## 3 Results and Discussion  
### Overall Performance of Partial Nitrification on Treating Livestock Wastewater  
The IASBR and CASBR were operated for 120 days, and the COD removal rate reached a steady state after 15 days. The average COD removal rate was 87.6 \(\pm\) 1.7% and 89.7 \(\pm\) 3.4% in IASBR and CASBR, respectively. In the effluent, BOD\({}_{5}\) concentration was only 20 \(\pm\) 13 mg/L. The effluent was added into a flask with activated sludge taken from the IASBR, and then it was continuously aerated for two days. There was no evident decrease in COD, indicating that most of the remaining COD in the effluent was non-biodegradable organic matter, such as cellulose and hemicelluloses [3]. Nitrite accumulation occurred immediately in the beginning period (Figure 2) due to the activated sludge in IASBR and CASBR seeded from a partial nitrification reactor. However, during this period, it was observed that the effluent NH\({}_{4}\)+N level rose with a significant pH decrease (from 7.9 in Day 1 to 6.1 in Day 10). It shows that the alkalinity in the livestock wastewater was not sufficient to sustain a stable pH. Thus, an additional 4.5 g/L of alkaline solution (NaHCO\({}_{3}\)) was added to the digestate liquid from Day 13. The nitrification activity recovered immediately, and after Day 30, the effluent NH\({}_{4}\)+-N concentration dropped to below 10 mg/L with a stable pH of 8.1 \(\pm\) 0.2. In two SBRs, NO\({}_{2}\)\({}^{-}\)-N accumulated and more than 90 mg/L of NO\({}_{2}\)\({}^{-}\)-N appeared in the effluent as soon as the reactor operation commenced. In the steady state, effluent NO\({}_{2}\)\({}^{-}\)-N concentration increased to 544 \(\pm\) 36 mg/L and 492 \(\pm\) 65 mg/L in IASBR and CASBR, and an average nitrite accumulation efficiency of 78.4 \(\pm\) 1.3% and 64.7 \(\pm\) 3.2% was achieved, respectively. It was observed that the accumulation efficiency of effluent NO\({}_{2}\)\({}^{-}\)-N in CASBR was lower than that of IASBR. The nitration activity recovered when the partial nitrification sludge experienced continuous aeration. Therefore, the oxidation of NO\({}_{2}\)\({}^{-}\)-N to NO\({}_{3}\)\({}^{-}\)-N was encouraged in CASBR, and NOB activity partly recovered to lead to an increase in the NO\({}_{3}\)\({}^{-}\)-N concentration in the effluent from Day 38. Batch experiments show that in the NH\({}_{4}\)\({}^{+}\)-N oxidation process, the SOUR of IASBR and CASBR was 13.6 and 9.5 mg O\({}_{2}\)/(g biomass-h), and as for the NO\({}_{2}\)\({}^{-}\)-N oxidation, the SOUR of IASBR and CASBR was 1.4 and 2.0 mg O\({}_{2}\)/(g biomass-h), respectively. In IASBR and CASBR, the SOUR of NH\({}_{4}\)\({}^{+}\)-N was over more than that of NO\({}_{2}\)\({}^{-}\)-N. This means that the capability of nitrite accumulation was actively maintained under the intermittent aeration and continuous aeration conditions. The additional alkalinity supplement might have been the important factor promoting partial nitrification in two SBRs. It caused the pH values in two SBRs to range from 7.9-8.4. When pH is above 7.5, it may inhibit the growth and activity of NOB [17; 18]. It was observed that the partial nitrification efficiency in IASBR was 21.2% higher than that in CASBR, and the SOUR ratio of NH\({}_{4}\)\({}^{+}\)-N oxidation to NO\({}_{2}\)\({}^{-}\)-N oxidation reached 9.71:1 in IASBR. Under the intermittent aeration condition, the control strategy of alternating aeration and non-aeration could provide a suitable environment for partial nitrification. In order to investigate the influence of intermittent aeration on nitrite accumulation, under the condition of alternating 0, 10, 20, 30 or 40 min non-aeration periods with continuous 50 min aeration period conditions, the capability of partial nitrification was examined through batch experiments (Figure 3). Besides the 0 min non-aeration condition, after the commencement of aeration, the increase in nitrate was retarded for 12, 18, 18 and 21 min, respectively.