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T a n g e n t i a l F l o w S e p a r a t i o n : Tangential Flow Separation: A N e w W a y T o T r e a t W a s t e A New Way To Treat Waste Can "leaky hoses" really be considered an innovation? The answer is yes — when applied (ingeniously) for liquid/solids separation. By J.H. Wakefield M ost water treatment professionals are familiar with the categorization of waste treatment processes into three general segments: 1.) sedimentation/ settling of particulates in the waste stream and concomitant separation of these from the liquid phase of the waste stream as a first category; 2.) biological treatment technologies of one sort or another on the separated liquid phase as a second category; and 3.) tertiary treatment (usually chemical or physical in nature) to ensure removal of specific waste products that must be removed before any resulting effluent is discharged into the environment. There are a plethora of different devices and processes that arise to address these various segments of the waste treatment process. In the following explanation, we shall examine a new technology that is applicable to both the initial segment (solid particulate removal) as well as to the final "polishing" process of the waste stream. Understanding Particulate Removal Before we get started, it might be a good idea to review the basics of particulate removal in the entire wastewater treatment process. Particulates are assigned to categories depending on their physical state and sometimes even their chemical properties. The categories are: settleable solids, suspended solids, colloidal particulates ("solutions"), and soluble solids (i.e., solvated solids with the standard solvent being water, although other solvents are occasionally encountered). Settleable solids depend on size and density as separation characteristics, but the process is also dependent upon temperature and particularly the velocity of the waste stream. Turbulence is also a variable, though it, too, is generally treated as a velocity-related parameter. Suspended solids are those of smaller size that do not readily settle out, so one normally utilizes coagulants/flocculants to effect the process. Coagulants develop a sticky, gummy mass that usually consists of a gel-like hydroxide (aluminum, iron, or even calcium) to entrap these smaller particulates as they become enmeshed in the formed matrix. A further step is completed as these trapped and bound particulates are sequestered in the matrix formed, and flocculants increase the extent of the matrix. Colloidal particulates are those that are so small they do not spontaneously separate, and they carry an adsorbed electrostatic charge surrounding the particulate. This charged "coating" is referred to as the Stern Layer and has a thickness of a single hydrated ionic layer tightly attached by electrostatic forces to the colloidal particulate forming a first inner layer of charges. Because the colloidal particulate is electrically charged, it attracts ions and other colloidal particulates of the opposite sign. The particle and the attached ions of opposite sign form an electrostatic double layer. Additional ions of opposite sign to that of the colloidal particulates also accumulate next to the Stern Layer. These form a diffuse layer. These colloids are agglomerated by essentially tampering with these electrostatic charges in one way or another. Depending on the nature of a particular colloidal particulate, they may be "salted out," bridged by means of various polymers, removed by ion-exchange mechanisms, and/or flocculated by any and all of the above. The last category of particulates to be removed is solvated particulates (usually molecular in nature, but not always) that are chemically treated — that is, reacted with some other functional group that results in their being bound or sequestered in the gel matrix. It is here that ion-exchange becomes a predominant "player." Keep in mind that no matter what the nature of these individual particulates is, they are "converted" from being a 22 wateronline.com ■ Water Innovations The big advantage of this tangential flow separator technology is that it lowers the cost by controlling the cost of the polymers necessary and by allowing the use of zeolites (specifically, clinoptilolites) to both remove and bind a variety of offending ionic species.