An Overview of Water And Waste-Water Treatment

We may organize water treatment technologies into three general areas: Physical Methods, Chemical Methods, and Energy Intensive Methods. Physical methods of wastewater treatment represent a body of technologies that we refer largely to as solid-liquid separations techniques, of which filtration plays a dominant role. Filtration technology can be broken into two general categories - conventional and non-conventional. This technology is an integral component of drinking water and wastewater treatment applications. It is, however, but one unit process within a modern water treatment plant scheme, whereby there are a multitude of equipment and technology options to select from depending upon the ultimate goals of treatment. To understand the role of filtration, it is important to make distinctions not only with the other technologies employed in the cleaning and purification of industrial and municipal waters, but also with the objectives of different unit processes.

Chemical methods of treatment rely upon the chemical interactions of the contaminants we wish to remove from water, and the application of chemicals that either aid in the separation of contaminants from water, or assist in the destruction or neutralization of harmful effects associated with contaminants. Chemical treatment methods are applied both as stand-alone technologies, and as an integral part of the treatment process with physical methods. 

Among the energy intensive technologies, thermal methods have a dual role in water treatment applications. They can be applied as a means of sterilization, thus providing high quality drinking water, and/or these technologies can be applied to the processing of the solid wastes or sludge, generated from water treatment applications. In the latter cases, thermal methods can be applied in essentially the same manner as they are applied to conditioning water, namely to sterilize sludge contaminated with organic contaminants, and/or these technologies can be applied to volume reduction. Volume reduction is a key step in water treatment operations, because ultimately there is a tradeoff between polluted water and hazardous solid waste.

Energy intensive technologies include electrochemical techniques, which by and large are applied to drlnking water applications. They represent both sterilization and conditioning of water to achieve a palatable quality. All three of these technology groups can be combined in water treatment, or they may be used in select combinations depending upon the objectives of water treatment. Among each of the general technology classes, there is a range of both hardware and individual technologies that one may select from. The selection of not only the proper unit process and hardware from within each technology group, but the optimum combinations of hardware and unit processes from the four groups depends upon such factors as:

1. How clean the final water effluent from our plant must be;
2. The quantities and nature of the influent water we need to treat;
3. The physical and chemical properties of the pollutants we need to remove or render neutral in the effluent water;
4. The physical, chemical and thermodynamic properties of the solid wastes generated from treating water; and
5. The cost of treating water, including the cost of treating, processing and finding a home for the solid wastes.

To understand this better, let us step back and start from a very fundamental viewpoint. All processes are comprised of a number of unit processes, which are in turn made up of unit operations. Unit processes are distinct stages of a manufacturing operation. They each focus on one stage in a series of stages, successfully bringing a product to its final form. In this regard, a wastewater treatment plant, whether industrial, a municipal wastewater treatment facility, or a drinlung water purification plant, is no different than, say, a synthetic rubber manufacturing plant or an oil refinery. In the case of a rubber producing plant, various unit processes are applied to making intermediate forms of the product, which ultimately is in a final form of a rubber bale, that is sold to the consumer.

The individual unit processes in this case are comprised of: (1) a catalyst reparation stage - a pre-preparation stage for monomers and catalyst additives; (2) polymerization - where an intermediate stage of the product is synthesized in the form of a latex or polymer suspended as a dilute solution in a hydrocarbon diluent; (3) followed by finishing - where the rubber is dried, residual diluent is removed and recovered, and the rubber is dried and compressed into a bale and packaged for sale. Each of these unit process operations are in turn comprised of individual unit operations, whereby a particular technology or group pf technologies are applied, which, in turn, define a piece of equipment that is used along the production line. Drinking water and wastewater treatment plants are essentially no different. There are individual unit processes that comprise each of these types of plants that are applied in a succession of operations, with each stage aimed at improving the quality of the water as established by a set of product-performance criteria. The criteria focuses on the quality of the final water, which in the case of drinking water is established based upon legal criteria (e.g., the Safe Drinking Water Act, SDWA), and if non-potable or process plant water, may be operational criteria (e.g., nonbrackish waters to prevent scaling of heat exchange equipment). The number and complexity of unit processes and in turn unit operations comprising a water purification or wastewater treatment facility are functions of the legal and operational requirements of the treated water, the nature and degree of contamination of the incoming water (raw water to the plant), and the quantities of water to be processed. This means then, that water treatment facilities from a design and operational standpoints vary, but they do rely on overlapping and even identical unit processes.

If we start with the first technology group, then filtration should be thought of as both a unit process and a unit operation within a water treatment facility. As a separate unit process, its objective is quite clear: namely, to remove suspended solids. When we combine this technology with chemical methods and apply sedimentation and clarification (other physical separation methods), we can extend the technology to removing dissolved particulate matter as well. The particulate matter may be biological, microbial or chemical in nature, As such, the operation stands alone within its own block within the overall manufacturing train of the plant. Examples of this would be the roughening and polishing stages of water treatment. In turn, we may select or specify specific pieces of filtration equipment for these unit processes.

The above gives us somewhat of an idea of the potential complexity of choosing the optimum group of technologies and hardware needed in treating water. To develop a cost-effective design, we need to understand not only what each of the unit processes are, but obtain a working knowledge of the operating basis and ranges for the individual hardware. That, indeed, is the objective of this book; namely, to take a close look at the equipment options available to us in each technology group, but not individually. Rather, to achieve an integrated and well thought out design, we need to understand how unit processes and unit operations compliment each other in the overall design.