Remediation Of Organic Contaminants By Bioventing And Cometabolic Bioventing And Mathematic Simulations

Bioventing is an innovatory remediation technique removes biodegradable organic contaminants from the unsaturated zone. Firstly, a conceptual model is presented for describing comprehensive transport, rate limited interphase mass transfer, biodegradation, substrate interactions and cometabolism present during bioventing. The microorganisms are initially obtained from Dagang Oil Field, Tianjin, China and acclimated for about two months. Growth conditions for microorganisms are determined by experiments. Mixed microorganisms are isolated and screen out the species for degrading contaminants faster. Permeability was measured by TST-70 soil penetrate apparatus, and adsorption partition coefficients on benzene, toluene and p-xylene are measured from batch experiment. Monod coefficients (the maximum specific substrate utilization rate and half-saturation coefficient) and the yield coefficient Y are measured from batch experiments and fitted by least-square method. The volatilization and biodegradation of single contaminant (toluene) and multi-contaminants (BTX) from unsaturated soil during bioventing is studied. The experiment is done in a stainless steel column. The interactions between the contaminants are also explored by batch experiment. In gas-aqueous-solid system, 40% of removed contaminant was due to biodegradation. However, only 18% of contaminant was removed by biodegradation in gas-aqueous-NAPL-solid system. Based on conceptual model, one-dimensional model was used to forecast the remediation process of soil column experiment, and simulation results fitted well with experimental data. The effect of concentrations of toluene and TCE and temperature on cometabolic biodegradation is discussed. Results show that no TCE degradation occurred in the absence of toluene. Higher concentration of TCE may be more toxic to microorganisms. Low temperature becomes the limiting factor and the thermally enhanced biodegradation will be required. Cometabolic bioventing for removal of TCE in unsaturated zone was studied in the soil column using methane as the growth substrate. Among the total removed TCE, about 14% and 10% of TCE were biodegraded when TCE was existed in aqueous phase and NAPL phase respectively. The model of cometabolic bioventing is presented and simulations are found to provide reasonable agreement with experimental data. A two-dimensional numerical model is developed for simulating the flow, transport and biodegradation of toluene during bioventing process in the unsaturated zone. The changes of fluid fields with time were obtained and results from simulation show that the required time to develop the steady fluid fields is both about 1 hour. Numerical solutions are obtained with a Galerkin finite element approach. In this approach, the flow, transport and biodegradation equations are solved separately using Operator Splitting method. Effect of different operating conditions on remediation efficiency is detailedly studied. Compared two-dimensional simulations with data measured at site, the model has proved accurate and can be applied to full-scale studies.