I am not an aeration expert, and have no knowledge to add compared to the excellent advice our experts do give out on that topic.
However, I have "pipeline" experience, and there are several "best practices" for pipeline design that can significantly impact the efficiency of your aeration system.
1.) Water Build-up in the Pipeline
Consider your air pump early on a summer morning - pulling in air from near ground level. You have walked around many times and observed all of the dew on the grass. At that point your pump may be pulling in air with 70-90% relative humidity. That is a lot of water vapor going through your pump.
That air will be hot coming out of the air pump compressor so all of the water will be in the vapor phase. However, that air will cool significantly travelling down your air pipeline. (The soil temperature at depth at any location is close to the average temperature of your climate, so the ground will be "cool" relative to the summer air temperatures.)
Typically, the interior wall will start to "sweat" as the water vapor cools and condenses. These tiny droplets will continue to move along in the direction of the air flow. This is NOT a problem for a properly sloped pipeline.
However, this is a potentially large problem for a pipeline that contains any significant dips where the line goes down and then back up.
If there is a dip or sag in the pipeline, then those water droplets will move to that point and then begin to accumulate in the line. Over time, that entire dip will become filled with water! In a freezing climate and a pipeline not buried below the frost line, then that section of pipe will freeze. This is bad for two reasons. First, it will stop air flow to your pond diffusers. Secondly, your pump will be working at its highest possible pressure to try and clear the blockage. It usually will not be able to clear the blockage, so you will be creating severe wear on your air pump unless you have a high-pressure cut off switch.
Even if you are in a warm climate, the water-filled dip in the pipeline will steal efficiency from your aeration system. Typically, the pressure on the pump side will build up until it can push through the water and send the air on down the line. However, at typical aeration system flow rates and pressures, it does NOT push through at the full diameter of the pipeline. For example, in a 1" diameter pipeline it may be able to push through a stream of air only about a 1/4" thick at the upper boundary of the pipeline. For a twenty foot long dip, this is the equivalent of running a 20' section of 1/4" diameter pipe in the middle of your system. That always causes frictional pressure losses in the pipeline and a gauge at the pond end of the pipeline would now read a significantly lower pressure compared to the gauge at the air pump.
Solution #1: Always build your pipelines to have a gentle downward slope to the edge of the pond. You only need 1-2 degrees to keep those water droplets moving down to the diffusers so that they are blown out with the air.
If you have a gentle rise in the land at some point between your air pump station and your pond, then do NOT trench the whole route to be X inches below ground level. When you hit the rise in the ground level, trench that section deeper (relative to ground level) so that your pipeline run sustains a gentle downward slope.
Likewise, if you hit a soft spot while trenching or excavating, then check it with your longest level. If you did create a dip, then back-fill to ensure the proper slope through that section.
Solution #2: There are some situations that force you to have a dip in the line. Sometimes it may be a deep draw on your property, or it could even be due to man-made objects that cannot be moved. In that situation, pipelines use something called a blow-off valve to enable fluids to be removed from the dip in the line.
Basically a tee is installed in the pipeline at the very lowest point. A riser is then brought above the ground surface and a valve is installed. It works much better if a valve is also installed at the pond end of the pipeline. To operate, close the valve at the pond, and let the air pump pressure the whole pipeline up to the maximum "safe" pressure. You then throw the blow off valve wide open. This should create a big slug-flow of water, which should then turn to a mist as your hear the pipeline pressure drop to near 0 psi. Close the valve and repeat as many times as you are able to kick out fluid.
In summary, a gently downward slope on your pipeline is your best solution. However, a blow-off valve can help if you are forced to deviate from the optimal slope design.
