Zero effluent operation

Generally, wastewater is produced at the end of a batch and then reused for the processing of a subsequent batch of material. In general, the unit operation considered in wastewater minimisation in batch processes both consumes and produces water. Furthermore, the operations considered are generally mass transfer type processes, where mass is transferred to the water stream due to the operation occurring in a unit. In such operations wastewater reuse between the various units is governed by timing considerations and inlet and outlet concentration limitations. However, in processes where water consumption and wastewater production do not occur in the same operation, a unique opportunity arises in that the wastewater could be reused in the operations that consume water. 

The type of operation in which such an opportunity arises occurs often where product is produced, with water as a large constituent, and wastewater is generated 

Majozi, Batch Chemical Process Integration, DOI 10.1007/978-90-481-2588-3 8, Springer Science+Business Media B.V. 2010 

Reuse of this nature has a number of advantages. Firstly, wastewater produced is reduced since it is reused in product. In situations where the amount of water used in product is more than the amount of wastewater produced, it is then possible that all the water is reused and there is no effluent from a cleaning operation. Furthermore, the reuse allows for the capturing of the product residue that is left in the processing unit. This could account for substantial economic gain. Lastly, the amount of water that is used in product is reduced and water that would normally be discarded is now used in product, which also allows for some economic gain. 

The formulations presented in this section are based on the type of wastewater reuse described above, where wastewater is produced from a cleaning operation and reused in product. However, due to the fact that product integrity is a high priority a number of conditions must be ensured. Firstly, the reuse of wastewater contaminated with a certain residue can only be reused in compatible product. In most instances, this is the same product as the residue. Secondly, wastewater containing different contaminants cannot be stored at the same time in the same storage vessel. Finally, the water used for a cleaning operation has to be of the same quality as water used in product. 

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A natural progression from the scheduling of zero effluent operations is the derivation of a formulation that synthesises batch plants operating in the zero effluent mode of operation. The problem to be solved is slightly different to the general scheduling problem addressed in the formulation presented previously. 

The zero effluent synthesis formulation not only determines the optimal number and size of the storage and processing units, but also determines the schedule that will allow the resulting plant to operate in a near zero effluent operation. This is beneficial since scheduling considerations are taken into account during the plant synthesis phase. The exact problem considered in this formulation is given in the following section. 

The type of operation considered in the zero effluent methodology means that the amount of time points used for an operation has to increase. This is due to the fact that there is a processing step and a cleaning step associated with each batch of product. Normally two time points are used to describe a task in a unit. The first time point is used when the task commences in a unit and the second when the task terminates in a unit. In the type of operation considered in the zero effluent models, three time points are used. At the first time point the raw material processing task commences. The raw material processing step ends at the second time point, where the final product is removed and the cleaning operation commences. At the third time point the cleaning operation comes to an end and wastewater is produced. 

The methodology deals with two types of problems, namely, the wastewater minimisation problem within a given plant structure and the plant synthesis problem. Each of these is dealt with in the form of two mathematical formulations. The first mathematical formulation deals with the scheduling of an existing operation as to produce near zero effluent. The second mathematical formulation deals with the... 

Illustrative Examples Using the Zero Effluent Mode of Operation... 

Two illustrative examples are presented. The first deals with the scheduling of a small batch process operating in zero effluent mode and the second deals with the synthesis of a small batch operation. 

Fig. 8.2 Process diagram for first illustrative example operating in zero effluent mode...

The methodology presented above deals with the scheduling and synthesis of operations operating in a near zero effluent fashion. The effluent is reduced by reusing wastewater as part of product formulation. Reuse of water in this manner allows, under the correct conditions, the generation of near zero effluent. 

Two formulations were derived. The first deals with minimising the amount of effluent produced from an operation where wastewater can be reused in product formulation and the plant structure is known. The minimisation is achieved by scheduling the operation in such a manner as to maximise the opportunity for wastewater reuse. The second deals with the synthesis of a batch process operating in zero effluent mode. The formulation determines the number and size of processing and storage vessels as to minimise the cost of the equipment and the amount of effluent produced from the resulting operation, while achieving the required production. 

The zero effluent synthesis formulation was applied to a second illustrative example. In the example the number of processing units and the size of the central storage vessel were not known. The resulting plant required only 3 processing units and no storage vessel. The resulting schedule produced 68% less effluent than the same operation without wastewater reuse. 

Norris, P. J. (1998). Water reclamation operations in a zero effluent mill. TAPPI Proc.— Environmental Conf. Exhibit Proc. 1998 TAPPI Int. Environmental Conf. Exhibition, Part 3, April 5-8 1998, Vancouver, Canada, 3, 1045-1050, TAPPI Press, Norcross, GA. 

Past methodologies for wastewater rninirnisation in batch processes have been mainly focused on mass transfer based operations. In such operations water is consumed at the beginning of a unit operation and produced at the end. Reuse between different units is governed by availability of wastewater and the concentration of the contaminants present in the wastewater. Also, operations do exist where wastewater is produced as a result of a cleaning operation. If products produced from such operations require water as a raw material, it should be possible to reuse the wastewater as part of product formulation, since the wastewater is only contaminated with the residue in the previous batch of the same or another compatible product. The wastewater, when reused in this manner, is significantly reduced, hence the plant can operate in a near zero effluent fashion. Furthermore, the residue present in the wastewater is recovered, which could provide substantial economic benefits. 

A methodology for the synthesis of batch plants incorporating the zero effluent mode of operation has been presented. In the zero effluent mode of operation, the wastewater generated in the operation is reused as a constituent in a batch of subsequent compatible product. The methodology determines the optimal size and number of processing vessels and wastewater storage vessels. 

The management of impurities is an important issue both in Process Design and Operation. A key point is the interaction between these two aspects. Strict environmental regulations forbid the dump of harmful materials in the environment. Therefore waste minimisation, aiming to zero effluents, is a fundamental feature of sustainable process design. The increased number of recycles in integrated plants, makes the control and the operation much more difficult. 

As the plant to be optimized considers a process operating at steady state, then the variation of the phase concentrations with time is zero. For this reason, the mathematical model that describes the plant is a set of ordinary differential equations, as the phase concentrations depend only on the module axial position. In the tanks, the concentrations are constant. The differential-algebraic nonlinear optimization (DNLP) problem PI to be solved includes the ordinary differential equations that represent the mass balances for the phases in the membrane module. The objective function to be maximized is the amount of metal processed FeC , where Fe is the effluent flow rate whose Cr(VI) concentration after dilution from wastewaters is C . The problem has the following form ... 

Downwind of a gas fiare operated by the East Bay Municipal District Sewage Treatment Plant in Oakland, a maximum of 110 ng/meter was detected compared with an upwind ambient reading of zero. No mercury vapor was detected in the air immediately over the effluent water leaving the filtration treatment plant nor was any detected downwind of other sewage treatment plants in the area. 

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