Our team were called in when a large treatment plant had failed. The plastic media was covered in a thick sludge and therefore the blowers were burning themselves out. The media was removed and washed clean.
Once the tank was emptied and jet washed the media was put back in and re-charged with final effluent. New blowers were installed and the system is working perfectly again.
If you have a treatment plant remember to get it emptied and checked over.