Can deviated nasal septum give you constant post nasal drip and very bad breath? if so what is the treatment?

No: A deviated septum causes a mechanical blockage of breathing on the side of the deviation. It is true that the tissue on the opposite side may become enlarged and block the opposite side as well. But post nasal drip and foul breath is more likely a manifestation of other problems in the nose or sinuses. See an ENT doctor for a more specific diagnosis and treatment.

Unlikely: Having had a deviated septum myself, (car accident) this would very unlikely be the cause of your symptoms. A deviated septum typically only affect breathing through the nose.

Sinusitis: Post nasal strip and bad breath are more likely a result of a sinus infection rather than a deviated septum. However, a badly deviated septum can contribute to some patient having an increased frequency of sinus infections or contributing to a chronic sinus infection. This is bc a badly deviated septum can obstruct the drainage pathway of your sinuses.