Successful startup of the single-stage PN-A (partial nitrification-anammox) process by controlling the oxygen supply

Enhanced prediction of partial nitrification-anammox process in wastewater treatment by developing an attention-based deep learning network

In the process of partial nitrification and anaerobic ammonia oxidation (anammox) for nitrogen removal, the process offers simple metabolic pathways, low operating costs, and high nitrogenous loading rates. However, since the partial nitrification-anammox (PN-anammox) process combines partial nitrification and anammox reactions within the same reactor, strict control of dissolved oxygen (DO) is essential. Additionally, assessing treatment performance through chemical measurement involves time lag, making it challenging to recover the biological process when issue arise, especially in the PN-anammox process, where strict DO control and the sensitivity of anammox bacteria to conditions and substrates demand timely intervention. Therefore, modelling for PN-anammox process is of great significance. Because of traditional modelling methods have limitations in this process, in this study, deep learning networks were applied to model and predict the process by constructing a dataset based on long-term experiments. Specifically, Long Short-Term Memory Network (LSTM) and Densely Connected Convolutional Network (DenseNet) were employed, and an enhanced Attention-based DenseNet (AttentionNet) was developed to further improve the prediction performance. These networks were utilized for long-term continuous PN-anammox reactors for nitrogen removal in low-strength wastewater. The results demonstrated that both DenseNet and the proposed AttentionNet effectively modeled the PN-anammox processes, even under conditions of unstable influent quality and relatively poor treatment performance. The mean relative error (MRE) for DenseNet was under 15%, while AttentionNet achieved an MRE of approximately 10% or less, highlighting the superior performance of the Attention layer. These findings were further validated by Bland-Altman analysis. Additionally, further analysis of AttentionNet showed that predictions for pH, nitrite nitrogen, and total nitrogen in the effluent were more accurate than those for other output parameters.

How biofilm and granular sludge cope with dissolved oxygen exposure in anammox process: Performance, bioaccumulation characteristics and bacterial evolution

In order to study the resistance mechanisms of biofilm and granular sludge to various dissolved oxygen (DO) exposures in anaerobic ammonium oxidation (anammox) process, a biofilm - granular sludge anammox reactor was established and operated. Experimental results showed that DO levels of ≤0.41 mg L-1 hardly affected the total nitrogen removal efficiency (TNRE). Higher DO levels of 1.96-2.08 mg L-1 promoted biomass disintegration and decreased specific anammox activity and extracellular polymeric substance (EPS) levels in granular sludge, but did not decrease EPS significantly in biofilm. The relative abundance of anammox genus Candidatus Kuenenia in granular sludge and biofilm decreased to 13.93% and 1.93%, respectively. NO3--N was accumulated due to the increased NOB genus Nitrospira in granular sludge and biofilm. The inhibition effects of 1.96-2.08 mg L-1 DO on anammox system were reversible, and the TNRE was quickly restored to (82.21 ± 2.39)% with AnAOB accumulation after removing aeration. This study provided theoretical support for the development of coupled biological nitrogen removal system based on anammox with other aerobic processes.

Start-up and optimization of a one-stage partial nitrification-anammox (PN-A) process treating low ammonium concentration wastewater: experimental results and modeling investigation

Although the partial nitrification-anammox process (PN-A) has achieved great success in nitrogen removal for the high ammonium concentration wastewater, its application is still limited in low ammonium concentration wastewater treatment due to its instability and low nitrogen removal efficiency. In this study, a sequencing batch reactor (SBR) with continuous aeration was employed to enrich ammonia oxidation bacteria (AOB) and suppress nitrite oxidation bacteria (NOB) first; then, the SBR was operated intermittently aerated SBR (IASBR), to which the anammox granular sludge (AMX) was added to achieve complete autotrophic nitrogen removal under low influent ammonium concentration of 100 mg/L. A mathematical model was used to optimize the IASBR aeration strategy to achieve sub-optimal nitrogen removal. The experimental results showed that high nitrite accumulation efficiency (above 80%) in the SBR and a fast start-up within 100 days and a stable TN (total nitrogen) removal efficiency of 70% were achieved in the IASBR. Meanwhile, the simulation results indicated that keeping aeration duration at 4 h, k_La (oxygen transfer coefficient) at 50 day^-1, or aeration duration at 2.5 h, k_La at 80 day^-1 could obtain a higher total nitrogen removal efficiency (TNR) (TNR > 80%), and the TN removal could also be improved by increasing hydraulic retention time (HRT) under the optimal oxygen supply rate.

Genome-centered metagenomics illuminates adaptations of core members to a partial Nitritation-Anammox bioreactor under periodic microaeration

The partial nitritation-anaerobic ammonium oxidation (anammox; PN-A) process has been considered a sustainable method for wastewater ammonium removal, with recent attempts to treat low-strength wastewater. However, how microbes adapt to the alternate microaerobic-anoxic operation of the process when treating low ammonium concentrations remains poorly understood. In this study, we applied a metagenomic approach to determine the genomic contents of core members in a PN-A reactor treating inorganic ammonium wastewater at loading as low as 0.0192 kg-N/m³/day. The metabolic traits of metagenome-assembled genomes from 18 core species were analyzed. Taxonomically diverse ammonia oxidizers, including two Nitrosomonas species, a comammox Nitrospira species, a novel Chloroflexota-related species, and two anammox bacteria, Ca. Brocadia and Ca. Jettenia, accounted for the PN-A reactions. The characteristics of a series of genes encoding class II ribonucleotide reductase, high-affinity bd-type terminal oxidase, and diverse antioxidant enzymes revealed that comammox Nitrospira has a superior adaptation ability over the competitors, which may confer the privileged partnership with anammox bacteria in the PN-A reactor. This finding is supported by the long-term monitoring experiment, showing the predominance of the comammox Nitrospira in the ammonia-oxidizing community. Metagenomic analysis of seven heterotrophs suggested that nitrate reduction is a common capability in potentially using endogenous carbohydrates and peptides to enhance nitrogen removals. The prevalence of class II ribonucleotide reductase and antioxidant enzymes genes may grant the adaptation to cyclically microaerobic/anoxic environments. The predominant heterotroph is affiliated with Chloroflexota; its genome encodes complete pathways for synthesizing vitamin B6 and methionine. By contrast, other than the two growth factors, Nitrospira and anammox bacteria are complementary to produce various vitamins and amino acids. Besides, the novel Chloroflexota-related ammonia oxidizer lacks corresponding genes for detoxifying the reactive oxygen species and thus requires the aid of co-existing members to alleviate oxidative stress. The analysis results forecast the exchanges of substrates and nutrients as well as the collective alleviation of oxidative stress among the core populations. The new findings of the genomic features and predicted microbial interplay shed light on microbial adaptation to intermittent microaeration specific to the PN-A reactor, which may aid in improving its application to low-strength ammonium wastewater.

Large-scale (500 kg N/day) two-stage partial nitritation/anammox (PN/A) process for liquid-ammonia mercerization wastewater treatment: Rapid start-up and long-term operational performance

The nitrogen pollution control of liquid-ammonia mercerization wastewater (LMWW) is one of the typical obstacle restricting the sustainability of textile industry. In this study, a 500 kg N/day two-stage partial nitritation/anammox (PN/A) process containing PN reactor filled with zeolite and biofilm anammox reactors was successfully started up in 45 days and operated stably with high shock resistance over one year for LMWW treatment. The large-scale process achieved an average ammonium removal efficiency (94.3 ± 2.3%), total nitrogen removal efficiency (89.4 ± 2.7%) and nitrogen removal rate (1.003 ± 0.386 kg N/m³/day) during one year engineering operation. Simultaneous denitrification was revealed by the contribution of 5.2% total nitrogen removed. High-throughput sequencing results showed that Nitrosomonas was the most dominant genus in PN reactor, and Ca. Anammoxoglobus and Ca. Kuenenia were the functional bacteria for nitrogen removal in anammox reactors. Compared to traditional nitrification-denitrification process, the large-scale process reduced a total operational cost of 46.03 CNY/kg N for LMWW. This study revealed the proposed process was quite reliable with fast start-up and high impact resistance to overcome the obstacle of nitrogen pollution control for LMWW economically and conducive to the sustainable development for textile industry.