Potential directions for future development of mainstream partial nitrification-anammox processes: Ammonia-oxidizing archaea as novel functional microorganisms providing nitrite.

The application of ammonia-oxidizing archaea (AOA)-based partial nitrification-anammox (PN-A) for mainstream wastewater treatment has attracted research interest because AOA can maintain higher activity in low-temperature environments and they have higher affinity for oxygen and ammonia-nitrogen compared with ammonia-oxidizing bacteria (AOB), thus facilitating stabilized nitrite production, deep removal of low-ammonia, and nitrite-oxidizing bacteria suppression. Moreover, the low affinity of AOA for ammonia makes them more tolerant to N-shock loading and more efficiently integrated with anaerobic ammonium oxidation (anammox). Based on the limitations of the AOB-based PN-A process, this review comprehensively summarizes the potential and significance of AOA for nitrite supply, then gives strategies and influencing factors for replacing AOB with AOA. Additionally, the methods and key influences on the coupling of AOA and anammox are explored. Finally, this review proposes four AOA-based oxygen- or ammonia-limited autotrophic nitritation/denitrification processes to address the low effluent quality and instability of mainstream PN-A processes.

Low strength wastewater anammox start-up and stable operation by inoculating sponge-iron sludge: Cooperation of biological iron and iron bacteria.

The application of anaerobic ammonium oxidation (Anammox) technology in low-strength wastewater treatment still faces difficult in-situ start-ups and unstable operations. Sponge-iron sludge (R1) was used as a novel inoculum to provide a promising solution. Conventional activated sludge (R0) was used as the control. However, little is known about the feasibility and performance during the start-up and operation of Anammox combined with biological iron and iron bacteria in an iron sludge system. Anammox was successfully started both in R1 (87 days) and R0 (89 days) with a low-strength influent (with a nitrogen loading rate (NLR) of 43.64 ± 0.41 g N/(m3⋅d)). During long-term operation, the R0 nevertheless produced higher nitrates (9.7 ± 0.1 mg/L) than expected. In contrast, R1 presented no excess nitrate production (2.1 ± 0.06 mg/L). The total inorganic nitrogen (TIN) removal efficiency increased from 78.2 ± 7.1% in R0 to 86.1 ± 4.3% in R1. The iron sludge in R1 was divided equally into three parts and three different nitrogen-feeding methods were used over the 34 days of operation, as follows: first using a mixture of ammonium (27.15 ± 1.0 mg/L) and nitrite (32.7 ± 1.7 mg/L), then only ammonium (27.15 ± 1.0 mg/L) and lastly only nitrite (32.7 ± 1.7 mg/L) as the influent. R1 was a coupled system composed of Anammox, Feammox, and NOx--dependent Fe(II) oxidation (NDFO). The contribution of Feammox and NDFO to TIN removal was 27.1 ± 1.2% and 31.9 ± 0.7%. However, Anammox was the primary nitrogen transformation pathway. X-ray diffraction (XRD) analysis shows that iron hydroxide (Fe(OH)3) and iron oxide hydroxide (FeOOH) were generated in R1. The produced Fe(OH)3 and FeOOH were capable of participating in Feammox and formed a Fe(II)/Fe(III) cycle which further removed nitrogen. Therefore, a highly stable and impressive nitrogen removal performance was demonstrated in the iron sludge Anammox system under the cooperation of biological iron and iron bacteria. The study considered the enrichment of norank_c_OM190, Desulfuromonas, and Thiobacillus and their contribution to the Anammox, Feammox, and NDFO processes, respectively. This study provides a new perspective for the start-up and stable operation of low-strength wastewater Anammox engineering applications.

Use of sponge iron as an indirect electron donor to provide ferrous iron for nitrate-dependent ferrous oxidation processes: Denitrification performance and mechanism.

Sponge iron (SI) can serve as an indirect electron donor to provide Fe(II) for the nitrate-dependent ferrous oxidation (NDFO) process, producing OH- and magnetite. The SI-NDFO system mainly uses Fe(OH)2 as an electron donor, achieving a TN reduction rate of 0.42 mg-TN/(gVSS·h) for a period of at least 90 days. The enrichment of iron-oxidizing bacteria and the competition of iron-carbon micro-electrolysis for reaction sites on the surface of SI are the main reasons for the improvement of total nitrogen removal efficiency (TNRE). With an influent NO3--N concentration of 50 mg/L and a SI concentration of 50 g/L (at pH 5.0 and 30 °C), the TNRE reached a maximum level of 38.28%. In addition, reducing the pH environment was found to improve the denitrification efficiency of the SI-NDFO system, although denitrification stability was also reduced as a result. Overall, the SI-mediated NDFO process is a promising technique.