of 3500 mg kg-1. Within 30 d the concentration dropped to about 500 mg kg-1, but during the following period (up to day 56) no significant degradation was measured.
Figure 11.6b shows a batch degradation curve for a heavily polluted harbor sediment, as measured on the pilot scale (650 L) in an air-agitated slurry reactor (the dual injected turbulent separation (DITS) reactor). The DITS reactor was originally the first reactor unit in the Slurry Decontamination Process (Fig. 11.7). But for this experiment the DITS reactor was disconnected and used as a standalone batch reactor. The batch process showed a decrease from 10000 mg kg-1 of mineral oil contaminant to 6000-7000 mg kg-1.
Other batch processes have been carried out (see also Section 11.3.3). Generalizing the results of these batch experiments enables a 'typical' batch curve to be constructed (Fig. 11.8): extensive breakdown in the first few days is followed by slower degradation during the second stage. In this stage only a small percentage of the contaminant is degraded over a relatively long time period.
The exact shape of the curve and the conversion level at which breakdown stops vary with the composition and age of the solids and the type of contaminant.
To understand the typical batch curve, three major phenomena have to be considered:
In batch processing the biomass, growing on a complex substrate, has to adapt continuously to different components, since the 'easy' parts are degraded first (Teschner and Wehner, 1985). This pattern inevitably leads to increasing difficulties for the resident microbial population. The adaptation rate of the population to the available contaminants is insufficient.
The contaminant desorption kinetics are of such an order that at lower concentrations there is a limited 'driving force' for the adsorbed contaminants to be released
from the solids (Di Toro and Horzempa, 1982). Since microbial breakdown proceeds only if the contaminant is dissolved in the water phase, this type of kinetics does not allow for easy breakdown at low concentrations of contaminant. • During batch processing. inhibitory side products of the microbial breakdown may be increasingly released into the medium; in addition, the physical condition of the solids may change (e.g., attrition), which may lead to unfavorable conditions for the microorganisms. An example would be the drop in pH due to humification processes, which can accompany contaminant breakdown.
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