New Aerator for Wastewater Treatment

The Plunging Jet Aerator

used in

Wastewater Treatment

By: Brian James and Sung Lee

Introduction

This presentation was created by two Chemical Engineering majors for the course: Introduction to Biochemical Engineering. Wastewater treatment is very large industry throughout the world, and even though the basic technology is well known, significant improvements are still being made today.

The most common method of wastewater treatment uses an activated sludge process. The activated sludge process is a biochemical process in which aerobic bacteria consume the organic pollutants in wastewater. Since the bacteria are aerobic, their efficiency of consumption is very dependent upon the amount of available oxygen dissolved in the liquid sludge.

The air required for maximum production from a biochemical process can be in excess of one reactor volume of air per minute, so an improvement in aeration efficiency can significantly reduce the costs of operation. Plunging jet aerators provide a very effective and inexpensive method of dissolving air into the aeration tank for bacteria growth. We will discuss different variations of geometry and flowrate, and their effect on the efficiency of the plunging jet aerator.

Activated Sludge Process

This method of wastewater treatment involves three major steps. The primary treatment stage consists of a simple separation between dense sludge-which is sent to an incinerator-and the remaining effluent which then undergoes secondary treatment. Secondary treatment is where the biochemical consumption of organic material takes place. The microorganisms feast on the biomass in the aeration vessel. This is where extensive aeration is needed for the bacteria to consume the organic wastes. Tertiary treatment can be simple or extensive depending on the extent of pollution and the local requirements for water purity. Its purpose is to remove inorganic pollutants as well as any organic mass not removed by the primary and secondary stages.

Aeration

To achieve adequate aeration , many biochemical processes use spargers or air-lift fermenters, which pump air into the bottom of the reactor. Both methods tend rely in part on the turbulence from the bubbles to provide mixing. In wastewater treatment, however, the reactor media is very viscous due to the heavy sludge of organic matter. A better approach to these more common techniques is a plunging jet aerator.



Principle of Plunging Jet Aeration

The phenomenon of plunging jet aeration occurs quite often in nature. A waterfall is an excellent example. The "jet" of water falling vertically toward the surface of the water carries air inside the stream as well as along the sides. When the falling stream reaches the fluid surface, it's momentum carries small air bubbles with it into the reactor medium. The momentum of the liquid stream can be sufficient to carry small bubbles completely to the bottom of the vessel.

Aeration Reaction Vessel


Gas Entraiment

A stream of liquid falling toward a level surface of that liquid will pull the surrounding air along with it. It will indent the surface of the liquid to form a trumpet-like shape. If the velocity of the stream is great enough, air bubbles will be pulled down into the liquid. This happens for two main reasons:
1. Air that is trapped between the edge of the falling stream and the trumpet-shaped surface profile and is carried below the surface.
2. The turbulence/surface instabilities of the falling stream become mixed with air in edy type currents and are carried far below the surface.

Plunging Jet Stream

One can observe the phenomenon of minimum entrainment velocity by watching a kitchen faucet. When the water faucet is turned on "low", the falling liquid appears clear, and can be observed to decrease in diameter as it falls. This is due to the velocity increase of the water as it "falls" from the faucet. When the faucet is turned on "high", the stream appears cloudy with bubbles, and increases in diameter as it falls.
The difference in how much air is carried with each stream can be seen if they are allowed to fall onto a calm surface of water. The slow stream will not create a significant amount of bubbles, but the quicker stream will carry bubbles far below the liquid surface.

Optimum Performance

The characteristics of the falling stream are determined in large part by the nozzle through which it leaves. For optimum performance of a plunging jet aerator, the factors of nozzle diameter and nozzle length must be taken into account. Since the ultimate goal of the jet aeration is the mass transport of oxygen into the sludge, the most effective bubbles will have a small diameter and a long residence time in the sludge. The maximum rates of mass transfer are achieved at higher velocities, with large diameter nozzles, and with any additional turbulence that can be added by the nozzle.

Conclusions

Plunging jet aeration provides a very simple, effective, and inexpensive way of dissolving air in sludge for wastewater treatment. It stems from the natural occurance of gas entrainment into a falling liquid stream. It operates by simply pumping the sludge in a recycle fashion through a carefully designed nozzle. Hopefully, this method can significantly reduce the operating costs of many wastewater treatment and other related processes.
RPI Biochemical Engineering: Student Projects
HTML by Brian James....jamesb3@rpi.edu