Pipework and levels are perfect and everything is working as it should (in theory).
I increased the rate from 5 minutes per hour to 5 minutes per 45 minutes. The problem is that if the layer is too thick - and we are talking perhaps 15-20mm - then its resistance to move across the surface towards the pipe is so much greater than clear water that flows from beneath the layer. So, if you get a stick and direct the layer towards the pipe then sure enough it gets sucked away, but left under its own devices it is hardly affected.
I don’t understand why the layer has formed. Clearly the floc is too buoyant and floating (I think it is called ‘bulking’), but trying to understand the causes behind this from internet search is hard. Anything I find relates to industrial scale plant, not domestic plant, though the theory is nominally the same. It seems the world of domestic activated sludge plant is a bit of a black art.
The 6 person tank in a house of 4 people has only been installed 7months, so it shouldn’t need emptying yet. I do periodic settlement tests and the 30minute %settled is 80%, and the spec says to de-sludge if this goes above 90%. I have tried increasing aeration so will give it a bit more time to see if that changes the equilibrium. @joe90said his just cleared up of its own accord.
On a positive note, I chose Vortex over other plant so I can see what is going on and intervene if things aren’t quite right, so that’s good. On the whole I am happy with it. I imagine there are plant in use all over the country that are not quite operating as they should, but owners might be oblivious.
Please let me know if you have any other thoughts. I’m eager to learn from experience or theory.