This study was conducted to evaluate the capability of sulfonated lignin (S-lignin) to induce the flocculation of anaerobic sludge in upflow wastewater treatment. Two upflow anaerobic sludge bed-filter (UBF) reactors fed with a synthetic sugar waste were operated at a specific loading rate of 0.6 to 1.8 g of chemical oxygen demand (COD) per g of volatile suspended solid (VSS) per day (d). One of these reactors was stepwise supplemented with a spent liquor rich in S-lignin from a neutral sulfite semi-chemical (NSSC) pulping plant to reach an addition rate of 0.17 g S-lignin/g VSS d; 50 % of the S-lignin was adsorbed onto the biomass. This resulted in pre-granulation of the anaerobic sludge: 50 to 60% of the biomass was in particles > 0.3 mm versus 70 to 80% with a size < 75 µm in the control reactor. In addition, the S-lignin supplementation slightly enhanced the specific metabolic activities of the biomass. Calcium ion addition following the S-lignin treatment enhanced the stability and the density of the aggregates.

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