Ä HOMEPOWR ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ECHOMAIL011 Ä Msg : 1 of 9 + 2 From : Scott Parks 1:343/70 16 Sep 92 14:27:00 To : All Subj : Windpower Aerators 1/3 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Wind-driven aerator/mixers: another alternative for small communities? by Franklin R. Schutz, P.E. Research Assistant Professor West Virginia University April 1992 Small Flows Free energy... Why not harness the power of the wind to provide the oxygen required for wastewater treatment? Why invest in power lines and pay monthly electric bills if you don't have to? Wind-driven aerator/mixers, if practical, could provide small community wastewater systems with a low-cost alternative between an operator-responsive, electrically powered mechanical aeration system and a totally passive wind- and sun-powered facultative lagoon. According to manufacturers and suppliers, wind-driven aerator/mixers are not a substitute for electrically-driven mechanical aerators. They cannot normally provide the higher levels of energy needed to maintain the rates of oxygen transfer and solids suspension required in complete mix aerated lagoons, with their relatively high per acre organic loadings and relatively short detention times. However, in facultative lagoons, where relatively high rates of oxygen transfer and corresponding energy levels are not required, wind-driven aerator/mixers can, according to their manufacturers, enhance oxygen transfer, accelerate processing, and improve circulation. According to operators and engineers familiar with them, wind-driven aerator/mixers can be effective for the control of odor and surface matting of algae and also enhance pond circulation and solids distribution. They appear to be particularly effective in facultative lagoons that are at, near, or over their organic design capacity; suffer from poor circulation; or are subject to seasonal oxygen depletion caused by ice or solids cover. How they work Unlike conventional mechanical aerators, which enhance oxygen transfer to the lagoon through the creation of active gas/liquid interfaces (i.e., throwing the lagoon contents into the air or drawing ambient air down under the lagoon surface), wind-driven aerator/mixers increase the level of available oxygen within the lagoon by enhancing circulation. The typical wind-driven aerator/mixer consists of a turbine fan unit connected to an impeller housed in a three-foot long subsurface housing. As the wind blows, the turbine rotates and the impeller draws oxygen-poor wastewater from below the surface up through the impeller housing to the surface where it displaces and forces downward the oxygen-rich wastewater previously located at the surface. By the time the oxygen content of the former surface water, now at the bottom, is near depletion, it is once again drawn upward by the aerator/mixer, and the cycle continues. Because the rate of oxygen transfer at the pond surface is driven by the gradient or difference in partial oxygen pressures between the pond surface and the adjacent layer of ambient air, replacing the comparatively oxygen-rich surface water with oxygen-poor lower pond water increases this gradient and hence increases the rate of oxygen transfer. At the lagoon surface/ambient air interface, the rate of oxygen transfer is also improved by breaking up such gas exchange barriers as floating solids, scum layers, algae mats, and winter ice. Early development The concept of wind-driven aeration was explored by a number of inventors and innovators in the late 1970s, several of whom are still in the business. Most current wind-driven aerator/mixers are small units for use in irrigation ponds, recreational ponds, canals, and farm dugouts. Lake Aid Systems of Bismarck, North Dakota, is one manufacturer producing the larger units designed for use in wastewater treatment lagoons. According to Wayne Ruzicka, Lake Aid System's design assistance engineer, the company's various wind-driven aerator/mixers were developed by Richard Aide, the company's founder, in 1977 for the North Dakota Game and Fish Department. They were initially used as a means of increasing circulation in remote ponds and lakes to eliminate seasonal fish kills. Wind-driven aerator/mixers have been applied to drinking water storage reservoirs, irrigation ponds, and wastewater treatment lagoons. Along with standard electric model aerators, Lake Aid Systems currently manufactures two basic models of the wind-driven aerator/mixer: the Pond I, intended for use in low organically loaded irrigation ponds and tertiary treatment lagoons; and the Mark 3, a larger, industrial-grade unit intended for highly loaded primary cells of facultative wastewater treatment lagoons. According to Ruzicka, the cost of the smaller Pond I aerator is approximately $3,000 and the larger Mark 3, $12,000 to $15,000. >>> Continued to next message --- GEcho 1.00/beta * Origin: The Helix/Intentional Future/HST-DS/(206)783-6368 (1:343/70) Ä HOMEPOWR ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ECHOMAIL011 Ä Msg : 2 of 9 - 1 + 3 From : Scott Parks 1:343/70 16 Sep 92 14:27:00 To : All Subj : Windpower Aerators 2/3 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ >>> Continued from previous message Current uses According to Neil Whittey, chief executive officer of Lake Aid Systems, full- scale units have only been in production for the past three years. "There are now between 75 and 100 wastewater treatment lagoons using wind-driven aeration/mixing equipment," says Whittey. Some of these wind-driven units are equipped with auxiliary electric motors to keep them going during periods of low wind velocity or are activated by dissolved oxygen probes. The size of their installations, according to Whittey, varies from units as small as those serving roadside rest areas to systems as large as a four million gallons per day (mgd) facultative lagoon under construction in Louisiana. At present, the majority of these wind-driven units, according to Whittey, are being installed in the midwest, Mississippi, Louisiana, and Canada for wastewater treatment, while Great Britain is installing them for drinking water reservoirs. A totally wind-driven aerator/mixer, according to Ruzicka, can be installed at any open location without windbreaks, shelter belts, or significant wind protection that is exposed to a minimum wind speed of four miles per hour at least 75 percent of the time. According to Ruzicka, the aerator/mixer is designed like an airplane wing to dissipate energy and is unaffected by high winds during storms. In winds of up to 75 and 80 miles per hour the turbine reaches a maximum speed and will not turn any faster, he says. Historic wind speed and duration data are generally used to determine if a totally wind-driven aerator/mixer is feasible. This data is normally available from local offices of the National Weather Service. If wind data suggests concern, the electric motor backup is generally used, said Ruzicka. According to Whittey, because the newest models have been in production for only a few years and typical electric aerator design data does not readily apply when specifying aerator/mixers because of the unique way they operate and their multi-purpose design objective, there is no readily convertible or standardized oxygen transfer data on which to totally base design loadings or lagoon sizing. The average design for typical wastewater lagoons calls for one Mark 3 unit installed for every four to eight acres of lagoon surface area, he says. However, this will vary based on the specifics of each application. The lack of a specific per hour oxygen transfer rate for the wind-driven models (due to variations in wind speed and other conditions) does not, however, detract from their effective use, says Whittey. The best available basis of design are the results of previous installations during the past 12 years in a variety of waste applications, said Whittey. "Most facultative lagoon problems stem from poor mixing, uneven influent solids dispersion, surface solids matting, and poor photosynthetic oxygen production caused by excessive turbidity within the upper depths of the pond," says Whittey. "Wind-driven aerator/mixers can be used to reduce short circuiting, improve poor circulation, and eliminate dead areas. The use of a wind-driven aerator/mixer adds predictability and control to the operation of a facultative lagoon." Approximately half of Lake Aid System's wind-driven aerator/mixers have been installed to improve the performance of existing facultative wastewater lagoons, according to Whittey. The other half have been incorporated into the designs of new lagoons. There is no reason wind-driven aerator/mixers cannot be used in conjunction with conventional electrically-driven mechanical aerators to enhance circulation, Whittey says. In fact, several of their aerator/mixers have been installed in the same ponds as mechanical aerators, says Whittey. Although the capital cost of Lake Aid System's aerator/mixers are comparable to conventional electrically driven units, at a power cost of seven cents per kilowatt hour, one of their units can often pay for itself in about three years of wind-driven operation, according to Whittey. What they can and cannot do According to Whittey, the size of a new lagoon should not be decreased to an overloaded condition based on the installation of wind-driven aerator/mixers. In new designs, the units are used to insure the lagoon will work as designed, says Whittey. The wind-driven aerator/mixers allow the lagoon to resemble more closely the theoretical conditions upon which the design criteria are based (i.e., more uniform solids distribution throughout each portion of the cell). Aeration and mixing can reduce or ameliorate the effects of such common lagoon problems as oddly shaped cells with dead corners and poor circulation, influent solids deposition, and thermal stratification, he says. In retrofit applications, however, the use of wind-driven aerator/mixers has allowed lagoons that were significantly underdesigned or overloaded to operate normally with dischargeable quality effluent and reduced operation problems, says Whittey. "In a few actual installations, lagoons were only 50 percent of the normal design size," says Whittey. "Despite these impressive results, Lake Aid Systems still recommends sizing new lagoons at accepted design criteria if wind-driven aerator/mixers are to be used." >>> Continued to next message --- GEcho 1.00/beta * Origin: The Helix/Intentional Future/HST-DS/(206)783-6368 (1:343/70) Ä HOMEPOWR ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ECHOMAIL011 Ä Msg : 3 of 9 - 2 From : Scott Parks 1:343/70 16 Sep 92 14:27:00 To : All Subj : Windpower Aerators 3/3 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ >>> Continued from previous message The regulator's, engineer's, and owner's dilemma While the use of wind-driven aerator/mixers is a potentially intriguing and useful concept, how should the regulator, consulting engineer, and potential owner of the equipment approach them? Are the potential benefits of wind- driven aerator/mixers worth taking the risk of going out on a limb to permit, specify, and purchase an as yet unproven technology? Do its theoretical benefits justify the potential downside risks of poor performance and failure? Perhaps in a few more years, with additional operating experience, some experimental data, and a few studies on the subject, there will be more information available on how to utilize these wind-driven aerator/mixers to design better facultative lagoons. In the meantime, there's free energy out there just waiting to be put to work treating wastewater. If there are other manufacturers who produce wind-driven aerators/mixers specifically for wastewater treatment, we would appreciate hearing of them. Please send information to the Small Flows Editor. --- GEcho 1.00/beta * Origin: The Helix/Intentional Future/HST-DS/(206)783-6368 (1:343/70)