The bleaching operation of a kraft pulping process generates a large amount of effluents with high organic loads. The affinity of biomass-based fly ash as a low-cost adsorbent for reducing lignin concentration, chemical oxygen demand (COD) and turbidity of the bleaching effluents prior to a biological treatment process was studied in this work. The results showed that the optimized conditions for obtaining the maximum removals of lignin (81.6%) and COD (67.9%) were the fly ash dosage of 6 wt% in the effluent and the treatment time of 30 min and temperature of 30 degrees C. The results also showed that the settling rate of the fly ash in fly ash/effluent suspension systems increased as fly ash dosage increased. Charge neutralization played a key role at a low fly ash dosage (less than 2 wt%) for removing lignocellulosic flocs smaller than 10 mu m, while complexion of small flocs to larger ones (with the size of 30-500 mu m) occurred at the higher fly ash dosage in the effluent. Coagulation and adsorption were the main phenomena for the removals of COD, lignin and turbidity at a higher fly ash dosage and a prolonged treatment time.