WO1997000123A1

WO1997000123A1 - Dual aeration and cooling system

Info

Publication number: WO1997000123A1
Application number: PCT/US1996/009928
Authority: WO
Original assignee: Clean-Flo Laboratories, Inc.
Priority date: 1995-06-19
Filing date: 1996-06-19
Publication date: 1997-01-03

Abstract

The total energy required to treat wastewater in lagoons, ponds, etc., is less when surface aerators and bottom-located water circulators are used concurrently than when either the aerators or circulators are used alone.

Description

DUAL AERATION AND COOLING SYSTEM

Background of the Invention

It is common to treat wastewater by oxygenating it, using either surface aerators, which spray the water into the air, or paddle wheels, brushes, or drums, all of which mix water and air with various degrees of efficiency. If the water is too hot to meet discharge requirements (e.g., where a power company wants to dis¬ charge cooling water into a river), such aeration also helps cool it. One alternative to a surface aerator is a stationary or floating as- pirator, which forces air into the water. Another alternative is a porous device that diffuses bubbles of air into the water. A variety of devices for transferring oxygen to water are described in Metcalf & Eddy, Inc., Wastewater Engineering, 3d edition, McGraw Hill, Inc., New York, NY 1991, e.g., in Chapters 6 and 10. In standard test tanks, surface aerators typically transfer about 2.1 to 2.4 lbs. of oxygen per horsepower-hour (1.3 to 1.4 kg oxygen per kilowatt-hour) hen the same surface aerators are placed in sewage treatment lagoons, they circulate water in their immediate area, the oxygen transfer efficiency decreasing to about 1.0 - 1.2 lbs./hp-hr (0.6 to 0.7 kg per kW-hr.), the remainder of the lagoon remaining stagnant and anaerobic. Although an in¬ sufficient degree of aeration or cooling is frequently obtained, and although the procedure is energy-intensive, the only way heretofore recognized for improving the situation has been to install addition- al aerators, still further increasing the energy consumption.

It is known to position diffusers on the bottom of eu- trophic lakes, etc., air being pumped to the diffusers from a com¬ pressor located on shore. The diffusers are perforated, permitting air bubbles to rise, oxygenating the water to a limited extent and helping circulate the water; see, e.g., U.S. Patent No.5, 340,508. When diffusers are so designed that bubbles are released in streamline (as opposed to turbulent) flow, a eutrophic lake can be efficiently and effectively restored to usable condition as water at the bottom of the lake is moved to the top, where it is oxygenated by contact with the atmosphere. (Diffusers employing this principle of streamline flow are available from Clean-Flo Laboratories, Inc., located in Plymouth, Minnesota.) Diffusers require the same amount of power as surface-located aerators to achieve the same degree of oxygenation. Diffusers have rarely if ever been used in conjunction with surface aerators or aspirators, and then only to provide an auxiliary means of oxygen transfer. It is believed that diffusers have never been used for the purpose of either reducing the number of surface aerators so as to reduce energy consumption or increasing the efficiency of the surface aerators. Further, to the best of applicant's knowledge and belief, diffusers releasing bubbles in streamline flow have never been used in conjunction with sur¬ face aerators for any purpose.

Brief Summary

The present invention provides a means for reducing energy consumption and increasing cooling and cleaning efficiency in oxygenating waste water ponds and sewage lagoons or cooling lagoons. The invention relies on a combination of (1) surface-lo¬ cated aerators or aspirators and (2) water circulators or destratifi- cation systems, the latter terms including propellers but pref-er- ably referring to bottom-located diffusers, especially diffusers that release bubbles in streamline flow. The water circulators transport waste water from the bottom of the pond or lagoon to the surface, returning the surface water to the bottom, and the aerators or as¬ pirators are thus able to oxygenate a far greater volume of waste water and to cool the entire volume of waste water more efficiently. When used in this manner, the water circulators do not contribute any significant oxygenation of the wastewater; as previously indi- cated, to do so would consume as much energy as that used by the surface aerator. Applicant has found that the total energy require¬ ments for this combination are reduced significantly below the en¬ ergy requirements predicted by established design criteria, some- times as much as 50% or more. While seemingly a simple combi¬ nation of known equipment, it is believed that it has never been previously utilized to reduce the total amount of energy required for oxygenation, as it surely would have if the significance of the combination had been recognized. Hundreds of thousands of waste treatment plants are currently operating at perhaps half of their potential efficiency and will benefit greatly from the present invention.

Brief Description of the Drawing

Understanding of the invention will be enhanced by reference to the accompanying somewhat stylized drawing, showing a waste water pond employing a variety of surface aerators and as - pirators in combination with a plurality of bottom-located air dif¬ fusers that release bubbles in streamline flow.

Detailed Description

Referring to the drawing, waste water treatment lagoon

10 contains waste water 30, which has exposed surface 1 1 and bot¬ tom 12. Located at surface 11, typically attached to mounts or floats (neither shown) are surface aerator 13. aspirator 14, foun¬ tain aerator 15, and paddle wheel aerator 16. (It is, of course, ex- tremely unlikely that more than one type of aerator or aspirator would be used on a given pond at the same time.) Systems em¬ ploying equipment of this general type are in widespread use.

To the equipment described in the preceding para¬ graph, the present invention adds bottom-positioned air diffusers 17, shore-based air compressor 18 supplying air to diffusers 17 through lines 19. Diffusers 17 exude bubbles 20, which rise through waste water 30, carrying water from bottom 12 toward the aerators or aspirators at surface 11 and thus increasing the volume of waste water 30 available for aeration or cooling. When bottom-positioned air diffusers are added to the conventionally employed surface-positioned aerators or aspirators, cooling of heated water is greatly increased and the rate of oxygenation of waste water is also increased. Many commercial diffusers exude bubbles that carry water in turbulent flow, but if diffusers 17 are constructed in a manner such that bubbles 20 carry waste water 30 in streamline flow, as shown by arrows 31, waste water 30 is more efficiently rolled over and mixed.

In practicing the invention, the following procedure may be followed in designing aeration equipment. First, the num- ber of surface aerators required for aerating the lagoon is deter¬ mined, using the standard published oxygen transfer rate for the particular type of aerator in waste water. Approximately half of this recommended number of surface aerators is used in practicing this invention. Next the retention time of the lagoon is calculated by dividing its volume by its flow rate. The number of bottom dif¬ fusers or other type of destratification devices required to destratify the lagoon 5 to 10 times during one retention time period is then installed. The number of diffusers required is on the order of 10 - 25% of the number that would be required to oxygenate the lagoon if no aerators were employed. Following this procedure will reduce the amount of energy consumption resulting from using only sur¬ face aerators by 30 to almost 50%. After the system is installed, it may be found that either some of the surface aerators or some of the destratification systems can be turned off, still meeting the waste water discharge requirements while obtaining even greater efficiency. Exampi 1

A power company located in the eastern United States wishes to discharge 500,000 gallons (about 1,900,000 liters) of cooling water per minute into a river. Unfortunately, however, the water is 8° F. (about 4.4° C.) above the temperature permitted by the Environmental Protection Agency (EPA), which would fine the power company $30,000 per month, or $360,000 per year. To avoid payment of the fine, the power company does not discharge the water into the existing 11,000,000-gallon (about 40,000,000-liter) canal running directly to the river, with a transit time of about 22 minutes. Instead, the water is pumped through a large pipe to a distant location where the river empties into a bay, which provides a sufficiently large heat sink that the EPA does not object to the elevated water temperature. Unfortunately, however, this requires a 4000-hp (about 3,000-kW) pump. In accordance with the inven¬ tion, there could be installed in the canal, 140 10-hp (about 7.5- kW) fountains and 140 3-HP (about 2.2-kW) bottom air diffusers under the fountains. During the 22 minutes required for the water to travel the length of the canal, the temperature would be lowered to an acceptable level, and the power consumption reduced over 2500 hp (4000 -1400 - 52 = 2548 hp), or about 1900 kW, represent¬ ing a saving of more than 63% in power consumption.

Example 2

The discharge from a tomato paste factory was stored in a pond in which surface aerators were installed. The water was black and had an extremely offensive odor. Within 24 hours after bottom air diffusers were installed, the water had turned brown and all odor had vanished. Example 3

A well-known beverage company utilized five 25-hp (about 19 kW) surface aspirator -type aerators to meet its dissolved oxygen and biochemical oxygen demand discharge at times, but often failed to meet the requirements. When twenty 0.375-hp (0.285 kW) streamline flow bottom-based diffusers were placed under the surface aerators, the discharge requirements were met and exceeded. When three of the surface aerators were turned off, the discharge requirements were still exceeded at all times. Energy consumption was reduced from 125 hp (93 kW) to 57.5 hp (43 kW), a saving of 54%.

Tlie principles of this invention can also be applied in aquiculture ponds, where a fish farmer may install four 15-hp (about 11.2-kW) paddle wheels to aerate the surface water. Be¬ cause there is essentially no oxygen below the upper one or two feet (30-60 cm) ofwater, fish are compelled to live near the surface. Fish kills resulting from oxygen depletion are common, where the farmer may lose up to a million dollars at a time. Two gases that are highly toxic to fish, ammonia and hydrogen sulfide, are present in high amounts because of the anaerobic subsurface water, fur- ther contributing to fish kills. Since most pathogenic bacteria are either anaerobic or capable of living in low oxygen conditions, pathogenic bacterial outbreaks are common in aquiculture. A still further problem arising from using only surface aerators in fish ponds is the growth of cyanophyceae (blue-green algae), which gives the fish a bad taste. By placing bottom-positioned air dif¬ fusers in aquiculture ponds, at least half of the paddle wheel aera¬ tors can be turned off, and all of the above problems eliminated.

Claims

l. A body of water having an exposed upper surface and a bottom, with aerators located at the surface and water circu- lators positioned on the bottom .whereby water at the bottom is moved to the surface, the same degree of oxygenation and cooling being accomplished with less total energy consumption than if only surface aerators were utilized.

2. The combination of claim 1 wherein the water circu- lators are air diffusers, provided with air from an on-shore com- pressor.

3. The combination of claim 2, wherein the air diffuser releases bubbles in streamline flow.

4. The combination of claim 3 wherein the body of water is a sewage treatment lagoon.

5. The combination of claim 3 wherein the body of water is the heated water discharged from a cooling system.

6. The combination of claim 3 wherein the body of water is an aquiculture pond.

7. An improved method of oxygenating and cooling a body of water comprising installing both surface aerators and bot- tom-based air diffusers, whereby water is moved upward from the bottom by the diffusers and discharged into the air by the aerators, the energy requirement being significantly less than where either surface aerators or air diffusers are used alone.

8. The method of claim 7 wherein the diffusers release air bubbles in streamline flow.

9. An improved method of oxygenating or cooling a body of water having surface aerators already in place, comprising the steps of adjusting the number of surface aerators to approxi- mately half of the number indicated by published design standards and adding bottom-based air diffusers in a number also equal to approximately half of the number indicated by design standards, whereby circulation throughout the body of water is greatly im- proved, and the total energy required by the aerators and diffusers is significantly less than that required by the either the aerators or the diffusers alone when the number indicated by published design standards is employed.

10. The method of claim 9 wherein the diffusers release air bubbles in streamline flow.

11. The method of claim 10 wherein the number of surface aerators used is approximately one-half the normal design requirement, and the number of destratification systems used is enough to invert the water 5 to 10 times during one retention peri- od