The technology of jet aeration and jet mixing involves combining two fluid streams in a common mixing chamber. One stream is typically a liquid and the second stream is usually a gas. With jet mixing the second stream is another liquid, often entrained from the tank itself. The basic components of the jet aeration or jet mixing system are:
- Dual concentric jet nozzle.
- Piping for the two fluid streams.
- Pumps to create fine bubbles and circulate the tank.
- Blowers, for jet aeration systems, to supply the pressurized air flow.
One reason jet aeration is more effective than other methods of aeration is because the system utilizes multiple oxygen transfer zones.
- High contact zone within the nozzles: Oxygen transfer begins when a stream of recirculated liquid from the inner nozzle comes in contact with a stream of pressurized gas, creating an intense mixing action in the chamber between the inner and outer nozzle. The intimate contact between the gas and liquid streams results in micron-size bubbles.
- High pressure zone at the tank bottom: Next, oxygen transfer continues as a plume of fine bubbles, from 0.1 to 1 mm in diameter, is ejected horizontally through the outer nozzle into the main tank volume. The horizontal travel of the plume maintains the gas/liquid transfer interface for a much longer period of time than conventional diffused air systems. This high pressure condition gives the gas bubbles a greater opportunity to dissolve in the liquid, increasing the oxygen absorption efficiency. In addition, injection of the plume into the tank volume thoroughly mixes the tank and keeps the solids in suspension.
- Buoyancy bubble rise: As the momentum of the horizontal plume dissipates, the bubbles, now 1 to 3 mm in diameter, rise to the surface creating an air lift effect allowing for further molecular dispersion and absorption of oxygen.
- Surface agitation: To further enhance oxygen transfer, the liquid momentum near the tank surface draws the air bubbles from the tank surface and redistributes it through the tank volume. Since all aeration and mixing occurs below the surface, there is no mist or spray problems. There are also no icing problems because the jet aerators eliminate thermal stratification and prevent freezing.
Jet aeration systems yield a higher alpha factor than fine pore membrane and ceramic type aeration systems. Because of the high shear within the jet nozzles, jet aerators produce a high surface renewal at the gas/liquid interface.
Most wastewaters have surfactants present. The surfactants create a resistance to oxygen transfer at the gas/liquid interface. The film thickness is the smallest (lowest resistance) with high shear aeration devices such as jet aerators and surface aerators.
In actual wastewater treatment plants, due to the high alpha factor achieved by the jet aerators, a lower design standard oxygen is required than with fine pore diffused aeration systems. This is one of the benefits of using jet aerators in wastewater generated from pulp and paper, pharmaceutical, chemical and brewery industries. These wastewaters normally have surfactants present in them and therefore jet aerators are very efficient. The total energy consumed for treatment of these wastewaters is 20% to 30% lower with jet aeration systems than with fine pore diffused aeration systems.