In that style of filter with bio-wheels, most of the bacteria is on the wheel not the cartridges. Years back I used to email the designer of the those wheels. They do two things other filters don't. Because the bacteria need oxygen, that design insures the maximum availability. It is a form of wet/dry. The other thing is the pleated design encourages the bio-film to be more evenly distributed over the media. When a bio-film starts to get thick, it is subject to shearing off from the water flow. The thicker the film and the stronger the current, the more likely shearing becomes.
The downside to this design is a power or filter failure leaves those wheels high and dry. if this is long enough, the bacteria can start to die off. Almost 30 year ago when I set up my first tank, I had an Emperor 400 on it. When it finally failed a number of years later and I ordered a new one, I so disliked how it had been cheapened that I returned it. I was already moving to AquaClears. At my peak I had about 30 of these running. I have since discoverd the benefits of Poret foam. I am now down to about 17 ACs and have been reducing hang ons with time. I have every size they make.
