Effect of ferric salt addition on UV/electro-chlorine advanced oxidation process.

This study discussed the effect of ferric salt addition on UV/electro-chlorine advanced oxidation process using a train of electrolytic and UV flow cells with an ozone-free low-pressure mercury vapour lamp (total irradiance:0.60 W at 254 nm). Ferric salt addition enhanced 1,4-dioxane degradation at an electrolytic current of 0.100 A. By contrast, an inhibitory effect of ferric salt addition was observed at a current of 0.500 A. The enhanced accumulation of free chlorine at a current of 0.500 A directly decreased the 1,4-dioxane degradation rate by scavenging reactive radicals like HO˙ and Cl˙. However, at an electrolytic current of 0.100 A, UV irradiance was relatively excessive for electrochemical chlorine production. The excess UV energy enhanced the photoreduction of FeOH2+, followed by the Fenton-type reaction of Fe2+ and HOCl, which produced HO˙ and consumed free chlorine. As a result, the free chlorine concentration decreased, and the reaction efficiency between the reactive radicals and 1,4-dioxane improved. Thus, the addition of ferric salt to a UV/electro-chlorine system is recommended when the UV irradiance in the system is excessive compared to the electrochemical chlorine supply.

Advanced oxidation mechanism of UV photolysis of electrochemically generated free bromine.

In recent times, some researchers have successfully demonstrated the efficacy of UV photolysis of electrochemically generate free chlorine (UV/electro-chlorine) as for an advanced oxidation process. Since bromine as well as chlorine is an element belonging to halogen, it is expected that UV photolysis of electrochemically generated free bromine (UV/electro-bromine) also shows an advanced oxidation effect. To elucidate the feasibility of UV/electro-bromine system, its advanced oxidation mechanism was investigated using radical probes of 1,4-dioxane and nitrobenzene. In contrast to the UV/electro-chlorine system, the advanced oxidation effect of UV/electro-bromine system was inhibited under acidic conditions due to the accumulation of photochemically inert Br2. The most abundant radical in UV/electro-bromine system was dibromine radical anion (Br2˙-) and the second-order reaction rate constant of Br2˙- with 1,4-dioxane was estimated to be 2.4 × 105 M-1 s-1. As a result of the abundance and the reactivity of Br2˙-, it was the main contributor to 1,4-dioxane degradation. On the other hand, nitrobenzene was mainly decomposed by direct UV photolysis because Br2˙- does not react with nitrobenzene. The contribution of hydroxyl radical (HO˙) to 1,4-dioxane degradation was much lower than that of Br2˙- because its concentration was 4-5 order of magnitude lower than that of Br2˙-. However, the HO˙ concentration elevated with a decrease in the concentration of bromide ion (Br-). Consequently, the reactivity of Br2˙- with pollutants and the Br- concentration have critical impacts on the advanced oxidation performance of UV/electro-bromine system.

Removal of linear alkylbenzene sulfonate (LAS) by a cetyltrimethylammonium bromide (CTAB)-aided coagulation-filtration process.

ABSTRACTThis paper reports on the removal of linear alkylbenzene sulfonate (LAS) by conventional and cetyltrimethylammonium bromide (CTAB)-aided coagulation-filtration processes. The addition of CTAB led to the formation of CTAB-LAS aggregates, whose surface charge was negative at a CTAB-LAS molar ratio < 1 and positive at a molar ratio > 1. The efficacy of the coagulation-filtration process strongly depended on the electrostatic interactions between hydroxide floc and both free LAS and CTAB-LAS aggregate. Since free LAS is anionic, its adsorption onto the hydroxide floc was enhanced when the floc was positively charged. Ferric hydroxide floc (pH < 6.0) and aluminum hydroxide floc (pH < 7.4) showed positive surface charges and were thereby effective in removing free LAS. The optimal coagulation pH for removing the CTAB-LAS aggregate depended on the CTAB/LAS molar ratio. A higher coagulation pH was effective in removing LAS under the CTAB/LAS molar ratio > 1. However, in this case, excessive free CTAB remained after coagulation-filtration. Therefore, the lower coagulation pH with a CTAB/LAS molar ratio < 1, with the hydroxide floc and CTAB-LAS aggregate being positively and negatively charged, respectively, was recommended for LAS removal. In chemical industry wastewater, the optimal CTAB dose was higher than that in synthetic wastewater due to coexistent chemicals scavenging CTAB. Consequently, coagulation-filtration using a ferric chloride coagulant under the CTAB/LAS molar ratio of 0.7-2.0 and pH 4.5 was recommended for LAS removal from chemical industry wastewater, where the observed LAS removal efficiency reached 91 ± 3% without any deterioration of COD removal.

Efficacy of an electrochemical flow cell introduced into the electrochemical Fenton-type process using a Cu(I)/HOCl system.

An electrochemical flow cell was introduced into the electrochemical Fenton-type process using a Cu(I)/HOCl system. The effects of the current density and the initial cupric ion (Cu2+) concentration on the process performance were discussed. The current efficiency of the process improved from 6.1% for an electrolytic tank system to 33% for the electrochemical flow cell system at a current density of 5.0 mA/cm2 and an initial Cu2+ concentration of 1.0 mM. The current efficiency increased to 58% for Cu2+ concentrations of 2.0 mM and beyond. The cathodic reduction of Cu2+ to the cuprous ion (Cu+) emerged as the rate-determining step in comparison to the anodic production of free chlorine. The introduction of the electrochemical flow cell enhanced the cathodic production of Cu+ by reinforcing the mass transfer of the Cu2+ to the cathode, and the detachment of micro bubbles generated electrochemically at the cathode surface. A decrease in the current density and an increase in the initial Cu2+ concentration also improved the current efficiency by promoting the cathodic production of Cu+. This involved the prevention of the cathodic reduction of protons to hydrogen gas and the elevation of the electrode potential of the cathodic reaction from Cu2+ to Cu+.

Effects of pH and coexisting chemicals on photolysis of perfluorooctane sulfonate using an excited xenon dimer lamp.

Vacuum ultraviolet (VUV) photolysis at the wavelength of 172 nm in a sulfate solution was introduced as a more efficient process for perfluorooctane sulfonate (PFOS) degradation than ultraviolet (UV) photolysis at 254 nm. The effects of pH and coexisting chemicals on VUV photolysis under the coexistence of 100 mM sulfate were investigated. VUV irradiation successfully degraded PFOS, and the degradation rate was 5.5 times higher than by UV photolysis. Direct VUV photolysis was inferred to mainly contribute to PFOS degradation. PFOS degradation by VUV irradiation was enhanced at pH less than 2 due to sulfate radicals generated via VUV photolysis of sulfate ions. Consequently, VUV photolysis was superior to UV photolysis for PFOS removal on both the removal rate and energy efficiency. However, the inorganic chemicals phosphoric acid and nitric acid strongly inhibited PFOS degradation, probably by masking PFOS from VUV rays by their VUV absorption. Accordingly, PFOS separation from inorganic materials such as phosphate and nitrate will be recommended for the application of VUV techniques for PFOS removal. In this research, organic solvent abstraction was inferred to be one of candidates for PFOS separation.

Methylene blue removal by carbonized textile sludge-based adsorbent.

Colored effluent and a large amount of sludge are major pollutant sources derived from textile industry activity. In this research, the idea for converting textile sludge into a potential adsorbent was conducted through a carbonization process in order to solve the colored effluent problem. Textile sludge was carbonized at a temperature ranging from 400 to 800 °C in the absence of oxygen. Maximum adsorption capacity of carbonized sludge for methylene blue removal reached 60.30 mg/g when the sludge was carbonized at 600 °C with specific surface area of 138.9 m2/g and no significant alteration was observed until 800 °C. Experimental research by using a real wastewater also showed that there was almost no disruption during adsorption of methylene blue into surface of carbonized sludge. While reactivation process revealed that the regeneration of carbonized sludge was applicable by secondary heating at the same carbonization temperature. Furthermore, the application of this research demonstrated that the carbonized textile sludge was a good adsorbent for methylene blue removal and had a capability to be reactivated.

Effects of three additives on the removal of perfluorooctane sulfonate (PFOS) by coagulation using ferric chloride or aluminum sulfate.

Perfluorooctanesulfonic acid and its salts (PFOS) are emerging contaminants with long half-lives in water and human bodies. Accordingly, PFOS removal from water streams is required for controlling the PFOS pollution. To provide a simple PFOS separation technology, effects of three additives, powdered activated carbon (PAC), gelatin, and cetyltrimethylammonium bromide (CTAB), on the PFOS removal by coagulation with ferric chloride or aluminum sulfate were investigated in this study. As a result, coagulation with PAC or CTAB addition was effective in the PFOS removal, though the conventional coagulation and coagulation with gelatin addition were ineffective. A PFOS removal efficiency of over 90% was observed for the CTAB dose of over 1.6 µM (0.58 mg/L) and for the PAC dose of over 40 mg/L, and that of over 95% was achieved by the CTAB dose of over 2.4 µM (0.87 mg/L), when the initial PFOS concentration was 1.84 µM. The positive effect of CTAB would be caused by micelle formation, which was enhanced by both the association of hydrophobic tails and the electrostatic attraction of hydrophilic heads of PFOS and CTAB. Thus, a linear cationic surfactant of CTAB was concluded to be an effective additive for the PFOS removal by coagulation.

Effects of environmental factors on microalgal biomass production in wastewater using cyanobacteria Aphanothece clathrata and Microcystis wesenbergii.

Effects of temperature, light, coexistent microbes, and dissolved matter on the growth of cyanobacteria Aphanothece clathrata and Microcystis wesenbergii were discussed using a batch incubation system in this research for microbial biomass production in wastewater. As a result, water temperature in the effluents from the municipal wastewater treatment plants in Tokyo was suitable for the growth of these cyanobacteria, though preheating of wastewater may be required for M. wesenbergii in winter. The dissolved matter in the treated wastewater did not affect the growth of A. clathrata and enhanced the growth of M. wesenbergii. However, the microbes in the treated wastewater attacked the cyanobacteria and inhibited their growth. Therefore, pretreatment of microbes might be required for biomass production in the treated wastewater. The maximum methane production potentials of A. clathrata and M. wesenbergii in the treated wastewater were estimated to be 13.5 and 2.12 L-CH4•m-2•day-1, respectively, when the depth of the bioreactor was set at 4.8 m for A. clathrata and 0.7 m for M. wesenbergii. The potential of A. clathrata was higher than that of grasses. Consequently, cyanobacteria, especially A. clathrata, may become a good biomass for bioenergy production.

Applicability of an electrochemical Fenton-type process to actual wastewater treatment.

The applicability of an electrochemical Fenton-type process (EF-HOCl-ReFe) to the treatment of three actual wastewaters, namely wastewater from an automobile factory (automobile wastewater), metal scrap-cleansing wastewater, and municipal wastewater, is discussed in this research. The EF-HOCl-ReFe successfully removed the chemical oxygen demand (COD) from automobile wastewater pre-treated by a coagulation process without any inhibition. The apparent current efficiency reached 86%, 46% of which was ascribed to the electrochemical Fenton-type mechanism. The metal scrap-cleansing wastewater had a yellow colour and high concentrations of COD (6550 mg/L) and Cl(-) (1560 mM). The EF-HOCl-ReFe could achieve almost complete COD removal and decolourization after 48 h of treatment, although a temporary intensification of colour was observed before the decolourization. The EF-HOCl-ReFe was also effective in the removal of 1,4-dioxane from municipal wastewater pre-treated by activated sludge and coagulation processes, which were unable to remove 1,4-dioxane. The 1,4-dioxane removal efficiency after 30 min of treatment reached 68.5%. Thus, the EF-HOCl-ReFe was applicable to the treatment of these actual wastewaters.

Effects of waste glass additions on quality of textile sludge-based bricks.

This research investigated the utilization of textile sludge as a substitute for clay in brick production. The addition of textile sludge to a brick specimen enhanced its pores, thus reducing the quality of the product. However, the addition of waste glass to brick production materials improved the quality of the brick in terms of both compressive strength and water absorption. Maximum compressive strength was observed with the following composition of waste materials: 30% textile sludge, 60% clay and 10% waste glass. The melting of waste glass clogged up pores on the brick, which improved water absorption performance and compressive strength. Moreover, a leaching test on a sludge-based brick to which 10% waste glass did not detect significant heavy metal compounds in leachates, with the product being in conformance with standard regulations. The recycling of textile sludge for brick production, when combined with waste glass additions, may thus be promising in terms of both product quality and environmental aspects.

Effect of pH and molar ratio of pollutant to oxidant on a photochemical advanced oxidation process using hypochlorite.

Ultra violet (UV)-photolysis of hydrogen peroxide (H2O2) is a conventional advanced oxidation process (AOP) and is advantageous in its simplicity, although H2O2 is costly. Accordingly, we tried to substitute H2O2 by hypochlorite in the photochemical AOP, and discussed the effect of pH and the molar ratio of a pollutant to hypochlorite on the process using 1,4-dioxane as a model pollutant. The photochemical treatment of hypochlorite solutions at a wavelength of 254 nm under various pH values revealed that the UV-photolysis of hypochlorous acid (HOCl) species mainly contributed to hydroxyl radical (•OH) production. The reaction efficiency, as defined by the molar ratio of removed 1,4-dioxane to consumed hypochlorite, deteriorated under higher pH levels due to the stronger radical scavenging effect of hypochlorite ion (ClO(-)) as compared to that of HOCl. The optimal pH for the UV-photolysis of hypochlorite as an AOP was found to be in the range of 3-6. The reaction efficiency at a high molar ratio of initial 1,4-dioxane to initial hypochlorite exceeded 100%, which was caused by the regeneration of HOCl from photochemically generated chlorine radicals (•Cl). Finally, the overall reaction of the UV-photolysis of HOCl was proposed on the basis of the radical reactions that were related to chlorine species, which suggested that the UV-photolysis of 1 mmol of HOCl stoichiometrically produced 2 mmol of •OH. Since the use of liquid chlorine is more economical than that of H2O2, the substitution of HOCl for H2O2 in the photochemical AOP was concluded to be feasible from the viewpoints of both stoichiometry and chemical costs.

Roughness and temperature effects on the filter media of a trickling filter for nitrification.

The performance of trickling filters using two types of plastic media with the same material, the same shape and different roughness was evaluated during a temperature-decreasing period to understand the roughness and temperature effects on the filter media. Real restaurant wastewater was used for the experiments. The chemical oxygen demand (COD) removal and nitrification performance of plastic media with a rough surface (LT-15) was superior to that with a smooth surface (KT-15). Because the biomass of microorganisms attached on the LT-15 was twice that attached on the KT-15, the larger biomass attached on the LT-15 was thought to be responsible for the higher performance. During the operation, the COD loading and water temperature varied in the range from 0.37 to 1.9 kg m(-3) d(-1) and 17.0--10.0 degrees C, respectively. However, the COD removal performance was not dependent on the COD loading or water temperature. On the contrary, the COD loading and the water temperature influenced the nitrification performance. Although a nitrification efficiency of 100% was recorded at a COD loading of 0.37 kg m(-3) d(-1), it deteriorated to 17-28% at higher COD loading. Moreover, a decline in the water temperature decreased the nitrification performance. The temperature-activity coefficient for nitrification was estimated to be 1.096. Based on this value, it was inferred that the COD loading should be set at less than 0.20 kg m(-3) d(-1) for the complete nitrification of the restaurant wastewater in winter, when the water temperature usually drops to around 10 degrees C.

Reusability of iron sludge as an iron source for the electrochemical Fenton-type process using Fe²+/HOCl system.

This paper reports on the reusability and the optimal reuse of iron-rich sludge for an electrochemical Fenton-type process using sequencing batch mode and separation batch mode reuse models. In the sequencing batch mode, processes of electrochemical treatment, neutralization, sedimentation, and re-dissolution of iron sludge were all performed in the same reactor, whereas the neutralization and sedimentation of iron sludge in the separation batch mode was carried out in an iron recovery tank separated from the electrochemical reactor. The effects of iron speciation at different pH levels were discussed. It was found that ferric ions at a pH ≤2.5 were suitable for this electrochemical Fenton-type process where ferrous ions acted as hydroxyl radical scavengers, generating brownish deposits on the cathode at pH 3. When the sequencing batch mode was applied, the current efficiency in the Fenton-type process after the iron recovery declined due to the formation of insoluble deposits on the electrodes, which decreased at lower pH. The deposits were mainly formed during the neutralization and sedimentation steps after the electrochemical process. Fortunately, iron from the sludge could be reused for the electrochemical process when the separation batch mode was used, with 100% iron recovery and no decline in current efficiency.

Bromate ion removal by electrochemical reduction using an activated carbon felt electrode.

The electrochemical removal of bromate ion (BrO3-) was investigated using a two-compartment electrolytic flow cell with activated carbon felt electrodes. Bromate ion removal and corresponding Br- increase was observed during electrochemical treatment whereas the activated carbon felt used possessed no catalytic effect on BrO3- reduction. The BrO3-reduction rate was accelerated at lower pH, which also improved current efficiency. Transition of chemical equilibrium of the BrO3- reductive reaction was theorized as the reason for pH dependency of the BrO3- reduction.The electrochemicaltreatment of BrO3- -contaminated tap water resulted in a rapid decrease in BrO3- concentration from 100 to 48 microg/L with a contact time of 9.2 s. Thus, electrochemical treatment allowed the rapid removal of BrO3-. However, competitive hydrogen evolution at the cathodes reduced current efficiency of BrO3- reduction. Standard potentials of corresponding anodic and cathodic reactions suggested that electrolysis at a terminal voltage less than 1.229 V would promote BrO3- reduction without hydrogen evolution. However, the activated carbon felt electrode did not function well at a terminal voltage of 1.0 V. Accordingly, the development of an electrode material with high catalytic activity will be required to improve current efficiency.

Ozonation combined with electrolysis of 1,4-dioxane using a two-compartment electrolytic flow cell with solid electrolyte.

Ozonation combined with electrolysis (ozone-electrolysis) is a new advanced oxidation process for water treatment. The advantages of ozone-electrolysis are (1) that reagents such as hydrogen peroxide or ferrous salts are unnecessary, (2) there is less influence from chromaticity, and (3) electric power is only required for operation. However, electrolysis has a serious limitation, in that it requires electrical conductivity (EC). This research is aimed at developing an ozone-electrolysis reactor that is applicable to wastewater with low EC using a cation exchange membrane as solid electrolyte. Moreover, experimental evidence of hydroxyl radical (.OH) generation via the cathodic reduction of ozone was obtained. Competitive kinetics analysis, based on the experimental data from the ozone-electrolysis of a mixed solution of 1,4-dioxane and tert-butyl alcohol, revealed that .OH contributed to 1,4-dioxane degradation. The ozone-electrolysis reactor was successfully applicable to degradation of 1,4-dioxane in both 1,4-dioxane solution (EC: less than 0.30 microS/cm) and a landfill leachate treated by a low-pressure reverse osmosis membrane (EC: 0.06 mS/cm). The use of a solid electrolyte was also very effective in reducing the electric power required for electrolysis.

Characteristics of electrolysis, ozonation, and their combination process on treatment of municipal wastewater.

The characteristics of municipal wastewater treatment by electrolysis, ozonation, and combination processes of electrolysis and aeration using three gaseous species (nitrogen [N2], oxygen [O2], and ozone [O3]) were discussed in this research using ruthenium oxide (RuO2)-coated titanium anodes and stainless-steel (SUS304) cathodes. Electrolysis and electrolysis with nitrogen aeration were characterized by a rapid decrease in 5-day biochemical oxygen demand (BODs) and total nitrogen and a slow decrease in chemical oxygen demand (COD). In contrast, ozonation, electrolysis with oxygen aeration, and electrolysis with ozone aeration were characterized by transformation of persistent organic matter to biodegradable matter and preservation of total nitrogen. The best energy efficiency in removing BOD5 and total nitrogen was demonstrated by electrolysis, as a result of direct anodic oxidation and indirect oxidation with free chlorine produced from the chloride ion (Cl-) at the anodes. However, electrolysis with ozone aeration was found to be superior to the other processes, in terms of its energy efficiency in removing COD and its ability to remove COD completely, as a result of hydroxyl radical (*OH) production via cathodic reduction of ozone.

Advanced oxidation effect of ozonation combined with electrolysis.

The advanced oxidation effect of ozonation combined with electrolysis (electrolysis-ozonation) was discussed through the treatment of 4-chlorobenzoic acid (4-CBA) as a hydroxyl radical probe. The mechanism of hydroxyl radical production by electrolysis-ozonation process was also estimated with a mathematical model. The experimental results revealed that the electrolysis-ozonation process had a synergistic effect on the degradation of 4-CBA. The advanced oxidation effect of electrolysis-ozonation was inferred from standard potentials of relevant electrochemical reactions and mathematical model analysis to be mainly attributed to ()O(3)(-) promotion of O(3) at the cathodes. An increase in electric current improved the degradation rate of 4-CBA. However, the pseudo-first order degradation rate constant reached a plateau at high electric current densities, as ()O(3)(-) promotion of O(3) at the cathodes was regulated by O(3) transport process from the bulk to the cathodes in the range exceeding an electric current density of 10 Am(-2). Accordingly, the balance of O(3) transport flux and electric current is important for the efficient operation of the electrolysis-ozonation reactor.

Advanced treatment of sewage by pre-coagulation and biological filtration process.

A pre-coagulation and bio-filtration process for advanced treatment of sewage was developed and experimentally discussed with a pilot plant. The bio-filtration unit consists of a denitrification filter, a nitrification filter with side stream to the denitrification filter, and a polishing filter with anoxic and aerobic parts. Concentrations of SS, T-COD(Cr), T-carbonaceous BOD, T-N and T-P in the effluent were stably kept at less than 3, 20, 5mg/L, 2mg N/L and 0.2mg P/L, respectively, and transparency at higher than 100 cm, under total hydraulic retention time of 3.2h in the bio-filtration parts (filter-bed). ORP in an anoxic tank before a nitrification tank should be at a low level of less than -120 mV to keep remaining NO(-)(x) - N less than 1mg N/L, but must be maintained at a level higher than -150 mV. The maximum nitrogen-loading rate under a water temperature of 18 degrees C should be less than 0.25 kg N/(m(3)-filter-bed.d). Concentrations of microorganisms kept in the reactors were as high as 4000-5000 mg COD/L-filter-bed. Denitrification activity of 0.4 or 0.7 kg N/(m(3)-filter-bed.d), and nitrification activity of 0.3 kg N/(m(3)-filter-bed.d) were obtained, respectively, under a water temperature of about 18 degrees C. Backwashing in each tank as well as methanol addition and aeration in the polishing filter were operated successfully by the automatic control systems. These results proved that this process is applicable to advanced treatment of sewage with easy maintenance.