A model activated sludge (AS) plant receiving pulping effluents was operated on two modes. One mode was the treatment in a dedicated plant receiving no inflow other than pulping effluents, seeded during start-up, and receiving a supplemental carbon source. The second mode was the co-treatment of pulping effluents with the municipal sewage, continuously seeded both by microorganisms and organics. Higher Adsorbable Organic Halide (AOX) removal was obtained at lower Solids Retention Times (SRTs) in the co-treating AS, whereas opposite was hue for the dedicated AS, i.e. high AOX removals at long SRTs. The AOX removal mechanism was mineralization in both modes.