Scottish slang for "The filth"

I'm researching slang in Irvine Welsh's books, and since I'm not a native English speaker and have never been to Edinburgh, I'd like to hear from more knowledgeable readers of "The Filth" about street slang and how it's implemented in the book. On a scale of 5, how offensive is the word "filth" to a police officer? Is it considered a swear word? What other slang words were used in the book? I've also read that, in addition to hooligans using the word to refer to the police, it's also used within the British police specifically for the homicide department, where Bruce works. I'd like to know if this is true or not.