Development of mathematical models for clean up of Cr (VI) contaminated aquifers using bioremediation – A report by Indian Institute of Technology Madras

 .  In-situ bioremediation is an attractive alternative for this purpose. To achieve this objective, bacterial strains were isolated and enriched from the contaminated site of Tamil Nadu Chromates and Chemicals Limited (TCCL) premises, Ranipet, Tamil Nadu, India.

The study indicates the following -

• The strain which was isolated from the highly contaminated location had shown high Cr(VI) reduction potential.
• Cr(VI) reduction was evaluated both in aerobic and anaerobic conditions. Though the aerobic system performed better than the anaerobic one, further study were carried out in the anaerobic condition due to its economic viability.
• At higher initial concentration, Cr(VI) reduction was not complete even after 108 hrs, however, specific Cr(VI) reduction, unit weight of Cr reduced/unit weight of biomass was greater at higher concentration. 
• It was found that a bacterial concentration of 15+1.0 mg/g of soil (wet weight) 50 mg of molasses /g of soil as carbon source were required for the maximum Cr(VI) reduction. 
• Cr(VI) reduction studies were also carried out with Chromium Reducing Bacteria (CRB), Sulphate Reducing Bacteria (SRB) and Iron Reducing Bacteria (IRB), individually and in combination. 
• Biokinetic parameters such as maximum specific growth rate (μmax), half saturation constant (Ks), yield coefficient (YT) and inhibition coefficient (Ki) for individual cultures were evaluated.
• Bench scale soil column experiments were conducted to study the effectiveness of Cr(VI) containment in aquifers using in-situ bio-transformation. Batch adsorption studies were carried out to estimate the adsorption capacities of two different soils for Cr(VI) and Cr(III). Transport studies indicated that it would not be possible to contain Cr(VI) by adsorption alone. Results showed that Cr(VI) concentration was zero in wells located downstream of the biobarrier and in the influence zone of the well even after 75 days indicating that both technologies are effective in containing the plume.
• A one-dimensional mathematical model was initially developed based on one-dimensional advection-dispersion reaction equations for Cr(VI) and molasses in saturated, homogeneous porous medium. The applicability of the model, for studying effectiveness of reactive-zone method for chromium containment, is demonstrated through an illustrative example.
• Optimal design of trench type bio-barriers for the containment of Cr(VI) in aquifers was also attempted. The management problem for the optimal design of bio-barriers was formulated. A simulation optimization code using Genetic Algorithms was developed for this purpose.
• The one-dimensional simulation model is linked with the Genetic algorithm to obtain the optimal solution. Based on the optimal solutions obtained, the design charts were prepared. The results presented here indicate only a general trend in the optimal design parameters.