Performance and microbial characterization of aerobic granular sludge in a sequencing batch reactor performing simultaneous nitrification, denitrification and phosphorus removal with varying C/N ratios

Metaproteomics, metagenomics and 16S rRNA sequencing provide different perspectives on the aerobic granular sludge microbiome

The tremendous progress in sequencing technologies has made DNA sequencing routine for microbiome studies. Additionally, advances in mass spectrometric techniques have extended conventional proteomics into the field of microbial ecology. However, systematic studies that provide a better understanding of the complementary nature of these 'omics' approaches, particularly for complex environments such as wastewater treatment sludge, are urgently needed. Here, we describe a comparative metaomics study on aerobic granular sludge from three different wastewater treatment plants. For this, we employed metaproteomics, whole metagenome, and 16S rRNA amplicon sequencing to study the same granule material with uniform size. We furthermore compare the taxonomic profiles using the Genome Taxonomy Database (GTDB) to enhance the comparability between the different approaches. Though the major taxonomies were consistently identified in the different aerobic granular sludge samples, the taxonomic composition obtained by the different omics techniques varied significantly at the lower taxonomic levels, which impacts the interpretation of the nutrient removal processes. Nevertheless, as demonstrated by metaproteomics, the genera that were consistently identified in all techniques cover the majority of the protein biomass. The established metaomics data and the contig classification pipeline are publicly available, which provides a valuable resource for further studies on metabolic processes in aerobic granular sludge.

Performance of anaerobic/oxic/anoxic simultaneous nitrification, denitrification and phosphorus removal system overwhelmingly dominated by Candidatus_Competibacter: Effect of aeration time

The anaerobic/oxic/anoxic simultaneous nitrification, denitrification and phosphorus removal process (AOA-SNDPR) is a promising technology for enhanced biological wastewater treatment and in situ sludge reduction. Herein, the effects of aeration time (90, 75, 60, 45, and 30 min, respectively) on AOA-SNDPR were evaluated including simultaneous nutrients removal, sludge characteristics, and microbial community evolution, where the role of a denitrifying glycogen accumulating organisms, Candidatus_Competibacter, was re-explored given its overwhelming dominance. Results revealed that nitrogen removal was more vulnerable, and a moderate aeration period of 45-60 min mostly favored nutrients removal. Low observed sludge yields (Y_obs) were obtained with decreased aeration (as low as 0.02 g MLSS/g COD), while MLVSS/MLSS got increased. The dominance of Candidatus_Competibacter was proven to be the key to endogenous denitrifying and in situ sludge reduction. This study would aid the more carbon- and energy-efficient aeration strategy for AOA-SNDPR systems treating low-strength municipal wastewater.

Bacterial-algae biofilm enhance MABR adapting a wider COD/N ratios wastewater: Performance and mechanism

Although membrane aerated biofilm reactor (MABR) is promising in nitrogen removal due to its counter-diffusion biofilms structure, it still cannot adapt a wider COD/N ratios wastewater. In this condition, expanding the MABR applicability range in different COD/N ratio wastewater is necessary. In this study, a bacterial-algae biofilm, instead of bacteria biofilm, supporting membrane aerated biofilm reactor (MABAR) was constructed, and the performance was compared to MABR. Results showed that the total nitrogen (TN) removal efficiency was promoted significantly in MABAR regardless of the COD/N ratio. Compared to MABR, effluent TN concentration in COD/N ratio of 2, 5, and 8 declined by 14.34 mg/L, 0.50 mg/L, and 12.10 mg/L, respectively. Nitrification inhibition test suggested that algae assimilation made an obvious contribution (at least 18.18 mg/L) to the NH₄⁺-N removal in MABAR. Besides, redundancy analysis (RDA) indicates that MABAR has a negative correlation with Nitrospirae but is positively correlated with NH₄⁺-N removal load. These results are consistent with the kinetics result that algae assimilation, instead of nitrification-denitrification, is responsible for the nitrogen removal in MABAR. Therefore, the change of nitrogen removal route further gave MABAR excellent adaptability and impact resistance to address wastewater with different COD/N ratios, which is conducive to its wide application.

Characteristics of nutrients removal under partial denitrification initiated by different initial nitrate concentration

The partial denitrification (PD) is a very promising process developed in the last decade, to study the comprehensive influence of influent carbon to nitrogen (C/N) on the activated sludge system under PD, six sequencing batch reactors (SBRs) were operated in parallel at C/N of 2.75, 3.30, 4.13, 5.50, 8.25 and 16.50, the nitrogen removal, phosphorus removal and sludge settleability of PD were investigated. The results showed that PD was observed treating synthetic wastewater in all the six SBRs, and the nitrite accumulation rate (NAR) was highest at C/N of 5.50 (NAR of 82.30%). However, due to the alternate inhibition of NO₂^--N and free nitrous acid (FNA) produced by a limited carbon source, both the sludge settleability and phosphorus removal deteriorated. The average SVI at C/N of 8.25 was 130% lower than C/N of 3.30, and the average amount of PO₄^3--P released at C/N of 16.5 was 189% higher than C/N of 2.75. Kinetic analysis showed that the denitrification kinetics of PD and complete denitrification were similar, and the nitrite accumulation was caused by the difference between nitrate reduction rate and nitrite reduction rate. Variations of on-line parameters (pH and ORP) revealed that nitrite accumulation could be indicated by judging the nitrate turning point and nitrite turning point on pH and ORP curves, which provided guidance for the setup of PD.

Multistability and Reversibility of Aerobic Granular Sludge Microbial Communities Upon Changes From Simple to Complex Synthetic Wastewater and Back

Aerobic granular sludge (AGS) is a promising alternative wastewater treatment to the conventional activated sludge system allowing space and energy saving. Basic understanding of AGS has mainly been obtained using simple wastewater containing acetate and propionate as carbon source. Yet, the aspect and performances of AGS grown in such model systems are different from those obtained in reactor treating real wastewater. The impact of fermentable and hydrolyzable compounds on already formed AGS was assessed separately by changing the composition of the influent from simple wastewater containing volatile fatty acids to complex monomeric wastewater containing amino acids and glucose, and then to complex polymeric wastewater containing also starch and peptone. The reversibility of the observed changes was assessed by changing the composition of the wastewater from complex monomeric back to simple. The introduction of fermentable compounds in the influent left the settling properties and nutrient removal performance unchanged, but had a significant impact on the bacterial community. The proportion of Gammaproteobacteria diminished to the benefit of Actinobacteria and the Saccharibateria phylum. On the other hand, the introduction of polymeric compounds altered the settling properties and denitrification efficiency, but induced smaller changes in the bacterial community. The changes induced by the wastewater transition were only partly reversed. Seven distinct stables states of the bacterial community were detected during the 921 days of experiment, four of them observed with the complex monomeric wastewater. The transitions between these states were not only caused by wastewater changes but also by operation failures and other incidences. However, the nutrient removal performance and settling properties of the AGS were globally maintained due to the functional redundancy of its bacterial community.

The effect of Np-magnetite on the granulation process of an SBR reactor used for domestic wastewater treatment

This study investigated the effect of magnetite nanoparticles (Np-magnetite) added to a pilot-scale sequencing batch reactor (SBR) treating domestic wastewater, to improve aerobic granular sludge (AGS) formation and the effects of granule disintegration. Np-magnetite additions (75 mg L^-1) were made during the start-up of the reactor and repeated after 100 and 170 days, when granule disintegration was observed. From the first Np-magnetite addition, SVI₅ was reduced from 1315 to 85 mL g^-1. The granular biomass was observed on the 56th day, when 57% of the granules presented diameters bigger than 212 µm. The 100-day disintegration episode disturbed the granular biomass, reducing the volatile suspended solids by 51%, increasing the SVI values to above 200 mL g^-1. Np-magnetite addition recovered all the granular biomass parameters to the values observed before disintegration. The treatment efficiency was stable during operation of the reactor for nutrients (52.8 ± 23.4% NH₄⁺-N; 54.5 ± 12.2% PO₄^3--P) and carbonaceous organic matter (71.7 ± 12.7% BOD₅; 77.5 ± 10.0% COD_t). Np-magnetite addition changed the microbial community of the granular sludge, analysed via high-throughput 16S RNA sequencing, and recovered the treatment efficiency previously disturbed by the disintegration processes. These results indicate the potential of Np-magnetite as an agent for sludge aggregation in an aerobic granular reactor.