For the EN-12566-3 test, the smallest plant in the range is the one that must be sent for testing. This is because it is believed that the smallest tank size will be the one that gives the poorest performance results.
This SHOULD be true, if the treatment tank size is expanded in a linear fashion, i.e if a 4 person tank size needs to be 2000 litres, then an 8 person needs to be 4000 litres.
Unfortunately, what SHOULD happen with the design of the larger tanks, more often than not, doesn't. There are very few manufacturers who design their larger plants this way, as the larger plants do not need to be tested. These linear design ones tend to be the more expensive sewage treatment plants.
Instead, many manufacturers produce larger tanks that have smaller compartments throughout, thus making them cheaper to make and buy. As there is no testing required for these, they are not, officially, doing anything wrong, but the performance cannot be the same as the one stated on the EN-12566-3 Certificate and on the brochure. This is because the retention time for the digestion of the pollutants in the biozone also reduces as the volume per person reduces. If the tested plant had a biozone retention time of 50 hours, but the 20 person one has a retention time of only 26 hours, then there is no way that the sewage can be cleaned to the same degree.
This Is wrong.
This also costs you money. Everyone produces the same amount of sludge per day in a treatment plant. If the 4 person system needs emptying once a year, then a larger tank, with a smaller capacity per person, needs emptying more often, at a rate depending on the size reduction.
This, often vastly decreased emptying interval, needs to be explained to the customer, but often is not.