Akira Hirata

Extracellular polymeric substances responsible for bacterial adhesion onto solid surface

The influence of extracellular polymeric substances (EPS) on bacterial cell adhesion onto solid surfaces was investigated using 27 heterotrophic bacterial strains isolated from a wastewater treatment reactor. Cell adhesion onto glass beads was carried out by the packed-bed method and the results were discussed in terms of the amount of each EPS component produced and cell surface characteristics such as zeta potential and hydrophobicity. Protein and polysaccharides accounted for 75-89% of the EPS composition, indicating that they are the major EPS components. Among the polysaccharides, the amounts of hexose, hexosamine and ketose were relatively high in EPS-rich strains. For EPS-poor strains, the efficiency of cell adhesion onto glass beads increased as the absolute values of zeta potential decreased, suggesting that electrostatic interaction suppresses cell adhesion efficiency. On the other hand, the amounts of hexose and pentose exhibited good correlations with cell adhesiveness for EPS-rich strains, indicating that polymeric interaction due to the EPS covering on the cell surface promoted cell adhesion. It was concluded that, if the EPS amount is relatively small, cell adhesion onto solid surfaces is inhibited by electrostatic interaction, and if it is relatively large, cell adhesion is enhanced by polymeric interaction.

Simultaneous nitrification and denitrification by controlling vertical and horizontal microenvironment in a membrane-aerated biofilm reactor

Nitrogen and carbon components in domestic modified wastewater were completely removed by simultaneous nitrification and denitrification using a membrane-aerated biofilm reactor where biofilm was fixed on a hollow-fiber membrane. To measure the spatial distribution of pH, ammonium and nitrate ions and to observe microbes inside the biofilm fixed on the membrane, microelectrodes and the fluorescence in situ hybridization (FISH) method were applied. Due to plug flow in the vertical direction (from the bottom to the top of the reactor), ammonium nitrogen was gradually removed and negligible nitrate nitrogen was detected throughout the reactor. FISH revealed that ammonia-oxidizing bacteria were mainly distributed inside the biofilm and other bacteria, which included denitrifying bacteria, were mainly distributed outside the biofilm and over the suspended sludge. In order to characterize bacterial activity in the vertical direction of the reactor, nitrification rates at lower, central and upper points were calculated using microelectrode data. The nitrification rate at the lower point was 7 and 125 times higher than those at the central and upper points, respectively. These results show that the removal of carbon and nitrogen compounds was accomplished efficiently by using various kinds of bacteria distributed vertically and horizontally in a single reactor.

Nitrogen removal characteristics and biofilm analysis of a membrane-aerated biofilm reactor applicable to high-strength nitrogenous wastewater treatment

A membrane-aerated biofilm reactor (MABR) capable of simultaneous nitrification and denitrification in a single reactor vessel was developed to investigate the characteristics of nitrogen removal from high-strength nitrogenous wastewater, and biofilm analysis using microelectrodes and the fluorescence in situ hybridization (FISH) technique was performed. Mean removal percentages of total organic carbon (TOC) and nitrogen were 96% and 83% at removal rates of 5.76 g-C m(-2) d(-1) and 4.48 g-N m(-2) d(-1), respectively. For stable removal efficiency, constant washing of the biofilm was needed. Dissolved oxygen microelectrode measurement revealed that the biofilm thickness was about 1600 microm, and that oxygen penetrated about 300 to 700 microm, from the outer surface of the membrane. Furthermore, FISH analysis revealed that ammonia-oxidizing bacteria (AOB) were located near the outer surface of the membrane, whereas other bacteria were located from the inner to the outer part of the biofilm. Combining these results demonstrated that simultaneous nitrification and denitrification occurred in the biofilm of the MABR system. In addition, stoichiometric analysis revealed that after 130 d(-1), the free ammonia (FA) concentration ranged within the concentration causing inhibition of the growth of nitrite oxidizing bacteria (NOB) and that AOB consumed 86% of the oxygen supplied through the intra-membrane. These results indicate that nitrogen removal not via nitrate but via nitrite was mainly achieved in the MABR system.

Influence of extracellular polymers on electrokinetic properties of heterotrophic bacterial cells examined by soft particle electrophoresis theory

Abstract In this study, influence of extracellular polymeric substance (EPS) on the bacterial electrokinetics and cell adhesion to glass beads was examined for four kinds of heterotrophic bacteria (A1, A2, B1 and B2 strains) isolated from a biological reactor treating domestic wastewater. Electrophoretic mobility data were analyzed by Ohshimas soft particle theory. Cell adhesion to glass beads was carried out by packed-bed method, and the data were interpreted as the cell collision efficiency. Electrophoretic mobility measurements showed that all four strains had soft particle characters. EPS covering on cell surface increased the softness of the cell surface and decreased the negative surface charge density around the cell surface. For A1, A2 and B1 strains, both surface potential based on soft particle theory Ψ 0 and conventional zeta potential ζ of intact were lower than those of washed cells. While ζ increased from −31.6 to −27.2 mV when EPS was removed, Ψ 0 estimated from soft particle altered from −11.0 to −14.9 mV in the case of B2 strain, indicating EPS covering resulted in the increase of cell surface potential. Cell collision efficiencies are in qualitative agreement with the result of the cell surface potential evaluation based on soft particle analysis.

Dynamic modeling and simulation of a three-phase fluidized bed batch process for wastewater treatment

Abstract Batch operation with different starting substrate concentrations (biological oxygen demand (BOD5)) and biomass concentrations (mixed liquor volatile solid (MLVS)) were performed on a completely mixed three-phase fluidized bed reactor used in treating simulated domestic waste water. The mixed culture microorganisms used were contained in a biofilm attaching to cement balls. The data obtained were fitted to five different kinetic rate equations. The kinetic parameters of each model were obtained using a Gauss–Newton nonlinear regression analysis method. It was found that the Haldane model incorporating endogenous metabolic consumption (Endo–Haldanc model), best described the kinetics of the biological reaction inside the three-phase fluidized bed reactor. This is due to the fact that this model can both account for the effects of cell death and/or endogenous metabolism at low concentration and of substrate inhibition at high concentration.

Kinetics of tetrachloroethylene (PCE) gas degradation and byproducts formation during UV/H2O2 treatment in UV-bubble column reactor

The UV/H 2 O 2 process is commonly used for the remediation of drinking and groundwater pollution with chlorinated volatile organic compounds. In direct treatment, its efficiency can be lowered in the presence of radical scavengers and/or UV light absorbers. This research is focused on the improvement of the UV/H 2 O 2 photolytic process, due to the reduction of OH radical scavengers and UV absorbers in the reacting system. Degradation of tetrachloroethylene (PCE) gas, which was absorbed into a bubble column reactor equipped with UV light (UV-BCR), containing distilled water and H 2 O 2 as the reacting medium, was studied in a one flow-through mode. Degradation of PCE in the liquid phase was found to follow pseudo-first-order kinetics and apparent rate constants in the order of 0.02 s -1 have been observed. An absorption-reaction model based on slow reaction in the bulk liquid was proposed, and it fitted the experimental data reasonably well. However, when sequence making internal (designed as to prolong the gas pathway through the reactor) was in the system in addition to the relatively high PCE concentrations (500 ppm), production of the byproduct chloroform was observed, indicating that some reaction might have occurred in the gas phase. The addition of term for a reaction in the gas phase was able to account for the reaction due to direct absorption of PCE.