Aeration system and method with tapered nozzle

An improved system for mixing gas with waste water in which the water is pumped through a plurality of mixing chambers into which the gas is injected at a step region to form parallel streams of gas and water. An extending chamber contains the parallel streams as the interface between them becomes unstable, breaks down creating vortices and produces tiny bubbles which mix with the water. The extending chamber is divided into two sections with at least the section remote from the step surface tapered inwardly at a rate of 11.degree.-22.degree. to permit operation at higher air flow rates without blowing the bubble forming vortices out of the chamber.

The invention relates to an improved method, and submerged system for 
efficiently mixing gas with waste water. 
Industrial waste, sewage and the like are commonly purified by pumping the 
liquid into a large pond, tank or basin where a bacteria population 
consumes the inorganic and organic material. Because the dissolved oxygen 
in the waste water is usually insufficient to support the required 
population of bacteria, the water must be aerated. This can be done with a 
surface aerating machine which has beaters extending into the waste water 
from above the water surface to agitate the water and incorporate air. 
Alternatively, air can be diffused through the bottom of the basin, for 
example, through a porous medium. Surface aerators are not efficient and 
cause certain mechanical problems. The energy loss of diffusing air is 
also great and a diffused system is not suitable for installation in an 
existing pond. 
The waste water can also be aerated by pumping through submerged tubes with 
Venturi openings through which air is drawn or pumped into the tubes to 
create turbulent mixing. 
The present invention relates to an improved system for mixing a gas such 
as oxygen or air with waste water in a body. A plurality of mixing 
chambers are disposed below the surface of the waste water and the water 
in the body is pumped from an inlet to an outlet. A suitable gas, such as 
oxygen or air containing oxygen, is injected into each of the mixing 
chambers at a step region to form parallel streams of air and water in an 
extending chamber. As the two streams move down the extending chamber, the 
interface between the two streams becomes unstable and waves form which 
attach to the sides of the chamber. This causes large frictional stresses, 
creating tiny bubbles which mix with the water. Since the water and air 
essentially flow in the same direction, no energy is wasted in turbulence 
and the system is energy efficient. Systems of this type are described and 
claimed in co-pending application Ser. No. 598,871, filed July 24, 1975, 
now abandoned. 
According to an improved aspect of this system, the extending chamber is 
inwardly tapered in the downstream direction to ensure that the vortices 
created by the mixing do not extend out of the chamber which would reduce 
the efficiency of the mixing. Thus, the device operates at a high and 
constant efficiency over a wide range of air flow rates. Further, helical 
vanes (not shown) can be provided in the injection passages for the gas to 
create greater wave generating conditions which extend the operating range 
of the device to greater air flow rates. 
The extending chamber is divided into a first section extending from the 
step region and a second inwardly tapered section extending downwardly 
from the first section. This second section is shorter than the first and 
tapered at a rate greater than any taper of the first section. It is 
desired that both sections be tapered, but if only the second section is 
tapered the results are satisfactory, and the cost of manufacture is less 
when machining techniques rather than molding techniques are used. 
However, when molding in plastic, tapering of the first section aids in 
removal from the mold. The first section is preferably non-diverging. The 
length of the first section along the flow direction is preferably between 
one and ten times the diameter at the step region and the length of the 
second section along the flow direction is preferably between one-eighth 
and one times the diameter at the intersection of the sections. The taper 
of the second section is preferably between 11.degree. and 22.degree.. 
This system can be quickly and easily installed in any existing aeration 
pond or tank without the need for the system to be shut down for an 
extended period and without the need for the pond or tank to be drained, a 
project which is difficult or impossible to accomplish in most instances. 
The system can, in fact, be installed and operating within a few minutes. 
In comparison with diffused air type devices and surface aeration systems, 
the energy required to incorporate a given amount of oxygen into the water 
is much less. Because little energy is wasted in turbulent mixing, the 
present invention is more energy efficient than are Venturi, jet or 
impingement type systems which depend on turbulent mixing. Further, the 
bubbles which are produced are tiny, thus creating a good environment for 
effective use of oxygen by the bacteria within the pond or basin. Many of 
the other disadvantages of surface aerators and diffusion type devices are 
also avoided. 
Other objects and purposes of the invention will be clear from the 
following detailed description of the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS: 
Reference is now made to FIGS. 1 and 2 which illustrate a preferred 
embodiment of the invention. This embodiment is disclosed and claimed in a 
co-pending application entitled WASTE TREATMENT APATUS Dkt. 36, Ser. 
No. 953,215 filed Oct. 20, 1978. Floating platform 20 is adjustably 
suspended from aeration device 22, in a body of waste water. Aeration 
device 22 is comprised of a plurality of jet aerators 24, for example, 
twenty-six jet aerators radially extending outward from a water manifold 
26 having a dome upper surface with an access port 36. The dome surface 
withstands the upward pressure of the water and supports the heavy pump 
above it. A bottom bracket 38 allows device 22 to sit on the bottom of the 
body if desired. Access port 36 allows epoxy coating the interior surfaces 
of the dome during fabrication and removal of tools and debris after 
welding. 
Water is pumped radially outward through an internal passage in each of 
these jet aerators. Air from a second manifold 28 disposed below manifold 
26 and separated therefrom by a suitable partition of diaphragm (not 
shown) is injected into each of the nozzles to form parallel water and air 
streams. The interface between these two streams in the nozzle passage 
becomes unstable, creating vortices and forming tiny bubbles which are 
intimately and efficiently mixed with the water being pumped through the 
passage. The operation of this type of jet aerator and detailed structure 
thereof are further described in U.S. Patent application Ser. No. 863,588, 
filed Dec. 22, 1977, now U.S. Pat. No. 4,157,304, and Ser. No. 863,587, 
filed Dec. 22, 1977, now U.S. Pat. No. 4,152,259, both the invention of 
the present applicant and in Ser. No. 598,871, filed July 24, 1975, now 
abandoned, the co-invention of the present applicant and a second 
inventor. The disclosure of each of these applications is hereby 
incorporated by reference into the present application. 
Water is supplied to the manifold by a conventional submersible pump 29, 
for example, a 14 horsepower submersible pump, via a neck portion conduit 
30. Screen 37 covers the pump inlet and filters debris in the body. 
Electric line 39 powers pump 29. Air is supplied to the manifold 28 via 
conduit 32 from a source of pressurized air shown schematically as source 
34 and located above the surface of the water, for example, on land. 
Platform 20 is designed and includes a number of features which make the 
platform a stable, desirable, and effective support for suspending an 
aeration device. Base 40 provides the positive buoyancy required to 
support the aeration device 22. Base 40 is preferably closed on the top, 
bottom and all sides and contains conventional foam material 41, part of 
which can be seen in the partially broken away part of FIG. 1. Since the 
bottom is closed, the waste water cannot break down foam material 41. Any 
suitable foam can be employed. Safety rails 43 extend about the top 
surface of base 40. Base 40 can be made of any suitable material, and 
fibreglass over a metal frame has been found to be particularly 
satisfactory. 
Conduit 32 extends through the interior of hollow base 40 between 
peripheral surface and the central opening and is a structural part of 
base 40. Conduit 32 is fibreglass sealed where it enters and leaves the 
interior of base 40, and is preferably arranged so that its center line is 
located at the water line. Conduit 32 connects to flexible hose 33 which 
in turn connects to manifold 28. Thus, the conduit 32 exerts no force on 
the platform, and it functions like an outrigger to increase the stability 
of the platform. 
Base 40 is provided with a rectangular central opening 42 exposing the 
waste water surface to the atmosphere. This opening is desirable to 
prevent rolling and pitching of the platform during operation and 
particularly during back-flush operation in which considerable flow of 
water and entrained air in an upward directly occurs. In addition, the 
platform is preferably dimensioned so that its smallest dimension is at 
least equal to the nozzle to nozzle dimension of the aeration unit. In 
order to provide stability, the cross-sectional area of opening 42 must be 
as large as the intake area of the pump. A platform so dimensioned 
provides an effective aeration pattern since some of the aerated stream is 
trapped under the floating platform as small bubbles to cap and increase 
residence time and absorption. However, if the aeration unit is too small, 
then large pockets of air tend to form underneath the platform, reducing 
absorption and decreasing efficiency. It has been found that this is 
avoided if the distance a from the outlets of the most separated nozzles 
is at least about equal to or greater than the smallest horizontal 
platform dimensions b. The distance a can be slightly smaller than b, 
perhaps a few feet smaller, when higher jet velocities are used. In the 
illustrated embodiment the distance a is considerably larger than distance 
b. 
Aeration unit 22 is connected to the platform by four stabilizing bars 44, 
46, 48, and 50 which are welded to the dome upper surface of manifold 26 
and can be readily adjusted on the platform to move the aeration unit in a 
vertical direction. Bars 44, 46, 48 and 50 prevent rotation rolling and 
pitching. At least two bars are needed and four are preferred. Winch 52 is 
suspended from an A-frame 54 made of aluminum and connected to the dome 26 
by capable 55 at four separated locations which are joined above the 
center of gravity of the submerged unit as shown. The A-frame retrieval 
winch 52 allows aeration device 22 to be raised high enough for 
maintenance. Thus, to move the aeration unit up and down it is only 
necessary to use the winch. Guy wires 56, 58, 60 and 62 attach floating 
platform 20 to solid supports for wind stabilization (not shown). 
FIG. 3 illustrates one device 24 in detail. Waste water flows from the 
inlet through passage 100 into the extending chamber 102. At the entry of 
passage 100 into chamber 102, a step region 104 is provided which includes 
gas passages. The passages inject gas at an angle between roughly 
11.degree. and 221/2.degree.. 
Thus, two parallel streams of gas and waste water are created as shown in 
FIG. 3. As the streams move along the chamber 102, the friction between 
them causes waves to form and air thus trapped in the waves to disperse 
into tiny bubbles. Since the air and gas streams move in the same 
direction, effective mixing is achieved at minimum energy consumption. 
The extending chamber is divided into first section 110 and second section 
112. Section 112 is tapered between 11.degree. and 22.degree., whereas in 
the illustrated embodiment, section 110 is a straight cylinder. The length 
x of the first section is between one and ten times the distance c and the 
length y of the second section, between one-eighth and one times the 
distance d. If the length y is less than about one-eighth, circulating 
eddies are created which decrease efficiency. As the length y increases 
greater than about one times, efficiency decreases. If the length x is 
less than one, then the vortices tend to blow out of the tube. If the 
length x is greater than ten, coalescence results and efficiency declines. 
Many changes and modifications in the above-described embodiments can be 
carried out without departing from the scope of the present invention, 
that scope being intended to be limited only by the scope of the appended 
claims.