Alkalinity and ORP changes at nitrification and denitrification in a sequencing batch reactor (SBR)

Nitrification-denitrification under different operation conditions (COD, NH4+, oxygen concentration and flow rate) was studied in a SBR system (treatment capacity: 450 gallon). Results showed that nitrification could still proceed well under high influent COD concentration (1300 mg/L), due to long retention time. However, insufficient denitrification at low COD ( 30 mg/L) in effluent. Dissolved oxygen (DO) was a major factor for nitrogen removal. Nitrification could not complete at low DO ( 5 mg/L) during aeration period inhibited the denitrification in the following anoxic phase. High flow rate led to short retention time in the SBR, which directly reduced the nitrification efficiency. High DO (or long aeration time) was found to compensate this short retention time, with a great improvement of nitrification.Effluent alkalinity changed with the extents of nitrification and denitrification. It was clear that more alkalinity was consumed during high-extent nitrification, and more alkalinity was produced during high-extent denitrification. Redox potential (ORP) also changed with the extents of nitrification and denitrification. High COD concentration in the SBR resulted in more negative ORP and better denitrification values in anoxic phase. Higher ORP was related with higher oxygen concentrations. However, ORP change was not as good as alkalinity change related with nitrogen removal in the SBR.Nitrogen removal was also tested under allylthiourea (ATU) shock. Under the same ATU dosage, the SBR system had 20-50% higher nitrogen removal efficiency than batch test. Nitrification decreased with ATU dosage increase, and recovered with a week after the end of ATU shock. The experiment showed that ATU did not kill nitrifying bacteria, just inhibited their activity. Alkalinity was found to increase with ATU shock, due to less alkalinity consumption, and started dropping during the recovery of nitrification, indicating that alkalinity indicated the extent of nitrification in the SBR.