Wastewater Storage Minimisation through the Exploitation of Inherent Storage

Abstract

Abstract The need for wastewater minimisation in batch processes is gaining importance as environmental pressures are forcing industry to seek means to reduce effluent. One of the most economical means of reducing effluent is through wastewater recycle/reuse. Wastewater recycle/reuse in batch processes is either done in the form of direct reuse, from unit to unit, or indirect reuse through intermediate storage. The reuse of wastewater through intermediate storage allows for the bypassing of the inherent time dimension present in batch processes. Intermediate storage, however, occupies a substantial amount of space. This is undesirable in batch operations since these operations are usually undertaken in limited spaces. In general, a trade-off exists between the size of wastewater storage and the savings achieved through wastewater reuse. However, an unexplored wastewater storage option exists in form of idle processing units. In any batch process there are processing units that are idle for most of the time within the time horizon of interest. These units are those units that are not the bottleneck in the process recipe. The fact that the processing units are idle means that the full return on investment is not being realised. Furthermore, any processing unit is in essence a storage vessel. Therefore, idle processing units can be used as wastewater storage vessels. In doing this the utilisation of the idle processing units increases, thus increasing the return on capital investment. The size of the required intermediate storage vessel also decreases and in certain cases, the usage of inherent storage increases reuse possibilities. This concept was first considered by Majozi et al .[1]. The authors considered the usage of inherent storage in their graphical wastewater minimisation technique. The mathematical method presented in this paper deals with wastewater minimisation in batch processes through the usage of inherent storage in idle processing vessels. The method proposed determines the minimum wastewater target and the corresponding minimum size of the central storage vessel through the usage of inherent storage.