Electrochemical sulfide oxidation from domestic wastewater using mixed metal-coated titanium electrodes

Hydrogen sulfide generation is a major issue in sewer management. A novel method based on electrochemical sulfide oxidation was recently shown to be highly effective for sulfide removal from synthetic and real sewage. Here, we compare the performance of five different mixed metal oxide (MMO) coated titanium electrode materials for the electrochemical removal of sulfide from domestic wastewater. All electrode materials performed similarly in terms of sulfide removal, removing 78 ± 5%, 77 ± 1%, 85 ± 4%, 84 ± 1%, and 83 ± 2% at a current density of 10 mA/cm² using Ta/Ir, Ru/Ir, Pt/Ir, SnO₂ and PbO₂, respectively. Elevated chloride concentrations, often observed in coastal areas, did not entail any significant difference in performance. Independent of the electrode material used, sulfide oxidation by in situ generated oxygen was the predominant reaction mechanism. Passivation of the electrode surface by deposition of elemental sulfur did not occur. However, scaling was observed in the cathode compartment. This study shows that all the MMO coated titanium electrode materials studied are suitable anodic materials for sulfide removal from wastewater. Ta/Ir and Pt/Ir coated titanium electrodes seem the most suitable electrodes since they possess the lowest overpotential for oxygen evolution, are stable at low chloride concentration and are already used in full scale applications.     

Long-term field test of an electrochemical method for sulfide removal from sewage

Corrosion caused by hydrogen sulfide leads to significant costs for the rehabilitation or replacement of corroded sewer pipes. Conventional methods to prevent sewer corrosion normally involve the dosing of significant amounts of chemicals with the associated transport and storage costs as well as considerable maintenance and control requirement. Recently, a novel chemical free method for sulfide abatement based on electrochemical sulfide oxidation was shown to be highly effective for the removal of sulfide from synthetic and real sewage. Here, we report on the electrochemical removal of sulfide using Ta/Ir and Pt/Ir coated titanium electrodes under simulated sewer conditions during field trials. The results showed that sulfide can successfully be removed to levels below the normal target value at the end of a simulated rising main (i.e. <1 mg/L). A coulombic efficiency for dissolved oxygen generation of ∼60% was obtained and was independent of the current density. Scaling of the electrode and the membrane was observed in the cathode compartment and as a result the cell potentials increased over time. The cathode potentials returned to their original potential after switching the polarity every two days, but a more frequent switching would be needed to reduce the energy requirements of the system. Accelerated lifetime experiments indicated that a lifetime of 6.0 ± 1.9 years can be expected under polarity switching conditions at a pH of 14 and significantly longer at lower pH values. As operating the system without switching simplifies construction as well as operation, the choice whether to switch or not will in practice depend on operational cost (higher/lower energy) versus capital cost (reactor and peripherals). Irrespective of the approach, our study demonstrates that electrochemical sulfide control in sewer systems may be an attractive new option.     

Electrochemical sulfide removal from synthetic and real domestic wastewater at high current densities

Hydrogen sulfide generation is the key cause of sewer pipe corrosion, one of the major issues in water infrastructure. Current abatement strategies typically involve addition of various types of chemicals to the wastewater, which incurs large operational costs. The transport, storage and application of these chemicals also constitute occupational and safety hazards. In this study, we investigated high rate electrochemical oxidation of sulfide at Ir/Ta mixed metal oxide (MMO) coated titanium electrodes as a means to remove sulfide from wastewater. Both synthetic and real wastewaters were used in the experiments. Electrochemical sulfide oxidation by means of indirect oxidation with in-situ produced oxygen appeared to be the main reaction mechanism at Ir/Ta MMO coated titanium electrodes. The maximum obtained sulfide removal rate was 11.8 ± 1.7 g S m⁻² projected anode surface h⁻¹ using domestic wastewater at sulfide concentrations of ≥30 mg L⁻¹ or higher. The final products of the oxidation were sulfate, thiosulfate and elemental sulfur. Chloride and acetate concentrations did not entail differences in sulfide removal, nor were the latter two components affected by the electrochemical oxidation. Hence, the use of electrodes to generate oxygen in sewer systems may constitute a promising method for reagent-free removal of sulfide from wastewater.     

Nondeterministic endless control system for active polarizationcontrol

An active polarization-state control system is described for use in coherent fiber-optic communication systems. The active control system uses four piezostack fiber squeezers to enable control of the state of polarization (SOP) of the light over an unlimited range. The response time of the system is small enough to enable accurate tracking of changes in the SOP at a rate of up to 25 revolutions per second in the Poincare sphere representation. Unlimited control is achieved with a nondeterministic control algorithm. This makes the system insensitive to irregularities and unbounded instabilities of the control devices (e.g. drift). Experiments show that the SOP is accurately controlled by the system; the penalty loss due to residual mismatch in SOP is kept well below 1 dB even under dynamic circumstances     

In-situ caustic generation from sewage: The impact of caustic strength and sewage composition

Periodic caustic dosage is a commonly used method by the water industry to elevate pH levels and deactivate sewer biofilms responsible for hydrogen sulfide generation. Caustic (NaOH) can be generated in-situ from sewage using a divided electrochemical cell, which avoids the need for transport, handling and storage of concentrated caustic solutions. In this study, we investigated the impact of caustic strength in the cathode compartment and the impact of sodium concentration in sewage on the Coulombic efficiency (CE) for caustic generation. The CE was found to be independent of the caustic strength produced in the range of up to ∼3 wt%. Results showed that a caustic solution of ∼3 wt% could be produced directly from sewage at a CE of up to 75 ± 0.5%. The sodium concentration in sewage had a significant impact on the CE for caustic generation as well as on the energy requirements of the system, with a higher sodium concentration leading to a higher CE and lower energy consumption. The proton, calcium, magnesium and ammonium concentrations in sewage affected the CE for caustic generation, especially at low sodium concentrations. Economical assessment based on the experimental results indicated that sulfide control in sewers using electrochemically-generated caustic from sewage is an economically attractive strategy.     

Temperature control in very low birthweight infants during first five days of life

AIM: To determine ranges for skin temperatures in infants weighing under 1000 g in the first five days of life. METHOD: Abdominal skin and foot temperatures were automatically collected each second, averaged over 1 minute and stored on computer. A computer program analysed the data in 83 babies weighing under 1000 g at birth over the first five days of life and expressed the temperatures as means and standard deviation. The temperature patterns seen in these babies were also visually analysed. The relation between an increasing abdominal skin-foot temperature difference and other signs of hypovolaemia was also studied. RESULTS: These babies all had similar temperature patterns. Just after birth there was little ability to vasoconstrict in the presence of cold stress and the babies behaved more like poikilothermic animals. Vasomotor tone developed in the first three days, resulting in a stabilisation of the abdominal skin temperature to a mean of 36.9 degrees C and a widening of the central-peripheral temperature difference (Td) to a mean of 1.0 degree C. A Td of > 2 degrees C was associated with other evidence of hypovolaemia for only 11% of the time. CONCLUSIONS: Infants weighing under 1000 g have poor vasomotor control at birth and are at increased risk from cold stress. After the first two to three days of life, monitoring the central-peripheral temperature difference gives an early indication of cold stress.