Water Online the Magazine gives Water & Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.
Issue link: http://wateronline.epubxp.com/i/529674
involved. The key to removing these particulates is to find what charge they carry, and, if possible, what engendered the charge. This is addressed by providing an opposite charge to aggregate the particulates (opposite charges attract). How this is accomplished may require a coagulant/flocculant that is itself oppositely charged. Most of the microparticulates carry a negative charge; therefore, coagulants/flocculants should usually have a positive surface charge. Many of the Bentonites do have a positive surface charge (remember they are ion-exchangers); if a more or differently charged surface is required, the addition of an aluminum salt can alter the structure of the Bentonite or can even be sequestered in other additions to the mixture, such as the various zeolites (which are also ion-exchangers [water softeners]). The bases resulting from aluminum and ferric/ferrous iron (aluminum and iron hydroxides) are commonly used as coagulants/ flocculants in water treatment and wastewater plants presently. The tangential flow separator system is designed to add the necessary amount of coagulants/flocculants in order to aggregate the waste stream to an acceptable degree. This is affected by an air-driven mixer that injects the flocculant stream concurrently by mixing it into the waste stream. This mixer can be adapted to deliver any surface charge necessary to effect coagulation/ flocculation. Exactly how and at what step prior to filtration depends on the nature of the waste stream, as well as the physical parameters such as velocity, temperature, and particulate charge on the suspended particulates within the waste stream. The tangential flow separator is then "charged" as the flocculant-treated waste stream enters the lumen of the separatory part. At this point, the solids are moved "down the line," and the fluid fraction is separated and directed tangentially to the flow. This action results in a filtration situation in which the liquid fraction is flowing more or less perpendicular to the linearly directed flow of the solids as they proceed down the lumen. This results in the tangential flow separator acting as a "leaky pipe" for the fluid phase removal and as a solids concentrator for the solids moving down the separator's lumen. Evidence regarding the separation efficacy resides in the before-and-after values of various impurities "of interest" in a particular waste stream that are determined from the seperated liquid phase. Of course, the solid phase likewise shows the efficacy of the process by the amount of fluid retained in the cake and by the amount of coagulant/flocculant comprising the solid phase of the treated solids emanating from the lumen. As flocculant inclusion into this cake is quite important from a regulatory viewpoint (yet limited as a processing aid), the amount present in the final cake determines the efficacy of the process as a whole. 26 wateronline.com ■ Water Innovations Dr. J.H. Wakefield is a consulting scientist/engineer with more than 30 years of experience in water/wastewater treatment. He holds advanced degrees in microbiology and physical/analytical chemistry and has been a practicing chemical and environmental engineer for many years. About The Author TERTIARYTREATMENT Advanced controls optimize the performance of the tangential flow separator's physical processes.