Improved water mixing was what fixed that particular MBBR scenario.
Increased water circulation promotes improved water mixing....
Improved water mixing allows the pollutants to move where they need to move so to get "cleaned"...
Generally, in higher organic systems, mixing (aided by water circulation) becomes more important...
...lack of proper water mixing allows "poor water quality zones" to be created... called "irrotational zones".. These zones can be seen, due to the settled debris piles, around big rocks or jutted out corners/sections of a pond...
...this even happens in shallow systems as well as high fish density aquaculture tanks... and the fish movements don't always force these zones to get properly mixed...
"Irrotational zones" (not talking about stratification) have lower water velocities which prevents proper mixing, this zone has poorer water quality (including reduced oxygen), and creates a dormant environment to allow for significant amount of organics to settle and collect.
...too much force from water jets and suctions can also aid in the creation of "irrotational zones"...
A major reason there are so many fish kills from ammonia or carbon dioxide or lack of oxygen after deep winter freezes... is due to a lack of proper water mixing... even though they still may have a hole in the ice... since the water mixing helps to reduce the ammonia, carbon dioxide, and equalizes the oxygen.
This is why always important to keep the water moving... whether by pump or aeration...
The lack of proper water mixing also occurs in Lakes and mud ponds as well... in Lakes and mud ponds, the water mixing is so poor that the "irrotational zones" turn into stratification layers... which is good for fish in a Lake until organic pollutants and carbon dioxide start to significantly build up in these layers creating reduced oxygen and other problems....
...this is why small Lakes and mud ponds often utilize aeration devices...
...whether it is large fountains or actual air diffusers or a machine driven (or tractor PTO driven) paddle aerator.
LATE EDIT :::
Thought I would mention really quick...
I have found in my small pond... proper mixing is much more difficult to manage...
...cause... in a small pond like mine (an oval design with a large island in middle), the increased water mixing in the pond easily disturbs the water's surface area thus disturbing proper functioning of my Skimmer device...
So... I had to increase the "mixing mechanisms" up into many more, smaller outlets (having 15 instead of just 6) in the pond and significantly increased the pond turnover rate per hour (to 5x).. and I have a primary and secondary suction for the pump... secondary suction with a pile of 1~1.5 inch river rock over the pump's suction so to "disperse" the suction (rather than focused in one particular area) and primary suction located where the submserible pump is placed (in an irrigation box in the pond).
I still have some irrotational zones, but not as many nor as big of irrotational zones as I had before.
I have come to the conclusion that... after all of my efforts... would have been better to construct the pond design like a large stream, rather than in a rugged shape... otherwise, the only way I think to truly remove all irrotational zones is by having a properly constructed concrete koi pond (which is basically a swimming pool) or an aquaculture circular tank and I don't like the aesthetics of either of those two... so I'll be happy with what I have...
