Degreasing aeration tank
If this small plant processes with purely domestic effluent, an initial degreaser is rarely necessary. In some cases though, more recent installations are equipped with a drum screen that removes some of the grit and grease. Without further info, I would look at the following:
If the persistent foam is actually mainly fat, then it could be that you have a restaurant or caterer whose kitchen is not equipped with the right sized trap or they are not emptying and maintaining it properly. In this case, the foam is often whitish to light brown. The first thing to do is investigate the upstream network where these fats will have left traces; don't use any miracle solutions s to remove grease in your tank just try to identify the source.
Foam that look greasy on the surface of a small aeration tank, with dark brown activated sludge and light brown foam, indicates you have an urgent situation that requires you to ascertain whether there is excessive sludge content. This phenomenon is even more pronounced if recirculation is insufficient, and greasy foams will also form on the clarifier. In this case, the only choice you have is to empty out the sludge.
And filamentous bacteria can—for various reasons such as an imbalance in the organic load—add to the problem and even create it. I recently experienced this on a larger plant whose discharges looked like pancake batter.
These are some of the avenues to explore as a first step.
Just to add to Darkwater's comments about a disruption in the biological balance, you might want to perform the V30 test to a) check that the decantation performance remains satisfactory and plan a sludge discharge, b) observe the behavior of a portion of the supernatant, and c) evaluate the composition of the floc.
Supernatants can become a breeding ground for filamentous bacteria (such as Nocardia) if the biological phase is operating under inadequate treatment conditions (mass loading, O2 control, recirculation management, etc.).
As explained by Darkwater, considering the size of the installation, your best bet is to start by investigating the source of the incoming fat and fine tuning your operation, before investing in equipment or taking remedial action that may not actually solve the problem (applying reagents or other miracle cures).
Thanks for your great answers.
I like the idea of a caterer or restaurant because I know there are three of them in our town.
An analysis will be carried out to check the presence of fat—and no miracle cures, understood!