Actually, it's a global limitation. It has to do with the fact that the discharge standards are of such a low density that you're essentially looking for one organism in swimming pool volumes of water. In order to test that standard, you have to be able to filter those very large volumes of water without any loss or error, or causing death to the organisms by filtering that much water. Essentially you're talking about such large volumes of water that we physically can't filter it to look for such a small number of organisms.
