Steady State and Dynamic Biofilm Characteristics Associated with Phenol Degradation in Pulsed Plate Bioreactor

Abstract

Phenol is considered to be one of the major pollutants in effluents from various industries. Biofilm reactors are the engineered systems extensively used for continuous biodegradation of phenol in wastewater. Stable and efficient operation of any biofilm reactor depends on the biofilm characteristics. The main components of biofilms are microorganisms and exopolymeric substances (EPS). The knowledge of biofilm characteristics and the factors that influence its formation and detachment behaviour would lead to better understanding, optimization, control and stability of biofilm reactors. In the present study, the steady state and start-up performance of pulsed plate bioreactor (PPBR) in continuous phenol degradation using Pseudomonas desmolyticum (NCIM 2112) cells immobilized on granular activated carbon (GAC) as the cell carrier material was investigated and the associated biofilm characteristics were evaluated under various operating conditions of the bioreactor such as frequency and amplitude of pulsation, cell carrier loading, dilution rate and influent phenol concentration. The physical characteristics of the biofilm such as its morphology, thickness, density and biomass dry weight; chemical characteristics such as the composition of EPS in terms of its protein, carbohydrate and humic substance content were evaluated and the effect of shear inducing conditions was investigated. Dynamics of biofilm in PPBR during the start up dominantly included three phases of accumulation, compaction and plateau. The PPBR could be continuously operated under stable conditions for degradation of phenol with influent concentration of upto 800 ppm with dilution rate of 0.33 h-1 at frequency of 0.08s-1 and amplitude of 5 cm to achieve greater than 99.5% degradation. Though stable operation of PPBR was possible at higher inhibitory concentrations of 1200 ppm, it is not recommended to operate at these concentrations owing to lower efficiency of phenol degradation. The characteristics of biofilm have been found to be influenced by the PPBR operating conditions and they play a dominant role in controlling the biofilm structure and integrity thus leading to stable operation of the bioreactor. Conditions which resulted in the formation of thinner, denser and compact biofilms with uniform and smooth surfaces have always led to better performance of PPBR in terms of phenol biodegradation. Artificial neural network model has been developed to predict the biofilm characteristics at steady state and during the start-up. The present study has successfully demonstrated that PPBR is an efficient biofilm reactor and can be employed in wastewater treatment for phenol degradation without much operational difficulties associated with controlling the biofilm thickness and stability.