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Review: Alternative energy from food processingwastes

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Impact Factor: 1.31 · DOI: 10.1002/ep.10312

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Review: Alternative Energy from

Food Processing WastesBrett Digman and Dong-Shik KimDepartment of Chemical and Environmental Engineering, University of Toledo, Toledo, OH 43606; [email protected] (forcorrespondence)

Published online 14 October 2008 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10312

Food processes generate large amounts of various waste types. The environmental impact of these wastes is generally high if disposed untreated due to their high BOD and COD contents. By utilizing the high

potential energy available in food processing wastes, sustainable energy can be obtained while reducing the waste disposal costs and impact on the environ-ment. According to the unique compositions of vari-ous food processing wastes, proper conversion meth-ods must be selected to optimize the economic and environmental benefits. In this review, food process-ing wastes are categorized in six groups, and various energy conversion technologies are addressed in asso-

ciation with them. Future direction for better utiliz-ing food process wastes in alternative energy genera-tion is also suggested. 2008 American Institute of Chemical Engineers Environ Prog, 27: 524–537, 2008

Keywords: food processes, food process wastes, al-ternative energy, incineration, gasifiction, pyrolysis,anaerobic digestion, fermenttion

INTRODUCTION

Food processing wastes play a very unique posi-tion in the production of renewable energy and envi-ronmental sustainability. Because of the high contentof organic components, proteins, oils and fats, and

polysaccharides, the potential energy available infood processing wastes is high and various energy generation techniques may be used. Compared toraw biomass, e.g., corn stalk, wood chips, switchgrass, etc., food processing wastes can be more read-ily converted to various forms of energy with lesspretreatment steps in conversion processes. Also,food processing wastes may have an adverse impacton the environment when they are discarded withoutproper treatments. Therefore, utilizing food process-ing wastes for generating energy not only produces

revenue gains by recovering the energy and reducingthe cost for waste treatment, but also helps enhanceenvironmental sustainability.

There are many types of food processing wastesand it is difficult to categorize them into a fewgroups. Each category would have a wide variety of chemical components and physical characteristicsmaking it hard to generalize how to process the

wastes to generate alternative energy. In this review,as rough as it may be, food processing wastes arecategorized according to the final processed products:dairy products (such as milk, cheese, and butter), seafoods, meat and poultry products, edible oils, brew-

ery products, and confectionary products (such aspotato chips and fries). Table 1 shows the compari-son of chemical compositions between the food proc-essing wastes.

As the food supply has increased over the years,the amount of food processing waste has alsoincreased. For example, according to the 2007 Dairy Market News [3], milk production increased 2.2% in2007 from 2006, reaching 185.7 billion pounds in theUnited States. As 80–90% of milk is collected as whey separated from all the dairy products, it means thatmore than 148 billion pounds of whey were pro-duced in the United States in 2007. Although 0.7%

whey protein is recovered for human consumption,and some of the whey wastes are used for fertilizerand animal feed, the rest of it is discarded to waste-

water treatment facilities or landfills [4]. Recently,dairy wastes were used for producing value addedchemicals, such as propionic acid [5] and lactic acid[6, 7]. As oil prices increase, and the supply remainsunstable, it may be advantageous to utilize the highenergy available in food processing wastes, such ascheese whey waste, by converting it to alternativeenergy. Furthermore, there have been many new

waste treatment technologies developed recently, which may well be applied directly to food processing 2008 American Institute of Chemical Engineers

524 December 2008 Environmental Progress (Vol.27, No.4) DOI 10.1002/ep



Table 1. Comparison of chemical compositions of food processing wastewater [1].

Food processing WastesCharacteristics and selected

elements of the waste

Dairy processing Cheese/whey plant wastewater BOD5: 377–2214 mg/LCOD: 189–6219 mg/LFOG (mg/L)NH4-N: 0.7–28.5 mg/L

Cheese processing waste BOD5: (mg/L)COD: 63,300 mg/LFOG: 2.6 g/L

Whey wastewater BOD5: 35,000 mg/LCOD: (mg/L)FOG: 0.8 g/LNH4-N: (mg/L)

Raw cheese whey waste BOD5:(mg/L)COD: 68,814 mg/LFOG:(g/L)NH4-N: 64.3 mg/L

Sea food processing (cannedand preserved)

Farm-raised catfish processing wastewater

BOD5: 340 mg/LCOD: 700 mg/LFOG: 200 mg/L

Breaded shrimp processing wastewater

BOD5: 720 mg/LCOD: 1200 mg/LFOG:(mg/L)

Tuna processing BOD5: 700 mg/LCOD: 1600 mg/LFOG: 250 mg/L

Herring BOD5: 1200–1600 mg/LCOD: 3,000–10,000mg/LFOG: 600–5000 mg/L

Scallops BOD5: 300–1100mg/LCOD: 27–4000 mg/LFOG: 15–25 mg/L

Meat processing Cattle BOD5: 448–7237 mg/LFOG: 250 mg/L

Hog BOD5: 1000–2200 mg/LCOD: 3000 mg/L

Mixed BOD5: 400–11,000 mg/LCOD: 583–18,768 mg/L

Edible oil processing Palm oil wastewater BOD5: 25,000 mg/LCOD: 50,000 mg/LOG: 6000 mg/LNH4: 35 mg/L

Olive oil BOD5: 4000–100,000 mg/LCOD: 15,000–225,000 mg/L

Confectionary food processing Potato peeling (steam) BOD5: 62 60 mg/LCOD: 10,000 mg/L

French fry plant (plantcomposite)

BOD5: 1150 mg/LCOD: 1790 mg/L

Soft drink processing wastewater BOD5: 600–4500 mg/LCOD: 1200–8000 mg/L

Bakery process wastewater(Bread plant)

BOD5: 155–620 mg/LCOD: 1500 mg/LG: 60–80 mg/L

Brewery processing [2] Wastewater from spent grain BOD5: 1000 mg/LCOD: 33,000 mg/LTSS: 4800 mg/L

BOD5: five day biological oxygen demand, COD: chemical oxygen demand, FOG: fats, oil, and grease, OG: oiland grease, G: grease, TSS: total suspended solid, NH4: ammonia, NH4-N: ammonia/nitrogen.

Environmental Progress (Vol.27, No.4) DOI 10.1002/ep December 2008 525



wastes. Some old technologies have even been modi-fied and revived, and become available for effectiveprocessing of the wastes.

In this review, we focus on the recent progress inalternative energy generation techniques for foodprocessing wastes, and the various energy forms thatcan possibly be extracted from them. Based on thisreview, we suggest the future direction for more

effective ways of utilizing food processing wastes foralternative energy generation.

THERMAL ENERGY PROCESSES

Incineration Thermal processing is probably the most popular

way of generating heat from wastes in general. Incin-eration of municipal wastes has been used in many parts of the world [8–10]. For hazardous industrialand medical wastes, incineration is currently the mostcommon process used [8, 9]. The heat generated by burning the wastes can be used to operate steam tur-bines for energy production or for heat exchangers

used to heat up process streams in industry [11, 12].Utilization of thermal energy from waste incineratorsis reported for many cases [13–15].

There are four incinerator types commonly used inindustry: fixed-hearth, rotary-kiln, liquid injection,and fluidized bed [16, 17]. Fixed-hearth incineratorsare used extensively for medical and municipal wasteincineration. The rotary-kiln incinerators are used by municipalities and by large industrial plants [15]. Co-combustion of wastes, in particular in coal-firedpower plants, is the single largest growing conversionroute for biomass wastes in many EU countries (e.g.,in Spain, Germany, and the Netherlands) [18–20].Food processing wastes can be readily used in the

co-combustion process only after the moisture con-tent is reduced enough.

However, very few reports on incineration of foodprocessing wastes are found in the literature [21, 22].Unlike general municipal wastes and raw biomasssuch as wood and grass, most food processing wastesare not proper for burning because of the high mois-ture content and non-combustible components [23– 25]. An increasing concern of adverse environmentalimpact from emissions is another reason for the lowuse of incineration. Many problems have beenreported recently concerning problems with hazard-ous pollutants such as TSP and dioxins [26–28].

Pyrolysis and Gasification Considering the environmental concerns and low

energy utilization efficiency of incineration (typically about 15% [29]), pyrolysis, gasification, or both com-bined appear to be a better option for thermal proc-essing of food processing wastes. In pyrolysis andgasification processes, high temperatures are used tobreak down the wastes containing mostly hydrocar-bons with no (pyrolysis) or less oxygen than inciner-ation (gasification) [30–32]. The pyrolysis processdegrades the wastes to produce char, or ash, pyroly-sis oil, and synthetic gas (i.e., syngas) at 500–10008C

without oxygen. The gasification process decomposesthe hydrocarbons left into a syngas using a controlledamount of oxygen at above 10008C.

As both processes rely on carbon-based wastes,they are regarded as appropriate for food processing

wastes and food scraps. Both produce a syngas madeup mainly of carbon monoxide and hydrogen (85%),

with small amounts of carbon dioxide and methane.

Fast pyrolysis produces 75% bio-oil, of which theheating value is around 17 MJ/kg [33]. The syngascan be used to generate steam or electricity througha fuel cell. Moreover, these processes can mitigate airemissions by using no or low oxygen. It is also easierto control emissions because they are scrubbed toremove contaminants [32].

Despite many advantages over traditional incinera-tion [34–36], pyrolysis and gasification of food proc-essing wastes still seems to have a long way to go tosatisfy the energy profitability. Currently, there aremore than 100 pyrolysis facilities operating or or-dered around the world capable of processing over 4million tons of waste per year. Some plants—particu-

larly in Europe and Japan—have been operatingcommercially for more than 5 yr. However, many of the proprietary systems currently being promotedhave only operated so far as small scale pilots. Therehave also been some noteworthy problems in partic-ular projects over the last 5 yr, which raise concernsabout operational reliability. Compared with the heat-ing values of gasoline, 43.5 MJ/kg (Low Heating

Value) and ethanol, 26.7 MJ/kg [37], the heating valueof pyrolysis-oil and the amount of syngas from pyrol-

ysis is considered too low to be economically feasi-ble, as it cannot make up for the energy spent in theprocess, and as a result, the total cost of pyrolysis-oilis 10–100% more than fossil fuel.

A pyrolysis process, Siemens Westinghouse’s‘‘Schwel-Brenn/Thermal waste recycling’’ process [38]operates at 4508C for about an h, which is followedby high temperature combustion (13008C) and steamgeneration at 4008C, 40 bar. The plant in Furth, Ger-many, processed 100–150 kilotons/yr, but shut downon August 1998 after an accident with pyrolysis gas[39]. A small pyrolysis process, R21 process, devel-oped by Mitsui Engineering and Shipbuilding, canprocess 150 tons of municipal solid waste per yearand demonstrates good dioxin removal [40].

Although mass-burn incineration is a proven tech-nology, the effectiveness of full-size commercial py-rolysis and gasification has not yet been fully demon-

strated. Although several gasification systems havebeen designed and constructed in the past two deca-des, most have been demonstration and laboratory-scale systems. Large scale demonstration plants usingcoal and wood chips in the United States experiencedtechnological problems and are no longer operatingbecause of operating and financial problems [41].There are currently no commercial-scale solid wastegasification systems operating in the United States,except for a few proof-of-concept facilities. Some gas-ification/pyrolysis plants were built and operated inEurope in the early 1980s. The DOE Clean Coal Dem-onstration Project helped to construct three integrated




gasification process with combined cycle (IGCC) plantsin the 1990s: Wabash River Power Station in West TerreHaute, Indiana, Polk Power Station in Tampa, Florida,and Pinon Pine in Reno, Nevada. In the Reno demon-stration project, researchers found that IGCC technol-ogy at the time would not work more than 300 feet(100 m) above sea level [42]. The plant failed.

The first generation of IGCC plants polluted air

less than contemporary coal-based technology, butthey polluted water: For example, the Wabash RiverPlant ‘‘routinely’’ violated its water permit, because itemitted arsenic, selenium, and cyanide. The WabashRiver Generating Station, however, is now wholly owned and operated by the Wabash River Power

Association, and currently operates as one of thecleanest solid fuel power plants in the world.

Currently, gasification seems to be the closest tofull-size commercialization among the other alterna-tive energy conversion options. IGCC utilizes the syn-gas directly for steam turbines. With the optimizedprocess design of integrating a gasification unit withthe electrical power generating plant’s combined

cycle, IGCC provides competitive operational costsfor generating electricity with efficiencies up to 59%.In the case of combined heat and power generation,the efficiency can increase to 85%. There are cur-rently only two IGCC plants generating power in theUnited States. However, several new IGCC plants areexpected to come online in the United States in the2012–2020 time frame.

The main problem for IGCC is its extremely highcapital cost, upwards of $3,593/kW [43]. Official USgovernment figures give more optimistic estimates[44] of $1,491/kW installed capacity (2005 dollars)

versus $1290 for a conventional clean coal facility.This is about 20% greater cost than a conventional

pulverized coal plant. The U.S. Department of Energy and many states offer subsidies for clean coal tech-nology projects that could help to bridge the costgap. However, the per megawatt-hour cost of anIGCC plant versus a pulverized coal plant comingonline in 2010 would be $56 versus $52. IGCCbecomes even more attractive when you include thecosts of carbon capture and sequestration, IGCCbecomes $79 per megawatt h versus $95 per mega-

watt h for pulverized coal [45]. When the environmental concern on toxic pollu-

tants and environmental hormones is great, pyrolysisand gasification can be a good choice for hazardous

wastes disposal. In addition, with the advent of

$1001/barrel crude oil and $81/MM Btu natural gas,the economics of gasification once again make sense. A modest investment in capital equipment can be off-set by a 1–2 yr savings—especially when making useof fuels that simply cannot be directly combusted in aclean manner. As demonstrated in De Filippis et al . [46],the energy content of syngas produced from wastefeedstock (except in the case of MSW from low-incomecountries) shows potential for generating electric powerfrom gasification. It is possible to achieve the actualenergy gain depending on the type of feedstock.

There seems to be no gasification/pyrolysis proc-esses that have been solely developed for food proc-

essing wastes. However, we can gain insight from thereports on these processes designed for other wastes,not to mention for biomass. With profits from theenergy gain and less environmental burdens in theequation, the processes may well be more economi-cally attractive than the existing incineration or sludgeprocesses for food processing wastes. As the energy efficiency of both processes are getting better [47],

the energy profitability may increase, especially whenthe syngas is used with a fuel cell to produce electric-ity. The use of pyrolysis and gasification may resultin a net energy gain, especially when the cost for

waste disposal or treatment is high. Therefore, it isnecessary to carefully examine the chemical composi-tions of the waste, including moisture content inorder to optimize the benefit from the process. Forexample, hydrogen-rich syngas was successfully pro-duced in steam gasification of food wastes [48]. Anup-to-date and in-depth discussion as well as casestudies on gasification are well presented in Faaij [49].

CHEMICAL ENERGY PROCESSES

Methanol Partly due to the oil crises, biomass-derived syngas

has become an important part of alternative energy since the 1980s, and pressurized gasification formethanol production from biomass was tested anddeveloped in France and Sweden [49]. After a fewunsuccessful operations in industries in Finland andformer East Germany [50], renewed attention in pro-ducing methanol and hydrogen as transport fuelsusing gasification technology has arisen, in particularthe Fischer-Tropsch process has been recently revived. Despite its techno-economic potential, thereare technological challenges such as more effective

gas cleaning to protect the downstream catalyticprocess. Once clean syngas is available, the knownprocess technology can be used to produce metha-nol, Fischer-Tropsch diesel oil, and hydrogen. How-ever, strict gas cleaning is the main challenge, andscale-up and integration of processes have not beensufficiently investigated. Be that as it may, liquid meth-anol production and the well-established Fischer-Tropsch process combined with electricity generationhave been demonstrated efficient and economically profitable [51–53]. Therefore, these processes should beconsidered as a possible option for effective energy production from food process wastes.

Biodiesel Biodiesel production using waste oil and grease

have recently become more attractive. For the foodprocessing wastes that have high contents of oil andgrease, the production of biodiesel from them is anattractive option. The food processes that generatehigh oil and grease wastes are vegetable refinery, ani-mal rendering, fish and meat processes, and all theprocesses that include frying steps [54].

Chemically, biodiesel is known as monoalkylesters of fatty acids. The production of biodiesel by transesterification process to form fatty acid alkyl




esters using acid and base catalysts has been industri-ally accepted for its high conversion and reactionrates [55]. Downstream processing costs and environ-mental problems associated with biodiesel productionand byproducts recovery have led to the search foralternative production methods and alternative sub-

strates [56, 57]. One of the highly regarded processesis the enzymatic method. Enzymatic reactions involv-ing lipases can be an excellent alternative to producebiodiesel through a process commonly referred to asalcoholysis, a form of transesterification reaction, orthrough an interesterification (ester interchange) reac-tion [58, 59].

Although the high enzyme price is a major barrierfor commercialization of enzymatic biodiesel production[60], enzymatic processing is a promising option for theoil and fat waste producing industries as higher effi-ciency, low cost enzymes are being developed throughprotein engineering and the use of recombinant DNA technology to produce large quantities of enzymes at

cheaper prices [61]. The use of immobilized enzymesand immobilized cells may lower the overall cost, whilepresenting less downstream processing problemsto biodiesel production. In addition, the enzymaticapproach is environmentally friendly (considered a‘‘green reaction’’), and needs to be explored for indus-trial production of biodiesel.

BIOLOGICAL ENERGY PROCESSES

Methane Anaerobic digestion has been used for decades to

produce methane from organic wastes. As shown in

Table 1, food processing wastes consist mostly of or-ganic components making anaerobic digestion agood choice for alternative energy generation. Thereare many anaerobic digesters being used throughoutthe world for livestock wastes, such as feces fromcows, pigs, and chickens [62–64], and food process-

ing wastes [65, 66]. To achieve effective energy recov-ery, various processes have been developed: up-flowanaerobic sludge blanket (UASB) [67, 68], anaerobicfluidized bed reactor (AFBR) [69, 70], anaerobicattached film expanded bed reactor (AAFEB) [71, 72],all designed to improve cell retention and the two-phase digestion process [73] optimizing acidogenesisand methanogenesis.

Anaerobic digesters are widely used for methaneproduction from food processing wastes [66, 74–76].For example, wastewaters from food processing fac-tories, which produce beer, sugar, fruit jams, andpotatoes, are processed for methane fermentation [74,77, 78]. As is well known, anaerobic digestion pro-

ceeds through three different microbial mechanisms(Figure 1 [79]): 1. acidogenesis, 2. acetogenesis, and3. methanogenesis [80, 81]. In acidogenesis, acido-genic bacteria produce extracellular enzymes for hy-drolysis of all organic solids and dissolve colloids.Carbohydrates are hydrolyzed to mono- and disac-charides, proteins to amino acids, and lipids to fatty acids. These compounds are transformed to acetateand longer chain fatty acids as well as CO2 and H2.In acetogenesis, low molecular weight fatty acidssuch as butyrate and propionate are transformed toacetate, CO2, and H2 by acetogenic bacteria. The pro-duced hydrogen should be oxidized by other anaero-

Figure 1. Anaerobic metabolism.




bic bacteria; otherwise the propionate concentration would continually increase. Then, hydrogen and ace-tate are utilized by methanogenic bacteria producingmethane and CO2.

There are several major issues to be addressed formore efficient methane production from food proc-essing wastes: protein content, the presence of salt,solid materials, dry organic wastes containing less

than 80% moisture, and lipids characterized as eitherfats or oils and grease. Protein inhibits methane pro-duction after being converted to ammonia [82] fromnitrogen-rich feedstock, such as fish processing resi-dues [83], dairy wastewater [84], or high-solid anaero-bic digester liquor [85, 86]. High levels of ammonia

were noted to form, which could inhibit anaerobicdigestion operation [87, 88]. Ammonia toxicity hasbeen extensively discussed on both acetogenesis andmethanogenesis phases [89–92] or solely on theacidogenesis phase [93–97]. The volatile fatty acids(VFA) and pH significantly suppress hydrolysis of or-ganic substrates [98–100]. However, few works inves-tigate the inhibitory effects of ammonium at different

pHs on protein or lipid hydrolysis [84, 101].One way to resolve the ammonia problem wasproposed by Michaelsen et al . [102] They suggestedpassing the biogas through an acid scrubber beforerecycling it into the digester. The return gas will serveto mix the digester content and allow the process tobe suitable for both single- and dual-stage digestion.In a study of the removal of protein in excess sludgefrom wastewater treatment plants, it was found thatnot only the quality of the biogas was improved butso was the rate of generation [103].

High salt concentration in many food processing wastes also inhibits anaerobic digestion because of the presence of cations [104–109]. The treatment of

saline and hypersaline wastewater could represent asmuch as 5% of worldwide effluent treatment require-ments. It has already been reported that a sodiumconcentration exceeding 10 g/L strongly inhibitsmethanogenesis [109–111].

Despite obstacles created by high salinity, a certainnumber of processes have been used successfully forthe anaerobic treatment of saline wastewater. Someof them used a halophilic inoculum [112–115],

whereas others required the adaptation of a non-hal-ophilic inoculum to increase salt concentrations [107,108, 116–120].

Lipid-rich wastes from meat and fish processingindustries, edible oil, and dairy industries pose a chal-

lenge to methanogenic processes. Despite a high the-oretical methane yield from lipids as compared withother organic compounds, methanogenic processes

with lipid-rich wastes are less stable and not able toaccommodate lower substrate-loading rates [121, 122].This is reported to be partly because of the toxicity of long-chain fatty acids (LCFA) to anaerobicmicrobes [123], and partly because of its absorptiononto the biomass and flowing out of the reactor. Inaddition, because the methanogenic b-oxidation of LCFA is carried out by syntrophic LCFA-degrading,hydrogen- (and/or formate-) producing fermentativebacteria and hydrogenotrophic methanogens, LCFA-

degrading anaerobes can gain only a small amount of energy through syntrophic reactions, and thus, theirgrowth is generally slow. The oxidation of LCFA isthermodynamically unfavorable in such environmentsunless the consumption of reducing equivalents(hydrogen and/or formate) is coupled with oxidation,because of the syntrophic metabolism and toxicity of LCFA isolation of LCFA-degrading bacteria has

been difficult and not many species/subspecies arereported [124]. Therefore, efficient degradation of these LCFA is essential for the successful treatment of lipid-rich wastes in methanogenic processes.

Recent approaches include the separation of lipidsand fatty acids from wastes by UASB reactors [121]and the use of anaerobic thermophilic strains [125].

When designing or selecting a process for lipid-rich wastes, it seems reasonable to carefully examine thecost and efficiency between the production processesof biodiesel and methane.

Ethanol Ethanol has gained huge interest in both industry and research as a plausible renewable energy sourcein the future. Food processing wastes, as shown inTable 1, have a great potential for ethanol produc-tion. Wastes that have high contents of hydrocarbon,such as brewery wastes and potato chip process

wastes, can be good sources of ethanol. Unlike thegeneral biomass wastes such as corn stalk, woodchip, switch grass, etc., food processing wastes haveless pretreatment problems, which is one of the majorchallenges to overcome in lowering the price of thefuel ethanol [126]. For food processing wastes, pre-treatment is not needed as much.

The greatest concern in ethanol production fromfood processing wastes (or biomass wastes in gen-eral) is probably the production cost. The final mar-ket price of fuel ethanol must be lower than that of gasoline. There are many ways to lower the produc-tion cost of ethanol. One of them is to use genetically modified microorganisms. Common wild-type bacte-ria such as yeast and zymomonas only convert su-crose and hexose sugars. Many pentose sugars suchas xylose, ribose, and arabinose, which are as abun-dant as hexose sugars in biomass, are not properly used. By utilizing all the available sugars, ethanolproduction yield can increase. A great number of studies have been performed to mutate the cells with

some of the mutant cells being used in industry. E. coli KO11, E. coli SL40, E . coli FBR3, E. coli LY01, Zymomonas CP4 (pZB5), and Saccharomyces 1400(pLNH32) are some of them to name a few. E. coli KO11 was reported to be successfully used in abrewery company and in some research applications[127–129]. These mutant cells are not only capable of converting various sugars, but also have stronger re-sistance to ethanol and acidity [130–132]. Some havehigher temperature tolerance [131], which makes itpossible to control the fermentation kinetics and tooperate at a higher temperature for better simultane-ous enzymatic hydrolysis and fermentation.




Hydrogen Hydrogen can be directly or indirectly produced

from biological processes of food processing wastes.Interest in biohydrogen has resurfaced over the lastdecade as hydrogen has taken as a future energy alternative and the fuel-cell capacity has recently accelerated. Although there are a great number of articles on biohydrogen from biomass in general,

there are only a few articles published on biohydro-gen production from food processing wastes as of 2007. One of them is about successful hydrogen pro-duction from brewery wastewater, which achieves62% hydrogen content [133]. Hydrogen fermentationof dairy wastes [134, 135] and olive or palm oil proc-essing wastes [136, 137] are also available.

There is a wide range of approaches to producehydrogen depending on the microbiologic metabo-lism. These include direct and indirect biophotolysis,photo-fermentation, and dark-fermentation [138]. A previous study showed that photosynthesis-based sys-tems do not produce H2 at rates that are sufficientenough to meet the goal of providing enough H2 to

power a 1 kW proton exchange membrane fuel cell(PEMFC) on a continuous basis [139]. However, itdoes not mean that these processes should be aban-doned. Photosynthetic bacteria such as Rhodospirilla-ceae gelatinosus can grow in the dark using CO asthe sole carbon source to generate ATP with the con-comitant release of H2 and CO2 [140, 141]. The oxida-tion of CO to CO2 with the release of H2 occurs via a water–gas shift reaction. This process provides very promising results [139].

Dark fermentation is regarded as having greatpotential in the development of practical biohydro-gen systems. As carbohydrates are the preferred sub-

strate for hydrogen-producing bacteria, food process-ing wastes with high contents of carbohydrates canbe a good feedstock for dark fermentation. One of the several major problems of dark fermentation isthe product gas composition. Although direct andindirect photolysis systems produce pure H2, darkfermentation produces a mixed biogas consisting of primarily H2 and carbon dioxide and lesser amountsof methane, CO, and H2S. The gas composition may be a technical challenge when using the biogas infuel cells. Another problem is the low hydrogen pro-duction yields, typically 10–20% stoichiometrically.

The advantages of biohydrogen from food proc-essing wastes are so great that it cannot be marginal-

ized in renewable energy research. Many technolo-gies are under investigation. As mentioned above, theuse of mixed cultures of photosynthetic and anaero-bic fermentation bacteria would be an option toenhance the yield [142]. Another plausible approachis to produce a mixture of hydrogen and methane ina two-state process [143]. The first step would pro-duce hydrogen and organic acids, which would beconverted to methane in a second fermentation stage.The hydrogen–methane mixture may be at an advant-age over pure methane as a fuel because it signifi-cantly reduces air pollutants in internal combustionengines. Effective process designs are reported to

evolve hydrogen more rapidly. For example, theimmobilization of bacteria on fibers was reported toprovide long-term stable hydrogen evolution at a rea-sonable rate [144].

Although there is no commercial biohydrogenprocess in association with food processing wastes, itis exciting to see the recent technological progress inbiohydrogen production accompanying fuel cell tech-

nology. The economics of hydrogen fermentationseems to be favorable at even less than stoichiometric yields. Compared to methane fermentation of whicha cost range of approximately $3–$8 per MMBTU,hydrogen produced by the same type of fermentationequipment could be sold for as much as $15 perMMBTU-depending on location, scale, and other fac-tors [145]. The ultimate goal for hydrogen fermenta-tion research is to achieve the production yield of 60–80%, which can sustain economic feasibility.

OTHER PROCESSES

There are many other energy conversion processesthat can be used for food processing wastes. Thermal

plasma waste treatment systems use plasma arctorches that produce ionized gases with arc centerlinetemperatures as high as 20,000 F. The thermal energy is transferred to the material to be treated throughradiation, convection, and conduction. Thus, theplasma arc torch provides the energy necessary tomelt inorganic material and breakdown organic mate-rial so that it will react to form harmless byproducts[146]. Plasma waste treatment systems are being usedin many industries all over the world, primarily fortoxic wastes. The advantages of plasma technology are the complete destruction of the wastes into harm-less materials and syngas production. The generatedsyngas from the waste can be possibly converted into

electric energy, supplying the torches and the plantand also generate additional revenues for the projectfrom the sale of the net electric production to thelocal utilities [147].

Microbial fuel cells (MFC) directly produce electric-ity through the catalytic reaction of microorganismsgrowing on organic matters [148, 149]. Although theinterest in microbial fuel cells was relatively highin the 1960s, the study of microbial fuel cells wanedas the cost of other energy sources remained low andthe available microbial fuel cells lacked efficiency and long-term stability. However, in the past 4–5 yr,there has been a resurgence in microbial fuel cellresearch. Recent advances in system design, materials

for electrodes, and microbial species, have made itpossible for MFC to compete with conventionalpower sources [150]. Many food processing wastes

with high contents of carbohydrates and sugar can bedirectly processed with MFC’s.

CONCLUSIONS

Food processing wastes have great potential forproducing alternative energy. High contents of carbo-hydrates, proteins, and lipids can be used for varioustypes of energy. However, careful analysis of themain components of the waste and economic analysis




of the possible conversion process must be per-formed to effectively use the food processing wastes.For example, the wastewaters from dairy processescan be used for either production of value addedchemicals such as lactic acid and propionic acid [5,151], or they can be also used to produce ethanol,methane, or hydrogen [84, 135]. Potato chip processand brewery wastes can be used for animal feed orfertilizer, and for ethanol or methane production [152,153].

When designing the conversion process for food

processing wastes, all the possible problems andaccompanying extra costs must be considered too.For example, anaerobic systems are usually wellsuited to the treatment of the high organic substrate

waste contents, because they can achieve a highdegree of BOD removal at a significantly lower costthan other processes as well as the advantage of methane production. However, as stated before, foroil- and fat-rich wastes, they need special equipmentand additional treatment processes with extra costs.

Also, anaerobic treatment suffers from the disadvant-age of odor generation and large amounts of sus-pended matters in the influent require more sophisti-

cated reactor design. Therefore, to select a processthat brings in economic and environmental benefits,the process design must include economic analysesthat include capital costs, operating cost, and returnon investigation (ROI) of the final products.

Once the energy conversion method is deter-mined, it must be well analyzed between differentforms of energy to optimize the potential of the

wastes. Wastes with high contents of protein and/orfatty acids may better opt hydrogen fermentation orgasification than methane or ethanol production.Process modeling and optimization must also be per-formed [154].

Energy conversion processes are suggested forfood processing wastes and research articles relatedto the process are listed in Table 2.

FUTURE DIRECTION

It appears that many investigations have been per-formed on the integration of different processes tomaximize the production yield and minimize thepotential problems: Fermentation process is com-bined with catalytic reforming [158], gasification with

fuel cell [155, 157], and anaerobic digestion with gasi-fication [159]. Integrated energy conversion systemsmust also be considered for conversion of food proc-essing wastes into energy.

Mixed cultures of different bacterial species formulti-energy production have been studied recently,for example, simultaneous production of biogas andhydrogen from solid food processing [160] and mixedculture anaerobic digestion for simultaneous produc-tion of methane and hydrogen [161].

Processes of mixed feedstock are also thought tobe useful approaches for food processing wastes and

will be used more in the future. Dairy manure, bio-solids (sludge), and food wastes are co-digested to

produce methane or hydrogen [162], and mixed wastes of liquids and solids are used to producerenewable energy and fertilizers [163]. Co-firing of biomass and coal in a coal gasification or in a pyroly-sis/gasification system is thought to be a promisingapproach [156, 164].

Novel approaches such as solid-state fermentations(SSF) [165] may be a better option for solid foodprocessing wastes. Compared with submerged fer-mentation, SSF can provide high productivities,extended stability of products and low productioncosts [166]. Although there are challenges in scalingup with increasing progress and application of

Table 2. Selected conversion processes for food processing wastes.

Food processing Waste

characteristics Energy conversion process

Dairy processing High lactose and salts Ethanol fermentation [129]Hydrogen by anaerobic digestion [134, 135]

High lipids Methane by anaerobic digestion [68]Sea food processing

(canned andpreserved)

High lipids Methane by anaerobic digestion [82, 107, 114, 122]

High protein Biodiesel process [54, 55]

Meat processing Syngas and oil by pyrolysis/gasification [48]Edible oil processing High oil Biodiesel [54, 59]

High carbohydrates(grain wastes)

Methane by anaerobic digestion [121, 122]Biohydrogen by anaerobic digestion [137, 143]Syngas by gasificaiton [46, 155]Hydrogen by pyrolysis/gasification [32]

Confectionary foodprocessing

High carbohydrates Syngas and oil by pyrolysis [30, 31]Syngas by pyrolysis/gasification [156]Methane by anaerobic digestion [68, 69, 70, 72]

Brewery processing Ethanol fermentation [126]Biohydrogen by anaerobic digestion [133]Hydrogen by anaerobic digestion and gasification [157]




rational methods in engineering, SSF will achievehigher levels in standardization and reproducibility inenergy production from food processing wastes inthe future.

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