12 hours is not long time.Try ...

12 hours is not long time.Try to keep D.O at least 2.0 mg/l at shut down. Hydrogen sulfide is always a problem if MLSS are high & D.O. is less than 2.0 at shut down.I have used Sodium Nitrate many times on large and small plants for H2S control. It will release and dissolve fast. Fertilizer 16-0-0. It would take as much as 30 mg/L at first then back off to 10-20 mg/L. Hydrogen Sulfide gas is a strong wind that will have every elected official calling you. Hand held H2S meters are low cost. Plan ahead and have many bags on hand. Always use caution when H2S is too strong you can not smell it. Good luck.