Uniformity of Air Distribution CFD Model at a Large WWTP

"Passive Air Distribution" is a phrase that you will hear if you hang around with diffuser manufacturers enough. There are other perks to spending time with us, but I'm going to try to keep this brief.

What it means is that, in an ideal aeration system, the piping, diffuser headlosses and installed elevations should be so perfect, that you can leave all valves open and get uniform air to each grid in the system. The main benefit of operating this way would be the reduction of blower pressure with the corresponding power savings.

So let's see how we did on one of our systems, at least theoretically by modeling the flow rate using computational fluid dynamics (CFD). Below are plan and section views of a wastewater treatment plant where SSI provided the aeration system in the Middle East. There are four blowers on the right side of the drawing, feeding yard and drop pipes in two aeration basins.

The modeled operating conditions were 16,040 Nm3/hr of air at a pressure of 628 mbar. Diffuser submergence was 5.25m and rhe airflow rate per 9" disc diffuser was 4.77 Nm3/hr, which represents the peak condition. We stop the simulation at the bottom of each submerged drop pipe, and the outputs we will get are mass flow rates, in kg/s at the bottom of each drop pipe.

Velocity from the blowers:

Contours as the air passes in the yard pipe from one tank to the next:

Boundary layer, at the neck-down:

Drop Pipe Labels in Tank 1:

Drop pipe labels in Tank 2:

Mass Flow Rate at each drop pipe:

Graphical display of uniformity at this operating condition:

It's difficult to conclude anything from a single theoretical model, but it looks like valve manufacturers still have plenty of job security.