Reactor development and process optimisation for the bioremediation of phenolic wastewaters by trametes species

Abstract

Contamination of soils, groundwater, sediments, surface water and air with hazardous and toxic chemicals is one of the major problems facing the industrialised world today. Of the various types of industry-generated effluents, those containing organic pollutants such as phenols are generally difficult to remediate. There is a need to develop new technologies that emphasize the destruction of these pollutants rather than their disposal. The bioremediation of phenolic effluents by white rot fungi, and the oxidative enzymes produced by them, has been the topic of much research and is widely reported in literature. This study focuses on the integration of an optimised process of enzyme production/ phenol degradation by the fungi Trametes versicolor and Trametes pubescens with an Airlift Loop Reactor found suitable for the large- scale fermentation of these organisms. Using a comparison of the growth media recommended in literature as a basis for further experimentation, a 'nutrient sufficient' medium directed towards rapid biomass accumulation and increased lignolytic enzyme (laccase) production was developed. Enzyme activity was further increased by screening a variety of known inducers and comparing them with additions of small concentrations of stripped gas liquor from a Fischer-Tropsch plant, the target effluent in this study. In this way laccase production was increased by 700%. The removal from solution of the principal effluent monomers (phenol, p-, m- and o-cresol) was studied, with up to 20% v/v of the effluent monomers being removed in flask culture. An attempt was made to characterise the effect of this effluent on the morphology and physiology on the two Trametes species, with particular emphasis on laccase production. Large scale, cost-effective applications of white-rot fungi to continuous treatment of liquid effluent has been hindered by the lack of suitable bioreactor systems. A hollow fibre membrane bioreactor and a trickle filter were considered as supports for immobilised biofilms of T. versicolor and laccase production and pollutant degradation were demonstrated in both reactor configurations. The need for a simple, cost effective, yet simple to upscale reactor system led to the investigation and development of an airlift loop reactor (ALR). These reactors have well defined flow patterns, high liquid velocities and yet provide a relatively low shear environment ideal for the growth of T. pubescens. The reactor configuration and aeration rate were optimised using recognised chemical engineering principles and favourable feeding strategies, the effects of inducers/precursors and the timing of effluent addition were ascertained. Increased final values for growth (10 g/L dry mass) and enzyme production (12000 U/L) as well as effluent degradation (5% v/v/day) were achieved in the ALR in fermentations exceeding two weeks. This study demonstrated, in the ALRJT pubescens system, the simplicity of design, low cost of operation and high performance that are essential for a successful biological wastewater treatment scheme.