[Effect of Influent C/N Ratio on the Nutrient Removal Characteristics of SNEDPR Systems]

To determine the performance of nitrogen and phosphorus removal within a simultaneous nitrification endogenous denitrification system (SNEDPR), an extended anaerobic/low aerobic (dissolved oxygen:0.5-2.0 mg·L^-1)-operated sequencing batch reactor (SBR) was fed with simulation wastewater. The SBR was initiated under a constant influent C/N ratio of 10, with the simultaneous enrichment of polyphosphate-accumulating organisms (PAOs). It was then investigated at different influent C/N ratios of 10, 7.5, 5, and 2.5. The experimental results indicated that, when the influent C/N ratio was 10, SNEDPR could be successfully started up. The effluent PO₄^3--P and total nitrogen (TN) concentrations were 0.1 mg·L^-1 and 8.1 mg·L^-1. PO₄^3--P efficiency, TN efficiency, and SNED efficiency were 99.79%, 89.38%, and 58.0%, respectively. When the influent C/N ratio increased from 5 to 10, the nitrogen and phosphorus removal performance of the system improved with PRA, and SNED efficiency increased from 16.0 m·L^-1 and 48.0% to 24.4 mg·L^-1 and 69.2%, respectively. When the C/N ratio was 10, the TN and PO₄^3--P removal efficiencies increased to 94.5% and 100%, respectfully. When the C/N ratio was decreased to 2.5, the nitrogen and phosphorus removal performance of the system decreased. The PRA and SNED efficiencies were only 1.36 mg·L^-1 and 10%, respectively. During the stable phase of the system (C/N ratio were 10, 7.5 and 5), SNED efficiency reached to 85.9%, with the average effluent concentration of NH₄⁺-N, _x^--N, and PO₄^3--P being 0.0, 8.1, and 0.1 mg·L^-1, respectively.

[Nitrite Accumulation Characteristics of Partial Denitrification in Different Sludge Sources Using Sodium Acetate as Carbon Source]

In order to explore the characteristics of nitrite accumulation during the operational period of partial denitrification in different sludge sources using sodium acetate as a carbon source, No.1 SBR and No.2 SBR were used to inoculate with surplus sludge taken separately from a secondary sedimentation tank of a sewage treatment plant and simultaneous nitrification and denitrifying phosphorus removal system. By reasonably controlling the initial nitrate concentration and anoxic time, partial denitrification was realized. The carbon and nitrogen removal characteristics under different initial COD and NO₃^--N concentrations were investigated. The results showed that, using sodium acetate as the carbon source, the partial denitrification process in No.1 SBR and No.2 SBR sludge successfully began in 21 d and 20 d, respectively. The accumulation of NO₂^--N and nitrite accumulation rate (NAR) in reactors were maintained at high levels (12.61 mg·L^-1, 79.76% and 13.85 mg·L^-1, 87.60%, respectively). When the initial NO₃^--N concentration of No.2 SBR was 20 mg·L^-1 and the initial COD concentration increased from 60 mg·L^-1 to 140 mg·L^-1, the operation time for achieving the highest NO₂^--N accumulation in the system was shortened from 160 min to 6 min. The NO₃^--N ratio of the denitrification rate (in VSS) increased from 3.84 mg·(g·h)^-1 to 7.35 mg·(g·h)^-1. Increased initial COD concentration was beneficial to the accumulation of NO₂^--N during partial denitrification. When the initial COD concentration of No.2 SBR was 100 mg·L^-1 and the initial NO₃^--N concentration increased from 20 mg·L^-1 to 30 mg·L^-1, NAR was maintained above 90% and up to 100% (the initial NO₃^--N concentration was 25 mg·L^-1). When the initial NO₃^--N concentration was ≥ 35 mg·L^-1, insufficient COD caused NO₃^--N to be completely reduced to NO₂^--N. Under different initial COD concentrations (80, 100, or 120 mg·L^-1) and different initial NO₃^--N concentrations (20, 25, 30, or 40 mg·L^-1), the nitrogen and carbon removal and partial denitrification performance of the No.2 SBR was better than that of No.1 SBR.

[Operating Characteristics of a DPR-SNED System Treating Low C/N Municipal Wastewater and Nitrate-containing Sewage]

In order to realize the simultaneous treatment of low C/N municipal wastewater and high nitrate wastewater, a sequencing batch reactor (SBR), inoculated with activated sludge, was used to initiate the denitrifying phosphorus removal coupled with simultaneous nitrification and endogenous denitrification (DPR-SNED). The anaerobic/anoxic/hypoxic durations and dissolved oxygen (DO) concentration were appropriately controlled, and the nitrogen and phosphorus removal characteristics were examined. The experimental results demonstrated that, in the anaerobic/hypoxia operation mode, with an anaerobic duration of 3 h and DO concentration of 0.5-1.0 mg·L^-1, the simultaneous nitrification of phosphorus removal (SNEDPR) system successfully began in 60 d. The effluent PO₄^3--P concentration was below 0.5 mg·L^-1, the nutrient and COD removal efficiencies were stably maintained above 90% and 80%, respectively, and the SNED efficiency and COD_ins efficiency reached 70% and 95%, respectively. When the operation mode was anaerobic/anoxic/hypoxic and nitrate-containing sewage was added at the beginning of the anoxic stage, DPR-SNED was achieved with the effluent PO₄^3--P concentration<0.5 mg·L^-1, nutrient and COD removal efficiencies above 88% and 90%, respectively, and SNED efficiency and COD_ins efficiency maintained at 62% and 90%, respectively. After the successful initiation of DPR-SNED, enhanced intracellular carbons storage was achieved by phosphorus-and glycogen-accumulating organisms using the limited carbons in raw municipal wastewater to provide sufficient carbon sources for subsequent nutrient removal. In addition, the endogenous partial denitrification ensured the efficient nitrogen removal performance of the DPR-SNED system at low C/N conditions (average 4).

[Denitrification and Phosphorus Removal from Low C/N Urban Sewage Based on a Post-Partial Denitrification AOA-SBR Process]

This study focuses on the investigation of the nitrogen (N) and phosphorus (P) removal characteristics of a combination of enhanced phosphorus removal (EBPR) with simultaneous partial nitrification endogenous denitrification (SPND) and post-partial denitrification process. An anaerobic/aerobic/anoxic (A/O/A) operated sequencing batch reactor (SBR) fed with urban sewage was optimized by regulating the aeration rate and anoxic time. Based on this optimization, deep-level nitrogen and phosphorus removals from low C/N urban sewage could be realized. The experimental results show that the effluent PO₄^3--P concentration decreased from 0.06 mg·L^-1 to 0 mg·L^-1, the effluent NH₄⁺-N, NO₂^--N, and NO₃^--N concentrations gradually decreased from 0.18, 18.79, and 0.08 mg·L^-1 to 0, 16.46, and 0.05 mg·L^-1, respectively, and the TN removal efficiency increased from 72.69% to 77.97% when the aeration rate decreased from 1.0 L·min^-1 to 0.6 L·min^-1 and the anoxic duration was 180 min. With the reduction of the aeration rate, the SPND phenomenon became notable and the SND rate increased from 19.18% to 31.20%. When the anoxic duration was extended from 180 min to 420 min, the effluent PO₄^3--P, NH₄⁺-N, and NO₃^--N concentrations stabilized at~0, 0, and 0.03 mg·L^-1, respectively. The effluent NO₂^--N concentration was as low as 3.06 mg·L^-1, the SND rate was~32.21%, the TN removal performance gradually improved, and the TN removal efficiency was as high as 99.42%. Thus, deep-level nitrogen and phosphorus removals could be realized with the SPNDPR-PD system.