Aeration and Water Circulation
There are three basic types of circulation available for your pond. The first form of circulation in general use is a floating fountain.
As we've mentioned proper water circulation is critical to an optimized pond ecosystem. So far was have explained how your medium sized pond (½ to 5 acres) has a very different ecosystem from the lake you visited last summer, or from the Koi pond in the back yard. These ponds are unique and are built to add aesthetic appeal to their setting. Regrettably, they often double as water retention basins for street and landscape run-off. This run-off adds nutrients, dirt and other contaminants that disrupt the ponds ecosystem by increasing the nutrient load. Without proper circulation these beautiful reflections ponds will turn into green, dirty, smelly adversaries. The residents or patrons that were intended to enjoy the pond are now repelled by it, all the while making vague comments about the health department. What these lakes need is a consistent treatment program, and proper circulation to evenly distribute the water chemistry, water temperature, oxygen and other gasses.
These fountains will move a great deal of water and boast wonderful turn over rates. The drawback, as the diagram above shows, is that these fountains provide a very small area of circulation, leaving the remainder of the lake un-circulated. These fountains will add substantial amounts of oxygen and facilitate transfer of gasses, but only near the surface of the lake.
Remember that the area that most needs circulation and oxygen is the bottom of the water column near the benthic zone. These fountains can be fairly expensive to install with each unit costing $3000.00 or more. Large ponds may need more than one fountain to even begin to help the circulation in the lake. In the end, these fountains will help with the circulation while adding a new water feature that may or may not add to the aesthetic appeal of the water feature. They will not provide adequate circulation to completely mix the water in the pond.
The second type of circulation is set up with a water pump with a water intake near the lake bottom. The pump moves water from the intake and discharges into a waterfall or fountain in the lake. As the above diagrams shows this only provides limited circulation in a channel through the lake with dead areas outside the area of water flow. With the proper pump size, this arrangement will provide an acceptable water turn over rate but the entire lake is not mixed and there is very little transfer of oxygen or other gasses. This system is designed for aesthetic appeal and not necessarily with the ecosystem in mind.
If you must rely on either of these systems it would be best to consider using both. By combining the water circulation provided by the pump system and the transfer of oxygen and other gasses provided by the fountains you can achieve a reasonable circulation system.
The third choice in circulation is aeration and is actually the most simple and basic. An air compressor is installed on the lake edge with air lines extending to the lake bottom. Tiny bubbles of air float to the surface transferring needed oxygen into the water throughout the water column. This flow also carries water and creates a permanent circular circulation pattern. It is also important to note that oxygen transfer actually occurs from air bubbles in the water. This is why the other two types do not transfer as much oxygen; they do not actually create bubbles as much as simple water flow.
Aeration, on the other hand, is all about bubbles. The circulation is produced by bubbles rather than creating bubbles as a side effect of water movement. In short, aeration is better, more economical and more efficient than the other systems. The fountains are more easily installed but aeration is worth the extra effort. One more point to consider is the actual diffusers used to create the bubbles. Smaller bubbles carry more water with them and transfer more oxygen. A very large bubble from a central location will not provide as much oxygen transfer but it will create some degree of surface movement by creating ripples around the discharge area. Some people feel this large bubble looks better in the lake because of the added surface action. In the end, smaller, more diffused bubbles actually do more for your pond. When you purchase an aeration system, check the specks and consider the cost. An aeration system can be very simple and inexpensive and still do an excellent job on circulation.
Basically it is much less expensive to move air in your pond than to move water. Aeration, will not of course, add any kind of water feature above the surface of the pond, but you can add a fountain or waterfall for their aesthetic appeal while letting the aerators do the real work.
When you look at these choices, you may want to use more than one option. You should consider each choice in assessing the total circulation in your pond. Both water circulation and oxygen transfer are important to your pond. Very often, a system will be set up with aeration on the pond bottom and either a waterfall or a floating fountain to add a water display above the surface, along with additional water turnover.
The things you need to consider when installing any system are cost to install, cost to run and maintain, the total area that will be circulated in the lake and the turnover rate. With good aeration and overall circulation, a turnover rate of once every three days will usually be adequate. Dollar for dollar you can't beat aeration for overall effectiveness.
If you're building a new pond, it's not difficult to build in a nice circulation system using water circulation and/or aeration diffusers on the lake bottom. All this will be installed as you build the pond, which makes things easy. So, if you are building a pond, do not cut costs now by leaving out a proper circulation system. Your pond needs circulation to establish an ecosystem so you need the circulation equipment installed. It is much easier to do it right in the beginning than to try to add more later. However, most of you probably are not building a new pond. You likely have a pond that has been around a while, hasn't had a lot of care and has a few maintenances challenges. That's why you're reading this paper.
Let's look at the setup for each of these choices. Each of these systems will need a power source, breakers and time clock. The floating fountains all come with a pre-built electrical panel including breakers, motor overload, time clock and gfi. Just add power. The water pump or air compressor will need these items installed as part of the set up. The floating fountains are easy to install. Everything comes in one installation kit. You just float the unit into the pond with a special submersible power cord attached, set anchor weights or tie lines to hold it in place, set up the electrical panel, attach the power cord to the panel, turn on the power and watch your new water feature go. This is why you see these fountains in so many artificial ponds, they're easy. These fountains are fairly maintenance free. The units come with at least a one year warrantee with some up to three years. You can expect the motor to run at least three years under normal conditions. The cost of these units can be $3,000.00 each or more for a two-horsepower unit. The problem is that all this water movement is on the lake surface and the bottom of the lake is left un-circulated.
Installing a circulation system using a water pump has two choices; an above ground pump or a submersible pump. The two diagrams above show these options. You still need the electrical panel, time clock and breakers for each of these but then these choices diverge in the installation process.
The submersible pump is simple to install. There is a broad range of choices on pump type you can use but these fountains are usually set up using a ½ or ¾ hp pump. Larger submersible pumps get very expensive, so most people don't chose to use them in this manner. This doesn't move a lot of water volume so the fountain really is more of a visual effect rather than viable water circulation. The pump sits near the lake bottom where it pumps water up a PVC pipe and discharges in a single stream or through a fountain head. The head can be a threaded PVC cap with ½ “holes drilled in it to create the spray effect shown in the diagram.
Note that these fountain heads can clog up and require cleaning, adding a new maintenance item to the pond-keepers duties. These submersible pumps will often pick up leaves and small fish and shoot them up the pipe to the fountain head where they lodge and require removal. That is why you need a threaded fountain head, don't glue the head or you won't be able to clean it. You'll probably need to dive into the lake to install these pumps. Don't set the pump right on the lake bottom because it will suck up bottom sediment and could clog up. The pump should be set inside a container then put on the pond bottom. A plastic 55 gallon drum cut off at about 2 feet works well. Put the pump inside the cut off barrel, include a cinder block for additional weight and set them on the bottom. Now the water intake is coming from 2 feet off the bottom since the water has to go over the edge of the barrel. Another choice is to simply build a platform on the lake bottom, with a few cinder blocks for the pump to sit on. Just as long as the pump is not sitting directly in the bottom sediment you'll be okay.
Installing a circulation system using an above-ground pump can be very difficult after the pond is already built and filled with water. It's easy to see why. You need to install an intake pipe near the bottom of the lake. Multiple intake pipes in different locations are better to spread the water circulation pattern. Next you need to decide where the water will discharge. Do you want a waterfall, or a fountain like we described above, or a small stream, or perhaps just an underwater jet. Again multiple discharges in different locations will increase the circulation pattern. All this is a lot of plumbing to install underwater in an existing pond. The highlight of this setup is that the intake and discharge need to be as far from each other as possible to create circulation throughout the pond. Remember the water will flow in a channel between the intake and discharge; the areas outside the channel will still be dead areas. This system can be set up with as large a pump as is needed to produce adequate water turn over. Two to five horsepower pumps are common on medium sized ponds. These pumps are much less expensive than submersibles of equal horsepower but the cost differential is offset by all the plumbing needed with this system.
The bottom line is that the water circulation does not always add thorough circulation to the system. The water will move in channels, and the transfer of oxygen and other gasses is limited. Remember that gasses are actually transferred by air bubbles in the water, not simply by water movement. After you understand your treatment options and set up a maintenance program, you must provide the needed circulation to make your program work. Without proper circulation even the best treatment choices will have limited success.
