Characteristics of nitrifying granules developed in an air pulsing SBR

The r/K selection theory and its application in biological wastewater treatment processes

Understanding the characteristics of functional organisms is the key to managing and updating biological processes for wastewater treatment. This review, for the first time, systematically characterized two typical types of strategists in wastewater treatment ecosystems via the r/K selection theory and provided novel strategies for selectively enriching microbial community. Functional organisms involved in nitrification (e.g., Nitrosomonas and Nitrosococcus), anammox (Candidatus Brocadia), and methanogenesis (Methanosarcinaceae) are identified as r-strategists with fast growth capacities and low substrate affinities. These r-strategists can achieve high pollutant removal loading rates. On the other hand, other organisms such as Nitrosospira spp., Candidatus Kuenenia, and Methanosaetaceae, are characterized as K-strategists with slow growth rates but high substrate affinities, which can decrease the pollutant concentration to low levels. More importantly, K-strategists may play crucial roles in the biodegradation of recalcitrant organic pollutants. The food-to-microorganism ratio, mass transfer, cell size, and biomass morphology are the key factors determining the selection of r-/K-strategists. These factors can be related with operating parameters (e.g., solids and hydraulic retention time), biomass morphology (biofilm or granules), and operating modes (continuous-flow or sequencing batch), etc., to achieve the efficient acclimation of targeted r-/K-strategists. For practical applications, the concept of substrate flux was put forward to further benefit the selective enrichment of r-/K-strategists, fulfilling effective management and improvement of engineered pollution control bioprocesses. Finally, the future perspectives regarding the development of the r/K selection theory in wastewater treatment processes were discussed.

A review of quorum sensing improving partial nitritation-anammox process: Functions, mechanisms and prospects

Partial nitritation-anammox (PNA) is a promising and energy-efficient process for the sustainable nitrogen removal. However, its wide applications are still limited by the long start-up period and instability of long-term operation. Quorum sensing (QS), as a way of cell-to-cell communication generally regulating various microbial behaviors, has been increasingly investigated in PNA process, because QS may substantially manipulate the metabolism of microorganisms and overcome the limitations of PNA process. This critical review provides a comprehensive analysis of QS in PNA systems, and identifies the challenges and opportunities for the optimization of PNA process based on QS. The analysis is grouped based on the configurations of PNA process, including partial nitritation, anammox and single-stage PNA systems. QS is confirmed to regulate various properties of PNA systems, including microbial activity, microbial growth rate, microbial aggregation, microbial interactions and the robustness under adverse conditions. Major challenges in the mechanisms of QS, such as QS circuits, target genes and the response to environmental inputs, are identified. Potential applications of QS, such as short-term addition of certain acyl-homoserine lactones (AHLs) or substances containing AHLs, transient unfavorable conditions to stimulate the secretion of AHLs, are also proposed. This review focuses on the theoretical and practical cognation for QS in PNA systems, and serves as a stepping stone for further QS-based strategies to enhance nitrogen removal through PNA process.

Flocs in disguise? High granule abundance found in continuous-flow activated sludge treatment plants

To date, high performance of full-scale aerobic granular sludge (AGS) technology has been demonstrated on a global scale. Its further integration with existing continuous flow activated sludge (CFAS) treatment plants is the next logical step. All granular sludge reactors operated in sequencing batch reactors (SBR) mode with anaerobic feeding conditions select for growth of phosphorus and glycogen accumulating organisms (PAO and GAO, respectively), which are known to enhance sludge settling characteristics. Therefore, we hypothesized that AGS are commonly present at full-scale CFAS processes with enhanced biological phosphorus removal (EBPR) and low sludge volume index (SVI). This hypothesis was confirmed at 13 EBPR plants, where granules were found present (at plants where SVI was lower than 100 ml/g) with a strong correlation between high granule abundance and low SVI. A wide range of granule abundance was found among the plants, ranging from 0.5% to as high as 80%. Evaluations of the EBPR plant process configurations showed that high granule abundances may be related to selector design features such as high anaerobic food to mass (F/M) ratios, unmixed in-line fermentation, and high influent soluble COD fraction. Granules were also observed at a non-EBPR plant with an aerobic selector receiving high F/M feeds. Quantitative PCR and 16S rRNA gene sequencing analyses revealed higher relative gene abundance of Accumulibacter PAO and Competibacter GAO in the granules over flocs, as well as a correlation between granule abundance and some possible EPS producers such as Flavobacterium and Competibacter. Our results indicated that process configurations that select for slow-growing or EPS-producing heterotrophs play an important role for granule formation in full-scale CFAS systems as previously shown in SBR configurations.

AHL-mediated quorum sensing regulates the variations of microbial community and sludge properties of aerobic granular sludge under low organic loading

Aerobic granular sludge (AGS) is promising in wastewater treatment. However, the formation and existence of AGS under low organic loading rate (OLR) is still not fully understood due to a knowledge gap in the variations and correlations of N-acyl-homoserine lactones (AHLs), the microbial community, extracellular polymeric substances (EPS) and other physiochemical granule properties. This study comprehensively investigated the AHL-mediated quorum sensing (QS) and microbial community characters in the AGS fed with ammonium-rich wastewater under a low OLR of 0.15 kg COD (m³ d)^-1. The results showed that the AGS appeared within 90 days, and the size of mature granules was over 700 μm with strong settleability and ammonium removal performance. More tightly-bound extracellular polysaccharide and tightly-bound extracelluar protein were produced in the larger AGS. C10-HSL and C12-HSL gradually became dominant in sludge, and short-chain AHLs dominated in water. EPS producers and autotrophic nitrifiers were successfully retained in the AGS under low OLR. AHL-mediated QS utilized C10-HSL, C12-HSL and 3OC6-HSL as the critical AHLs to regulate the TB-EPS in aerobic granulation, and autotrophic nitrifiers may perform interspecific communication with C10-HSL. The correlations of bacterial genera with AGS properties and AHLs were complex due to the dynamic fluctuations of microbial composition and other variable factors in the mixed-culture system. These findings confirmed the participation of AHL-mediated QS in the regulation of microbial community characters and AGS properties under low OLR, which may provide guidance for the operation of AGS systems under low OLR from a microbiological viewpoint.

Rapid start-up of partial nitritation and simultaneously phosphorus removal (PNSPR) granular sludge reactor treating low-strength domestic sewage

Obtaining desirable partial nitritation (PN) is crucial for successful application of the combined PN and anammox process. In this study, the partial nitritation and simultaneously phosphorus removal (PNSPR)¹ granular sludge reactor treating low-strength domestic sewage was rapidly started up in 67days through seeding denitrifying phosphorus removal (DPR)² sludge. The nitrite/ammonium ratio in effluent was approximately 1 and the nitrite accumulation rate (NAR) was more than 95%, about 93% of orthophosphate was removed. The DPR sludge rich in phosphate accumulating organisms (PAOs) with few nitrifying bacteria could promote the achievement of PNSPR. Quantitative microbial analysis showed that the ammonium oxidizing bacteria (AOB) gene ratio in sludge increased from 0.21% to 3.43%, while nitrite oxidizing bacteria (NOB) gradually decreased to 0.07%. The average particle size of sludge increased from 114 to 421μm, indicating the formation of PNSPR granules. The high phosphorus content in sludge and phosphorus removal facilitated rapid granulation.

Influence of high temperature on the performance of aerobic granular sludge in biological treatment of wastewater

The effect of temperature on the efficiency of organics and nutrients removal during the cultivation of aerobic granular sludge (AGS) in biological treatment of synthetic wastewater was studied. With this aim, three 3 L sequencing batch reactors (SBRs) with influent loading rate of 1.6 COD g (L d)^-1 were operated at different high temperatures (30, 40 and 50 °C) for simultaneous COD, phosphate and ammonia removal at a complete cycle time of 3 h. The systems were successfully started up and progressed to steady state at different cultivation periods. The statistical comparison of COD, phosphate and ammonia for effluent from the three SBRs revealed that there was a significant difference between groups of all the working temperatures of the bioreactors. The AGS cultivated at different high temperatures also positively correlated with the accumulation of elements including carbon, oxygen, phosphorus, silicon, iron, aluminium, calcium and magnesium that played important roles in the granulation process.

Cell adhesion, ammonia removal and granulation of autotrophic nitrifying sludge facilitated by N-acyl-homoserine lactones

In this study, six N-acyl-homoserine lactone (AHL) molecules (C6-HSL, C8-HSL, C10-HSL, 3-oxo-C6-HSL, 3-oxo-C8-HSL and 3-oxo-C10-HSL) were each dosed into a bioreactor and seeded using autotrophic nitrifying sludge (ANS). The effects of the AHLs on cell adhesion, nitrification and sludge granulation were investigated. The results indicated that the efficiencies of cell adhesion and ammonia removal both had a close correlation with the side chain length and β position substituent group of the AHLs. The best-performing AHL in terms of accelerating bacterial attached-growth was 3-oxo-C6-HSL, whereas C6-HSL outperformed the others in terms of the ammonia degradation rate. The addition of 3-oxo-C6-HSL or C6-HSL increased the biomass growth rate, microbial activity, extracellular proteins and nitrifying bacteria, which can accelerate the formation of nitrifying granules. Consequently, selecting AHL molecules that could improve bacteria in attached-growth mode and nitrification efficiency simultaneously will most likely facilitate the rapid granulation of nitrifying sludge.

Performance of aerobic granular sludge in a sequencing batch bioreactor for slaughterhouse wastewater treatment

Lab-scale experiment was conducted to investigate the formation and characteristics of aerobic granular sludge for biological nutrient removal of slaughterhouse wastewater. Experimental results showed that removal performances of chemical oxygen demand (COD), ammonia and phosphate were enhanced with sludge granulation, and their removal efficiencies reached 95.1%, 99.3% and 83.5%, respectively. The aerobic granular sludge was matured after 90days cultivation, and protein-like substances were the main components. Simultaneously, the mass ratio of proteins and polysaccharides (PN/PS) was enhanced to 2.5 from 1.7. The granules with particle sizes of 0.6-1.2 and 1.2-1.8mm, accounting for 69.6%, were benefit for the growth of ammonia oxidizing bacteria (AOB) and nitrate oxidizing bacteria (NOB), and corresponding specific oxygen demand rates (SOUR) of AOB and NOB were 31.4 and 23.3mgO2/gMLSSh, respectively.

Rapid formation of nitrifying granules treating high-strength ammonium wastewater in a sequencing batch reactor

Short initial settling time and rapidly increased ammonium nitrogen loading were employed to cultivate nitrifying granular sludge treating inorganic wastewater with 1000 mg/L ammonium nitrogen. It was found that the nitrifying granule-dominant sludge was formed in a sequencing batch reactor (SBR) with influent ammonium concentration increased from 200 to 1000 mg N/L within 55 days. During the following 155-day operation period, nitrifying granules exhibited good performance with an ammonium removal efficiency of 99%. In the meantime, sludge volume index (SVI) decreased from 92 to 15 mL/g and the mean size of the nitrifying granules increased from 106 to 369 μm. Mixed liquor suspended solids (MLSS) decreased from the initial 6.4 to around 3 g/L during the granulation period and increased to over 10 g/L at the end of the operation. The long-term stability of nitrifying granules and the reactor performance were not negatively affected by inhibition from free ammonia (FA) and free nitrous acid (FNA) in this study. This makes the granule sludge technology promising in treating high-strength ammonium wastewater in practice.

Nitrification of anaerobic digester effluent for nitrogen management at swine farms

Anaerobic digester effluent collected from a swine farm was nitrified in lab-scale bioreactors mimicking an innovative manure management system to understand factors relevant to a successful start-up. The effects of digester effluent strength, aeration time, nitrifying sludge inoculation, and initial pH control on the startup of the nitrification system were investigated. The results showed that initial NH4+-N concentrations higher than 800 mg L−1 could severely inhibit the onset of nitrification if relying on native bacterial populations. When aeration time was increased from 4 to 12 h d−1, ammonium oxidation occurred earlier, but nitrite oxidation was delayed. However, the delay was not observed when bioaugmentation with nitrifying sludge was conducted. Initial addition of acid for pH control was unsuccessful as initial depletion of alkalinity eventually resulted in self-inhibition of nitrification cause by decreasing pH over time. Overall, these results provide guidance on how to effectively start large-scale innovative animal waste treatment systems.

Nitrogen-converting communities in aerobic granules at different hydraulic retention times (HRTs) and operational modes

This study determined how the activity and number of nitrogen-converting microorganisms varied with changes in hydraulic retention time (HRT) and the operating regime of aerobic granular sequencing batch reactors (GSBRs) treating high-nitrogen wastewater. Continuously aerated (O-mode) GSBRs were operated at HRTs of 10-, 13- and 19-h. Then the same reactors were operated at identical HRTs but the cycles started with an anoxic phase (A/O mode). To investigate the microbial communities, DNA- and RNA-based relative real-time PCR was used. In all experimental reactors ammonium was fully removed with a removal rate up to 75 mg N–NH₄⁺/(L·h), and nitrification efficiency was above 90 %. The efficiency of the removal of oxidized nitrogen forms decreased with the lengthening of HRT. The study found that variable oxic conditions (A/O mode) in the GSBR cycle stimulated the simultaneous activity of ammonium oxidizing bacteria (AOB), N₂O-reducers, and Anammox bacteria in aerobic granules. With both modes, the activity of nitrogen-converting bacteria was highest with a 13-h HRT. Shortening HRT, resulted in higher chemical oxygen demand and nitrogen loadings, which favored the growth of Anammox microorganisms in granules and caused a decrease in the number of AOB. With all HRTs, the number of Anammox microorganisms was about 1.5-times higher in A/O mode than in O mode.

Influence of temperature on the partial nitritation of reject water in a granular sequencing batch reactor

Two Granular Sequencing Batch Reactors were operated to perform partial nitrification of sludge reject water at different temperatures, from 25-41 degrees C. Every temperature was fixed for about a month in order to evaluate the nitritation rate, morphological features of aggregates and bacterial populations. The optimum temperature was found between 33 and 37 degrees C in terms of nitritation rate. Morphological features of granules did not show significant changes with temperature in the range between 28 and 37 degrees C; Feret diameter remained at 5.8 +/- 0.7mm and roundness was 0.76 +/- 0.02. Lower temperatures promoted the appearance of filamentous bacteria, leading to an increase of the sludge volume index (SVI) and a consequent reduction of biomass concentration. When the temperature was increased to 39 degrees C, more than the 80% of aggregates showed a diameter higher than 6mm but density decreased from 28 to 19 g VSS L(-1), resulting in an increase of the SVI from 33 to 80 mL g(-1). The establishment of 41 degrees C caused a rapid destabilization of the system and nitritation activity disappeared. Bacterial populations did not experience significant changes during the experimental period and Nitrosomonas was the dominant species at all the temperatures assayed.

Partial nitrification using aerobic granules in continuous-flow reactor: rapid startup

This study applied a novel strategy to rapid startup of partial nitrification in continuous-flow reactor using aerobic granules. Mature aerobic granules were first cultivated in a sequencing batch reactor at high chemical oxygen demand in 16 days. The strains including the Pseudoxanthomonas mexicana strain were enriched in cultivated granules to enhance their structural stability. Then the cultivated granules were incubated in a continuous-flow reactor with influent chemical oxygen deamnad being stepped decreased from 1,500 ± 100 (0-19 days) to 750 ± 50 (20-30 days), and then to 350 ± 50 mg l(-1) (31-50 days); while in the final stage 350 mg l(-1) bicarbonate was also supplied. Using this strategy the ammonia-oxidizing bacterium, Nitrosomonas europaea, was enriched in the incubated granules to achieve partial nitrification efficiency of 85-90% since 36 days and onwards. The partial nitrification granules were successfully harvested after 52 days, a period much shorter than those reported in literature.

Cultivation of aerobic granular sludge for rubber wastewater treatment

Aerobic granular sludge (AGS) was successfully cultivated at 27±1 °C and pH 7.0±1 during the treatment of rubber wastewater using a sequential batch reactor system mode with complete cycle time of 3 h. Results showed aerobic granular sludge had an excellent settling ability and exhibited exceptional performance in the organics and nutrients removal from rubber wastewater. Regular, dense and fast settling granule (average diameter, 1.5 mm; settling velocity, 33 m h(-1); and sludge volume index, 22.3 mL g(-1)) were developed in a single reactor. In addition, 96.5% COD removal efficiency was observed in the system at the end of the granulation period, while its ammonia and total nitrogen removal efficiencies were up to 94.7% and 89.4%, respectively. The study demonstrated the capabilities of AGS development in a single, high and slender column type-bioreactor for the treatment of rubber wastewater.

Enhanced aerobic nitrifying granulation by static magnetic field

One of the main challenging issues for aerobic nitrifying granules in treating high strength ammonia wastewater is the long granulation time required for activated sludge to transform into aerobic granules. The present study provides a novel strategy for enhancing aerobic nitrifying granulation by applying an intensity of 48.0mT static magnetic field. The element analysis showed that the applied magnetic field could promote the accumulation of iron compounds in the sludge. And then the aggregation of iron decreased the full granulation time from 41 to 25days by enhancing the setting properties of granules and stimulating the secretion of extracellular polymeric substances (EPS). Long-term, cycle experiments and fluorescence in-situ hybridization (FISH) analysis proved that an intensity of 48.0mT magnetic field could enhance the activities and growth of nitrite-oxidizing bacteria (NOB). These findings suggest that magnetic field is helpful and reliable for accelerating the aerobic nitrifying granulation.

Effect of wastewater COD/N ratio on aerobic nitrifying sludge granulation and microbial population shift

The effect of COD/N ratio on the granulation process and microbial population succession was investigated. Four identical sequencing batch reactors, R1, R2, R3 and R4, were operated with various initial COD/N ratios ranging from 0/200 to 800/200 (m/m). Ethanol was fed as the source of COD. Aerobic granules were successfully cultivated in R2 and R3, operating with the COD/N ratio of 200/200 and 400/200, respectively. Scanning electron microscope observations indicated that short rod-shaped and spherical bacteria were dominant in R2, while granules produced in R3 were surrounded with a large amount of filamentous bacteria. The average specific nitritation rate in R2 and R3 were 0.019 and 0.008 mg N/(mg MLVSS x hr), respectively. Fluorescence in situ hybridization results demonstrated that nitrifying bacteria population was enriched remarkably in R2. It indicated that nitrification ability and nitrifying bacteria population were enriched remarkably at low COD/N ratio. However, no granules were formed in R1 and R4 which might attribute to either limited or excessive extracellular polymeric substances production. This study contributed to a better understanding of the role of COD/N ratio in nitrifying sludge granulation.

Bioremediation of 2-chlorophenol containing wastewater by aerobic granules-kinetics and toxicity

2-Chlorophenol (2-CP) degrading aerobic granules were cultivated in a sequencing batch reactor (SBR) in presence of glucose. The organic loading rate (OLR) was increased from 6.9 to 9.7 kg COD m(-3)d(-1) (1150-1617 mg L(-1)COD per cycle) during the experiment. The alkalinity (1000 mg L(-1) as CaCO(3)) was maintained throughout the experiment. The specific cell growth rate was found to be 0.013 d(-1). A COD removal efficiency of 94% was achieved after steady state at 8h HRT (hydraulic retention time). FTIR, UV, GC, GC/MS studies confirmed that the biodegradation of 2-CP occurs via chlorocatechol (modified ortho-cleavage) pathway. Biodegradation kinetics followed the Haldane model with kinetic parameters: V(max)=840 mg2-CPgMLVSS(-1)d(-1), K(s)=24.61 mg L(-1), K(i)=315.02 mg L(-1). Abiotic losses of 2-CP due to volatilization and photo degradation by sunlight were less than 3% and the results of genotoxicity showed that the degradation products are eco-friendly.

Bioaugmentation as a tool for improving the start-up and stability of a pilot-scale partial nitrification biofilm airlift reactor

The effectiveness of bioaugmentation in the improvement of the start-up of a biofilm airlift reactor to perform partial nitrification was investigated. Two identical biofilm airlift reactors were inoculated. The non-bioaugmented reactor (NB-reactor) was inoculated with conventional activated sludge, whereas the bioaugmented reactor (B-reactor) was seeded with the same conventional activated sludge but bioaugmented with nitrifying activated sludge from a pilot plant performing full nitritation under stable conditions (100% oxidation of influent ammonium to nitrite). The fraction of specialized nitrifying activated sludge in the inoculum of the B-reactor was only 6% (measured as dry matter). To simplify comparison of the results, operational parameters were equivalent for both reactors. Partial nitrification was achieved significantly faster in the B-reactor, showing a very stable operation. The results obtained by fluorescence in situ hybridization assays showed that the specialized nitrifying biomass added to the B-reactor remained in the biofilm throughout the start-up period.

Performance of high-loaded ANAMMOX UASB reactors containing granular sludge

The performance of high-loaded anaerobic ammonium oxidizing (ANAMMOX) upflow anaerobic sludge bed (UASB) reactors was investigated. Two ANAMMOX reactors (R1 with and R2 without effluent recycling, respectively) were fed with relatively low nitrite concentration of 240 mg-N L(-1) with subsequent progressive increase in the nitrogen loading rate (NLR) by shortening the hydraulic retention time (HRT) till the end of the experiment. A super high-rate performance with nitrogen removal rate (NRR) of 74.3-76.7 kg-N m(-3) day(-1) was accomplished in the lab-scale ANAMMOX UASB reactors, which was 3 times of the highest reported value. The biomass concentrations in the reactors were as high as 42.0-57.7 g-VSS L(-1) with the specific ANAMMOX activity (SAA) approaching to 5.6 kg-N kg-VSS(-1) day(-1). The high SAA and high biomass concentration were regarded as the key factors for the super high-rate performance. ANAMMOX granules were observed in the reactors with settling velocities of 73-88 m h(-1). The ANAMMOX granules were found to contain a plenty of extracellular polymers (ECPs) such as 71.8-112.1 mg g-VSS(-1) of polysaccharides (PS) and 164.4-298.2 mg g-VSS(-1) of proteins (PN). High content of hemachrome (6.8-10.3 μmol g-VSS(-1)) was detected in the ANAMMOX granules, which is supposed to be attributed to their unique carmine color.