To overcome some of the limitations of batch processing, continuous operation in a plug-flow system is considered. As the contaminated solids travel through the system, specific conditions in the successive reactors develop. A practical way of achieving a plug flow system is to use a sequence of batch reactors (Fig. 11.10). In an experimental cascade of three stirred-batch slurry reactors, the contaminated solids (oil contamination) are periodically transferred to the next step (only part of the reactor slurry content is transferred, the so-called 'slug') (Apitz et al., 1994).
Figure 11.11 shows the breakdown pattern in the transferred slug during its residence in each individual reactor. The authors explained "that each successive stage of the cascade maintains a microbial consortium that is optimized to consume organic compounds of increasing complexity. When compared to the batch process the biocascade was shown to be more effective, both in terms of the rate and degree of degradation" (Apitz et al., 1994).
Fig. 11.11 Level of contamination in one slug of material traveling through the sequenced batch process (three-step biocascade). Successive steps reflect the degradation of more recalcitrant components (Apistz et al., 1994; with permission).
In the Netherlands a continuous plug-flow system has been achieved with the Slurry Decontamination Process (SDP) (Fig. 11.7). This process contains four major unit operations (Kleijntjens, 1991).
1. The contaminated solids are mixed with (process) water to form a slurry and are sized by passage over a vibrating screen. In this wet sieving step, debris is removed and a slurry having the proper density (about 30 w/w%) is prepared.
2. In the first reactor/separator (a tapered air-lifted bioreactor: the DITS reactor), the sand fractions are removed by means of a fluidized bed. Extensive organic material is removed by fine screening of the light material. In addition, agglomerates of contaminated fines are demolished owing to the power input and are thereby opened to biological breakdown (inoculation with the active biomass also takes place here).
3. In a second reactor stage, the contaminated fine fraction is treated. The second stage consists of a cascade of interconnected bioreactors (ISB cascade).
4. A dewatering stage completes the process; the water released is partly recirculated as process water to mix fresh solids into a slurry.
Figure 11.12 shows the configuration of the DITS bioreactor (Luyben and Kleijntjens, 1988). In the tapered bioreactor system, energy is introduced at the bottom by the simultaneous injection of compressed air and slurry that was recycled from the reactor content itself. Shown are the dual injectors, the settlers (used in the recycle flow), and the tapered vessel. This system has been built on scales of 400 L, 800 L, and 4 m3. It has been operated for 2.5 years to test various solid waste streams. The integral process was operated semicontinuously on a pilot scale (3 m3 working reactor volume).
Figure 11.13 shows the experimental results for a heavily polluted harbor sediment during a steady-state period of six weeks. Nutrients (nitrogen, phosphorous, and potassium) were added and the temperature was kept at 30 °C. The steady-state PAH concentration in the solids is shown as a function of time. The upper symbols show the feed concentrations in the slurry mill, which have an average of about 350 mg kg-1 (the input data are scattered because of the heterogeneous feedstock). In the DITS reactor the steady-state concentration dropped to around 100 mg kg-1 (first part of the microbial breakdown).
In the ISB cascade the average concentration dropped to 30-40 mg kg-1. After de-watering, the final concentration increased somewhat in the filter cake. The overall
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