Conrad Kleinholz and George Luker

Two natural mechanisms add oxygen to water. The first is diffusion from the atmosphere. Second is uptake by water of the oxygen released by aquatic vegetation during photosynthesis. The dissolved oxygen (DO) content of pond water fluctuates on seasonal and daily cycles. The amount of oxygen dissolved in water is greatest when the water is cold, and least when hot. DO levels fluctuate seasonally due to water’s ability to dissolve oxygen. During cold weather, when the water temperature is less than 60^o Fahrenheit, DO levels are usually sufficient to maintain aquatic life. As the water temperature increases during spring, DO decreases. At the same time, the metabolic rate of organisms in the pond increases, as seen in the Figure below.

During late spring, when solar radiation heats the surface of the pond faster than the heat can be transferred to deeper water, the pond stratifies into a warm upper layer and a cooler lower layer. The upper layer remains fairly responsive to weather conditions, warming and cooling with ambient air temperature, while the lower layer slowly loses heat to the pond bottom, and may lose all oxygen during decay of organic material in pond sediments.

The warm, upper layer also supports all of the vegetation, whether phytoplankton or rooted plants. During daylight, aquatic plants release oxygen to the water during photosynthesis. This photosynthetic oxygen, together with oxygen that diffuses into the water from the atmosphere, sustains the fish and other aquatic organisms which live in the pond. Respiration by the total aquatic community continuously uses the dissolved oxygen for normal metabolic functions.

If photosynthesis is reduced or stopped due to overcast skies, a severe storm event, or the input of some toxic substance, the dissolved oxygen can be reduced to levels which no longer support healthy aquatic life. If a storm event causes a large or rapid inflow of runoff, the incoming water may mix the upper, aerobic layer and the lower, anaerobic layer. The mixing may cause total depletion of dissolved oxygen in the pond due to oxygen demand from the anaerobic portion of the pond.

The dissolved oxygen dynamics of a pond are further complicated if the pond is not regularly flushed with runoff, or if the pond receives excess nutrient input in the form of lawn, pasture, or feed lot runoff. Concentrations of waterfowl, particularly geese, can add large nutrient loads to ponds. And, feeding fish in the pond also adds nutrients. The resultant accumulation of living and dead materials can cause chronic or catastrophic low dissolved oxygen levels if photosynthesis is reduced, or the pond is suddenly mixed during warm weather.

Aeration can maintain satisfactory dissolved oxygen levels, and should reduce nutrient concentrations to safe levels. Aeration of the entire water column will increase respiration in the sediment surface, thus increasing the rate at which nitrogenous wastes are degraded and diffused back to the atmosphere. Phosphorus will be adsorbed by sediments at an increased rate, reducing its availability to phytoplankton. The phytoplankton populations are then reduced to safe levels. Finally, aeration of the entire water column prevents oxygen stratification and fish kills caused by oxygen depletion from sudden mixing of aerated and anaerobic layers of water.

Aerators mix oxygen-containing air with water. They either pump water into the air, or diffuse air into the water. This document addresses air diffusion aerators. They are less expensive to purchase and maintain than mechanical aerators, and when properly used, will keep dissolved oxygen at safe levels in ponds. Air diffusion delivers air beneath the pond surface. As the air rises through the water column, oxygen contained in the air bubbles diffuses into the surrounding water. Efficiency is increased by reducing air bubble size, or by increasing the water depth at which the air bubbles are released. Air blowers commonly deliver air through porous stones made from fused silica or alumina, membrane diffusers of EPDM, a synthetic rubber, or with air lifts. Air lifts are less efficient at oxygen transfer than air stones, but require much less maintenance and move much more water than either air stones or membrane diffusers.

There are three types of air pumps used to aerate ponds. Regenerative blowers are used to force air to depths less than 4 feet. Regenerative blower systems use floating air lines, and air lifts, suspended air stones or membrane diffusers to compensate for low air pressure. Oilless rotary vane compressors and rocking or linear diaphragm air pumps are used when it is desirable to keep air lines and air stones or membrane diffusers near the pond bottom. Oilless pumps are necessary to prevent contamination of the aeration system and pond with lubricating oil. Both systems use a weighted air line connected to one or more grids of stones or membranes. The airlines may be self-weighted, or be connected to individual weights, such as bricks or concrete blocks. The grids may be placed in water up to 20' deep with rotary vane pumps and up to 30' deep with diaphragm or rocking piston pumps.

All three types of air pumps need to be located in a building or weatherproof container. The air is conveyed to the pond bank in buried PVC pipe. Schedule 40 pressure pipe should be used for diaphragm pumps, but PVC cellular core DWV pipe will work for rotary vane pumps and regenerative blowers.

Cellular core PVC is approximately half the cost of pressure PVC. From the pond bank, the air goes into floating hose (layflat) for regenerative blowers, or polyethylene tubing for use with rotary vane or diaphragm pumps. The tubing can be purchased self-weighted, or can be submerged by attaching bricks or other weights along the length of the tubing to the air stone manifold(s).

Air stones and membrane diffusers become fouled with a film of small organisms or accumulation of precipitated minerals during use. Maintenance entails removing the air stones from their grid and cleaning them by soaking in muriatic acid or a concentrated beach or lye solution, then rinsing in tap water. Membrane diffusers are cleaned with a stiff-bristled brush. Air lifts can be used to replace air stones when a regenerative blower is used. Air lifts are less efficient oxygenators than stones or membranes, but do not usually require cleaning.

An air lift system is constructed using 2" Schedule 40 PVC pressure pipe. Drill a single 1/4" inlet 30" from one end of the pipe, and cut the pipe no longer than 12" less than the lowest normal water level at the proposed location for the air lift. Attach a 12" x 12" polystyrene or equivalent float to the upper end of the airlift. Use a 12" section of the remaining pipe, and attach it to the top of the airlift with a 90^o fitting.

Regenerative blowers should be used to aerate ponds less than 10' maximum depth. Both rotary vane and piston compressors are more efficient than regenerative blowers for destratifying and aerating deep ponds, since a single, high pressure air stone manifold can destratify deep ponds much better than an airlift system. Ponds 10 to 20' deep can be aerated with either diaphragm or rotary vane pumps, while those greater than 20' deep should be aerated with piston compressors.

The three aeration systems described here are compared in Table 1 below. These systems are adequate to aerate a one-acre pond less than 10' deep. The operating cost estimates were based on 365-day operation, with electrical cost of $0.11 per kilowatt hour. Continuous use was planned to maintain low nutrient levels once they have been attained, and because the aerators can also prevent portions or all of the pond from freezing during winter.

Slightly larger rotary vane and piston compressors than listed in Table 1 can also be used to aerate ponds up to 3 surface acres, and up to 40' deep. Thus, the cost per volume of water aerated can be much less than indicated in Table 1.

Table 1. Specifications and costs for three diffused air aeration systems for recreational fish ponds.

Each of these aeration systems is available from aquaculture suppliers in kit form, or you can purchase the components separately from several instate supply houses. More suppliers are available online by entering ‘pond aeration’ in a search engine. For additional information and current component lists, contact the Langston University Aquaculture Program at (405) 466-3836.

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