Effects of seasonal temperature variations on phosphorus removal, recovery, and key metabolic pathways in the suspended biofilm

Cultivation of phosphate-accumulating biofilm: Study of the effects of acyl-homoserine lactones (AHLs) and cyclic dimeric guanosine monophosphate (c-di-GMP) on the formation of biofilm and the enhancement of phosphate metabolism capacity

This study investigated the mechanisms of microbial growth and metabolism during biofilm cultivation in the biofilm sequencing batch reactor (BSBR) process for phosphate (P) enrichment. The results showed that the sludge discharge was key to biofilm growth, as it terminated the competition for carbon (C) source between the nascent biofilm and the activated sludge. For the tested reactor, after the sludge discharge on 18 d, P metabolism and C source utilization improved significantly, and the biofilm grew rapidly. The P concentration of the recovery liquid reached up to 157.08 mg/L, which was sufficient for further P recovery via mineralization. Meta-omics methods were used to analyze metabolic pathways and functional genes in microbial growth during biofilm cultivation. It appeared that the sludge discharge activated the key genes of P metabolism and inhibited the key genes of C metabolism, which strengthened the polyphosphate-accumulating metabolism (PAM) as a result. The sludge discharge not only changed the types of polyphosphate-accumulating organisms (PAOs) but also promoted the growth of dominant PAOs. Before the sludge discharge, the necessary metabolic abilities that were spread among different microorganisms gradually concentrated into a small number of PAOs, and after the sludge discharge, they further concentrated into Candidatus_Contendobacter (P3) and Candidatus_Accumulibacter (P17). The messenger molecule C-di-GMP, produced mostly by P3 and P17, facilitated P enrichment by regulating cellular P and C metabolism. The glycogen-accumulating organism (GAO) Candidatus_Competibacter secreted N-Acyl homoserine lactones (AHLs), which stimulated the secretion of protein in extracellular polymeric substances (EPS), thus promoting the adhesion of microorganisms to biofilm and improving P metabolism via EPS-based P adsorption. Under the combined action of the dominant GAOs and PAOs, AHLs and C-di-GMP mediated QS to promote biofilm development and P enrichment. The research provides theoretical support for the cultivation of biofilm and its wider application.

A novel process based on powder carriers demonstrates robustness in nitrogen and phosphorus removal from real municipal wastewater

The development of efficient and low-consumption wastewater upgrading process is currently at the forefront of the wastewater treatment field. In this study, a novel wastewater treatment process based on powder carriers was proposed. Three systems, namely the activated sludge (AS) system, powder carrier (PC) system, and moving bed biofilm reactor (MBBR) system, were established and operated for over 140 days to treat real municipal wastewater. The characteristics and differences between the three systems were comprehensively investigated. The results suggested that the PC system exhibited notable advantages in nitrogen and phosphorus removal, especially under high influent load and low aeration conditions. The PC system, characterized by a higher nitrification rate compared to the MBBR system and a higher denitrification rate compared to the AS system, contributed to the stable nitrogen removal performance. The particle size of the zoogloea increased under the linkage of the powder carriers, and the mean size of micro-granules reached 170.88 μm. Large number of hydrophobic functional groups on sludge surface, coupled with increased protein content in EPS, further promoted sludge aggregation. Micro-granules formation improved settling performance and enhanced the abundance and activity of functional microbes. A significant enrichment in denitrifying bacteria and denitrifying phosphorus accumulating bacteria was observed in PC system. Up-regulation of the napA, narG, and nosZ genes was responsible for efficient nitrogen removal of the PC system. Moreover, a higher abundance in polyphosphate phosphotransferase (2.11 %) was found in PC system compared with AS and MBBR systems. The increase in the enzymes associated with poly-β-hydroxybutyrate (PHB) synthesis metabolism in PC system provided the energy for denitrification and phosphorus removal processes.

Metagenomics, metatranscriptomics, and proteomics reveal the metabolic mechanism of biofilm sequencing batch reactor with higher phosphate enrichment capacity under low phosphorus load

The biofilm sequencing batch reactor (BSBR) process has higher phosphate recovery efficiency and enrichment multiple when the phosphorus load is lower, but the mechanism of phosphate enrichment at low phosphorus load remains unclear. In this study, we operated two BSBR operating under low and high phosphorus load (0.012 and 0.032 kg/(m3·d)) respectively, and used metagenomic, metatranscriptomic, and proteomics methods to analyze the community structure of the phosphorus accumulating organisms (PAOs) in the biofilm, the transcription and protein expression of key functional genes and enzymes, and the metabolism of intracellular polymers. Compared with at high phosphorus load, the BSBR at low phosphorus load have different PAOs and fewer types of PAOs, but in both cases the PAOs must have the PHA, PPX, Pst, and acs genes to become dominant. Some key differences in the metabolism of PAOs from the BSBR with different phosphorus load can be identified as follows. When the phosphorus load is low, the adenosine triphosphoric acid (ATP) and NAD(P)H in the anaerobic stage come from the TCA cycle and the second half of the EMP pathway. The key genes that are upregulated include GAPDH, PGK, ENO, ppdk in the EMP pathway, actP in acetate metabolism, phnB in polyhydroxybutyrate (PHB) synthesis, and aceA, mdh, sdhA, and IDH1 in the TCA cycle. In the meantime, the ccr gene in the PHV pathway is inhibited. As a result, the metabolism of the PAOs features low glycogen with high PHB, Pupt, Prel, and low PHV. That is, more ATP and NAD(P)H flow to phosphorus enrichment metabolism, thus allowing the highly efficient enrichment of phosphorus from low concentration phosphate thanks to the higher abundance of PAOs. The current results provide theoretical support and a new technical option for the enrichment and recovery of low concentrations of phosphate from wastewater by the BSBR process.

A new insight in enhancing phosphate enrichment in biofilm process: Comparison of the key metabolic pathways in highly-efficient and dominant PAOs based on metagenomics

The formation of dominant phosphate-accumulating organisms (PAOs) is essential for the high enrichment of phosphate in biofilm sequencing batch reactors (BSBR) for phosphorus recovery. The dominant PAOs in the biofilm process have not been isolated and purified, and the key metabolic pathways that promote the formation of dominant PAOs are still unclear. In this study, four strains of highly-efficient PAOs were obtained by an innovative isolation procedure. The relationship between the abundance of highly-efficient and dominant PAOs and the phosphate removal ability was compared. We found that the abundance of PAOs was positively correlated with the phosphate removal efficiency in vitro pure culture and complex biofilm process. Metagenomics analysis revealed that compared with highly-efficient PAOs cultured in vitro, dominant PAOs in biofilms had unique key metabolic pathways, F-ATPases and N-Acyl homoserine lactones (AHLs). F-ATPases are important for maintaining the proton motive force (PMF) required for the uptake of carbon sources by PAOs, and AHLs are participating in phosphate metabolism through quorum sensing (QS) mediated secretion of extracellular polymeric substance (EPS). The formation of dominant PAOs was promoted by optimizing carbon source uptake and phosphate metabolism. This study revealed that the difficult isolation of dominant PAOs was due to the AHLs-mediated QS, and we identified the key pathways regulating the formation of dominant PAOs in biofilms through genomics analysis. Our findings provide insights in enhancing phosphate enrichment in BSBR by modulating the components of microbial community under the low concentration of carbon source consumption.

Metagenomics reveals the metabolism of polyphosphate-accumulating organisms in biofilm sequencing batch reactor: A new model

This study assessed the impact of the operating conditions of the biofilm sequencing batch reactor (BSBR) on the community structure and the growth/metabolic pathways of its polyphosphate-accumulating organisms (PAOs). There are significant difference with reference to the enhanced biological phosphorus removal (EBPR) process. The leading PAOs in BSBR generally are capable of high affinity acetate metabolism, gluconeogenesis, and low affinity phosphate transport, and have various carbon source supplementation pathways to ensure the efficient circulation of energy and reducing power. A new model of the metabolic mechanism of PAOs in the BSBR was formulated, which features low glycogen metabolism with simultaneous gluconeogenesis and glycogenolysis and differs significantly from the classic mechanism based on Candidatus_Accumulibacter and Tetrasphaera. The findings will assist the efficient recovery of low concentration phosphate in municipal wastewater.

Biological nutrients removal performance under starvation stress: Efficacy deterioration and recovery

Biological nutrients removal performance affected by starvation stress was investigated via the addition of pre-anoxic stage to SBR. COD removal efficiency maintained at around 90% regardless of the starvation stress. Starvation stress presented significant impact on nitrogen and phosphorus removal, with noticeable reduction of TN removal and remarkable deterioration of TP removal as prolonging the pre-anoxic time, which was mainly attributed to the integrative effect of carbon source competition, depression of denitrification and invalid P release as well as the variation of microbial community. It was notable that starvation stress exerted distinct evolution on microbial community. The improvement in relative abundance of the certain genera relating to denitrification was the main reason for the partial recovery of nitrogen removal after eliminating stress starvation. The promotion of P uptake capacity accompanied with the relief of invalid P release and the enriched DPAOs accounted for the complete recovery of phosphorus removal.

Phosphorus recovery from a pilot-scale grate furnace: influencing factors beyond wet chemical leaching conditions

Phosphorus is a non-renewable resource going to be exhausted in the future. Sewage sludge ash is a promising secondary raw material due to its high phosphorus content. In this work, the distribution of 19 elements in bottom and cyclone ashes from pilot-scale grate furnace have been monitored to determine the suitability for the phosphorus acid extraction. Moreover, the influence of some parameters beyond wet chemical leaching conditions were investigated. Experimental results showed that bottom ash presented lower contamination in comparison to cyclone ash and low co-dissolution of heavy metals (especially Cr, Pb and Ni), while high phosphorus extraction efficiencies (76-86%) were achieved. High Al content in the bottom ash (9.4%) negatively affected the phosphorus extraction efficiency as well as loss on ignition, while the particle size reduction was necessary for ensuring a suitable contact surface. The typology of precipitating agents did not strongly affect the phosphorus precipitation, while pH was the key parameter. At pH 3.5-5, phosphorus precipitation efficiencies higher than 90% were achieved, with a mean phosphorus content in the recovered material equal to 16-17%, comparable to commercial fertilizers. Instead, the co-precipitation of Fe and Al had a detrimental effect on the recovered material, indicating the need for additional treatments.

Study on community structure and metabolic mechanism of dominant polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs) in suspended biofilm based on phosphate recovery

Biofilm sequencing batch reactor (BSBR) can achieve efficient phosphate (P) removal and enrichment, but its process performance and metabolic mechanisms for P removal and enrichment of municipal wastewater remain largely unclear. In the present study, we assessed the P removal and enrichment of municipal wastewater at influent P concentrations of 2.5 mg/L and 10 mg/L. The efficiency of P removal and enzyme activity in polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs) were compared, and the growth and metabolic characteristics of dominant PAOs and GAOs at different influent P concentrations were studied with the macro-sequencing technology. The results showed that the P recovery efficiencies were 70.03% and 76.19% when the influent P concentration was 2.5 mg/L and 10 mg/L in BSBR, respectively, and the maximum P concentration of recovery liquid was 81.29 mg/L and 173.12 mg/L, respectively. There were no phosphate kinase (PPK) and phosphate hydrolase (PPX) in extracellular polymeric substances (EPS). The dominant PAOs were Candidatus_Contendobacter, Dechloromonas, and Flavobacterium, and the dominant GAO was Candidatus_Competibacter. The abundance of Candidatus_Contendobacter was the highest with the most potential contribution to P removal. PAOs had competitive advantages in carbon (C) source uptake, glycogen metabolism, P metabolism, and adenosine triphosphate (ATP) metabolism. HMP was unique to PAOs, EMP had the highest abundance in glycogen metabolism, and ED was contained in PAOs of BSBR. These results indicated that BSBR provided sufficient reducing power and ATP for PAOs through different glycogen decomposition pathways to promote P uptake and obtained competitive advantages in P metabolism, C source uptake, and ATP utilization to achieve efficient P removal and enrichment. Collectively, our current findings provided valuable insights into the P removal and enrichment mechanism of BSBR in municipal sewage.

Microbial population changes and metabolic shift of candidatus accumulibacter under low temperature and limiting polyphosphate

This study explored the microbial population dynamics of Accumulibacter (Acc) at low temperature and metabolic shift to limiting polyphosphate (Poly-P) in enhanced biological phosphorus removal (EBPR) system. The Accumulibacter-enriched EBPR systems, fed with acetate (HAc) and propionate (HPr) at 10 ± 1 °C respectively, were operated for 60 days in two identical SBR reactors (SBR-1 and SBR-2). The phosphorus removal performance in two systems was stable at 10 ± 1 °C, while the microbial community structure changed. Compared with the population structure in seed sludge, Accumulibacter clades reduced in the HAc system, while Acc I increased significantly in the HPr system. Low temperature was beneficial to the formation of granular sludge in the EBPR system, and the sludge granulation in the HAc system was more homogeneous than that in the HPr system. Accumulibacter in the HPr system can get ATP through glycogen accumulating metabolism (GAM) under limiting Poly-P condition at 10 ± 1 °C, while that in the HAc system cannot. This work suggests that poly-P levels can affect the metabolic pathway of Accumulibacter in EBPR systems under low temperature.