Clean water is needed by chemical-process plants for a wide range of process-related and auxiliary uses. Likewise wide is the range of options available to the plant for cleaning up the raw, incoming water. Here are guidelines for making the right choice
Depending on their location and on other factors, chemical-process- industries (CPI) plants obtain their process water from a diverse range of sources (see Process Water Supply — the Big Picture Chem. Eng., May 2005, pp. 32–34.).
The water from most, if not all, of these sources requires some, if not a great deal of, contaminant removal onsite in order to make the water suitable for use. A wide array of contaminant-removal technologies is available, so it is important to choose the ones that are most appropriate for the situation at hand. Once the choices are made, certain underlying principles can help get the process design of the treatment unit off to a good start.
All water supplies contain contaminants. The type of contaminant can vary greatly, and the contaminant concentration may range from extremely low (as in the case of highly pure water requiring final polishing before use in semiconductor manufacture), to very high (as in a typical wastewater stream that is to be recycled). What constitutes a contaminant depends entirely on the application. For drinking water, for instance, the contaminants are those defined (for the U.S.) by the Safe Drinking Water Act, whereas for semiconductor rinsing, anything other than H20 is a contaminant, and the concentrations of such contaminants must be as close to zero as possible. The uses of water at CPI plants (which range from petroleum refineries to cement mills to breweries) are wide-ranging, so it is not possible to generalize the definition of “contaminant.”
It is virtually impossible to make water free of any and all contaminants. The goal of a treatment process is to reduce the contaminant levels to the extent required by the application. To more easily address that removal, it is convenient to look at contaminants by category (Table 1). Removal technologies No single technology effectively removes all contaminants. The challenge is to specify and design a system utilizing a combination of technologies that provide the optimal removal to meet the particular, use-specific water quality requirements for the situation at hand.
In general, a water treatment system consists of three basic components: pretreatment, primary treatment and posttreatment. Pretreatment technologies typically protect the primary treatment technologies from such problems as fouling and chemical degradation. The primary technologies bring the water supply to its desired quality level, and the posttreatment technologies are designed to keep the water supply at that quality level during storage and distribution. Accordingly, the key selection must be that of the primary treatment technology or technologies. This decision, in turn, will dictate the selection of pretreatment technologies. The choice of posttreatment technologies will be dictated by the need for storage and distribution, the instantaneous flow requirements, and water quality maintenance issues.
Table 2 summarizes several waterpurification processes with regard to their effectiveness in removing a particular class of contaminants. Because of the wide range of contaminants in each contaminant class, there are certainly exceptions to the effectiveness of a particular treatment process; with that in mind, this table serves as a useful and basically accurate guide.
Filtration removes suspended solid contaminants mechanically, by use of a porous medium that allows water to pass while retaining the solids. In general, filters used for water treatment consist of either (a) bed filters, namely, containers that are partially filled with a porous bed of inert particles, or (b) manufactured cartridges or bags, typically constructed of a synthetic porous fabric, and usually designed to process smaller flowrates than a bed filter.
Some bed filters or cartridge filters are classified as adsorptive filters, as discussed below. Precoat filters (see below) also serve for water pretreatment. The unit of measurement used in connection with filtration for contaminant removal is the micrometer, μm, equivalent to one-millionth of a meter (the smallest particle that can be seen by the unaided eye measures 40 μm). Bed filters. In a bed filter, the water pressure itself provides the force needed for passage through the filter medium or media. Many types of inert, uniformly sized materials can serve as the media. The bed filter shown in Figure 1 employs three: coal, sand and garnet. The flow of water can be either from bottom up or from top down. Typical bed filters can remove suspended solids down to about 10 μm.
An advantage of bed filters is that they can be backwashed to remove entrapped solids, and, therefore, these filters have very long lifetimes. Obviously, they must be taken offline during backwashing.