Spp Simon Ottengraf

Verification studies of a simplified model for the removal of dichloromethane from waste gases using a biological trickling filter (part II)

The removal of dichloromethane from waste gases in a biological trickling filter was studied experimentally as well as theoretically within the concentration range of 0–10,000 ppm. A stable dichloromethane elimination performance was achieved during two years of operation, while the start-up of the system only amounted to several weeks at constant inlet concentrations. The trickling filter system was operated co-currently as well as counter-currently.However, experimental and theoretical results revealed that the relative flow direction of the mobile phases did not significantly affect the elimination performance. Moreover, it was found that the gas-liquid mass-transfer resistance in the trickling filter bed applied was negligible, which leaves the biological process inside the biofilm to be the rate limiting step.A simplified model was developed, the “Uniform-Concentration-Model”, which showed to predict the filter performance close to the numerical solutions of the model equations. This model gives an analytical expression for the degree of conversion and can thus be easily applied in practice.The dichloromethane eliminating performance of the trickling filter described in this paper, is reflected by a maximum dichloromethane elimination capacity ECmax=157 g/(m3 · h) and a critical liquid concentration Clcr=45 g/m3 at a superficial liquid velocity of 3.6 m/h, inpendent of the gas velocity and temperature.

The effect of pH profiles in methanogenic aggregates on the kinetics of acetate conversion

Due to the conversion of acetic acid into the weaker carbonic acid and methane, the pH inside methanogenic aggregates is higher than in the bulk liquid. The pH profiles in aggregates were measured with pH microelectrodes. These profiles strongly determine the macro-kinetics of the aggregate, by their influence on the values of the growth parameters Ks and μmax. Acetate transport resistances were not limiting for the conversion rate in methanogenic aggregates. Nevertheless, the effectiveness factor η did not approach unity, but amounted to 0.57–0.62 in the acetate concentration range relevant for most methanogenic reactors. The value of η is determined almost entirely by the pH profiles inside the aggregates. It was concluded that for the physical/mathematical description of the conversion in methanogenic aggregates, information on the pH gradients and the pH dependency of the growth parameters is indispensable. Batch experiments showed that acetate uptake by aggregates was not coupled directly to methanogenesis. Consumed acetate was not converted instantaneously to methane, suggesting the conversion to proceed via a pool of acetate or reserve material.

Emission of microorganisms from biofilters

Experiments are reported on the discharge of microbial germs by biofilter systems used for the treatment of waste gases containing volatile organic compounds. The systems investigated concern six full-scale filter installations located in the Netherlands in several branches of industry, as well as a laboratory-scale installation used for modelling the discharge process. It is concluded that the number of microbial germs (mainly bacteria and to a much smaller extent moulds) in the outlet gas of the different full scale biofilters varies between 103 and 104 m−3, a number which is only slightly higher than the number encountered in open air and of the same order of magnitude encountered in indoor air. It is furthermore concluded that the concentration of microorganisms of a highly contaminated inlet gas is considerably reduced by the filtration process. On the basis of the experiments performed in the laboratory-scale filter bed, it is shown that the effect of the gas velocity on the discharge process results from two distinctive mechanisms: capture and emission. A theoretical model is presented describing the rate processes of both mechanisms. The model presented and the experimentally determined data agree rather well.

The influence of NaCl on the degradation rate of dichloromethane by Hyphomicrobium sp

The degradation of dichloromethane by the pure strainHyphomicrobium GJ21 and by an enrichment culture, isolated from a continuously operating biological trickling filter system, as well as the corresponding growth rates of these organisms were investigated in several batch experiments. By fitting the experimental data to generally accepted theoretical expressions for microbial growth, the maximum growth rates were determined. The effect of NaCl was investigated at salt concentrations varying from 0 to 1000 mM. Furthermore the dichloromethane degradation was investigated separately in experiments in which a high initial biomass concentration was applied. The results show that microbial growth is strongly inhibited by increased NaCl concentrations (50% reduction of μmax at 200–250 mM NaCl), while a certain degree of adaptation has taken place within an operational system eliminating dichloromethane. A critical NaCl concentration for growth of 600 mM was found for the microbial culture isolated from an operational trickling filter, while a value of 375 mM was found for the pure cultureHyphomicrobium GJ21. The substrate degradation appears to be much less susceptible to inhibition by NaCl. Even at 800 mM NaCl relatively high substrate degradation rates are still observed, although this process is again dependent on the NaCl concentration. Here the substrate elimination is due to the maintenance requirements of the microorganisms. The inhibition of the dichloromethane elimination was also investigated in a laboratory scale trickling filter. The results of these experiments confirmed those obtained in the batch experiments. At NaCl concentrations exceeding 600 mM a considerable elimination of dichloromethane was still observed for during several months of operation. These observations indicate that the inhibition of microbial growth offers a significant control parameter against excessive biomass growth in biological trickling filters for waste gas treatment.

Continuous sedimentation theory: Effects of density gradients and velocity profiles on sedimentation efficiency

Abstract A theory is presented on continuous sedimentation. In case the solids concentration is small and uniformly distributed over the inlet height, the theory predicts independent sedimentation efficiencies on velocity distributions in a longitudinal vertical plane. A velocity profile in a horizontal plane on the other hand will have a negative effect on the efficiency. Experiments, carried out on a laboratory-scale model, have shown that even smal density differences in the basin can have a significant effect on the velocity distribution. The measured efficiencies are in good agreement with the theory

Longitudinal dispersion in oblong aerated systems

Abstract A scale model study to the flow and mixing phenomena has been carried out in oblong aeration basins, where a transversal circulating flow of the liquid is introduced by dispersing air along one side of the basin. A semi-empirical correlation of dimensionless numbers has been developed, which is considered to give a reasonable prediction of the rate of longitudinal mixing. Experiments carried out in commercial basins have shown to be in good agreement with the presented model.

Heat treatment and low pressure oxidation of aqueous sewage sludge

Abstract The subject of study was the effect of heat treatment and low pressure oxidation on: • —the specific filtration resistance. • —the COD, N and P content of the suspension. Residence time distributions of the industrial oxidation reactor show considerable axial mixing of the continuous liquid phase and a high gas fraction (30%), which is probably caused by the existence of a foam layer. Test runs at 165 and 185°C were carried out to compare specific filtration resistance of the treated sludge and the steam consumption of a heat treatment (Zimpro) unit at these two temperatures. Specific filtration resistance of the sludge treated at 165°C was 8 times higher than that of the treatment at 185°C. Moreover 20% more steam was consumed at the lower temperature. COD reduction of the sludge was in good agreement with the theoretical model.