Make your process water pay for itself

Environmental protection, rising costs for wastewater-treatment, and at many sites a shortage of fresh water are all persuasive motives for reducing raw water consumption and wastewater discharge at a chemical-process plant. Maximizing the re-use of water within the plant can be of great help. Systematic strategies for such maximization can lower freshwater usage and wastewater discharges by 50% or more, while also significantly reducing capital investment in treatment facilities. The typical base case or starting point for such improvements appears in a figure which shows a conventional water network in a process plant. After undergoing initial treatment, the incoming water goes in parallel streams to the various individual process units, as well as to the utility system for steam production and for use in cooling towers. Wastewater streams from the processes, along with blowdown and condensate losses from the utility system, are usually collected together and the combined stream fed to a wastewater treatment facility prior to discharge. There are two basic strategies for reducing water demand in such a plant. One strategy consists of modifying individual process and utility units to reduce their inherent need for water. Examples include replacing water cooling with air cooling, improving controls of boilermore » and cooling-tower blowdowns, and increasing the number of stages in an extraction unit that employs water as its extractant. In the other basic strategy, which is the main focus of this article, the engineer seeks opportunities to use the outlet water from one operation to satisfy the water requirement of another or the same operation. In some cases, the water may require some regeneration prior to re-use. Examples of regeneration include pH adjustment, filtration, membrane separation, sour-water stripping and ion exchange.« less