FESOP stands for Federally Enforceable State Operating Permit. To get a FESOP, you go to your state agency and agree to certain voluntary restrictions on your operation in exchange for opting out of Title V. You basically reduce your PTE level by self-imposing restrictions on your operation. For example, you may tell the state agency that you will only operate two shifts a day and 50 weeks a year. By using this operating scenario to calculate your PTE (instead of the usual 24/7/365 formula for PTE), you may fall below the threshold that would require you to get a federal Title V permit.
Or you may tell the agency that you will only use certain type of solvents or paints that contain much lower levels of VOC in your operation in order to reduce yor PTE to below the Title V threshold.
The state will then issue you a state operating permit with your self-imposed restrictions (hours, usages, products or combination) as conditions. If you violate these conditions, the federal government (EPA) has the right to take enforcement action against you even though what you have is a state operating permit – hence the term “federally enforceable”. The reason is that those restrictions are the underlying reasons why you are able to avoid Title V.
So if you are going to get a FESOP, you want to make sure that you WILL be able to meet your own operating restrictions.