(637d) Biphasic Biodegradation of Phenol in an Osmotic Membrane Bioreactor Using Trioctylphosphine Oxide Impregnated Membranes As the Partitioning Phase

A membrane bioreactor (MBR) refers to the integration of a membrane filtration process into a bioreactor operating with suspended microorganisms. These bioreactors combine biodegradation and clarification, two stages in conventional activated sludge wastewater treatment process, into one single step. MBRs offer several advantages over conventional bioreactors such as high biomass concentration in the bioreactor, elimination of cell washout concerns even at high influent flow rate, high effluent quality and a low bioreactor footprint. Although MBRs are typically operated using microfiltration or ultrafiltration membranes, a new bioreactor configuration has recently been developed based on the use of forward osmosis (FO) membranes. This configuration, which is known as an osmotic membrane bioreactor (OMBR), offers several advantages over conventional MBRs: higher rejection, better effluent quality, low or no hydraulic pressure requirement, low energy consumption and low biofouling tendency.

OMBRs have been used successfully in the treatment of municipal wastewater. However, the application of OMBR in industrial wastewater treatment, especially those containing toxic and recalcitrant pollutants such as aromatic compounds, has been very limited. The primary challenge in the biodegradation of these aromatic compounds arises from their high concentration in the wastewater, which exerts substrate inhibition on the biodegrading microorganisms resulting in low or no cell growth and metabolism. This problem is aggravated in the OMBR due to the excellent rejection properties of the FO membranes, which results in high accumulation of the pollutants and an increase in the severity of their inhibitory effects. Therefore, in order to facilitate biodegradation of toxic industrial wastewater in an OMBR, the microorganisms must be protected against the detrimental effects of substrate inhibition.

In this research, biodegradation of phenol by Pseudomonas putidawas investigated in an OMBR at inhibitory phenol concentrations of 600-2500 mg/L. Trioctylphosphine oxide (TOPO) was impregnated into polypropylene membranes and added into the OMBR to alleviate substrate inhibition. When phenol concentration in the OMBR was high, the TOPO impregnated membranes absorbed phenol to maintain a sub-inhibitory phenol concentration in the bioreactor. When phenol was metabolized and its concentration in the liquid medium decreased, it was desorbed from the absorbents and was degraded by the microorganisms. Thus, the TOPO-impregnated membranes acted as a partitioning phase for phenol sequestration and prevented the microorganisms from substrate inhibition in the beginning of the bioreactor operation, and also during bioreactor operation when phenol concentration was spiked.

In the one month of continuous operation using synthetic wastewater, the OMBR was fed phenolic wastewater at 600, 1000, 2000 and 2500 mg/L concentrations and phenol concentrations up to 2000 mg/L were completely metabolized by P. putida. Biomass concentration, pH, dissolved oxygen and salinity in the bioreactor were regularly measured. While biomass concentration in the OMBR changed with the feed phenol concentration, pH and DO values were relatively stable, and the salinity of the medium increased steadily throughout the operating period. Phenol concentrations in the bioreactor and in the effluent were regularly monitored and phenol was not detected in the effluent once the OMBR achieved steady state. The permeate flux through the FO membranes was initially quite high but it gradually decreased due to an increase in the bioreactor salinity as well as due to membrane biofouling. Cell attachment on the FO membranes was characterized and a washing cycle was developed to recover membrane performance by removing the attached cells from the membranes through osmotic backwashing. The results from the OMBR operation suggest that the OMBR coupled with a partitioning phase such as TOPO-impregnated membranes can be used for rapid and effective biodegradation of toxic and recalcitrant pollutants.