A Two-stage Batch Sorption Optimized Design for Dye Removal to Minimize Contact Time

In recent years many cheap, widely-available materials have been identi® ed as suitable adsorbents for the removal of colour from wastewaters. The sorption of various dyes on peat , onto pith and onto wood has been studied. However, only limited application of such data has been directed towards the design of adsorption treatment systems such as batch adsorber design8,9 or ® xed bed adsober design. Batch sorber design has mainly concentrated on reducing adsorbent costs, which is particularly relevant when expensive sorbent materials such as active carbon, silica, zeolites and resins are used. But for cheaper adsorbents minimizing the contact time for a ® xed percentage of pollution removal using a ® xed mass of adsorbent will result in being able to process more batches of polluted wastewater per day, thus enabling the required treatment plant items to be reduced in size, with a decrease in the plant capital cost. The cost and performance of product/equipment/system or the mode of application are always of concern to control the process ef® ciency. Therefore the sorption capacity and required contact time are two of the most important parameters to understand in a sorption process. Equilibrium analysis is fundamental to evaluate the af® nity or capacity of a sorbent. However, thermodynamic data can only predict the ® nal state of a system from an initial nonequilibrium mode. It is therefore important to determine how sorption rates depend on the concentrations of sorbate in solution and how rates are affected by sorption capacity or by the character of sorbent in terms of kinetics. From the kinetics analysis, the solute uptake rateÐ which determines the residence time required for completion of the sorption reactionÐ may be analysed and established. This approach has been adopted and is presented in the present paper. This paper studies the sorption of Basic Blue 69 (BB69) and Acid Blue 25 (AB25) dyes onto two biosorbents (wood and peat) and develops a two-stage batch adsorber design model. A design analysis method has been developed to predict the percentage of dye removal at various times of contact for a ® xed mass of adsorbent. The model enables the total contact time to be minimized to achieve a ® xed percentage of BB69 and AB25 dyes removal using a ® xed mass of biosorbent material. This minimum contact time enables the minimum size of the batch sorption equipment to be speci® ed and therefore minimize capital investment costs.