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Wastewater treatment in molasses-based alcohol distilleries for CODand color removal: A review

Y. Satyawali, M. Balakrishnan�

TERI University, Darbari Seth Block, India Habitat Centre, Lodhi Road, New Delhi 110 003, India

Abstract

Molasses-based distilleries are one of the most polluting industries generating large volumes of high strength wastewater. Different

processes covering anaerobic, aerobic as well as physico-chemical methods have been employed to treat this effluent. Anaerobic

treatment is the most attractive primary treatment due to over 80% BOD removal combined with energy recovery in the form of biogas.

Further treatment to reduce residual organic load and color includes various: (i) biological methods employing different fungi, bacteria

and algae, and (ii) physico-chemical methods such as adsorption, coagulation/precipitation, oxidation and membrane filtration.

This work presents a review of the existing status and advances in biological and physico-chemical methods applied to the treatment of

molasses-based distillery wastewater. Both laboratory and pilot/industrial studies have been considered. Furthermore, limitations in the

existing processes have been summarized and potential areas for further investigations have been discussed.

Keywords: Decolorization; Distillery; Molasses; Spentwash

1. Introduction

Ethanol manufacture from molasses generates largevolumes of high strength wastewater that is of seriousenvironmental concern. The effluent is characterized byextremely high chemical oxygen demand (COD)(80,000–100,000mg/l) and biochemical oxygen demand(BOD) (40,000–50,000mg/l), apart from low pH, strongodor and dark brown color (Central Pollution ControlBoard (CPCB) 1994, 2003). In India, which is the second-largest producer of ethanol in Asia with a projected annualproduction of about 2300 million liters in 2006–07(Subramanian et al., 2005), alcohol distilleries are ratedas one of the 17 most polluting industries. Apart from highorganic content, distillery wastewater also contains nu-trients in the form of nitrogen (1660–4200mg/l), phos-phorus (225–3038mg/l) and potassium (9600–17,475mg/l)(Mahimairaja and Bolan, 2004) that can lead to eutrophi-cation of water bodies. Further, its dark color hinders

ing author. Tel.: +91 11 2468 2100; fax: +91 11 2468 2144.

ess: malinib@teri.res.in (M. Balakrishnan).

photosynthesis by blocking sunlight and is thereforedeleterious to aquatic life (FitzGibbon et al., 1998). Studieson water quality of a river contaminated with distilleryeffluent displayed high BOD values of 1600–21,000mg/lwithin a 8 km radius (Baruah et al., 1993). Adequatetreatment is therefore imperative before the effluent isdischarged. In addition to pollution, increasingly stringentenvironmental regulations are forcing distilleries to im-prove existing treatment and also explore alternativemethods of effluent management. For instance, Indiandistilleries were stipulated to achieve zero discharge ofspentwash to inland surface water by December 2005(Uppal, 2004).In an earlier review on this subject, Sheehan and

Greenfield (1980) discussed treatment options practicedin the 1970s. More recently, Wilkie et al. (2000) haveexamined characteristics and anaerobic treatment ofeffluent obtained from different feedstock used for ethanolmanufacture. This review focuses on the advances inmolasses-based distillery wastewater treatment in thelast two decades and the emerging technologies in thisfield.

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2. Process description

Alcohol manufacture in distilleries consists of four mainsteps viz. feed preparation, fermentation, distillation andpackaging (Fig. 1).

2.1. Feed preparation

Ethanol can be produced from a wide range of feedstock.These include sugar-based (cane and beet molasses, canejuice), starch-based (corn, wheat, cassava, rice, barley) andcellulosic (crop residues, sugarcane bagasse, wood, muni-cipal solid wastes) materials. Details of sugar, starch andlignocellulosic biomass-based feedstock and their pre-treatment steps have been reviewed earlier (Wilkie et al.,2000). In general, sugar-based feedstock containing readilyavailable fermentable sugars are preferred since starch andcellulosic substrates involve an additional pre-treatmentstep to convert starch into fermentable sugars. Thus, canejuice is a commonly used substrate in Brazil (Tano andBuzato, 2003) while Indian distilleries almost exclusivelyuse sugarcane molasses. Overall, nearly 61% of worldethanol production is from sugar crops (Berg, 2004).

The composition of molasses varies with the variety ofcane, the agro climatic conditions of the region, sugar

Yeast Pre-fermenter

Fermenter

Analyzer

column

Diluted

molassesCO2

Spentwash

Rectification

column

Spentlees

Alcohol

Blending &

maturation

Dehydration

(molecular sieve)

Potable

alcohol

Power

alcohol

Industrial

alcohol

Bottling

Fig. 1. Process description.

manufacturing process and handling and storage (God-bole, 2002). Table 1 summarizes the chemical compositionof beet and cane molasses. Molasses is diluted to about20–25 brix (measurement of sugar concentration in asolution) and its pH adjusted, if required, before fermenta-tion. In India, about 90% of the molasses produced in canesugar manufacture is consumed in ethanol production(Billore et al., 2001).

2.2. Fermentation

Yeast culture is prepared in the laboratory andpropagated in a series of fermenters, each about 10 timeslarger than the previous one. The feed is inoculated withabout 10% by volume of yeast (Saccharomyces cerevisiae)inoculum. This is an anaerobic process carried out undercontrolled conditions of temperature and pH whereinreducing sugars are broken down to ethyl alcohol andcarbon dioxide. The reaction is exothermic. To maintainthe temperature between 25 and 32 1C plate heat exchan-gers are used; alternatively some units spray cooling wateron the fermenter walls. Fermentation can be carried out ineither batch or continuous mode (CPCB, 2003). Fermenta-tion time for batch operation is typically 24–36 h with anefficiency of about 95%. Continuous operation, involvinghigher sugar concentration and an osmotolerant variety ofyeast, is faster (16–24 h fermentation time) but theefficiency is marginally lower (T.R. Sreekrishnan, pers.comm.). The resulting broth contains 6–8% alcohol. Thesludge (mainly yeast cells) is separated by settling and

Table 1

Composition of cane and beet molasses (Curtin, 1983; Chen and Chou,

1993; Godbole, 2002)

Property Cane molasses Beet molasses

Brix (%) 79.5 79.5

85–92b

Specific gravity 1.41 1.41

1.38–1.52a

Total solids (%) 75.0 77.0

75–88a

Total sugars (%) 46.0 48.0

44–60a

50–90b

Crude protein (%) 3.0 6.0

2.5–4.5b

Total fat (%) 0.0 0.0

Total fiber (%) 0.0 0.0

Ash (%) 8.1 8.7

7–15b

Calcium (%) 0.8 0.2

Phosphorus (%) 0.08 0.03

Potassium (%) 2.4 4.7

Sodium (%) 0.2 1.0

Chlorine (%) 1.4 0.9

Sulfur (%) 0.5 0.5

aGodbole (2002).bChen and Chou (1993).

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discharged from the bottom, while the cell free fermenta-tion broth is sent for distillation.

Apart from yeast, the bacterium Zymomonas mobilis hasalso been investigated for ethanol production (Tao et al.,2005). The organism follows a simple catabolic pathway(Toma et al., 2003) and has advantages over S. cerevisiae

because of its higher sugar uptake rate, lower biomassyields and higher ethanol production (Lin and Tanaka,2006). Immobilization of Zymomonas cells has been triedon support materials such as pectin because immobilizedwhole cells retain higher microbial activity (Siva Kesavaet al., 1996). However, formation of byproducts such asacetoin, glycerol, acetate, and lactate under anaerobicconditions is a drawback associated with Z. mobilis.During its growth on sucrose medium, Z. mobilis alsoleads to formation of levan, a polymer of fructose units.Further, like yeast, Zymomonas is unable to convertcomplex carbohydrates like cellulose, hemicellulose andstarch to ethanol. To overcome these constraints, geneticmanipulation of Z. mobilis has been attempted by severalworkers. For instance, the hydrolytic and isomerase genesfrom recombinant Escherichia coli have been transferred toZ. mobilis, resulting in utilization of xylose, mannose,lactose and arabinose as the carbon source (Gunasekaranand Raj, 1999). Earlier, Picataggio et al. (1996) reportedthe development of genetically altered Zymomonas strainto ferment pentoses and glucose, obtained by hydrolysis ofhemicellulose and cellulose, to produce ethanol. Here, Z.

mobilis was transformed with combination of E. coli genesfor xylose isomerase, xylulokinase, transaldolase, transke-tolase, L-arabinose isomerase, L-ribulokinase, and L-ribu-lose-5-phosphate 4-epimerase. In another work aimed atxylose utilization combined with enhanced ethanol yield, astrain derived from Z. mobilis ATCC31821 was developed(Zhang, 2000). The strain comprised of exogenous genesencoding xylose isomerase, xylulokinase, transaldolase andtransketolase.

2.3. Distillation

Distillation is a two-stage process and is typically carriedout in a series of bubble cap fractionating columns. Thefirst stage consists of the analyzer column and is followedby rectification columns. The cell free fermentation broth

Table 2

Sample quantities and characteristics of wastewater streams generated in an I

Parameter Specific wastewater

generation (kl/kl alcohol)

Color

Spent wash 14.4 Dark brown

Fermenter cleaning 0.6 Yellow

Fermenter cooling 0.4 Colorless

Condenser cooling 2.88 Colorless

Floor wash 0.8 Colorless

Bottling plant 14 Hazy

Other 0.8 Pale yellow

(wash) is preheated to about 90 1C by heat exchange withthe effluent (‘‘spentwash’’) and then sent to the degasifyingsection of the analyzer column. Here, the liquor is heatedby live steam and fractionated to give about 40–45%alcohol. The bottom discharge from the analyzer column isthe spentwash. The alcohol vapors are led to therectification column where by reflux action, �96% alcoholis tapped, cooled and collected. The condensed water fromthis stage, known as ‘‘spentlees’’ is usually pumped back tothe analyzer column.

2.4. Packaging

Rectified spirit (�96% ethanol by volume) is marketeddirectly for the manufacture of chemicals such as aceticacid, acetone, oxalic acid and absolute alcohol. Denaturedethanol for industrial and laboratory use typically contains60–95% ethanol as well as between 1% to 5% each ofmethanol, isopropanol, methyl isobutyl ketone (MIBK),ethyl acetate, etc. (Skerratt, 2004).For beverages, the alcohol is matured and blended with

malt alcohol (for manufacture of whisky) and diluted torequisite strength to obtain the desired type of liquor. Thisis bottled appropriately in a bottling plant. Anhydrousethanol for fuel-blending applications (‘‘power alcohol’’)requires concentration of the ethanol to499.5wt% purity.The ethanol dehydration is typically done using molecularsieves; however, pervaporation has also been employed inBrazil and India for this purpose (Mitsui & Co., 2003).

3. Wastewater generation and characteristics

Table 2 lists the major wastewater streams generated atdifferent stages in the alcohol manufacturing process.Table 3 summarizes the typical characteristics of spentwashgenerated in Indian distilleries using sugarcane molasses.Values for beet molasses-based effluent are given forcomparison. The main source of wastewater generation isthe distillation step wherein large volumes of dark browneffluent (termed as spentwash, stillage, slop or vinasse) isgenerated in the temperature range of 71–81 1C (Yeoh,1997; Nandy et al., 2002; Patil et al., 2003). Thecharacteristics of the spentwash depend on the rawmaterial used (Mall and Kumar, 1997); also, it is estimated

ndian distillery (S. Majumdar, pers. comm.)

pH Suspended solids

(mg/l)

BOD (mg/l) COD (mg/l)

4.6 615 36,500 82,080

3.5 3000 4000 16,500

6.3 220 105 750

9.2 400 45 425

7.3 175 100 200

7.6 150 10 250

8.1 100 30 250

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Table 3

Characteristics of spentwash generated from various feedstock (Pathade, 1999; Wilkie et al., 2000; Mahimairaja and Bolan, 2004)

Characteristics Feedstock

Cane molasses Beet molasses

Pathade (1999) Mahimairaja and Bolan (2004) Wilkie et al. (2000)

COD (mg/l) 65,000–130, 000 104,000–134,400 91,100

BOD (mg/l) 30,000–70,000 46,100–96,000 44,900

COD/BOD ratio 2.49 1.95

Total solids (mg/l) 30,000–100,000

Total suspended solids (mg/l) 350

Total dissolved solids (mg/l) 80,000 79,000–87,990

Total nitrogen (mg/l) 1000–2000 1660–4200 3569

Total phosphorus (mg/l) 800–1200 225–3038 163

Potassium (mg/l) 8000–12,000 9600–17,475 10,030

Sulfur as SO4 (mg/l) 2000–6000 3240–3425 3716

pH 3–5.4 3.9–4.3 5.35

Spentwash

Biomethanation

Biocomposting Aerobic

treatement

Solar

dryingEvaporation /

incineration

Tertiary treatmentDilution

Irrigation

Surface water

discharge

Fig. 2. Spentwash treatment options.

that 88% of the molasses constituents end up as waste (Jainet al., 2002). Molasses spentwash has very high levels ofBOD, COD, COD/BOD ratio as well as high potassium,phosphorus and sulfate content (Table 3). In addition, canemolasses spentwash contains low molecular weight com-pounds such as lactic acid, glycerol, ethanol and acetic acid(Wilkie et al., 2000).

Cane molasses also contains around 2% of a dark brownpigment called melanoidins that impart color to thespentwash (Kalavathi et al., 2001). Melanoidins are lowand high molecular weight polymers formed as one of thefinal products of Maillard reaction, which is a non-enzymatic browning reaction resulting from the reactionof reducing sugars and amino compounds (Martins andvan Boekel, 2004). This reaction proceeds effectively attemperatures above 50 1C and pH 4–7. The structure ofmelanoidins is still not well known (Rivero-Perez et al.,2002). Only 6–7% degradation of the melanoidins isachieved in the conventional anaerobic–aerobic effluenttreatment process (Gonzalez et al., 2000). Due to theirantioxidant properties, melanoidins are toxic to manymicroorganisms involved in wastewater treatment (Siria-nuntapiboon et al., 2004a). Apart from melanoidins,spentwash contains other colorants such as phenolics,caramel and melanin. Phenolics are more pronounced incane molasses wastewater whereas melanin is significant inbeet molasses (Godshall, 1999).

4. Effluent treatment

Till the early 1970s, land disposal was practiced as one ofthe main treatment options, since it was found to enhanceyield of certain crops. For example, in Brazil, vinassegenerated from sugarcane juice fermentation is mainly usedas a fertilizer due to its high nitrogen, phosphorus andorganic content. Its use is further reported to increasesugarcane productivity; furthermore under controlledconditions, the effluent is capable of replacing application

of inorganic fertilizers (Cortez and Perez, 1997; Rodrıguez,2000). However, for the high strength molasses-basedspentwash, the odor, putrefaction and unpleasant land-scape due to unsystematic disposal are concerns in landapplication. In addition, this option is subject to landavailability in the vicinity of the distillery; also, it isessential that the disposal site be located in a low–mediumrainfall area (Sheehan and Greenfield, 1980). More recentinvestigations have indicated that land disposal of distilleryeffluent can lead to groundwater contamination (Joshi,1999). Deep well disposal is another option but limitedunderground storage and specific geological location limitsthis alternative. Other disposal methods like evaporation ofspentwash to produce animal feed and incineration ofspentwash for potash recovery have also been practiced(Sheehan and Greenfield, 1980; Wilkie et al., 2000).Fig. 2 presents the options currently employed for

molasses spentwash treatment. The salient features of theexisting options as well as the advances in this field arediscussed in the following sections.

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4.1. Biological treatment

4.1.1. Anaerobic process

The high organic content of molasses spentwash makesanaerobic treatment attractive in comparison to directaerobic treatment. Therefore, biomethanation is theprimary treatment step and is often followed by two-stageaerobic treatment before discharge into a water body or onland for irrigation (Nandy et al., 2002). Aerobic treatmentalone is not feasible due to the high energy consumptionfor aeration, cooling, etc. Moreover, 50% of the COD isconverted to sludge after aerobic treatment (Sennitt, 2005).In contrast, anaerobic treatment converts over half of theeffluent COD into biogas (Wilkie et al., 2000). Anaerobictreatment can be successfully operated at high organicloading rates; also, the biogas thus generated can beutilized for steam generation in the boilers thereby meetingthe energy demands of the unit (Nandy et al., 2002).Further, low nutrient requirements and stabilized sludgeproduction are other associated benefits (Jimenez et al.,2004).

The performance and treatment efficiency of anaerobicprocess can be influenced both by inoculum source andfeed pre-treatment. In particular, thermal treatment ofwastewaters can result in rapid degradation of organicmatter leading to lower hydraulic residence time (HRT),higher loading rate and BOD reduction. Moreover themethane content and calorific value of biogas producedfrom thermophilic systems was higher (Vlissidis andZouboulis, 1993). On the contrary, in shake flask studieswith an initial COD loading of 20,000mg/l using biogasplant sludge, Dhar et al. (1998a) observed a COD removalof just 6.5% with thermally pre-treated spentwash whereasuntreated influent displayed 30.4% COD removal. Sig-nificant improvement was observed using inoculum fromanaerobic lagoon with 27.2% COD reduction withthermally pre-treated wastewater and 51% reduction withuntreated wastewater. Thermal pre-treatment changes thebiodegradability of wastewater; thus, it acts as an entirelynew feedstock for which the inoculum has to beacclimatized afresh. Further, the temperature of thermalpre-treatment is also important. After 150 d adaptationperiod, wastewater treated at lower temperature (170 1C)showed 66% COD reduction (Dhar et al., 1998b). This wasnearly twice the removal obtained with treatment at 230 1C.Further, addition of micronutrients (iron, boron andmolybdenum) eliminated the long adaptation periods.

Anaerobic lagoons are the simplest option for theanaerobic treatment of distillery spentwash. Subba Rao(1972) reported that employing two anaerobic lagoons inseries resulted in final BOD levels up to 600mg/l. However,large area requirement, odor problem and chances ofground water pollution restrict its usage (Pathade, 1999).Though anaerobic lagoons are still employed in Indiandistilleries, high rate anaerobic reactors are more popular(Lata et al., 2002). These reactors offer the advantage ofseparating the hydraulic retention time (HRT) from solids

retention time (SRT) so that slow growing anaerobicmicroorganisms can remain in the reactor independent ofwastewater flow. Table 4 summarizes the performance ofvarious anaerobic reactors covering both laboratorystudies and pilot/commercial scale operations for treatmentof molasses-based distillery wastewaters.

4.1.1.1. Suspended bed reactor. Upflow anaerobic sludgeblanket (UASB) reactor is the most popular high ratedigester that has been utilized for anaerobic treatment ofvarious types of industrial wastewaters (Akunna andClark, 2000; Syutsubo et al., 1997). Treatment by a UASBreactor resulted in 75% COD removal in sugarcanemolasses spentwash and 90% COD reduction in whiskypot ale (Goodwin and Stuart, 1994; Sanchez Riera et al.,1985). However, dilution is required before treatment dueto the presence of some inhibitory substances such as sulfurcompounds, potassium and calcium ions and free hydrogenions left in solution after pH correction (Sanchez Rieraet al., 1985). Wolmarans and de Villiers (2002) havereported a similar COD removal efficiency of greater than90% over three seasons in a UASB plant treating distillerywastewater.Most of the practical UASB systems are operated under

mesophilic conditions; however, thermophilic operationresults in higher methanogenic activity. Mesophilicallygrown sludge utilized in thermophilic UASB as a seedingmaterial leads to prompt start up and stable operation with85% COD removal efficiency at a loading of 30 kg COD/m3 d (Syutsubo et al., 1997). There are also reports on thecultivation of thermophilic granular sludge for the seedingof thermophilic UASB reactor. Wiegant et al. (1985)reported the cultivation of thermophilic sludge on sucrosefor a period of 4 months. The system after adaptation wasable to take high COD loadings (86.4 kg/m3 d) and resultedin 60% COD removal efficiency. In another study in athermophilic UASB reactor, Harada et al. (1996) reported39–67% COD removal, with a corresponding BODremoval of over 80%. The results suggested that thewastewater contained high concentration of refractilecompounds; this, in turn, affected the microbial populationin the sludge granules. Generally, the predominant generaof methanogens in granular sludge are Methanobacterium,Methanobrevibacter, Methanothrix and Methanosarcina

(Bhatti et al., 1997); however, the predominance ofMethanothrix in granular sludge is most essential for theestablishment of a high performance UASB process. In thisstudy, abundance of Methanosarcina sp. was observedwhereas Methanothrix sp. was present to a lesser extentthereby indicating that the latter are more sensitive torefractile compounds (Harada et al., 1996).

4.1.1.2. Fixed bed reactor. This involves immobilizationof microorganisms on some inert support to limit the lossof biomass and enhance the bacterial activity per unit ofreactor volume. Moreover it provides higher COD removalat low HRT and better tolerance to toxic and organic

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Table 4

Performance of various anaerobic reactors for molasses distillery wastewater

Reactor configuration COD loading

(kg COD/

m3day)

HRT (Days) % COD

reduction

% BOD

reduction

References

UASB 24 75 Sanchez Riera et al. (1985)

UASBa 15 2.1 90 Goodwin and Stuart (1994)

UASBb 18 490 Wolmarans and de Villiers (2002)

Thermophilic UASB Up to 86.4 60 Wiegant et al. (1985)

Thermophilic UASB Up to 28 39–67 480 Harada et al. (1996)

Thermophilic UASB Up to 30 0.3 87 Syutsubo et al. (1997)

Two-stage anaerobic treatmenta Blonskaja et al. (2003)

Anaerobic filter 2.5–5.1 10–19 54

UASB 0.6–2.5 20–39 93

Downflow fixed film reactor 14.2–20.4 3.3–2.5 85–97 60–73 Bories et al. (1988)

Two-phase thermophilic process 65 85 Yeoh (1997)

Acidogenesis 4.6–20.0 2

Methanogenesis 15.2

Diphasic (upflow) fixed film

reactor (clay brick granules

support)

22 3 71.8 Seth et al. (1995)

Diphasic (upflow) fixed film

reactor (granular activated

carbon support)

21.3 4 67.1 Goyal et al. (1996)

Upflow anaerobic filter (UAF)a 20 76 Tokuda et al. (1999)

Hybrid baffled reactor 20 77 Boopathy and Tilche (1991)

Downflow fluidized bed reactor

with ground perliteb17 kg TOC/m3 d 0.35 75–95% TOC Garcıa-Bernet et al. (1998)

Downflow fluidized bed reactor

with ground perliteb4.5 3.3–1.3 85 Garcia-Calderon et al., (1998)

Downflow filter 8 55–85 Athanasopoulos (1987)

Two-stage bioreactor(anaerobic) 7 71 86 Vlissidis and Zouboulis (1993)

Ist stage (upflow sludge bed

reactor)

11

IInd stage (batch operated

bioreactor, flocculator,

precipitator)

0.10

Anaerobic contact filter (in series) 4 73–98 Vijayaraghavan and Ramanujam (2000)

Granular bed anaerobic baffled

reactor (GRABBR)a4.75 82–90 90 Akunna and Clark (2000)

Upflow blanket filter 9–11 11–12 70 Bardiya et al. (1995)

aMalt whisky wastewater.bWinery wastewater.

shock loadings. In anaerobic contact filters, variouspacking materials, viz. polyurethane, clay brick, granularactivated carbon (GAC), polyvinyl chloride (PVC) plasticmedia have been employed resulting in 67–98% reductionin COD (Bories et al., 1988; Seth et al., 1995; Goyal et al.,1996; Vijayaraghavan and Ramanujam, 2000).

GAC as support media is relatively expensive butbecause of its adsorptive properties, it contributes towardsimproved process stability. The interference by sulfate,unionized sulfite and total hydrogen sulfide in anaerobicfilters is reported to be negligible (Vijayaraghavan andRamanujam, 2000). It was observed that the percentagesulfate removal increased with increasing HRT from 2 to5 d. This may be due to the utilization of sulfate as anutrient by microorganisms present in anaerobic contactfilter and their conversion to sulfide by sulfate reducingbacteria (SRB) under anaerobic conditions. However at

higher sulfate concentration (426mg/l), the removaldecreased, possibly due to low SRB population incomparison to methanogens. Also the removal of sulfidewas explained by stripping of hydrogen sulfide from liquidto vapor phase by the carbon dioxide and methanegenerated during the anaerobic process.In another study, Tokuda et al. (1999) performed

anaerobic treatment of undiluted whisky pot ale using anupflow anaerobic filter (UAF) packed with special supporttype (Pelia 4555, Herding GmbH, Germany) whichresulted in 76% COD removal. The pilot system alsoconsisted of a decanter, dephosphatation or magnesiumammonium phosphate (MAP) (MgNH4PO4) reactor,denitrification reactor, nitrification reactor and sedimenta-tion tank for the reduction of nitrogen and phosphate.Downflow filter using plastic PVC as support materialhas been employed for the treatment of beet molasses

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wastewater (Athanasopoulos, 1987). The system resulted in55–85% reduction in COD. Also, though high sulfideconcentration (4250mg) was inhibitory to the system, itwas not toxic at higher loadings (44 kgCOD/m3 d)probably due to high stripping of H2S.

4.1.1.3. Fluidized bed reactor. Fluidized bed reactorscontain an appropriate media such as sand, gravel orplastics for bacterial attachment and growth. The reactorscan be operated either in the upflow or downflow modes,with fluidization being realized by applying high fluidvelocities, normally by effluent recycling. Studies ondistillery effluent treatment in a downflow fluidized bedsystem using ground perlite (an expanded volcanic rock)resulted in 75–95% reduction in carbon content (Garcıa-Bernet et al., 1998). The utilization of ground perlite as acarrier material was advantageous in the downflowconfiguration because it requires low fluidization velocitieswhich preclude the possibility of clogging.

4.1.1.4. Two-stage processes and hybrid reactors. A two-stage process with an anaerobic filter followed by a UASBreactor was investigated by Blonskaja et al. (2003). Theacidogenic and methanogenic phases were clearly separatedensuring better conditions for the methanogens. CODreduction was 54% and 93% in the first and second stage,respectively. In another study on a two-phase thermophilicsystem, 65% COD reduction combined with a three-foldincrease in biogas yield over a single phase system wasobserved (Yeoh, 1997). Boopathy and Tilche (1991) studiedthe anaerobic digestion of 3–12 times diluted beet molasseswastewater, without pH adjustment, in a hybrid anaerobicbaffled reactor (HABR). Additional nitrogen and phos-phorus were provided in the form of urea (0.007 g/g ofCOD) and diammonium hydrogen phosphate (0.0006 g/gof COD). The reactor consisted of three chambers and afinal settler. 77% COD removal at a loading rate of20 kgCOD/m3 d was obtained.

Several variations of the UASB reactor have beeninvestigated for distillery wastewater treatment. In large-scale operations, highly variable process wastewater flowsmakes it difficult to maintain suitable inlet UASB flow rate;further, prevention of the loss of low density granules isalso important. To overcome these problems, Akunna andClark (2000) used granular-bed anaerobic baffled reactor(GRABBR). The reactor consisted of 10 equal compart-ments, each of which was further divided into two withsuitable baffles. Acidogenesis was found to be predominantin the compartments near the inlet and methanogenesis inthose located near the outlet. 82–90% COD reduction wasobserved at a HRT of 4 d. Yet another modification isupflow blanket filter (UBF) in which the packing is limitedto 5–10% height of the reactor. This configuration resultedin 70% COD removal in sugarcane molasses distilleryspentwash (Bardiya et al., 1995).

Vlissidis and Zouboulis (1993) have investigated thethermophilic anaerobic treatment of wastewater from the

processing of beet molasses. The process consisted of twostages: anaerobic digestion in upflow sludge bed reactorfollowed by coagulation–flocculation with lime. The HRTwas 11 d in the bioreactor and 2.5 h in the flocculator–pre-cipitator tank. On an average, the overall treatment schemeresulted in 86% BOD and 71% COD removal. Biogasproduced in the anaerobic reactor had a methane contentof 76%. This configuration was reported to be efficient intreating undiluted wastewaters.

4.1.2. Aerobic treatment

The post-anaerobic treatment stage effluent still has highorganic loading and is dark brown in color, hence it isgenerally followed by a secondary, aerobic treatment. Solardrying of biomethanated spentwash is one option but thelarge land area requirement limits this practice. Further, inIndia, solar drying beds become non-functional during therainy season (Nandy et al., 2002). The other treatmentoptions that have been demonstrated for biomethanateddistillery effluent are described below.

�

Aquaculture: Post-biomethanted effluent has been usedfor pisciculture near Chennai city in southern India. Thebiodigested effluent, which is a rich growth medium, isdirected to bioconversion ponds after which it is spreadin about 6 ha of fishponds. The BOD is reduced tonearly zero and the initiative yields about 50 tons perhectare per year of fish (Vorion Chemicals & DistilleriesLtd., 1999). � Constructed wetlands (CWs): Billore et al. (2001) have

demonstrated a four-celled horizontal subsurface flow(HSF) CW for the treatment of distillery effluent afteranaerobic treatment. The post-anaerobic treated effluenthad a BOD of about 2500mg/l and a COD of nearly14,000mg/l. A pre-treatment chamber filled with gravelwas used to capture the suspended solids. All the cellswere filled with gravel up to varying heights and cellsthree and four supported the plants Typha latipholia andPhragmites karka respectively. The overall retentiontime was 14.4 d and the treatment resulted in 64% COD,85% BOD, 42% total solids and 79% phosphoruscontent reduction. In another study, a laboratory scaleCW employing T. latipholia was used to treat diluteddistillery effluent (Trivedy and Nakate, 2000). A rootzone of 1.5� 0.3� 0.3m, filled with 75% sand andgravel and 25% soil was used and the diluted effluentwas applied after 4 weeks of planting. The systemresulted in 76% COD reduction in 7 d which increasedmarginally to 78% COD reduction in 10 d. The BODreduction was 22% and 47% on days 7 and 10,respectively. In yet another instance, a distillery innorthern India is presently employing CW for polishingthe effluent prior to land discharge for irrigation in thesurrounding paddy fields (M.K. Pilania, pers. comm.).The effluent is initially subjected to primary treatmentwhich includes settling and anaerobic digestion ina structured media attached growth (SMAG)-type

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anaerobic reactor. The primary treated effluent, with aCOD of 28,000–35,000mg/l, is subjected to two-stageaeration to bring down the COD to 400mg/l. There-after, it is directed to a CW before final discharge.

� Biocomposting: The spentwash, either directly, or after

biomethanation is sprayed in a controlled manner onsugarcane pressmud. The latter is the filter cakeobtained during juice clarification in the manufactureof sugar. Biocomposting is an aerobic, thermophilicprocess resulting in a product rich in humus which isthus used as a fertilizer. This is a popular optionadopted by several Indian distilleries attached to sugarmills with adequate land availability.

The most common post-biomethanation step is theactivated sludge process wherein research efforts aretargeted at improvements in the reactor configurationand performance. For instance, aerobic sequencingbatch reactor (SBR) is reported to be a promising solu-tion for the treatment of effluents originating fromsmall wineries (Torrijos and Moletta, 1997). The treatmentsystem consisted of a primary settling tank, an interme-diate retention trough, two storage tanks and an aerobictreatment tank. A start up period of 7 d was given tothe aerobic reactor and the system resulted in 93% CODand 97.5% BOD removal. Another configuration thathas been examined is the rotating biological reactor(RBR) (de Bazua et al., 1991). The system consisted of300 l anaerobic fluidized bed reactor coupled with a 3000 lRBR. Both the reactors were tested with diluted rawvinasse with a COD of 60–70 g/l. At a HRT of 2 dand COD loading up to 20 kg COD m3/day, the anaerobicunit resulted in 70% COD removal while a lower46% COD removal was obtained in the aerobic step.Further testing in the aerobic system was planned withthe effluent from anaerobic reactor but no results werereported.

The activated sludge process and its variations utilizemixed cultures. To enhance the efficiency of aerobicsystems, several workers have focused on treatment bypure cultures. Further, aerobic treatment has also beenexamined as a precursor to anaerobic treatment. In studieson both beet spentwash and molasses, aerobic pre-treatment of beet spentwash with Penicillium decumbens

resulted in about 74% reduction in phenolics content and40% reduction in color (Jimenez et al., 2003). Anaerobicdigestion without aerobic pre-treatment resulted in a sharpdrop in COD removal efficiencies with decreasing HRT.The organic matter removal was marginally higher for beetmolasses previously fermented with P. decumbens. Theanaerobic reaction followed first-order kinetics and the rateconstant decreased on increasing the organic loading withuntreated molasses; however, it remained almost constantwith pre-treated molasses (Jimenez et al., 2003, 2004).Geotrichum candidum is another species that resulted inpartial elimination of phenolic inhibitors such as gentisicacid, gallic acid, quercetin, p-coumaric acid, etc., thereby

enhancing the effectiveness of anaerobic process (Borjaet al., 1993).The following sections discuss pure culture studies on

molasses distillery wastewater targeting both COD reduc-tion and effluent decolorization.

4.1.2.1. Fungal treatment. White rot fungus secretingligninolytic enzymes are capable of degrading xenobioticsand organopollutants. Phanerochaete chrysosporium andTrametes versicolor are the most widely studied amongthese (Gonzalez et al., 2000). P. chrysosporium JAG 40resulted in 80% decolorization of diluted syntheticmelanoidin (absorbance unit of 3.5 at 475 nm), as well aswith 6.25% anaerobically digested spentwash (Kumar etal., 1998; Dahiya et al., 2001a). T. versicolor produces a47 kDa extracellular enzyme identified as peroxidase whichis involved in mineralization of melanoidins. The fungusresulted in 82% decolorization of 12.5% anaerobical-ly–aerobically treated effluent. Of this, 90% color wasremoved biologically and the rest by adsorption on themycelium (Dehorter and Blondeau, 1993; Benito et al.,1997). In addition, treatment by Trametes species I-62(CECT 20197) detoxifies the effluent by degrading furanderivatives as observed by gas chromatography analysis(Gonzalez et al., 2000).Decolorization of melanoidin pigment has also been

reported by extracellular H2O2 and peroxidase producedby Coriolus hirsutus (Miyata et al., 1998). For 6.25%anaerobically digested spentwash, this species showed71–75% reduction in color and 90% reduction in COD(Kumar et al., 1998). Further, Coriolus versicolor Ps4adecolorizes the effluent by decomposition of melanoidinsand not by the partial transformation of chromophores.The decolorizing activity was attributed to an intracellularenzyme which is induced in the presence of melanoidinpigment (Aoshima et al., 1985). Treatment of biodigesteddistillery wastewater by C. versicolor has also beeninvestigated by Chopra et al. (2004). The fungus was ableto reduce both COD and color up to 53% in 8 d; however,glucose and peptone were required as additional nutrientsources.In another study, Raghukumar et al. (2004) used a

marine fungus, Flavodon flavus for the combined decolor-ization and detoxification of 10% molasses spentwash. Itwas suspected that Maillard reaction also resulted in theformation of pyrogenic compounds like polycyclic aro-matic hydrocarbons (PAHs) that are toxic to estuarine fish.Treatment by F. flavus detoxified the effluent by 68%reduction in PAH and resulted in 73% color removal. Thefungus was more effective in decolorizing raw molassesspentwash than the anaerobically and aerobically treatedstreams. This was possibly due to changes in the chemicalstructure of the melanoidin pigments during anaerobic andaerobic treatment. However, the oxygen demand of thefungus was reportedly high.The effects of filamentous fungi have also been studied

on distillery wastewater. These are comparatively slow

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growing species and more susceptible to infection but theproduction of a series of extracellular hydrolytic enzymesmakes it easier for them to grow on starch and cellulosesubstrates. Among the filamentous species, Aspergillus sp.is the most popular (Friedrich, 2004). A. niveus and A. niger

resulted in 60–69% reduction in color and 75–95% CODremoval; also, the treated effluent enhanced the seedlinggrowth in Zea mays (Angayarkanni et al., 2003; Miranda etal., 1996). Immobilized fungal isolate of A. niger UM2resulted in a 72% decolorization of diluted syntheticmelanoidin (absorbance unit of 3.5 at 475 nm) and 80%decolorization of 50% biodigested effluent (Patil et al.,2003). Similarly, a thermophilic strain of A. fumigatus G-2-6 decolorized 75% of melanoidin pigment solution at45 1C. Gel filtration chromatography revealed that largemolecular weight fractions of melanoidins, in particular,were degraded rapidly (Ohmomo et al., 1987).

Apart from white-rot and filamentous fungi, yeast hasalso been investigated for distillery wastewater treatment.Yeast is characterized by quick growth and is lesssusceptible to contamination by other microorganisms;further, yeast produces biomass with high nutritive value(Friedrich, 2004). The yeast Citeromyces WR-43-6 resultedin high and stable removal efficiency in both color intensityand organic matter. The removal efficiencies for dilutedspentwash (absorbance unit of 3.5 at 475 nm) were 75% forcolor intensity and 76% for BOD (Sirianuntapiboon et al.,2004a). Shojaosadati et al. (1999) optimized the growthconditions for single cell protein (SCP) production andCOD reduction by the use of Hansenula sp. in sugar beetstillage. They concluded that production of SCP fromstillage is one of the most promising options. Besides, theyeast was also found to utilize lactate and acetate that areinhibitory to ethanol production. As a result, the treatedeffluent could be used as dilution water for fermentationthereby reducing the residual stillage volume by 70%.Another strain of Hansenula anomala J 45-N-5 and I-44isolated from soil, resulted in 74% reduction in totalorganic carbon (TOC) (Moriya et al., 1990). S. cerevisiae

also provides promising results on a larger scale. The use ofpure culture of S. cerevisiae resulted in 82.7% decoloriza-tion in the 10% anaerobically treated distillery effluentalong with 84% reduction in COD (Selim et al., 1991;Rajor et al., 2002). It was also reported that the nitrogenpresent in the distillery effluent was sufficient for thegrowth of yeast.

The dye decolorizing fungus G. candidum Dec 1immobilized on polyurethane foam resulted in 80%removal in color in diluted molasses solution (40–50 g/l)(Kim and Shoda, 1999). In case of pure culture experi-ments, Candida utilis and Trichoderma viridiae each showedless than 65% reduction in COD whereas C. utilis and A.

niger together resulted in 89% COD removal (Nudel et al.,1987). This reduction was from sugarcane stillage-basedmedia with an initial COD of 40–75 g/kg. The fungusMycelia sterilia D90 resulted in 91% decolorization of rawspentwash. The color intensity (in terms of absorbance unit

at 475 nm) was originally 47 for the raw spentwash but itwas diluted to a value of 3.5 before use (Sirianuntapiboonet al., 1988). However, a lower color removal of 60–65%was obtained for the wastewater from anaerobic andaerobic ponds. This was possibly due to either theformation of some toxic compounds during anaerobicand aerobic treatment or the inability of the strain toattack the color causing compound due to a change in theirstructure during anaerobic and aerobic treatment.

4.1.2.2. Bacterial treatment. Treatment of distillery was-tewater by the use of Pseudomonas putida followed byAeromonas sp. in a two-stage bioreactor resulted in CODas well as color reduction (Ghosh et al., 2002). P. putida

produces hydrogen peroxide which is a strong decolorizingagent. Since the organism cannot use spentwash as a sourceof carbon, 1% w/v glucose supplement was provided alongwith 12.5% spentwash. Aeromonas sp. utilizes the carbo-naceous compounds present in spentwash as the solecarbon source, thereby eventually reducing the effluentCOD by 66% in a 24 h period. P. putida also resulted in44% COD removal accompanied by 60% color reduction.In another study on predigested distillery effluent withAeromonas formicans, 57% COD reduction and 55%decrease in color was observed after 72 h (Jain et al.,2000). The color removal efficiency increased up to 68%using the bacteria immobilized on calcium alginate beads;however, the COD reduction remained unchanged withlonger incubation period of up to 96 h.

P. fluorescence immobilized on porous cellulose carrierresulted in 66% color removal with non-sterile dilutedspentwash (absorbance unit of 3.5 at 475 nm) and 90%decolorization with sterile samples at 30 1C over a 4 dperiod (Dahiya et al., 2001b). The decolorization efficiencywas further increased to 94% with cellulose carrier coatedwith collagen. These immobilized cells could be reused butthe efficacy of color removal was reduced. In anotherstudy, three different bacterial strains Xanthomonas fragar-

iae, Bacillus megaterium and Bacillus cereus were used bothin free form as well as after immobilization on calciumalginate beads for the treatment of 33% predigesteddistillery effluent (Jain et al., 2002). B. cereus resulted inmaximum COD (81%) and color (75%) reduction in freeform. The reduction efficiencies increased marginally withimmobilization.The decolorization activity of acetogenic bacteria has

been reported for the first time by Sirianuntapiboon et al.(2004b). Acetogenic bacteria is capable of oxidativedecomposition of melanoidins thereby removing lowmolecular weight compounds in untreated molasses spent-wash and almost all the low and high molecular weightcompounds in anaerobically treated molasses spentwash.Nearly 76% decolorization, which is possibly due to asugar oxidase, has been observed. The nitrifying bacteriaNitrosococcus oceanus is capable of detoxifying thespentwash accompanied by a reduction in the chloridecontent (Arora et al., 1992). However, no explanation was

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provided for this observation. The treated wastewater leadsto better growth of rice plant due to adequate nitrogencontent and can therefore be used as a low cost fertilizer. Inyet another investigation, two aerobic bacterial strains TA2 and TA 4 have been isolated from sites contaminatedwith anaerobically treated distillery effluent (Asthana et al.,2001). These bacteria, which were identified to be Gram-negative and Gram-positive, respectively, resulted in 66%and 62% BOD reduction in anaerobically treated spent-wash. However the reduction in BOD was found to behigher (80%) when the two were used together; further thecombination resulted in 76% color removal after 72 h.

More recently, the decolorization of four types ofsynthetic melanoidins i.e., glucose–glutamic-acid (GGA),glucose–aspartic-acid (GAA), sucrose–glutamic acid(SGA), and sucrose–aspartic-acid (SAA), were investigatedusing three different isolates, viz. Bacillus thuringiensis,Bacillus brevis and Bacillus sp. (Kumar and Chandra,2006). The degree of decolorization of the melanoidinsseparately by each isolate was in the 1–31% range;however, when used collectively, these isolates resulted inup to 50% decolorization due to the enhanced effect ofcoordinated metabolic interactions. The results alsoindicated that the GAA polymer was the most recalcitrantamong the melanoidins tested.

The biodegradability of spentwash can be enhanced byenzymatic pre-treatment prior to the aerobic step (Sangaveand Pandit, 2006a). After 24 h of treatment with Gram-positive bacterium ASN6, the COD reduction of cellulasepre-treated spentwash was 28.8% in comparison to 18.3%for untreated effluent. This was explained by the fact thatpre-treatment affected the metabolic value (microbialacceptability) by generating intermediate hydrolysis pro-ducts from the parent cellulosic compounds present in thespentwash. The biodegradability was further enhanced bycombined ultrasound and enzymatic pre-treatment result-ing in 62.2% COD reduction after 36 h as compared to39.4% COD removal for the untreated effluent (Sangaveand Pandit, 2006b). The enhancement in biodegradabilitywas attributed to molecular transformation of effluentconstituents by ultrasound pre-treatment.

4.1.2.3. Algal treatment. The treatment of anaerobicallytreated 10% distillery effluent using the microalga Chlor-

ella vulgaris followed by Lemna minuscula resulted in 52%reduction in color (Valderrama et al., 2002). In anotherstudy, Kalavathi et al. (2001) examined the degradation of5% melanoidin by the marine cyanobacterium Oscillatoria

boryana BDU 92181. The organism was found to releasehydrogen peroxide, hydroxyl ions and molecular oxygenduring photosynthesis resulting in 60% decolorization ofdistillery effluent. In addition, this study suggested thatcyanobacteria could use melanoidin as a better nitrogensource than carbon. Further, cyanobacteria also excretecolloidal substances like lipopolysaccharides, proteins,polyhydroxybutyrate (PHB), polyhydroxy-alkanoates(PHA), etc. These compounds possess COO� and ester

sulphate (OSO3�) groups that can form complexes with

cationic sites thereby resulting in flocculation of organicmatter in the effluent. It was observed that the strainOscillatoria resulted in almost complete color removal(96%) whereas Lyngbya and Synechocystis were lesseffective resulting in 81 and 26% color reduction,respectively (Patel et al., 2001). The consortium of thethree strains showed a maximum decolorization of 98%.This was attributed to adsorption in the initial stagesfollowed by degradation of organic compounds whichdominated in the subsequent stages.

4.2. Physico-chemical treatment

Sugarcane molasses spentwash after biological treatmentby both anaerobic and aerobic method can still have aBOD of 250–500mg/l (Mall and Kumar, 1997). Also, eventhough biological treatment results in significant CODremoval, the effluent still retains the dark color (Inanc etal., 1999). The color imparting melanoidins are barelyaffected by conventional biological treatment such asmethane fermentation and the activated sludge process(Migo et al., 1993). Further, multistage biological treat-ment reduces the organic load but intensifies the color dueto re-polymerization of colored compounds (Pena et al.,2003). In this context, various physico-chemical treatmentoptions have been explored.

4.2.1. Adsorption

Activated carbon is a widely used adsorbent for theremoval of organic pollutants from wastewater but therelatively high cost restricts its usage. Decolorization ofsynthetic melanoidin using commercially available acti-vated carbon as well as activated carbon produced fromsugarcane bagasse was investigated by Bernardo et al.(1997). The adsorptive capacity of the different activatedcarbons was found to be quite comparable. Chemicallymodified bagasse using 2-diethylaminoethyl chloride hy-drochloride and 3-chloro-2-hydroxypropyltrimethylammo-nium chloride was capable of decolorizing dilutedspentwash (Mane et al., 2006). 0.6 g of chemically modifiedbagasse in contact with 100ml 1:4 (v/v) spentwash:watersolution resulted in 50% decolorization after 4 h contactwith intermittent swirling.Significant decolorization was observed in packed bed

studies on anaerobically treated spentwash using commercialactivated charcoal with a surface area of 1400m2/g (Chandraand Pandey, 2000). Almost complete decolorization (499%)was obtained with 70% of the eluted sample, which alsodisplayed over 90% BOD and COD removal. In contrast,other workers have reported adsorption by activated carbonto be ineffective in the treatment of distillery effluent (Sekarand Murthy, 1998; Mandal et al., 2003). Adsorption bycommercially available powdered activated carbons resultedin only 18% color removal; however, combined treatmentusing coagulation–flocculation with polyelectrolyte followed

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by adsorption resulted in almost complete decolorization(Sekar and Murthy, 1998).

Low cost adsorbents such as pyorchar (activated carbonboth in granular and powdered form, manufactured frompaper mill sludge) and bagasse flyash have also beenstudied for this application. Ramteke et al. (1989) reportedcolor removal up to 98% with pyorchar. However, toachieve the same level of color removal, larger doses of theindigenously prepared powdered and granular pyorcharwere required in comparison to commercial activatedcarbon. Mall and Kumar (1997) compared the colorremoval using commercial activated carbon and bagasseflyash. 58% color removal was reported with 30 g/l ofbagasse flyash and 80.7% with 20 g/l of commercialactivated carbon. Since the bagasse flyash has high carboncontent and the adsorbed organic material further in-creases its heating value, the spent adsorbent can be usedfor making fire briquettes. Yet another adsorbent that hasbeen examined is the natural carbohydrate polymerchitosan derived from the exoskeleton of crustaceans.Lalov et al. (2000) studied the treatment of distillerywastewater using chitosan as an anion exchanger. At anoptimum dosage of 10 g/l and 30min contact time, 98%color and 99% COD removal was observed.

4.2.2. Coagulation and flocculation

Inanc et al. (1999) reported that coagulation with alumand iron salts was not effective for color removal. Theyexplored lime and ozone treatment with anaerobicallydigested effluent. The optimum dosage of lime was foundto be 10 g/l resulting in 82.5% COD removal and 67.6%reduction in color in a 30min period. These findings are indisagreement with those of Migo et al. (1993) who used acommercial inorganic flocculent, a polymer of ferrichydroxysulfate with a chemical formula [Fe2 (OH)n(SO4)3�n/2]m for the treatment of molasses wastewater.The treatment resulted in around 87% decolorization forbiodigested effluents; however an excess of flocculenthindered the process due to increase in turbidity andTOC content. FeCl3 and AlCl3 were also tested fordecolorization of biodigested effluent and showed similarremoval efficiencies. About 93% reduction in color and76% reduction in TOC were achieved when either FeCl3 orAlCl3 was used alone. The process was independent ofchloride and sulfate ion concentration but was adverselyaffected by high fluoride concentration. However in thepresence of high flocculent concentration (40 g/l), additionof 30 g/l CaO enhanced the decolorization process resultingin 93% color removal. This was attributed to the ability ofcalcium ions to destabilize the negatively charged mela-noidins; further, formation of calcium fluoride (CaF2) alsoprecipitates the fluoride ions.

Almost complete color removal (98%) of biologicallytreated distillery effluent has been reported with conven-tional coagulants such as ferrous sulfate, ferric sulfate andalum under alkaline conditions (Pandey et al., 2003). Thebest results were obtained using Percol 47, a commercial

organic anionic polyelectrolyte, in combination withferrous sulfate and lime. The combination resulted in99% reduction in color and 87 and 92% reduction in CODand BOD, respectively. Similar findings have also beenreported by Mandal et al. (2003).Coagulation studies on spentwash after anaerobic–aero-

bic treatment have also been conducted using bleachingpowder followed by aluminum sulfate (Chandra and Singh,1999). The optimum dosage was 5 g/l bleaching powderfollowed by 3 g/l of aluminum sulfate that resulted in 96%removal in color, accompanied by up to 97% reduction inBOD and COD.Non-conventional coagulants namely wastewater from

an iron pickling industry which is rich in iron and chlorideions and titanium ore processing industry containingsignificant amounts of iron and sulfate ions have also beenexamined (Pandey et al., 2003). The iron pickling waste-water gave better results with 92% COD removal,combined with over 98% color removal. Though thetitanium processing wastewater exhibited similar colorremoval levels, the COD and BOD reductions wereperceptibly lower.

4.2.3. Oxidation process

Ozone destroys hazardous organic contaminants and hasbeen applied for the treatment of dyes, phenolics,pesticides, etc. (Pena et al., 2003). Oxidation by ozonecould achieve 80% decolorization for biologically treatedspentwash with simultaneous 15–25% COD reduction. Italso resulted in improved biodegradability of the effluent.However, ozone only transforms the chromophore groupsbut does not degrade the dark colored polymeric com-pounds in the effluent (Alfafara et al., 2000; Pena, et al.,2003). Similarly, oxidation of the effluent with chlorineresulted in 497% color removal but the color reappearedafter a few days (Mandal et al., 2003). Ozone incombination with UV radiation enhanced spentwashdegradation in terms of COD; however, ozone withhydrogen peroxide showed only marginal reduction evenon a very dilute effluent (Beltran et al., 1997).In another study, Sangave and Pandit (2004) employed

sonication of distillery wastewater as a pre-treatment stepto convert complex molecules into a more utilizable formby cavitation. Samples exposed to 2 h ultrasound pre-treatment displayed 44% COD removal after 72 h ofaerobic oxidation compared to 25% COD reduction shownby untreated samples. These results are contrary to those ofMandal et al. (2003) who concluded ultrasonic treatment tobe ineffective for distillery spentwash treatment.A combination of wet air oxidation and adsorption has

been successfully used to demonstrate the removal ofsulfates from distillery wastewater. Studies were done in acounter current reactor containing 25 cm base of smallcrushed stones supporting a 20 cm column of bagasse ashas an adsorbent (Gaikwad and Naik, 2000). The waste-water was applied from the top of the reactor and air wassupplied at the rate of 1.0 l/min. The treatment removed

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57% COD, 72% BOD, 83% TOC and 94% sulfates. Wetair oxidation has been recommended as part of a combinedprocess scheme for treating anaerobically digested spent-wash (Dhale and Mahajani, 2000). The post-anaerobiceffluent was thermally pre-treated at 150 1C under pressurein the absence of air. This was followed by soda-limetreatment, after which the effluent underwent a 2 h wetoxidation at 225 1C. 95% color removal was obtained inthis scheme. Another option is photocatalytic oxidationthat has been studied using solar radiation and TiO2 as thephotocatalyst (Kulkarni, 1998). Use of TiO2 was found tobe very effective as the destructive oxidation process leadsto complete mineralization of effluent to CO2 and H2O. Upto 97% degradation of organic contaminants was achievedin 90min.

Pikaev et al. (2001) studied combined electron beam andcoagulation treatment of distillery slops from distilleriesprocessing grain, potato, beet and some other plantmaterials. Humic compounds and lignin derivatives con-stitute the major portion of this dark brown wastewater.The distillery wastewater was diluted with municipalwastewater in the ratio of 3:4, irradiated with electronbeam and then coagulated with Fe2(SO4)3. The opticalabsorption in UV region was decreased by 65–70% afterthis treatment. The cost was found to be less than theexisting method wherein the effluent was transported about20 km via pipeline to a facility for biological treatmentfollowed by sedimentation. The treatment cost was0.45–0.65 US$/m3 which dropped to 0.25 US$/m3 usingcombined electronic-beam and coagulation method.

4.2.4. Membrane treatment

Pre-treatment of spentwash with ceramic membranesprior to anaerobic digestion is reported to halve the CODfrom 36,000 to 18,000mg/l (Chang et al., 1994). The totalmembrane area was 0.2m2 and the system was operated ata fluid velocity of 6.08m/s and 0.5 bar transmembranepressure. In addition to COD reduction, the pre-treatmentalso improved the efficiency of the anaerobic processpossibly due to the removal of inhibiting substances.Kumaresan et al. (2003) employed emulsion liquidmembrane (ELM) technique in a batch process forspentwash treatment. Water–oil–water type of emulsionwas used to separate and concentrate the solutes resultingin 86% and 97% decrease in COD and BOD, respectively.Electrodialysis has been explored for desalting spentwashusing cation and anion exchange membranes resulting in50–60% reduction in potassium content (de Wilde, 1987).In another study, Vlyssides et al. (1997) reported thetreatment of vinasse from beet molasses by electrodialysisusing a stainless steel cathode, titanium alloy anode and4% w/v NaCl as electrolytic agent. Up to 88% CODreduction at pH 9.5 was obtained; however, the CODremoval percentage decreased at higher wastewaterfeeding rates.

In addition, reverse osmosis (RO) has also beenemployed for distillery wastewater treatment. A unit in

western India is currently processing effluent obtained afteranaerobic digestion, followed by hold-up in a tankmaintained under aerobic conditions, in a RO system(B.P. Agrawal, pers. comm.). 290m3/d of RO treatedeffluent is mixed with 300m3/d of fresh water and used inthe process for various operations like molasses dilution(290m3/d), make-up water for cooling tower (178m3/d),fermenter washing (45m3/d), etc. Yet another unit insouthern India is employing disc and tube RO modules fordirect treatment of the anaerobically digested spentwash(M. Prabhakar Rao, pers. comm.). The permeate isdischarged while the concentrate is used for biocompostingwith sugarcane pressmud. In a recent study, Nataraj et al.(2006) reported pilot trials on distillery spentwash using ahybrid nanofiltration (NF) and RO process. Both the NFand RO stages employed thin film composite (TFC)membranes in spiral wound configuration with moduledimensions of 2.5 inches diameter and 21 inches length. NFwas primarily effective in removing the color and colloidalparticles accompanied by 80%, 95% and 45% reduction intotal dissolved solids (TDS), conductivity and chlorideconcentration, respectively, at an optimum feed pressure of30–50 bar. The subsequent RO operation at a feed pressureof 50 bar resulted in 99% reduction each in COD,potassium and residual TDS.

4.2.5. Evaporation/combustion

Molasses spentwash containing 4% solids can beconcentrated to a maximum of 40% solids in a quintu-ple-effect evaporation system with thermal vapor recom-pression (Bhandari et al., 2004; Gulati, 2004). Thecondensate with a COD of 280mg/l can be used infermenters. The concentrated mother liquor is spray driedusing hot air at 180 1C to obtain a desiccated powder with acalorific value of around 3200 kcal/kg. The powder istypically mixed with 20% agricultural waste and burnt in aboiler. The use of recirculating fluidized bed (RCFB)incinerator is recommended to overcome the constraintsdue to stickiness of spentwash and its high sulfate content(Alappat and Rane, 1995). Combustion is also an effectivemethod of on-site vinasse disposal as it is accompanied byproduction of potassium-rich ash (Cortez and Perez, 1997)that can be used for land application.

5. Discussion

A range of biological and physico-chemical methodshave been investigated for the treatment of wastewaterfrom molasses-based distilleries. Because of the very highCOD, anaerobic treatment with biogas recovery isemployed extensively as the first treatment step. Anaerobiclagoons are still used; however, most Indian distilleriesemploy high rate digesters wherein the HRT is decoupledfrom the SRT thereby retaining the slow growinganaerobic microorganisms in the reactor even at highwastewater flow. Biomethanation reduces the organicpollution load and brings down BOD to 80–95% of the

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original value; however, the biodigested effluent stillcontains BOD in the range of 5000–10,000mg/l (Pathade,1999). Moreover, anaerobic treatment is a slow process andtypically requires long start-up periods. In addition, theproblem of color associated with this effluent not onlyremains unsolved (Patil et al., 2003) but actually getsaggravated since the color causing melanoidin pigmentintensifies under anaerobic conditions (Pena et al., 2003).Therefore anaerobically treated effluent is darker in colorcompared to untreated spentwash and needs several-folddilution by fresh water prior to discharge. The currentpractice of using diluted, biodigested spentwash forirrigation (‘‘ferti-irrigation’’) by a large number of Indiandistilleries is reportedly causing gradual soil darkening(Kumar et al., 1998). Yet another limitation is the need todilute the spentwash prior to biomethanation itself. Mostdistilleries follow 1–3-fold dilution to ensure properoperation of the biomethanation plant. Since 8–15 lspentwash is generated per liter of ethanol, dilutionincreases the load on the effluent treatment plant besidesconsuming considerable amount of fresh water for thispurpose.

Biological treatment using aerobic processes like acti-vated sludge, biocomposting etc. is presently practiced byvarious molasses-based distilleries. Due to the largevolumes generated, only a part of the total spentwash getsconsumed in biocomposting. Biocomposting utilizes su-garcane pressmud as the filler material; thus it is typicallyemployed by distilleries attached to sugar mills. Since sugarmanufacturing is a seasonal operation, pressmud avail-ability is often a constraint. Further, biocompostingrequires large amount of land; also, it cannot be carriedout during the rainy season.

Though aerobic treatment like the conventional acti-vated sludge process leads to significant reduction in COD,the process is energy intensive and the color removal is stillinadequate. Thus several pure cultures of fungi, bacteriaand algae have been investigated specifically for theirability to decolorize the effluent as discussed earlier. In allinstances, supplementation with either nitrogen or carbonsource is almost always necessary because the microbialspecies are not able to utilize the spentwash as the solecarbon source. Further, high dilution (typically up to 1:10fold for untreated spentwash and 1:16–1:2 fold forbiomethanated spentwash) is required for optimal micro-bial activity. In addition, these studies are mostly limited tolaboratory scale investigations and no pilot/commercialscale operations are reported as yet.

Physico-chemical treatment, viz. adsorption, coagula-tion/flocculation, oxidation processes, membrane treat-ment have been examined with particular emphasis oneffluent decolorization. Though these techniques areeffective for both color removal as well as reduction inorganic loading, sludge generation and disposal is aconstraint in coagulation/flocculation and adsorption.Also, the cost of chemicals, adsorbents and membranes isa deterrent to the adoption of these methods (Rajor et al.,

2002). Membrane operations like microfiltration/ultrafil-tration for spentwash treatment are characterized bysignificant membrane fouling that limits its applicability(Jain and Balakrishnan, 2004). Decolorization throughchemical treatment with ozone and chlorine leads totemporary color reduction because of transformation ofthe chromophore groups so these are not preferredsolutions.Thus, solutions for effective management of molasses-

based distillery wastewaters are still evolving. Some of thegaps are highlighted below:

�

Biomethanation of spentwash is well established com-mercially; however, there is scope for several operationalimprovements. These include adapting the system totreat the spentwash without any dilution, ensuringshorter start-up periods and degrading refractile com-ponents to improve the anaerobic treatment efficiency.Also, a better understanding of the re-polymerization ofcolor-causing compounds during anaerobic digestionwould assist in the subsequent decolorization steps. � Biocomposting with sugarcane pressmud is increasingly

being adopted by a number of sugar complexes as amethod for disposing the biodigested spentwash. How-ever, pressmud availability is limited; thus, alternativematerials like rice husk, wood chips, bagasse pith etc.have been suggested (CPCB, 1994). This requiresexploring readily available local filler materials thatcan be utilized for this purpose. Also, the issue ofmanure quality, possibly to match the requirements oflocal crops, can be addressed.

� The structure and characteristics of the color-causing

components (melanoidins) is still not fully understood.This has consequently hindered the development of anappropriate process scheme for their removal.

� It is established that several microorganisms (bacteria,

fungi, algae), especially in pure cultures, display alimited ability to decolorize the spentwash. A betterunderstanding of the microbial enzymes/activities re-sponsible for the degradation of melanoidins wouldcontribute to enhancing the efficiency of the decoloriza-tion process.

� Investigations on pure culture aerobic systems that can

result in both COD and color removal have beenconfined exclusively to laboratory scale set-ups. Theissues of appropriate system design, including scale-up,have not been addressed. In this context, systems likemembrane bioreactors that have lower sludge produc-tion can be considered. Also, issues like minimizingnutrient supplementation, avoiding feed dilution andoperation under non-sterile conditions should be exam-ined. These points are particularly significant intranslating these studies into field applications.

� Adsorbents like activated carbon that result in almost

complete decolorization are not cost effective fortreating the enormous volumes of spentwash typicallygenerated in a distillery. Thus, there is scope for

ARTICLE IN PRESS

examining low cost adsorbents, including wastes gener-ated in other industrial processes/operations. In thiscontext, appropriate sludge disposal methods shouldalso be examined.

� Membrane processes like nanofiltration and RO can

result in significant color removal thereby permittingreuse of the treated effluent. Since spentwash is acomplex, multicomponent stream that is known to causeconsiderable fouling, an understanding of the compo-nents those are primarily responsible for this phenom-enon would assist in appropriate feed pre-treatment, forthe efficient operation of the membrane system.

6. Conclusion

The review indicates that a comprehensive treatmentscheme for molasses distillery wastewater leading toeffective removal of both organics and color is notcurrently available. The properties of the color-causingmelanoidins and their transformation during anaerobictreatment are also not fully understood. Biological treat-ment, especially with pure cultures, appears promising andpossibly cost-effective for color removal; however, theinitiatives are mainly confined to laboratory trials. Physico-chemical methods are capable of both color and organicload reduction; consequently, in spite of the cost, evenhigh-end options like membrane filtration are being field-tested. There is thus an urgent need to address thelimitations in the existing methods and to developintegrated treatment processes that provide a completesolution to the treatment of wastewater from molasses-based distilleries.

Acknowledgement

Y. Satyawali gratefully acknowledges the financialsupport in the form of Junior Research Fellowshipprovided by University Grants Commission, New Delhi,India.

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