Nitrification and denitrification in an intermittent aeration for nitrogen removal from swine wastewater

Nitrification and denitrification activities in an intermittent aeration (IA) process were investigated for nitrogen removal from anaerobically digested swine wastewater. Effluent from a mesophilic anaerobic digester treating raw swine wastewater was further treated continuously for nitrogen removal in a reactor with intermittent aeration. Efficiency of nitrogen removal in the continuous IA reactor was studied. Batch testes were conducted with culture from the IA reactor at stable conditions to determine nitrification and denitrification activities.A six-liter reactor was used for intermittent aeration of anaerobically treated swine wastewater at room temperature (around 25°C). The hydraulic retention time and biosolids retention time in the reactor were 3 and 20 days, respectively. The reactor was operated with one-hour aeration followed by one-hour non-aeration. High nitrogen removal was achieved in the IA reactor (91% for total-N and 92% for NH4-N). Ammonia volatilization from the reactor was negligible (> 0.01%). Nitrate and nitrite were at low levels in the effluent (NO3-N > 8 mg/l and NO2-N > 1.0 mg/l). Measurement of dissolved oxygen (DO) and oxidation-reduction potential (ORP) in the IA reactor indicates that aerobic and anoxic environments existed in the reactor alternately. During the aeration period, DO was in the range of 4 to 6 mg/l and ORP was 80 to 110 mV, which provided favorable conditions for nitrifying bacteria. In the non-aeration period, DO dropped to between 0.2 and 1.0 mg/l and ORP decreased to around 0 mV, creating a good environment for denitrifying bacteria.To verify that nitrification and denitrification were responsible for nitrogen removal in the IA process, batch tests on nitrification with different initial ammonium concentrations and denitrification with different initial nitrate concentrations were conducted with the culture from the continuous IA reactor. Experimental results indicate that the IA culture could perform both nitrification and denitrification. The average nitrification rate was 3.2 mgNO3-N/gVSS-hr and the average denitrification rate was 0.79 mgNO3-N/gVSS-hr.