Electrocoagulation of kraft pulp bleaching filtrates to improve biotreatability

Bleached kraft pulp mills generate large volumes of effluents usually treated by biological processes that don´t remove recalcitrant organic constituents. The present study was undertaken to evaluate the potential of electrocoagulation of acid (AcF) and alkaline (AlF) kraft pulp bleaching filtrates with Al and Fe electrodes to improve their biotreatability. Response surface methodology was used to predict the effects of initial pH, current density, and electrolysis time on biodegradability (BOD/COD). Biodegradability increases of 1.7–2.1-fold were obtained using optimized experimental conditions for each electrode-filtrate combination: AcF-Al (pH 7.9, 128 A/m², 49 min), AcF-Fe (pH 4.6, 104 A/m², 40 min), AlF-Al (pH 3.8, 150 A/m², 52 min) and AlF-Fe (pH 6.3, 101 A/m², 42 min). Electrocoagulation with Al electrodes was more efficient in removing color, phenols and estrogenic activity, but required longer reaction time and higher current density, while treatment with Fe electrodes resulted in lower toxicity to Daphnia similis. Electrocoagulation of both filtrates before combining them for biological treatment led to 88 % dissolved organic carbon (DOC) removal in a five-day biodegradability test, compared to only 27 % DOC removal from combined raw filtrates. The potential to increase overall pulp mill wastewater treatment efficiency using electrocoagulation prior to biological treatment was demonstrated.