Investigation of dry and wet deposition of air pollution in a rural area

The inorganic composition of dry and wet atmospheric depositions is reported from a rural sampling site near the city of Rome. Cellulose filter paper impregnated with an alkaline solution, and acetate cellulose pre-filters were used for the collection of SO₂ and suspended particulate matter, respectively. A rain-activated wet/dry precipitation collector was utilized.A seasonal trend of SO₂ was observed and analysis of particulate for H⁺, SO₄²⁻, NO₃⁻ and NH₄⁺ revealed that a significant amount of free strong acidity was present on particles. Fifty percent of precipitation pH values were below 5.6.On an annual basis the data show that dry acid deposition is more important for the area investigated.     

Estimation of sulphate ions in water and industrial wastes using a solid state membrane electrode

A solid membrane electrode selective to sulphate ions, has been prepared from hydrous thorium oxide gel with polystyrene as binder. Although the electrode is non-Nernstian in nature it can be utilized to estimate sulphate ions in the concentration range 10^?1–10^?4 M. The useful pH range for this assembly is 6–10. It can also be used in partially non-aqueous systems. The membrane electrode shows good selectivity to SP₄^?2 ions and there is practically no interference of a large number of anions and cations. It has been used to estimate SO₄^?2 ions in wastes from Paper and Pulp and Tannery industries.     

Determination of phosphate in wastewaters by ion-exclusion chromatography with flow coulometric detection

Ion-exclusion chromatography of PO₄³⁻ in wastewaters has been investigated on a cation exchange resin in the H⁺-form. Phosphate ion was chromatographed by ion-exclusion as H₃PO₄ formed by cation exchange, and monitored with a flow coulometric detector based on the electrochemical reaction of H⁺ ion from H₃PO₄ and p-benzoquinone at a constant applied potential (+0.45 V vs Ag-AgI) or a conductometric detector. A reasonable separation of PO₄³ from strong acid anions (Cl⁻ SO₄²⁻ and NO₃⁻) and CO₃²⁻ as coexisting anions could be accomplished by isocratic elution with 60% (v/v) acetone-water. The calibration graph for PO₄³⁻ with the flow coulometric detector was linear over the concentration range 1–150 ppm (slope = 0.982), but not with the conductometric detector. The agreement of the analytical results of PO₄³⁻ between the ion-exclusion chromatography with the flow coulometric detector and the colorimetry (molybdenum-blue method) was excellent for some industrial wastewater and domestic sewage samples.     

Études des transformations des formes du carbone, azote, phosphore, soufre et des principaux éléments minéraux au cours de la mesure de la demande totale en oxygene—III transformation des formes du soufre

Previous experiments carried out with the laboratory TOD meter Ionics 225 of the DOW Chèmical made it possible (after a high temperature catalytic action) to characterize the stable forms of organic and inorganic carbon and nitrogen (NH₄⁺, NO₂⁻, NO₃⁻), and the principal cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) in the course of the total oxygen demand (TOD) measurement.The object of this study is firstly to compare the oxidation capability of different techniques of organic pollution (particularly the COD and TOD) in relation to the constituent elements of the organic matter C, N, P, S, and to calculate the possible interferences of the inorganic compounds at the time of TOD test.These investigations warrant the application of this technique to measure the amount of organic pollution in relatively mineralized conditions (Industrial wastewater, sea-water…). The present publication is concerned more with the study of the transformation of the organic and inorganic sulphur forms (S²⁻, SO₃²⁻. SO₄²⁻) in the course of the TOD measurement.The study of the oxidizability of the organic sulphur compound type C_xH_yO_zS, made it possible to establish a specific relation with a ratio of 0–50 mg of organic sulphur l⁻¹, between the oxygen demand of this element [TOD (S)] and its concentration (TOD (S) = 0.97 [S]).These tests showed a partial oxidation of the sulphur to SO₂ and SO₃ as the literature claimed. On the other hand, the oxidation of the same compounds during the COD tests varies greatly and although it is not possible to establish a correlation between these two measurements, as applies in the case of organic nitrogen, nevertheless these experiments showed a greater reliability of the TOD compared with the COD in the oxidation of organic matter in general. We then carried out experiments on the different mineral forms of sulphur in order to distinguish the possible effects and to recommend simple improvements.A relative study on sulphate ions had been carried out with standard solutions which have the same TOD (the basic TOD is obtained using potassium phthalate acid) and the same increasing concentration of the salt M₂SO₄ type. The experiments showed that the basic TOD decreases when the concentration of sulphate ions is increased (Fig. 3). Therefore, the interference is negative and taking into consideration the specific oxygen demand of the cation, we can propose an evaluation of this interference (ΔTOD (SO₄²⁻) = 0.203 [SO₄²⁻]). The same experiments have been conducted with a salt of M₂SO₃ type and similar results obtained (Fig. 5).The specific interference of the sulphite ion is negative and can be estimated by the following equation (ΔTOD (SO₃²⁻) = 0.132 [SO₃²⁻]). In both cases, we have to note that the transformation of these inorganic anions occurs between those relative to the theoretical dissociation reaction corresponding to the appearance of the oxide SO₂ and SO₃. For sulphurous on the contrary, the interference is positive and therefore corresponds to an extra oxygen demand (Fig. 8).The experiments were conducted directly with the M₂S salts (M representing K or Na) in aqueous solution.The evaluation of this interference had been made in the consideration of two concentration ranges of the sulphurous ions (0–35 mg S²⁻ l⁻¹): TOD (S²⁻) = 0.4 [S²⁻] and (35–100 mg S²⁻ l⁻¹): TOD (S²⁻) = 1.2 [S²⁻] − 30.Therefore this study confirms a better oxidation of the organic matter by TOD test in comparison with COD test.But sulphate and sulfite have a negative interference in the TOD measurement, whereas sulphurous is positive.The evaluation model of these interferences allows a correction to be made of the TOD value or to verify TOD measurement of organic pollution obtained by this technique.     

Mobility of Cr(VI) anions in the phase of the amphoteric inorganic ion exchanger

The method of impedance spectroscopy has been used to investigate hydrogel and xerogel of hydrated zirconium dioxide containing Cr(VI) obtained from cation-and anion-substituted forms of an ion exchanger. It is shown that a complex composition of the amphoteric inorganic ion exchanger as well as protonization of its functional groups producing a substantial impact on the value of its electrical conductivity do not make it possible to assess mobility of sorbed ions. Nonetheless, the effective values of internal diffusion mobility of Cr(VI) may be obtained by the kinetic method: diffusion coefficients corresponding to the Cr(VI) → OH constitute 8.57 × 10^?12?6.85 × 10^?11 m²/s or hydrogel and 3.33 × 10^?13?5.45 × 10^?12 m²/s for xerogel depending on the sorbate concentration.     

Chemical composition of rainwater at a coastal town on the southwest of Europe: what changes in 20 years?

Rainwater was collected at the Portuguese west coast between September 2008 and September 2009, and analysed for pH, conductivity, and Cl⁻, NO₃⁻, SO₄²⁻, and NH₄⁺ concentrations. Results of rainwater chemical composition were compared with those obtained at the same site between 1986 and 1989. In both collection periods rainwater was predominantly (≈ 80%) associated to oceanic air masses. The rainwater concentration of H⁺ was in the same range as twenty years ago. A clear decrease of non sea salt sulphate (NSS-SO₄²⁻) was observed in 2008-2009 relatively to 1986-1989, not only in samples with origin in central and northern Europe, but also in samples from Atlantic Ocean and Mediterranean. This decrease indicates that SO₂ emissions were reduced, which may be due to a lower content of sulphur in oil by-products. A decrease was also observed in NH₄⁺ concentration in 2008-2009. On the contrary an increase of NO₃⁻ concentration was observed for samples of all origins in 2008-2009 relatively to 1986-1989, which was particularly high (more than 3 fold) for samples with origin in Atlantic Ocean, suggesting the incorporation of this ion by rainout at the sampling site. The contribution of local sources is indeed suggested by the moderate negative correlation of NH₄⁺, NO₃⁻ and NSS-SO₄²⁻ with rainwater volume. The high increase of NO₃⁻ concentration can be attributed to the increase of local vehicular and industrial emissions in the sampling area.     

Modeling of coagulation-microfiltration hybrid process for treatment of oily wastewater using ceramic membranes

In this novel paper, fouling mechanisms of Mullite-alumina ceramic membranes in treatment of oily wastewaters in hybrid coagulation—microfiltration (MF) process have been presented. Hermia’s models for cross flow filtration were used to investigate the fouling mechanisms of membranes at different time intervals with various coagulant concentrations. Four coagulant ((ferrous chloride (FeCl₂ · 4H₂O), ferrous sulfate (FeSO₄ · 7H₂O), aluminum chloride (AlCl₃ · 6H₂O) and aluminum sulfate (Al₂(SO₄ ^?)₃ · 18H₂O)) plus equal concentration of calcium hydroxide in form of calcium hydroxide (Ca(OH)₂) were evaluated in the coagulation—MF hybrid process at different concentrations (0, 50, 100 and 200 ppm). To determine whether the data agree with any of the considered models, the coefficient of determination (R₂) of each plot for one model was compared with the others. In addition, average prediction errors of models are calculated. The results showed that the cake filtration model can be applied for prediction of the permeation flux decline for MF and coagulation—MF hybrid process with the best and worst average error equal to 0.96% and 5.78% respectively. Results indicated that by increasing time in filtration pore blocking behavior changes and one model cannot predict pore blocking behavior in all filtration time with very good preciseness.     

Investigating the potential role of ammonia in ion chemistry of fine particulate matter formation for an urban environment

To investigate the potential role of ammonia in ion chemistry of PM_2.5 aerosol, measurements of PM_2.5 (particulate matter having aerodynamic diameter < 2.5 µm) along with its ionic speciation and gaseous pollutants (sulfur dioxide (SO₂), nitrogen oxides (NO_x), ammonia (NH₃) and nitric acid (HNO₃)) were undertaken in two seasons (summer and winter) of 2007-2008 at four sampling sites in Kanpur, an urban-industrial city in the Ganga basin, India. Mean concentrations of water-soluble ions were observed in the following order (i) summer: SO₄²⁻ (26.3 µg m^− 3) > NO₃⁻ (16.8) > NH₄⁺ (15.1) > Ca²⁺ (4.1) > Na⁺ (2.4) > K⁺ (2.1 µg m^− 3) and (ii) winter: SO₄²⁻ (28.9 µg m^− 3) > NO₃⁻ (23.0) > NH₄⁺ (16.4) > Ca²⁺(3.4) > K⁺(3.3) > Na⁺ (3.2 µg m^− 3). The mean molar ratio of NH₄⁺ to SO₄²⁻ was 2.8 ± 0.6 (mostly >2), indicated abundance of NH₃ to neutralize H₂SO₄. The excess of NH₄⁺ was inferred to be associated with NO₃⁻ and Cl⁻. Higher sulfur conversion ratio (F_s: 58%) than nitrogen conversion ratio (F_n: 39%) indicated that SO₄²⁻ was the preferred secondary species to NO₃⁻. The charge balance for the ion chemistry of PM_2.5 revealed that compounds formed from ammonia as precursor are (NH₄)₂SO₄, NH₄NO₃ and NH₄Cl. This study conclusively established that while there are higher contributions of NH₄⁺, SO₄²⁻ to PM_2.5 in summer but for nitrates (in particulate phase), it is the winter season, which is critical because of low temperatures that drives the reaction between ammonia and HNO₃ in forward direction for enhanced nitrate formation. In summary, inorganic secondary aerosol formation accounted for 30% mass of PM_2.5 and any particulate control strategy should include optimal control of primary precursor gases including ammonia.     

Atmospheric contamination of archaeological monuments in the Agra region (India)

An analysis of water-soluble samples collected from marble and sandstone of monuments for different ions have been done. The combustion, manufacturing and other polluting operations existing within Agra area have been investigated. The measurements of flue gases amounting to 3.63 × 10⁹ S.C.F. indicate atmospheric contamination and deterioration of archaeological monuments of Agra. It has been found that the principal sources of air contamination are the 325 iron foundries and 3 railway shunting yards located within 0.3 to 3.0 Km. of the main monuments. The topographical and micrometeorological conditions of the city have tended to favour and aggravate the concentration of effluents in the surrounding air of the monuments. The annual average existing level of SO₂ ranges from 16 to 20 micrograms/m³. The seasonal distribution of SO₂ and suspended particulate matter in the air at Taj Mahal, Red Fort and Sikandra have been discussed and illustrated. It has been observed that there is substantial sulphur dioxide contamination existing at Agra. The maximum concentration of SO₄^2? and NO₃^? amounting 0.46 and 0.38 respectively by weight percentage found existing at Red Fort cause efflorescences of sandstone.     

Electro-Fenton degradation of synthetic dyes

The electrochemical removal of a synthetic solution containing 120 mg L⁻¹ of alizarin red has been studied by electro-Fenton process using a gas-diffusion cathode to produce in situ hydrogen peroxide by oxygen reduction. The effect of operating conditions such as Fe²⁺ concentration, applied current, solution pH and temperature on the efficacy of the process was investigated. It is shown that alizarin red and its products may be effectively degraded by the OH radicals produced by the reaction between the Fe²⁺ ions and the electrogenerated H₂O₂.After 4 h of electrolysis COD removal was only 45% when no ferrous ions were added to the solutions, while the presence of ferrous ion greatly improved COD removal up to more than 90%. In particular 1.0 mM was the optimal concentration of ferrous ions and the single step or the stepwise addition of Fe²⁺ ions enables the same COD removal. The oxidation rate increased with increasing of current density and temperature and with decreasing of pH. The UV-vis analysis indicated that the discoloration of the solution occurs simultaneous with the destruction of aromatic rings and alizarin red is oxidised firstly to colourless intermediates (mainly phthalic acid, small carbonyl species) and then to carbon dioxide.     

Direct determination of total fluoride in the environment of three major sources (aluminium,fertiliser and steel plants)

The development of a specific and sensitive technique for measurements of total fluoride in the atmospheric environment of aluminium, fertiliser and steel plants is described.This technique is based on collection of gaseous and particulate fluoride with sodium hydroxide impregnated filters.Total fluoride is solubilised with dilute acid solution (0.125 M H₂SO₄ or 0.25 M HCL). After addition of a buffer (1 M sodium citrate solution), F^? is determine using a specific electrode.This rapid, specific and sensitive technique yields results comparable to those with the fusion distillation method.     

Effect of COD/SO4ratio and Fe(II) under the variable hydraulic retention time (HRT) on fermentative hydrogen production

The effect of chemical oxygen demand/sulfate (COD/SO₄²⁻) ratio on fermentative hydrogen production using enriched mixed microflora has been studied. The chemostat system maintained with a substrate (glucose) concentration of 15 g COD L⁻¹ exhibited stable H₂ production at inlet sulfate concentrations of 0-20 g L⁻¹ during 282 days. The tested COD/SO₄²⁻ ratios ranged from 150 to 0.75 (with control) at pH 5.5 with hydraulic retention time (HRT) of 24, 12 and 6 h. The hydrogen production at HRT 6 h and pH 5.5 was not influenced by decreasing the COD/SO₄²⁻ ratio from 150 to 15 (with control) followed by noticeable increase at COD/SO₄²⁻ ratios of 5 and 3, but it was slightly decreased when the COD/SO₄²⁻ ratio further decreased to 1.5 and 0.75. These results indicate that high sulfate concentrations (up to 20,000 mg L⁻¹) would not interfere with hydrogen production under the investigated experimental conditions. Maximum hydrogen production was 2.95, 4.60 and 9.40 L day⁻¹ with hydrogen yields of 2.0, 1.8 and 1.6 mol H₂ mol⁻¹ glucose at HRTs of 24, 12 and 6 h, respectively. The volatile fatty acid (VFA) fraction produced during the reaction was in the order of butyrate > acetate > ethanol > propionate in all experiments. Fluorescence In Situ Hybridization (FISH) analysis indicated the presence of Clostridium spp., Clostridium butyricum, Clostridium perfringens and Ruminococcus flavefaciens as hydrogen producing bacteria (HPB) and absence of sulfate reducing bacteria (SRB) in our study.     

Methods for estimation of long-term non-carbonate neutralisation of acid rock drainage

In the long-term phase of an acid rock drainage (ARD) evolution profile, after any short-term neutralisation capacity provided by carbonate minerals is exhausted, the net acid release is a product of a declining acid generation rate (AGR) and a slower, long-term acid neutralisation rate mainly provided by gangue silicate minerals. At some point, the AGR and the non-carbonate acid neutralisation rate (ANRnc) will be similar. Matching of the AGR and ANRnc near 10 mg H₂SO₄/kg/week is demonstrated in data from 10-year columns. This long-term neutralisation is not measured at present in any accepted assessment tests. Methods to estimate ANRnc, based on silicate mineralogy and solution assays from long-term column leach tests, are compared. Good agreement is demonstrated between rates measured from the solution assay data and those calculated from mineralogy using kinetic databases. More rigorous analysis of the leachate chemistry of selected long-term leach tests also suggests possible cover design criteria based on the maximum AGR that will maintain a pH > 4 in leachate from ARD materials. The data show a distinct break at an AGR of 3 mg H₂SO₄/kg/week, below which no leachate pH is less than 4. The results indicate that an AGR of 10 t H₂SO₄/ha/year is conservative and a suitable cover design target for ARD control that would be matched by ANRnc.     

A quantitative investigation of the reaction of ozone with p-toluenesulfonic acid in aqueous solution as a model compound for anionic detergents

The reaction of ozone with p-toluenesulfonic acid (PTA) at initial pH 3 and 12 in aqueous solutions (25°C) has been studied at initial concentration 1 mmol l⁻¹ and ozone dose is 24 mg min⁻¹ 1. and 11 mg min⁻¹ 1. respectively. The substrate elimination follow a zero order rate law. A 98% p-toluenesulfonic acid reduction requires at least 7 mol O₃ per mol PTA, however to remove 100% PTA the consumption of ozone increases to 16 mmol O₃ per mmol PTA. At this point a 28% reduction of DOC and a 74% COD reduction was achieved.The PTA decomposition is quicker at higher ozone flow rate, but the specific ozone consumption increases also. As oxidation products the following compounds were identified and their quantitative variations as function of ozonation time were measured: methylglyoxal, acetic acid, formic acid, pyruvic acid, oxalic acid, H₂SO₄ and H₂O₂. As byproduct mesoxalic acid was identified. At pH 12 lactic acid as a further oxidation product was observed.Balances of carbon, sulfur and methyl as well of the acid equivalents indicate one or more intermediates with a sulfonic acid group. These intermediates with a proportion of about 20% disappear after 100% PTA elimination. On account of these results a reaction mechanism is discussed.     

A simple manifold flow injection analysis for determining phosphorus in the presence of arsenate

A simple manifold flow injection analysis (FIA) for determining phosphorus in the presence of arsenate in water, by the on-line reduction of As(V) to As(III) using sodium sulfite. The FIA method is applicable for the determination of phosphorus in water samples containing less than 0.37 μg·mL^?1 of AsO₄ ^3?. A solution obtained by mixing 6.30 g of Na₂ SO₃ (1 M), 5.0 ml H₂SO₄ 18 M completed up to 50 ml with deionized distilled water was used to reduce on line As(V) to As(III), using a reactor of 0.30 m. The limit of detection is 0.05584 ± 0.00167 μg·mL^?1 P-PO₄ ^3? and sampling frequency is 45 samples per hour. It is a simple and low cost methodology, easily applicable in the determination of phosphorus in samples of water contaminated with arsenate.     

Photolytic reaction mechanism and impacts of coexisting substances on photodegradation of bisphenol A by Bi2WO6 in water

Bi₂WO₆ displayed great photolytic degradation efficiency to bisphenol A (BPA) under simulated solar light irradiation but its reaction mechanism and the impacts of coexisting substances on the degradation remain unclear. In present study, the reaction mechanism was investigated using DMPO spin-trapping ESR spectra and experiments with scavengers of hydroxyl radicals (OH) and holes. The results supported that hole oxidation mainly governed the photodegradation process. As a common humic substance in natural water, humic acid accelerated the degradation of BPA when its concentration was 1 mg/L, while the photodegradation was impeded with the increase of humic acid concentration in the range of 5-20 mg/L. Almost all anions, including NO₃⁻, HCO₃⁻, Cl⁻, SO₄²⁻ inhibited the degradation of BPA by Bi₂WO₆ and their inhibition effects followed the order of SO₄²⁻ > Cl⁻ > HCO₃⁻ > NO₃⁻. Cations of Na⁺, K⁺, Ca²⁺ and Mg²⁺ displayed slight suppressing effect on BPA degradation mainly due to the impact of Cl⁻ coexisting in the solution. However, Cu²⁺ hindered the BPA photodegradation heavily. Fe³⁺ and H₂O₂ affected the photodegradation in a complicated way: they suppressed or promoted the photodegradation depending on their concentrations. This could be the result of competition between photolyitc hole generated by Bi₂WO₆ and OH produced by Fe³⁺ or H₂O_2.     

Durability of sulfur concrete in various aggressive environments

Durability of sulfur concrete with different fillers, as well as Portland cement concrete, was tested in the solutions of HCl, H₂SO₄, and NaCl. Regarding mass changes, in the solutions of HCl and H₂SO₄ sulfur concrete with talc and fly ash exhibited higher durability, while in NaCl samples with alumina and microsilica were better. The type of filler did not affect durability regarding compressive strength. Strength loss was higher in the solution of HCl comparing to H₂SO₄, while negligible in NaCl which is in accordance with apparent porosity increase. Portland cement concrete after two months lost 20% of mass.     

Fluorine pollution in the marmara region,Turkey. Production methods for fluorine removal and recovery

The importance of the removal and recovery of fluorine from Turkish fertilizer plant effluents has been emphasized. Even though the economic evaluation of the processes developed are not based on capital and operating costs for the processes there are strong economic incentives to adopt recovery apart from clear environmental needs. Selection of the methods for the removal of fluorine through production processes specifically the dependence of the choice of waste treatment process on available H₂SiF₆ concentration have been discussed. The production of acid grade calcium fluoride and sodium fluosilicate from waste fluosilicic acid solutions were studied.The optimum process conditions were determined. It has been found that it is possible to produce these industrially valuable fluorine compounds from 2.5 to 4.5% H₂SiF₆ containing waste waters and in appropriate conditions both processes are effective in removing 90–95% of the fluorine present in waste fluosilicic acid solutions. It has been shown that for the production of acid grade calcium fluoride and sodium fluosilicate the P₂O₅ content of waste fluosilicic acid solutions should be < 250 mg P₂O₅ I⁻¹ and 300 mg P₂O₅ I⁻¹ respectively. It has been suggested to make pilot plant studies for the industrial application of the processes.     

The effects of a whole-watershed calcium addition on the chemistry of stream storm events at the Hubbard Brook Experimental Forest in NH, USA

Patterns of storm runoff chemistry from a wollastonite (calcium-silicate mineral, CaSiO₃) treated watershed (W1) were compared with a reference watershed (W6) at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire (NH), USA to investigate the role of Ca²⁺ supply in the acid–base status of stream chemistry. In the summer of 2003, six storm events were studied in W1 and W6 to evaluate the effects of the wollastonite treatment on the episodic acidification of stream waters. Although mean values of Ca²⁺ concentrations decreased slightly from 33.8 to 31.7 μmol/L with increasing stream discharge in W1 during the events, the mean value of acid neutralizing capacity (ANC) was positive (1.2 μeq/L) during storm events, compared to negative values (− 0.2 μeq/L) in W6. This pattern is presumably due to enhanced Ca²⁺ supply in W1 (20.7 to 29.0% of dissolved Ca²⁺ derived from the added wollastonite) to stream water as a result of interflow along shallow flowpaths. In addition, the application of wollastonite increased pH and dissolved silica (H₄SiO₄) concentrations, and decreased the concentration of inorganic monomeric Al (Al_i) in W1 in comparison with W6 during storm events. Despite an increase in SO₄²⁻ concentration, likely due to desorption of sulfate from soil after the treatment, the watershed showed an increase in ANC compared to the reference watershed, serving to mitigate episodic acidification.     

Effect of COD/SO(4)(2-) ratio and sulfide on thermophilic (55 degrees C) sulfate reduction during the acidification of sucrose at pH 6

This study investigated the effect of the COD/SO₄²⁻ ratio (4 and 1) and the sulfide concentration on the performance of thermophilic (55 °C) acidifying (pH 6) upflow anaerobic sludge bed reactors fed with sucrose at an organic loading rate of 4.5 g COD l_reactor⁻¹ day⁻¹. Sulfate reduction efficiencies amounted to 65% and 25-35% for the COD/SO₄²⁻ ratios of 4 and 1, respectively. Acidification was complete at all the tested conditions and the electron flow was similar at the two COD/SO₄²⁻ ratios applied. The stepwise decrease of the sulfide concentrations in the reactors with a COD/SO₄²⁻ ratio of 1 by N₂ stripping caused an immediate stepwise increase in the sulfate reduction efficiencies, indicating a reversible inhibition by sulfide. The degree of reversibility was, however, affected by the growth conditions of the sludge. Acidifying sludge pre-grown at pH 6, at a COD/SO₄²⁻ ratio of 9 and exposed for 150 days to 115 mg l⁻¹ sulfide, showed a slower recovery from the sulfide inhibition than a freshly harvested sludge from a full scale treatment plant (pH 7 and COD/SO₄²⁻=9.5) exposed for a 70 days to 200 mg l⁻¹ sulfide. In the latter case, the decrease of the sulfide concentration from 200 to 45 mg l⁻¹ (35 mg l⁻¹ undissociated sulfide) by N₂ stripping caused an immediate increase of the sulfate reduction efficiency from 35% to 96%.