Zero-effluent processing plants

Generally, the process liquid effluents released in the metallurgical industry are manageable both in quantity and quality. The process liquid effluent consists of water containing small amounts of dissolved solids, and extensive recycling of the effluent is carried out with a view not only to obtain zero effluent discharge but also to minimize freshwater input to the plant... 

Gouws, J., Majozi, T., 2008. Impact of multiple storage in wastewater minimisation for multicontaminant batch plants towards zero effluent. Ind. Eng. Chem. Res., 47 369-379 Majozi, T., Zhu, X., 2001. A novel continuous-time MILP formulation for multipurpose batch plants. 1. Short-term scheduling. Ind. Eng. Chem. Res. 40(23) 5935-5949 Quesada, I., Grossmann, I. E., 1995. Global optimization of bilinear process networks with multicomponent flows. Comput. Chem. Eng. 19 1219-1242... 

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. 

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. 

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. 

Better environmental performance. Modem process design should aim to zero-effluent plants by minimisation of gaseous emissions and of process waste, including wastewater. 

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. 

There are other such examples. Many unsaturated soils are known to convert to N2(g> via NO3 (i.e., nitrification then denitrification). They achieve this because there are local oxic and anoxic environments in the soil waters that, respectively, allow nitrification and denitrification to proceed. In stratified lakes, nitrification may occur in the oxygenated epilimnion (upper layer) and denitrification in the hypolimnion (bottom water) and in the sediment pore water where dissolved oxygen concentrations fall to zero. The nitrification and denitrification process is important in preserving the fishery in Indian Creek Reservoir in the Sierra Nevada mountains. This reservoir is fed by the tertiary effluent from the City of South Lake Tahoe sewage treatment plant. The effluent has at times contained 15 to 20 mg NH4 N/liter. Levels of ammonia of this magnitude are toxic to fish, yet in the reservoir there is a thriving fishery. This is achieved because the top waters of the lake nitrify the ammonia to nitrate and this is reduced to N2(g) by the anoxic bottom waters. The summer concentrations of nitrogen species of the reservoir are approximately 4 mg NOa -N/Iiter and 4 mg NH4-N/liter. 

The cold and the Demethanizer column section are cooled down and pressurized. Deethanizer vapors are flared at the overhead of Deethanizer reflux drum until Acetylene content is zero in the Acetylene reactor effluent. Effluents are diverted to C2 splitter column and flaring is gradually discontinued. The flaring operation at startup and shut-down typically lasts 7-8 days at QChem s process plants. This is typical of most processing plants in Qatar and elsewhere. 

Membrane technology may become essential if zero-discharge mills become a requirement or legislation on water use becomes very restrictive. The type of membrane fractionation required varies according to the use that is to be made of the treated water. This issue is addressed in Chapter 35, which describes the apphcation of membrane processes in the pulp and paper industry for treatment of the effluent generated. Chapter 36 focuses on the apphcation of membrane bioreactors in wastewater treatment. Chapter 37 describes the apphcations of hollow fiber contactors in membrane-assisted solvent extraction for the recovery of metallic pollutants. The apphcations of membrane contactors in the treatment of gaseous waste streams are presented in Chapter 38. Chapter 39 deals with an important development in the strip dispersion technique for actinide recovery/metal separation. Chapter 40 focuses on electrically enhanced membrane separation and catalysis. Chapter 41 contains important case studies on the treatment of effluent in the leather industry. The case studies cover the work carried out at pilot plant level with membrane bioreactors and reverse osmosis. Development in nanofiltration and a case study on the recovery of impurity-free sodium thiocyanate in the acrylic industry are described in Chapter 42. 

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 ... 

Not all power plant designs fit into an upstream or downstream category. Integrated systems let carbon move through the entire process, but they prevent normal dilution of the output flue gas, so that the effluent is concentrated CO2. While most of the plants in this category are still in an early development phase, they promise to combine high efficiency, virtually zero atmospheric pollution, and complete capture of all CO2. All avoid the intake of air. 

Perhaps the most exciting promise of PAC is to make process lines so efficient and so well-controlled that zero-defect products are released together with a decrease in effluent. Imagine the good will such efficient plants might generate in the community. 

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