| Keep an equalization pond well-mixed and aerobic. By keeping the solids and water thoroughly mixed, the solids move on to the plant instead of settling to the bottom. |
Long distance circulators are different from any other reservoir equipment in that the adjustable intake takes advantage of the manner in which water forms thin horizontal layers in ponds, and allows a precise horizontal cross-section of water to be circulated throughout the entire pond footprint. This circulated zone can be indexed to the top of the pond, such as a setting to circulate just the top X feet of the pond, or the intake hose can be set to circulate the entire depth of the pond regardless of how full the pond is.
This article discusses how this unique characteristic allows long distance mixers to control odors in two types of wastewater treatment ponds, equalization ponds and anaerobic ponds.
Equalization Ponds
An equalization pond in front of a wastewater treatment plant acts as a shock absorber by temporarily holding excess inflow during rain events until the water can go through the treatment plant.
For example, an equalization basin may be designed to hold eight million gallons of water when full at eight feet deep, but normally be operated with two million gallons in the pond at a depth of three to four feet, with one million gallons per day entering the pond and going through to the treatment plant. During a heavy rain, the equalization pond may quickly fill to the depth of eight feet and then, a few days later, be back down to three feet.
In this type of pond, without thorough mixing, the organic solids entering the pond will tend to settle to the bottom and, over time, the anaerobic digestive process at the bottom can create ongoing odor problems. But by keeping the solids and water thoroughly mixed, the solids move on to the plant instead of settling to the bottom, and the detention time of both the water and the solids is too short for the anaerobic process to create an odor problem.
In these ponds, the unique design of a long distance circulator, set with the intake hose all the way to the bottom of the pond, will automatically allow full mixing of the pond as it goes through depth changes, without any adjustment being needed and without damaging the bottom of the pond in any way.
Anaerobic Ponds
Some wastewater treatment ponds, such as waste sludge storage ponds in activated sludge systems, are purposely designed for anaerobic digestion and thus produce sulfides and odors continuously throughout most of the pond depth. To eliminate odors emanating from these ponds, operators can maintain an oxygenated layer of water at the surface of the pond, sometimes called an "odor cap." When sulfide gas bubbles rise toward the surface of the pond, they are instantly oxidized to non-odorous sulfate as they pass through the oxygenated odor cap.
A thin odor cap at the top of the pond, even 1 inch thick, is all that is required to neutralize sulfide odors. But a thin odor cap can be disturbed by wind, which would then let odors escape, so usually it is best to have a one- to three-foot thick odor cap.
Floating circulators are ideal for creating an odor cap on top of the pond, because their intake hoses can be set for a shallow water depth, usually 1-3 feet, and because one machine will circulate to the edge of the pond in all directions. Consequently only one machine is usually needed per pond. Circulators maintain the oxygen in the odor cap through two mechanisms, (a) capture of photosynthetic oxygen during the daylight hours, and (b) surface re-aeration during the nighttime hours. Since the machine is not bringing up any BOD loading from deeper water, because there is no turbulence, usually there is no problem in maintaining oxygen in the odor cap 24 hours per day.
