FIG. 7.35.7 Activated-sludge waste wastewater treatment at a paper mill in Fors, Sweden.

produces newsprint and linerboard from thermo-mechan-ical pulp (TMP) and chemico-thermo-mechanical paper (CTMP). The periodic production of CTMP caused an effluent overload.

The activated-sludge plant had been designed for 14,000 m3/day of clarified TMP effluent with 1590 kg/day BOD. CTMP production raised this BOD load to 5000 kg/day. The schematic of the treatment plant in Figure 7.35.7 shows that the increased BOD load reduced its removal efficiency from 90 to 70%. More than twice the BOD was removed at the overload conditions than at the design conditions; 3500 versus 1431 kg/day. The mean hydraulic residence time fell from 8.6 to 3.9 hr, and the MLVSS level fell from 1500 to 1063 ppm. The effluent settler was less effective in handling the increased sludge load.

Four 300 m3/hr VENTOXAL systems were capable of adding 180 kg/hr of oxygen. This addition doubled the aeration capacity, raising the BOD removal efficiency to 95%. Figure 7.35.8 shows that the MLVSS concentration in the aeration tank now reached almost 7000 ppm. As previously discussed and shown in Figure 7.35.1, the use of pure oxygen enhances sludge settling velocities.

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