Chemical Reduction of Nitrate by Zero-Valent Iron: Shrinking-Core versus Surface Kinetics Models.

Zero valent iron (ZVI) is being used in permeable reactive barriers (PRB) for the removal of oxidant contaminants, from nitrate to chlorinated organics. A sound design of these barriers requires a good understanding of kinetics. Here we present a study of the kinetics of nitrate reduction under relatively low values of pH, from 2 to 4.5. We use a particle size of 0.42 mm, which is within the recommended size for PRBs (0.2 mm to 2.0 mm). In order to avoid possible mass-transfer limitations, a well-stirred reactor coupled with a fluidized bed reactor was used. The experiments were performed at constant pH values using a pH controller that allows to accurately track the amount of acid added. Since the reduction of H + to H 2 by the oxidation of ZVI will always be present for these pH values, blank experiments (without nitrate) were performed and the rate of this H + reduction obtained. This rate of reduction was studied using three kinetic models: a regular empirical one, the Shrinking-Core Model (SCM), and the Surface Kinetics Model (SKM). The best performance was obtained from the SKM model. Therefore, this model was also used to study the results for the nitrate reduction, also with satisfactory results. In both cases, some assumptions are introduced to maintain a moderate number of fitting parameters.

Anaerobic co-digestion of municipal sewage sludge and fruit/vegetable waste: effect of different mixtures on digester stability and methane yield.

The anaerobic co-digestion of fruit and vegetable waste (FVW) and municipal sewage sludge (MSS) is investigated under mesophilic conditions. This was done at a constant hydraulic retention time (HRT) similar to that typically used at waste water treatment plant digesters, 20 days. The effects on digester performance of the FVW:MSS ratio and the organic loading rate (OLR) were examined. Initially the digester was fed with MSS from wastewater treatment plants (WWTP) with an average OLR of 1.03 kgVS (m3 d)-1. The co-digestion of MSS and FVW was performed at various ratios of FVW in the mixture, while increasing the OLRs, from 1.03 to 4.78 kgVS (m3 d)-1. The experimental specific methane production (SMP) was 0.303 m3 (kgVS)-1 for MSS and 0.403 m3 (kgVS)-1 for FVW as single substrate. This value varied for co-digestion with a maximum of 0.445 m3 (kgVS)-1 for a FVW:MSS ratio of 40:60. Alkalinity and pH values remain relatively constant regardless the different FVW:MSS ratios fed. As this ratio increases, the removal of the volatile solids (VS) increased from 38.7% to 82%. The average methane content of digester biogas was about 62-64%.

Quantification of methane emissions in a Mediterranean landfill (Southern Spain). A combination of flux chambers and geostatistical methods.

In this study, landfill gas emissions from a landfill located in southern Spain were estimated using static surface flux chambers and applying and comparing four geostatistical methods; ordinary kriging, lognormal kriging, intrinsic random functions of order K and indicator kriging. This paper presents the methodology used to calculate methane (and carbon dioxide) emissions from a landfill in southern Spain. Static flux chambers were used to estimate emissions through the sealing layer of a landfill assuming that the geospatial mean best expresses the average value of these emissions. This study considers several geostatistical methods for obtaining the corresponding spatial estimation, using measurements obtained from static flux chambers and finding the best proven results. The most appropriate geostatistical analysis method was found to be indicator kriging and lognormal kriging because of the simplicity of its implementation and the transformation of the flux measurements. Methane surface emissions (100 g·m-2·d-1) and visualization of the hotspots were significant enough to result in the placement of a new cover across the entire landfill. This additional cover had an immediate impact on the effectiveness of the recovery system and increased LFG collection flow rates by 15% with an increase in CH4 concentration in the collected gas from 50% to 60%.

Production of biogas from co-digestion of livestock and agricultural residues: A case study.

This study was undertaken to determine the possible changes in the digester yield and performance for the anaerobic co-digestion under mesophilic conditions of strawberry residues (SRs) together with pig manure (PM). The first part of this paper deals with the digestion of SR as a single substrate. For organic loading rates (OLRs) of 4.4 (g L-1 d-1) or less, the experimental specific biogas and methane productions are 0.588 and 0.231 L g-1, respectively. When higher OLRs (5.5 g L-1 d-1) are used the digester fails due to acidification. In the second part, the co-digestion of both residues is explored using a wide variety of SR:PM ratios and OLRs of 5.5 g L-1 d-1 with good stability. Therefore, it is demonstrated that co-digestion allows the improvement of the treatment capacity as compared with SR as a single residue. The methane and biogas productions increase as the SR:PM ratio increases. It may be concluded that, when a digester works with a certain OLR, the performance for co-digestion is always better than for single substrates because the presence of PM provides a better stability and the presence of SR improves the biogas and methane production.

Mobility and fate of carbetamide in an agricultural soil.

The objective of this work is to gain a better understanding of the fate of carbetamide, as a representative herbicide, after its soil application. To reach this goal, batch and column laboratory experiments were performed and a transport model was proposed consistent with these results. Then field-scale experiments were carried out for two years and the results compared with those that would be obtained from the transport model, once the degradation terms were introduced. All this is done for four different scenarios: first, considering that the soil is under its natural condition; second, the soil is amended with organic carbon by the addition of stabilized sewage sludge; third, considering that the percolating aqueous phase contains a significant quantity of surfactant [Linear Alkyl Benzene Sulfonate, (LAS)]; and fourth, the scenario in which the sewage sludge and the surfactant are present simultaneously. The Freundlich model yields a good fit to the data of the sorption isotherms obtained from batch equilibrium experiments, but the isotherms are close to linear. The batch sorption/desorption kinetic data together with the column and field results indicate that the retention kinetics are quite fast and local equilibrium can be assumed for the description of the sorption phenomenon. Results also prove that carbetamide is moderately retained in the original soil with a mean value of the partition coefficient of carbetamide about 0.46 (L kg(-1)). When the soil is amended with sewage sludge, this coefficient is somewhat lower, about 0.40 (L kg(-1)). A further decrease is observed 0.32 L kg(-1)) when the surfactant (LAS) at critical micelle concentration (CMC) is used. The two-region model yields a good reproduction of the results of carbetamide mobility in the soil, both at the laboratory scale and at the field scale.