The reasoning is that a UV is supposed to cause the micro organisms (that are green) to flocculate (clump together) and these get trapped in the bio filter, if you have the UV after the bio filter you are dumping the "clumped" organisms back in the pond.
This is not the mechanism by which UV disinfection occurs. UV has nothing to do with clumping and cannot affect pathogens in this manner.
Disinfection occurs when the DNA and RNA molecules of the pathogen absorbs UV photons. The most effective frequency is in the neighborhood of 250 nanometers.
The photon bombardment and absorption of the radiation energy affects the "Base Pairs" of the DNA.
Here is a schematic of a typical DNA molecule showing the famous "Double Helix" shape. The Base Pairs are the horizontal ladder steps having the symbols A, T, C, G:
All of what we are or what we will become, physically and chemically, is encoded by the sequence that the base pairs occur in. When a cell needs to reproduce or to perform a function the information on how to accomplish that function must be decoded by reading the sequence of Base Pairs, then replicating that sequence, and finally, carrying out the instructions given by that segment of the sequence.
In schematic above can be seen a molecule called "Thymine" (the purple T). It usually bonds with another molecule called "Adenine" (light grey A), thus forming an "Adenine-Thymine Base Pair."
Because the molecules are tightly packed together, absorption of UV into the DNA causes two adjacent Thymine molecules to fuse together, creating something called a "Thymine Dimer". This new molecule changes the sequence of the Base Pairs, thus causing a mutation. This new mutated set of instructions usually disrupts the function of the cell and prevents reproduction or replication of cells because the coded sequence of "how to be", "what to look like" and "what to do" become completely nonsensical, so the organism dies, or at best, cannot reproduce.
In fact, clumping is a mechanism used by microorganism to evade the effects of UV radiation because they can hide from the photon bombardment.
Theoretically, therefore, it makes much more sense to mount the UV bulb in the area of least turbid water possible which ensures maximum exposure and dwell time of the radiation upon the microorganism. There is no way any mechanical filter will remove microorganisms.
Cheers,