Effects of black liquor shocks on activated sludge treatment of bleached kraft pulp mill wastewater

Abstract

Kraft pulp mills use activated sludge systems to remove organic matter from effluents. Process streams may appear as toxic spills in treatment plant effluents, such as black liquor, which is toxic to microorganisms of the activated sludge. The present study evaluates the effects of black liquor shocks in activated sludge systems. Four black liquor shocks from 883 to 3,225 mg chemical oxygen demand−COD L−1 were applied during 24 hours in a continuously operating lab−scale activated sludge system. Removal efficiencies of COD, color and specific compounds were determined. Moreover, specific oxygen uptake rate (SOUR), sludge volumetric index (SVI) and indicator microorganisms were evaluated. Results show that the addition of black liquor caused an increase in COD removal (76−67%) immediately post shock; followed two days later by a decrease (−19−50%). On the other hand, SOUR ranged between 0.152 and 0.336 mgO2 g−1 volatile suspended solids−VSS• min−1 during shocks, but the initial value was reestablished at hour 24. When the COD concentration of the shock was higher than 1,014 mg/L, the abundance of stalked ciliates and rotifers dropped. Finally, no changes in SVI were observed, with values remaining in the range 65.8−40.2 mL g−1 total suspended solids−TSS during the entire operating process. Based on the results, the principal conclusion is that the activated sludge system with the biomass adapted to the kraft pulp effluent could resist a black liquor shock with 3,225 mgCOD L−1 of concentration during 24 h, under this study's conditions.