Performance and operational strategy of simultaneous nitrification, denitrification, and phosphorus removal system under the condition of low organic loading rate in wet weather.

A pilot-scale aerobic granular sequencing batch reactor (SBR) with domestic wastewater was operated to evaluate the effects of the low organic loading rate (OLR) due to wet weather flow conditions on simultaneous nitrification, denitrification, and phosphorus removal (SNDPR). As the OLR decreased from 0.85 to 0.43 kg COD m-3 d-1, the total nitrogen (TN) and total phosphorus (TP) removal efficiencies decreased from 84.0% and 94.1% to 51.3% and 73.8%, respectively, the sludge volume index (SVI) increased from 42.3 to 85.5 mL g-1, and the average granular size decreased from 1022 to 742 µm; however, no sludge disintegration and biomass loss were observed. The poor nutrient removal efficiencies and settling ability were due to the shrinking anoxic zone and substrate scarcity inside the granules, wherein the activity decay of ammonia-oxidizing bacteria and overgrowth of filamentous bacteria played an important role. Alternating the aeration intensity was effective in enhancing nitrogen removal and sludge settling by improving the anoxic activity in granules and inhibiting the proliferation of filamentous bacteria. Returning 20% of sludge from the end of one anaerobic stage to the beginning of the next anaerobic stage (midway sludge return) was beneficial for phosphorus removal as it improved phosphorus storage by phosphorus-accumulating bacteria. A smaller granular size with stronger stability and better nutrient removal performance was the new steady state of the SNDPR system under wet-weather flow conditions.

[Effect of Different Ratios of Anaerobic Time and Aeration Time on the Operation of a Continuous-Flow Reactor with Partial Nitrification Granules].

A continuous flow reactor was inoculated at 25-28℃ with mature partial nitrification granular sludge. In order to avoid the granular sludge being crushed because of the sludge backflow through the peristaltic pump, a built-in sedimentation zone was used for internal backflow. The experiment investigated the influence of the different anaerobic time to aeration time ratio (1:1, 2:1, and 1:2) on the stability of a continuous-flow partial nitrification granular sludge system. The results showed that when the controlled anaerobic time to aeration time ratio was 1:1 and 2:1, the partial nitrification performance was good and the nitrite accumulation rates were 85.2% and 94.5%, respectively. When the controlled anaerobic time to aeration time ratio was 1:2, the partial nitrification performance gradually deteriorated, and the ammonia nitrogen removal rate and nitrite accumulation rate at the end of the stage decreased to 64.1% and 58.7%, respectively. Batch test results showed that intermittent aeration and continuous aeration can better inhibit the relative activity of NOB in the partial nitrification system to a certain extent. The longer the anaerobic time, the better the NOB activity inhibition. However, too long an anaerobic time will also lead to ammonia nitrogen removal rate. In the process of partial nitrification, the long-term stable operation of continuous flow partial nitrification process can be realized by 1:1 and coordinated control of other control conditions. An analysis of sludge performance indicated that in the anaerobic time to aeration time ratio range of 1:1-1:2, the longer the anaerobic time, the more stable the granular sludge structure. The shorter the anaerobic time, the smaller the selection pressure in the reactor, resulting in poor sludge sedimentation performance and partial disintegration of partial nitrification granular sludge. An EPS chemical analysis and a three-dimensional fluorescence spectroscopic analysis showed that the PN content was higher and the PN/PS value was higher when the anaerobic time and aeration time ratio was 2:1.

[Effect of On/Off Aeration Time Ratio Under High Frequency On/Off Aeration on Performance of Nitrosated Granular Sludge].

The inhibition of the on/off aeration time ratio on nitrite oxidizing bacteria (NOBs) in intermittent aeration is the key to accumulating nitrite. This study explores the stable operation of nitrosated granular sludge under the same frequency of aeration and different combinations of stop exposure time. At 25-28℃, aerobic granular sludge was inoculated into a sequencing batch reactor (SBR) (R0) and acclimatized to nitrosated granular sludge by intermittent aeration and dissolved oxygen (DO) restriction. The effects of different on/off aeration time ratios in one sub-aeration cycle (3 min:3 min; 4 min:2 min; 2 min:4 min) on the performance of nitrosated granular sludge were analyzed using three same sets of SBRs (R1, R2, and R3). Experiments showed that the nitrite accumulation rates of R1 and R2 were higher, with average nitrite accumulation rates of 89% and 95%, respectively. The nitrosation performance of R3 was poor, and the nitrite accumulation rate at the end of the operation cycle was only 57%. The results showed that when the off-aeration time was between 2 and 4 minutes, the longer the off-aeration time was, the more stable the granular sludge structure was; hence, a good nitrosation performance could be maintained. When the off aeration time was less than 2 minutes, the alternation of anaerobic and aerobic environment could not be fully realized, which resulted in the sludge tending to stay in an environment of continuous aeration. Thus, the environmental advantage of intermittent aeration was lost; the NOBs could not be inhibited, and the granular sludge was disintegrated thereby leading to the deterioration of the nitrosation performance. Additionally, the longer aeration time contributed to the development of irregularly shaped particles. The longer the off-aeration time was, the higher the PN/PS value was, which was beneficial to the enhancement of the hydrophobicity of the particle surface and the sludge settling performance.

[Effect of Step Aeration on a Municipal Sewage Aerobic Granular Sludge System].

In order to achieve simultaneous nitrogen and phosphorus removal in low-C/N urban sewage, a sequencing batch reactor (SBR) was operated in anaerobic/aerobic (A/O) mode. Keeping the total aeration volume at 900 L, the aeration strategy was adjusted. The uniform aeration of 2.81 L·(h·L)-1 was changed to "high/low aeration" with high strength 4.22 L·(h·L)-1before low strength 1.88 L·(h·L)-1, and "low/high aeration" with low strength 1.88 L·(h·L)-1 before high strength 4.22 L·(h·L)-1. The experiment investigated the nitrogen and phosphorus removal performance and sludge characteristics of the system under different aeration strategies. The results showed that the nitrogen and phosphorus removal performances of the system under high/low aeration were the best. The effluent concentrations of NH4+-N, NO2--N, NO3--N, and TP were 0, 0.15, 8.12, and 0.04 mg·L-1, respectively. The removal rates of total nitrogen (TN) and total phosphorus (TP) were as high as 78.33% and 99.19%, respectively. Simultaneous nitrification endogenous denitrification (SNED) was clear, with the SNED ratio at 77.08%. Compared with uniform aeration, the system nitrification rate and denitrification rate increased, and the denitrification rate reached 14.33 mg·(g·h)-1, which was the maximum value during the whole operation; the solidity, sedimentation performance, and stability of granular sludge were improved, and the sludge volume index (SVI) was 23.49 mL·g-1. After adjusting the aeration strategy to low/high aeration, the nitrogen and phosphorus removal performance of the system deteriorated, and the removal rates of TN and TP were reduced to 51.26% and 58.32%, respectively. However, the system had the best nitrification performance with ammonia oxidation rate and nitrate production rate at 14.92 mg·(g·h)-1and 7.50 mg·(g·h)-1, respectively, which were their maximum values during the whole operation. Simultaneously, the filamentous bacteria in the granular sludge multiplied, the granular structure became loose, the sedimentation and stability all worsened, and the SVI rose to 40.76 mL·g-1.

[Startup and Stabilization of Nitrosation in an Anoxic-aerobic Continuous-flow Reactor with Granules].

Matured nitrosation granules were inoculated to an anoxic-aerobic continuous-flow reactor at room temperature (17-19℃). The startup and stabilization of nitrosation with granules were studied. The results show that the nitrosation of the continuous-flow was successfully achieved with an average nitrite accumulation rate (NAR) above 95%. With the increase of the dissolved oxygen (DO) content from (3±0.2) to (4.5±0.2) mg·L-1 in the aerobic zone, the NAR remained above 95%. The effect of the hydraulic retention time (HRT) of the continuous-flow reactor was investigated. The short HRT (8.4 h) sped up the circulation of the sludge particles in the continuous-flow system such that the broken granular sludge could not be integrated in time, resulting in the deterioration of granular sludge settling and the loss of sludge granules. The performance of the system was restored with the increase of the HRT to 12.2 h and the continuous-flow system stabilized. The ammonia removal efficiency and NAR were 86.7% and 96.2% on day 166, respectively.

[Effect of Different Ratios of Anaerobic Time and Aeration Time on the Formation of Partial Nitrification Granules].

Flocculent sludge with good nitritation performance and MLSS and SVI values of 3500 mg ·L-1 and 162 mL ·g-1, respectively, was inoculated in the R1, R2, and R3 groups of the same SBR reactors at room temperature (22-28℃). The effects of different anaerobic time and aeration time ratios (1:2, 1:1, and 2:1) on the formation of partial nitrification granular sludge were studied. The results showed that granules in three reactors were successfully formed at 103 d, 82 d, and 64 d. The nitrosation rate of R1, R2, and R3 were 86%, 98%, and 96%, respectively, and SVI decreased to 89, 75, and 58 mL ·g-1 and MLSS increased to 2314, 2781, and 2946 mg ·L-1, respectively, at the 85 d. The larger ratio of anaerobic time and aeration time was more conducive to sludge granulation, better settling, and nitritation performance. When the anaerobic time and aeration time ratio was 1:2, the nitritation performance of the sludge decreased since the NOB could not be suppressed, and the settling performance of the sludge became better and then worse. In addition, at the larger ratio of anaerobic time and aeration time, there was more PN and PS from microbial secretion. The PN of R1, R2, and R3 was smaller at the end of the anaerobic phase compared to that of the aerobic phase in the early stage of granular formation, but were reversed in the stabilization stage. The PS of the anaerobic phase was larger than that of the aerobic stage over the entire process.