calcium rich media (aragonite), mainly CaCO₃, is forced into contact with water injected with carbon dioxide (CO₂) in order to create carbonic acid(H₂CO₃). This increases the solubility of the calcium carbonate. The reaction frees the calcium and carbonate, supplying the aquarium with water rich in Ca²⁺ and CO₃^2-, important for maintaining both alkalinity and calcium levels.

Bubble Counter

The bubble counter is used to measure the rate of carbon dioxide introduced to the system. The flow rate of carbon dioxide must be monitored so that the maximum amount of gas dissolves into the solution with a minimum left unconsumed. A needle valve or solenoid valve is used to regulate the CO₂ bubble rate. Valves with precise adjustment abilities will improve bubble control characteristics.

Feed Pump

The feed pump controls the volume of water exchange in the system. This is important because a high rate of water flow into the reactor reduces it's efficiency, thus resulting in underproduction and a waste of CO₂.

A number of methods exist to supply the reactor with water from the tank. Some reactors siphon water into the input of the reactor's re-circulation pump. A potential complication is the medium in the reactor becoming compacted, increasing back pressure onto the pump and reducing water into the reactor. Placing a gate or needle valve on the reactor's outlet side will improve flow characteristics compared to control from the inlet side.

Alternately, peristaltic pumps are effective operating against pressure, capable of supplying a precisely adjustable and continuous flow over a variety of flow rates with minimal maintenance.

pH Control

The pH control is connected to a probe in the reactor and adjusts the rate at which the calcium media dissolves. This probe constantly monitors the pH level within the calcium reactor. The pH range for the typical calcium reactor is 6.5 - 6.8. When the pH rises above a certain level, a valve opens, allowing carbon dioxide to enter the reactor. The control also closes the valve as the pH falls below this level.

Many pH controllers also incorporate an interface for an air pump. This air pump is usually connected to an airstone located within the sump or main tank. If the probe detects a pH level below the set range, the pump will activate and the resulting bubbles raise the pH by helping to dissipate the CO₂ gas.

See also

• Aquarium
• Protein skimmer
• Filter

References

• Reefkeeping.com article on calcium reactors
• Georgia Tech information on calcium reactor