Air carrier Flight Attendant reported a malfunctioning toilet resulted in harsh fumes released into the cabin that caused physiological reactions among crew and passengers.

Narrative

As we were taxing prior to the pilots asking the FA's to take there jump seats I noticed that one of the toilets was not flushing so I brought it to the attention of the other FA's - we then checked all toilets in the aft of the AC- we realized then that only the 2L/outboard toilet was flushing. I immediately brought this to the attention of the forward galley & contacted the pilots. Who along w/the purser came to check the toilets as well as the tanks (which upon pre-departure) were in acceptable ranges.We then returned to the airport (hard stand) & awaited maintenance to board-it was determined that perhaps one of the sensors in the tanks were misreading g or that the gauges were defective. So it was decided by maintenance that they would again attempt to clear the hold g tanks-this was done and as this took place a horrendous stench filled the cabin-the decision was finally made to deplane the customers...we of course were still onboard (specifically those assigned to the aft of the AC) & exposed for quite sometime (3hrs+) to the harsh sewage/urine gases in the A/C which reached as far as economy + & beyond...The following day when we returned the stench was still very present & we had to relocate all the passengers forward of the economy cabin... many of us experienced coughing; headaches; and other symptoms-l personally had couching to the point of nausea as well as loosing continence...headaches; weakness; upper respiratory irritation & diarrhea that has continued all day today.