[Shortening SRT of Intermittent Gradient Aeration to Realize Nitrogen and Phosphorus Removal in Short-range SNEDPR System].

This study explored the effect of sludge retention time (SRT) on ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) under intermittent gradient aeration, as well as the effect of the short-range nitrification endogenous denitrification phosphorus removal system on the treatment of low C/N ratio domestic sewage. In this study, an SBR reactor was used to cultivate aerobic granular sludge, using actual domestic sewage as the influent substrate. As the SRT decreased from 50 d to 30 d, the specific ammonia oxidation rate increased from 3.16 mg·(g·h)-1to 4.38 mg·(g·h)-1, and the specific nitrite oxidation rate decreased from 3.4 mg·(g·h)-1to 1.8 mg·(g·h)-1. The activity of NOB decreased by about 44%, resulting in short-range nitrification within the system. With an SRT of 30 d, the maximum nitrite accumulation was 6.93 mg·L-1. Because the reduced SRT led to a slight decrease in sludge concentration within the system, an aeration reduction strategy was adopted after 40 d, according to the DO curve. When the final SRT was 30 d, the effluent COD concentration was 40.76 mg·L-1, the TN concentration was 12.4 mg·L-1, the TP concentration was 0.31 mg·L-1, and the simultaneous removal of C, N and P was realized. Thus, a stable short-range nitrification endogenous denitrification phosphorus removal system was finally obtained. At the same time, the EPS content of aerobic granular sludge was negatively correlated with SRT, the protein content increased from 66.7 mg·g-1 to 95.1 mg·g-1, and the polysaccharide content remained in the range of 12.1-17.2 mg·g-1, indicating that the decreased SRT had a great effect on the protein content. With an SRT of 30 d, the PN/PS value was maintained at approximately 6.2, and the structural stability of aerobic granular sludge can be maintained under such conditions.

[Nitrogen and Phosphorus Removal from Domestic Sewage Aerobic Granular Sludge Under Intermittent Gradient Aeration].

In this study, three SBR reactors R1, R2, and R3 were set up and operated using (A/O)3-SBR gradient aeration, (A/O)3-SBR constant aeration, and the conventional (A/O)-SBR mode, respectively. The nutrient removal performance and aerobic granular sludge characteristics under these aeration modes were explored using real municipal wastewater as the influent matrix. The experimental results revealed that for the R1, R2, and R3 particles during the stable period, the average removal rate of COD was 88.68%, 89.05%, and 88.96%, respectively, the average removal rate of TN was 76.97%, 71.99%, and 64.92%, respectively, the average removal rate of TP was 96.28%, 85.05%, and 78.97%, respectively, and the proportion of denitrifying phosphorus accumulating bacteria to phosphorus accumulating bacteria was 25.52%, 19.60%, and 12.77%, respectively. The results showed that the operation mode of anaerobic, aerobic, and anoxic was more conducive to the enrichment of denitrifying phosphorus accumulating bacteria (DPAOs), and that the gradient aeration was more enriched than the constant aeration mode, which is of great significance to low-intensity municipal domestic sewage treatment with an insufficient carbon source. At the same time, the dissolved oxygen in the aeration section of R1 was reduced step-by-step, which improved the simultaneous nitrification and denitrification rates of particles and the utilization rate of the internal carbon source, which was beneficial for the efficient removal of TN. The particle size of the three groups of reactors was 727.368, 815.072, and 895.041 µm respectively. As the transfer rate of the matrix decreased with particle size, the microorganisms in R2 and R3 may have caused anaerobic respiration to release harmful gas, thus damaging the particle structure, such that the particles in R2 and R3 were less dense than those in R1. In addition, the PN/PS values of R1, R2, and R3 were 6.31, 5.63, and 4.83, respectively, and the EPS content (in terms of VSS) was 103.97, 92.22, and 76.98 mg·g-1, respectively, at the time of particle stabilization, which revealed that the mode of intermittent gradient aeration was beneficial to stimulate the secretion of EPS. This was especially the case for the secretion of PN, which increased the PN/PS value, enhanced the cell hydrophobicity, and made the particles dense and stable.

[Effects of Different Intermittent Aeration Strategies on the Start-up of SNAD Process].

In this experiment, two reactors, R1 and R2, were inoculated with anaerobic ammonium oxide sludge at room temperature (20℃±3℃). R1 employed intermittent constant aeration while R2 employed intermittent gradient aeration to start up the SNAD granular sludge process, to study the effects of different intermittent aeration ways. The results showed that the time for R2 to stabilize the performance in each stage was shorter, and the actual start-up speed was faster; after successful startup, the eigenvalues of R1 and R2 were 6.46 and 10.34, respectively. The suppression effect for NOB is better in R2; the DO fluctuation in a typical cycle was stable in R2 while the DO in R1 gradually rose overall and reached over 0.5mg·L-1 by the end of the cycle. R2 can maintain low DO more stably and inhibit NOB proliferation; after successful startup, the PN/PS values in the R1 and R2 reached 2.745 and 2.823, respectively, and the particle size reached 365.8 µm and 402.1 µm, respectively. The stability and sedimentation of the particles in R2 were stronger and the particle size increased faster.