Application of Filtration to Wastewater Treatment

General discussions on the applications of filtration to wastewater flows were given in previous chapters. These discussions have provided the reader with an understanding of the objectives of wastewater treatment and how filtration can be used as one of the unit operations for solids removal. Filtration must, however, be used in conjunction with other treatment technologies or operations, and does not necessarily represent the primary treatment method. It is beyond the scope of this volume to discuss the other techniques used in the treatmait of wastewater flows. For general information and backgroimd on other unit operations and processes used with filtration in the treatment of wastewaters, the reader may refer to the suggested references at the end of this chapter. 

Subsequent chapters address the application of filtration techniques to wastewater treatment in some detail. For now, only some general comments and terminology are introduced as part of this introductory chapter. 

The first two categories, clarifying and crossflow filters, have been very well developed and optimized for use in biotechnology and standard wastewater treatment applications. Equipment is easily available for these applications, whether as small 0.2 micron sterilizing filter used to terminally sterilize 100 ml of product solution, or a small 500 ml crossflow filter used to concentrate a small amount of antibody solution. Many vendors of this equipment to wastewater treatment applications have their origins in the CPI (Chemical Process Industries), and have incorporated many of the scale-up and optimization properties developed in much larger units used in large scale chemical production. As a result, these two filtration unit operations are one of the most optimized and efficient used in wastewater treatment. 

Chaufer, B. and Deratani, A. (1988). Removal of metal ions by complexation-ultra filtration using water-soluble macro-molecules Perspective of application to wastewater treatment. Nuclear and Chemicals Waste Management 9, 175. 

The third category, cake filters, although well developed in many wastewater treatment applications, are the least developed of the filtration equipment use by the Biotech Industry. In the organic synthesis laboratory sometimes very simple equipment like a funnel and filter paper is used to accomplish this operation. Some other operations used for this filtration step in the lab are more sophisticated, but many are very labor intensive and limit the capacity of the overall production process itself. As a result, there is a need for optimization of the cake filtration equipment used in biotechnology. Cake filtration equipment is available in batch and continuous modes. Following are several examples of cake filtration units ... 

Different technologies have been developed in recent years to treat the wastewaters contaminated with heavy metals. Chemical precipitation, coagulation-flocculation, flotation, ion exchange, and membrane filtration can be employed to remove heavy metals from contaminated wastewater.6 However, they have inherent limitations in application mainly due to the lack of economical feasibility for the treatment of large volumes of water with a low metal concentration. Furthermore, the major disadvantage of conventional technologies is the production of sludge.9... 

Vourch et al49 studied the applicability of the RO process for the dairy industry wastewater. The treated wastewater total organic carbon (TOC) was <7 mg/L. It was found that in order to treat a flow of 100 m3/d, 540 m2 of the RO unit is required with 95% water recovery. Dead-end NF and RO were studied for the treatment of dairy wastewater.50 Permeate COD, monovalent ion rejection, and multivalent ion rejection for the dead-end NF were reported as 173-1095 mg/L, 50-84%, and 92.4-99.9%, respectively. When it comes to the dead-end RO membranes, the values for permeate COD, monovalent ion removal, and multivalent ion removal were 45-120 mg/L, >93.8%, and 99.6%, respectively. Membrane filtration technology can be better utilized as a tertiary treatment technology and the resultant effluent quality will be high. There can be situations where the treated effluents can be reused (especially if RO is used for the treatment). 

Defiance, L. and Jaffrin, M.Y. (1999) Comparison between filtrations at fixed transmembrane pressure and fixed permeate flux application to a membrane bioreactor used for wastewater treatment. Journal of Membrane Science, 152, 203-210. 

The final system, shown in Figure 30.4D, is the continuous system with a partial (PRF) or complete (RF) cell recycle. It is similar to the continuous system, but cells are returned to the fermentor by means of a biomass separation device. Cross-filtration units, centrifuges, and settling tanks have all been used for biomass separation.22 In the partial cell recycle fermentor, a steady state is achieved as in the continuous system. This process is typically used to increase the productivity of the system and is used commonly in wastewater treatment and ethanol production type applications. 

Filtration has not found extensive use in fats and oils wastewater treatment application however, it provides an effective treatment process, particularly as an alternative to DAF as a trimmer device. Another advantage of filtration is its familiarity to refinery personnel as it is used in other in-plant processes. The principal disadvantage of filtration is that it also produces another waste sludge to be disposed of. 

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