Varadarajan Ravindran

Hybrid membrane filtration process for leachate treatment

Abstract A hybrid technology known as the ultrafiltration-biologically active carbon (UF-BAC) process that amalgamates adsorption, biodegradation and membrane filtration is found to be highly efficient for treatment of landfill leachates. The process employs bioactive powdered activated carbon (PAC) with a leachate-acclimated microbial culture for the simultaneous sorption and biodegradation of organic constituents. Tubular cross-flow ultrafiltration membrane modules separate out colloids and microorganisms, and a high quality permeate is obtained. Batch biokinetic studies were performed for the two leachates to evaluate their extent of biodegradability and biodegradation kinetics. The process efficiencies for both leachates were in the range of 95–98% in terms of TOC removal, and exceeded 97% for specific organic pollutants. The UF-BAC process compared well with the PACT process in terms of organic removal, and produced higher quality effluent in terms of suspended solids (100% removal). The study demonstrated that addition of 1% PAC mitigated permeate flux deterioration attributed to membrane fouling and concentration polarization, and enhanced permeate transport. Possible mechanisms for flux amelioration are discussed.

GAC adsorber design protocol for the removal of off-flavors

Abstract This paper investigates the effectiveness of granular activated carbon (GAC) for removing off-flavor compounds of water—geosmin and 2-methylisoborneol (MIB). More specifically, it focuses on the development of an appropriate modeling approach and experimental protocol for the design of fixed-bed GAC adsorbers. Adsorption equilibrium, rate and long-term mini-column studies were conducted on a bench-scale for geosmin and MIB to estimate the equilibrium and mass-transfer parameters required for adsorber modeling. The dispersed flow homogeneous surface diffusion model (DFHSDM) was used for the prediction/simulation of the adsorber dynamics. Scale-up procedures based on dimensional analysis and similitude were employed for the design of full-scale adsorbers from bench-scale adsorbers, and for performance forecasting of full-scale adsorbers under different operating conditions. More importantly, operation and maintenance (O & M) costs were estimated for full-scale adsorbers directed at the removal of off flavor compounds, based on carbon utilization rates and disposal costs. These estimates were obtained for different plant capacities and empty bed contact times (EBCTs).

Bioadsorber efficiency, design, and performance forecasting for alachlor removal

This study discusses a mathematical modeling and design protocol for bioactive granular activated carbon (GAC) adsorbers employed for purification of drinking water contaminated by chlorinated pesticides, exemplified by alachlor. A thin biofilm model is discussed that incorporates the following phenomenological aspects: film transfer from the bulk fluid to the adsorbent particles, diffusion through the biofilm immobilized on adsorbent surface, adsorption of the contaminant into the adsorbent particle. The modeling approach involved independent laboratory-scale experiments to determine the model input parameters. These experiments included adsorption isotherm studies, adsorption rate studies, and biokinetic studies. Bioactive expanded-bed adsorber experiments were conducted to obtain realistic experimental data for determining the ability of the model for predicting adsorber dynamics under different operating conditions. The model equations were solved using a computationally efficient hybrid numerical technique combining orthogonal collocation and finite difference methods. The model provided accurate predictions of adsorber dynamics for bioactive and non-bioactive scenarios. Sensitivity analyses demonstrated the significance of various model parameters, and focussed on enhancement in certain key parameters to improve the overall process efficiency. Scale-up simulation studies for bioactive and non-bioactive adsorbers provided comparisons between their performances, and illustrated the advantages of bioregeneration for enhancing their effective service life spans. Isolation of microbial species revealed that fungal strains were more efficient than bacterial strains in metabolizing alachlor. Microbial degradation pathways for alachlor were proposed and confirmed by the detection of biotransformation metabolites and byproducts using gas chromatography/mass spectrometry.

Emissions of Volatile and Semi-Volatile Organic Compounds and Particulate Matter from Hot Asphalts

Asphalts are widely used in paving of roads, and water-proof sealing of building roofs, tanks and containers. This study evaluated the qualitative and quantitative characteristics of emissions from hot asphalts and bitumen that included reactive organic gases (ROGs) and particulate matter (PM). The ROGs consisted of several volatile organic compounds (VOCs), and semi-volatile organic compounds (SVOCs) of environmental concern. The latter included several polynuclear aromatic hydrocarbons (PAHs) and alkanes. An experimental laboratory testing, sampling and analysis protocol was developed for obtaining efficient and cost-effective a priori estimates of asphalt emissions. The investigation identified and quantified the emissions of organics and evaluated the magnitudes as well as particle size distributions of PM emissions. The study demonstrated that the asphalt type and temperature greatly affected the emission characteristics, and that several organic compounds emitted were partitioned between gaseous and...

Modeling of bioactive carbon adsorbers: a hybrid weighted residual-finite difference numerical technique

Phenomenological mathematical models incorporating adsorption, mass transfer, and biofilm degradation were developed for performance prediction/simulation of bioactive carbon fixed-bed and fluidized-bed adsorbers in wastewater treatment. The model equations were solved by a numerical technique combining a weighted residual technique such as orthogonal collocation with finite difference method. This hybrid technique was numerically consistent and stable and provided accurate solutions at computing times lower than those corresponding to pure orthogonal collocation. The bioadsorber model parameters were independently determined from carefully designed laboratory-scale experiments and correlations. The model predictions of bioadsorber effluent concentration profiles were in strong agreement with the experimental data, illustrating the good predictive capability of the model. Sensitivity studies were performed to identify the influence of model parameters on the bioactive adsorber dynamics.

Biological denitrification of reverse osmosis brine concentrates: II. Fluidized bed adsorber reactor studies

This phase of the study (Part II) investigates the application of a high-rate fluidized bed adsorber reactor (FBAR) process for biological denitrification of reverse osmosis (RO) brine concentrate....

Predictive Modeling for Bioactive Fluidized Bed and Stationary Bed Reactors: Application to Dairy Wastewater

Bioadsorber models were formulated, and a systematic protocol was developed for adsorber modeling and design. These models were used for forecasting the performance of bioactive and non bioactive reactors in two configurations: fluidized bed reactor (with effluent recycle), and stationary bed reactor. The models incorporated important phenomena associated with adsorption and biofilm degradation, and used total organic carbon (TOC) as a lumped parameter for measuring the wastewater strength. The protocol could be easily applied to bioactive reactors with non-adsorbing media such as sand, polymers and glass. The protocol included the development of experimental techniques for evaluating model parameters. Laboratory-scale adsorption equilibrium and rate studies were conducted to evaluate the equilibrium and mass-transfer parameters, respectively. Biokinetic experiments were performed to determine the biodegradation parameters including kinetic coefficients for biological growth and decay, and the yield coeff...

Thermal Destruction of Chlorinated Hydrocarbons by Reductive Pyrolysis

Abstract A rigorous thesmochemical analysis of the pyrolysis of aromatic chlorides shows that destruction efficiencies in excess of 99.999 percent are possible in a reducing atmosphere at temperatures near 1000 K. This process used for the destruction of chlorinated organic ompounds yields non-halogenated hydrocarbons and hydrochloric acid as reaction products. Qualitative batch experiments on chloromethane and polychlorinated biphenyls (PCBs) showed that such efficiencies could be obtained in residence times below 7 minutes at 1273 K. Qualitative continuous flow experiments demonstrated that these efficiencies are actually obtained in residence times less than 1.8 seconds for chloromethane and 4.4 seconds for the PCB mixture Arochlor 1254. A simple kinetic analysis shows that the Aroclor 1254 radicals are produced by the decomposition of hydrocarbon solvents [n-hexane and iso-octane) used to provide the reducing atmosphere. The difference in residence times between batch and continuous flow reactors is e...