Xiao-yan Li

Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: A review

A review concerning the definition, extraction, characterization, production and functions of extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment reactors is given in this paper. EPS are a complex high-molecular-weight mixture of polymers excreted by microorganisms, produced from cell lysis and adsorbed organic matter from wastewater. They are a major component in microbial aggregates for keeping them together in a three-dimensional matrix. Their characteristics (e.g., adsorption abilities, biodegradability and hydrophilicity/hydrophobicity) and the contents of the main components (e.g., carbohydrates, proteins, humic substances and nucleic acids) in EPS are found to crucially affect the properties of microbial aggregates, such as mass transfer, surface characteristics, adsorption ability, stability, the formation of microbial aggregates etc. However, as EPS are very complex, the knowledge regarding EPS is far from complete and much work is still required to fully understand their precise roles in the biological treatment process.

Electro-catalytic oxidation of phenol on several metal-oxide electrodes in aqueous solution

Elecrtochemical degradation of phenol was evaluated at five typical anodes for mineralization to carbon dioxide or for being a pre-treatment method in toxic aromatic compounds. Three kinds of RuO(2)-base electrodes were prepared by thermal deposition, which were coated by the oxides of Ru or by Ru, Sn and Sb or by Ru, Sn, Sb and Gd on Ti metal surface, respectively. Another electrode Ti/ PbO(2) was prepared by electro-deposition method with PbO(2) coated on Ti. A Pt electrode was chosen for comparison. Characteristics of the typical five electrodes were investigated by cyclic voltammetry, SEM and its degradation ability for phenol. Performance for phenol degradation of the three RuO(2) electrodes lie in: Ti/Sb-Sn-RuO(2)-Gd> Ti/Sb-Sn-RuO(2)> Ti/RuO(2) and the electrode with beta-PbO(2) coating was superior to RuO(2)-based electrodes and Pt electrode. Aromatic ring opening take place at all researched electrodes and it is supposed that electrolysis run stop at different intermediates, such as benzoquinone, maleic acid, etc. Under the present experimental conditions, whole mineralization to CO(2) takes place only in the beta-PbO(2) anode. A pathway of electrochemical degradation of phenol was suggested based on the experimental analysis.

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.

Electron microscopic investigation of the bactericidal action of electrochemical disinfection in comparison with chlorination, ozonation and fenton reaction

Abstract Laboratory experiments were carried out to investigate the mechanisms of electrochemical (EC) disinfection of artificial wastewater contaminated by Escherichia coli culture. Comparative disinfection tests with chlorine, ozone and hydroxyl ( OH − ) radicals produced by the Fenton reaction were also conducted. It was demonstrated that the EC process was highly effective for wastewater disinfection. Investigation with scanning electron microscopy (SEM) showed different appearances of damage to in the surface morphology and structure of the cells after different forms of disinfection. Substantial leakage of intracellular materials was found for the E. coli cells after EC disinfection, which was also observed for the cells treated by the Fenton reaction. However, such cell lysis was noticeable to a less extent for the ozonated cells and hardly noticeable for the chlorinated cells. Electron microscopic examination suggested that the cells were likely inactivated during the EC process by the chemical products with an oxidising power similar to that of hydroxyl radicals and much stronger than that of chlorine. The SEM results support the hypothesis that the predominant killing action of EC disinfection is provided by high-energy intermediate EC products. Therefore, in addition to electro-chlorination, the great capacity of EC disinfection may be attributable to the generation of short-lived germicidal agents, such as free radicals.

Electrochemical degradation of bisphenol A on different anodes.

Laboratory experiments were carried out on the kinetics, pathways and mechanisms of electrochemical (EC) degradation of bisphenol A (BPA) on four types of anodes, Ti/boron-doped diamond (BDD), Ti/Sb-SnO(2), Ti/RuO(2) and Pt. There were considerable differences among the anodes in their effectiveness and performance of BPA electrolysis. BPA was readily destructed at the Ti/Sb-SnO(2) and Ti/BDD anodes, the Pt anode had a moderate ability to remove BPA, and the Ti/RuO(2) anode was incapable of effectively oxidising BPA. The intermediate products of EC degradation of BPA were detected and quantified by high-performance liquid chromatography (HPLC), and a general BPA degradation pathway was proposed based on the analytical results. It was suggested that OH radicals produced by water electrolysis attacked BPA to form hydroxylated BPA derivatives that were then transformed into one-ring aromatic compounds. These compounds underwent ring breakage, which led to the formation of aliphatic acids that were eventually mineralised by electrolysis to CO(2). Compared to the Pt and Ti/RuO(2) anodes, the Ti/Sb-SnO(2) and Ti/BDD anodes were found to have higher oxygen evolution potentials and higher anodic potentials for BPA electrolysis under the same current condition. However, the stability and durability of the Ti/Sb-SnO(2) anode still needs to be greatly improved for actual application. In comparison, with its high durability and good reactivity for organic oxidation, the Ti/BDD anode appears to be the more promising one for the effective EC treatment of BPA and similar endocrine disrupting chemical (EDC) pollutants.

Membrane bioreactor for the drinking water treatment of polluted surface water supplies.

A laboratory membrane bioreactor (MBR) using a submerged polyethylene hollow-fibre membrane module with a pore size of 0.4 microm and a total surface area of 0.2 m2 was used for treating a raw water supply slightly polluted by domestic sewage. The feeding influent had a total organic carbon (TOC) level of 3-5 mg/L and an ammonia nitrogen (NH(3)-N) concentration of 3-4 mg/L. The MBR ran continuously for more than 500 days, with a hydraulic retention time (HRT) as short as 1h or less. Sufficient organic degradation and complete nitrification were achieved in the MBR effluent, which normally had a TOC of less than 2 mg/L and a NH(3)-N of lower than 0.2 mg/L. The process was also highly effective for eliminating conventional water impurities, as demonstrated by decreases in turbidity from 4.50+/-1.11 to 0.08+/-0.03 NTU, in total coliforms from 10(5)/mL to less than 5/mL and in UV(254) absorbance from 0.098+/-0.019 to 0.036+/-0.007 cm(-1). With the MBR treatment, the 3-day trihalomethane formation potential (THMFP) was significantly reduced from 239.5+/-43.8 to 60.4+/-23.1 microg/L. The initial chlorine demand for disinfection decreased from 22.3+/-5.1 to 0.5+/-0. 1mg/L. The biostability of the effluent improved considerably as the assimilable organic carbon (AOC) decreased from 134.5+/-52.7 to 25.3+/-19.9 microg/L. All of these water quality parameters show the superior quality of the MBR-treated water, which was comparable to or even better than the local tap water. Molecular size distribution analysis and the hydrophobic characterisation of the MBR effluent, in comparison to the filtered liquor from the bioreactor, suggest that the MBR had an enhanced filtration mechanism. A sludge layer on the membrane surface could have functioned as an additional barrier to the passage of typical THM precursors, such as large organic molecules and hydrophobic compounds. These results indicate that the MBR with a short HRT could be developed as an effective biological water treatment process to address the urgent need of many developing countries that are plagued by the serious contamination of surface water resources.

Permeability of fractal aggregates

It is well known that the permeability and density of an aggregate decreases with its size, affecting its settling velocity and coagulation rate (rate of particle capture) with other particles. This change in aggregate density with size can be described by fractal scaling relationships. Two distinctly different fractal scaling approaches, however, have been used to describe aggregate permeability. In one approach (single-particle-fractal model), the permeability is calculated by assuming primary particles are uniformly distributed in the aggregate. In the other approach (cluster-fractal model), it is assumed that aggregates are composed of primary particles separated into individual clusters that are less permeable than the aggregate. The overall permeability of the aggregate is dependent on the number and sizes of these clusters. Using three different permeability correlations (Brinkman, Happel and Carmen-Kozeny), it is demonstrated through comparison with aggregate settling velocity data that the single-particle-fractal model does not provide realistic predictions of settling velocity as a function of aggregate size. In addition, it is shown that the Carmen-Kozeny permeability equation does not produce realistic settling velocity relationships. The transport settling velocity and capture rate of sinking aggregates in natural and engineered environments should therefore only be calculated using the Happel or Brinkman equations and a cluster-fractal model.

Sorption and desorption of antibiotic tetracycline on marine sediments

Tetracycline is commonly used for human therapy and veterinary purposes as well as agricultural feed additives. In this study, batch experiments were carried out to investigate the sorption behaviour of tetracycline on marine sediments. The sediment samples were collected from Victoria Harbour, Hong Kong. Sorption isotherms of tetracycline on marine sediments can be well described by a Freudlich model. The calculated K(f) varied from 1.12 to 2.34Lg(-1). After H(2)O(2) oxidation for removing the organic carbon from marine sediments, the K(f) values were reduced by more than 80%, but the organic carbon normalized sorption constant averaged 213.1Lg(-1) for the H(2)O(2)-treated sediments, which was higher than 98.3Lg(-1) for the raw marine sediments. The calculated hysteresis coefficient H ranged from 0.79 to 0.90 indicating that there is a hysteresis in desorption. The sorption of tetracycline on marine sediments was found to decrease with an increase of pH and salinity. These research findings are of importance to an assessment of the fate and transport of tetracycline and other similar antibiotics in seawater-sediment systems.

Selective removals of heavy metals (Pb2+, Cu2+, and Cd2+) from wastewater by gelation with alginate for effective metal recovery

A novel method that uses the aqueous sodium alginate solution for direct gelation with metal ions is developed for effective removal and recovery of heavy metals from industrial wastewater. The experimental study was conducted on Pb(2+), Cu(2+), and Cd(2+) as the model heavy metals. The results show that gels can be formed rapidly between the metals and alginate in less than 10 min and the gelation rates fit well with the pseudo second-order kinetic model. The optimum dosing ratio of alginate to the metal ions was found to be between 2:1 and 3:1 for removing Pb(2+) and around 4:1 for removing Cu(2+) and Cd(2+) from wastewater, and the metal removal efficiency by gelation increased as the solution pH increased. Alginate exhibited a higher gelation affinity toward Pb(2+) than Cu(2+) and Cd(2+), which allowed a selective removal of Pb(2+) from the wastewater in the presence of Cu(2+) and Cd(2+) ions. Chemical analysis of the gels suggests that the gelation mainly occurred between the metal ions and the -COO(-) and -OH groups on alginate. By simple calcination of the metal-laden gels at 700 °C for 1 h, the heavy metals can be well recovered as valuable resources. The metals obtained after the thermal treatment are in the form of PbO, CuO, and CdO nanopowders with crystal sizes of around 150, 50, and 100 nm, respectively.

Membrane (RO-UF) filtration for antibiotic wastewater treatment and recovery of antibiotics

Abstract A membrane system including reverse osmosis (RO) and ultrafiltration (UF) was proposed and evaluated for the treatment of an oxytetracycline (OTC) waste liquor. Using RO filtration with a volume reduction coefficient of 3.5, the organic content in its permeate was decreased from COD∼10 000 mg/l to less than 200 mg/l, while OTC was reduced from more than 1000 mg/l to lower than 80 mg/l. Oxytetracycline was concentrated more than 3 times to 3000–4000 mg/l in the retentate. Due to likely the high concentration of large biopolymers interacting with OTC molecules, OTC could not be successfully recovered from the RO retentate by conventional crystallization. With additional treatment of ultrafiltration by 3K membranes, OTC crystallization and recovery from the RO retentate were significantly improved with a recovery ratio of more than 60% and a purity of higher than 80%. Detailed studies indicated that most large biopolymers in the OTC liquor were polysaccharides that would either hinder the growth of OTC crystals or precipitate together with OTC molecules. Removal of these large biopolymers by ultrafiltration could improve the condition remarkably for OTC crystallization. Therefore, the RO-UF membrane process can be developed as an effective alternative for the treatment of antibiotic wastewater as well as the recovery of antibiotics from the waste liquor.