STUDIES ON ADSORPTION AND BIOREGENERATION PROCESSES IN GAC SYSTEMS

Abstract

Aqueous phenolic effluents are relatively common industrial wastes, being produced in several industries and operations. Adsorption on activated carbon, and biological degradation may be used separately for removing phenolic compounds from wastewaters. However, their scopes are limited. Use of activated carbon entails prohibitive installation and operation costs, and is energy intensive, whereas treatment processes based on biological degradation arc sensitive to high phenol concentration, and pH. One treatment alternative, Bioactive Activated Carbon (BAC) has been shown to be viable in treating high strength phenolic liquor [Lin and Weber (1992)]. In this treatment option, a bioligical growth is encouraged on granular activated carbon (GAC) and both the removal mechanisms arc benefited by being in contact with each other. The bacteria are saved from the toxicity of high concentration of phenols by adsorption, whereas the service life of activated carbon is prolonged by degrading the phenol remained in the liquid medium and that desorbed from the activated carbon. This process of regeneration of activated carbon is called bioregeneration. Phenolic compounds present in effluent may differ in their adsorbability and biodcgradability. The literature survey revealed that most of the research works pertaining to activated carbon, biodegradation and the bioactive activated carbon have been conducted with phenol as target compound. Therefore, the present research work has been focussed to study some of aspects of these phenolic compounds and bioactive activated carbon systems. These include the adsorption of these compounds, namely phenol, 4-nitrophcnol, resorcinol, and catechol, on activated carbon, the degradation kinetics of phenol and catechol using P. putida (MTCC 1194) and removal kinetics of phenol and catechol in bioactive activated carbon. The results obtained in the thesis have been described in the following paragraphs. Adsorption isotherm and kinetics studies for four phenolic compounds on activated carbon in Basal Salt Medium at 30 °C were carried out in completely mixed batch reactor. In kinetics experiments, the initial concentrations were varied from 1000 mg/1 to 500 mg/1 with fixed carbon dose of 5 g/1. The four phenolic compounds showed the similar adsorption behaviour. Approximately 50-60% of the ultimate capacity was realized within one hour of contact. Further analysis of the kinetics data using Weber-Morris plot indicate that the rate of adsorption was intraparlicle diffusion controlled. From these plots, it may be concluded that there were three regions of change of rate of adsoiption, termed as rapid, medium, and slow. The values of effective diffusion coefficients calculated using Vermeulen (1953) approximation were of almost same magnitude and order (10"13 cm2/s) at all concentrations and for four phenolic compounds. The percent removal was found to increase with the decreasing initial concentration, however, the uptake of the compounds on the activated carbon decreases with the decreasing initial concentrations. 10 g/1 of carbon dose was required to remove 95% of the initially present (1000 mg/1) phenol, resorcinol and catechol whereas the corresponding carbon dose in case of 4-nitrophenol was found 6 g/1. This implies the high affinity of 4-nitrophenol to activated carbon. Adsorption isotherm data were fitted to Langmuir, Freundlich, Redlich-Peterson, Radke- Prausnitz, Toth, and Fritz-Schlunder isotherm models in order to represent these data by a single isotherm model. Statistical analysis showed that the Redlich-Peterson, Radke-Prausnitz and Fritz-Schlunder model could represent experimental data well within 12% of the maximum deviation and the correlation coefficient more than or equal to 0.98. The relatively simple two parameter model of Freundlich could represent the data of resorcinol and phenol equally well. Langmuir's model gave the poorest fit in all the cases. The adsorption capacities of the four phenolic compounds were in the order: 4-nitrophenol > catechol > phenol > resorcinol. Batch growth studies were performed to study the removal of phenol and catechol using P. putida (MTCC 1194). The Basal Salt Medium (BSM) was used as growth medium. The lyophilized culture of P. putida (MTCC 1194) were revived and could be acclimatized to phenol and catechol upto 1000 mg/1 and 500 mg/1 respectively over a period of three months. However, the same strain could not be acclimatized to the as low as 1 mg/1 initial concentration of 4- nitrophenol and resorcinol. The wcll-acclimalizcd culture of P. putida (MTCC 1194) degraded the initial phenol concentration of 1000 mg/1 and initial catechol concentration of 500 mg/1 completely in 162 hours and 94 hours respectively. Further, batch growth studies indicate the presence oflag phases at higher concentration of compounds. The higher the concentration, the longer the lag phase was observed. The trend of specific growth rate with initial concentrations of both phenol and catechol showed that these two are inhibitory compounds. Monod's, and Linearized Haldane's model could not represent the growth kinetics over the concentration range studied. However, Haldane's growth kinetics could be fitted to the growth kinetics data well for the entire concentration range studied. Statistical analysis indicate that the maximum deviation and the correlation coefficient, R2 for the fit of this model to the data of phenol arc 9.24 % and 0.99 respectively, and the corresponding values in case of catechol are 12.85% and 0.99. Further, the decay coefficient have been found 0.0056 hi'1 and 0.0067 hr'1 for growth on phenol and catechol respectively . Besides, the yield coefficient for growth on phenol and catechol were found 0.65 mg/mg and 0.51 mg/mg respectively. The bacterial strain P. putida (MTCC 1194) acclimatized to phenol and catechol separately could be immobilized on activated carbon. Most of the bacteria have been found to be adsorbed within fifteen hours of contact and reached equilibrium in 24 hours in both the cases. The removal of phenol at initial concentrations of 900, 1000, 1200 and 1400mg/1 and of catechol at initial concentrations of 400, 500, 600 and 700 mg/1 was complete, unlike biological degradation, where the phenol and catechol could not be degraded beyond 1000 mg/1 and 500 mg/1. The removal of phenol and catechol started immediately without any lag phase. The photomicrograph of biofilm coated activated carbon indicated the presence of dense growth of bacterial mass in the pits and crevices, scattered bacteria over the smooth surface of activated carbon and those grown on the surface of the macropore. The thickness of biofilms formed on the activated carbon have been estimated to be in the range 20-45 pm. The bioregencration study indicates that there was 45-60% regeneration of activated carbon. It is our view that the outcome of this research work shall be useful in the modelling and design of bioactive activated carbon units treating effluents containing high concentrations of phenol and catechol.