complex in every sense.
equipment is complicated, the process is hard to control.
It's bio filtration that I'm talking of. Used in sewage treatment etc as well.
You break all the toxins in the water using bacteria which you control. But before that, you filter the water using the usual sand/gravel filters to filter out any solids. Then the water goes through UV to make sure nothing alive gets through into the bio filter. Then the bio filter does it's thing. Then the water goes through UV again, so no bacteria leaves the bio filter media. And you need to continuously monitor water quality and also make sure the bacteria is doing okay. Starting up the filer is also time consuming and complex. So controlling such a process is quite complicated, not impossible though, just really difficult.