Treating thermomechanical pulping wastewater with biomass-based fly ash: Modeling and experimental studies

In the present study, biomass based fly ash was used to remove lignocellulose from the spent liquor (SL) of a thermomechanical pulping process. The effect of independent parameters, e.g. time, temperature and dosage, on the performance of fly ash in treating SL was studied using an optimal design of response surface methodology. Under the optimal conditions (time 160 min, temperature 40 °C and fly ash dosage of 150 g/L), the removals of turbidity, COD and lignin were 75.1%, 61.3% and 63.5%, respectively. Furthermore, the sedimentation and drainability of fly ash/SL systems were investigated to separate fly ash from SL after the treatment. The sedimentation analysis showed that fly ash would not be settled from the system fast, but sedimentation was improved at a higher temperature of 60 °C than 30 °C. Interestingly, the drainability analysis showed that fly ash could be separated from the SL by filtration. Increasing the fly ash dosage from 120 g/L to 180 g/L reduced the average filtrate rate from 95 mL/min to 25 mL/min, and it was improved from 35 mL/min to 75 mL/min with the temperature increasing from 25 °C to 75 °C. Agitation intensity did not affect the filtration performance, but it increased the solid content of the filtrate from 0.30 wt.% to 0.35 wt.%.