The biodisk (obsolete I thought now) or aerator pump systems fall into the category of 'microstations' (tiny sewerage processing plants). They both work in the same way by allowing additional air to get to the sewerage to break it down. The advantage is the outflow contains no unprocessed sewerage so the soakaways never sludge up and here the outflow can be discharged straight into a water course (the manufacturers say you can drink it). The disadvantage is the continuous power consumption, which is about 50 watts. Also they are not suitable for discontinuous operation as they need to 'feed' all the time (or the flora die), so no good for holiday cottages. They are small and need to be emptied regularly. I was advised every 3 -12 months. The air pumps can get noisy very quickly. So a microstation is great for a small plot with poor drainage which is continuously occupied.
A normal septic tank consists of two or more compartments where the sewerage separates under gravity and breaks down somewhat using the air already in the water. A crust forms on the surface and the heavy stuff drops to the bottom. The outflow contains untreated particles which must go to a soak away. The soakaway will gradually sludge up, which takes over 20 years perhaps. The tanks need emptying every 2 -20 years depending on size or use. An improvement is a second tank which further filters or processes the outflow to protect the soakaways or considerably reduce the size necessary. This is the Ecoflow I mentioned earlier which uses coconut matting to trap the fine particles to the extent that 96% are removed. The French allow that system, because of its efficiency, also to discharge to a watercourse.