Membrane Processes for Advanced Wastewater Treatment

An overview about membrane processes that can be used in wastewater treatment has been presented in Chapter 3; the general characteristics of various processes determine the applications of membrane processes in wastewater treatment. Membrane filtration can theoretically replace conventional processes such as secondary sedimentation, flocculation, settling basin, and granular filtration all together. In reality, however, the applications of membrane filtration in wastewater treatment are carefully implemented. Membrane filtration is usually placed after the secondary treatment as the wastewater has ridded itself of the majority of suspended particulates and FOG; cartridge filters or carbon filters are often used before the membrane unit for extending the working life cycle of the membrane material, which is very susceptible to fouling or forming an adsorbed layer by lipids, proteins, silicates, and other minuscule substances.

Wastewater Treatment Membrane
Figure 5.4. Classification of membrane filtration based on size exclusion.

Low-strength wastewater from food and agricultural processing can be treated with membrane filtration alone, provided that some forms of pre-treatment (filtration) precede membrane filtration. For example, steep or soaking water from grain processing can be treated with a microfiltration unit; wastewaters from milk and cheese processing, including cleaning water and evaporator condensate, may be filtered with ultrafiltration or a combination of microfiltration and ultrafiltration; and oil/water emulsion may be separated with a ceramic membrane filtration unit. A number of membrane filtration processes that are capable of retaining or removing certain materials are classified based on their size exclusion capability, as shown in Fig. 5.4.

Reverse osmosis (RO) is often associated with water treatment or ultrapure water production because of its ability to retain the dissolved ions (in the case of ultrapure water production, RO serves as pretreatment for ion exchange deionization). One successful commercial application of reverse osmosis is desalination of seawater or brackish water and production of bottled water. RO is also used with some success in removing arsenic from drinking water sources. RO may also be used to desalt effluents from a wastewater treatment plant to reduce the salt concentration before discharge. RO is widely used in food processing as a concentration process and recovery process of useful components in food wastewaters; it is conceivable that RO can also be used to reduce the volume of the wastewater in food wastewater treatment (for example, RO may be used to treat nutrients such as nitrate or phosphate) or to produce recycled water for reuse in food processing operations.

Like RO, electrodialysis (ED) is used to desalt impaired waters or remove ions such as nitrate, arsenic, and phosphate. ED may also be used to separate acids from food wastewaters. Recovery of carboxylic acids (such as acetic, citric, and lactic) is a known application of ED in food and agricultural processing.

As explained in Chapter 3, there are four common types of membrane module designs available for membrane processes; however, not all module types are suitable for all membrane processes. Table 5.1 provides a guideline for selecting modules for membrane processes.

One current interest in applications of membrane processes in food wastewater treatment is recovery of valuable commodities from food wastewater streams. Chapter 8 is devoted entirely to the recovery of useful materials and energy from food and agricultural wastewaters including using membrane-based technologies to achieve the objectives of the recovery.

One problem that has hindered the widespread use of membrane technology is the noticeable occurrence of concentration polarization/fouling in membrane processes. The detrimental effect of concentration polarization and/or membrane fouling add significant costs to the operator. In membrane filtration processes, concentration polarization is formed as the result of the rapid accumulation of retained solutes near the membrane surface to the point that the concentration of macromolecule solute reaches the gel-forming concentration and the retained molecules diffuse back into the bulk fluid. The cause of concentration polarization in perva-poration or electrodialysis is slightly different from that of membrane filtration in that it is triggered by the relatively slow diffusional mass transfer rates of solutes or ions from the bulk to the membrane surface.

Membrane fouling is commonly observed as the membrane flux is continuously declining after a period of time of operation. This is usually an irreversible, partially concentration-dependent, and time-dependent phenomenon, which distinguishes it from concentration polarization. The identification of membrane fouling often relies on operator experience, performing fouling tests with lab-scale static filtration experiments or silt

Table 5.1. Membrane modules for common membrane processes.

Plate-and-

Spiral

Process

Tubular

Hollow Fiber

Frame

Wound

Microfiltration

Good

Not suitable

Good

Not suitable

Ultrafiltration

Good

Adequate

Good

Adequate

Nanofiltration

Good

Good

Good

Adequate

Reverse Osmosis

Adequate

Good

Adequate

Good

Pervaporation

Adequate

Good

Good

Good

Electrodialysis

Not suitable

Not suitable

Good

Not suitable

density index (SDI) measurement, and the membrane vendor's recommendations. Membrane fouling is intimately related to concentration polarization, but the two are not exactly interchangeable in our description of membrane performance deterioration. The exact cause of membrane fouling is very complex and therefore difficult to depict in full confidence with available theoretical understandings. Fouling is influenced by a number of chemical and physical parameters, such as concentration, size of particulates, pore size distribution, temperature, pH, ionic strength, and specific interactions (hydrophobic interaction, hydrogen bonding, dipoledipole interactions).

Membrane fouling can be greatly reduced in several ways. One effective way is to provide pretreatment to the feed liquids. Some simple adjustments, such as varying pH values and using hydrophilic membrane materials, can also provide some relief from membrane fouling. There are also persistent interests in modifying membrane properties to minimize the membrane-fouling tendency around the world. Because membrane fouling is intimately associated with the concentration polarization phenomenon, any action taken to minimize concentration polarization will also help reduce membrane fouling. Fouled membranes can be cleaned and they regain some of the original performance. Frequent cleaning and washing with detergents will inevitably lead to the demise of the membrane. There are three basic types of cleaning methods currently used: hydraulic flushing (back-flushing), mechanical cleaning (only in tubular systems) with sponge balls, and chemical washing. When using chemicals to perform defouling, caution must be observed because many polymeric membrane materials are susceptible to chlorine, high pH solutions, organic solvents, and other chemicals.

160 Food and Agricultural Wastewater Utilization and Treatment VOC Removal with Pervaporation

As described in Chapter 3, pervaporation is an energy-efficient technology that has been used commercially for alcohol dehydration, VOC removal from contaminated water, and hydrocarbon separations. The driving force of pervaporation processes is the chemical potential difference across the membrane between the feed and permeate; the performance of pervapora-tion is not restricted by vapor-liquid equilibrium like distillation (Dutta et al., 1996). Recently, it has been shown to be a valuable tool for value-added wastewater treatment through flavor and aroma recovery from food processing by-products (e.g., Karlsson and Tragardh, 1996; Peng and Liu, 2003c). The application of pervaporation in VOC removal has also been intensively researched (e.g., Jiang et al., 1997; Hitchens et al., 2001; Vane et al., 1999, 2001a; Peng and Liu, 2003a, 2003b; Liu and Peng 2006). These VOC removal research programs have led to several successful field demonstrations (e.g., Alvarez et al., 2001; Vane et al., 2001b).

In general, pervaporation processes can be easily adapted to VOC removal in water or wastewater because of their energy efficiency and targeted removal without introducing additional chemicals or new pollutants in different forms (e.g., carbon adsorption and air stripping). Almost all VOCs can be removed with pervaporation; however, VOCs of particular interest including petroleum-based solvents, such as benzene, toluene, ethyl benzene, and xylenes (BTEX), and chlorinated solvents, such as trichloro-ethylene (TCE) and tetrachloroethylene (PCE) are particularly well suited for pervaporation removal. The water solubilities of these compounds are low; therefore, the amount of VOCs dissolved in water is too small to be economically removed from water by conventional chemical process separation technologies such as distillation. In the past, air stripping and/or activated carbon treatments were deployed for the task; however, the former is susceptible to fouling and merely turns a water pollution problem into an air pollution issue, and the latter needs costly regeneration steps and may not be suitable for VOCs that are easily displaced by other organic compounds. Over the decades, a growing literature has been added to the knowledge base of VOC removal with pervaporation.

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