A COMPARISON OF EXPANDED-BED GAC REACTOR DESIGNS FOR THE TREATMENT OF REFRACTORY INHIBITORY WASTEWATERS

A hybrid granular activated carbon (GAC) expanded-bed reactor designed to decouple biological removal from physical removal and a GAC expanded-bed reactor were studied under similar operating conditions. The hybrid GAC reactor consisted of a GAC expanded-bed reactor which was retrofitted with a GAC adsorber containing 11.1% of the total GAC mass in the system. A synthetic wastewater composed of acetate and 3-ethylphenol (3-EP) at influent concentrations of 5 g acetate/l and 1.5-2.5 g 3-EP/l was used at COD loadings of 18.7 23.8 kg COD/m3. Partial replacement of GAC in both reactors facilitated the physical removal of 3-EP. GAC was replaced from only the side adsorber in the hybrid GAC reactor while GAC was replaced directly from the GAC reactor. Previous studies indicated that increasing GAC replacement rates washed biomass out of the GAC reactor while the hybrid reactor was resilient to changes in operating conditions. This study compared the reactors while maintaining a constant GAC replacement rate with an increasing influent loading of the inhibitory/refractory compound, 3-EP. The performance of the GAC reactor deteriorated with increasing 3-EP loading rates until reactor failure occurred due to inhibition. The hybrid reactor responded to increases in the influent 3-EP loading rate with greater than 60% biological removal of 3-EP and only minor changes in effluent quality. Termination of GAC replacement in the hybrid reactor allowed for almost complete biological removal of 3-EP in the hybrid reactor. The resilience of the hybrid GAC reactor was attributed to the accumulation and maintenance of a stable microbial population capable of degrading an inhibitory substrate.