Girgis F. Nakhla

Adsorption isotherms: illusive capacity and role of oxygen

This paper examines the influence of molecular oxygen on the adsorptive capacity of GAC. A new experimental procedure for determining adsorption isotherms is introduced. This procedure, denoted as “anaerobic”, differs from the currently used techniques, denoted as “aerobic”, in that oxygen is repeatedly purged from the test environment. The results show that the capacity of GAC for the retention of o-cresol can increase up to 3-fold in the presence of oxygen when compared to the anaerobic capacity. The same trend is observed for the adsorption of phenol and 3-ethylphenol. It is shown that this increase in capacity cannot be attributed to biological degradation of these adsorbates in the presence of oxygen. It is speculated that this phenomenon is due to some chemical reactions between the adsorbates and molecular oxygen that are catalyzed by the activated carbon surface and occur at a different time scale than physical adsorption. Initial portions of breakthrough curves for o-cresol are very accurately predicted using capacities depicted by the anaerobic isotherm, while the total GAC adsorptive capacity for o-cresol, as determined from breakthrough experiments, appears to agree closely with the capacity predicted from the aerobic isotherm.

Dissolved oxygen effects on equilibrium and kinetics of phenolics adsorption by activated carbon.

Phenol and o-cresol have been shown to undergo oxygeninduced polymerization reactions on activated carbon that enhance their adsorption. Four different levels of dissolved oxygen (DO) were involved in the equilibrium and kinetics studies undertaken. It was found that for both phenol and o-cresol, the adsorptive capacities increase with the increase in DO concentration. The quantities of dimers and trimers formed on the carbon surface were a function of the DO level. Phenol recovery efficiencies around 70% and 25% were found for anoric and oxic isotherms, respectively. The additional capacity attained under oxic conditions was limited by the masses of DO and granular activated carbon (GAC) in the test environment

Phenol degradation by pseudomonas aeruginosa

Abstract Bench scale batch reactors were used to acclimatize and selectively enrich phenol degrading microorganisms, using raw domestic sewage as a seed of heterogeneous culture. During acclimatization process, phenol was used as the sole carbon source and the feed concentration was varied from 20 mg/l to 500 mg/l. After the acclimatization, which was attested by low effluent phenol (<1 mg/l), samples where taken from the sludge as well as the effluent and bacteriologically analyzed. It was found that Pseudomonas aeruginosa to be the predominant microbail species for phenol degradalion in this particular case.

Anaerobic toxic wastes treatment : dilution effects

Abstract The impact of waste strength on the treatability of toxic wastes such as coal gasification wastewater by the anaerobic GAC reactor operating with periodic GAC replacement was assessed by operating three units treating 30%, 60% and full strength wastewater. At a COD loading of 4.7 kg/m 3 d performance was unimpacted by dilution at all the GAC replacement rates investigated in this study to define the minimum adsorptive capacity required to overcome the wastewater toxicity, with all three reactors achieving over 94% COD removal, more than 99.9% phenol removal, 98–99% cresols removal, and virtually complete removal of dimethyl phenols. The full strength wastewater was not treatable at COD loading rates higher than 10 kg/m 3 d while the reactors treating the diluted wastewater maintained a COD removal efficiency of about 90% and over 99.5% reduction of phenolic compounds at a loading of 32–34 kg COD/m 3 d. At this loading with a biological solids residence time of less than 21 d and hydraulic retention times of 4–8 h, biological activity was not affected at all by the toxicity of the waste.

Oxygen‐induced enhancement of the adsorpttve capacity of activated charcoal

Abstract Adsorption isotherms for phenol and o‐cresol on activated charcoal at neutral pH and several dissolved oxygen concentrations were conducted at 23°C. Significant improvement in capacities were observed with increasing dissolved oxygen concentrations for the two adsorbates. These statistically‐significant additional capacities were not due to biological activities but merely due to surface chemical reactions. The improvement in capacity was directly related to the amount of oxygen per unit mass of GAC.

Simplified analysis of biodegradation kinetics of phenolic compounds by heterogeneous cultures

Abstract A simplified approach to the determination of biokinetic parameters for phenol and ortho‐cresol degradation by heterogeneous microbial cultures, which involved a modified kinetic model and inoculating acclimated microbial cultures with low substrate concentrations, was undertaken. The growth rate data for the two compounds conformed to the Monod model with the maximum specific growth rate, μmax, and the half‐saturation concentration, Ks, for phenol ranging from 0.44 to 3.3 days‐1 and from 5.9 to 18 mg 1‐1 respectively. The corresponding values for o‐cresol were 0.7 to 2.0 days‐1 and 9.3 to 40.7 mg 1‐1. The bacterial yield coefficients for phenol and o‐cresol were 0.53 and 0.60 mg VSS (mg substrate)‐1, respectively with corresponding decay rates of 0.16 and 0.13 days‐1. The simplified approach yielded results that are comparable to those of more sophisticated experimental and mathematical techniques.

Effect of solution pH on the kinetics of phenolics uptake on granular activated carbon

Abstract The impact of solution pH on the kinetics of adsorption and adsorption-reaction combination of phenol and o-cresol on Granular Activated Carbon (GAC) was evaluated at room temperature (21°C). Batch experiments were performed under oxic and anoxic conditions at pH values of 3, 7, and 11. The results showed that the equilibration time forphysical adsorption increased with the increase in pH and occurred in the time range of (7.5–11) days for the adsorption-reaction combination (oxic cases). The polymerization reactions lagged by about 10 h under all of the pH values. Diffusivity coefficients in the oxic cases increased inversely with pH while the highest difference between oxic and anoxic diffusivities was observed at neutral pH. The homogeneous surface diffusion model (HSDM) predicted accurately the anoxic batches and deviated from the oxic data under all pH conditions. Under controlled dissolved oxygen and pH conditions, the isotherm and batch equilibrium capacities matched with maximum deviation of 4%. The effect of solution pH on the rate of the DO-induced enhancement was found to follow a simplified form of self retardant reaction model. The aforementioned model showed strong predictive capability for the formation of polymers with time.

Modeling of the temperature variation effects on the polymerization reactions of phenolics on granular activated carbon

The effect of temperature variation on the kinetics of adsorption and adsorption-reaction combination of phenol and o-cresol on granular activated carbon (GAC) was evaluated throughout this study. Batch experiments were performed under oxic and anoxic conditions at temperatures of 8, 21, and 35°C. The results showed that the equilibration time for physical adsorption increased with the decrease in temperature and occurred in the time range of 7.5-11 days for the adsorption-reaction combination (oxic cases). The polymerization reactions lagged by about 10 hours under all of the temperature values. Diffusivity coefficients in the anoxic cases increased proportionally with temperature, with the highest difference between oxic and anoxic diffusivities at 35°C. The homogeneous surface diffusion model (HSDM) accurately predicted the anoxic batches and deviated from the oxic data. Temperature increases the adsorption and polymerization reaction rates, and thus the predictivity of the HSDM for both oxic and anoxic batches improves. Under controlled dissolved oxygen (DO) and temperature conditions, the isotherm and batch equilibrium capacities matched with a maximum deviation of 4%. The effect of temperature on the rate of the DO-induced enhancement was found to follow a self-retardant reaction model. The developed model showed strong predictive capability for the temporal formation of polymers.

Start‐up of sequencing batch reactors for toxic wastewater treatment

Abstract The start‐up of sequencing batch reactors (SBR) for the treatment of phenol‐bearing wastewater was achieved in about a week without any seeding, using the culture present in raw sewage only. The mixed liquor suspended solids (MLSS) concentration was grown to about 3000 mg/1 without showing any inhibition effects at phenol concentrations as high as 800 mg/1. The effluent concentrations of phenol were consistently less than 0.5 mg/1 during most of the start‐up phase. The response of the SBR to shock loadings of phenol at concentrations up to 1600 mg/I was favorable and over 75% phenol removal was maintained at a hydraulic retention time of 1 day.

State‐of‐the‐art review of bioremediation studies

Abstract The state of the art bioremediation has been rapidly expanding during the last decade. Numerous articles are being published in a remarkable number of journals. Biodegradability of all possible organic pollutants is being tested in field and lab. New technologies are being discovered to biodegrade even the most recalcitrant organic chemicals. Many new books on bioremediation studies arc also being written and edited. Information is scattered in books, journals, conferences and many other sources. The objective of this paper is to summarize the available existing literature on bioremediation studies. Issues covered are bioremediation in context with other remediation technologies, biodegradability of different organic compounds under different condition as reported in lab and fields studies, factors affecting bioremediation, and review of existing literature on modeling and case studies.