My understanding is that convential filtration must achieve a turbidity level of less than 5 NTU and not more than 0.5 NTU in more than 5% of the measurements taken each month.
You should probably contact the state agency for drinking water where you are located to confirm.
The "new" LT2 rule for drinking water has not been promulgated yet by the EPA. It should be out in October or November. The LT2 rule addresses cryptosporidium with regards to methods of treatment, such as filtration, UV, ozone, etc. You will have to know what bin you fall in, in order to determine final NTU, in order to get disinfection credits through filtration.
