Upgrading TMP Whitewater for Internal Recycle: Biological and Physicochemical Treatment and Effects on Pulp Quality

The removal of contaminants from thermomechanical pulp (TMP) mill whitewater systems will allow mills to achieve better system closure. Research conducted at Forest Research has centered on developing a high-rate system to upgrade mill whitewater for recycling within the pulp mill without causing negative effects to process operations or pulp quality. The resultant 'biokidney' is a combination of a thermophilic aerobic moving bed biological reactor (MBBR) and dissolved air flotation (DAF). The thermophilic reactor reduced soluble COD, DOC and extractives by 45, 44 and 59%, respectively. However, a parallel study has indicated higher removals are possible. When operated at the high COD load (20 kg COD/m3/d) the reactor effluent was darker in colour and contained more suspended solids than the untreated whitewater. The study investigated flotation, ultrafiltration, filtration, settling or coagulation for the removal of this material. Flotation with chemical coagulation and flocculation was determined to be the most suitable post-treatment, and could achieve TSS, colour and resin removals of 95, 66 and 99%, respectively. Residual colour in the effluent could be removed by standard pulp bleaching agents. The effect of the treated whitewater on pulp properties was evaluated using handsheets and brightness pads. Handsheets prepared in the upgraded whitewater (MBBR and DAF) showed improved tear, tensile and burst, compared with handsheets made in untreated whitewater, and was comparable to handsheets prepared in deionised water. Brightness was lower than in the control handsheets. The brightness loss could be prevented by bleaching the treated whitewater or removed by washing the pads in cold deionised water. Overall, the 'biokidney' shows potential for in-mill treatment of TMP whitewater, allowing reductions in fresh water and effluent volumes, and is unlikely to impair pulp mill operations or product quality.