Performance and Characteristics of Aerobic Granular Sludge Degrading 2,4,6-Trichlorophenol at Different Volumetric Organic Loading Rates

Aerobic granular sludge represents a promising option for the treatment of wastewater contaminated by 2,4,6-trichlorophenol (2,4,6-TCP). Nevertheless, the need for high volumetric organic loading rates (vOLR) still constitutes a practical drawback, since the supply of an external carbon source for the treatment of low-strength industrial wastewater would imply significant costs at full-scale. This study investigates the possibility to sustain 2,4,6-TCP degrading aerobic granules with low vOLR, preserving long-term process performance. Sodium acetate was used as external carbon source for aerobic granules degrading 2,4,6-TCP (20 mg/L), and its concentration in the influent was progressively reduced from 1000 to 420 mg/L. The effects on process performance, granule properties and microbial population dynamics were investigated: complete 2,4,6-TCP removal was maintained, specific removal rates increased from 2.2 +/- 0.5 to 4.3 +/- 0.4 mg 2,4,6-TCP/g VSS per hour (mg(2,4,6-TCP)/g(VSS).h) and a shift of granule diameter toward smallest size fractions occurred. Moreover, the qualitative evaluation of polyhydroxyalkanoates (PHA) inclusions in microbial cells suggested their active role in 2,4,6-TCP degradation. The results showed that for low-strength wastewater containing toxic substances like 2,4,6-TCP, high concentrations of the external carbon source are required during the process start-up, but not in the long-term operation, thus reducing the operating costs significantly without compromising process performance: this represents an important step toward process scale-up.