In waste paper processing mills, entirely closed process water systems cause high specific polluting problems, deteriorate the quality of the paper produced and corrode the equipment.

Operating an anaerobic fluidized bed on part of the circulating water could make it possible to achieve lower dissolved organic concentration in the closed process water system.

Laboratory experiments have been carried out according to the following flow-sheet : Water circuit; Acidification tank; Fluidized bed.

When the anaerobic treatment is added to the water circuit, COD removal is 75% in the circuit and 65% in the acidification tank. Consequently, until reaching steady state, the organic load entering the fluidized bed decreases continuously from 28.75 to 9.86 kg COD/m3 d−l without disturbing biomass activity.

From a mass balance, a mathematical model is drawn to check the COD removal in the water circuit and study the transitory period.

Efficient fluidized bed performances are achieved even with high organic load (28.75 kgCOD/m3d−1) 52% to 75% COD removal are obtained in the water circuit respectively within 75 and 24 hours. The treated water represents a very small part of the circulating water. To achieve 52% COD removal the equivalent of 0.5 m3/t produced paper is treated. For a 75% COD removal efficiency, treated flow rate is 2.6 m3/t produced paper.

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