An Integrated Bioreactor Activated Carbon Adsorption and Polysulfone Hollow Fiber Membrane Cell Immobilisation for Cometabolic Transformation of 4-Chlorophenol

Kai Chee Loh, Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117576, Singapore and Ye Wang, Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Dr 4, Singapore, Singapore.

The composition of wastes involved in environmental pollution is often very complicated; toxic compounds (both growth and non-growth substrates) and non-toxic, easily biodegradable compounds co-exist. Cell growth on multiple substrates can differ greatly from that on individual compounds due to substrate interactions involved. Some of the most common chlorinated solvents used industrially are known to be biodegraded through cometabolic pathways, in which the transformation of the chlorinated compounds by the micro-organisms have to be facilitated by the presence of a specific growth substrate. A well-known cometabolism system is the biodegradation of phenol and 4-chlorophenol (4-cp) by Pseudomonas putida. Here, phenol is the specific growth substrate, while 4-cp is the non-growth supporting substrate which is cometabolically transformed. Since phenol and 4-cp are structurally similar, competitive inhibition is observed during the biodegradation process. As a result, the efficiency and efficacy of the waste water treatment are severely retarded. This is further exacerbated by the fact that phenol, at high concentrations, exert substrate inhibition on the cells. In order to reduce/eliminate these substrate inhibition interactions, one proposed method is to separate the two compounds in the waste stream, and separately biodegrade them in different reactors. This, while feasible, would inadvertently add cost to the overall process.

In consideration of the inhibition effects of 4-chlorophenol (4-cp) on phenol degradation and the substrate inhibition of phenol at high concentrations in the cometabolic transformation of 4-cp, an integrated bioreactor system incorporating adsorption on granular activated carbon and hollow fiber membrane cell immobilization was fabricated and investigated. Polysulfone hollow fiber membranes were used to immobilize the cells (Pseudomonas putida ATCC 49451) within the sponge-like porous regions, while activated carbon was provided to reduce the solution concentrations of the two substrates. Under batch operation of the bioreactor, simultaneous transformation of 1600mg/L phenol and 200mg/L 4-cp could be achieved. At these concentrations, the cells were not able to grow (let alone degrade) in free suspension. By immobilizing the cells in the hollow fiber membranes, the cells could tolerate much higher concentrations of both phenol and 4-cp. Subsequently, the integrated bioreactor system was operated under semi-continuous mode at various feed rates to demonstrate its efficiency and long-term sustainable operation. The experimental results show that the bioreactor system could effectively remove 1000mg/L phenol and 400mg/L 4-cp at feed rates up to 40mL/h in the semi-continuous feed mode.