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Application maintain an oxygenated layer of water at the surface will circulate to the edge of the pond in all directions. of the pond, sometimes called an "odor cap." When Consequently, only one machine is usually needed per sulfide gas bubbles rise toward the surface of the pond. Circulators maintain the oxygen in the odor cap pond, they are instantly oxidized to nonodorous through two mechanisms, (a) capture of photosynthetic sulfate as they pass through the oxygenated odor cap. oxygen during the daylight hours, and (b) surface A thin odor cap at the top of the pond, even 1-inch re-aeration during the nighttime hours. Since the thick, is all that is required to neutralize sulfide machine is not bringing up any BOD loading from odors. But a thin odor cap deeper water, because can be disturbed by wind, there is no turbulence, which would then let odors usually there is no problem escape, so usually it is best in maintaining oxygen in Whatever the situation, to have a 1- to 3-feet thick the odor cap 24 hours per circulators can be a viable tool odor cap. day. in both aerobic and anaerobic Some floating aerators, Regardless of what type such as brush aerators, have of equipment is used to pond types. a shallow, mostly horizontal create and maintain an mixing effect, and these odor cap on an anaerobic are often the best type of pond, three practices are aerator to use to create an crucial to proper odor odor cap on an anaerobic pond. However, because control in an anaerobic pond: they generally send out a turbulent flow which reaches several feet deeper than desired, they can bring deep 1. Inflow to the pond should be kept below the BOD (biochemical oxygen demand) material up into odor cap so that the influent BOD does not use up the odor cap zone, which causes oxygen depletion. the oxygen in the odor cap. If the odor-control plan For that reason, and also because these aerators calls for a 3-foot-thick odor cap at the top of the pond, push water in only one direction, oftentimes several the inflow should be brought in horizontally at 4 feet machines are needed for each pond in order to supply deep or deeper. If an old pond is converted to sludge enough oxygen to maintain the odor cap, which leads storage and the horizontal inflow pipe is not deep to higher capital and operating costs. enough, then add a baffle or 45-degree elbow with As illustrated below, floating circulators are ideal a short pipe extension so that the inflow water does for creating an odor cap on top of the pond, because come in below the odor cap. And in ponds where the their intake hoses can be set for a shallow water influent water enters vertically at the bottom of the depth, usually 1 to 3 feet, and because one machine pond, place a deflector, similar in shape to a card table, over the influent pipe to Anaerobic Pond deflect incoming water into a horizontal pattern, instead of allowing it to shoot up to the surface of the pond. 2. Similarly, the effluent should also be drawn off the pond from below the odor cap. This prevents the oxygenated water that composes the odor cap from being drained off the pond. If an old pond is being converted to use for sludge storage and it has a skimming outlet pipe, devise an antiskimming baffle, or 45-degree elbow and short pipe extension, to allow water Only the top 2 or 3 feet of the pond are mixed, effectively "capping" the anaerobic waters below. 16 wateronline.com ■ Water Online The Magazine