The Biodegradation of 4-Chlorophenol in a Moving Bed Biofilm Reactor Using Response Surface Methodology: Effect of Biogenic Substrate and Kinetic Evaluation

4-Chlorophenol (4-CP) is a persistent organic pollutant commonly found in petrochemical effluents. It causes toxic, carcinogenic and mutagenic effects on human beings and aquatic lives. Therefore, an environmentally benign and cost-effective approach is needed against such pollutants. In this direction, the chlorophenol degrading bacterial consortium consisting of Bacillus flexus GS1 IIT (BHU) and Bacillus cereus GS2 IIT (BHU) was isolated from a refinery site. A composite biocarrier namely polypropylene-polyurethane foam (PP-PUF) was developed for bacterial cells immobilization purpose. A lab-scale moving bed biofilm reactor (MBBR) packed with Bacillus sp. immobilized PP-PUF biocarrier was employed to analyse the effect of peptone on biodegradation of 4-CP. The statistical tool, i.e. response surface methodology (RSM), was used to optimize the process variables (4-CP concentration, peptone concentration and hydraulic retention time). The higher values of peptone concentration and hydraulic retention time were found to be favourable for maximum removal of 4-CP. At the optimized process conditions, the maximum removals of 4-CP and chemical oxygen demand (COD) were obtained to be 91.07 and 75.29%, respectively. In addition, three kinetic models, i.e. second-order, Monod and modified Stover-Kincannon models, were employed to investigate the behaviour of MBBR during 4-CP biodegradation. The high regression coefficients obtained by the second-order and modified Stover-Kincannon models showed better accuracy for estimating substrate degradation kinetics. The phytotoxicity study supported that the Vigna radiata seeds germinated in treated wastewater showed higher growth (i.e. radicle and plumule) than the untreated wastewater.

A Computational Approach to Incorporate Metabolite Inhibition in the Growth Kinetics of Indigenous Bacterial Strain Bacillus subtilis MN372379 in the Treatment of Wastewater Containing Congo Red Dye

A rigorous knowledge of the bacterial growth kinetics is essential for the scaling-up and optimization of biodegradation process conditions in a bioreactor. Although a great deal of literature is available on the modeling of bacterial growth kinetics considering the inhibition at high substrate-loading, the inhibition caused by toxic metabolic byproducts was not accounted in the bacterial growth kinetics. This work primarily aimed at developing a parametric bacterial growth model to account for metabolite inhibition, indicated by a decelerating log-phase growth, which was rarely discussed in the previous studies. An efficient azo-dye degrading bacterium (Bacillus subtilis MN372379) was isolated from the sludge-waste nearby a carpet-dyeing unit. The isolated bacterial strain was used to decolorize the simulated wastewater containing Congo red dye. This study proposed a computational approach to calculate specific bacterial growth rate time-averaged over the entire sigmoidal log phase (including the decelerating phase) for incorporating the effect of metabolite-inhibition, in contrast to the conventional studies where only the initial part (accelerating) of log phase was considered. The nature of metabolite inhibition was also determined and found to be non-competitive. Next, the computed time-averaged specific bacterial growth rate was incorporated into three substrate inhibition models to account for both, the metabolite and substrate inhibitions, and subsequently their kinetic parameters were also determined. Finally, the initial dye concentration and inoculum size were optimized to yield maximum dye utilization rate. This study paves the way for predicting bacterial growth kinetic with improved accuracy to enable a better optimization of bioreactors at the industrial scale.

Collective removal of phenol and ammonia in a moving bed biofilm reactor using modified bio-carriers: Process optimization and kinetic study

The performance of a moving bed biofilm reactor (MBBR) with bio-carriers made of polypropylene-polyurethane foam (PP-PUF) was evaluated for the collective removal of phenol and ammonia. Three independent variables, including pH (5.0-8.0), retention time (2.0-12.0 h), and airflow rate (0.8-3.5 L/min) were optimized using central composite design (CCD) of response surface methodology (RSM). The maximum removal of phenol and ammonia was obtained to be 92.6, and 91.8%, respectively, in addition to the removal of 72.3% in the chemical oxygen demand (COD) level at optimum conditions. First-order and second-order kinetic models were analyzed to evaluate the pollutants removal kinetics in a MBBR. Finally, a second-order model was found to be appropriate for predicting reaction kinetics. The values of second-order rate constants were obtained to be 2.35, 0.25, and 1.85 L2/gVSS gCOD h for phenol, COD, and ammonia removal, respectively.

London-van der Waals Force Field of a Chemically Patterned Surface To Enable Selective Adhesion

The London-van der Waals (L-vdW) force between a particle and a surface strongly depends on the topography and the chemical properties of the interacting surfaces. Although a great deal of work has been done to understand the effect of topographical heterogeneity on the L-vdW adhesion, the role of chemical heterogeneity has been discussed only rarely. This study makes an attempt to quantify the magnitude and range of the L-vdW force acting on a spherical particle in the vicinity of a chemically patterned surface. Specifically, an ideal system of a smooth spherical particle approaching a surface composed of parallel stripes of chemically distinct materials with different Hamaker constants is considered. The L-vdW forces for such systems are determined by solving the London dispersion potential for the entire volumes of the adhering bodies from first principles, using Hamaker's microscopic approach. The computational results elucidate that a chemical interface can apply a tangential L-vdW force, in addition to the normal L-vdW force, on nearby particles. This can cause lateral motion of particles neighboring a chemically inhomogeneous surface. The magnitude of the tangential L-vdW force is found to be maximum when the particle is centered at the interface and shows a gradual drop as it moves away from this location. The magnitude and range of the tangential L-vdW force can be large for large colloidal particles in close contact with a chemically patterned surface whose materials have distinct Hamaker constants. This study suggests that the tangential L-vdW force field generated by a chemical interface can be utilized as a tool to manipulate the path of an approaching particle to facilitate selective adhesion.

Approximate scheme for calculating van der Waals interactions between finite cylindrical volume elements

A successful approach to calculating van der Waals (vdW) forces between irregular bodies is to divide the bodies into small cylindrical volume elements and integrate the vdW interactions between opposing elements. In this context it has been common to use Hamaker's expression for parallel plates to approximate the vdW interactions between the opposing elements. This present study shows that Hamaker's vdW expression for parallel plates does not accurately describe the vdW interactions for co-axial cylinders having a ratio of cylinder radius to separation distance (R/D) of 10 or less. This restricts the systems that can be simulated using this technique and explicitly excludes consideration of topographical or compositional variations at the nanoscale for surfaces that are in contact or within a few nm of contact. To address this limitation, approximate analytical expressions for nonretarded vdW forces between finite cylinders in different orientations are derived and are shown to produce a high level of agreement with forces calculated using full numerical solutions of the corresponding Hamaker's equations. The expressions developed here allow accurate calculation of vdW forces in systems where particles are in contact or within a few nm of contact with surfaces and the particles and/or surfaces have heterogeneous nanoscale morphology or composition. These calculations can be performed at comparatively low computational cost compared to the full numerical solution of Hamaker's equations.

Modeling and validation of the van der Waals force during the adhesion of nanoscale objects to rough surfaces: a detailed description

The interactions between nanoparticles and rough surfaces are of great scientific and engineering importance and have numerous applications in surface science and biotechnology. Surface geometry and roughness play crucial roles in observed particle adhesion forces. We previously developed a model and simulation approach to describe adhesion between microscale bodies. This work provides detailed descriptions of the modeling framework, with associated experimental validation, applied to nanoscale systems. The physical systems of interest include nanoscale silicon nitride adhering to different surfaces in both dry and aqueous environments. To perform the modeling work, precise descriptions of the geometry of the particle and the roughness of the particle and substrate were generated. By superimposing the roughness and geometry models for the particle and the substrate, it was possible to precisely describe the spatial configurations of the adhering surfaces. The interacting surfaces were then discretized, and the adhesion force between the two surfaces was calculated by using Hamaker's additive approach, based on van der Waals interactions. In the experimental work, an atomic force microscope (AFM) was used to measure the adhesion force (pull-off force) between nanoscale silicon nitride cantilever tips and a range of substrates in different environments. The measured and predicted force distributions were compared, and good agreement was observed between theory and experiment.