[Effect of Sludge Retention Time and pH on the Denitrifying Phosphorus Removal Process].

In this work, the effects of the sludge retention time (SRT, 35, 25, or 15 d) and pH (7.5, 8.0, 8.5) on denitrifying phosphorus removal were investigated using denitrifying phosphorus bacteria (DPBs) enriched in a sequencing batch reactor (SBR). The results indicated that shortening the SRT from 35 d to 25 d resulted in a decrease in the mixed liquor volatile suspended solids (MLVSS) from 2821 to 2301 mg·L-1, while the sludge loading rate (F/M) increased from 0.256 kg·(kg·d)-1to 0.312 kg·(kg·d)-1. Although the quantity of net phosphorus release and net phosphorus uptake decreased at this stage, the rates of anaerobic phosphorus release, anoxic phosphorus absorption, and denitrification reached their highest levels with values of 25.07, 15.92, and 9.45 mg·(g·h)-1, respectively, due to the increased sludge activity. Consequently, the phosphorus content of the sludge increased from 4.78% to 5.33%, and the removal rate of PO43--P was stable at above 95% with an average effluent PO43--P concentration below 0.5 mg·L-1. When the SRT was further shortened to 15 d, the MLVSS decreased to values as low as 1448 mg·L-1, and the proportion of DPBs in the phosphorus accumulating organisms (PAOs) decreased from 82.4% to 65.7%, indicating that the DPBs were gradually washed out from the system due to the excessively short SRT. At this stage, the phosphorus content of sludge decreased to 3.43%, while the rates of phosphorus release, phosphorus absorption, and denitrification also decreased to some extent. When the pH was increased (7.5-8.0), the anaerobic phosphorus release rate and the anoxic phosphorus absorption rate also increased, and reached 25.86 mg·(g·h)-1 and 16.62 mg·(g·h)-1, respectively, at a pH of 8.0. When the pH exceeded 8.0, the phosphorus removal efficiency dropped rapidly, supposedly due to phosphorus chemical precipitation.

[Start-up and Stable Operation of ABR-MBR Denitrifying Phosphorus Removal Process].

The nitrogen and phosphorus removal characteristics during the start-up and the long-term operational stability of an anaerobic/anoxic (A/A) ABR coupled aerobic MBR system treating low C/N domestic wastewater were investigated. The results showed that the denitrifying phosphorus bacteria (DPBs) were successfully enriched within 46 d by controlling the nitrate recycling ratio (increasing from 150% to 300%), with a temperature of 30℃±2℃, volume loading rate of 0.8 kg·(m3·d)-1 and sludge reflux ratio of 80% in the ABR, sludge retention time (SRT) in the denitrifying phosphorus removal functional area of 25 d, and the dissolved oxygen (DO) of 1-2 mg·L-1 in the MBR. The net phosphorus release and phosphorus uptake of DPBs reached 20.56 mg·L-1and 27.74 mg·L-1, respectively. Batch tests demonstrated that about 84.8% of phosphorus-accumulating organisms (PAOs) could use NO3--N as an electron acceptor for denitrifying phosphorus removal. After 50 d of stable operation after the successful system start-up, the average removal rates of COD, NH4+-N, TN, and PO43--P were 91.8%, 99.0%, 71.5%, and 94.2%, respectively. The results also suggested that 0.83 mg·L-1NO3--N was consumed per 1 mg·L-1 PO43--P removed during the denitrifying phosphorus removal, indicating that the simultaneous nitrogen and phosphorus removal was achieved in the ABR-MBR system.

[Effect of C/N and Sludge Concentration on the pH-Regulated Nitrosation System].

pH is one of the most important means of control for the realization and stability of the nitrosation system. To study the change rule of pH values of the nitrosation system and the influence of pollution removal and transformation at different pH under the conditions of different C/N (0, 1, 2, 3,4) and sludge concentrations (sludge amount:water content was 1:6, 1:3, 1:1), batch tests were conducted with tapered bottles using sodium acetate as the carbon source and inoculated with mature nitrosation sludge. The results showed that the higher the C/N, the higher the pH increment and the denitrification efficiency at the same sludge concentration. At the same C/N, a higher sludge concentration corresponded to a smaller pH increment but a higher denitrification efficiency. The removal and transformation of carbon and nitrogen was highly correlated with pH changes in the reaction system, and the denitrification and nitrosation reactions were in sequence. Throughout the operational period of the system, as pH increased, the specific organic matter removal rate was 7-16 times as much as when pH decreased. However, as pH decreased, the specific ammonia oxidation rate (SAOR) was 1-20 times that of when pH increased. When pH was less than 6.1, the system lost its ability to oxidize ammonia-nitrogen. The highest removal efficiency of carbon and nitrogen in the system was achieved when C/N was 4. Ammonia transformation 80% COD removal at the three sludge concentrations took 480, 350, and 300 min, respectively. Under different conditions, the proportion of nitrosation in the system remained above 50% and the concentration of NO3--N remained below 5 mg·L-1, which indicated that the system was dominated by nitrosation.

[Effect of Substrate Concentration on SAD Collaborative Nitrogen and Carbon Removal Efficiency in an ABR Reactor].

In order to solve the problem of declining total nitrogen (TN) removal caused by anaerobic ammonium oxidation (ANAMMOX) and the suppression of organic matter for ANAMMOX, the anaerobic baffled reactor (ABR), inoculating ANAMMOX sludge and anaerobic sludge from a municipal WWTP, was selected to construct system of ANAMMOX coupled denitrification (SAD) by the control of different substrate concentration. The SAD was constructed to study the effects of different influent substrates (COD, NO2--N, NH4+-N) on the performance of nitrogen and carbon removal in the coupled system and pollutant removal rules. The results showed that the coupling reaction was achieved in the ABR reactor and the inhibitory effect of organic compounds on anaerobic ammonium oxidation bacteria (AAOB) was relieved. When influent concentrations of COD, NO2--N, and NH4+-N were 260, 185, and 100 mg·L-1, respectively, which equates to a ratio of 2.6∶1.85∶1, the concentrations of these substances in the effluent decreased to 10, 1.0, and 0.9 mg·L-1, respectively. The TN removal rate reached 99%, hence stable system operation and ultra-low emissions of carbon and nitrogen pollutants were achieved. Under different conditions of substrate concentrations and ratios, the targeted pollutants were generally eliminated in the first compartment, in which the removal rate reached higher than 75%, and ANAMMOX held the dominant position in the SAD coupled system.

[Effect of Volume Loading Rate (VLR) on Denitrifying Phosphorus Removal by the ABR-MBR Process].

The effect of volume loading rate (VLR) on denitrifying phosphorus removal was investigated in a continuous-flow ABR-MBR combined process treating domestic wastewater to arrive at optimum process parameters. In the experiment, the VLR of the ABR was set at 0.76, 1.01, 1.51, and 2.27 kg·(m3·d)-1. The removal of carbon, nitrogen, and phosphorus in the system and the effect of the VLR in the MBR on nitrification performance were observed for each VLR of the ABR. The results showed that under the condition when the VLR of the ABR was 1.51 kg·(m3·d)-1, the amount of COD removal in the A2 chamber was the largest, and shortcut nitrification was achieved in the MBR when the VLR of the MBR was 1.51 kg·(m3·d)-1. Meanwhile, the removal efficiency of NH4+-N and TN reached more than 90% and 72%, respectively, the anaerobic P-release and anoxic P-uptake were 7.41 mg·L-1and 15.42 mg·L-1, respectively, and the concentration of PO43--P in effluent was lower than 0.5 mg·L-1, which indicated that the shortcut nitrification was more conducive to strengthening the performance of denitrifying phosphorus removal in the ABR-MBR system.

[Realization of Shortcut Nitrification in the ABR-MBR Process Treating Domestic Wastewater].

The operational control conditions for realizing shortcut nitrification in the membrane bioreactor (MBR) process was investigated in a lab-scale anaerobic baffled reactor (ABR)-MBR combined system, which laid the foundation for further research on the performance of denitrification phosphorus removal in the system. The experimental results under different conditions showed that shortcut nitrification in the MBR was achieved by controlling the dissolved oxygen (DO) concentration to low levels (0.5-1.0 mg·L-1 to 0.3-0.7 mg·L-1) and changing the effective volume of the MBR to control hydraulic retention time (HRT), with the HRT in the ABR equal to 6 h, sludge reflux ratio of 100%, NOx--N reflux ratio of 300%, and temperature of 30℃±2℃. Finally, the shortcut nitrification deteriorated as the HRT in the MBR increased from 3 h to 5 h, with nitrite accumulation rate dramatically dropping from 60% to 15%. The analysis of the influencing factors of shortcut nitrification showed that maintaining low DO concentration (0.3-0.7 mg·L-1) and gradually shortening HRT were the key factors. The pH, free ammonia (FA), free nitrous acid (FNA), temperature, and sludge retention time (SRT) had a slightly positive effect on shortcut nitrification. During the period of shortcut nitrification, a stable and high efficiency removal of COD and NH4+-N were achieved, and the average concentration of the effluent of COD and NH4+-N, whose removal rates were above 90%, were below 50 mg·L-1 and 2 mg·L-1, respectively, and the removal efficiency of total nitrogen (TN) reached 72%.