## 3 Results and Discussion
### Overall Performance of Partial Nitrification on Treating Livestock Wastewater
NO\({}_{2}\)\({}^{-}\)-N oxidation was not activated immediately after aeration started when the non-aeration period was adopted [19]. After a \"lag time\" of NO\({}_{2}\)\({}^{-}\)-N oxidation, the activity of the NOB recovered, and the calculated NO\({}_{3}\)\({}^{-}\)-N accumulation rates were 0.0972, 0.0977, 0.0967 and 0.9930 mg NO\({}_{3}\)\({}^{-}\)-N/(L-min), respectively. Compared with 0.1030 mg NO\({}_{3}\)\({}^{-}\)-N/(L-min) in the 0 min non-aeration condition, which could denote continuous aeration condition, there was no evident decrease of NO\({}_{3}\)\({}^{-}\)-N accumulation rates in the intermittent aeration conditions. Thus, the intermittent aeration strategy was discovered to induce a lag reaction for NO\({}_{2}\)\({}^{-}\)-N oxidation in a short time, but had no further effects on the inhibition of NOB activity. The average ratios of NO\({}_{2}\)\({}^{-}\)-N accumulation rate to NO\({}_{3}\)\({}^{-}\)-N accumulation rate within the 50 min aeration period were investigated (Figure 4). In 10, 20, 30 and 40 min non-aeration conditions, the ratios were 1.28, 1.44, 1.51 and 1.35 times of that in 0 min condition, respectively. As for the 40 min non-aeration condition, the NH\({}_{4}\)\({}^{+}\)-N oxidation rate was only 0.294 mg NH\({}_{4}\)\({}^{+}\)-N/(L-min), compared with 0.358, 0.364, 0.356 and 0.346 mg NH\({}_{4}\)\({}^{+}\)-N/(L-min) in 0, 10, 20 and 30 min non-aeration conditions, respectively. Figure 3: Influence of aeration strategy on partial nitrification (\(\blacksquare\): 0 min non-aeration duration; \(\sim\) 10 min non-aeration duration; \(\blacktriangle\): 20 min non-aeration duration; \(\blacklozenge\): 30 min non-aeration duration; \(\blacklozenge\): 40 min non-aeration duration). This means that a slight inhibition of ammonium-oxidizing bacteria (AOB) could be carried out in the 40 min non-aeration condition. Thus, the optimal alternating non-aeration duration was 20-30 min for partial nitrification treatment of livestock wastewater. Sludge samples were taken from two SBRs to investigate the microbial population by 16S rRNA gene sequencing on Day 45 (partial nitrification set-up stage) and Day 109 (partial nitrification steady-state stage) compared with seed sludge (Figure 5). Except for unculturable bacteria, the determined microbial analysis results show that the dominant genus in the sludge samples was recognized to be capable of degrading a wide range of refractory organic pollutants; for example, _Comanomas_ (7.2-11.5% of total genera), _Thuera_ (3.4-6.7% of total genera), _Trupera_ (3.4-5.9% of total genera), and _Pseudomonas_ (0.7-4.5% of total genera), or of degrading celluloses, including _Cytophaga_ (2.8-4.9% of total genera) and _Pseudoxanthomonas_ (1.1-5.1% of total genera) [20; 21; 22; 23]. _Paracooccus_ (7.0-11.4% of total genera) was found to be the main denitrifier in two reactors [24]. Besides the nitrification genera (Figure 6), about 6.5-8.5% of the other genera might be species of bacteria indigenous to livestock manure, such as _Enterococcus_ and _Fastidiosiplina_. Figure 4: Ratio of NO\({}_{2}\)–N accumulation rate to NO\({}_{3}\)–N accumulation rate under the different non-aeration durations. Figure 5: Microbial population analysis of partial nitrification sludge by 16S rRNA gene sequencing. Nitrification genus analysis (Figure 6) shows that _Nitrosomonas_ was the key bacteria for NH4\({}^{+}\)-N oxidation in two SBRs, and was recognized as the main AOB species in IASBR and CASBR. Common NOB genera (including _Nitrobacter_ and _Nitrospira_) were not detected evidently. It shows NOB was inhibited by partial nitrification conditions. However, favorable conditions for NO2\({}^{-}\)-N oxidation actually occurred in IASBR and CASBR (Figure 2).