H. Van Langenhove

Treatment of waste gases contaminated with odorous sulfur compounds.

Due to their very low odor threshold value (ppbv range), high toxicity, and potential corrosive effect, the presence of volatile sulfur compounds in waste gases deserves special attention. These sulfur compounds mainly include hydrogen sulfide, dimethyl sulfide, dimethyl disulfide, meth-anethiol, carbon disulfide, and carbonyl sulfide. Contrary to natural emissions, anthropogenic emissions may contribute to local concentrations, strongly exceeding the odor threshold value. These anthropogenic sources mainly include these processes where anaerobic degradation of organic matter can occur (e.g., waste water treatment and composting plants) and processes where organic matter is heated (e.g., thermal sludge-treatment plants). Industrial applications of volatile sulfur compounds (e.g., for viscose rayon manufacturing) and their production during chemical reactions (e.g., in the Kraft paper pulping process) can also lead to high local atmospheric concentrations. An overview of abatement technologies for aerobic ...

Abatement of volatile organic sulfur compounds in odorous emissions from the bio-industry.

Compounds of interest in this work are methanethiol (MeSH), dimethyl sulfide (Me2S), dimethyl polysulfides (Me2Sx) and carbon disulfide (CS2) since these volatiles have been identified as predominant odorants in the emission of a wide range of activities in the bio-industry (e.g. aerobic waste water treatment plants, composting plants, rendering plants). In these processes, the occurrence of volatile organic sulfur compounds is mainly related to the presence of anaerobic microsites with consecutive fermentation of sulfur containing organic material and/or to the breakdown of the latter due to thermal heating. Due to the chemical complexity of these low-concentrated waste gas streams and the high flow rates to be handled, mainly biotechnological techniques and scrubbers can be used to control the odour emission. When using biofilters or trickling filters, inoculation with specific micro-organisms and pH-control strategies should be implemented to optimise the removal of volatile organic sulfur compounds. In scrubbers, chemical oxidation of the volatile organic sulfur compounds can be obtained by dosing hypochlorite, ozone or hydrogen peroxide to the scrubbing liquid. However, optimal operational conditions for each of these abatement techniques requires a further research in order to guarantee a long-term and efficient overall odour abatement.

Illustrations towards quantifying the sustainability of technology

This paper attempts to quantify the sustainability of technological processes. It is based on thermodynamics with energy carriers and materials (products, waste, etc.) expressed in the same calculable quantity–exergy (Joule). The results have three considerations. One factor reflects to that extent renewable resources are used. In addition, the technological efficiency has to be accounted for as it affects sustainability. Finally, the results take into account the generation of waste products and the exergy required for converting the waste into products which are harmless or assimilable in the ecosphere. The proposed measure of sustainability has been illustrated for two types of products. In the first illustration, ethanol production was studied. Two routes were investigated, one starting from fossil oil and the other from agricultural products. Additionally, a route based on the synthesis from carbon dioxide and hydrogen was examined, in which hydrogen was generated by splitting water with electricity from photovoltaic solar energy conversion. The second product studied was electricity, generated from the combustion of natural gas or from photovoltaic solar energy conversion. The merit of the obtained results are that they treat technological sustainability not only in qualitative but also in quantitative terms. The insights obtained can help to account for sustainability in the development of new concepts of chemical technology.

Elimination of hydrogen sulphide from odorous air by a wood bark biofilter

Abstract Wood bark, used as biofilter material, gives less back pressure than fibre peat or household compost and is therefore cheaper in variable costs than the other materials. With a filter bed height of 0.9 m and with a surface load of 65 m 3 m −2 h −1 , 10 ppm hydrogen sulphide is efficiently eliminated from odorous air by a wood bark biofilter. The most important parameter for a good functioning of the filter is the water content of the filter material. The optimum for wood bark is about 65%. Unnecessarily high back pressures are recorded during sprinkling water on top of the filter. Therefore, it seems preferable to condition the relative humidity of air entering the filter. If absorption and adsorption were the only factors in the biofilter mechanism, hydrogen sulphide would, in the given circumstances, break through after about 7 h. It is shown that hydrogen sulphide is oxidized to sulphate. According to literature data, chemical oxidation is too slow and therefore hydrogen sulphide oxidation in the biofilter is attributed to microbiological action.

The effect of inoculation and the type of carrier material used on the biofiltration of methyl sulphides

Low elimination capacities (less than 10 gm−3day−1) were observed for the odorant dimethyl sulphide (Me2S) when either wood bark or compost was used as the carrier material in a laboratory-scale biofilter. Enrichment experiments were set up by incubation of garden soil samples during 4 weeks with 100 ppm (v/v) headspace concentrations of both Me2S and dimethyl disulphide (Me2S2). After transfer to a mineral medium, Me2S- and Me2S2-degrading enrichment cultures were obtained for all five soil samples tested, both compounds being converted stoichiometrically to sulphuric acid. Upon inoculation of the laboratory-scale biofilter with one of these enrichment cultures (± 120 g cell dry weight m−3 reactor), the elimination capacity for Me2S increased in a 3-week period to 35 gm−3 day−1 and 680 gm−3 day−1 when wood bark and compost were used as the respective carrier materials. Both inoculated biofilters were able to degrade Me2S2, however the elimination capacities obtained for Me2S2 were lower (e.g. 24 g m−3 day−1 for the wood bark filter) compared to those for Me2S. For both inoculated biofilters, a gradual decrease of the elimination capacity for the methyl sulphides was observed as a result of acidification of the carrier material, suggesting that pH regulation is necessary if long-term biofiltration experiments are to be performed.

Ultrasonic degradation of trichloroethylene and chlorobenzene at micromolar concentrations: kinetics and modelling.

Although most papers in the field of sonochemical degradation of volatile organics in aqueous media describe experiments at the millimolar concentration range, this study focuses on the degradation kinetics of chlorobenzene (CB) and trichloroethylene (TCE) in the micromolar range. It was found that the reaction kinetics increase with decreasing initial substrate concentrations. For example, the pseudo-first-order reaction rate constant of CB increases by a factor of 14.3, if the initial concentration drops from 3440 to 1 microM. Previous work in the millimolar range has shown that the degradation of these volatiles is mainly due to pyrolytic reactions. The enhancement of the reaction kinetics at lower concentrations, in this work, could no longer be explained by this mechanism, even by taking into account the effect of the concentration of the solutes on the reaction temperature. Therefore, a new model was developed, incorporating gas phase OH radical induced degradation, next to pyrolysis. The model, fitting the experimental results, illustrated that at micromolar concentrations the OH radical induced degradation becomes significant. Simulations showed that at initial concentrations of CB > 1000 microM degradation is due to pyrolysis for over 99.97%, but it was also demonstrated that at concentrations between 1 and 5 microM, the OH radical mechanism contributed 48.5% of the total degradation.

Sonolysis of an aqueous mixture of trichloroethylene and chlorobenzene.

The effect of the initial concentration on the ultrasonic degradation of two volatile organic compounds trichloroethylene (TCE) and chlorobenzene (CB) was investigated. At higher concentrations, slower sonolysis rates were obtained due to the lowering of the average specific heat ratio gamma of the gas inside the cavitation bubbles. Furthermore, the effect of different concentrations of CB on the sonolysis of 3.34 mM TCE and the effect of different concentrations of TCE on the sonolysis of 3.44 mM CB was examined. The presence of CB lowered the sonolysis rate of TCE, while the sonolysis rate of CB did not decrease by TCE addition. An even higher sonolysis rate was obtained for 3.44 mM CB in the presence of 0.84 mM TCE than without TCE. The explanation for the different effects of both volatile organics on each others sonolysis rate is thought to be the difference in reaction rate of TCE and CB with the radicals formed during sonolysis. The effect of TCE on the sonolysis rate of CB by lowering the gamma value is compensated by an increased indirect degradation of CB by radicals formed out of TCE. The decreased thermal degradation and the increased indirect radical degradation of CB in the presence of TCE is demonstrated by determining the kinetics of the degradation products styrene and dichlorobenzene.

Occurrence patterns of pharmaceutical residues in wastewater, surface water and groundwater of Nairobi and Kisumu city, Kenya

Emerging organic contaminants have not received a lot of attention in developing countries, particularly Africa, although problems regarding water quantity and quality are often even more severe than in more developed regions. This study presents general water quality parameters as well as unique data on concentrations and loads of 24 pharmaceuticals including antibiotic, anti(retro)viral, analgesic, anti-inflammatory and psychiatric drugs in three wastewater treatment plants, three rivers and three groundwater wells in Nairobi and Kisumu. This allowed studying removal efficiencies in wastewater treatment, identifying important sources of pharmaceutical pollution and distinguishing dilution effects from natural attenuation in rivers. In general, antiretrovirals and antibiotics, being important in the treatment of common African diseases such as HIV and malaria, were in all matrices more prevalent as compared to the Western world. Wastewater stabilization ponds removed pharmaceuticals with an efficiency between 11 and 99%. Despite this large range, a different removal is observed for a number of compounds, as compared to more conventional activated sludge systems. Total concentrations in river water (up to 320 μg L(-1)) were similar or exceeded concentrations in untreated wastewater, with domestic discharges from slums, wastewater treatment plant effluent and waste dumpsites identified as important sources. In shallow wells situated next to pit latrines and used for drinking water, the recalcitrant antiretroviral nevirapine was measured at concentrations as high as 1-2 μg L(-1). Overall, distinct pharmaceutical contamination patterns as compared to the Western world can be concluded, which might be a trigger for further research in developing regions.

Long-term stability of a biofilter treating dimethyl sulphide

Abstract The biofiltration of dimethyl sulphide (Me2S) and other volatile sulphur compounds results in the accumulation of the metabolite sulphuric acid in the carrier material. Regeneration of an acidified (pH 4.7), Hyphomicrobium-MS3-inoculated compost biofilter degrading Me2S was not possible by trickling tap water (days 0–28) or a KH2PO4/K2HPO4 buffer solution (1.26 g PO3-4 l-1, pH 7) (days 29–47) over the bioreactor at a superficial liquid flow rate of 34 lm-2 day-1. Since the protons produced displaced nutrient cations (Na+, K+, Ca2+, Mg2+, NH+4) from the cation-exchange sites on the compost material, 95% of the SO2-4 was leached as the corresponding sulphate salts and not as sulphuric acid. Concomitantly, the pH of the compost material decreased from 4.7 to 3.9 over the 47 days rinsing period. Moreover, the rinsing procedure resulted in the leaching of essential microbial nutrients from the compost material, such as NH+4 (22.3% wash-out over the 47-day rinsing period) and PO3-4 (39.3% washout over the 28-day tap-water rinsing period). However, mixing limestone powder into the Me2S-degrading compost biofilter was a successful approach to controlling the pH in the optimal range for the inoculum Hyphomicrobium MS3 (pH 6–7). A stoichiometric neutralisation reaction (molar ratio CaCO3/H2SO4=1.1) was observed between the CaCO3 added and the metabolite of the Me2S degradation, while high elimination capacities (above 100 g Me2S m-3 day-1) were obtained over a prolonged (more than 100 days) period.

GC-MS identification of odorous volatiles in wastewater

GC-MS analyses of water samples from all unit processes of a wastewater treatment plant resulted in the identification of hydrocarbons, oxygen, nitrogen and sulphur containing organic compounds. Sample preparation included a purge and trap method for apolar volatiles and solvent extraction for polar ones. Because odour nuisance was registered in the neighbourhood of the plant, attention was focused on the identification of malodorous volatiles. Sulphides, polysulphides, thiols, alkylthiophenes, alkylthiothiophenes, lower organic acids, phenol and p-cresol, were the most important odour stimuli identified in the samples. From these analyses it was shown that some organic malodorants were already present in the industrial wastewater. Most sulphur compounds however were formed during transport of the industrial wastewater in mains leading to the works.