The petroleum refineries as well as the chemical and petrochemical industry make an intensive usage of water. As a result, wastewater streams containing several contaminants (phenols, sulfides, ammonia, benzene, oil, etc.) create an environmental pollution problem. Water cleanup has been the object of several retrofit programs in industry, and several legislation exist that regulate and establish goals for these efforts (e.g. Clean Water Act in the USA).
Wastewater has several contaminants that make it unsuitable for discharge. For example, the total organic carbon (TOC), the biochemical oxygen demand (BOD) and the chemical oxygen demands (COD) indicate the organic matter content. Oil and grease (O&G) and total petroleum hydrocarbons (TPH) give a measure of the presence of oil, grease and other hydrocarbons. The physical characteristics of the wastewater are also adjusted before disposal. These characteristics include the total suspended solids (TSS), pH, temperature, color and odor.
In compliance with the EPA Clean Water Act of 1977, wastewater must be treated before discharge (that is, end-of-pipe treatment). Several treatment options are taken into account depending on the sludge characterization. In other words: wastewater treatment procedures are based on the type and concentration of its contaminants. For decades, the primary concern has always focused on end-of-pipe wastewater treatment. End-of-pipe solutions have been seen as the sole remedy to meet the imposed discharge limits. Scarcity of water, rising energy costs and stricter regulations on industrial effluents have created a new and different view on water usage. When pollutants are selectively removed during the process, the wastewater can be reused and/or recycled. This produces a direct impact in the overall amount of fresh makeup water usage as well as in the amount of wastewater that reaches final treatment. Afterwards, the main concern shifted towards solutions that maximize water reuse. Zero water discharge cycles became a desired goal for grassroots design or retrofit. The concept of zero discharge, although plausible, politically correct and environmentally sound, may come at the expenses of additional capital expenditure. Therefore, as tradeoffs are present, it is a matter that should be further investigated.
Takama et al. (1980) used mathematical programming to solve a refinery example for water allocation. Wang and Smith (1994) presented a method for single contaminants based on targeting. IThe basic concept underlying the methodology is mass exchanger network (MEN) technology, which was in turn first proposed by El-Halwagi and Manousiouthakis (1989) and was applied to the removal of phenol from refinery wastewater (El-Halwagi and Manousiouthakis, 1990). Wang and Smith (1994) also explored options of regenerating wastewater even when the pollutant level has not reach end-of-pipe conditions, or has not be reused throughout the entire process. Dhole et al. (1996) popularized this methodology calling it the "water pinch". Several authors have studied the problem since. We have developed:
Necessary conditions of optimality for single and multiple contaminant problems.
Two procedures that can be implemented by hand to obtain optimal structures when only one contaminant is present
An Linear procedure for solving water allocation problems with regeneration and with heat integration included.
A tree-type branch and bound procedure for multiple contaminant problems.
Studies on Zero-Discharge cycles.
Software
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