BACKGROUND: There is a lack of systematic studies of biodegradation of mixtures of VOCs with different hydrophobicities in two-phase partitioning bioreactors (TPPBs). The role of tailored mixtures of solid polymers on the biodegradation of MEK (low hydrophobicity), toluene (moderate hydrophobicity) and hexane (high hydrophobicity) was evaluated under steady state operation and transient loading (2- and 3.6-fold 4 h step increase) in TPPBs. Two mixtures of polymer beads (A and B) were selected based on their 2 h partition coefficients for the target VOCs tested, biocompatibility and resistance to microbial attack. RESULTS: The addition of polymer mixture A (20%) into the bioreactor resulted in a severe microbial inhibition, likely due to the leaching of a polymer component, however, the presence of polymer mixture B (20%) supported removal efficiencies (REs) comparable with those recorded in the absence of polymers during steady state operation (hexane, toluene and MEK REs of 7%, 76% and 98%). However, the presence of polymer mixture B supported enhanced MEK REs during the 2-fold step increase, and increased toluene removal during the 3.6-fold step increase compared with the system without polymers. CONCLUSIONS: TPPBs with tailored polymer mixtures can improve process performance during VOC transient loadings, however, the interactions between the target VOCs and the solid polymers used should be a key selection criterion in order to avoid microbial inhibition during TPPB operation. (C) 2010 Society of Chemical Industry