BIODEGRADATION DYNAMICS OF PHENOLIC WASTEWATERS USING GLIOMASTIX INDICUS MTCC 3869

Abstract

Environmental pollution due to phenolic compounds is a major ecological problem as a result of their use in various industries. Phenol, resorcinol, and p-cresol are the phenolic compounds found in the effluents of many industries. They have harmful effects on every living being. Phenolic compounds present in a mixture cause problem during their biodegradation because different operating conditions like pH and temperature may be required for the biodegradation of different substrates. Hence, the analysis of substrate interaction taking place among these multiple substrates is a crucial step. Besides, the biodegradation technology fulfils the need of an economical and ecofriendly technology for the proper treatment of phenolic wastewaters. Use of biological treatment facility provides advantage of low capital and operating cost, no harmful by-product formation with simple installation of the treatment unit. Hence, in the present research work the biodegradation technique has been used for the removal of three phenolic compounds phenol, resorcinol, and p-cresol. Literature sources revealed that most of the authors reported the biodegradation of phenolic compounds using bacteria and fungi. The studies involving fungi for biodegradation have been conducted for the detection of metabolic pathway and only a few authors have worked on biodegradation kinetics. Most of the research on biodegradation kinetics is centred on biomass growth kinetics only. There is a lack of quantitative analysis of substrate expenditure for maintenance of biomass, effect of maintenance energy expenditure on biomass growth yield and the biodegradation dynamics of substrates. The aim of this research work was to investigate these aspects of biodegradation of phenol, resorcinol, and p-cresol in single and dual substrate biodegradation systems. The batch biodegradation experiments were conducted for phenol, resorcinol, and p-cresol as single and dual substrate systems using pure culture of filamentous fungus Gliomastix indicus MTCC 3869. Modified czapeck medium was used to carry out the experiments under the pH of 6 and temperature 28 °C. Biodegradation of phenol, resorcinol, and p-cresol was studied up to the initial concentration of 1000, 1300, and 700 mg/L, in the single substrate biodegradation system, starting from the lowest concentration of 10 mg/L. The dual substrate biodegradation was carried out using the three substrates as phenol — p—cresol and phenol — resorcinol, in three combinations for each dual substrate system (100 mg/L phenol in presence of 300 mg/L p—cresol/resorcinol, 200 mg/L phenol in presence of 200 mg/L p—cresol/resorcinol, 300 mg/L phenol in presence of 100 mg/L p—cresol/resorcinol). Initially the fungus was acclimatized, supplying 2% glucose in the medium with the toxic substrates. 5 % V/V inoculums was taken in 250 mL flasks with working volume of 50 mL for biodegradation kinetic experiments. The lag phase was found to be completed within 19, 15, and 24 h for phenol, resorcinol and p—cresol respectively. Five single substrate inhibition growth kinetic models were applied for the analysis of biomass growth kinetics of the fungus for the biodegradation of phenol, resorcinol, and p—cresol as single substrates. Predictions of Andrews and Noack model were found in best agreement with the experimental data of specific growth rate, for phenol and p—cresol. For resorcinol Yano model was found to be best fit to specific growth rate data. During the experimentation phenol, resorcinol, and p—cresol were observed inhibitory to biomass growth and self biodegradation beyond initial concentration of 70, 90, and 50 mg/L respectively. The maximum specific growth rate value 0.129 h-' at 70 mg/L of phenol, 0.132W' at 90 mg/L of resorcinol, and 0.102 h-' at 70 mg/L of p—cresol were achieved. Similarly maximum observed biomass growth yield values of 0.437 g/g, 0.443 g/g and 0.31 g/g were obtained for phenol, resorcinol, and p—cresol at their respective inhibitory initial concentrations. Eight models (M1, M2, M3, M4, M5, M6, M7, and M8) analogous to growth inhibition kinetic models were used to examine the single substrate degradation kinetics of phenol. M1 to M4 are specific degradation rate models and M5 to M8 are initial specific degradation rate models. M1 and M8 were found to be best fit to the experimental degradation data of phenol. Generally, initial specific degradation rate models are applicable when the substrate degradation rate is too slow that was not the case here, therefore only the specific degradation rate models were considered for resorcinol and p—cresol. The specific degradation rate models M3 and M1 were best fit models for the experimental degradation rate data of resorcinol and p—cresol respectively. A maintenance energy model for the estimation of substrate consumption for biomass maintenance was proposed. The minimum value of maintenance energy coefficient 0.020 h-1 at 70 mg/L of phenol, 0.0135 h-1 at 90 mg/L of resorcinol, and 0.0229 h-' was obtained at 50 mg/L of p-cresol. The maximum values of maintenance energy coefficient were obtained at 1000 mg/L, 1300 mg/L, and 700 mg/L for phenol, resorcinol, and p-cresol respectively. Beyond the inhibitory initial concentration the maintenance energy coefficient values tend' to increase with the initial concentration of the substrate due to the substrate inhibition. Therefore, it was found that the specific growth rate and observed biomass growth yield values are reduced beyond the inhibitory initial concentration of the substrate due to the increased substrate consumption for biomass maintenance during biodegradation process of toxic substrates as phenol, resorcinol, and p-cresol. Three models model - a, model - b and model - c were developed to investigate the biodegradation dynamics of single substrate degradation. The set of mathematical equations corresponding to each model were solved to get computed profiles of substrate biodegradation with time. The effect of variation in maintenance energy expenditure and observed biomass growth yield was incorporated in model - a. Model - b was based on the initial biodegradation rates and the variation of maintenance and observed biomass growth yield was not considered in case of model - c. Predictions of model - a were identified in best agreement with the experimental data in entire range of initial substrate concentration for phenol, resorcinol, and p-cresol. Predictions of model - b were not found in .agreement with the experimental data of degradation for any of the substrate. Predictions of model - c showed a bit agreement with experimental data of phenol and p-cresol in lower substrate concentration range only. For resorcinol, the predictions of model - c were not in close agreement with the experimental degradation data. Hence, the effect of maintenance energy variation is important to consider, for the study of biodegradation dynamics of inhibitory substrates as phenol, resorcinol, and p-cresol. The experiments on the two dual substrate biodegradation systems, phenol -- p-cresol, and phenol - resorcinol were carried out to study the substrate interaction during the biodegradation of the two substrates. The substrate interaction between phenol and p-cresol was studied using three combinations of these substrates; 100 mg/L phenol with 300 mg/L p-cresol, 200 mg/L phenol with 200 mg/L p-cresol, and 300 mg/L phenol with 100 mg/L p-cresol. Similarly, three combinations of phenol and resorcinol were used to study the substrate interaction between them. A model to describe the specific growth rate and substrate interaction was derived and solved by Levenberg-Marquardt nonlinear regression technique. Four types of substrate inhibitions were H tested. Interaction parameter values ('at = 0.044, Iu 2 = 1.17) revealed that the phenol inhibited the degradation of p-cresol more than the p-cresol caused inhibition to phenol degradation in the medium. For phenol - resorcinol degradation system the interaction parameter values (I= 1.09, 'a2 = 0.052) indicated that the resorcinol posed stronger inhibition to the phenol degradation in comparison to the inhibition caused by phenol to resorcinol degradation. The competitive cross inhibition was observed to be involved during the biodegradation of two homologous substrates phenol and p-cresol, phenol and resorcinol. For the study of substrate degradation kinetics with the substrate consumption as maintenance energy expenditure in dual substrate systems, a conceptual model was developed which incorporates the variation of maintenance energy expenditure and specific growth rate with the variation in the concentrations of the two substrates in their mixture. To the best of our knowledge, there is no other study available in the literature for the estimation of maintenance energy expenditure in the dual substrate systems. The work presented here, is a reasonable starting point for the development and validation of mathematical model to describe the maintenance energy expenditure and thereby the specific degradation rate for mixtures of two homologous substrates. The specific degradation rate values were estimated for each substrate phenol, resorcinol, and p-cresol in the two dual substrate degradation systems. The model has provided excellent predictions of substrate degradation rate with the variation in maintenance energy expenditure. Biodegradation of phenol, resorcinol, and p-cresol was modelled in dual substrate degradation system. A set of equations was developed by incorporating the maintenance energy and growth yield variation, and solved to get the computed time profiles of substrate degradation. The model predictions were very close to the experimental data well. It is our view that the proposed models for the biodegradation of phenolic compounds provide in depth knowledge of the biodegradation of organic pollutants, prediction of microbial growth, and substrate degradation dynamics for phenolic waste-water treatment in single as well as in the dual substrate systems, which may be useful for the design of a biodegradation facility.