Jet aerators use the ejector method of contacting gases and liquids. The jet consists of a double nozzle arrangement. Each jet has a primary nozzle, an intermediate high shear mixing chamber, and an outer secondary nozzle.
Mixed liquor, recirculated from the tank, moves through the primary inner nozzle where it becomes a high velocity, low pressure stream.
Low pressure gas enters the mixing chamber perpendicularly to the high velocity liquid stream. Intense contacting and mixing of the two streams occurs in the high shear mixing chamber. The intimate contact between gas and liquid streams results in the formation of micron-size bubbles.
The fine bubble gas/liquid mist jets out through the secondary nozzle into the main tank volume. The high velocity plume from the secondary nozzle travels horizontally, spreading throughout the tank volume before riding to the surface.
The horizontal travel of the plume maintains high pressure conditions for a longer 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 transfer efficiency.
Injection of the plume into the tank volume provides all the necessary mixing energy. The expanding plume’s powerful movement creates fine eddy currents in the surrounding liquid, thoroughly mixing the tank and keeping the MLSS solids in suspension.
High velocity gradients form within the tank volume and enhance mixing and solids suspension. The gradients are of a random nature, insuring the uniformity of the aerated liquid and the thorough suspension of solids.
When the initial horizontal momentum of the plume dissipates, it rises to the top of the liquid surface. This produces an air lift effect that further mixes the tank contents.
The jet plume also creates good molecular dispersion, sending oxygen molecules to microorganisms much faster. Such intimate mixing of reactants assures effective process operation.
Mixing Systems' submerged jet aeration and jet mixing systems are easier and less expensive to install than comparable diffused aeration systems. All equipment is supplied in prefabricated sections that are easily assembled using field joints or flanged connections. In addition, the jet aeration system contains no in-basin moving parts. All mechanical assemblies such as pumps and blowers are placed outside the tanks where they are easy to service. Combined with an optional pneumatic backflush designed to eliminate clogging, the systems are virtually maintenance free.
Aeration is often considered to be the heart of the effluent treatment system at most wastewater treatment plants. The correct selection of an aeration system is crucial because the operation of the aeration system requires about 80% of the total operating power cost of a waste water treatment plant.
Mixing Systems, Inc. jet aeration systems provide both environmentally conscious and cost effective operation. Built of quality components with an established reputation for reliability, Mixing Systems' jet aeration systems have shown energy reductions of up to forty percent over other aeration methods. In addition, during periods of low service demand, air flow rates can be reduced by controlling and varying the blower output. By regulating the air flow to the aeration system, oxygen transfer rates can be controlled without affecting the mixing efficiency or solids suspension and additional energy savings are achieved.
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.