An-jie Li

Microbial population dynamics during aerobic sludge granulation at different organic loading rates

Laboratory experiments were carried out to investigate the evolution of the bacterial community during aerobic sludge granulation. The experiments were conducted in three 2.4L sequencing batch reactors (SBRs) that were seeded with activated sludge and fed with glucose-based synthetic wastewater. Three different influent organic concentrations were introduced into the three SBRs, R1, R2 and R3, resulting in chemical oxygen demand (COD) loading rates of 1.5 (R1), 3.0 (R2) and 4.5 (R3)kg/m(3)d, respectively. Changes in bacterial diversity throughout the granulation process were monitored and analysed using polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) techniques. The experimental results demonstrate that glucose-fed aerobic granules could be formed without significant presence of filamentous bacteria. Granules formed at different loading rates had different morphology, structural properties and bacterial species. A higher loading rate resulted in faster formation of larger and loose granules, while a lower loading rate resulted in slower formation of smaller and more tightly packed granules. The biomass underwent a dynamic transformation in terms of bacterial species richness and dominance during the granulation process. The reactor with the highest substrate loading rate had the lowest species diversity, while the reactor with the lowest substrate loading rate had the highest species diversity. Different dominant species of beta- and gamma-Proteobacteria and Flavobacterium within the granule communities from the three different SBRs were confirmed by analysis of 16S rDNA sequences of the PCR products separated by DGGE. It is apparent that a few common bacterial species play an important role in the formation and growth of aerobic granules and help sustain the granular sludge structure in the bioreactors.

Selective sludge discharge as the determining factor in SBR aerobic granulation: numerical modelling and experimental verification.

Numerical simulation and laboratory experiments were conducted to investigate the determining factor and the underlying mechanism in aerobic sludge granulation in a sequencing batch reactor (SBR). In the numerical simulation, a sectional approach was used to develop a model to describe the biomass dynamics during the granulation process. The growth of different classes of the SBR sludge with different substrate uptake rates and different sludge discharge ratios was simulated. The results indicate that the selective discharge of slow-settling sludge flocs is the key determining factor for granulation. In the laboratory study, experiments were conducted with two identical 2.4-L SBRs, R1 and R2, using different sludge discharge methods - the selective discharge of slow-settling sludge flocs for R1, and mixed, unselective sludge discharge for R2. The SBRs were fed with glucose-based synthetic wastewater at a chemical oxygen demand (COD) loading rate of 1.5 kg/m(3)-d. The evolution of the microbial community during the experimental process was monitored using the molecular techniques of polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE) and clone library analysis. Sludge granulation was achieved in less than three weeks in R1, whereas the sludge in R2 remained in the form of flocs. However, some bacterial species had a significant presence in both the R1 granules and the R2 flocs. The results suggest that aerobic granulation may not require the dominance of any particular species. Small and loose sludge flocs were found to have an advantage over larger and dense granules in substrate uptake. Thus, discharge of loose flocs would remove these competitors from the system and makes the substrate more available for uptake and utilisation by biomass in the attached-growth form, resulting in sludge granulation.

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.

Synergy of N-(3-oxohexanoyl)-l-homoserine lactone and tryptophan-like outer extracellular substances in granular sludge dominated by aerobic ammonia-oxidizing bacteria

Nitrogen removal via nitrite is an energy-saving method for high-strength ammonia wastewater treatment. A better understanding of the formation of granular sludge dominated by aerobic ammonia-oxidizing bacteria (AerAOB) could facilitate the improved use of rapid sludge granulation for nitritation. In this study, AerAOB-dominated activated sludge (NAS) and granular sludge (NGS) produced different N-scyl-homoserine lactones (AHLs). N-(3-oxohexanoyl)-l-homoserinelactone (OHHL), only released from NGS, was shown to accelerate sludge aggregation by increasing the biomass growth rate, microbial activity, extracellular protein, and AerAOB biomass. For both NAS and NGS, sludge cells were glued together by inner extracellular polymeric substances (EPSs) with similar components to form microcolony. Different from the characterized negative effect of NAS’s outer-EPS on cell adhesion, the outer-EPS of NGS played a positive role in the attached growth of AerAOB-dominated sludge and contained more tryptophan-like substances. More interesting, OHHL enhanced the yields of tryptophan-like substances after mixing with the outer-EPS of NGS, enhancing cell adhesion. In a word, OHHL and more tryptophan-like substances were produced in the process of granulation under the selective sludge discharge condition, which was proved to be able to accelerate NAS granulation. Therefore, the sludge granulation process for nitritation can be improved by increasing the levels of OHHL and tryptophan in the initial startup stage. The appropriate engineering strategy should be further studied to facilitate the actual application of granular sludge for nitrogen removal on a large scale.