Tomohide Watanabe

Selective reduction of nitrate to nitrogen gas in a biofilm-electrode reactor

Abstract Microbial denitrification and the occurrence of neutralization in a denitrifying biofilm-electrode reactor (BER) using an amorphous carbon anode has been experimentally demonstrated. In this study, the BER was operated over one year and measurements of influent and effluent ionic species were made at different electric currents to evaluate the predominant electrochemical and biological reactions. The ionic species measured were NO3−, NO2−, SO42−, Cl−, PO43−, NH4+, Na+, K+, Ca2+ and Mg2+, most of which are common constituents of surface water or groundwater. Concentrations of Na+, K+, SO42− and Cl− were almost the same in the influent and effluent. Removal efficiency of nitrate (NO3−) varied in the range of about 0 to 100%, depending on the electric current. Complete denitrification to N2 gas was readily achieved without accumulation of NO2−, N2O and NH4+. The concentration of Ca2+ and Mg2+ decreased due to deposition onto the surface of the electrode, but the calculation result from the solubility equilibrium of CaCO3, MgCO3 and CaMg(CO3)2 using the saturation index (SI) showed that the deposition could be hampered by the electrochemical neutralization in the reactor. Furthermore, the deposited calcium and magnesium could be redissolved immediately by changing the polarity of electrodes. From these results, it is concluded that a highly selective reduction of nitrate is operationally possible in the present BER, hence this process is a feasible alternative for the treatment of various nitrate-contaminated water.

Denitrification and neutralization treatment by direct feeding of an acidic wastewater containing copper ion and high-strength nitrate to a bio-electrochemical reactor process

The feasibility of the direct denitrification treatment of copper metal pickling wastewater by using a bio-electrochemical reactor process was investigated experimentally. Carbon electrodes were installed in the reactor as the anode and cathode and denitrifying microorganisms were fixed on the surface of the cathode. The reactor was continuously operated by applying an electric current and feeding acetate. In this reactor, copper ion removal and denitrification proceeded simultaneously and the pH value of the treated water was increased almost to neutral. The electric current that passed through the cathode contributed to the removal of the copper ion and the generation of hydrogen gas. The generated hydrogen gas as well as the added acetate was effectively utilized for denitrification. A theoretical evaluation of pH in the effluent suggested that the pH increase was mainly caused by the generation of hydroxyl ion during denitrification. In addition, the inorganic carbon species generated during denitrification with acetate and by the electrochemical oxidation of anodic carbon acted as a buffer to minimize a further increase of pH at higher nitrate removal efficiencies. These results demonstrated that copper ion removal, denitrification and neutralization could be achieved simultaneously by using a single bioelectrochemical reactor.

Increase of bond strength at interfacial transition zone by the use of fly ash

The effects of the addition of silica powder, silica fume, fly ash, and hemihydrated gypsum to the interface of new-to-old paste on the enhancement of bond strength were investigated experimentally. Seven-day bond strengths for the specimens with pozzolanic materials added to the interfacial zone, except for silica powder addition, were slightly smaller than that for control without any additives, whereas the strength of all specimens with added pozzolanic materials were higher than that of the control at 28 days. It was suggested that increased effect of the bond strength at interfacial zone depended on the SiO2 and CaO contents in the additives; higher SiO2 and/or lower CaO contents were preferred. On the other hand, when a high CaO content fly ash was coated to the interface with some amount of hemihydrated gypsum, the bond strength enhanced significantly at 7 days, as well as at 28 days. This result suggests that the structure of the interfacial zone can be modified sufficiently by controlling the chemical components of the additives; even the use of fly ashes consisted of relatively high CaO and low SiO2.

The effect of the physical structure of a porous Ca-based sorbent on its phosphorus removal capacity.

A macroporous calcium-based sorbent made from bentonite, calcium hydroxide and Yalloun coal was prepared and tested for its phosphorus removal performance. The coal was added as a solid pore-forming agent. Pores sizes as large as a few micron were generated when the Yalloun coal was incorporated at a relatively high proportion. These pores were formed due to the burning out of the added coal during the thermal treatment conducted at 700 degrees C. Results of phosphorus removal experiments showed that the increase of the amount of phosphorus removed could be positively correlated with the increase of the total pore volume and the mean pore size. Decreases in pore sizes and total pore volume were pronounced in sorbents used to remove phosphorus due to the formation of a Ca-phosphate mineral on their surfaces. It was suggested that the widely formed pores avoided the rapid plugging and contributed to maintain the diffusional transport of the substrate to a certain extent. Hydroxyapatite was formed and coated on the surface of the sorbent during phosphorus removal. On the basis of these results, it was concluded that modification of the physical structure of Ca-based sorbents by creating large pore sizes, revealed to be effective for increasing their phosphorus uptake.

The denitrification and neutralization performance of an electrochemically activated biofilm reactor used to treat nitrate-contaminated groundwater

Continuous denitrification of nitrate-contaminated groundwater containing dissolved oxygen (DO), SO42−, and no buffer was carried out with three identical electrochemically activated biofilm reactors. The reactors consisted primarily of denitrifying biofilm attached to the surface of the cathode, an amorphous carbon employed as the anode, and a DC power supply. In the reactors, denitrification and neutralization caused by H2 and CO2 produced from the cathode and the anode, respectively, occurred simultaneously when an electric current was applied. A complete-mix reactor model coupled with a biofilm-electrode model was developed in conjunction with a limiting-current theory. When the biofilms were sufficiently acclimated and adapted to the electric current, the denitrification performance calculated using the model was in fairly good agreement with experimental results.

CO2 reduction to methane and acetate using a bio-electro reactor with immobilized methanogens and homoacetogens on electrodes

A new method of CO 2 reduction to methane and/or acetate was investigated by a bio-electro reactor which consisted of cathodic electrodes with immobilized bacteria on the surface and anodic electrodes. Biological CO 2 reduction to methane and/or acetate occurred with H 2 gas produced by the electrolysis of water on the cathodic electrode when an electric current was applied. A stable methane production was observed under continuous operations and about 80% of the theoretical quantity of H 2 gas based on Faradays law was utilized for methane production. CO 2 was also effectively converted to acetate when methane production was inhibited. The CO 2 reduction was carried out and controlled by the applied electric current in the bio-electro reactor system

Characterization of wastewater treatment by two microbial fuel cells in continuous flow operation.

A two serially connected single-chamber microbial fuel cell (MFC) was applied to the treatment of diluted molasses wastewater in a continuous operation mode. In addition, the effect of series and parallel connection between the anodes and the cathode on power generation was investigated experimentally. The two serially connected MFC process achieved 79.8% of chemical oxygen demand removal and 11.6% of Coulombic efficiency when the hydraulic retention time of the whole process was 26 h. The power densities were 0.54, 0.34 and 0.40 W m−3 when electrodes were in individual connection, serial connection and parallel connection modes, respectively. A high open circuit voltage was obtained in the serial connection. Power density decreased at low organic loading rates (OLR) due to the shortage of organic matter. Power generation efficiency tended to decrease as a result of enhancement of methane fermentation at high OLRs. Therefore, high power density and efficiency can be achieved by using a suitable OLR range.

Inhibitory effect of gamma-irradiated chitosan on the growth of denitrifiers.

In order to find an environmentally benign substitute to hazardous inhibitory agents, the inhibitory effect of γ-irradiated chitosans against a mixed culture of denitrifying bacteria was experimentally evaluated. Unlike other studies using pure aerobic cultures, the observed effect was not a complete inhibition but a transient inhibition reflected by prolonged lag phases and reduced growth rates. Raw chitosan under acid conditions (pH 6.3) exerted the strongest inhibition followed by the 100 kGy and 500 kGy irradiated chitosans, respectively. Therefore, because the molecular weight of chitosan decreases with the degree of γ-irradiation, the inhibitory properties of chitosan due to its high molecular weight were more relevant than the inhibitory properties gained due to the modification of the surface charge and/or chemical structure by γ-irradiation. High dosage of γ-irradiated appeared to increase the growth of mixed denitrifying bacteria in acid pH media. However, in neutral pH media, high dosage of γ-irradiation appeared to enhance the inhibitory effect of chitosan.

Antibacterial Activity of Gamma‐irradiated Chitosan Against Denitrifying Bacteria

In order to find an environmentally benign substitute to hazardous inhibitory agents, the inhibitory effect of γ‐irradiated chitosans against a mixed culture of denitrifying bacteria was experimentally evaluated. Unlike other studies using pure aerobic cultures, the observed effect was not a complete inhibition but a transient inhibition reflected by prolonged lag phases and reduced growth rates. Raw chitosan under acid conditions (pH 6.3) exerted the strongest inhibition followed by the 100 kGy and 500 kGy irradiated chitosans respectively. Therefore because the molecular weight of chitosan decreases with the degree of γ‐irradiation, the inhibitory properties of chitosan due to its high molecular weight were more relevant than the inhibitory properties gained due to the modification of the surface charge and/or chemical structure by γ‐irradiation. High dosage of γ‐irradiated appeared to increase the growth of mixed denitrifying bacteria in acid pH media. However, in neutral pH media, high dosage of γ‐irradiation appeared to enhance the inhibitory effect of chitosan.