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Dewatering for food waste
21 Dewatering for food waste Valérie Orsat and G S Vijaya Raghavan, McGill University, Canada
21.1 Introduction The food industry produces a variety of wastes which require handling in an environmentally-friendly and sustainable way. Depending on the type of waste, numerous waste handling alternatives are available from fermentation, separation, biofuel conversion, composting, extraction and more. Choosing the right waste handling process can help to meet environmental regulations and provide a useful by-product for further processing, recovery or animal consumption. A lower moisture content of the waste material reduces the cost of transport due to reduced volume and weight. This chapter discusses the concentration of solids from food waste using dewatering techniques. The reduction of moisture content offers flexibility in terms of handling, shelf-life and subsequent use of the waste. A liquid/solid separation process involves multiple steps: pretreatment, thickening, separation and post-treatment (Trias et al., 2004). Common dewatering processes use mechanical means of separation such as screens, screw presses, belt presses, vacuum filters and centrifuges, which can all be combined with additional forces to remove the water such as an electric field, ultrasonics, vibrations, chemical treatments, etc. In any dewatering application there is a definite advantage of combining multiple dewatering fields to promote the synergy of separation forces (Muralidhara, 1990). The selection of an adequate dewatering process depends on numerous factors such as the type and quantity of the waste product, the end-use of the dewatered/dried solids and environmental and economic considerations. In general, food wastes contain large amounts of organic materials,
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high biochemical oxygen demand and high variations in pH (Kroyer, 1995). With a dewatering process, the underlying advantage is that the water is removed in the liquid state. The lack of a phase change renders the process less energy-intensive and in some instances may improve the end-product quality. Dewatering lowers the moisture content to a level not low enough for shelf stability and thus the dewatered material requires a finish drying treatment or further processing.
21.2 Waste conditioning When considering the adoption of a dewatering process for any given waste material, the initial characteristics of that sludge will govern the choice to be made and the handling process adopted. Waste characteristics of particular interest are particle size, particle charge, pH, organic content and viscosity (Ormeci, 2007; Ruiz et al., 2007). The smaller the particle size, the greater is the water retention from the larger specific surface area leading to more unfavourable dewatering conditions (Kolish et al., 2005). In a liquid/solid waste mixture, the water content to be expressed can be present as bulk or free water, as capillary bound water or as adsorbed bound water. In certain cases, the sludge may be too liquid and may require a chemical pre-treatment to improve the size of the solid particles through flocculation/agglomeration, or the waste may be too solid where the water is retained, for example, within the cellular structure of the plant-based waste material. In the latter case, a grinding process may help release some of that moisture, or a freezing pre-treatment may serve the purpose through cellular breakdown. A pretreatment by freezing studied by Zhou et al. (2001) gave significantly enhanced water removal. In the case of mechanical dewatering of chopped alfalfa, the best water extraction results were obtained with previously macerated alfalfa which released the cellular bound moisture content (Sinha et al., 2000). In the case of kelp dewatering, the slurry is highly viscous and requires a chemical treatment (calcium chloride) to release the water (Lightfoot and Raghavan 1995; Orsat et al., 1999). Enzymatic pre-treatment was studied by Dursun et al. (2006) and shown to weaken the sludge structure, thus improving the filtration process.
21.3 Thickening In the case of liquid wastes, a thickening process prior to dewatering can reduce equipment size requirements and ensure higher throughput during dewatering (Kukenberger, 1996). Thickening processes include gravity thickening and dissolved air flotation.
614 Handbook of water and energy management in food processing 21.3.1 Gravity thickening Gravity thickening is traditionally conducted in cone-shaped bottom circular tanks equipped with collectors or scrapers placed at the bottom for solids collection. The solids slowly settle to the bottom of the tank from the gravity pull from their own weight. Gravity thickening is simple to operate and maintain while the solids collected at the bottom of the tank reach between 4 and 6 % of total solids (US EPA, 2003).
21.3.2 Dissolved air flotation and electroflotation separation Dissolved air flotation (DAF) is a liquid/solid separation process for liquid suspended colloidal mixtures. DAF is principally used for the clarification of wastewaters which contain suspended solids. DAF is used widely for the recovery of valuable solids from food processing wastewaters, especially from the meat processing industry (Le Roux and Lanting, 2000). DAF involves the dissolution of air in the waste mixture at a high pressure to achieve saturation. By bringing the pressure of the mixture back to atmospheric, the air in the form of very small bubbles rises to the product’s surface carrying with it the colloidal particles which can be recovered. Improving the DAF process, depending on the type of waste material, may require the addition of a chemical flocculant or coagulant (such as a polyelectrolyte) as a pretreatment step for the waste slurry (Ng et al., 1988; Genovese and Gonzalez, 1994). In general a DAF system consists of a flocculator tank, a flotation tank and a pressure vessel. Its operation can be continuous or intermittent and it can be designed and constructed to meet the requirements of a variety of wastewaters in terms of characteristics of its suspended solids and the plant volume requirement for separation (Viitasaari et al., 1995). Similarly, electroflotation removes suspended solids using gas bubbles obtained through water electrolysis. For that purpose, electrodes are placed at the bottom of the tank containing the wastewater (Fig. 21.1). As current is passed through the electrodes, the water is electrolyzed producing bubbles of gaseous hydrogen and oxygen. As the bubbles work their way up to the top of the tank they entrain suspended particles to the surface of the liquid waste where they can be skimmed. Electroflotation is more effective in thickening when compared with gravity thickening. It is an interesting alternative to the separation of low-density suspended solids, especially for its simplicity of equipment design and operation (Choi et al., 2005).
21.4
Dewatering methods
21.4.1 Belt filter press Belt filter press systems usually include a gravity drainage feeding section and a mechanically applied pressure belt arrangement. In gravity drainage, through simple screens, a large portion of free water is removed. Pressure is
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Skimmer
Surface
Electrodes + –
Gas bubbles
Fig. 21.1
Suspended particles
Schematic of an electroflotation process.
then applied at an increasing rate on the waste contained between supporting porous belt material (Demetrakakes, 1996). The dewatered waste cake is removed from the belt with scrapers. In certain arrangements, a small vacuum must be applied (4–6 kPa) to facilitate the removal of water accumulating at the surface of the belts (Snyman et al., 2000). A pre-flocculation step is often considered to suit particular waste applications that are too liquid and to maximize dewatering efficiency right from the start of the process during gravity drainage. In roller press dewatering, the waste material is pressed between rotating roller drums, where the single belt material serves only for conveying. The bottom rollers are perforated to allow for drainage of the pressed liquid. The basic system is composed of a top roller which presses down onto two bottom rollers with the drums rotating so as to facilitate the passage through of the material using a conveyor belt (Orsat et al., 1999). This system can be well adapted to combine with electro-osmotic dewatering. Design improvements were investigated by Kauppila et al. (2001) for roller groove geometries. It appears that larger groove angles can help reduce further the moisture content during roller pressing of sugar cane bagasse.
21.4.2 Screw press dewatering In a screw press, the material is introduced in a perforated chamber where an endless screw forces the material along the length of the chamber towards discharge. The pressure force of the screw drives the water out through the perforations of the holding chamber. For this type of dewatering process, the waste feed must have a certain particle size large enough not to clog the perforations of the holding system and to flow through without excessive resistance.
616 Handbook of water and energy management in food processing When feeding through becomes more problematic due to the waste product’s characteristics (particles size, viscosity, etc.), a twin screw press may be more appropriate (Fig. 21.2). The two press screws are designed to compress the product as they rotate in opposite directions, which prevent the waste material from rotating with the screws and clogging up the system. Twin screw dewatering has been successfully developed and has proven efficient for citrus waste and for oil extraction from agricultural products (Isobe et al., 1997). Screw press dewatering of citrus pulp is practised in the industry to yield a higher dry matter pulp and a liquid fraction high in soluble solids. The liquid fraction is further processed to produce a citrus molasses, whereas the citrus pulp can be used as animal feed for ruminants thus fulfilling the requirement of high fibre content leading to high digestibility (Crawshaw, 2001).
21.4.3 Rotary and centrifugal presses A centrifugal dewatering system consists of a basket or a solid bowl and a conveyor, both of which can rotate at high speed. As the bowl rotates, the heavier solids gravitate to the bowl wall where they accumulate. The separation of solids from the liquid depends on the G-force, time and permeability of the waste mass (Leung, 1998). The Rotary Press manufactured by Fournier Industries Inc. Quebec, Canada (www.rotarypress.com) offers the industry interesting dewatering equipment (Fig. 21.3). The waste material is fed into a rectangular channel and rotated between two parallel revolving stainless steel chrome plated screens. The filtrate passes through the screens as the particulate sludge advances within
Fig. 21.2
Twin screw arrangement (Vincent Corporation, Florida, USA,www.vincentcorp.com).
Dewatering for food waste
Screen Cake formation
Seal Cover Interior of channel Outlet for filtrate
A
A Filtrate Filtrate section A–A
Fig. 21.3
Restriction zone
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Sludge Static mechanical component Rotating mechanical component Dewatering sludge (cake) Filtrate flow
Sludge feed Cake extrusion
Schematic of the rotary press manufactured by Fournier Industries Inc. (Fournier Industries Inc., 2003).
the channel. The sludge continues to dewater as it travels through the channel, eventually forming a cake near the outlet side of the press. The frictional force of the slow-moving screens, coupled with the outlet restriction, results in the extrusion of low-moisture material.
21.4.4 Membrane filter press A membrane filter press comprises a series of filter plates held tightly together by pressure (Fig. 21.4). The filter plates have a filtration drainage surface that supports a filter medium, in most cases a polypropylene filter cloth held in place by a more rigid polypropylene structure. The mixed solid/liquid waste is pumped into the chambers under pressure. The filtered liquid passes through the filter cloth, against the drainage surface of the plates, and is directed towards discharge collectors. The pressure gradient between the cake and the filter material provides the driving force for the flow. Solids are retained on the filter cloth forming a filter cake. The filter plates are separated and the filter cake is discharged. At this stage a vacuum step may be introduced to further reduce the moisture content. In a study by El-Shafey et al. (2004), brewer’s spent grain was dewatered to a low moisture level of 20–30 % when combining membrane filter pressing (5 bar) with vacuum drying.
21.4.5 Hydraulic press With a hydraulic press system, the holding unit consists of a cylinder equipped with a flexible drainage system composed of a piston where the sludge solids are held back by a filter cloth. The filtration pressure is gradually increased with each pressing step from the piston. The piston and cylinder are in
618 Handbook of water and energy management in food processing
Fig. 21.4 An Andritz Netzsch Filter Press (www.andritz.com/de/ ANONIDZ5DC4CBD110F65B48/ep/ep-products-main/ep-products-mechanical-r_b, last visited February 2008).
constant movement causing new drainage capillaries to be formed in the filter cake ensuring dewatering efficiency (Kolish et al., 2005).
21.4.6 Electro-osmotic dewatering Electro-osmosis is caused by the electrical double layer that exists at the interface of suspended particles subjected to an applied voltage across a solid–liquid mixture. In waste slurries, the solid particles possess a slight electric charge known as the zeta potential. Hence, when exposed to an electric field, the charged particles and the liquid fraction are entrained to move in opposite directions, one towards the anode, the other towards the cathode (Orsat et al., 1996). On one hand, electro-phoresis is the movement of charged particles within solution under the influence of an electrical field, and on the other hand with electro-osmosis, the electric field causes the movement of the electrically neutral solution (Weber and Stahl, 2002). The position of the electrodes is selected in order to promote the gravity flow of water (Chen and Mujumdar, 2002). The product’s properties and mainly its zeta potential will dictate the position of the negative and positive electrodes so as to favour dewatering with gravity flow. The zeta potential of a material is dependent on its composition and the ion concentration of the surrounding fluid. With an increase in the ion concentration of the surrounding fluid there is an improvement of the coagulation of the suspended particle at the price of a decrease of the zeta potential and thus the electro-osmotic flow. As the waste is dewatered by electro-osmosis, a layer of the waste near
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one of the electrodes has a greater water removal causing an increase in the local electrical resistance which hinders the dewatering process (Yoshida and Yasuda, 1992). To overcome this drawback, an increase in mechanical pressure can limit the negative effect of the formation of an unsaturated layer. Evidence of this was found for the electro-osmotic dewatering of food waste, where best results were obtained when combining highest pressure and highest electric field applied since the electro-osmotic flow is proportional to the current density and electric field strength with the movement of dissolved ions within the solid suspension (Gazbar et al., 1994; Orsat et al., 1996; AlAsheh et al., 2004). A schematic presentation of combined pressure and electro-osmotic dewatering apparatus is presented in Fig. 21.5. An increase in salinity from 5000 to 20 000 ppm can increase the dewatered cake solid content by 3–7 % due to the increased conductivity of the slurry allowing an increase in the electric current. This benefit subsequently cancels out with the ensuing decrease in the zeta potential (Chen et al., 2003). Electro-osmotic dewatering can be conducted in DC or AC electric field with varying results. In general, the use of an alternating or intermittent electric field helps to reduce the electrical contact resistance which occurs at the dewatering front with an increased dewatering yield (Yoshida et al., 1999; Iwata, 2000; Yoshida, 2000). In most applications of electro-osmotic dewatering, a vertical electric field is usually applied along with mechanical pressure; however, Zhou et al. Applied mechanical pressure
Top electrode Bed of waste material Electric power source
Filter material Perforated bottom electrode
Liquid
Fig. 21.5
Schematic of a combined pressure and electro-osmotic dewatering cell.
620 Handbook of water and energy management in food processing (2001) proposed a modification with a horizontal electric field which yielded comparable results to a vertical field in terms of dewatering efficiency while offering an alternative design for equipment construction. Furthermore, the application of a horizontal electric field may facilitate the discharge of the gases emitted by oxidation (oxygen) at the anode and by reduction (hydrogen) at the cathode (Jin et al., 2003). A rotating anode arrangement was studied for dewatering fine particle suspensions by Ho and Chen (2001). The system operated at variable speed. The rotating anode mixed the waste material which prevented the formation of a dry layer and thus increased the electric current and the dewatering efficiency. The electro-osmotic dewatering process may be operated at constant voltage or constant current (Orsat et al., 1996; Banerjee and Law, 1998). In constant voltage mode, the flowrate of expressed liquid increases with voltage applied. In such a case, heating of the sludge bed occurs due to joule heating which causes vaporization of part of the water (Banerjee and Law, 1998). In constant current mode, the electric resistance of the bed increases with time and current settings in a quadratic relation. Overall, electro-osmotic dewatering can remove a significant amount of water from a waste suspension at a fraction of the energy consumption that would have been required to vaporize it.
21.5 Combining dewatering methods 21.5.1 Electro-osmotic belt filter Mechanical dewatering removes mostly free water from waste material, while electro-osmotic dewatering aims at pushing more tightly bound water out of the solid matrix. In the case of combined electro-osmosis with a belt filter, the waste material must pass between stainless steel or carbon fiber coated woven belts that serve as the electrode. In certain cases, to prove effective, the waste material may require pre-conditioning with an electrolyte to achieve appropriate conductivity for an efficient combined electro-osmotic belt press dewatering (Hwang and Min, 2003). The total amount of water removed by combined pressure and electro-osmotic dewatering is a measure of both effects with high significance (Vijh, 1999).
21.5.2 Ultrasonic and vibrations Ultrasonic energy may be used to improve the efficiency and capacity of traditional separation/dewatering methods. The ultrasonic vibrations can help the agglomeration of particles to facilitate their collection in the separation process. Furthermore high-intensity ultrasonic energy causes alternative contraction and expansion of a solid–liquid mixture which facilitates the migration of moisture through the porous channels acting as a sponge (Gallego-
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Juarez et al., 1999; Riera-Franco de Sarabia et al., 2000). In some cases, high-intensity ultrasounds can produce cavitation which efficiently moves moisture away. Typically the sound transducers operate in the 10–40 kHz range, and they can be coupled with belt filter press, rotary or centrifuge dewatering (Swamy et al., 1983). Additional dewatering efficiency can also be achieved by adding vibrations to existing press equipment, especially when dewatering viscous or thixotropic waste products. A vibratory action improves the capillary channels in dewatering cakes.
21.5.3 Electro-acoustic dewatering Some applications have been experimentally developed for combining electrical and acoustic fields to enhance dewatering. The results have demonstrated that the acoustic field has little to bring in terms of improving dewatering in comparison to the significant improvement in the rate of dewatering brought about by electro-osmosis at a fraction of the cost of acoustic equipment (Smythe and Wakeman, 2000; Wakeman and Smythe, 2000). In some applications where clogging or fouling of a filter material is common, the application of optimal sound waves (at the distance prescribed by the wavelength) may keep the filtration surface clear of debris accumulation (Tuori, 1992).
21.5.4 Vapour pressure dewatering Vapour pressure dewatering involves mechanical pressure dewatering coupled with a contact drying process (involving a thermal treatment). The waste material is compressed by mechanical means while vaporization of the moisture occurs within the draining capillaries of the waste mass with an indirect application of heat through a heated plate. The indirect application of heat to the dewatering material causes a pressure buildup with the vaporization of the liquid. The temperature gradient in the filter cake causes vaporization and condensation effects within the capillaries, thus improving the dewatering process (Korger and Stahl, 1993). With pressure differences of 1–3 bar, Peuker and Stahl (2001) obtained a residual moisture content of 16 % for the steam pressure filtration of a saturated waste cake. In this study, the waste cake was exposed directly to a steam atmosphere in a closed environment. The steam pushes the liquid out of the pores while the cake is being mechanical compressed (Peuker and Stahl, 2001).
21.6 An environmental and economic choice In conventional drying, the latent heat supplied to vaporize the water content is a considerable energy sink. Moisture reduction by dewatering and/or drying may be an expensive option for waste handling since the concentrated material
622 Handbook of water and energy management in food processing may have very little improvement in its monetary value as a dried product. The processor may also have on their hands a nutrient-rich filtrate liquid component that also becomes an environmental issue and an additional expense for disposal as an effluent in municipal collectors. In some cases, the filtrate has commercial value. In the UK, the expressed juice from brewer spent grain dewatering is being marketed as a liquid protein feed for hogs (Crawshaw, 2001). Penzim Produce, a wholesale and retail fruit and vegetable distributor located in New York City, was generating approximately 2370 t of landfill waste per year and spending over $16 000 per month to dispose of the waste. Before each refuse collection, the perishable produce emitted foul odors, especially during summer months. In collaboration with New York Wa$teMatch (New York City’s materials exchange and solid waste reduction program) and Earth Conserve Management Consultants, the fresh produce company has installed dewatering machinery to reduce the volume of organic material from 8–10 cubic yards a day to 1.5–2 cubic yards a day. Through use of dewatering machinery, Penzim expects to reduce its waste stream by 480 t/y and save about $17 000 y in waste removal and disposal costs (New York Wa$te Match www.wastematch.org). A simple screw press may represent a small investment to reduce the expensive cost of hauling large quantities of water from the processing plant to the farm or landfill. Dewatering means are gaining attention in answer to rising environmental concerns and the rising cost of transport (fuel).
21.7 Conclusion and future trends Dewatering represents an alternative to thermal drying that is more energyefficient. Combining different dewatering forces (pressure, acoustic, electric, chemical, etc.) offers potential for increased yields and improved quality of both liquid and solid fractions, for further processing or application development. Dewatering of biomass is advisable both technically and economically and should be strongly considered for the handling of agrifood waste. Today and is future years the concept of ‘combined field separation’ should be emphasized where two or more fields/properties are exploited in a single equipment for optimized energy usage and synergy of separation effects (Muralidhara, 1992). Future development will no doubt further encourage the coupling of two or more dewatering mechanisms in a single process (Aziz et al., 2006; Abu-Orf et al., 2007). Developments have been reported recently on the coupling of thermal energy to a filter press system. The combination improved the dewaterability of the waste material by reducing the binding power of water (Lee, 2006; Lee et al., 2006). Chuvaree et al. (2006) are proposing a novel design combining filtration, vacuum pressure and thermal drying in a single process.
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Waste conditioning prior to dewatering with enzymes and/or thermal treatments has great scope for future developments at it offers an excellent potential to improve the dewaterability of waste materials while being economically and environmentally friendly. Microwave pre-treatments are showing great potential to increase waste dewaterability by improving the filterability through the capillary matrix (Seehra et al., 2007).
21.8 Sources of further information and advice To find additional information for industrial applications of dewatering to handle food waste, it is advised to conduct a thorough patent search. Furthermore, a large array of manufacturers of dewatering presses have internet sites showcasing their equipment and technology which are worth a look, for example Fluid Technology Inc. (www.fluidtechnologyinc.com), Vincent Corporation (www.vincentcorp.com), Anhydro (www.foodprocessingtechnology.com) among many. A good comprehensive review of the dewatering of biosolids was published in a book by Spellman (1997). Moisture from food waste can be removed in a number of ways, all with their own advantages and disadvantages, as well summarized by Anlauf (2006) in a review of recent developments in solid–liquid separation processes.
21.9 References Abu-Orf M, Junnier R, Mah J and Dentel S (2007) Demonstration of combined dewatering and thermal vacuum drying of municipal residuals, Journal of Residuals Science and Technology, 4(1), 25–34. Al-Asheh A, Jumah R, Banat F and Al-Zou’Bi K (2004) Direct current electro-osmosis dewatering of tomato paste suspension, Trans IChemE Part C, 82(C3), 193–200. Anlauf H (2006) Recent development in research and machinery of solid–liquid separation processes, Drying Technology, 24(10), 1235–1241. Aziz A A A, Dixon D R, Usher S P and Scales P J (2006) Electrically enhanced dewatering of particulate suspensions, Colloids & Surfaces A – Physicochemical & Engineering Aspects, 290(1–3), 194–205. Banerjee S and Law E (1998) Electro-osmotically enhanced drying of biomass, IEEE Trans Industry Applications, 34(5), 992–999. Chen G and Mujumdar A S (2002) Application of electrical fields in dewatering and drying, Developments in Chemical Engineering and Mineral Processing, 10(3/4), 439–441. Chen G, Lai K C K and Lo I M C (2003) Behavior of electro-osmotic dewatering of biological sludge with salinity, Separation Science and Technology, 38(4), 903–915. Choi Y G, Kim H S, Park Y H, Jeong S H, Son D H, Oh Y K and Yeom I T (2005) Improvement of the thickening and dewatering characteristics of activated sludge by electroflotation (EF), Water Science and Technology, 52(10–11), 219–226. Chuvaree R, Nishida N, Otani Y and Tanaka T (2006) Filter press dryer for filtration/ squeezing and drying of slurries, Journal of Chemical Engineering of Japan, 39(3), 298–304.
624 Handbook of water and energy management in food processing Crawshaw R (2001) Co-product Feeds: Animal Feeds from the Food and Drinks Industries, Nottingham, Nottingham University Press. Demetrakakes P (1996) Wringing success, Food Processing, 57(12), 73–74. Dursun D, Turkmen M, Abu-Orf M and Dentel S K (2006) Enhanced sludge conditioning by enzyme pre-treatment: comparison of laboratory and pilot scale dewatering results, Water Science and Technology, 54(5), 33–41. El-Shafey E I, Gameiro M L F, Correia P F M and de Carvalho J M R (2004) Dewatering brewer’s spent grain using a membrane filter press: a pilot plant study, Separation Science and Technology, 39(14), 3237–3261. Fournier Industries Inc. (2003), Effective, economic and easy dewatering of sludges, Filtration and Separation, 40(3), 26–27. Gallego-Juarez J A, Rodriguez-Corral G, Galvex Moraleda J C and Yang T S (1999) A new high-intensity ultrasonic technology for food dehydration, Drying Technology, 17(3), 597–608. Gazbar S, Abadie J M and Colin F (1994) Combined action of electro-osmotic drainage and mechanical compression on sludge dewatering, Water Science and Technology, 30(8), 169–175. Genovese C V and Gonzalez J F (1994) Evaluation of dissolved air flotation applied to fish filleting wastewater, Bioresource Technology, 50(2), 175–179. Ho M Y and Chen G (2001) Enhanced electro-osmotic dewatering of fine particle suspension using a rotating anode, Industrial and Engineering Chemistry Research, 40(8), 1859– 1863. Hwang S and Min KS (2003), Improved sludge dewatering by addition of electro-osmosis to belt filter press, Journal of Environmental Engineering and Science, 2(2), 149–153. Isobe S, Zuber F, Manebag E S, Lite L, Uemura K and Noguchi A (1997) Solid-liquid separation of agricultural products using a twin screw press and electro-osmosis, Japan Agricultural Research Quarterly, 31(2), 137–146. Iwata M (2000) Final moisture distribution in materials after electro-osmotic dewatering, Journal of Chemical Engineering of Japan, 33(2), 308–312. Jin W H, Liu Z and Ding F X (2003) Broth dewatering in a horizontal electric field, Separation Science and Technology, 38(4), 767–778. Kauppila D J, Loughran J G and Kent G A (2001) Fundamental studies into the dewatering of prepared cane and bagasse between grooved surfaces, Proceedings Australian Society of Sugar Cane Technologists, 3, 437–443. Kolish G, Boehler M, Arancibia F C, Pinnow D and Krauss W (2005) A new approach to improve sludge dewatering using a semi-continuous hydraulic press system, Water Science and Technology, 52(10–11), 211–218. Korger V and Stahl W (1993) Vapour pressure dewatering – A new technique in the field of mechanical and thermal solid-liquid separation, Aufbereitungs-Technik, 34(11), 555–563. Kroyer G Th (1995) Impact of food processing on the environment – An overview, Lebensm. – Wiss. U.-Technol, 28(6), 547–552. Kukenberger R J (1996) Conditioning and dewatering, in Girovich M J (ed.), Biosolids Treatment and Management, New York, Marcel Dekker, 131–164. Le Roux L D and Lanting J (2000) Recovery of salable by-products from meat processing plant effluent with the dissolved air flotation process, in Moore J A (ed.), Proceedings 8th International Symposium on Animal, Agricultural and Food Processing Wastes, St Joseph, M I ASABE Publication, 68–75. Lee J E (2006) Thermal dewatering to reduce the water content of sludge, Drying Technology, 24(2), 225–232. Lee D J, Lai J Y and Mujumdar A S (2006) Moisture distribution and dewatering efficiency for wet materials, Drying Technology, 24(10), 1201–1208. Leung W W F (1998) Torque requirement for high solids centrifugal sludge dewatering, Filtration and Separation, 35(9), 883–887.
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Lightfoot D and Raghavan G S V (1995) Combined field dewatering of seaweed with a roller press, Applied Engineering in Agriculture, 11(2), 291–295. Muralidhara H S (1990) Combined fields dewatering techniques, in Roques M and Mujumdar A S (eds) Drying’89, New York, Hemisphere Publishing Corporation, 70–75. Muralidhara H S (1992) Advanced dewatering techniques and their impact on drying technologies, in Mujumdar A S (ed.), Drying’92, Amsterdam, Elsevier Science Publishers, 200–208. Ng W J, Goh A C C and Tay J H (1988) Palm oil mill effluent treatment – liquid–solid separation with dissolved air flotation, Biological Wastes, 25(4), 257–268. Ormeci B (2007) Optimization of a full-scale dewatering operation based on the rheological characteristics of wastewater sludge, Water Research, 41(6), 1243–1252. Orsat V, Raghavan G S V and Norris E R (1996) Food processing waste dewatering by electro-osmosis, Canadian Agricultural Engineering, 38(1), 63–67. Orsat V, Raghavan G S V, Sotocinal S, Lightfoot D G and Gopalakrishnan S (1999) Roller press for electro-osmotic dewatering of biomaterials, Drying Technology, 17(3), 523–538. Peuker U A and Stahl W (2001) Steam pressure filtration: mechanical-thermal dewatering process, Drying Technology, 19(5), 807–848. Riera-Franco de Sarabia E, Gallego-Juarex J A, Rodriguez-Corral G, Elvira-Segura L and Gonzalez-Gomez I (2000) Application of high-power ultrasound to enhance fluid/ solid particle separation processes, Ultrasonics, 38(1–8), 642–646. Ruiz T, Wisniewski C, Kaosol T and Persin F (2007) Influence of organic content in dewatering and shrinkage of urban residual sludge under controlled atmospheric drying, Process Safety and Environmental Protection, 85(B1), 104–110. Seehra M S, Kalra A and Manivannan A (2007) Dewatering of fine coal slurried by selective heating with microwaves, Fuel, 86(5–6), 829–834. Sinha S, Sokhansanj S, Crerar W J, Yang W, Tabil L G, Khoshtaghaza M H and Patil R T (2000) Mechanical dewatering of chopped alfalfa using an experimental piston– cylinder assembly, Canadian Agricultural Engineering, 42(3), 153–156. Smythe M C and Wakeman R J (2000) The use of acoustic fields as a filtration and dewatering aid, Ultrasonics, 38(1–8) 657–661. Snyman H G, Forssman P, Kafaar A and Smollen M (2000) The feasibility of electroosmotic belt filter dewatering technology at pilot scale, Water Science and Technology, 41(8), 137–144. Spellman F R (1997) Dewatering Biosolids, Lancaster, PA, Technomic. Swamy K M, Rao A R K and Narsimhan K S (1983) Acoustics aids dewatering, Ultrasonics, 21(6), 280–281. Trias M, Mortula M M, Hu Z and Gagnon G A (2004) Optimizing settling conditions for treatment of liquid hog manure, Environmental Technology, 25, 957–965. Tuori T K (1992) Enhancing the filtration of bio/filter sludge by the electro-acoustic method, in Mujumdar A S (ed.), Drying’92, Amsterdam, Elsevier 1897–1913. US EPA (2003) Biosolids Technology Fact Sheet: Gravity Thickening, EPA 832-F-03022, Washington, DC, United States Environmental Protection Agency. Viitasaari M, Jokela P and Heinänen J (1995) Dissolved air flotation in the treatment of industrial wastewaters with a special emphasis on forest and foodstuff industries, Water Science and Technology, 31(3–4), 299–313. Vijh A K (1999) The significance of current observed during combined field and pressure electro-osmotic dewatering of clays, Drying Technology, 17(3), 555–563. Wakeman R J and Smythe M C (2000) Clarifying filtration of fine particle suspensions aided by electrical and acoustic fields, Trans IChemE, 78(A), 125–135. Weber K and Stahl W (2002) Improvement of filtration kinetics by pressure filtration, Separation and Purification Technology, 26(1), 69–80. Yoshida H (2000) Electro-osmotic dewatering under intermittent power application by rectification of A.C. Electric field, Journal of Chemical Engineering of Japan, 33(1), 134–140.
626 Handbook of water and energy management in food processing Yoshida H, Kitajyo K and Nakayama M (1999) Electro-osmotic dewatering under A.C. electric field with periodic reversals of electrode polarity, Drying Technology, 17(3), 539–554. Yoshida H and Yasuda A (1992) Analysis of pressurized electro-osmotic dewatering of semi-solid sludge, in: Mujumdar A S (ed.) Drying’92, Amsterdam, Elsevier Science Publishers, 1814–1821. Zhou J, Liu Z, She P and Ding F (2001) Water removal from sludge in a horizontal electric field, Drying Technology, 19(3–4), 627–638.
21.1 Introduction The food industry produces a variety of wastes which require handling in an environmentally-friendly and sustainable way. Depending on the type of waste, numerous waste handling alternatives are available from fermentation, separation, biofuel conversion, composting, extraction and more. Choosing the right waste handling process can help to meet environmental regulations and provide a useful by-product for further processing, recovery or animal consumption. A lower moisture content of the waste material reduces the cost of transport due to reduced volume and weight. This chapter discusses the concentration of solids from food waste using dewatering techniques. The reduction of moisture content offers flexibility in terms of handling, shelf-life and subsequent use of the waste. A liquid/solid separation process involves multiple steps: pretreatment, thickening, separation and post-treatment (Trias et al., 2004). Common dewatering processes use mechanical means of separation such as screens, screw presses, belt presses, vacuum filters and centrifuges, which can all be combined with additional forces to remove the water such as an electric field, ultrasonics, vibrations, chemical treatments, etc. In any dewatering application there is a definite advantage of combining multiple dewatering fields to promote the synergy of separation forces (Muralidhara, 1990). The selection of an adequate dewatering process depends on numerous factors such as the type and quantity of the waste product, the end-use of the dewatered/dried solids and environmental and economic considerations. In general, food wastes contain large amounts of organic materials,
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high biochemical oxygen demand and high variations in pH (Kroyer, 1995). With a dewatering process, the underlying advantage is that the water is removed in the liquid state. The lack of a phase change renders the process less energy-intensive and in some instances may improve the end-product quality. Dewatering lowers the moisture content to a level not low enough for shelf stability and thus the dewatered material requires a finish drying treatment or further processing.
21.2 Waste conditioning When considering the adoption of a dewatering process for any given waste material, the initial characteristics of that sludge will govern the choice to be made and the handling process adopted. Waste characteristics of particular interest are particle size, particle charge, pH, organic content and viscosity (Ormeci, 2007; Ruiz et al., 2007). The smaller the particle size, the greater is the water retention from the larger specific surface area leading to more unfavourable dewatering conditions (Kolish et al., 2005). In a liquid/solid waste mixture, the water content to be expressed can be present as bulk or free water, as capillary bound water or as adsorbed bound water. In certain cases, the sludge may be too liquid and may require a chemical pre-treatment to improve the size of the solid particles through flocculation/agglomeration, or the waste may be too solid where the water is retained, for example, within the cellular structure of the plant-based waste material. In the latter case, a grinding process may help release some of that moisture, or a freezing pre-treatment may serve the purpose through cellular breakdown. A pretreatment by freezing studied by Zhou et al. (2001) gave significantly enhanced water removal. In the case of mechanical dewatering of chopped alfalfa, the best water extraction results were obtained with previously macerated alfalfa which released the cellular bound moisture content (Sinha et al., 2000). In the case of kelp dewatering, the slurry is highly viscous and requires a chemical treatment (calcium chloride) to release the water (Lightfoot and Raghavan 1995; Orsat et al., 1999). Enzymatic pre-treatment was studied by Dursun et al. (2006) and shown to weaken the sludge structure, thus improving the filtration process.
21.3 Thickening In the case of liquid wastes, a thickening process prior to dewatering can reduce equipment size requirements and ensure higher throughput during dewatering (Kukenberger, 1996). Thickening processes include gravity thickening and dissolved air flotation.
614 Handbook of water and energy management in food processing 21.3.1 Gravity thickening Gravity thickening is traditionally conducted in cone-shaped bottom circular tanks equipped with collectors or scrapers placed at the bottom for solids collection. The solids slowly settle to the bottom of the tank from the gravity pull from their own weight. Gravity thickening is simple to operate and maintain while the solids collected at the bottom of the tank reach between 4 and 6 % of total solids (US EPA, 2003).
21.3.2 Dissolved air flotation and electroflotation separation Dissolved air flotation (DAF) is a liquid/solid separation process for liquid suspended colloidal mixtures. DAF is principally used for the clarification of wastewaters which contain suspended solids. DAF is used widely for the recovery of valuable solids from food processing wastewaters, especially from the meat processing industry (Le Roux and Lanting, 2000). DAF involves the dissolution of air in the waste mixture at a high pressure to achieve saturation. By bringing the pressure of the mixture back to atmospheric, the air in the form of very small bubbles rises to the product’s surface carrying with it the colloidal particles which can be recovered. Improving the DAF process, depending on the type of waste material, may require the addition of a chemical flocculant or coagulant (such as a polyelectrolyte) as a pretreatment step for the waste slurry (Ng et al., 1988; Genovese and Gonzalez, 1994). In general a DAF system consists of a flocculator tank, a flotation tank and a pressure vessel. Its operation can be continuous or intermittent and it can be designed and constructed to meet the requirements of a variety of wastewaters in terms of characteristics of its suspended solids and the plant volume requirement for separation (Viitasaari et al., 1995). Similarly, electroflotation removes suspended solids using gas bubbles obtained through water electrolysis. For that purpose, electrodes are placed at the bottom of the tank containing the wastewater (Fig. 21.1). As current is passed through the electrodes, the water is electrolyzed producing bubbles of gaseous hydrogen and oxygen. As the bubbles work their way up to the top of the tank they entrain suspended particles to the surface of the liquid waste where they can be skimmed. Electroflotation is more effective in thickening when compared with gravity thickening. It is an interesting alternative to the separation of low-density suspended solids, especially for its simplicity of equipment design and operation (Choi et al., 2005).
21.4
Dewatering methods
21.4.1 Belt filter press Belt filter press systems usually include a gravity drainage feeding section and a mechanically applied pressure belt arrangement. In gravity drainage, through simple screens, a large portion of free water is removed. Pressure is
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Skimmer
Surface
Electrodes + –
Gas bubbles
Fig. 21.1
Suspended particles
Schematic of an electroflotation process.
then applied at an increasing rate on the waste contained between supporting porous belt material (Demetrakakes, 1996). The dewatered waste cake is removed from the belt with scrapers. In certain arrangements, a small vacuum must be applied (4–6 kPa) to facilitate the removal of water accumulating at the surface of the belts (Snyman et al., 2000). A pre-flocculation step is often considered to suit particular waste applications that are too liquid and to maximize dewatering efficiency right from the start of the process during gravity drainage. In roller press dewatering, the waste material is pressed between rotating roller drums, where the single belt material serves only for conveying. The bottom rollers are perforated to allow for drainage of the pressed liquid. The basic system is composed of a top roller which presses down onto two bottom rollers with the drums rotating so as to facilitate the passage through of the material using a conveyor belt (Orsat et al., 1999). This system can be well adapted to combine with electro-osmotic dewatering. Design improvements were investigated by Kauppila et al. (2001) for roller groove geometries. It appears that larger groove angles can help reduce further the moisture content during roller pressing of sugar cane bagasse.
21.4.2 Screw press dewatering In a screw press, the material is introduced in a perforated chamber where an endless screw forces the material along the length of the chamber towards discharge. The pressure force of the screw drives the water out through the perforations of the holding chamber. For this type of dewatering process, the waste feed must have a certain particle size large enough not to clog the perforations of the holding system and to flow through without excessive resistance.
616 Handbook of water and energy management in food processing When feeding through becomes more problematic due to the waste product’s characteristics (particles size, viscosity, etc.), a twin screw press may be more appropriate (Fig. 21.2). The two press screws are designed to compress the product as they rotate in opposite directions, which prevent the waste material from rotating with the screws and clogging up the system. Twin screw dewatering has been successfully developed and has proven efficient for citrus waste and for oil extraction from agricultural products (Isobe et al., 1997). Screw press dewatering of citrus pulp is practised in the industry to yield a higher dry matter pulp and a liquid fraction high in soluble solids. The liquid fraction is further processed to produce a citrus molasses, whereas the citrus pulp can be used as animal feed for ruminants thus fulfilling the requirement of high fibre content leading to high digestibility (Crawshaw, 2001).
21.4.3 Rotary and centrifugal presses A centrifugal dewatering system consists of a basket or a solid bowl and a conveyor, both of which can rotate at high speed. As the bowl rotates, the heavier solids gravitate to the bowl wall where they accumulate. The separation of solids from the liquid depends on the G-force, time and permeability of the waste mass (Leung, 1998). The Rotary Press manufactured by Fournier Industries Inc. Quebec, Canada (www.rotarypress.com) offers the industry interesting dewatering equipment (Fig. 21.3). The waste material is fed into a rectangular channel and rotated between two parallel revolving stainless steel chrome plated screens. The filtrate passes through the screens as the particulate sludge advances within
Fig. 21.2
Twin screw arrangement (Vincent Corporation, Florida, USA,www.vincentcorp.com).
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Screen Cake formation
Seal Cover Interior of channel Outlet for filtrate
A
A Filtrate Filtrate section A–A
Fig. 21.3
Restriction zone
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Sludge Static mechanical component Rotating mechanical component Dewatering sludge (cake) Filtrate flow
Sludge feed Cake extrusion
Schematic of the rotary press manufactured by Fournier Industries Inc. (Fournier Industries Inc., 2003).
the channel. The sludge continues to dewater as it travels through the channel, eventually forming a cake near the outlet side of the press. The frictional force of the slow-moving screens, coupled with the outlet restriction, results in the extrusion of low-moisture material.
21.4.4 Membrane filter press A membrane filter press comprises a series of filter plates held tightly together by pressure (Fig. 21.4). The filter plates have a filtration drainage surface that supports a filter medium, in most cases a polypropylene filter cloth held in place by a more rigid polypropylene structure. The mixed solid/liquid waste is pumped into the chambers under pressure. The filtered liquid passes through the filter cloth, against the drainage surface of the plates, and is directed towards discharge collectors. The pressure gradient between the cake and the filter material provides the driving force for the flow. Solids are retained on the filter cloth forming a filter cake. The filter plates are separated and the filter cake is discharged. At this stage a vacuum step may be introduced to further reduce the moisture content. In a study by El-Shafey et al. (2004), brewer’s spent grain was dewatered to a low moisture level of 20–30 % when combining membrane filter pressing (5 bar) with vacuum drying.
21.4.5 Hydraulic press With a hydraulic press system, the holding unit consists of a cylinder equipped with a flexible drainage system composed of a piston where the sludge solids are held back by a filter cloth. The filtration pressure is gradually increased with each pressing step from the piston. The piston and cylinder are in
618 Handbook of water and energy management in food processing
Fig. 21.4 An Andritz Netzsch Filter Press (www.andritz.com/de/ ANONIDZ5DC4CBD110F65B48/ep/ep-products-main/ep-products-mechanical-r_b, last visited February 2008).
constant movement causing new drainage capillaries to be formed in the filter cake ensuring dewatering efficiency (Kolish et al., 2005).
21.4.6 Electro-osmotic dewatering Electro-osmosis is caused by the electrical double layer that exists at the interface of suspended particles subjected to an applied voltage across a solid–liquid mixture. In waste slurries, the solid particles possess a slight electric charge known as the zeta potential. Hence, when exposed to an electric field, the charged particles and the liquid fraction are entrained to move in opposite directions, one towards the anode, the other towards the cathode (Orsat et al., 1996). On one hand, electro-phoresis is the movement of charged particles within solution under the influence of an electrical field, and on the other hand with electro-osmosis, the electric field causes the movement of the electrically neutral solution (Weber and Stahl, 2002). The position of the electrodes is selected in order to promote the gravity flow of water (Chen and Mujumdar, 2002). The product’s properties and mainly its zeta potential will dictate the position of the negative and positive electrodes so as to favour dewatering with gravity flow. The zeta potential of a material is dependent on its composition and the ion concentration of the surrounding fluid. With an increase in the ion concentration of the surrounding fluid there is an improvement of the coagulation of the suspended particle at the price of a decrease of the zeta potential and thus the electro-osmotic flow. As the waste is dewatered by electro-osmosis, a layer of the waste near
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one of the electrodes has a greater water removal causing an increase in the local electrical resistance which hinders the dewatering process (Yoshida and Yasuda, 1992). To overcome this drawback, an increase in mechanical pressure can limit the negative effect of the formation of an unsaturated layer. Evidence of this was found for the electro-osmotic dewatering of food waste, where best results were obtained when combining highest pressure and highest electric field applied since the electro-osmotic flow is proportional to the current density and electric field strength with the movement of dissolved ions within the solid suspension (Gazbar et al., 1994; Orsat et al., 1996; AlAsheh et al., 2004). A schematic presentation of combined pressure and electro-osmotic dewatering apparatus is presented in Fig. 21.5. An increase in salinity from 5000 to 20 000 ppm can increase the dewatered cake solid content by 3–7 % due to the increased conductivity of the slurry allowing an increase in the electric current. This benefit subsequently cancels out with the ensuing decrease in the zeta potential (Chen et al., 2003). Electro-osmotic dewatering can be conducted in DC or AC electric field with varying results. In general, the use of an alternating or intermittent electric field helps to reduce the electrical contact resistance which occurs at the dewatering front with an increased dewatering yield (Yoshida et al., 1999; Iwata, 2000; Yoshida, 2000). In most applications of electro-osmotic dewatering, a vertical electric field is usually applied along with mechanical pressure; however, Zhou et al. Applied mechanical pressure
Top electrode Bed of waste material Electric power source
Filter material Perforated bottom electrode
Liquid
Fig. 21.5
Schematic of a combined pressure and electro-osmotic dewatering cell.
620 Handbook of water and energy management in food processing (2001) proposed a modification with a horizontal electric field which yielded comparable results to a vertical field in terms of dewatering efficiency while offering an alternative design for equipment construction. Furthermore, the application of a horizontal electric field may facilitate the discharge of the gases emitted by oxidation (oxygen) at the anode and by reduction (hydrogen) at the cathode (Jin et al., 2003). A rotating anode arrangement was studied for dewatering fine particle suspensions by Ho and Chen (2001). The system operated at variable speed. The rotating anode mixed the waste material which prevented the formation of a dry layer and thus increased the electric current and the dewatering efficiency. The electro-osmotic dewatering process may be operated at constant voltage or constant current (Orsat et al., 1996; Banerjee and Law, 1998). In constant voltage mode, the flowrate of expressed liquid increases with voltage applied. In such a case, heating of the sludge bed occurs due to joule heating which causes vaporization of part of the water (Banerjee and Law, 1998). In constant current mode, the electric resistance of the bed increases with time and current settings in a quadratic relation. Overall, electro-osmotic dewatering can remove a significant amount of water from a waste suspension at a fraction of the energy consumption that would have been required to vaporize it.
21.5 Combining dewatering methods 21.5.1 Electro-osmotic belt filter Mechanical dewatering removes mostly free water from waste material, while electro-osmotic dewatering aims at pushing more tightly bound water out of the solid matrix. In the case of combined electro-osmosis with a belt filter, the waste material must pass between stainless steel or carbon fiber coated woven belts that serve as the electrode. In certain cases, to prove effective, the waste material may require pre-conditioning with an electrolyte to achieve appropriate conductivity for an efficient combined electro-osmotic belt press dewatering (Hwang and Min, 2003). The total amount of water removed by combined pressure and electro-osmotic dewatering is a measure of both effects with high significance (Vijh, 1999).
21.5.2 Ultrasonic and vibrations Ultrasonic energy may be used to improve the efficiency and capacity of traditional separation/dewatering methods. The ultrasonic vibrations can help the agglomeration of particles to facilitate their collection in the separation process. Furthermore high-intensity ultrasonic energy causes alternative contraction and expansion of a solid–liquid mixture which facilitates the migration of moisture through the porous channels acting as a sponge (Gallego-
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Juarez et al., 1999; Riera-Franco de Sarabia et al., 2000). In some cases, high-intensity ultrasounds can produce cavitation which efficiently moves moisture away. Typically the sound transducers operate in the 10–40 kHz range, and they can be coupled with belt filter press, rotary or centrifuge dewatering (Swamy et al., 1983). Additional dewatering efficiency can also be achieved by adding vibrations to existing press equipment, especially when dewatering viscous or thixotropic waste products. A vibratory action improves the capillary channels in dewatering cakes.
21.5.3 Electro-acoustic dewatering Some applications have been experimentally developed for combining electrical and acoustic fields to enhance dewatering. The results have demonstrated that the acoustic field has little to bring in terms of improving dewatering in comparison to the significant improvement in the rate of dewatering brought about by electro-osmosis at a fraction of the cost of acoustic equipment (Smythe and Wakeman, 2000; Wakeman and Smythe, 2000). In some applications where clogging or fouling of a filter material is common, the application of optimal sound waves (at the distance prescribed by the wavelength) may keep the filtration surface clear of debris accumulation (Tuori, 1992).
21.5.4 Vapour pressure dewatering Vapour pressure dewatering involves mechanical pressure dewatering coupled with a contact drying process (involving a thermal treatment). The waste material is compressed by mechanical means while vaporization of the moisture occurs within the draining capillaries of the waste mass with an indirect application of heat through a heated plate. The indirect application of heat to the dewatering material causes a pressure buildup with the vaporization of the liquid. The temperature gradient in the filter cake causes vaporization and condensation effects within the capillaries, thus improving the dewatering process (Korger and Stahl, 1993). With pressure differences of 1–3 bar, Peuker and Stahl (2001) obtained a residual moisture content of 16 % for the steam pressure filtration of a saturated waste cake. In this study, the waste cake was exposed directly to a steam atmosphere in a closed environment. The steam pushes the liquid out of the pores while the cake is being mechanical compressed (Peuker and Stahl, 2001).
21.6 An environmental and economic choice In conventional drying, the latent heat supplied to vaporize the water content is a considerable energy sink. Moisture reduction by dewatering and/or drying may be an expensive option for waste handling since the concentrated material
622 Handbook of water and energy management in food processing may have very little improvement in its monetary value as a dried product. The processor may also have on their hands a nutrient-rich filtrate liquid component that also becomes an environmental issue and an additional expense for disposal as an effluent in municipal collectors. In some cases, the filtrate has commercial value. In the UK, the expressed juice from brewer spent grain dewatering is being marketed as a liquid protein feed for hogs (Crawshaw, 2001). Penzim Produce, a wholesale and retail fruit and vegetable distributor located in New York City, was generating approximately 2370 t of landfill waste per year and spending over $16 000 per month to dispose of the waste. Before each refuse collection, the perishable produce emitted foul odors, especially during summer months. In collaboration with New York Wa$teMatch (New York City’s materials exchange and solid waste reduction program) and Earth Conserve Management Consultants, the fresh produce company has installed dewatering machinery to reduce the volume of organic material from 8–10 cubic yards a day to 1.5–2 cubic yards a day. Through use of dewatering machinery, Penzim expects to reduce its waste stream by 480 t/y and save about $17 000 y in waste removal and disposal costs (New York Wa$te Match www.wastematch.org). A simple screw press may represent a small investment to reduce the expensive cost of hauling large quantities of water from the processing plant to the farm or landfill. Dewatering means are gaining attention in answer to rising environmental concerns and the rising cost of transport (fuel).
21.7 Conclusion and future trends Dewatering represents an alternative to thermal drying that is more energyefficient. Combining different dewatering forces (pressure, acoustic, electric, chemical, etc.) offers potential for increased yields and improved quality of both liquid and solid fractions, for further processing or application development. Dewatering of biomass is advisable both technically and economically and should be strongly considered for the handling of agrifood waste. Today and is future years the concept of ‘combined field separation’ should be emphasized where two or more fields/properties are exploited in a single equipment for optimized energy usage and synergy of separation effects (Muralidhara, 1992). Future development will no doubt further encourage the coupling of two or more dewatering mechanisms in a single process (Aziz et al., 2006; Abu-Orf et al., 2007). Developments have been reported recently on the coupling of thermal energy to a filter press system. The combination improved the dewaterability of the waste material by reducing the binding power of water (Lee, 2006; Lee et al., 2006). Chuvaree et al. (2006) are proposing a novel design combining filtration, vacuum pressure and thermal drying in a single process.
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Waste conditioning prior to dewatering with enzymes and/or thermal treatments has great scope for future developments at it offers an excellent potential to improve the dewaterability of waste materials while being economically and environmentally friendly. Microwave pre-treatments are showing great potential to increase waste dewaterability by improving the filterability through the capillary matrix (Seehra et al., 2007).
21.8 Sources of further information and advice To find additional information for industrial applications of dewatering to handle food waste, it is advised to conduct a thorough patent search. Furthermore, a large array of manufacturers of dewatering presses have internet sites showcasing their equipment and technology which are worth a look, for example Fluid Technology Inc. (www.fluidtechnologyinc.com), Vincent Corporation (www.vincentcorp.com), Anhydro (www.foodprocessingtechnology.com) among many. A good comprehensive review of the dewatering of biosolids was published in a book by Spellman (1997). Moisture from food waste can be removed in a number of ways, all with their own advantages and disadvantages, as well summarized by Anlauf (2006) in a review of recent developments in solid–liquid separation processes.
21.9 References Abu-Orf M, Junnier R, Mah J and Dentel S (2007) Demonstration of combined dewatering and thermal vacuum drying of municipal residuals, Journal of Residuals Science and Technology, 4(1), 25–34. Al-Asheh A, Jumah R, Banat F and Al-Zou’Bi K (2004) Direct current electro-osmosis dewatering of tomato paste suspension, Trans IChemE Part C, 82(C3), 193–200. Anlauf H (2006) Recent development in research and machinery of solid–liquid separation processes, Drying Technology, 24(10), 1235–1241. Aziz A A A, Dixon D R, Usher S P and Scales P J (2006) Electrically enhanced dewatering of particulate suspensions, Colloids & Surfaces A – Physicochemical & Engineering Aspects, 290(1–3), 194–205. Banerjee S and Law E (1998) Electro-osmotically enhanced drying of biomass, IEEE Trans Industry Applications, 34(5), 992–999. Chen G and Mujumdar A S (2002) Application of electrical fields in dewatering and drying, Developments in Chemical Engineering and Mineral Processing, 10(3/4), 439–441. Chen G, Lai K C K and Lo I M C (2003) Behavior of electro-osmotic dewatering of biological sludge with salinity, Separation Science and Technology, 38(4), 903–915. Choi Y G, Kim H S, Park Y H, Jeong S H, Son D H, Oh Y K and Yeom I T (2005) Improvement of the thickening and dewatering characteristics of activated sludge by electroflotation (EF), Water Science and Technology, 52(10–11), 219–226. Chuvaree R, Nishida N, Otani Y and Tanaka T (2006) Filter press dryer for filtration/ squeezing and drying of slurries, Journal of Chemical Engineering of Japan, 39(3), 298–304.
624 Handbook of water and energy management in food processing Crawshaw R (2001) Co-product Feeds: Animal Feeds from the Food and Drinks Industries, Nottingham, Nottingham University Press. Demetrakakes P (1996) Wringing success, Food Processing, 57(12), 73–74. Dursun D, Turkmen M, Abu-Orf M and Dentel S K (2006) Enhanced sludge conditioning by enzyme pre-treatment: comparison of laboratory and pilot scale dewatering results, Water Science and Technology, 54(5), 33–41. El-Shafey E I, Gameiro M L F, Correia P F M and de Carvalho J M R (2004) Dewatering brewer’s spent grain using a membrane filter press: a pilot plant study, Separation Science and Technology, 39(14), 3237–3261. Fournier Industries Inc. (2003), Effective, economic and easy dewatering of sludges, Filtration and Separation, 40(3), 26–27. Gallego-Juarez J A, Rodriguez-Corral G, Galvex Moraleda J C and Yang T S (1999) A new high-intensity ultrasonic technology for food dehydration, Drying Technology, 17(3), 597–608. Gazbar S, Abadie J M and Colin F (1994) Combined action of electro-osmotic drainage and mechanical compression on sludge dewatering, Water Science and Technology, 30(8), 169–175. Genovese C V and Gonzalez J F (1994) Evaluation of dissolved air flotation applied to fish filleting wastewater, Bioresource Technology, 50(2), 175–179. Ho M Y and Chen G (2001) Enhanced electro-osmotic dewatering of fine particle suspension using a rotating anode, Industrial and Engineering Chemistry Research, 40(8), 1859– 1863. Hwang S and Min KS (2003), Improved sludge dewatering by addition of electro-osmosis to belt filter press, Journal of Environmental Engineering and Science, 2(2), 149–153. Isobe S, Zuber F, Manebag E S, Lite L, Uemura K and Noguchi A (1997) Solid-liquid separation of agricultural products using a twin screw press and electro-osmosis, Japan Agricultural Research Quarterly, 31(2), 137–146. Iwata M (2000) Final moisture distribution in materials after electro-osmotic dewatering, Journal of Chemical Engineering of Japan, 33(2), 308–312. Jin W H, Liu Z and Ding F X (2003) Broth dewatering in a horizontal electric field, Separation Science and Technology, 38(4), 767–778. Kauppila D J, Loughran J G and Kent G A (2001) Fundamental studies into the dewatering of prepared cane and bagasse between grooved surfaces, Proceedings Australian Society of Sugar Cane Technologists, 3, 437–443. Kolish G, Boehler M, Arancibia F C, Pinnow D and Krauss W (2005) A new approach to improve sludge dewatering using a semi-continuous hydraulic press system, Water Science and Technology, 52(10–11), 211–218. Korger V and Stahl W (1993) Vapour pressure dewatering – A new technique in the field of mechanical and thermal solid-liquid separation, Aufbereitungs-Technik, 34(11), 555–563. Kroyer G Th (1995) Impact of food processing on the environment – An overview, Lebensm. – Wiss. U.-Technol, 28(6), 547–552. Kukenberger R J (1996) Conditioning and dewatering, in Girovich M J (ed.), Biosolids Treatment and Management, New York, Marcel Dekker, 131–164. Le Roux L D and Lanting J (2000) Recovery of salable by-products from meat processing plant effluent with the dissolved air flotation process, in Moore J A (ed.), Proceedings 8th International Symposium on Animal, Agricultural and Food Processing Wastes, St Joseph, M I ASABE Publication, 68–75. Lee J E (2006) Thermal dewatering to reduce the water content of sludge, Drying Technology, 24(2), 225–232. Lee D J, Lai J Y and Mujumdar A S (2006) Moisture distribution and dewatering efficiency for wet materials, Drying Technology, 24(10), 1201–1208. Leung W W F (1998) Torque requirement for high solids centrifugal sludge dewatering, Filtration and Separation, 35(9), 883–887.
Dewatering for food waste
625
Lightfoot D and Raghavan G S V (1995) Combined field dewatering of seaweed with a roller press, Applied Engineering in Agriculture, 11(2), 291–295. Muralidhara H S (1990) Combined fields dewatering techniques, in Roques M and Mujumdar A S (eds) Drying’89, New York, Hemisphere Publishing Corporation, 70–75. Muralidhara H S (1992) Advanced dewatering techniques and their impact on drying technologies, in Mujumdar A S (ed.), Drying’92, Amsterdam, Elsevier Science Publishers, 200–208. Ng W J, Goh A C C and Tay J H (1988) Palm oil mill effluent treatment – liquid–solid separation with dissolved air flotation, Biological Wastes, 25(4), 257–268. Ormeci B (2007) Optimization of a full-scale dewatering operation based on the rheological characteristics of wastewater sludge, Water Research, 41(6), 1243–1252. Orsat V, Raghavan G S V and Norris E R (1996) Food processing waste dewatering by electro-osmosis, Canadian Agricultural Engineering, 38(1), 63–67. Orsat V, Raghavan G S V, Sotocinal S, Lightfoot D G and Gopalakrishnan S (1999) Roller press for electro-osmotic dewatering of biomaterials, Drying Technology, 17(3), 523–538. Peuker U A and Stahl W (2001) Steam pressure filtration: mechanical-thermal dewatering process, Drying Technology, 19(5), 807–848. Riera-Franco de Sarabia E, Gallego-Juarex J A, Rodriguez-Corral G, Elvira-Segura L and Gonzalez-Gomez I (2000) Application of high-power ultrasound to enhance fluid/ solid particle separation processes, Ultrasonics, 38(1–8), 642–646. Ruiz T, Wisniewski C, Kaosol T and Persin F (2007) Influence of organic content in dewatering and shrinkage of urban residual sludge under controlled atmospheric drying, Process Safety and Environmental Protection, 85(B1), 104–110. Seehra M S, Kalra A and Manivannan A (2007) Dewatering of fine coal slurried by selective heating with microwaves, Fuel, 86(5–6), 829–834. Sinha S, Sokhansanj S, Crerar W J, Yang W, Tabil L G, Khoshtaghaza M H and Patil R T (2000) Mechanical dewatering of chopped alfalfa using an experimental piston– cylinder assembly, Canadian Agricultural Engineering, 42(3), 153–156. Smythe M C and Wakeman R J (2000) The use of acoustic fields as a filtration and dewatering aid, Ultrasonics, 38(1–8) 657–661. Snyman H G, Forssman P, Kafaar A and Smollen M (2000) The feasibility of electroosmotic belt filter dewatering technology at pilot scale, Water Science and Technology, 41(8), 137–144. Spellman F R (1997) Dewatering Biosolids, Lancaster, PA, Technomic. Swamy K M, Rao A R K and Narsimhan K S (1983) Acoustics aids dewatering, Ultrasonics, 21(6), 280–281. Trias M, Mortula M M, Hu Z and Gagnon G A (2004) Optimizing settling conditions for treatment of liquid hog manure, Environmental Technology, 25, 957–965. Tuori T K (1992) Enhancing the filtration of bio/filter sludge by the electro-acoustic method, in Mujumdar A S (ed.), Drying’92, Amsterdam, Elsevier 1897–1913. US EPA (2003) Biosolids Technology Fact Sheet: Gravity Thickening, EPA 832-F-03022, Washington, DC, United States Environmental Protection Agency. Viitasaari M, Jokela P and Heinänen J (1995) Dissolved air flotation in the treatment of industrial wastewaters with a special emphasis on forest and foodstuff industries, Water Science and Technology, 31(3–4), 299–313. Vijh A K (1999) The significance of current observed during combined field and pressure electro-osmotic dewatering of clays, Drying Technology, 17(3), 555–563. Wakeman R J and Smythe M C (2000) Clarifying filtration of fine particle suspensions aided by electrical and acoustic fields, Trans IChemE, 78(A), 125–135. Weber K and Stahl W (2002) Improvement of filtration kinetics by pressure filtration, Separation and Purification Technology, 26(1), 69–80. Yoshida H (2000) Electro-osmotic dewatering under intermittent power application by rectification of A.C. Electric field, Journal of Chemical Engineering of Japan, 33(1), 134–140.
626 Handbook of water and energy management in food processing Yoshida H, Kitajyo K and Nakayama M (1999) Electro-osmotic dewatering under A.C. electric field with periodic reversals of electrode polarity, Drying Technology, 17(3), 539–554. Yoshida H and Yasuda A (1992) Analysis of pressurized electro-osmotic dewatering of semi-solid sludge, in: Mujumdar A S (ed.) Drying’92, Amsterdam, Elsevier Science Publishers, 1814–1821. Zhou J, Liu Z, She P and Ding F (2001) Water removal from sludge in a horizontal electric field, Drying Technology, 19(3–4), 627–638.