Gypsum needle crystallization from a suspension of calcium sulfate hemihydrate grains in an aqueous solution is studied on a mesoscopic scale. We build a simulation model, which assumes that gypsum formation is limited by heterogeneous nucleation and precipitation with an initial autocatalytic surface-controlled needle growth and a final diffusion-controlled growth. The model introduces a minimal number of parameters whose effect on growth dynamics and gypsum morphology is analyzed. We find that the increase of nucleus number per hemihydrate grain decreases induction time and needle length and increases needle entanglement, i.e., improves the mechanical properties of the material. The simulation results correctly reproduce experiments with and without treatment of the reactants. The simulation model can be used to predict the behavior of the reactive system in conditions that are not compatible with experimental observations.

Various processes that have been successfully developed for wastewater treatment (treatment of industrial wastes/ effluents) have been surveyed with special reference to biological treatment including design of bioreactors. Limitations of each process, design and performance characteristics of different kinds of bioreactors developed starting from stirred tanks to packed bed, fluidized bed, moving bed, semifluidized bed, inverse fluidized bed, sludge bed/sludge blanket and downflow stationary fixed film bioreactors have been highlighted. Utilization of membrane-based technology and liquid phase oxygen technology in wastewater treatment has also been analyzed. Both aerobic and anaerobic processes have been considered and possibilities of clubbing waste treatment with waste utilization (production of valuable products from waste streams) have also been surveyed and scrutinized.