Page 249 - Industrial Wastewater Treatment, Recycling and Reuse
P. 249

Novel Technologies for the Elimination of Pollutants  223




              190 C (374 F) and a pressure of 18 bar (256 psi) results in COD elimina-
              tion by 85%. The remaining COD is reduced aerobically by a further 90% in
              the biological wastewater treatment plant, thus resulting in a total COD
              elimination of 98%. Since the effluent contains up to 5% chlorides, the
              column has to be lined with titanium to prevent corrosion.
                 A further example from the pulp industry shows how, using the
              LOPROX process and relatively mild reaction conditions, long-chain
              molecules can be broken down and the resultant fragments then degraded
              easily. The untreated wastewater has a COD of 20,000–30,000 ppm and
              contains a maximum 50% of easily degradable substances. In trials with
              the LOPROX process, it was possible to reduce the COD to 8000–
              9000 ppm (66%) in about one hour in a one-step operation at a temperature


              of only 150 C( 300 F). In addition, there was a drastic improvement in
              the biodegradability of the effluent. Subsequent treatment in a one-step aer-
              obic test unit reduced the COD by a further 92%, resulting in total COD
              elimination of more than 97%.



              5.3 BAYER TOWER BIOLOGY (Holzer et al., 1992)
              Bayer Tower Biology has been used successfully for more than 20 years for
              the biological treatment of industrial effluents (Figure 5.9). This technology
              represents a new generation of wastewater treatment plants. Bio-oxidation
              is carried out in closed tanks holding a water column between 10 and 25 m
              (33 and 82 ft) high rather than in conventional open basins.
                                                                3
                 The activation tanks, with a volume of up to 15,000 m (4 million gal),
              are usually made from steel and are aerated with specially developed Bayer
              injectors. These are two-phase injectors in which air is dispersed into very
              fine bubbles by the kinetic energy of a pressure water stream.
                 A large number of these injectors are installed at regular intervals on the
              bottom of the activation tank. The wastewater itself is used as the pressure
              water for the operation of the injectors.
                 The injectors distribute the wastewater evenly over the whole of the bot-
              tom of the Tower Biology unit, simultaneously and intensively mixing it
              with atmospheric oxygen. The water/air stream emitted by the injector is
              aimed at the bottom of the tank to prevent sedimentation. This system also
              means that the air bubbles can rise through the full height of the tank,
              remaining in the liquid for the longest possible time.
                 The large gas/water interface resulting from the fineness of the bubbles,
              the long residence time of the bubbles as they rise through the tank, and the
   244   245   246   247   248   249   250   251   252   253   254