Aerobic granular sludge for efficient biotransformation of chalcogen SeIV and TeIV oxyanions: Biological nutrient removal and biogenesis of Se0 and Te0 nanostructures

Aerobic Se(IV) reducing bacteria and their reducing characteristics in estuarine sediment

Microorganisms play a critical role in the biogeochemical cycling of selenium in natural ecosystems, particularly in reducing selenite (Se(IV)) to element selenium (Se(0)) which reduces its mobility and bioavailability. However, Se(IV)-reducing bacteria and their reducing characteristics in estuarine sediments remain inadequately understood. In this study, the reduction of Se(IV) was confirmed to be microbially driven through the cultivation of a mixture of estuarine sediment and Se(IV) under aerobic conditions. Community analysis indicates that Bacillus was primarily involved in the reduction of Se(IV). A strain with high salt tolerance (7.5 % NaCl) and Se(IV) resistance (up to 200 mM), Bacillus cereus SD1, was isolated from an estuarine sediment. The reduction of Se(IV) occurred concomitantly with the onset of microbial growth, and reduction capacity increased approximately 5-fold by adjusting the pH. In addition, Se(IV) reduction in Bacillus cereus SD1 was significantly inhibited by sulfite, and the key enzyme activity tests revealed the possible presence of a sulfite reductase-mediated Se(IV) reduction pathway. These research findings provide new insights into the bioreducing characteristics and the biogeochemical cycling of selenium in estuarine environments.

Reduction of selenite to selenium nanoparticles by highly selenite-tolerant bacteria isolated from seleniferous soil

The microbial reduction of selenite to elemental selenium nanoparticles (SeNPs) is thought to be an effective detoxification process of selenite for many bacteria. In this study, Metasolibacillus sp. ES129 and Oceanobacillus sp. ES111 with high selenite reduction efficiency or tolerance were selected for systematic and comparative studies on their performance in selenite removal and valuable SeNPs recovery. The kinetic monitoring of selenite reduction showed that the highest transformation efficiency of selenite to SeNPs was achieved at a concentration of 4.24 mM for ES129 and 4.88 mM for ES111. Ultramicroscopic analysis suggested that the SeNPs produced by ES111 and ES129 had been formed in cytoplasm and subsequently released to extracellular space through cell lysis process. Furthermore, the transcriptome analysis indicated that the expression of genes involved in bacillithiol biosynthesis, selenocompound metabolism and proline metabolism were significantly up-regulated during selenite reduction, suggesting that the transformation of selenite to Se0 may involve multiple pathways. Besides, the up-regulation of genes associated with nucleotide excision repair and antioxidation-related enzymes may enhance the tolerance of bacteria to selenite. Generally, the exploration of selenite reduction and tolerance mechanisms of the highly selenite-tolerant bacteria is of great significance for the effective utilization of microorganisms for environmental remediation.

Pilot-scale aerobic granular sludge reactors with granular activated carbon for effective nitrogen and phosphorus removal from domestic wastewater

Aerobic granular sludge (AGS) is a breakthrough biotechnology of 21st century and an innovative alternative to activated sludge for treating wastewater. Concerns on long-start up periods for development of AGS and stability of granules are impeding its widespread implementation for treating low-strength domestic wastewater especially in tropical climate conditions. Addition of nucleating agents have been shown to improve development of AGS while treating low-strength wastewaters. There are no previous studies on AGS development and biological nutrient removal (BNR) in the presence of nucleating agents during treatment of real domestic wastewater. This study investigated AGS formation and BNR pathways while treating real domestic wastewater in a 2 m3 pilot-scale granular sequencing batch reactor (gSBR) operated without and with granular activated carbon (GAC) particles. The gSBRs were operated under tropical climate (T ≈ 30 °C) for >4-years to evaluate the effect of GAC addition on granulation, granular stability and BNR at pilot-scale. Formation of granules was observed within 3 months. MLSS values of 4 and 8 g/L were recorded within 6 months in gSBRs without and with GAC particles, respectively. The granules had an average size of 1.2 mm and SVI5 of 22 mL/g. Ammonium was mainly removed through nitrate formation in the gSBR without GAC. But, ammonium was removed by short-cut nitrification via nitrite due to washout of nitrite oxidizing bacteria in the presence of GAC. Phosphorus removal was much higher in gSBR with GAC due to the establishment of enhanced biological phosphorus removal (EBPR) pathway. After 3 months, the phosphorus removal efficiencies were at 15 % and 75 %, respectively, without and with GAC particles. The addition of GAC led to moderation in bacterial community and enrichment of polyphosphate-accumulating organisms. This is the first ever report on pilot-scale demonstration of AGS technology in the Indian sub-continent and GAC addition on BNR pathways.

Rapid, stable, and highly-efficient development of salt-tolerant aerobic granular sludge by inoculating magnetite-assisted mycelial pellets

Long cultivation time hinders the industrial applications of aerobic granular sludge (AGS) in treatment of hypersaline wastewater. Mycelial pellets (MPs) have been used to efficiently strengthen the flocculent sludge aggregation and accelerate the formation of AGS. However, the MPs-based AGS was easily crushed or fragmented into several small pieces/granules that brought the uncertainty and extended the transition process to form mature AGS. In this study, magnetite was used to strengthen MPs (halotolerant fungus Cladosporium tenuissimum NCSL-XY8), and co-culture and adsorption type of magnetite-assisted mycelial pellets (CMMPs and AMMPs) were prepared and used for acceleration of salt-tolerant aerobic granular sludge (SAGS) cultivation under 3% salinity conditions. Compared to inoculating MPs, the inoculation of either CMMPs or AMMPs could stably transition to mature SAGS without evident fragmentation, which obviously increased the certainty and stability of SAGS formation. Also, highly-efficient simultaneous nitrogen and carbon removal (∼98% TOC and ∼80% TN removal) could be reached in 8 days. Typically, the granules maintained perfect characteristics (D50 > 1300 μm, D10 > 350 μm, SVI30 < 45 mL/g, and SVI30/SVI5 = 1.0) during the whole cultivation/transition processes (Day 0-55) by using the inoculum of CMMPs. ITS rDNA sequencing revealed the inoculated fungus Cladosporium tenuissimum played key roles in the formation of SAGS. All the phenomena indicated the rapid, stable, and highly-efficient start-up of SAGS could be successfully realized by inoculating CMMPs.

Aerobic granular sludge formation and stability in enhanced biological phosphorus removal system under antibiotics pressure: Performance, granulation mechanism, and microbial successions

Wastewater containing antibiotics can pose a significant threat to biological wastewater treatment processes. This study investigated the establishment and stable operation of enhanced biological phosphorus removal (EBPR) by aerobic granular sludge (AGS) under mixed stress conditions induced by tetracycline (TC), sulfamethoxazole (SMX), ofloxacin (OFL), and roxithromycin (ROX). The results show that the AGS system was efficient in removing TP (98.0%), COD (96.1%), and NH₄⁺-N (99.6%). The average removal efficiencies of the four antibiotics were 79.17% (TC), 70.86% (SMX), 25.73% (OFL), and 88.93% (ROX), respectively. The microorganisms in the AGS system secreted more polysaccharides, which contributed to the reactor's tolerance to antibiotics and facilitated granulation by enhancing the production of protein, particularly loosely bound protein. Illumina MiSeq sequencing revealed that putative phosphate accumulating organisms (PAOs)-related genera (Pseudomonas and Flavobacterium) were enormously beneficial to the mature AGS for TP removal. Based on the analysis of extracellular polymeric substances, extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, and microbial community, a three-stage granulation mechanism was proposed including adaption to the stress environment, formation of early aggregates and maturation of PAOs enriched microbial granules. Overall, the study demonstrated the stability of EBPR-AGS under mixed antibiotics pressure, providing insight into the granulation mechanism and the potential use of AGS for wastewater treatment containing antibiotics.

Selenite reduction and biogenesis of selenium-nanoparticles by different size groups of aerobic granular sludge under aerobic conditions

Microbial transformations play a vital role in Se cycle in the environment and decrease the solubility and toxicity of Se oxyanions by converting to elemental selenium (Se⁰) nanostructures. Aerobic granular sludge (AGS) has attracted interest due to efficient reduction of selenite to biogenic Se⁰ (Bio-Se⁰) and retention in bioreactors. Here, selenite removal, biogenesis of Bio-Se⁰ and entrapment of Bio-Se⁰ by different size groups of aerobic granules were investigated to optimize biological treatment process for Se-laden wastewaters. Furthermore, a bacterial strain showing high selenite tolerance and reduction was isolated and characterized. Removal of selenite and conversion to Bio-Se⁰ were achieved by all the size groups of granules ranging from 0.12 mm to 2 mm and above. However, selenite reduction and Bio-Se⁰ formation were rapid and more efficient with large aerobic granules (≥0.5 mm). The formed Bio-Se⁰ was majorly associated with the large granules, due to better entrapment capabilities. In contrast, the Bio-Se⁰ formed by the small granules (≤0.2 mm) was distributed both in the granules and aqueous phase because of ineffective entrapment. Scanning electron microscope and energy dispersive X-ray (SEM-EDX) analysis confirmed formation of Se⁰ spheres and association with the granules. Efficient selenite reduction and entrapment of Bio-Se⁰ was related to prevalent anoxic/anaerobic zones in the large granules. A bacterial strain showing efficient SeO₃^2- reduction of up to 15 mM SeO₃^2- under aerobic conditions was identified as Microbacterium azadirachtae. SEM-EDX analysis confirmed the formation and entrapment of Se⁰ nanospheres (size: 100 ± 5 nm) in the extracellular matrix. The cells immobilized in alginate beads showed effective SeO₃^2- reduction and Bio-Se⁰ entrapment. Efficient reduction and immobilization of bio-transformed metalloids by large AGS and AGS-borne bacteria implicates prospective use in bioremediation of metal(loid) oxyanions and bio-recovery.

De novo granulation of sewage-borne microorganisms: A proof of concept on cultivating aerobic granular sludge without activated sludge and effective enhanced biological phosphorus removal

Long start-up periods for granulating activated sludge and concerns on granular stability are the bottlenecks reported during implementation of novel aerobic granular sludge (AGS) technology in municipal wastewater treatment plants. Here, de novo granulation of sewage-borne microorganisms without using activated sludge (AS) inoculum was investigated in bench-scale sequencing batch reactors (SBR). Data showed that formation of AGS from sewage-borne microorganisms was rapid and first granules appeared within one week. Granulation was indicated by appearance of biomass particles (size >0.12 mm), high biomass levels (∼8 g/L) and superior settling properties (SVI_30 min: 30 mL/g). Granulation process involved distinct stages like formation of aggregates, retention of aggregates, and growth of millimetre sized granules. Simultaneous COD, nitrogen and phosphorous removal was established within 10 days of start-up in the SBR without using AS inoculum. However, phosphorus removal became stable after 50 days of start-up. Total nitrogen (TN) and total phosphorus (TP) removals of 92% and 70%, respectively, were achieved from real domestic wastewater. Furthermore, addition of granular activated carbon (GAC) had improved both granulation and biological nutrient removals. Interestingly, phosphorus removal became quite stable within 10 days of start-up in the SBR operated with GAC particles. TN and TP removals were found to be higher at >98% and >94%, respectively, in GAC-augmented SBR. Removal of ammonia and phosphorus were mediated by nitritation-denitritation and enhanced biological phosphorus removal (EBPR) pathways, respectively. The bacterial diversity of AGS was lower than that of sewage. Quantitative PCR indicated enrichment of ammonia oxidizing bacteria, denitrifying bacteria and polyphosphate accumulating organisms during granulation. De novo granulation of sewage-borne microorganisms is a promising approach for rapidly cultivating AGS and establishing biological nutrient removal in sewage treatment plants.

Effects of oxytetracycline on aerobic granular sludge process: Granulation, biological nutrient removal and microbial community structure

Formation of aerobic granular sludge (AGS), process performance and microbial community structure were investigated in lab-scale sequencing batch reactors (SBR) operated without and with oxytetracycline (OTC). Granulation of activated sludge and appearance of AGS was observed in parallel SBRs operated without and with OTC. However, formation of well-settling aerobic granules was relatively faster in the SBR fed with 100 μg/L OTC and observed within 2 weeks of start-up. Ammonium, total nitrogen, and phosphorus removals were quickly established in the AGS cultivated without OTC. In contrast, nitrogen and phosphorus removals were lower in the OTC fed SBR. But, a gradual improvement in nitrogen and phosphorus removals was observed. After 45 days, nitrogen and phosphorous removals were stabilized at 99% and 70%, respectively, due to establishment of OTC-tolerant community. qPCR revealed the impact of OTC on ammonium oxidizing bacteria, polyphosphate accumulating organisms and their enrichment during exposure to OTC. Ammonium and phosphorus were majorly removed via nitritation-denitritation and enhanced biological phosphorus removal (EBPR) pathways, respectively, in the presence of OTC. Brevundimonas (35%), Thaurea (14%) sp. Ca. Competibacter (5.6%), and Ca. Accumulibacter (4.2%) were enriched in OTC-fed AGS. Of the two OTC-tolerant strains isolated, Micrococcus luteus exhibited growth and efficient OTC biotransformation at different OTC concentrations. Moreover, M. luteus was predominantly growing in the form of aggregates. Key traits such as tolerance, biotransformation and high autoaggregation ability allowed a niche for this strain in the granules. This work has important implications in understanding the effect of antibiotics on AGS and designing AGS based treatment for antibiotic-laden wastewaters.

Enhanced biological phosphorus removal in aerobic granular sludge reactors by granular activated carbon dosing

This study investigated the effects of granular activated carbon (GAC) addition on the enrichment of polyphosphate accumulating organisms (PAOs), stratification of PAOs in the co-existing GAC-biofilms and granules and biological nutrient removal (BNR) in aerobic granular sludge (AGS) reactors. It was found that BNR increased in the GAC-augmented system. Establishment of enhanced biological phosphorus removal (EBPR) pathway was faster with about 1.7 to 2-fold higher P removal in GAC system than control. EBPR biomass grown in the presence of GAC was segregated into different size fractions for determining BNR and stratification of microbial groups. It was found that EBPR was majorly associated with the large biomass (>0.5 mm) fraction, corroborating with higher abundance of PAOs. Higher P removals of 60 to 70% with characteristic EBPR profiles were observed in 0.5 mm fraction. In contrast, P removals by 0.25 mm fraction were lower at 20 to 35% without EBPR profiles. EBPR biomass (>0.5 mm) fraction was segregated into granules and GAC-biofilms for determining the role of GAC in PAOs enrichment. P release (2.5-3.5 mg L^-1 P) and P uptake (5-7 mg L^-1 P) were higher in the P removal profiles exhibited by GAC-biofilms. In contrast, P release and P uptake were lower with the granules. These differences in P removal profiles resulted in distinct net P removal efficiencies of 70 ± 5% and 50 ± 6% for GAC-biofilms and granules, respectively. These differences in P removals were corroborated by higher abundance of PAOs in the GAC-biofilms than co-existing granules. PAO clade-level enrichment was found to be dependent on substrate wherein acetate feeding enriched PAO clade I, while acetate-propionate feeding caused enrichment of both PAO clade I and II. These results suggest that GAC addition to AGS reactors can aid in enrichment of PAOs, reduce the start-up period for EBPR, and increase P removal efficiencies.

An overview on microalgal-bacterial granular consortia for resource recovery and wastewater treatment

Excessive generation of wastewater is a matter of concern around the globe. Wastewater treatment utilizing a microalgae-mediated process is considered an eco-friendly and sustainable method of wastewater treatment. However, low biomass productivity, costly harvesting process, and energy extensive cultivation process are the major bottleneck. The use of the microalgal-bacteria granular consortia (MBGC) process is economic and requires less energy. For efficient utilization of MBGC, knowledge of its structure, composition and interaction are important. Various microscopic, molecular and metabolomics techniques play a significant role in understating consortia structure and interaction between partners. Microalgal-bacteria granular consortia structure is affected by various cultivation parameters like pH, temperature, light intensity, salinity, and the presence of other pollutants in wastewater. In this article, a critical evaluation of recent literature was carried out to develop an understanding related to interaction behavior that can help to engineer consortia having efficient nutrient removal capacity with reduced energy consumption.

Enhancing biological nitrogen and phosphorus removal performance in aerobic granular sludge sequencing batch reactors by activated carbon particles

Long start-up periods for aerobic granular sludge (AGS) formation and establishment of P removal pathways are challenges for widespread implementation of AGS process. External additives such as activated carbon (AC) attracted interest for accelerating AGS formation. However, the roles of AC in granulation and biological nutrient removal (BNR) are not understood. Here, the role of AC was investigated in decreasing start-up periods in AGS formation and BNR under different carbon substrate conditions (i.e., acetate (HAc), propionate (HPr) and HAc-HPr) in sequencing batch reactors (SBRs). AC addition increased aggregation index and settleability of activated sludge (AS) inoculum which minimized AS washout from SBRs. AC addition hastened AGS formation and establishment of BNR pathways by facilitating AS retention and biofilm formation. Feeding HAc or HAc-HPr supported better granulation (MLSS: 6-7 g l^-1, SVI: 30-40 ml g^-1) than HPr (MLSS: 4 g l^-1, SVI: 70). The start-up periods for efficient total nitrogen (TN) removals were decreased to 22 and 16 d from 38 to 25 d, respectively, in AC augmented SBRs fed with either HAc or HAc-HPr. TN removals were higher at ≥95% in HAc or HAc-HPr fed SBRs. Total phosphorus (TP) removals were also higher in AC-augmented SBRs at 80% and ≥90% in HAc and HAc-HPr fed SBRs, respectively. In contrast, TN and TP removals were lower at 70% and 35%, respectively, in HPr fed SBR. Ammonium was primarily removed via nitritation-denitritation pathway. Phosphorus removal was at 1.7 to 2-fold higher in AC augmented SBRs and driven by enhanced biological phosphorus removal (EBPR) pathway. MiSeq sequencing and qPCR revealed higher enrichment of polyphosphate accumulating organisms (PAOs), denitrifying PAOs, and ammonia oxidizers in AC-augmented SBRs fed with HAc or HAc-HPr. This study demonstrates that AC addition can be considered for enrichment of PAOs and establishment of EBPR in aerobic granular SBRs.