If you have low DO, that means your ORP is also low. This is an indication that there are alot of organics in your water. You will need to inject oxygen into the clarifiers.
If organic materials are present a small amount of Archaea microbes would eliminate them and in the process make that oxygen more available. Most post readings indicate an O2 increase by a factor of 3. Oxygen is a good choice. Either pump in it or release the oxygen already there are clean the organic material at the same time. Hint: if you add Archaea at the beginning of the cleaning cycle you will be able to eliminate the addition of chlorine.