Shiyu Li

Characterization of the size-fractionated biomacromolecules: Tracking their role and fate in a membrane bioreactor

This article presents a study aimed at the fractionation and characterization of what is thought to be one of the most complex organic mixtures produced by activated sludge: biomacromolecules (BMM). Photometric quantification combined with excitation-emission matrix (EEM) fluorescence spectroscopy and nuclear magnetic resonance (NMR) measurements were used to characterize BMM in a membrane bioreactor (MBR) from a chemical perspective. Overall, the BMM in sludge supernatant were mainly present in three fractions: colloidal BMM (BMMc, >0.45 μm), biopolymeric BMM (BMMb, 0.45 μm-100 kDa) and low molecular weight (MW) fraction (<5 kDa). The analysis of fluorescence regional integration (FRI) showed that the organics in membrane permeate and those in the low-MW fraction of sludge supernatant were of similar chemical composition. The characterization by NMR suggested that the BMMc fraction had similar carbon content of proteins and polysaccharides. In contrast, the BMMb and the low-MW BMM were proved to be carbonaceous and aromatics, respectively. Moreover, because of the high MW and gelling property, polysaccharides were found to have a high potential to accumulate on the membranes. In addition, the lipids present in the BMMb of the sludge supernatant were demonstrated to be another important foulant due to their large size. Our results also indicated that aromatic proteins had a higher fouling propensity than tryptophan proteins though they were of similar size nature. This work could be useful for better understanding of the chemical nature of BMMs in MBRs.

A biofilm model for prediction of pollutant transformation in sewers

This study developed a new sewer biofilm model to simulate the pollutant transformation and biofilm variation in sewers under aerobic, anoxic and anaerobic conditions. The biofilm model can describe the activities of heterotrophic, autotrophic, and sulfate-reducing bacteria (SRB) in the biofilm as well as the variations in biofilm thickness, the spatial profiles of SRB population and biofilm density. The model can describe dynamic biofilm growth, multiple biomass evolution and competitions among organic oxidation, denitrification, nitrification, sulfate reduction and sulfide oxidation in a heterogeneous biofilm growing in a sewer. The model has been extensively verified by three different approaches, including direct verification by measurement of the spatial concentration profiles of dissolved oxygen, nitrate, ammonia, and hydrogen sulfide in sewer biofilm. The spatial distribution profile of SRB in sewer biofilm was determined from the fluorescent in situ hybridization (FISH) images taken by a confocal laser scanning microscope (CLSM) and were predicted well by the model.

Microbial transformation of biomacromolecules in a membrane bioreactor: implications for membrane fouling investigation.

Background The complex characteristics and unclear biological fate of biomacromolecules (BMM), including colloidal and soluble microbial products (SMP), extracellular polymeric substances (EPS) and membrane surface foulants (MSF), are crucial factors that limit our understanding of membrane fouling in membrane bioreactors (MBRs). Findings In this study, the microbial transformation of BMM was investigated in a lab-scale MBR by well-controlled bioassay tests. The results of experimental measurements and mathematical modeling show that SMP, EPS, and MSF had different biodegradation behaviors and kinetic models. Based on the multi-exponential G models, SMP were mainly composed of slowly biodegradable polysaccharides (PS), proteins (PN), and non-biodegradable humic substances (HS). In contrast, EPS contained a large number of readily biodegradable PN, slowly biodegradable PS and HS. MSF were dominated by slowly biodegradable PS, which had a degradation rate constant similar to that of SMP-PS, while degradation behaviors of MSF-PN and MSF-HS were much more similar to those of EPS-PN and EPS-HS, respectively. In addition, the large-molecular weight (MW) compounds (>100 kDa) in BMM were found to have a faster microbial transformation rate compared to the small-MW compounds (<5 kDa). The parallel factor (PARAFAC) modeling of three-dimensional fluorescence excitation-emission matrix (EEM) spectra showed that the tryptophan-like PN were one of the major fractions in the BMM and they were more readily biodegradable than the HS. Besides microbial mineralization, humification and hydrolysis could be viewed as two important biotransformation mechanisms of large-MW compounds during the biodegradation process. Significance The results of this work can aid in tracking the origin of membrane foulants from the perspective of the biotransformation behaviors of SMP, EPS, and MSF.

A novel nonwoven hybrid bioreactor (NWHBR) for enhancing simultaneous nitrification and denitrification

This study proposed a nonwoven hybrid bioreactor (NWHBR) in which the nonwoven fabric played dual roles as a biofilm carrier and membrane‐like separation of the flocculent sludge in the reactor. The results of long‐term monitoring demonstrated that the NWHBR could achieve simultaneous nitrification and denitrification (SND), with nearly complete ammonium removal and 80% removal of total nitrogen. The biofilm attached to the nonwoven fabric removed 27% of the chemical oxygen demand (COD) and 36% of the nitrate in the reactor, an enhanced elimination of nutrients that was attributed to the increased mass transfer within the biofilm due to permeate drag. The results of batch experiments showed that the flocculent sludge played a more dominant role in nitrification and denitrification (79% and 61%, respectively) than the biofilm (21% and 36%, respectively). The batch experiments also revealed that the enforced mass transfer, with an effluent recirculation rate of 4.3 L/m2h (which was the same as the flux during the reactors long‐term operation), improved the denitrification rate by 58% (i.e., from 9.0 to 14.2 mg‐NO  3− ‐N/h). Pyrosequencing of the 16S rRNA gene amplification revealed a high microbial diversity in both the flocculent sludge and biofilm, with Proteobacteria, Bacteroidetes and Chloroflexi as the dominant groups. A phylogenetic (P) test indicated that the NWHBR contained phylogenetically distinct microbial communities: those in the biofilm differed from those in the flocculent sludge. However, the communities on the exterior and interior of the biofilm were more similar to each other. Due to its good SND performance, low physical back‐washing frequency and low air‐to‐water ratio, the NWHBR represents an attractive alternative for the wider application of either low‐cost membrane bioreactors or biofilm reactors. Biotechnol. Bioeng. 2013; 110: 1903–1912.

Role of microorganism growth phase in the accumulation and characteristics of biomacromolecules (BMM) in a membrane bioreactor

The objective of this study was to highlight the significance of microorganism growth on the production of biomacromolecules (BMM) in a membrane bioreactor (MBR). During the MBR operation, both polysaccharides and proteins in the sludge supernatant were found to increase steadily in exponential growth phase (EGP) due to higher organic loading rates and microbial primary metabolism. Subsequently, both increased continuously and then decreased sharply in the following deceleration growth phase (DGP). Finally, the BMM maintained a low and steady level as the sludge reached stationary growth phase (SGP). The results of batch experiments showed that the sludge under DGP was prone to produce much more BMM than that under SGP as a result of a nutrients decrease and higher microbial secondary metabolism activities. Furthermore, large-molecular weight (MW) compounds (>100 kDa) made up the majority of BMM in the EGP and DGP. In contrast, the small-MW compounds (<5 kDa) became a more and more important BMM fraction as the microorganism growth shifted to the SGP. The characterization by three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy indicated that the fluorescence compounds in the BMM pool were mainly comprised of aromatic and tryptophan protein-like substances, and humic substances. The protein-like substances were related with microorganism growth more greatly than the humic substances. Generally, the microorganism physiological stages (i.e.growth/decay) play a crucial role in the formation of BMM in the MBRs.

A modified oxic-settling-anaerobic activated sludge process using gravity thickening for excess sludge reduction.

Oxic-settling-anaerobic process (OSA) was known as a cost-effective way to reduce the excess sludge production with simple upgrade of conventional activated sludge process (CAS). A low oxidation-reduction potential (ORP) level was the key factor to sludge decay and lysis in the sludge holding tank of the OSA process. However, the ORP control with nitrogen purge or chemical dosing in the OSA process would induce extra expense and complicate the operation. Hence, in this study, a sludge holding tank using gravity thickening was applied to OSA process to reduce the excess sludge production without any ORP control. Results showed that the modified OSA process not only reduced the excess sludge production effectively but also improved the sludge settleability without affected the treatment capacity. The reduction of the excess sludge production in the modified OSA process resulted from interactions among lots of factors. The key element of the process was the gravity thickening sludge holding tank.

A new oxic-settling-anaerobic (NOSA(®)) activated sludge process for minimizing excess sludge in secondary biological treatment plants: a pilot-scale evaluation of the absorption-biodegradation process.

This study compared the sludge reduction performance of a new oxic-settling-anaerobic (NOSA) process with that of a conventional adsorption-biodegradation process. A 50 m(3)/d pilot trial system with two different process configurations was operated for 6 months. The NOSA process functioned effectively in removing both chemical oxygen demand and nitrogen with the efficiencies of 86 and 92.5%, respectively, which reduced approximately 40% of the excess sludge. In this research, 0.77 kg volatile suspended solids/d sludge vanished in the anaerobic tank, which accounted for 58.9% of the total sludge loss in the NOSA process. Economic calculation suggests that the new process can dramatically upgrade the sludge reduction in wastewater treatment plants without a digestion device, and the investment for fundamental upgrading can be recovered in 5-6 years by cutting the costs of excess sludge dewatering and disposal treatment.