Efficient nitrite accumulation and elemental sulfur recovery in partial sulfide autotrophic denitrification system: Insights of seeding sludge, S/N ratio and flocculation strategy.

Partial sulfide autotrophic denitrification (PSAD) has been proposed as a promising process to achieve elemental sulfur recovery and nitrite accumulation, which is required for anaerobic ammonium oxidation reaction. This study investigated the effect of seeding sludge on the start-up performance of PSAD process, with different sludge taken from the oxidation zone (S-o) of wastewater treatment plants, partial denitrification reactor (S-PD), and anoxic/oxic reactor (S-A/O). The results showed that the PSAD process could be achieved rapidly in three systems on day 22, 29 and 26, respectively. In particular, the S-O system completed the start-up in the shortest time of 22 d, with NO3--N and S2- removal efficiency of 85.3% and 99.3%, respectively. Selected the S-O system to operate long term, the nitrite (NO2--N) and biological elemental sulfur (S0) accumulation efficiencies were systematically investigated under different S/N ratios (in a range of 0.71-1.2). The maximum NO2--N and S0 accumulation efficiencies were 85.2% and 73.5%, respectively, at the S/N ratio of 1.1. In addition, the separation and recovery of S0 in effluent was achieved by employing polyaluminum chloride (PAC) as a flocculant. Using 2D Gaussian function as quadratic model for the maximizing of S0 flocculant efficiency (SFR), an optimal condition of PAC dosage 7.92 mL/L and pH 5.14 was obtained, and the SFR reached 94.1%, under such conditions. The findings offered useful information to facilitate the application of the PSAD process.

[Partial Nitritation and Anaerobic Ammonia Oxidation Synergistic Denitrification to Remove Nitrogen and Carbon from Domestic Sewage].

A sequencing batch reactor-anaerobic sequencing batch reactor(SBR-ASBR) process was used to treat domestic sewage. In the SBR, the effects of the anoxic/aerobic time ratio and temperature on the realization of partial nitritation(PN) were investigated. In the ASBR, the effects of different COD/NO2--N(C/N) ratios on the removal of nitrogen and carbon using anaerobic ammonia oxidation(ANAMMOX) and denitrification were studied. The results illustrated that:① After three single cycles and on the 22nd day, the NO2--N accumulation rate(NiAR) was 98.06%, and the nitrate nitrogen generation rate(SNiPR, calculated as N/VSS) was 0.28g·(g·d)-1, and simultaneous nitrification and denitrification removal the TN and COD were 12.29 and 110.36mg·L-1, respectively(temperature=25℃, anoxic/aerobic time ratio=30 min:30 min). ② At an anoxic/aerobic time ratio of 30 min:30 min, the filamentous sludge bulked, the sludge activity decreased, and sludge settleability was poor at 15℃. Furthermore, the conversion rate of NH4+-N to NO2--N was 86.83%, indicating that the effluent NH4+-N concentration was too low to provide suitable matrix concentrations for ANAMMOX at 30℃. The effluent concentrations of NH4+-N and NO2--N were 31.58 mg·L-1 and 35.04mg·L-1, respectively, matching the ratio of the ANAMMOX substrate at 25℃. ③ The SBR-ASBR combined process showed good denitrification performance; the effluent TN, NH4+-N, and COD concentrations were stable at 13.13, 4.83, and 69.96mg·L-1, respectively, and the removal rates were 83.10%, 93.64%, and 75.11%, respectively. When the influent C/N of the ASBR was 2.5, 2.0, and 1.5, respectively, anaerobic ammonia oxidation and denitrification showed the best performance with respect to nitrogen and carbon removal with a C/N of 2.0. The effluent NH4+-N, NO2--N, NO3--N, and COD were 0.09, 0.25, 1.04, and 32.73 mg·L-1, respectively.

Simultaneous carbon and nitrogen removal by anaerobic ammonium oxidation and denitrification under different operating strategies.

Real domestic wastewater was treated initially in a sequencing batch reactor (SBR), with partial nitrification achieved before the effluent was used as the influent for an anaerobic ammonium oxidation (anammox) reactor (ASBR) system. The effects of three factors, hydraulic retention time (HRT), substrate (NO2-/NH4+) ratio, and the ratio of COD to NH4+ (C/N), on the removal of carbon and nitrogen by an anammox and denitrification process were investigated in the ASBR reactor at 24°C. The response surface methodology was used to explore the interactions of the three factors. The results indicated that the nitrogen and carbon removal efficiency was optimum when HRT, substrate ratio, and C/N ratio were 33 h, 1.4-1.6, and 3-5, respectively. The optimal removal rates of NH4+, NO2-, and COD were 96.30%, 97.79%, and 72.91%, respectively. The ΔNO2-/ΔNH4+ and ΔNO3-/ΔNH4+ ratios of the first two conditions were less than the theoretical anammox values of 1.32 and 0.26 due to heterotrophic denitrification. The optimum nitrogen and carbon removal efficiencies of the third condition could be realized by the synergistic effect of denitrification and the anammox process. Analysis of variance (ANOVA) results showed that when the HRT was 33.48 h, the substrate ratio was 1.46, and the C/N ratio was 4.28, the total nitrogen removal rate (TNR) was optimum (90.12 ± 0.1%), verified by parallel experiments.