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.

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.

Improving mechanical stability of anammox granules with organic stress by limited filamentous bulking.

Under organic stress, the limited filamentous bulking (FB) was demonstrated to improve anammox capability by inhibiting granule disintegration and washout. The accumulation of internal stress played a more important role than the adverse physicochemical properties (low viscoelasticity and hydrophobicity) of granules in limiting granular strength by consuming the granular elastic energy. Different from the floc-forming heterotrophic bacteria (HB) that stored its growth stress as internal stress by pushing the surrounded anammox micro-colonies outwards under the spatial constraint of elastic anammox "shell", the filamentous HB grew into a uniform network structure within granules, endowed granules low internal stress and acted as the granular skeleton due to its rich amyloid substance, which was benefited from the elimination of inhomogeneous growth and the consequent expansion competition for living space. Combined with the mechanical instability and sticking-spring models, controlling FB at limited level was effective for improving granular strength without affecting sludge-water separation.

Improving anammox performance by limited filamentous bulking for wastewater treatment with organic stress.

In this study, the filamentous bulking was demonstrated to improve anammox capability and anammox bacteria (AnAOB) population density under organic stress. The selective heterotrophic bacteria (HB) washout that involved in shear detachment, enmeshment and biomass washout was triggered. The microbial spatial distribution and granular detachment properties revealed that the filamentous bulking transferred the "location advantage" of HB from granules interior to surface, and endowed granular surface low shear tolerance for shear detachment, ultimately resulted in selective HB detachment. The detached filaments-mediated enmeshment provided additional selective pressure for free HB-flocs, eventually achieving the retention time differentiation between AnAOB (34 - 141 days) and HB (3 - 15 days), and a high anammox population density. Controlling dissolved oxygen level was crucial for regulating sludge bulking. Collectively, the filamentous bulking was developed as an effective anti-organic stress strategy to broaden the application of granular anammox process in actual wastewater treatment.