STUDIES ON THE TREATMENT OF PHENOLIC WASTEWATERS USING ADSORPTION AND IMMOBILIZED WHOLE CELLS

Abstract

Phenols originate from natural as well as industrial sources. These are found to be present in the effluents of anumber of industries such as oil refineries, coke oven plants, steel plants etc. The permissible limits for phenols in industrial effluents before being discharged into municipal sewers and surface waters are specified as 5.0 and 1.0 mg I1, respectively, by the regulatory agencies in India. In steel plants, coke oven plants, petroleum refineries etc., phenolic wastewaters are generated at several processing units with phenol concentration varying from 110 to 5000 mg l1. Various treatment processes used for the removal of and/or recovery of phenols include * hot air/gas or steam stripping, adsorption, ion exchange, solvent extraction, oxidation, phase transfer catalysis and biological processes. Biological processes are considered to be very cost-effective in comparison to physico-chemical processes, however, biological processes may not be sufficient for the removal ofphenolic compounds to the desired permissible limit of 1.0 mg 1' and polishing-stage treatment may be required. Large capital investment is required for the biological treatment of large volumes of wastewaters with low phenol concentration. Also, under utmost controlled conditions, the reliability of such systems are found to be around 40%. Among other treatment methods, at lower concentrations adsorption has been found to be most efficient for the removal of most organic compounds in wastewaters. For high strength and low volumes of wastewater, phenol removal by adsorption technique may be an alternative proposition. Granular/powdered activated carbon, is the most widely used adsorbent, as it has a good capacity for the adsorption of organic molecules. However, high cost ofactivated carbon and 10 - 15 percent loss during regeneration have been 11 deterrents in the utilization of activated carbon in the developing countries. This has led to search for cheaper alternative materials as adsorbents such as bagasse pith, peat, saw dust, coal fly ash and bagasse fly ash. Biological treatment for phenol removal is practised in a number of industries where the combined plant effluent is treated for the removal of BOD in addition to reduction in phenol concentration. Phenol is, however, not readily biodegradable. It is toxic to most types of microorganisms at sufficiently high concentrations and at low concentrations, it can be inhibitory to the growth-rate of even those species which have the metabolic capability of using it as a substrate for growth. Immobilized cell technology has been used in the bio-treatment of wastewaters. Many attempts have been made to degrade phenols and other aromatic compounds by microbial free cells. Inspite of the rather extensive use of phenol biooxidation process, only few research reports are found to deal with microbial kinetics in either pure or mixed culture systems using phenol as a carbon source. Not much is known about the degradation of phenols by immobilized microorganisms using polymer and calcium alginate entrapped cells. The present study has been undertaken with the objective to investigate the suitability of bagasse fly ash as a low-cost adsorbent for the removal of phenol, resorcinol and o-cresol as a replacement of conventional activated carbons. The biodegradation of phenol has also been extensively investigated with immobilized cell systems. Adsorption studies have been carried out for evaluating the suitability of bagasse fly ash as a low cost adsorbent. The effects of various operating parameters like pH, adsorbent dose, initial phenol concentration contact time and temperature on the removal of phenols was studied using bagasse fly ash (BFA) and/or ACL and ACC in batch experiments. The iii detailed characterization pertaining to physico-chemical, structural and morphological properties of these adsorbents has been carried out. Theeffect of adsorbent dose on uptake of phenolic compounds (phenol, resorcinol and o-cresol) onbagasse fly ash, ACL and ACC was studied. In all the cases, the percent removal ofphenolic compounds increases with increase in adsorbent concentration, while removal per unit mass ofadsorbent increases with the decrease in adsorbent concentration and equilibrium was found to be attained more rapidly at lower concentrations. The adsorption of the phenolic compounds is pH specific. pH <7 is found increase phenol removal. Reductions in adsorption at high pH is possibly due to incre ased solubility of phenolic compounds and the abundance of H+ ions which increases hindrance to diffusion of phenol ions. » The percent removal of phenol, resorcinol and o-cresol for different contact times (0-300 min) and for an initial concentration of 100 mg l1 with 10 g l1 of adsorbent was also studied. It was found that the rate of phenol removal is very rapid during initial 30 minutes and there-after the rate decreases. No significant change in percent removal was observed with change in contact times. For resorcinol, equilibrium times for bagasse fly ash, ACC and ACL were found to be 120 minutes and at 240 minutes the resorcinol removal efficiency by ACL, bagasse fly ash and ACC is 92.00 %, 75.50 %and 64.00%, respectively. Further, maximum removal of o-cresol by ACL and bagasse fly ash is in the initial 120 minutes of contact time and in the initial 90 minutes for ACC. The effect of initial concentration of phenol, resorcinol and o-cresol on the adsorption on bagasse fly ash shows that for any contact time the percent removal decreases with the increase in initial concentration of phenolic compounds. The straight line plot of(l-XA/XAe) versus time (t) for adsorption of phenol, resorcinol and o-cresol show the validity of the so called Lagergren equation during initial period only. iv Various steps involved in the adsorbate transport from the solution to the surface of the adsorbent particles have been dealt with by using the Weber-Morris plots- q versus t°5 - for the three adsorbents. The rate controlling parameters - k* and k" have been determined and it is found that macropore diffusion rate is much larger than the micropore diffusion rate. Five isotherm equations : Toth, Freundlich, Langmuir, Radke-Prausnitz and Redlich- Peterson have been fitted with the equilibrium adsorption data and based on%relative absolute error it is concluded that the Toth isotherm equation shows best fit followed by Radke-Prausnitz isotherm for bagasse fly ash, ACL and ACC. Fixed bed adsorption studies for bagasse fly ash has also been carried out. The effect of bed height and flow velocity on the breakthough for the bagasse fly ash has been studied. Wolborska and Bohart-Adams model have been used for the prediction of model specific parameters. Column studies showed that the break-through point depends upon the bed height and the flow rate. Wolborska model predicts breakthrough times with good accuracy. The bed depth-service time (BDST) plots showed applicability of Bohart-Adams equation. In the second part of the investigation, unstructured models have been developed for immobilized whole cells using Briggs-Haldane equation incorporating the substrate inhibitory effects for Gaden type I fermentations. Model equations have been developed in which effectiveness factor concept and the reaction contribution in liquid space as well as immobilization matrix have been included, for the exponential as well equilibrium stage growth phases. The resultant equations for the substrate-inhibition immobilized whole cell systems could be reduced to equations for free cell and for non-inhibitory immobilized cell system. A simple computer programme has been used based on the developed model equations for the estimation of biokinetic parameters : maximum specific growth rate, pmax; Monod constant, K„, ; substrate inhibition constant, Kj. and the effectiveness factor, n. In view of the potential ofPseudomonas putida for the degradation of phenolic wastewaters, this strain was selected to study the biodegradation ofphenol. The Pseudomonas putida cells were immobUized in calcium alginate and growing synthetic medium was employed. Growth pattern ofPseudomonas putida cells immobilized in calcium alginate matrix as represented by number ofcells, was investigated. Two distinct phases of growth- exponential and equilibrium stage growth phases have been observed. Detailed experimental investigations have been carried out with respect to effect of immobilized cell concentration, initial phenol concentration and gel bead size in a batch bioreactor for these two phases. The initial phenol concentration in the range of25-2000 mg V;calcium alginate bead size 2.50-4.67 mm (Sauter mean diameter) and immobilized cell concentration 12.56-254.51 mg V(of immobilization matrix) have been studied. At about 100 mgr1 and above, substrate inhibition has been observed which could be incorporated by inhibition constant K, having avalue of 120.23 ±20.32 (SD) mg Vin Briggs-Haldane equation. The maximum specific growth rate, pmax; Monod constant, K„,; were found to be 0.400 ±0.0096 (SD) h1 and 2.54 ±0.1097 (SD) mg l1, respectively. The effectiveness factor n, varied from 0.186 to 0.879 depending upon the cell loading, bead size, growth regime and the initial phenol concentration.