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Modified platinum electrodes: Electrochemical characteristics and behaviour in activated sludge
Abstract
Polished platinum electrodes were coated with platinum black and/or with organic polymers (Nafion, polyurethane) to accelerate the electron tranfer rate to the metal-solution interface and/or to constitute a protection barrier for the metal against neutral or anionic contaminants present in wastewater. The modified electrodes were electrochemically characterized (surface roughness and exchange current density) and their behaviour in activated sludge was studied. A platinum black deposit increases the roughness and the exchange current density. However, the response of these electrodes in an anoxic environment is very slow, which results in a low variation amplitude of the potential when they are placed in activated sludge alternately submitted to sequences of aeration and anoxia. A Nafion film provides partial protection against contaminants thus increasing electrode response reproducibility. A polyurethane film has no apparent protective effect. Copyright (C) 1996 IAWQ. Published by Elsevier Science Ltd.
... These reveal the end of the nitrification and denitrification processes (Plisson-Saune et al., 1996;Battistoni et al., 2003a, b). In particular, the ammonia disappearing during the aerobic cycle, identified by a flex-point in the DO profile with time, at the end of ammonia nitrification, and the nitrates disappearing (breakpoint) in the anoxic cycle, identified by a flex-point in the ORP profile with time, at the end of nitrate denitrification process, are the main evident observations (Craig et al., 1990;Heduit et al., 1996). ...
The feasibility of phosphate removal from the supernatant of anaerobically digested sludge by struvite (MAP, MgNH4PO4) crystallization in a fluidized-bed reactor (FBR) was studied. Quartz sand was used as seed material. Three successive steps were investigated to highlight the process: 1, natural aging of the supernatant, with phosphate concentrations in the range 18–164 mg/l, which gave good crystallization of struvite within 3 days; 2, the inhibiting effect of Mg and bicarbonate ions on hydroxyapatite (HAP, Ca5OH(PO4)3) formation, which leads to struvite formation under supersaturation conditions; and 3, phosphate removal via the process of complete crystallization on the seed material, and which is obtained by means of external continuous aeration. All runs were performed without the addition of chemicals.
... These reveal the end of the nitrification and denitrification processes [9] [10] [11] [12] [13]. In particular, the ammonia disappearing during the aerobic cycle, identified by a flex-point in the DO profile with time, at the end of ammonia nitrification, and the nitrates disappearing (breakpoint) in the anoxic cycle, identified by a flex-point in the ORP profile with time, at the end of nitrate denitrification process, are the main evident observations [14] [15]. ...
The necessity to produce treated wastewaters with high quality standards for both discharge or reusing, implies the adoption of very effective processes in the field of biological wastewater treatment. The Membrane Biological Reactor (MBR) has showed the capacity to remove COD, BOD, N, P and suspended solids as well as heavy metals and organic micropollutants so to obtain reusable water. In this study the automatically controlled alternate cycles process have been applied to a MBR, the process allows for the reuse purpose with a remarkable reduction of the operational costs. This paper regards the treatment of a real low loaded domestic wastewater (TSS = 136 mg/l, COD = 113 mg/l; TKN = 22 mg/l; Ptot = 2.4 mg/l) with low attitude to biological treatment. Despite the lack of necessary readily biodegradable organic carbon (RBCOD), the nitrogen removal was good (Edenitrification = 67%) thanks to the flexibility of the alternate cycles process to the low loading fluctuation. Moreover, the perfect retention capacity of the ultrafiltration membranes permits an high removal of heavy metals and polycyclic aromatic hydrocarbon (PAH). The effluent quality was suitable to reuse purpose according to the Italian law.
... The first condition, the ammonia breakpoint, is also observable in the ORP versus time profile, normally in the range 60 – 150 mV. The second condition, the nitrates breakpoint, usually happens in the range –40 – –60 mV (Wareham et al., 1993; Heduit et al., 1996). The last range is higher than the one observed in pre-denitrification/nitrification processes but, according to Deguin (1992) it needs to assure a good performance in nitrates reduction. ...
A simple mathematical model of an alternate oxic-anoxic process has been elaborated. It enables us to optimise the cycle time on the basis of maximum nitrates concentration in the effluent and the desired nitrogen removal performance. At the same time the model can be employed to verify the impact of the variations of flow rate and influent characteristics as well as the operational parameters of the process. Actually, the model confirms the process efficiency but its feasibility in real plants needs a local or remote process control. To verify these theoretical conclusions a real wastewater plant (700 PE) has been upgraded in an alternate oxic-anoxic process. It was implemented with software able to elaborate the data of dissolved oxygen concentration and oxidation reduction potential. Moreover, the evaluation of the flexing points was performed to manage mixer and blowers. A one-year experience of plant management allowed us to obtain very high nitrogen removal. However, the performances were different during wet or dry weather periods. The statistical analysis of probe signals evaluation confirmed the capability of the control device to detect the flexing points during the anoxic phase (70-94%). On the other hand, the capability of detecting the DO signal was lower, in particular when the oxygen demand was similar to the amount of supplied oxygen. The hourly variations of flow rate and mass loading determines different conditions for starting the anoxic phase: over aeration, over loading and the equivalence of oxygen demand and supply, are the main factors determining the blowers stopping.
A literature review is presented on modeling, instrumentation and measurements, automation, and optimization of wastewater treatment facilities. Papers on modeling discuss topics like biomass growth under enriched culture substrate-unlimited conditions; reduced activated sludge process; and primary and co-metabolic biodegradation, water surface and air bubble volatization, and adsorption; while those on instrumentation and measurements propose instruments such as bacterial biosensors and mass spectrometers and methods to measure biodegradable organic carbon, biochemical oxygen demand, wastewater fatty acids, and other relevant quantities.
During wastewater transportation, chemical and biological transformations may occur. Aerobic, anoxic, and anaerobic conditions exist depending on the presence or absence of dissolved oxygen and oxidized nitrogen compounds. The occurrence of anaerobic conditions has a negative effect
on wastewater transportation because of the usual subsequent sulfide generation process. In Tenerife, Spain, reclaimed wastewater used for agricultural purposes is transported via a completely filled gravity pipe, 0.6 m in diameter and 61 km long. Because of the long residence time (approximately
30 to 40 hours), anaerobic conditions typically occur after 5 to 10 hours of flow. Field studies during transportation were conducted from November 1994 to July 1997. This paper describes a study of the evolution of septic conditions (aerobic, anoxic, and anaerobic) and the relationship between
septicity and oxidation–reduction potential (ORP), which can predict the occurrence of anaerobic conditions.
A patented automatic control device was applied to management of an alternate oxic−anoxic process in a small wastewater treatment plant (700 PE). The control system enabled the optimal time−length of the aerobic and anoxic phases to be determined by analyzing the dissolved oxygen and the oxidation−reduction potential data. Moreover, also a time set point was introduced to establish the maximum length for the two phases. Results showed high performances in biological nitrogen removal (0.7−5.2 mg of NO3−N L-1 in the effluent) and a reliable control of the treatment process also during wet weather events. In comparison with extended aeration plants of similar size, lower energetic consumption was observed, generally <200 Wh PE-1 day-1. The automatic control device was a reliable system that gave a good performance in a small wastewater treatment plant with low investment and managing costs.
This paper describes the demonstrative scale application of a membrane biological reactor (MBR) for low loaded domestic wastewater with low attitude to biological treatment (carbon/nitrogen approximately 5). The biological process was managed by the automatically controlled alternate cycles allowing for re-use purposes with a remarkable reduction of the operational costs. The global process evaluation revealed the system capability of obtaining high nitrogen removal (effective nitrogen removal of 69%) thanks to its high flexibility related to the hourly loading fluctuation. Moreover, high removal of heavy metals and polycyclic aromatic hydrocarbons (PAH) was obtained due to the perfect retention capability of the membranes. In-depth studies were conducted to determine the process behaviour for activated sludge over aeration and with addition of exogenous carbon. Limitation of sludge over aeration and energy savings were observed with a gradient air supplying method. The addition of exogenous carbon (acetic acid up to carbon/nitrogen approximately 9) led to complete nitrogen removal (Ed = 96%) and permitted biological phosphorus uptake. In conclusion, it was been found that the coupled process alternated cycles-MBR had the capacity to remove COD, BOD, N, P and suspended solids, as well as heavy metals and organic micropollutants, resulting in high quality effluent suitable for re-use purposes.
The zero current potential of a platinum electrode in a biological medium (wastewater, activated sludge) is strongly dependent on the surface characteristics of the metal. It is also influenced by pH (probably Pt/PtO system), dissolved oxygen (O2/OH- system), and ionic forms of nitrogen (NO2-/NH4+ and NO3-/NO2-systems).
The experimental values of the coefficients relating the stabilized potential of a platinum electrode to the logarithm of the concentration of the elements under consideration (Nernst equations) are significantly different from the thermodynamic coefficients corresponding to each reaction. The platinum is thus not in equilibrium with the dissolved redox reactants and is likely subject to mixed potentials in which the adsorbed components play an important role.
The importance of dissolved oxygen level in determining the platinum electrode potential in activated sludge has been clearly demonstrated by current-potential curves plotted at different oxygen concentrations.
Tests have been carried out, in the laboratory and in full scale treatment plants,to define the relationship between the platinum electrode potential at equilibrium (Eh) and the dissolved oxygen [O2] concentration in the activated sludge. These two parameters obey a law of the form Eh = a + b log [O2]. The measured values of coefficents a and b differ widely to those found from the oxygen reduction reaction in water (a = 0.8 V at pH 7 and b = 15 mV per decade).
Factors a and b mainly depend on the sludge loading, the aeration conditions and the sludge concentration. Using non–polished stationary platinum ring electrodes, the following values of a. were obtained (at pH between 7 and 7.6) : + 410 mV/NHE for sludge aerated for several hours without feeding, + 265 mV/NHE for over-aerated/low-loaded sludge (Cm = 0.2 kg BOD.kg MLVSS−1. day−1) and + 180 mV/NHE for high-loaded activated sludge in plug-flow system (Cm = 1 kg BOD.kg MLVSS−1.day −1). Factor b would seem to lie between 55 and 65 mV when the sludge is continuously aerated without feeding.At low loads with excess aeration, it lies between 70 and 90 mV. When the medium is slightly septic at low dissolved oxygen concentrations (insufficient daily aeration time, high sludge concentration or aerators shut down for too long periods), factor b increases and can reach 200 mV. In the same way, at high loads, factor b can become 150 mV.
These results demonstrate the importance of dissolved oxygen concentration in the mechanisms which determine the metal electrode potentials in activated sludge. They also illustrate the role that other electroactive species play in the process. The type and concentration of these species depend on parameters such as the sludge loading, the overall oxygen supply, the aeration sequence and the sludge concentration.
This study was carried out at full scale on the St Mars la Jaille activated sludge system (Western France), which is one of the first successful biological excess phosphorus removal systems in Europe. The aim of the study was to show how the parameter oxidation reduction potential (ORP) can be used to optimise biological nitrogen and phosphorus removal in a two-series reactor system, by controlling air supply to the second aeration tank (for nitrification and denitrification) and monitoring anaerobiosis in the first non-aerated tank. It would appear that setting ORP regulation limits in the aeration tank is of utmost importance in control of the system. High levels of carbon, nitrogen and phosphorus removal were obtained for lower and upper ORP limits in the aeration tank of +200 and +400 mV respectively and +300 and +500 mV respectively (COD and SS removal > 95%; TKN removal > 90% and P removal > 80%). Furthermore, ORP recordings enabled the aerobic, anoxic and anaerobic phases of a cycle in the aeration tank, and the degree of anaerobiosis in the non-aerated tank, to be monitored.
Hydrogen chemisorption from dilute acidic solution onto Pt single crystal surfaces was examined using an electrochemical cell directly coupled to LEED/Auger analytical system. No pre-anodization was used prior to observing hydrogen adsorption by cyclic voltammetry so that clean surfaces having the ordered structures indicated by LEED were studied. The problem of contributions from non-ordered parts of the electrode like support wires and edges was solved by using a gold evaporation masking technique. The specific contribution of atomic imperfections to the voltammetry curve was deduced from the ordered and countable imperfections occurring on high Miller index single crystal surfaces that have a stepped structure. The H-Pt bond energy Has found to be structure sensitive, and sensitive both to local site geometry and long range order in the surface. The bond strength was found to vary systematically: n(111)x(100) > (100) > n(111)x(111) > (110) > (111). Distinct states for hydrogen at steps versus hydrogen on terraces could be distinguished. The (110) surface is shown to be a (111) vicinal, probably the [3(111) x 2(111)] microfacetted surface. The zero coverage heat of adsorption on the well-ordered (111) surface (48 kJ/mol) in solutions is the same as the value reported by Ertl and co-workers for adsorption on a (111) surface in vacuum. Adsorption Isotherms for hydrogen on the (111) and (100) surfaces is adequately fit by the classical model for immobile adsorption at single sites with nearest neighbor repulsive interaction.
The importance of dissolved oxygen level in determining the platinum electrode potential in activated sludge has been clearly demonstrated by current-potential curves plotted at different oxygen concentrations. Tests have been carried out, in the laboratory and in full scale treatment plants, to define the relationship between the platinum electrode potential at equilibrium (Eh) and the dissolved oxygen (O2) concentration in the activated sludge. These two parameters obey a law of the form Eh = a + b log (O2). The measured values of coefficients a and b differ widely to those found from the oxygen reduction reaction in water (a = 0.8 V at pH 7 and b = 15 mV per decade). Factors a and b mainly depend on the sludge loading, the aeration conditions and the sludge concentration. Using non-polished stationary platinum ring electrodes, the following values of a were obtained (at pH between 7 and 7.6): +410 mV/NHE for sludge aerated for several hours without feeding, +265 mV/NHE for over-aerated/low-loaded sludge (Cm = 0.2 kg BOD.kg MLVSS-1.day-1) and ) and +180 mV/NHE for high-loaded activated sludge in plug-flow system (Cm = 1 kg BOD.kg MLVSS-1 . day-1). Factor b would seem to lie between 55 and 65 mV when the sludge is continuously aerated without feeding. At low loads with excess aeration, it lies between 70 and 90 mV. When the medium is slightly septic at low dissolved oxygen concentrations (insufficient daily aeration time, high sludge concentration or aerators shut down for too long periods), factor b increases and can reach 200 mV. In the same way, at high loads, factor b can become 150 mV. These results demonstrate the importance of dissolved oxygen concentration in the mechanisms which determine the metal electrode potentials in activated sludge. They also illustrate the role that other electroactive species play in the process. The type and concentration of these species depend on parameters such as the sludge loading, the overall oxygen supply, of aeration sequence and the sludge concentration. Tests have been carried out, in the laboratory and in full scale treatment plants, to define the relationship between the platinum electrode potential at equilibrium (Eh) and the dissolved oxygen [O2] concentration in the activated sludge. These two parameters obey a law of the form Eh = a + b log [O2]. The measured values of coefficients a and b differ widely to those found from the oxygen reduction reaction in water (a = 0.8 V at pH 7 and b = 15 mV per decade). Factors a and b mainly depend on the sludge loading, the aeration conditions and the sludge concentration. Using non-polished stationary platinum ring electrodes, the values of a were obtained and are reported in the paper. The results demonstrate the importance of dissolved oxygen concentration in the mechanisms which determine the metal electrode potentials in activated sludge. They also illustrate the role that other electroactive species play in the process. The type and concentration of these species depend on parameters such as the sludge loading, the overall oxygen supply, the aeration sequence and the sludge concentration.
Gold and platinum were compared to ascertain how they expressed a stabilized potential in activated sludge. The comparison was based on electrochemical determination of the electron transfer rate (i.e. equilibrium exchange current density) and recording of potentials against time.
When both metals are treated in the same way, platinum gives equilibrium exchange current densities approx. 10 times higher than gold, both in aerated activated sludge and in treated water. For platinum, the equilibrium exchange current densities range from 0.1 to 0.25 µA/cm2 immediately after polishing and decrease during prolonged contact with activated sludge subjected to alternating aeration/anoxia sequences.
The lower kinetics of electron transfer on gold go together with significant differences in response:- In an aerobic medium a gold electrode potential is lower than that of a platinum electrode. In a strongly anaerobic medium, the reverse is true. Consequently, the amplitude of the potential variation between aerobic and anaerobic media is smaller for gold than for platinum. Under our experimental conditions this amplitude was approx 350 mV for gold and 850 mV for platinum.- The slopes of the linear relationships between potential and pH or potential and the logarithm of the dissolved oxygen concentration are two or three times greater for platinum than for gold.
Although the values obtained with platinum electrodes cannot represent a veritable equilibrium state, the platinum electrode zero-current potential would seem to be far more sensitive to variations in the medium than that of the gold electrode; it is, therefore, more suitable for use in activated sludge.
The adsorption of hydrogen on platinum electrodes is a very sensitive probe for the surface structures and their change after electrochemical adsorption-desorption of oxygen alone or combined with hydrogen which generally induces a redistribution of the hydrogen adsorption states. In many cases when the transformation takes place the redistribution of the states occurs with a constant total electrical charge.As the voltammetry of adsorbed hydrogen on platinum surfaces is similar to spectroscopy of the adspecies, a quantitative numerical treatment of the voltammograms has been developed in order to characterize in situ the various crystallographic surface structures by using the detailed distribution of the adsorption states at these surfaces. The multiplicity of peaks on the voltammograms has been ascribed to the presence of several hydrogen adsorption states, where the term “state” has a general significance linked to the intrinsic and induced heterogeneity of the surface.For the quantitative description of this adsorption equilibrium, as in the gas phase on solids, an adsorption isotherm has to be chosen. Because the peaks are symmetrical with various half-height widths for a one-electron transfer process, the Frumkin-Fowler isotherm has been selected which takes into account the interactions between adsorbed species.Three or four adsorption states have been obtained in the deconvolution of the voltammograms of Pt (100), depending on the surface order as a result of pretreatment applied to the surface. Significant relations between the proportions of hydrogen relative to each state have been found.The results of the calculation show that all the states in sulphuric medium are characterized by an attractive interaction constant.
This work documents the results from two lab-scale reactors digesting waste sludge in an aerobic-anoxic fashion. The control reactor “fixed” the length of the aerobic-anoxic cycle at 6 h, with each segment of the cycle set at 3 h each. The experimental reactor used ORP to control the total length of the cycle based upon the distinctive “nitrate breakpoint” occurring in the ORP-time profile. The reactors were evaluated on the basis of removals for TSS, VSS, nitrogen and phosphorus. In addition, the control stability achieved when the reactors were subjected to spikes of specific chemicals was monitored. The ORP-regulated reactor better accommodated the disturbances, as well as appeared to show an increase in the removal of nitrogen and solids.
A new miniaturized glucose oxidase based needle-type glucose microsensor has been developed for subcutaneous glucose monitoring. The sensor is equivalent in shape and size to a 26-guage needle (0.45-mm o.d.) and can be implanted with ease without any incision. The novel configuration greatly facilitates the deposition of enzyme and polymer films so that sensors with characteristics suitable for in vivo use (upper limit of linear range greater than 15 mM, response time less than 5 min, and sensitivity yielding a 5:1 signal-to-background ratio at normal basal glucose levels) can be prepared in high yield (greater than 60%). The sensor response is largely independent of oxygen tension in the normal physiological range. It also exhibits good selectivity against common interferences except for the exogenous drug acetaminophen.
The perfluorosulfonic acid polymer Nafion leads to highly reproducible electrode preparation and performance when used as a dialysis membrane material on glucose oxidase/platinum electrodes in the determination of glucose in whole blood. Protection from enzyme electrode fouling in whole blood was demonstrated for a 1.7 μm thick perfluorosulfonic acid polymer overcoat by 6 days of continuous in vitro measurement in fresh whole blood samples at 37°C, during which time the glucose calibration slope decreased by about 6%. Coating with the perfluorosulfonic acid polymer yields enzyme electrodes wih a linear response to glucose up to at least 28 mM and response times of 5-17 s, superior to those for electrodes coated with cellulose dialysis membranes.
Electrochemical methods John Wiley and Sons Design and in virro studies of a needle type glucose sensor for subcutaneous monitoring
- A J Bard
- L R Faulkner
Bard, A. J. and Faulkner, L. R. (Ed.) (1980). Electrochemical methods. John Wiley and Sons. Bindra, D. S., Zhang, Y., Wilson, G. S., Sternberg, R., Thevenot, D. R., Moatti, D. and Reach, G (1991). Design and in virro studies of a needle type glucose sensor for subcutaneous monitoring. Anal. Chem. 63, 1692-1696.