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Water minimization in the process industry is becoming increasingly important as environmental legislation becomes increasingly stringent and the awareness of the impact of industrial activities on the environment increases. Much work has been done on water minimization in continuous processes as evidenced by the detailed reviews of Bagajewicz (2000) and Foo (2009). Although water minimization for batch processes (batch water network in short) has been ignored in the past, it is steadily gaining more attention in research. An overview of the developments and methodologies proposed for batch water network is presented. The methodologies for water minimization can roughly be divided into insight-based and mathematical techniques. The former always consists of a two-step approach (targeting and design) in synthesizing a batch water network that features the minimum freshwater and wastewater flows for a given production schedule. The mathematical techniques, on the other hand, may be categorized into two subs...

Wastewater minimisation in batch plants is gaining importance due to intensifying environmental legislation and the gradual reduction in the number of freshwater sources. Intrinsic in the minimisation of wastewater in batch plants is the reuse of wastewater through intermediate storage vessels. However, the intermediate storage vessels take up unnecessary space which is undesirable in processes which are generally undertaken in limited spaces. Furthermore, in any batch process there are processing units that are not used extensively in the time horizon. In other words, these units remain idle for the major part of the time horizon, amounting to wasted return on capital investment. The idle processing units can be used as storage vessels, since any processing unit is, in essence, a storage vessel. In doing this one can reduce the size of the central storage and increase the utilisation of capital intensive processing units. The methodology presented in this paper deals with the minimisation of single contaminant wastewater by exploiting the inherent storage possibilities in idle processing units. The methodology is applied to two cases. In the first case the objective is to minimise the amount of effluent and the size of the central storage vessel through the usage of inherent storage, as commonly encountered in grassroot design. In the second case the objective is to determine the minimum wastewater target through the usage of both inherent storage and fixed central storage, as encountered in retrofit design.

To date, most work on water network synthesis has been focusing on a single water network. The increase of public awareness toward industrial ecology has inspired new research into interplant water integration (IPWI). In this context, each water network may be grouped according to the geographical location of the water-using processes or as different plants operated by different business entities. Water source(s) from one network may be reused/recycled to sink(s) in another network. In this work, two different IPWI schemes, that is, “direct” and “indirect” integration are analyzed using mathematical optimization techniques. In the former, water from different networks is integrated directly via cross-plant pipeline(s). A mixed integer linear program (MILP) model is formulated and solved to achieve a globally optimal solution. In the latter, water from different networks is integrated indirectly via a centralized utility hub. The centralized utility hub serves to collect and redistribute water to the indiv...

This paper presents the least constrained mass transfer mathematical formulation for freshwater minimization in multipurpose batch chemical processes with central reusable water storage. The mathematical formulation is an extension of the model developed by Majozi [T. Majozi, Wastewater minimization using central reusable water storage in batch processes, Computers and Chemical Engineering Journal 29 (7) (2005) 1631–1646]. In the latter model four scenarios were considered with various limitations or constraints. In the scenario presented in this paper only the mass load is fixed, whilst both the quantity of water used in a particular operation and outlet concentration are allowed to vary. In essence, fixing the mass load is more representative of the practical case. A solution procedure for the resultant nonconvex mixed integer nonlinear programming (MINLP) model is also presented. The solution procedure first involves reformulating the MINLP into a relaxed linear model (MILP). The MILP is first solved, the solution of which forms a feasible starting solution for the MINLP. Presented are two illustrative examples.

The mathematical technique presented in this paper deals with wastewater minimization within a multiple contaminant environment, where there are multiple storage vessels available for the storage of wastewater. In the multiple storage vessel situation, it is possible to dedicate certain storage vessels to the storage of wastewater with specific contaminants. The mathematical technique is extended to include operations where wastewater produced in one batch is reusable as feed for subsequent batches of the same product. This type of operation enables a plant to operate in an almost zero-effluent fashion. Product integrity is ensured by not allowing different types of wastewater to mix and storing each type of wastewater in a dedicated storage vessel. In both cases, the mathematical model determines the minimum wastewater target and the corresponding production schedule.

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.

Wastewater minimisation in chemical processes has always been the privilege of continuous rather than batch plants. However, this situation is steadily changing, since batch plants have a tendency to generate much more toxic effluent compared to their continuous counterparts which are usually encountered in bulk manufacturing. Past methodologies for wastewater minimisation in batch processes have focused on operations based on mass transfer. They do not take into consideration the reuse of wastewater as part of product formulation. Reusing wastewater in product formulation has the major advantage of negating much of the effluent produced, thereby enabling a process to operate in an almost zero-effluent manner. Presented in this paper is a mathematical technique for the simultaneous design and scheduling of batch operations operating in a near-zero-effluent manner. The technique determines the number and size of the processing vessels, while ensuring maximum water reuse in product. The technique was applied to an illustrative example, and an 80% savings in wastewater was achieved, with a corresponding plant design that achieves the required production.Keywords: zero-effluent, batch process, wastewater minimisation