A NOVEL METHOD FOR BIOLOGICAL TREATMENT OF BLEACHED KRAFT MILL WASTEWATERS

Removal of adsorbable organic halogen (AOX) and chemical oxygen demand (COD) from bleached kraft mill effluent was investigated in laborator y and pilot scale aerobic suspended carrier reactors and abiotic therm o-alkaline reactors. Laboratory treatment focused on determination of loading capacity of the suspended carrier process and operation of the process at elevated temperature and pH values in order to combine abi otic thermo-alkaline dechlorination and biological treatment in one re actor. At pH 7, 37-degrees-C and hydraulic retention times (HRTs) long er than 3.5 hours, a maximum COD removal of 55% was achieved in the su spended carrier process. The COD conversion rate at the minimum HRT wa s 2.6 kg COD/(m3d). The suspended carrier treatment was operated succ essfully at pH 9.0 and 45-degrees-C and at pH 7.0 and 50-degrees-C wit h over 50 percent COD removal with a HRT of four hours. The AOX remova l achieved at pH 9 and 45-degrees-C (50%) was significantly higher tha n the removal at pH 7 and 37-degrees-C (39%), most certainly because o f an increased abiotic dechlorination at the higher pH and temperature levels. Sequential thermo-alkaline and biological treatment were stud ied on a pilot scale. Thermo-alkaline treatment at pH 10, 54-degrees-C and a HRT of two hours followed by biological treatment at pH 8, 35-d egrees-C and a HRT of four hours, removed almost 80% of AOX and 50% of COD from the kraft mill effluent.