Treatment of Wastewater Containing DCM or DCA

DCM balances in activated sludge systems show that DCM is almost totally stripped by air (Gerber et al. 1979). Therefore, trickling filters are also unsuitable.

Only 11% of the DCM added were biodegraded, 75% were desorbed into the air and 14% remained in the effluent (Winkelbauer and Kohler 1989).

In further laboratory- and pilot-scale studies, fixed and fluidized bed reactors with solid particles as support material for bacteria were used. In fixed-bed reactors, a high fluid recycle rate is necessary for aeration in external absorption tanks (Kästner 1989) or for the addition of H2O2 by mixing (Stucki et al. 1992). Similarly, in two-phase fluidized-bed reactors, a high recycle rate must be used for oxygen addition and fluidization (Gälli 1986; Burgdorf et al. 1991; Stucki et al. 1992; Niemann 1993; Herbst 1995). In order to avoid total substrate desorption, oxygen can be added in the absorption tank using non-porous membranes (Herbst 1995). Figure 9.1 presents a laboratory-scale fluidized bed reactor for the mineralization of DCA and DCM. The production of HCl makes it necessary to control the pH to 7-8 by adding NaOH.

During growth, bacteria formed pellets with a diameter of 3-5 mm, which were kept fluidized in the reactor with the upflowing water containing the DCM or DCA. The pellets settled inside the enlarged head of the reactor; and the solid-free water passed a non-porous tubular membrane where oxygen was taken up by diffusion. This water was mixed with inflowing wastewater containing DCA or DCM before being recycled into the reactor.

Fig. 9.1 Laboratory-scale plant with fluidized-bed reactor and oxygenation membrane for bubble-free aerobic mineralization of dichloromethane (DCM) and dichloroethane (DCA; Herbst and Wiesmann 1996).

The avoidance of direct aeration by using a membrane prevented desorption of the volatile substrates. A special membrane bioreactor to mineralize DCA has been tested successfully (Freitas dos Santos and Livingston 1995). DCA was kept separated from the biofilm by a membrane and only the part of the reactor with the biofilm and without DCA was aerated. This kept the DCA concentration inside the region of the membrane and biofilm so low that almost none was stripped by air bubbles.

High bacterial concentrations growing on the surface of small sand particles (Galli 1986; Stucki et al. 1992; Niemann 1993), in porous glass particles (Burgdorf et al. 1991) or in dense flocs weighted by CaCO3 (Herbst 1995; Herbst and Wiesmann 1996) yielded high mean reaction rates of up to 1400 mg L-1 h-1 DCM and 400 mg L-1 h-1 DCA. HCl was formed during the reaction and a relatively large amount of NaOH or Ca(OH)2 had to be dosed. Lonjaret (1996) successfully tested a biotrickling filter for the treatment of DCM-loaded exhaust air.

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