Effects of hydrodynamic conditions on OH radical production at Ti/Pt anodes during electrochemical treatment.

The effects of hydrodynamic conditions on the production of hydroxyl radicals (*OH) during electrochemical treatment using a titanium coated by platinum anode and a stainless steel cathode are discussed in this paper. The sample used was ultra-pure water containing 200 mM of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The amount of *OH for treatment was measured using electron spin resonance spectrometry coupled with DMPO spin trapping. Under constant hydrodynamic conditions, the production rate of the *OH spin adduct (DMPO-OH) increased with the terminal potential. This result was caused by the change in the electric current density at the anode, which was proportional to the rate of production of DMPO-OH. Increasing the linear velocity of water in an electrolytic cell promoted the production of DMPO-OH by two effects, namely, by the enhancement of ion transportation and the promotion of DMPO transfer from the bulk to the anode. The former effect emerged when the DMPO concentration near the anode was not insignificant in comparison with the DMPO concentration in the bulk, that is, when the overall rate-determining step was the electron transfer at the anode. The latter effect emerged when the DMPO concentration near the anode was much lower than the DMPO concentration in the bulk, that is, when the overall rate-determining step was the diffusion of DMPO from the bulk to the anode. In addition, the latter effect was found to be proportional to the square root of the linear velocity of water in the electrolytic cell.

Removal and recovery of phosphate and ammonium as struvite from supernatant in anaerobic digestion.

Removal of phosphorus and nitrogen is required to prevent eutrophication problems in lakes and enclosed coastal seas. And recovery of phosphorus from wastewater has been attracting attention because of lack in phosphorus resources in the near future. In this study, reaction kinetics and design parameters of struvite production are experimentally investigated by using basic reaction type and a draft-tube type reactors. Struvite production rate, which is a very important parameter in reactor design and efficiency estimation, is formulated in an equation consisting of a rate constant (k2), and magnesium, phosphate and ammonium concentrations. The value of k2 is shown to be increased with struvite concentration and mixing intensity in the reactor. The developed equation is applied to the results obtained from the draft-tube type reactor experiments and verified for its applicability. High struvite concentration of 10-25% is maintained in the draft-tube reactor experiments. 92% removal and recovery efficiency with effluent phosphorus concentration of 17 mg/L is achieved under the conditions of 4 minutes reaction time, pH of 8.5 and Mg/P molar ratio of 1.1.

Application of membrane-coupled anaerobic volatile fatty acids fermentor for dissolved organics recovery from coagulated raw sludge.

To investigate the treatment performance of membrane-coupled anaerobic volatile fatty acids fermentor system, the effects of operational parameters for volatile fatty acids production were evaluated through experiments and a mathematical model. The volatile fatty acids recovery ratio was largely affected by the change of hydraulic retention time, reaching its maximum value at 12 hrs. Over the range of hydraulic retention time 8 to 96 hrs, the volatile fatty acids recovery ratio decreased with the increase of hydraulic retention time above 12 hrs, while the ratio of mineralization and gasification increased. Hydraulic retention time and membrane filtration ratio should be maintained less than 1 day and above 0.9, respectively, to attain over 40% of organic materials recovery ratio at 10 days of solids retention time. When the hydrolysis rate constant was 0.01 hr-1, the organic loading rate should be maintained at above 1.0 (kgC/m3/day) to attain over 45% of volatile fatty acids recovery ratio. Based on experimental and simulated results, membrane-coupled anaerobic volatile fatty acids fermentor system was thought to be effective for dissolved organics recovery from coagulated sewage sludge.

Quantitative comparison of forests and other areas with dry weather input loading in the Lake Biwa catchment area.

For the evaluation of pollutants loading to Lake Biwa, comprehensive river surveys on river mouths and forest sites were conducted 9 times from 1996 to 1999, on 25-40 main rivers in the Lake Biwa watershed. The main results obtained are as follows. (1) River water quality reflects regional characteristics a of their catchment areas, and the concentration difference among rivers ranged between 2-3 fold. (2) Concentration variation shows different patterns with time and location depending on water quality indices a used. (3) Indices related to organic matter and nutrients have lower correlation between forests and river mouths, but those related to ionic species showed strong correlation. (4) Flux comparison of forest and river mouth sites suggests that pollutants from catchment areas are conveyed to the lake not only through rivers but also underground. (5) In dry weather conditions, forests contribute 30% to the whole pollutants (TN, TP, and TCOD(Mn)) loading, and the remainder is derived mainly from paddy fields and residential/commercial zones. (6) Unit loading factors from forests are estimated as 640, 57 and 1200 kg/km2/y, respectively for TN, TP and TCOD(Mn), while those from other areas are estimated as 2,500, 208 and 4,200 kg/km2/y.

Prediction of a membrane-coupled anaerobic VFA fermenter efficiency using an empirical model.

An empirical model based on some statistical analysis for predicting produced volatile fatty acids (VFA) concentration was developed to establish reliable design conditions of a membrane-coupled anaerobic VFA fermenter (MCAVF) and assess its performance with influent organics concentration (Ci), membrane filtration ratio (phi) and hydraulic loading rate (HLR). The empirical model followed the same trend as the experimental data, which showed the effectiveness of the model. The relationship involving these three independent variables explained more than 90% of the variation in the dependent variable. A model explains that the produced VFA concentration is more sensitive to changes in influent organics concentration (Ci) and membrane filtration ratio (phi) than hydraulic loading rate (HLR). This empirical model can predict the optimum values of operation parameters on many scenarios. Due to its simplicity, the empirical model can be used to design and operate a membrane-coupled anaerobic VFAs fermenter.

Advanced treatment of sewage by pre-coagulation and biofilm process.

Treatment performance and operational parameters of a pre-coagulation and biofilm process were experimentally discussed with the pilot-scale plant treating actual municipal sewage. Perfect nitrification was accomplished within total hydraulic retention time (HRT) of 4 h at 20% volumetric added ratio of attached-growth media of biofilm in the aerobic tanks under temperature conditions as low as 17 degrees C. Treatment efficiencies in organic matter, T-N and T-P were high and stable with the total HRT of 8 h in the biological reactor. Concentrations of T-N and T-P in effluent were kept about 2 mgN/L and under 1 mgP/L, respectively. Attached growth bacterial mass and the potential activity of nitrification and denitrification in each tank in cold season were high enough to accomplish excellent treatment performance. Then the process was proved to be applicable to municipal sewage treatment for removal of nitrogen and phosphorus as well as organic matter.

Water quality characteristics of forest rivers around Lake Biwa.

Forest river surveys were carried out at upper streams of several rivers in the Lake Biwa watershed to understand the water quality characteristics of the rivers, and to find out their relationships with forest features such as geographical, geological and vegetational data. The results showed: (1) Forests have some purification functions for nitrogen and organic matter, but become sources for most of ionic species. (2) Main mineral species in forest rivers are Ca2+, Mg2+ and Na+, HCO3-, CO3(2-), Cl-, SO4(2-) and SiO2. (3) Loading from forests was 0.4-7 kg/km2/d for TN and 0.01-0.3 kg/km2/d for TP. (4) River quality reflects the properties of each forest, and is unique to the place, especially in ionic species such as Ca2+ and Cl-. (5) A cluster analysis successfully categorized ionic components into several groups.

Effective combination of microfiltration and intermittent ozonation for high permeation flux and VFAs recovery from coagulated raw sludge.

This research was conducted to investigate the effect of intermittent ozonation on permeation flux recovery and production for soluble organic materials in a membrane-coupled anaerobic VFAs fermenter (MCAVF) system. Flux recovery ratio exceeded 80% with 22.2 gO3 l-1 of ozone injection and then showed nearly 95% with 43 gO3 l-1 in batch experiment. Under the same ozone concentration, extending contact time was more effective than increasing bubbling dose rate for flux recovery. In continuous-flow operation, the value of average permeation flux was 0.69 m3 m-2 d-1 during without ozonation period, while average permeation flux for 70 days with intermittent ozone bubbling was 1.17 m3 m-2 d-1 that corresponds to 1.7 times that with no ozonation period. Moreover, in spite of intermittent ozone bubbling for 70 days, significant inhibition was not observed for volatile fatty acids (VFAs) producing bacteria and VFAs production. It was found that the average value of [permeates total organic carbon (TOC) concentration: Influent TOC concentration] with ozonation period was about 1.5 times higher than that of no ozonation period. Consequently, it is believed that a membrane-coupled anaerobic VFAs fermenter (MCAVF) process combined with intermittent ozonation is suitable to overcome the flux decline of membrane and simultaneously is useful to the recovery of soluble organic materials from coagulated raw sludge.

Effect of hydraulic loading rate on acidogenesis in a membrane-coupled anaerobic VFAs fermenter.

A membrane-coupled anaerobic VFAs fermenter (MCAVF) was operated with five different hydraulic loading rates (3, 2, 1, 0.5 and 0.25 day-1) to evaluate its influence on acid phase VFAs production from coagulated raw sludge. Results show that C2 to C5 volatile fatty acids were formed as predominant compounds. At constant solids retention time (SRT) of 10 days, VFAs production ratio moderately increased over the range of hydraulic loading rate (HLR) of 3 day-1 to 2 day-1, but significantly decreased at other HLR conditions. First order hydrolysis rate constant decreased with a decrease in HLR. The relatively high degradation percentages of carbohydrate and protein were observed, ranging from 25 to 47%, 27 to 47%, respectively. However, lipid and fiber component were degraded at much lower ratio than the previous other two organic materials. On the other hand, despite some variation in the minor acids in five HLR cases, VFAs speciation appeared to be independent of HLR. The results obtained emphasize that; at least in the range investigated, the optimal HLR for VFAs production and recovery from coagulated raw sludge is in the range of 2 day-1 and 1 day-1.

Role of active oxygen species in DNA damage by pentachlorophenol metabolites.

Pentachlorophenol (PCP) has been shown to be carcinogenic for mice, although it does not seem to be mutagenic in bacterial test systems. In this study, the mechanism of DNA damage by PCP metabolites in the presence of metals was investigated with a DNA sequencing technique using 32P-labeled DNA fragments and with an electrochemical detector coupled to an HPLC. The metabolite tetrachlorohydroquinone (TCHQ) caused DNA damage in the presence of Cu(II) but not in the presence of either Mn(II) or Fe(III). TCHQ plus Cu(II) frequently induced piperidine-labile sites at thymine residues and guanine residues. The most preferred sites were the thymine residues of the 5'-GTC-3' sequence. TCHQ increased 8-oxo-7,8-dihydro-2'-deoxyguanosine in calf thymus DNA in the presence of Cu(II). Typical OH scavengers showed no inhibitory effects on TCHQ- plus Cu(II)-induced DNA damage. Bathocuproine and catalase inhibited DNA damage, suggesting that Cu(I) and H2O2 have important roles in the production of active species causing DNA damage. Tetrachloro-p-benzoquinone (TCBQ) alone did not induce DNA damage in the presence of Cu(II), but addition of NADH induced DNA cleavage even in the absence of NADH-FMN oxidoreductase. UV-visible and ESR spectroscopies have demonstrated that TCHQ is rapidly autoxidized into semiquinone even in the absence of metal ions, indicating that the semiquinone radical itself is not the main active species inducing DNA damage. These results suggest that the semiquinone radical produced by the autoxidation of TCHQ and/or the reduction of TCBQ by NADH reacts with dioxygen to form superoxide and subsequently H2O2, which is activated by transition metals to cause DNA damage.