Separation and recovery technologies

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This section presents the theories of various physical and chemical separation technologies and the application of each for treatment of food processing waste/wastewater.

12.3.1 Physical and thermal processes

According to Kentish and Stevens (2001), physico-chemical processes are usually effective in reducing organic pollutants in the wastewater to levels

Fig. 12.1 Definition of a separation process.

Fig. 12.2 Common separation technologies for food processing wastes/

wastewaters.

Fig. 12.2 Common separation technologies for food processing wastes/

wastewaters.

suitable for discharge into public sewer systems or natural water sources, depending on the initial concentrations of the pollutants. The processes may be capable of recovering some of the nutrients either for recycling or to be marketed as byproducts, but they cannot significantly reduce heavy metals. Nor do they usually provide the selectivity necessary for producing pure products suitable for recycling or reuse. In the following sections, eight different separation processes and their example applications are described.

Screening

Screening is commonly used as a primary separation process to remove solid materials from food processing wastes and other waste streams. The separated solids can be converted into valuable products by other technologies such as drying. Screening is widely used in food processing operations, including seafood, vegetable, meat, poultry, winery, distillery and canning plants (Zaror, 1992). Many screen designs are appropriate for food processing wastewater, including static or inclined screens, rotating cylindrical screens and vibrating screens (Dearborn Environmental Consulting

Services, 1979). The choice among these designs depends on financial and technical considerations. According to Zhang and Westerman (1997), the performance of the screen separator depends on screen-opening size and characteristics of waste to be separated, such as total solids content and particle size distribution. Another important parameter is the ratio between the flow rate of waste and the available surface area for screening. Knowledge of the waste flow rate relative to the screen openings and area is required to achieve effective solid-liquid separation.

Sedimentation (also called gravity settling or clarification) Sedimentation is the separation of solids from a liquid by means of gravity. It depends on the differences in specific gravities between the suspended matter and the solution. Gravity clarifiers are often equipped with skimming mechanisms for removing floating materials such as grease and fibers. They are commonly used to treat waste streams from sugar beet, meat, fish and poultry processing operations. Sedimentation can sometimes be done naturally without adding chemicals. However, for many applications, use of chemical coagulants can be very helpful in enhancing the removal efficiency of suspended colloids (Soderquist and Montgomery, 1975). Theoretically, sedimentation can be modeled by the laws of relative motion between particles and the enclosing liquid (Pinheiro and Cabral, 1993).

Flotation

Flotation involves the separation of suspended matter from an aqueous solution due to the differences in their specific gravity. Some applications use gas bubbles to enhance the separation by attaching the bubbles to the solid particles, causing them to rise to the surface through the buoyant effect. Depending on the methods used to generate the gas bubbles, flotation technologies can be divided into dispersed-air, dissolved-air and electrolytic flotation (Svarovsky, 1990). Dissolved-air flotation is the most commonly used method (Rubio et al., 2002). For example, dissolved-air flotation is used to remove soluble proteins in soybean processing waste-water after precipitation and flocculation (Schneider et al., 1995). The size of a protein flotation unit depends on the protein concentration and the volume of wastewater to be treated (Zaror, 1992). Generally, the main factors affecting the design of a flotation process are the specific gas flow per unit volume of the liquid, the concentration of suspended matter and, in the case of protein separation, the pH of the liquid. The latter is due to the fact that the pH affects protein solubility.

Crystallization

Crystallization is the formation of solid particles within a homogeneous phase (McCabe et al., 1985). It may include the formation of solid particles in a vapor (e.g. in snow) or from an aqueous phase. It is a powerful technique for isolating pure substances from mixtures (Kennedy et al., 1993;

Prazybycien et al., 2004). Crystallization processes are currently used to isolate and purify a wide range of inorganic and organic constituents from food products and waste streams (Adler et al., 2000). One important application is protein lactose recovery. Prazybycien et al. (2004) mentioned that at least four factors define crystal morphology and the yield of the protein crystallization process: protein concentration, precipitant concentration, pH and temperature.

Impurities such as minerals and protein are believed to hinder crystallization of lactose and should be removed beforehand. For example, in separating lactose from whey, Guru and Zall (1991) found that K+, Ca2+ and PO43- had significant effects on lactose recovery. Their results also showed that seeding with fine lactose crystals increased recovery efficiency, whereas extending the aging period made little difference.

Mukhopadhyay et al. (2003) studied the removal of interfering substances - such as protein from whey, using chitosan as a coagulant. They found that first treating crude and heat-deproteinized whey with this material brought reductions in protein, ash and fat of 62-85%, 50-75% and 70-80% of the original content, respectively. The chitosan-treated whey was subjected to lactose separation. Lactose was precipitated using ethanol, or crystallized using crystalline lactose as seed. Lactose precipitated by ethanol had lower ash content (1.2 g/100 ml for crude whey and 0.1 g/100 ml for heat-deproteinized whey) than the lactose separated by crystallization (2.25 g/100 ml for crude whey and 1.57 g/100 ml for heat deproteinized whey). Lactose prepared from the heat-deproteinized whey had a purity of 99.89% and met the standard for pharmaceutical grade.

The most important parameters affecting the design and performance of a system for crystallization are temperature and pH of the liquid, concentration and solubility of the intended component, and the presence of nuclea-tion seed and its origin. For crystallization of a specific protein, Berry (1995) mentioned that seed solutions from the same protein, but from different species, may activate crystallization of that specific protein.

Centrifugation

Centrifugation is an effective pre-treatment process for food processing wastewaters. It separates suspended solids by increasing their gravitational forces. Stokes' law may be modified to apply to centrifuges as follows (MacConnell et al., 1990):

V = [(ps -p{)d 2m2 r] c 18ju where: Vc is the settling velocity of the particle due to equivalent gravitational force from the centrifuge; ps and pl are the densities of the solids and liquid, respectively; d is the diameter of the particle; w is the speed of rotation; r is the radius of rotation; and m is the viscosity of liquid.

Centrifugation is widely used in fishmeal plants to separate fish oil from processing wastes (Archer et al., 2001). Several different types of centrifuge are available - including basket, solid-bowl, countercurrent-flow and concurrent-flow systems (Philips, 1997). Centrifugation can be used as a pre-treatment before membrane separations to reduce membrane fouling and increase overall separation efficiencies. Turano et al. (2002) used cen-trifugation in combination with ultrafiltration in the treatment of waste-water from olive oil production, and noted that centrifugation was considered economic since the centrifuge was already used during the production of olive oil. Another application is the use of high-speed centrifuges for separating plasma (60-80%) and red cells (20-40%) from whole animal blood (Liu, 2002). The red cells contained 34-38% protein while the plasma contained 7-8% protein.

Centrifugation has also been used for recovering protein precipitates. Stavrinides et al. (1993) stated that operating the centrifuges at their maximum efficiencies, minimizing the losses of protein into the supernatant and minimizing the supernatant in the precipitate, were important considerations in keeping processing costs reasonable. Gómez-Juárez et al. (1999) used centrifugation as a unit operation prior to breakdown of red blood cells (hemolysis), enzymatic hydrolysis and ultrafiltration during the recovery of white protein concentrate from bovine-waste blood. Sachindra and Mahendrakar (2005) reported that centrifugation was an important step during the recovery, from shrimp wastes, of carotenoids, a group of oil-soluble pigments. Toyoshima et al. (2004) used centrifugation to separate sardine oil from surimi wastewater without heating or chemical refining. Their results showed that more than 70% of the oil could be recovered from surimi wastewater by using continuous centrifugation. Bough et al. (1976) successfully used centrifugation to separate solids that had been coagulated with chitosan in an effort to reduce suspended solids in waste-water produced in vegetable, poultry, meat, cheese, seafood and egg-breaking plants. The separated solids were used as animal feed additives.

Hydrocyclones

Hydrocyclone technology, which takes advantage of centrifugation forces, has been suggested as a practical technology in solid-liquid separation of biological materials. Ortega-Rivas (2004) explained the theory and essential parts of hydrocyclones. A hydrocyclone consists of a cono-cylindrical body that creates a vortex when a liquid is pumped through it. The vortex creates a centrifugal force which hurls the coarse particles from the cyclone center toward the walls, where they fall out through an underflow orifice. The fine particles remain around the central axis and exit the hydrocyclone with the upflow stream. As with centrifugation devices, the efficiencies of hydrocyclones depend on the difference in density of the solid particles and the liquid, plus liquid viscosity, diameter, rotation speed and radius of the particles. Compared with centrifuges, hydrocyclones are easier to manufacture, install, maintain and operate, but they have lower separation efficiencies. According to Curtis (1996), they can be very efficient and cost-effective ways of separating some food wastewaters, such as oily wastewater.

288 Handbook of waste management and co-product recovery Adsorption

Adsorption is considered to be an effective method for treatment of dilute effluents (Laufenberg et al., 2003). The mechanism here is the attachment of molecules from an aqueous solution or a gaseous phase onto a solid surface (the adsorbent) because of intermolecular attractive forces. Adsorbents attach atoms, molecules, ions and/or radicals from their surrounding gaseous or liquid phase onto their surfaces. The amount of the trace chemicals to be absorbed is proportional to the available surface area of the adsorbent. Therefore, commercial adsorbents are extremely porous, giving high surface areas per unit of mass. Adsorbents are divided into three classes: hydrocarbon materials, inorganic materials and synthetic polymers. A number of low-cost adsorbents have been used for wastewater treatment - including peanut and walnut shells, orange peel, wool fibers and corncobs (Laufenberg et al., 2003).

Much research has been done on the use of adsorption for recovery of various chemical substances (e.g. polyphenolics) from food processing wastes. Polyphenolics are highly valuable compounds that may be used as functional food ingredients and as natural antioxidants. Schieber et al. (2003) investigated the recovery of pectin and phenolic compounds from apple pomace. The recovery process included extraction of dried apple pomace with diluted mineral acid and adsorption of phenolic constituents by a hydrophobic styrene-divinylbenzene copolymerizate, which is a resin used for reducing the bitterness of citrus juices. Edris et al. (2003) studied the recovery of aromatic components by adsorption from wastewaters produced from aromatic plant distillations, using granular activated carbon as adsorbent. Recovery ranged from 44 to 90%, depending on the selectivity of compounds. The authors found that adsorption did not affect the structure of the recovered aromatic compounds. Moreover, the adsorbent had no effect on the chemical nature of the aromatic components. The important factors affecting the rate of adsorption are shown in Table 12.1 (Vasanth Kumar et al., 2004).

Table 12.1 Factors affecting the adsorption rate (Vasanth Kumar et al., 2004) Factors related to the adsorbent:

1 Dimensions of the adsorbent (i.e. diameter or surface area).

2 Structure of the adsorbent.

Factors related to the nature of the adsorbate (i.e. solute) and the solvent:

1 pH of the liquid.

2 Solubility of the solute.

3 Molecular size of the solute.

4 Molecule geometry.

5 Degree of ionization.

6 The presence of materials for surface tension modification.

Thermal processes: freezing, evaporation and drying Three thermal processes can be used as solid-liquid separation technologies. Freezing is one: as it occurs, ice crystals are formed from pure water molecules, while other molecules are rejected to crystal boundaries where they can be recovered. The process is energy intensive if freezing is manmade, but natural freezing can be economic in cold regions (Martel, 1990).

Evaporation, another thermal process, is used to concentrate a solution consisting of a nonvolatile solute and a volatile solvent (McCabe et al., 1985). A third thermal process, drying, is normally used to remove moisture from the waste. Like artificial freezing, evaporation and drying are energy-intensive processes only used in applications that recover products of high value. Gogus and Maskan (2006) studied the air drying of olive pomace, which could be used as an animal feed or as a raw material for the production of glycolipids, at low temperatures (60-80 °C) and found that an increase of both drying temperature and particle size decreased drying time.

Generally, the major factors that affect the choice of thermal technologies are the energy consumption and the required characteristics of the recovered product. The most important design parameters are: (1) operational temperature and pressure; (2) surface area for heat transfer; (3) type of media used for heating or cooling; (4) relative flow rates between the waste and the heat-transfer media and (5) design of the equipment used.

12.3.2 Chemical processes

Chemical separation processes use chemicals to bring about reactions that change the surface characteristics of solid particles, and change compounds from soluble to insoluble forms to enhance the separation. The chemical treatment processes are usually used in conjunction with physical processes. Three chemical treatment processes - precipitation, coagulation and colloidal gas aphrons - and examples of their application are described in the following sections.

Precipitation

Precipitation involves converting a soluble compound into an insoluble form by adding a chemical to an aqueous medium (Dearborn Environmental Consulting Services, 1979). Boychyn et al. (2000) reviewed the factors that affect the physical properties of the aggregates formed by precipitation. These are: type of precipitation reactor, precipitating reagent type, concentration and rate of precipitating reagent addition; method and extent of mixing; and residence time in the reactor. The choice of precipitating reagent depends on precipitation yield, selectivity, denaturation, viscosity and density of the suspension, and the end use of the final product (Stavrinides et al., 1993). Precipitation processes can be carried out either in batch or continuous operations. Garcia (1993) pointed out that two steps should be considered in designing a precipitation process: (1) selection of the precipitation method and (2) the dynamics of precipitation. Selecting the method includes selecting the precipitation agent and dosage, operation costs, product yield, purity of final product and evaluation of possible damage that could be caused to biomolecules by the precipitation agent.

Precipitation has various applications in treating food processing waste-water, including separation of soluble phosphorus, and recovery of sugar and protein from wastewater. Precipitation can easily be adapted to a large scale using simple equipment (Singh and Singh, 1996). In the Steffen process of the sugar beet industry, calcium is used to precipitate and recover residual sugar (as calcium sucrate) from molasses (Dearborn Environmental Consulting Services, 1979). In the case of protein recovery, precipitation results in a lower degree of protein purity than ion exchange, but its cost is relatively low too (Zaror, 1992). Proteins are brought to an insoluble state either by heat or by adjusting the composition of the solution (pH, ions, poly electrolytes, solvents) and then removed by solid-liquid separation techniques such as sedimentation or dissolved-air flotation (Hearn and Anspach, 1990).

Fernández and Fox (1997) studied the use of chitosan for the selective precipitation of proteins and peptides in cheese wastewater. They found that chitosan gave good fractionation of water-soluble extracts at pH 2, 3 and 4. At pH 5, 6 and 7 most of the nitrogen in the water-soluble extract remained soluble. Effective fractionation was obtained at pH 4.0. This approach could also be used to recover protein and peptides from cheese and other dairy industry wastewater. Thus, from an environmental point of view, the use of chitosan is preferred to chemical precipitation agents, because chitosan is a biologically based byproduct.

Protein can be precipitated by using one of the following methods (Zaror, 1992): isoelectric precipitation, salting out and heat precipitation. More details about the isoelectric and salting out methods are given below.

Isoelectric precipitation

Isoelectric precipitation refers to the separation of a protein from a solution when the protein loses its charge at its isoelectric point (pI), reducing its solubility. The surface charge of the protein is largely affected by the pH of the solution. Protein has a net positive charge at low pH levels and a negative charge at high pH levels. At the pI, it has no net charge. This leads to reduced solubility because the protein is unable to interact with the medium and will then fall out of solution (Righetti, 2004). In addition to using pH as a variable to control precipitation, zeta potential is used. The zeta potential is a measure of the electrostatic charge on the surface of particles suspended in a liquid (Singh and Singh, 1996).

Salting out

Adding large amounts of neutral salts (e.g. ammonium sulfate) and organic solvents (e.g. ethanol and acetone) can also lead to protein precipitation. The choice of a precipitating salt depends largely on its cost and solubility, and the stability of the protein (Zaror, 1992), while the choice of an organic solvent depends on its cost, its miscibility with water, and the effect on protein stability and solubility. According to Singh and Singh (1996), the major concerns with precipitating proteins using organic solvents are mis-cibility, the safety of solvent handling and the prevention of protein dena-turation. Non-ionic polymers and polyelectrolytes are also used for protein precipitation and stabilization in solution. Carboxymethylcellulose solutions have been used to recover proteins from whey, egg and muscle.

In addition to isoelectric and salting out precipitation, heat can also be an effective means of changing the physical properties of protein and bringing about coagulation/precipitation. The heat-enhanced separation method can be used when the biological properties of the protein are not important (Zaror, 1992).

Chemical coagulation

Coagulation generally is a process that causes destabilization and aggregation of colloidal particles in a solution. In chemical coagulation, insoluble colloidal particles are agglomerated by the action of a chemical additive to produce flocculant materials, which can then be removed by sedimentation or flotation (Dearborn Environmental Consulting Services, 1979). Flocculation is a method of aggregating the suspended fine particles into large flocs (Moudgil and Shah, 1986). Floc properties in a given application are determined by other steps such as sedimentation, filtration or floc flotation. The desired floc characteristics for specific applications are presented in Table 12.2 (Moudgil and Shah, 1986). Flocculation promotes contact between native or coagulated particles, thus increasing aggregate size (Pinheiro and Cabral, 1993). Although some coagulation can occur naturally by particle contact and agglomeration, chemicals increase the rate of agglomeration. The chemicals have charged ions that neutralize the surface charges on the surface of colloidal particles. Metal ions (e.g. ferric chloride, ferric sulfate) and polyelectrolytes are examples of the chemicals applied

Table 12.2 Floc characteristics for specific applications (Moudgil and Shah, 1986)

Separation technique

Desired characteristics of floc

Filtration

Porous, strong, permeable flocs

Sedimentation

Dense, strong, large, regular in shape

Centrifugation

Strong, dense, large flocs

Floc flotation

Low-density, strong, narrow size distribution

in wastewater treatment. Organic materials, such as chitosan, may also be used as coagulant agents. The choice of suitable chemicals depends on cost, effectiveness of coagulation, and the quality and final usage of the recovered byproduct.

Many factors influence the effectiveness of coagulation, including the characteristics and dosage of the coagulation chemical and the pH of the wastewater. In a study conducted by Genovese and González (1998), maximum solid removal of 31% and 27% from fish-filleting wastewater was achieved at pH 5.5 using FeCl3 and chitosan, respectively, both at 60 mg/l. A maximum solids removal of 31% was achieved using 60 mg/l Al2(SO4)3 at pH 7.2. Using fish scales as a coagulant, a maximum removal of 27% was achieved at pH 7.2 and a dosage of 40 mg/l. The researchers concluded that not only classic inorganic coagulants but also ground fish scales could be used as coagulants. Wibowo et al. (2005) mentioned that chitosan appeared to work by mechanical entrapment and electrostatic interaction of chitosan amino groups with anions on the proteins. They found that the effectiveness of chitosan for recovering soluble proteins from surimi wash water (SWW) was increased by adding alginate complex (Chi-Alg) and by adjusting treatment time. Flocculation of SWW protein using Chi-Alg at a concentration of 100 mg/l for 1 h achieved high protein adsorption and reduced turbidity.

Good clarification (up to 96%) of suspended solids in slaughterhouse wastewater was achieved using aluminum sulfate and a commercial polymer (Al-Mutairi et al., 2004). Selmer-Olsen et al. (1996) investigated chitosan as a coagulant for the treatment of dairy wastewater and reported that use of chitosan at pH 5.3 achieved 60% phosphorous removal and 90% chemical oxygen demand (COD) removal. These removals were similar to those achieved with carboxymethylcellulose, which is commonly used at pH 4.2. Thus using chitosan could reduce treatment costs by reducing the need for acid to lower the initial pH of dairy wastewater (i.e. pH > 9). Moreover, the sludge recovered when using chitosan can be used as an animal feed.

Meyssami and Kasaeian (2005) studied the optimum conditions for using air flotation to separate coagulated oil droplets from a model olive oil-water emulsion. Their results showed that a decrease of 90% in turbidity of the olive oil-water suspension could be achieved at an optimum pH of 6 and an optimum chitosan dosage of 50 mg/l. They found that neither starch nor ferric chloride proved to be an effective coagulant in reducing the turbidity of the emulsion samples. They also mentioned that at 20 °C, 1 min aeration time and chitosan concentration of 50 mg/l, the coagulated oil droplets were fragile. An optimum aeration rate of 3 l/min was established for the chitosan concentration of 100 mg/l, which corresponds to a reduction of more than 90% in the initial turbidity. Their results also indicated that temperature (10, 20, 30 and 40 °C) had little effect on the flotation process at an aeration flow rate, residence time and chitosan concentration of 3 l/min, 45 s and 100 mg/l, respectively.

Xu et al. (2001) evaluated a coagulation process using the coagulants lignosulfonate, carboxymethylcellulose, ferric chloride and bentonite to treat egg processing plant wastewater. Protein and fat recoveries were over 95% for all of these coagulants. Optimal pH values for achieving maximum removal efficiencies were 3.5 with lignosulfonate, 3.0 with carboxymethylcellulose, 8.0 with ferric chloride and 4.0 with bentonite. The optimal coagulant concentration for maximum byproduct recovery depended on the initial wastewater concentrations of protein, total solid and fat. The dried products contained high concentrations of protein (36-50%) and fat (3242%). The recovered byproducts could be used safely as livestock feed ingredients, especially when carboxymethylcellulose, lignosulfonate, chi-tosan and bentonite were used. These coagulants did not adversely affect the growth rate of animals.

Coagulation, like precipitation, is a practical process for reducing pollution and recovering byproducts from food processing wastewaters. The characteristics of the wastewater affect the choice and the optimum dosage of coagulants as well as the operational parameters. Although inorganic coagulants are widely used, organic coagulants such as chitosan and car-boxymethylcellulose are more attractive due to the fact that the recovered products can be safely used as an animal feed. Many factors affect the design of a chemical coagulation process, including wastewater characteristics (e.g. pH, protein content) and the type and dosage of the coagulant.

Colloidal gas aphrons

Colloidal gas aphrons (CGAs) are defined as microbubbles created by intense stirring (5000-10 000 rpm) of a surfactant solution (Sebba, 1987). These microbubbles differ from conventional foams. According to Sebba (1987), CGAs consist of a gaseous inner core surrounded by a thin aqueous surfactant film or shell composed of two surfactant layers and a third surfactant layer that stabilizes this structure. Figure 12.3 shows the structure of CGAs. They have high surface areas for adsorption of molecules by means of electrostatic and/or hydrophobic interactions. Their surface properties can be modified by changing the type of applied surfactant. Furthermore, CGAs show relatively high stability and short separation time from the bulk phase. CGAs can also be easily transferred by pumping and have significant cost advantages compared with membrane and chromato-graphic separation methods (Fuda et al., 2004). Jauregi and Varley (1999) summarized the properties of CGAs as follows:

• They expose a large interfacial area per unit volume for the adsorption of molecules.

• They exhibit relatively high stability, measured in terms of the time required for their collapse.

• They have flow properties similar to those of water.

• The aphron phase separates easily from the bulk liquid phase because of its buoyancy.

294 Handbook of waste management and co-product recovery Outer surface of shell Inner surface of sheN

294 Handbook of waste management and co-product recovery Outer surface of shell Inner surface of sheN

Electrical double layers

Viscous water Normal water

Fig. 12.3 Proposed structure of colloidal gas aphrons (adapted from Sebba (1987) and Jauregi and Varley (1999)).

Electrical double layers

Viscous water Normal water

Fig. 12.3 Proposed structure of colloidal gas aphrons (adapted from Sebba (1987) and Jauregi and Varley (1999)).

Jauregi and Varley (1999) mentioned four main areas for CGA application: (1) flotation for the removal of biological and nonbiological products; (2) protein recovery; (3) enhancement of oxygen mass transfer; (4) biore-mediation. In the application of CGAs for protein recovery, four subsequent steps are included (Jauregi and Varley, 1998): (1) generation of CGAs from a surfactant solution; (2) addition of CGAs to a protein solution and gentle mixing to allow protein adsorption at the aphron-liquid interface; (3) separation of the aphron phase from the bulk-liquid phase; (4) collapse of the aphron phase. The protein should be concentrated in the aphron phase. Removal of oil droplets from wastewater is another important application of CGAs. According to Sebba (1987), limonene can be recovered from citrus processing wastewater using CGAs. He considered the process a profitable one because of the low cost of CGA production. The cost of surfactant, needed to form CGAs, was the main cost of the process. However, costs can be reduced substantially by using cheap surfactants.

According to Amiri and Valsaraj (2004), CGAs offer particular advantages in the removal of ultra-fine particles. They postulated that it was because of the small molecular size of whey protein (less than a micron) that conventional flotation was not successful. They used sodium lauryl sulfate as a surfactant and found that the separation efficiency was increased by adding aphrons in different pulses. They attributed this to the high concentrations of surfactant remaining around the spinning disk in the aphron generator. Such high concentrations of surfactant decreased surface tension and enhanced mass transfer into solution, thus smaller aphron bubbles were produced and the CGA dispersion became more stable. Fuda et al. (2004) demonstrated that under certain conditions, CGAs could be applied for recovery and separation of lactoferrin (Lf) and lactoperoxidase (Lp) from sweet whey. Their results showed that the amount of total protein in the starting whey and the pH of the separation mixture are the main factors influencing the partitioning of the Lf and Lp fractions into the aphron phase. The best separation performance was achieved with conditions favoring electrostatic interactions (pH < pi and low ionic strength (IS)), whereas conditions favoring hydrophobic interactions (pH > pi and high IS) led to lower performance. Protein adsorption in the aphron phase mainly occurred via electrostatic interactions. Barnett and Lin (1981) used microgas dispersions (MGDs) to remove proteins from seafood processing waste. They showed that without addition of a synthetic surfactant to clam processing wastewater, a foam was produced that could be used after freeze-drying to reform an MGD. The protein produced would be used as an animal feed.

It is clear that the use of CGAs is an efficient and economic separation technique for protein recovery from food wastewater and for recovery of oily products from citrus and seafood processing wastewaters. CGAs may also be used for oil recovery from wastewaters produced during vegetable oil processing.

12.3.3 Other separation technologies

Other advanced technologies are being applied for separation purposes (Fig. 12.1). This section discusses reactive separation and alternating current electrocoagulation.

Reactive separation

Reactive separation refers to the process that combines reaction with separation in a single operation. This integrated design is intended to solve particularly difficult separation problems, such as thermally unstable systems, by means of selective and reversible reactions (Gaikar and Sharma, 1987). Another purpose is to improve a given reaction by overcoming chemical equilibrium limitations through selective separation and removal of reaction products, or by suppressing undesired side reactions (Gilles et al., 1996). Adler et al. (2000) noted that separative reactors are devices that can achieve chemical reaction and separation simultaneously. They may include adsorption and membrane reactors, reactive distillation, and biological reactor systems. Simultaneous reaction and separation offer certain advantages that cannot be matched by conventional processes. For one, they can both reduce capital investment and overcome reaction equilibrium limitations (Samant and Ng, 1998).

According to Blaschek (1992), microbial fermentation and cell-free enzyme-based systems are good options for converting food processing byproducts and wastes into valuable products. Reactor separators convert the substrate to products and simultaneously remove them by stripping them from the fermentation broth into the gas phase. Bioreactor separators should increase reaction rates and cell viabilities, leading to reductions in plant size and cost (BFPE, 1988). Dale et al. (1985) investigated the application of the immobilized cell reactor-separator (ICRS) to produce ethanol from whey lactose. Their main objective was to remove inhibitory compounds formed during the reaction so that reaction rates and microbial activities could be maintained. The ICRS consisted of two separate columns in which the fermenting broth was contacted by both the immobilized cells and a 'stripping' gas phase. The inlet substrate and the gas moved co-currently in the first column, which was called the enriching column. In this column some substrate was converted to a volatile product. Part of the product moved into the gas phase and later to a condenser, while the liquid phase moved to the second column, called the stripping column. Here the remaining substrate was converted to product while the product was stripped into the gas phase, resulting in a final exhausted liquid effluent containing ideally no substrate or product. Thus, the ICRS both converts the substrate to product and removes the product from the fermentation broth. High reaction rates were obtained due to high-density cell loading in the reactor. A separation efficiency of 98% was obtained in this system.

Reactive separation could be a promising technology for the recovery of gases and liquids such as hydrogen, methane and ethanol from food processing wastes. Separation of intermediate products, for example volatile fatty acids during anaerobic digestion, could be an important help in preventing reactor failures.

Electrocoagulation (EC)

EC is a rather complicated process. It involves a number of chemical and physical phenomena that use consumable electrodes to supply ions into the wastewater stream (Mollah et al., 2004). Three successive steps are involved: (1) formation of coagulants by electrolytic oxidation of the 'sacrificial electrode'; (2) destabilization of the contaminants, suspension of particulates and breakdown of emulsions; (3) aggregation of the destabilized phases to form flocs. Ryan et al. (1990) mentioned that alternating current electroco-agulation (AC/EC) could be used as an alternative to chemical flocculation for liquid-liquid separation. It can be applied for phase separation of waste-water containing suspended and emulsified oils. The main components of an AC/EC system are shown in Fig. 12.4 (Ryan et al., 1990).

To the authors' knowledge, this technology is not commonly used for food processing wastewaters. Adhoum and Monser (2004) studied the application of EC using aluminium electrodes for the treatment of olive mill wastewater. They found that increasing the current increased the

Fig. 12.4 Schematic of a continuous AC/EC system: 1, wastewater; 2, pump; 3, AC/EC coagulator; 4, air for mixing; 5, gas vent; 6, separator; 7, oil; 8, liquid; 9, solids (Ryan et al, 1990).

Fig. 12.4 Schematic of a continuous AC/EC system: 1, wastewater; 2, pump; 3, AC/EC coagulator; 4, air for mixing; 5, gas vent; 6, separator; 7, oil; 8, liquid; 9, solids (Ryan et al, 1990).

efficiency at an optimal pH of 4-6. After 5 min of application, the removal efficiencies for COD, polyphenol and dark color were 79, 91 and 95%, respectively. Chen et al. (2000) studied the application of EC for the treatment of restaurant wastewater and found that the removal efficiency of oil and grease exceeded 94% while the removal efficiency of COD was in the range 84.1-99.0%. EC can be used to neutralize wastewater pH as well. The important design parameters for AC/EC include - but are not limited to - electrode spacing, the material of the electrode, retention time, and current strength and frequency.

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