Inhibition of free ammonia to the granule-based enhanced biological phosphorus removal system and the recoverability

Insight into the mechanism of nutrients removal and response regulation of denitrifying phosphorus removal system under calcium ion stress

The influent quality is an important factor affecting the nutrients removal and operational stability of denitrifying phosphorus removal (DPR) system. This study investigated the effects of calcium ion (Ca2+) on the nutrients removal, nitrogen oxide (N2O) release, microbial community, and quorum sensing in DPR system. Results showed that high accumulation of Ca2+ had a significant impact on the carbon footprint of DPR system. Specifically, N2O release reached 2.11 mg/L under Ca2+ of 150 mg/L, which represented 214.93% increase compared to 0 mg/L of Ca2+. The DPR system demonstrated its adaptability to elevated Ca2+ concentrations by modifying key enzyme activities involved in nitrogen and phosphorus removal, altering the microbial community structure, and adjusting the type and content of signal molecules. These findings hold significant implications for understanding the stress mechanism of Ca2+ on DPR system, ultimately aiding in the maintenance and enhancement of stable operational performance in biological wastewater treatment process.

Insights on biological phosphorus removal with partial nitrification in single sludge system via sidestream free ammonia and free nitrous acid dosing

The sidestream sludge treatment by free ammonium (FA)/free nitrous acid (FNA) dosing was frequently demonstrated to maintain the nitrite pathway for the partial nitrification (PN) process. Nevertheless, the inhibitory effect of FA and FNA would severely influence polyphosphate accumulating organisms (PAOs), destroying the microbe-based phosphorus (P) removal. Therefore, a strategic evaluation was proposed to successfully achieve biological P removal with a partial nitrification process in a single sludge system by sidestream FA and FNA dosing. Through the long-term operation of 500 days, excellent phosphorus, ammonium and total nitrogen removal performance were achieved at 97.5 ± 2.6 %, 99.1 ± 1.0 % and 75.5 ± 0.4 %, respectively. Stable partial nitrification with a nitrite accumulation ratio (NAR) of 94.1 ± 3.4 was attained. The batch tests also reported the robust aerobic phosphorus uptake based on FA and FNA adapted sludge after exposure of FA and FNA, respectively, suggesting the FA and FNA treatment strategy could potentially offer the opportunity for the selection of PAOs, which synchronously have the tolerance to FA and FNA. Microbial community analysis suggested that Accumulibacter, Tetrasphaera, and Comamonadaceae collectively contributed to the phosphorus removal in this system. Summarily, the proposed work presents a novel and feasible strategy to integrate enhanced biological phosphorus removal (EBPR) and short-cut nitrogen cycling and bring the combined mainstream phosphorus removal and partial nitrification process closer to practical application.

Toward Integrating EBPR and the Short-Cut Nitrogen Removal Process in a One-Stage System for Treating High-Strength Wastewater

Enhanced biological phosphorus removal (EBPR) is an economical and sustainable process for phosphorus removal from wastewater. Despite the widespread application of EBPR for low-strength domestic wastewater treatment, limited investigations have been conducted to apply EBPR to the high-strength wastewaters, particularly, the integration of EBPR and the short-cut nitrogen removal process in the one-stage system remains challenging. Herein, we reported a novel proof-of-concept demonstration of integrating EBPR and nitritation (oxidation of ammonium to nitrite) in a one-stage sequencing batch reactor to achieve simultaneous high-strength phosphorus and short-cut nitrogen removal. Excellent EBPR performance of effluent 0.8 ± 1.0 mg P/L and >99% removal efficiency was achieved fed with synthetic high-strength phosphorus wastewater. Long-term sludge acclimation proved that the dominant polyphosphate accumulating organisms (PAOs), Candidatus Accumulibacter, could evolve to a specific subtype that can tolerate the nitrite inhibition as revealed by operational taxonomic unit (OTU)-based oligotyping analysis. The EBPR kinetic and stoichiometric evaluations combined with the amplicon sequencing proved that the Candidatus Competibacter, as the dominant glycogen accumulating organisms (GAOs), could well coexist with PAOs (15.3-24.9% and 14.2-33.1%, respectively) and did not deteriorate the EBPR performance. The nitrification activity assessment, amplicon sequencing, and functional-based gene marker quantification verified that the unexpected nitrite accumulation (10.7-21.0 mg N/L) in the high-strength EBPR system was likely caused by the nitritation process, in which the nitrite-oxidizing bacteria (NOB) were successfully out-selected (<0.1% relative abundance). We hypothesized that the introduction of the anaerobic phase with high VFA concentrations could be the potential selection force for achieving nitritation based on the literature review and our preliminary batch tests. This study sheds light on developing a new feasible technical route for integrating EBPR with short-cut nitrogen removal for efficient high-strength wastewater treatment.

The Inhibitory Effect of Free Nitrous Acid and Free Ammonia on the Anoxic Phosphorus Uptake Rate of Polyphosphate-Accumulating Organisms

The purpose of this study is to investigate the effect of free nitrous acid (FNA) and free ammonia (FA) on the anoxic phosphorus uptake rate (PUR) of polyphosphate-accumulating organisms (PAOs) via the utilization of nitrite. With this goal, upon developing a PAO-enriched culture in a sequential batch reactor, a series of batch experiments were conducted to examine the effects of nitrite and ammonium on the anoxic phosphorus uptake rate at different pH levels. According to the results, both free nitrous acid and free ammonia were found to inhibit anoxic PUR to a degree similar to their respective effects on aerobic PUR reported in previous studies, suggesting that phosphorus removal via the anoxic pathway may be just as susceptible as that via the aerobic pathway. The effect of FNA on anoxic PUR is optimally described by a non-competitive inhibition model with a KiFNA value of 1.6 μg N L−1, while the Levenspiel model with an SFA* value of 37 mg N L−1 provided the best fit for the FA effect on PAOs anoxic activities. The results of this study provide new insights regarding the viability of EBPR under high nitrogen loading conditions.

Inhibition of free nitrous acid and free ammonia on polyphosphate accumulating organisms: Evidence of insufficient phosphorus removal through nitritation-denitritation

The purpose of this study is to investigate the effect of Free Nitrous Acid (FNA) and Free Ammonia (FA) on enhanced biological phosphorus removal (EBPR) and in particular on the aerobic phosphorus uptake rate (PUR). To this end, a PAO-enriched biomass was developed at a lab-scale reactor in order to fuel a series of ex-situ batch experiments to test the effect of various nitrite or ammonium concentrations on the phosphorus uptake rate at different pH values. FNA was found to be a strong inhibitor of EBPR, in agreement with other studies with PUR being inhibited by 50 % under 1.5 μg HNO₂-N L^-1 and 100 % at 13 μg HNO₂-N L^-1. FA was also found to inhibit EBPR with PUR being inhibited by 50 % under 6.4 mg NH₃-N L^-1. The results of this study suggest that EBPR under high nitrogen loading alongside nitritation-denitritation may not be a viable option.

Resource recovery in aerobic granular sludge systems: is it feasible or still a long way to go?

Lately, wastewater treatment plants are much often being designed as wastewater-resource factories inserted in circular cities. Among biological treatment technologies, aerobic granular sludge (AGS), considered an evolution of activated sludge (AS), has received great attention regarding its resource recovery potential. This review presents the state-of-the-art concerning the influence of operational parameters on the recovery of alginate-like exopolysaccharides (ALE), tryptophan, phosphorus, and polyhydroxyalkanoates (PHA) from AGS systems. The carbon to nitrogen ratio was identified as a parameter that plays an important role for the optimal production of ALE, tryptophan, and PHA. The sludge retention time effect is more pronounced for the production of ALE and tryptophan. Additionally, salinity levels in the bioreactors can potentially be manipulated to increase ALE and phosphorus yields simultaneously. Some existing knowledge gaps in the scientific literature concerning the recovery of these resources from AGS were also identified. Regarding industrial applications, tryptophan has the longest way to go. On the other hand, ALE production/recovery could be considered the most mature process if we take into account that existing alternatives for phosphorus and PHA production/recovery are optimized for activated sludge rather than granular sludge. Consequently, to maintain the same effectiveness, these processes likely could not be applied to AGS without undergoing some modification. Therefore, investigating to what extent these adaptations are necessary and designing alternatives is essential.

Long-term stability of a non-adapted aerobic granular sludge process treating fish canning wastewater associated to EPS producers in the core microbiome

The tolerance of aerobic granular sludge (AGS) to variable wastewater composition is perceived as one of its greatest advantages compared to other aerobic processes. However, research studies select optimal operational conditions for evaluating AGS performance, such as the use of pre-adapted biomass and the control of wastewater composition. In this study, non-adapted granular sludge was used to treat fish canning wastewater presenting highly variable organic, nutrient and salt levels over a period of ca. 8 months. Despite salt levels up to 14 g NaCl L^-1, the organic loading rate (OLR) was found to be the main factor driving AGS performance. Throughout the first months of operation, the OLR was generally lower than 1.2 kg COD m^-3 day^-1, resulting in stable nitrification and low COD and phosphorous levels at the outlet. An increase in OLR up to 2.3 kg COD m^-3 day^-1 disturbed nitrification and COD and phosphate removal, but a decrease to average values between 1 and 1.6 kg COD m^-3 day^-1 led to resuming of those processes. Most of the bacteria present in the AGS core microbiome were associated to extracellular polymeric substances (EPS) production, such as Thauera and Paracoccus, which increased during the higher OLR period. Ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) species were detected in AGS biomass; while AOB were identified throughout the operation, NOB were no further identified after the period of increased OLR. Different polyphosphate-accumulating organisms (PAOs) were detected along the process: CandidatusAccumulibacter, Tetrasphaera and Gemmatimonas. A non-adapted granular sludge was able to treat the fish canning wastewater and to tolerate salinity fluctuations up to 14 g L^-1. Overall, a high microbial diversity associated to EPS producers allowed to preserve bacterial groups responsible for nutrients removal, contributing to the adaptation and long-term stability of the AGS system.

Effect of sulfamethoxazole and oxytetracycline on enhanced biological phosphorus removal and bacterial community structure

The individual and combined effects of sulfamethoxazole (SMX) and oxytetracycline (OTC) on an enhanced biological phosphorus removal (EBPR) system was investigated. OTC at 5 mg/L resulted in filamentous bulking with a collapse of EBPR system. P removal decreased to 44.8% and COD was mostly removed during the aerobic phase. SMX and OTC had antagonistic effects in EBPR system. The inhibitory effect of SMX and SMX + OTC on P removal, COD removal, glycogen transformation and extracellular polymeric substances content was reversible with prolonged operation, accompanied with increase of polyphosphate accumulating organisms. The presence of nitrification inhibitor allylthiourea, high pH and low tetX abundance limited the removal of SMX and OTC. The bacterial community structure, antibiotic resistance genes abundances and genes functions were also investigated by metagenomic analysis. The results of this study offer insights into the individual and combined environmental risks of SMX and OTC, and their impact on EBPR.

Effect of fulvic acid on bioreactor performance and on microbial populations within the anammox process

The long-term effect of fulvic acid (FA) on bioreactor performance and on microbial populations within the anammox process were firstly investigated in this study. The average nitrogen removal rate showed an upward trend when the influent TOC concentration of FA was 25.2-65.1 mg/L. However, when FA was increased to 80.3 mg/L, the reactor performance was slightly inhibited. In addition, judging from the particle size and settling properties, FA can promote anammox sludge granulation. After 53 days of exposure to FA, the genus Anaerolineaincreased in number, while Denitratisoma decreased. Candidatus Jettenia and Candidatus Kuenenia survived and enriched in the changed environment, potentially due to the interaction between anammox bacteria and some heterotrophic bacteria, which could protect anammox bacteria from adverse environments. These results indicate that FA can change the bacterial community and trigger different microbial interaction mechanisms within the anammox reactor.

Chronic responses of aerobic granules to the presence of graphene oxide in sequencing batch reactors

The chronic responses of aerobic granular sludge (AGS) to the presence of graphene oxide nanoparticles (GO NPs) (5, 15, 25, 35, 45, 55, 65, 75, 85, and 95 mg/L of GO NPs for 7 days) during biological wastewater treatment processes were investigated. Bioreactor performance, extracellular polymeric substance (EPS) secretion, and microbial community characteristics were assessed. The results showed that the effects of GO NPs on bioreactor performances were dependent on the dose applied and the duration for which it was applied. At concentrations of 55, 75, and 95 mg/L, GO NPs considerably inhibited the efficiency of organic matter and ammonia removal; however, nitrite and nitrate removal rates were unchanged. Biological phosphorus removal decreased even when only low concentrations of GO NPs were used. The secretion of EPS, which could alleviate the toxicity of GO NPs, also changed. The increased amount of nanoparticles also resulted in significant changes to the bacterial community structure. Based on the amplicon sequencing of 16S rRNA genes, Paracoccus sp., Klebsiella sp., and Acidovorax species were identified as the most tolerant strains.

Rapid cultivation and stability of autotrophic nitrifying granular sludge

Autotrophic nitrifying granular sludge (ANGS) was cultivated by gradually decreasing the influent organics and adding exogenous nitrifying bacteria. Under the strategy, ANGS was domesticated within 36 days. Stability of the seed heterotrophic granules decreased significantly during conversion of organic wastewater to inorganic ammonia wastewater. Obvious granular breakage was observed during these days. However, the granular debris still had good settlement performance. With microbes gradually acclimated to the new environment, the debris provided a large number of carriers for the attached growth of the exogenous nitrifying bacteria, and they replaced the heterotrophic bacteria and became the dominant species. The domesticated ANGS showed good nitrification performance during the 37th to the 183rd day (ammonia nitrogen load between 0.28 and 0.29 kg/m³ · d). The removal rate of ammonia nitrogen was usually more than 95%, and nitrite accumulation rate was always larger than 50%. However, nitrification ability was gradually lost with the increase of the ammonia nitrogen load (0.3-0.64 kg/m³ · d) from the 184th day, and it almost approached the influent ammonia nitrogen at the 269th day. Interestingly, good structure stability of the ANGS was maintained during long-term operation, and the ANGS became smoother and denser at the end of the experiment.

Pilot-scale investigation on nutrient removal characteristics of mineral-rich aerobic granular sludge: Identification of uncommon mechanisms

This study investigated nutrient removal characteristics and the related pathways in aerobic granular reactors using three pilot-scale granular sequencing batch reactors (GSBRs) treating wastewaters of diverse carbon and nutrient strength. The GSBRs were operated with alternating (AN/O/AX/O_SBR and AN/O_SBR) and purely-aerobic (O_SBR) operation modes. Mineral-rich aerobic granules with hydroxyapatite (HAp) core were cultivated in all the three GSBRs. The highest nitrogen removal efficiency (75%) was achieved in AN/O/AX/O_SBR and O_SBR and the lowest (22%) in AN/O_SBR, establishing a quasi-linear relationship with organic loading rate (OLR). Phosphorus removal efficiencies of 55-63% were achieved in the GSBRs despite different influent PO₄-P concentrations. Heterotrophic nitrification and biologically-induced phosphate precipitation (BIPP) became the dominant nutrient depletion pathways, contributing 61-84% and 39-96% to overall ammonium nitrogen and phosphorus removal, respectively. A direct relation was noted between heterotrophic nitrification efficiency (η_{Heterotrophic nitrification}) and nutrient availability, as nitrification efficiencies of 18 and 64% were observed for COD:N_inf of 5 and 20, respectively. Whereas, BIPP efficiency (η_BIPP) established inverse relation with (COD:P)_inf and (Ca:P)_inf and direct relation with phosphorus concentration beyond microbial growth requirement. Core heterotrophic nitrifiers and bio-calcifying species were identified as {Thauera and Flavobacterium} and {Flavobacterium, Acinetobacter, Pseudomonas, and Corynebacterium}, respectively. Ca-P crystallization was proposed to be via phosphate precipitation on calcite surfaces. Granulation mechanism was proposed as crystallization on bio-aggregates' periphery and then crystal growth toward the core.

The roles of free ammonia (FA) in biological wastewater treatment processes: A review

Free ammonia (FA) can pose inhibitory and/or biocidal effects on a variety of microorganisms involved in different biological wastewater treatment process, which is widely presented in wastewater treatment plants (WWTPs) due to the high levels of ammonium in the systems. This review article gives the up-to-date status on several essential roles of FA in biological wastewater treatment processes: the impacts of FA, mechanisms of FA roles, modeling of FA impacts, and implications of FA for wastewater treatment. Specifically, the impacts of FA on both wastewater and sludge treatment lines were firstly summarized, including nitrification, denitrification, anaerobic ammonium oxidation (Anammox), enhanced biological phosphorus removal and anaerobic processes. The involved mechanisms were then analyzed, which indicated FA inhibition can slow specific microbial activities or even reconfigure the microbial community structure, likely due to negative impacts of FA on intracellular pH, specific enzymes and extracellular polymeric substances (EPS), thus causing cell inactivation/lysis. Mathematical models describing the impact of FA on both wastewater and sludge treatment processes were also explored to facilitate process optimization. Finally, the key implications of FA were identified, that is FA can be leveraged to substantially enhance the biodegradability of secondary sludge, which would further improve biological nutrient removal and enhance renewable energy production.

Sulfamethazine (SMZ) affects fermentative short-chain fatty acids production from waste activated sludge

In this study, the impact of sulfamethazine (SMZ), a typical Pharmaceuticals and Personal Care Products (PPCPs) existed in waste activated sludge (WAS), on the production of short-chain fatty acids (SCFA) during sludge anaerobic fermentation was investigated for the first time. Experimental results showed that the production of SCFA was positively affected by SMZ, which increased from 571 mg COD/L in the blank to 989 mg COD/L in the 24 mg/kg VSS SMZ reactor, causing 1.73-fold of increase. Mechanism exploration revealed that SMZ enhanced the disruption of both extracellular polymeric substances (EPS) and cell envelop, thereby benefiting the solubilization of sludge. It was also found that SMZ benefited the acidogenesis and acetogenesis processes but inhibited methonogenesis process, which was another reason for the enhanced SCFA production. Microbial analysis revealed that the abundance of anaerobic functional microorganisms in the SMZ reactor was more advantageous to acetic acid production.

Approach of describing dynamic production of volatile fatty acids from sludge alkaline fermentation

In this work, a mathematical model was developed to describe the dynamics of fermentation products in sludge alkaline fermentation systems for the first time. In this model, the impacts of alkaline fermentation on sludge disintegration, hydrolysis, acidogenesis, acetogenesis, and methanogenesis processes are specifically considered for describing the high-level formation of fermentation products. The model proposed successfully reproduced the experimental data obtained from five independent sludge alkaline fermentation studies. The modeling results showed that alkaline fermentation largely facilitated the disintegration, acidogenesis, and acetogenesis processes and severely inhibited methanogenesis process. With the pH increase from 7.0 to 10.0, the disintegration, acidogenesis, and acetogenesis processes respectively increased by 53%, 1030%, and 30% while methane production decreased by 3800%. However, no substantial effect on hydrolysis process was found. The model also indicated that the pathway of acetoclastic methanogenesis was more severely inhibited by alkaline condition than that of hydrogentrophic methanogenesis.

Understanding the performance of microbial community induced by ZnO nanoparticles in enhanced biological phosphorus removal system and its recoverability

In this study, the impacts of ZnO Nanoparticles (NPs) on the microbial community in enhanced biological phosphorus removal (EBPR) system and its recoverability were investigated. High-throughput sequencing was applied to study the microbial community shift. Results show that the species richness in the EBPR system was reduced under the condition of ZnO NPs with high concentration (above 6mg/L). Evolution analysis suggests that higher concentration ZnO NPs induced more microbial community shift. According to the analysis on genus level, Competibacter was more impressionable than Accumulibacter after exposure to 2mg/L ZnO NPs. Nonetheless, this phenomenon could not be found as the concentration of ZnO NPs got higher (above 6mg/L). Accumulibacter could reach to the initial level after recover for 20days, whereas Competibacter could not recover even when the concentration of ZnO NPs was only 2mg/L. Interestingly, although the phosphorus removal (P-removal) process was re-achieved, the microbial community in reactors was irreversible.

Characterizing the free ammonia exposure to the nutrients removal in activated sludge systems

As a by-product during liquid production, the liquor wastewater exhibits tremendous environmental risks and may cause undesirable effects to the biological systems due to the high concentration of ammonia.

Short-term performance of enhanced biological phosphorus removal (EBPR) system exposed to erythromycin (ERY) and oxytetracycline (OTC)

The effects of Erythromycin (ERY) and oxytetracycline (OTC), including individual and combinative effect, on enhanced biological phosphorus removal (EBPR) system within a short-term (24h) were evaluated in this study. Results showed that the P-removal efficiency decreased to 34.6% and 0.0% under the effect of ERY (10mg/L) and OTC (10mg/L) for 24h. OTC concentration higher than 5mg/L was sufficient to cause serious adverse impact on the EBPR performance. While the performance of EBPR system will be impacted by ERY above 10mg/L. OTC, due to its special antibacterial action to the gram-negative bacteria which most PAOs belong to, has more serious negative effect on the EBPR performance than ERY does. Moreover, in the combined antibiotics test, neither synergistic nor antagonistic effect was detected between ERY and OTC. Finally, ERY (10mg/L) and OTC (10mg/L) could inhibit the microorganisms' activity, while couldn't induce serious microorganisms death within 24h.

Startup and long term operation of enhanced biological phosphorus removal in continuous-flow reactor with granules

The startup and long term operation of enhanced biological phosphorus removal (EBPR) in a continuous-flow reactor (CFR) with granules were investigated in this study. Through reducing the settling time from 9min to 3min gradually, the startup of EBPR in a CFR with granules was successfully realized in 16days. Under continuous-flow operation, the granules with good phosphorus and COD removal performance were stably operated for more than 6months. And the granules were characterized with particle size of around 960μm, loose structure and good settling ability. During the startup phase, polysaccharides (PS) was secreted excessively by microorganisms to resist the influence from the variation of operational mode. Results of relative quantitative PCR indicated that granules dominated by polyphosphate-accumulating organisms (PAOs) were easier accumulated in the CFR because more excellent settling ability was needed in the system.

The shift of the microbial community in activated sludge with calcium treatment and its implication to sludge settleability

The sludge settleability is of prime importance for the activated sludge process. The effect of calcium ion on the biological performance of sludge was investigated in a lab-scale activated sludge system with varying Ca(2+) concentration. Results indicated that addition of 150mg/L Ca(2+) to the influent significantly improved the settling characteristics and metabolic reactivity of activated sludge in the bioreactors. Analyses using denaturing gradient gel electrophoresis (DGGE) and 16S rRNA sequencing showed that a significant difference in the presence of certain bacterial groups between the sludge systems with 150mg/L Ca(2+) and those with 0-100mg/L Ca(2+) addition. Ca(2+) also increased the production of the extracellular polymeric substance (EPS) and facilitated the development of microbial cluster in the bioreactor. Study showed that an addition of 150mg/L Ca(2+) to the influent provides a simple approach to improve the settling properties of activated sludge and maintain high pollutant removal efficiency.

Identification of inorganic and organic species of phosphorus and its bio-availability in nitrifying aerobic granular sludge

Phosphorus (P) recovery from sewage sludge is necessary for a sustainable development of the environment and thus the society due to gradual depletion of non-renewable P resources. Aerobic granular sludge is a promising biotechnology for wastewater treatment, which could achieve P-rich granules during simultaneous nitrification and denitrification processes. This study aimed to disclose the changes in inorganic and organic P species and their correlation with P mobility and bio-availability in aerobic granules. Two identical square reactors were used to cultivate aerobic granules, which were operated for 120 days with influent ammonia nitrogen (NH₄-N) of 100 mg/L before day 60 and then increased to 200 mg/L during the subsequent 60 days (chemical oxygen demand (COD) was kept constant at 600 mg/L). The aerobic granules exhibited excellent COD removal and nitrification efficiency. Results showed that inorganic P (IP) was about 61.4-67.7% of total P (TP) and non-apatite inorganic P (NAIP) occupied 61.9-70.2% of IP in the granules. The enrichment amount of NAIP and apatite P (AP) in the granules had strongly positive relationship with the contents of metal ions, i.e. Fe and Ca, respectively accumulated in the granules. X-ray diffraction (XRD) analysis and solution index calculation demonstrated that hydroxyapatite (Ca₅(PO₄)₃(OH)) and iron phosphate (Fe₇(PO₄)₆) were the major P minerals in the granules. Organic P (OP) content maintained around 7.5 mg per gram of biomass in the aerobic granules during the 120 days' operation. Monoester phosphate (21.8% of TP in extract), diester phosphate (1.8%) and phosphonate (0.1%) were identified as OP species by Phosphorus-31 nuclear magnetic resonance (³¹P NMR). The proportion of NAIP + OP to TP was about 80% in the granules, implying high potentially mobile and bio-available P was stored in the nitrifying aerobic granules. The present results provide a new insight into the characteristics of P species in aerobic granules, which could be helpful for developing P removal and recovery techniques through biological wastewater treatment.

The effect of COD loading on the granule-based enhanced biological phosphorus removal system and the recoverability

In this study, the effect of varied COD loading (200, 400, 500, 600 and 800 mg L(-1)) on stability and recoverability of granule-based enhanced biological phosphorus removal (EBPR) system was investigated during continuously 53-d operation. Results showed that COD loading higher than 500 mg L(-1) could obviously deteriorate the granular EBPR system and result in sludge bulking with filamentous bacteria. High COD loading also changed the transformation patterns of poly-β-hydroxyalkanoates (PHAs) and glycogen in metabolism process of polyphosphate-accumulating organisms (PAOs) and inhibited the EPS secretion, which completely destroyed the stability and integrality of granules. Results of FISH indicated that glycogen-accumulating organisms (GAOs) and other microorganisms had a competitive advantage over PAOs with higher COD loading. The community composition and EBPR performance were recovered irreversibly in long time operation when COD loading was higher than 500 mg L(-1).

Use low direct current electric field to augment nitrification and structural stability of aerobic granular sludge when treating low COD/NH4-N wastewater

This study explored the effect of low direct current (DC, ⩽5 V) electric field on aerobic granules by using two identical square sequencing batch reactors (SBRs) when influent chemical oxygen demand to ammonia nitrogen (COD/NH4-N) ratio decreased from 6 to 1.2 (with COD fixed at 600 mg/L). Results showed that the granules from the electro-augmentation SBR had a more compact and dense microbial structure with enriched Actinobacteria, Bacteroidia, Betaproteobacteria, and Nitrospira. Compared to the control, these granules produced 17.4% and 21.5% more proteins and polysaccharides respectively in the extracellular polymeric substances (EPS), with their integrity coefficient stabilized at 3.3-5.2% during 120 days' operation. Interestingly, more di- and tri-valent metals were accumulated in the electro-augmented granules. In addition, greater than 99% nitrification especially with stably high nitratation efficiency was realized in this new system.