Wastewater treatment in molasses-based alcohol distilleries for COD

ARTIC AR TICLE LE IN PR PRESS ESS

Wastewater treatment in molasses-based alcohol distilleries for COD and color removal: A review Y. Satyawali, M. Balakrishnanà 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 processes covering anaerobic, anaerobic, aerobic aerobic as well as physico-c physico-chemi hemical cal methods methods have been employed employed to treat this effluent. effluent. Anaerobic 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 Introduction

Ethano Ethanoll manufa manufactu cture re from from molas molasses ses genera generates tes large large volume volumess of high high streng strength th wastew wastewate aterr that that is of seriou seriouss environmen environmental tal concern. concern. The effluent effluent is characterize characterized d by extr extrem emel ely y high high chem hemical ical oxyg oxygen en dem demand (CO (COD) (80,00 (80,000–1 0–100, 00,000 000 mg/l) mg/l) and biochem biochemica icall oxygen oxygen demand demand (BOD) (BOD) (40,00 (40,000–5 0–50,0 0,000 00 mg/l), mg/l), apart apart from from low pH, strong strong odor odor and and dark dark brow brown n colo colorr (Centr Central al Poll Pollution ution Contr Control ol Board (CPCB) 1994, 2003). 2003). In India, which is the secondlargest producer of ethanol in Asia with a projected annual prod produc ucti tion on of abou aboutt 2300 2300 mill millio ion n lite liters rs in 2006 2006–0 –07 7 (Subra Subramani manian an et al., 2005), 2005), alcoho alcoholl distil distiller leries ies are rated rated as one of the 17 most polluting industries. Apart from high organi organicc conten content, t, distil distiller lery y wastew wastewate aterr also also contai contains ns nutrient trientss in the form form of nitrog nitrogen en (1660– (1660–420 4200 0 mg/l), mg/l), phosphosphorus phorus (225–3038 (225–3038 mg/l) and potassium potassium (9600–17,475 (9600–17,475 mg/l) (Mahimairaja and Bolan, 2004) 2004) that can lead to eutrophication cation of water water bodies bodies.. Furth Further, er, its dark dark color color hinders hinders Ã

Corresp Correspond onding ing author. author. Tel.: +91 11 2468 2468 2100; 2100; fax: +91 11 24682144. E-mail address: [email protected] (M. Balakrishnan).

phot photos osyn ynth thes esis is by bloc blocki king ng sunl sunlig ight ht and and is ther therefo efore re deleterious to aquatic life (FitzGibbon ( FitzGibbon et al., 1998). 1998 ). Studies on water water qualit quality y of a river river contam contamina inated ted with with distil distiller lery y effluen effluentt displa displayed yed high high BOD BOD val values ues of 160 1600–2 0–21,0 1,000 00 mg/l mg/l withi thin a 8 km radi radius us (Ba Baru ruah ah et al al., ., 19 1993 93). ). Adequa Adequate te treatm treatment ent is theref therefore ore impera imperativ tivee before before the effluen effluentt is discharged. In addition to pollution, increasingly stringent environmen environmental tal regulation regulationss are forcing forcing distilleri distilleries es to improv provee exis existin ting g trea treatm tmen entt and and also also expl explor oree alte altern rnat ativ ivee method methodss of effluen effluentt manage managemen ment. t. For For instanc instance, e, Indian Indian distil distiller leries ies were were stipul stipulate ated d to achiev achievee zero zero discha discharge rge of  spen spentw twas ash h to inla inland nd surfa surface ce wate waterr by Dece Decemb mber er 2005 2005 (Uppal, 2004). 2004). In an earl earlie ierr revi review ew on this this subj subjec ect, t, She Sheeha ehan n and Greenfield Green field (1980 (1980)) discussed discussed treatment treatment options options practiced practiced in the 197 1970s. 0s. More More recent recently, ly, Wil Wilkie kie et al. (2000) (2000) have exam examin ined ed char charac acter teris isti tics cs and and anae anaero robi bicc treat treatme ment nt of  effluent effluent obtained obtained from different different feedstock feedstock used for ethanol manu manufa fact ctur ure. e. This This revi review ew focu focuse sess on the the adva advanc nces es in molas molasses ses-ba -based sed distil distiller lery y wastew wastewate aterr treatm treatment ent in the last last two decade decadess and the emergi emerging ng techno technolog logies ies in this this field.

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2. Process description

Alcohol manufacture in distilleries consists of four main steps viz. feed preparation, fermentation, distillation and packaging (Fig. 1).  2.1. Feed preparation Ethanol can be produced from a wide range of feedstock. These include sugar-based (cane and beet molasses, cane  juice), starch-based (corn, wheat, cassava, rice, barley) and cellulosic (crop residues, sugarcane bagasse, wood, municipal solid wastes) materials. Details of sugar, starch and lignocellulosic biomass-based feedstock and their pretreatment steps have been reviewed earlier (Wilkie et al., 2000). In general, sugar-based feedstock containing readily available fermentable sugars are preferred since starch and cellulosic substrates involve an additional pre-treatment step to convert starch into fermentable sugars. Thus, cane   juice is a commonly used substrate in Brazil (Tano and Buzato, 2003) while Indian distilleries almost exclusively use sugarcane molasses. Overall, nearly 61% of world ethanol production is from sugar crops (Berg, 2004). The composition of molasses varies with the variety of  cane, the agro climatic conditions of the region, sugar

Yeast

Diluted molasses

Pre-fermenter

Fermenter

CO2

manufacturing process and handling and storage (Godbole, 2002). Table 1 summarizes the chemical composition of beet and cane molasses. Molasses is diluted to about 20–25 brix (measurement of sugar concentration in a solution) and its pH adjusted, if required, before fermentation. In India, about 90% of the molasses produced in cane sugar manufacture is consumed in ethanol production (Billore et al., 2001).  2.2. Fermentation Yeast culture is prepared in the laboratory and propagated in a series of fermenters, each about 10 times larger than the previous one. The feed is inoculated with about 10% by volume of yeast (Saccharomyces cerevisiae) inoculum. This is an anaerobic process carried out under controlled conditions of temperature and pH wherein reducing sugars are broken down to ethyl alcohol and carbon dioxide. The reaction is exothermic. To maintain the temperature between 25 and 32 C plate heat exchangers are used; alternatively some units spray cooling water on the fermenter walls. Fermentation can be carried out in either batch or continuous mode (CPCB, 2003). Fermentation time for batch operation is typically 24–36 h with an efficiency of about 95%. Continuous operation, involving higher sugar concentration and an osmotolerant variety of  yeast, is faster (16–24h fermentation time) but the efficiency is marginally lower (T.R. Sreekrishnan, pers. comm.). The resulting broth contains 6–8% alcohol. The sludge (mainly yeast cells) is separated by settling and 1

Table 1 Composition of cane and beet molasses (Curtin, 1983; Chen and Chou, 1993; Godbole, 2002)

Analyzer column Spentwash Rectification column

Property

Cane molasses

Beet molasses

Brix (%)

79.5 85–92 b 1.41 1.38–1.52a 75.0 75–88 a 46.0 44–60 a 50–90 b 3.0 2.5–4.5b 0.0 0.0 8.1 7–15 b 0.8 0.08 2.4 0.2 1.4 0.5

79.5

Specific gravity Spentlees

Total solids (%) Alcohol

Total sugars (%)

Crude protein (%) Blending & maturation

Dehydration (molecular sieve)

Potable alcohol

Power alcohol

Bottling

Total fat (%) Total fiber (%) Ash (%) Industrial alcohol

Calcium (%) Phosphorus (%) Potassium (%) Sodium (%) Chlorine (%) Sulfur (%) a

Fig. 1. Process description.

Godbole (2002). Chen and Chou (1993)

b

1.41 77.0 48.0

6.0 0.0 0.0 8.7 0.2 0.03 4.7 1.0 0.9 0.5

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discharged from the bottom, while the cell free fermentation broth is sent for distillation. Apart from yeast, the bacterium Zymomonas mobilis has also 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 biomass yields and higher ethanol production (Lin and Tanaka, 2006). Immobilization of  Zymomonas cells has been tried on support materials such as pectin because immobilized whole cells retain higher microbial activity (Siva Kesava et al., 1996). However, formation of byproducts such as acetoin, glycerol, acetate, and lactate under anaerobic conditions is a drawback associated with Z. mobilis. During its growth on sucrose medium, Z. mobilis also leads to formation of levan, a polymer of fructose units. Further, like yeast, Zymomonas is unable to convert complex carbohydrates like cellulose, hemicellulose and starch to ethanol. To overcome these constraints, genetic manipulation of  Z. mobilis has been attempted by several workers. For instance, the hydrolytic and isomerase genes from recombinant Escherichia coli have been transferred to Z. mobilis, resulting in utilization of xylose, mannose, lactose and arabinose as the carbon source (Gunasekaran and Raj, 1999). Earlier, Picataggio et al. (1996) reported the development of genetically altered Zymomonas strain to ferment pentoses and glucose, obtained by hydrolysis of  hemicellulose and cellulose, to produce ethanol. Here, Z. mobilis was transformed with combination of  E. coli  genes for xylose isomerase, xylulokinase, transaldolase, transketolase, L-arabinose isomerase, L-ribulokinase, and L-ribulose-5-phosphate 4-epimerase. In another work aimed at xylose utilization combined with enhanced ethanol yield, a strain derived from Z. mobilis ATCC31821 was developed (Zhang, 2000). The strain comprised of exogenous genes encoding xylose isomerase, xylulokinase, transaldolase and transketolase.   2.3. Distillation Distillation is a two-stage process and is typically carried out in a series of bubble cap fractionating columns. The first stage consists of the analyzer column and is followed by rectification columns. The cell free fermentation broth

(wash) is preheated to about 90 C by heat exchange with the effluent (‘‘spentwash’’) and then sent to the degasifying section of the analyzer column. Here, the liquor is heated by live steam and fractionated to give about 40–45% alcohol. The bottom discharge from the analyzer column is the spentwash. The alcohol vapors are led to the rectification column where by reflux action, $96% alcohol is tapped, cooled and collected. The condensed water from this stage, known as ‘‘spentlees’’ is usually pumped back to the analyzer column. 1

 2.4. Packaging Rectified spirit ($96% ethanol by volume) is marketed directly for the manufacture of chemicals such as acetic acid, acetone, oxalic acid and absolute alcohol. Denatured ethanol for industrial and laboratory use typically contains 60–95% ethanol as well as between 1% to 5% each of  methanol, 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 to requisite strength to obtain the desired type of liquor. This is bottled appropriately in a bottling plant. Anhydrous ethanol for fuel-blending applications (‘‘power alcohol’’) requires concentration of the ethanol to 499.5 wt% purity. The ethanol dehydration is typically done using molecular sieves; however, pervaporation has also been employed in Brazil and India for this purpose (Mitsui & Co., 2003). 3. Wastewater generation and characteristics

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

Table 2 Sample quantities and characteristics of wastewater streams generated in an Indian distillery (S. Majumdar, pers. comm.) Parameter

Specific wastewater generation (kl/kl alcohol)

Color

pH

Suspended solids (mg/l)

BOD (mg/l)

COD (mg/l)

Spent wash Fermenter cleaning Fermenter cooling Condenser cooling Floor wash Bottling plant Other

14.4 0.6 0.4 2.88 0.8 14 0.8

Dark brown Yellow Colorless Colorless Colorless Hazy Pale yellow

4.6 3.5 6.3 9.2 7.3 7.6 8.1

615 3000 220 400 175 150 100

36,500 4000 105 45 100 10 30

82,080 16,500 750 425 200 250 250

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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

COD (mg/l) BOD (mg/l) COD/BOD ratio Total solids (mg/l) Total suspended solids (mg/l) Total dissolved solids (mg/l) Total nitrogen (mg/l) Total phosphorus (mg/l) Potassium (mg/l) Sulfur as SO4 (mg/l) pH

Beet molasses

Pathade (1999)

Mahimairaja and Bolan (2004)

Wilkie et al. (2000)

65,000–130, 000 30,000–70,000 2.49 30,000–100,000 350 80,000 1000–2000 800–1200 8000–12,000 2000–6000 3–5.4

104,000–134,400 46,100–96,000

91,100 44,900 1.95

that 88% of the molasses constituents end up as waste (Jain et al., 2002). Molasses spentwash has very high levels of  BOD, COD, COD/BOD ratio as well as high potassium, phosphorus and sulfate content (Table 3). In addition, cane molasses spentwash contains low molecular weight compounds such as lactic acid, glycerol, ethanol and acetic acid (Wilkie et al., 2000). Cane molasses also contains around 2% of a dark brown pigment called melanoidins that impart color to the spentwash (Kalavathi et al., 2001). Melanoidins are low and high molecular weight polymers formed as one of the final products of Maillard reaction, which is a nonenzymatic browning reaction resulting from the reaction of reducing sugars and amino compounds (Martins and van Boekel, 2004). This reaction proceeds effectively at temperatures above 50 C and pH 4–7. The structure of  melanoidins is still not well known (Rivero-Pe ´rez et al., 2002). Only 6–7% degradation of the melanoidins is achieved in the conventional anaerobic–aerobic effluent treatment process (Gonzalez et al., 2000). Due to their antioxidant properties, melanoidins are toxic to many microorganisms involved in wastewater treatment (Sirianuntapiboon et al., 2004a). Apart from melanoidins, spentwash contains other colorants such as phenolics, caramel and melanin. Phenolics are more pronounced in cane molasses wastewater whereas melanin is significant in beet molasses (Godshall, 1999). 1

4. Effluent treatment

Till the early 1970s, land disposal was practiced as one of  the main treatment options, since it was found to enhance yield of certain crops. For example, in Brazil, vinasse generated from sugarcane juice fermentation is mainly used as a fertilizer due to its high nitrogen, phosphorus and organic content. Its use is further reported to increase sugarcane productivity; furthermore under controlled conditions, the effluent is capable of replacing application

79,000–87,990 1660–4200 225–3038 9600–17,475 3240–3425 3.9–4.3

3569 163 10,030 3716 5.35

Spentwash

Biomethanation

Biocomposting

Solar drying

Aerobic treatement

Dilution

Evaporation / incineration

Tertiary treatment

Surface water discharge Irrigation

Fig. 2. Spentwash treatment options.

of inorganic fertilizers (Cortez and Pere ´z, 1997; Rodrı ´guez, 2000). However, for the high strength molasses-based spentwash, the odor, putrefaction and unpleasant landscape due to unsystematic disposal are concerns in land application. In addition, this option is subject to land availability in the vicinity of the distillery; also, it is essential that the disposal site be located in a low–medium rainfall area (Sheehan and Greenfield, 1980). More recent investigations have indicated that land disposal of distillery effluent can lead to groundwater contamination (Joshi, 1999). Deep well disposal is another option but limited underground storage and specific geological location limits this alternative. Other disposal methods like evaporation of  spentwash to produce animal feed and incineration of  spentwash 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 the existing options as well as the advances in this field are discussed in the following sections.

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4.1. Biological treatment 4.1.1. Anaerobic process The high organic content of molasses spentwash makes anaerobic treatment attractive in comparison to direct aerobic treatment. Therefore, biomethanation is the primary treatment step and is often followed by two-stage aerobic treatment before discharge into a water body or on land for irrigation (Nandy et al., 2002). Aerobic treatment alone is not feasible due to the high energy consumption for aeration, cooling, etc. Moreover, 50% of the COD is converted to sludge after aerobic treatment (Sennitt, 2005). In contrast, anaerobic treatment converts over half of the effluent COD into biogas (Wilkie et al., 2000). Anaerobic treatment can be successfully operated at high organic loading rates; also, the biogas thus generated can be utilized for steam generation in the boilers thereby meeting the energy demands of the unit (Nandy et al., 2002). Further, low nutrient requirements and stabilized sludge production are other associated benefits (Jime ´ nez et al., 2004). The performance and treatment efficiency of anaerobic process can be influenced both by inoculum source and feed pre-treatment. In particular, thermal treatment of  wastewaters can result in rapid degradation of organic matter leading to lower hydraulic residence time (HRT), higher loading rate and BOD reduction. Moreover the methane content and calorific value of biogas produced from thermophilic systems was higher (Vlissidis and Zouboulis, 1993). On the contrary, in shake flask studies with an initial COD loading of 20,000 mg/l using biogas plant sludge, Dhar et al. (1998a) observed a COD removal of just 6.5% with thermally pre-treated spentwash whereas untreated influent displayed 30.4% COD removal. Significant improvement was observed using inoculum from anaerobic lagoon with 27.2% COD reduction with thermally pre-treated wastewater and 51% reduction with untreated wastewater. Thermal pre-treatment changes the biodegradability of wastewater; thus, it acts as an entirely new feedstock for which the inoculum has to be acclimatized afresh. Further, the temperature of thermal pre-treatment is also important. After 150 d adaptation period, wastewater treated at lower temperature (170 C) showed 66% COD reduction (Dhar et al., 1998b). This was nearly twice the removal obtained with treatment at 230 C. Further, addition of micronutrients (iron, boron and molybdenum) eliminated the long adaptation periods. Anaerobic lagoons are the simplest option for the anaerobic treatment of distillery spentwash. Subba Rao (1972) reported that employing two anaerobic lagoons in series resulted in final BOD levels up to 600 mg/l. However, large area requirement, odor problem and chances of  ground water pollution restrict its usage (Pathade, 1999). Though anaerobic lagoons are still employed in Indian distilleries, high rate anaerobic reactors are more popular (Lata et al., 2002). These reactors offer the advantage of  separating the hydraulic retention time (HRT) from solids 1

1

retention time (SRT) so that slow growing anaerobic microorganisms can remain in the reactor independent of  wastewater flow. Table 4 summarizes the performance of  various anaerobic reactors covering both laboratory studies and pilot/commercial scale operations for treatment of molasses-based distillery wastewaters. 4.1.1.1. Suspended bed reactor. Upflow anaerobic sludge blanket (UASB) reactor is the most popular high rate digester that has been utilized for anaerobic treatment of  various types of industrial wastewaters (Akunna and Clark, 2000; Syutsubo et al., 1997). Treatment by a UASB reactor resulted in 75% COD removal in sugarcane molasses spentwash and 90% COD reduction in whisky pot ale (Goodwin and Stuart, 1994; Sa ´nchez Riera et al., 1985). However, dilution is required before treatment due to the presence of some inhibitory substances such as sulfur compounds, potassium and calcium ions and free hydrogen ions left in solution after pH correction (Sa ´nchez Riera et al., 1985). Wolmarans and de Villiers (2002) have reported a similar COD removal efficiency of greater than 90% over three seasons in a UASB plant treating distillery wastewater. Most of the practical UASB systems are operated under mesophilic conditions; however, thermophilic operation results in higher methanogenic activity. Mesophilically grown sludge utilized in thermophilic UASB as a seeding material leads to prompt start up and stable operation with 85% COD removal efficiency at a loading of 30 kg COD/ m3 d (Syutsubo et al., 1997). There are also reports on the cultivation of thermophilic granular sludge for the seeding of thermophilic UASB reactor. Wiegant et al. (1985) reported the cultivation of thermophilic sludge on sucrose for a period of 4 months. The system after adaptation was able to take high COD loadings (86.4 kg/m3 d) and resulted in 60% COD removal efficiency. In another study in a thermophilic UASB reactor, Harada et al. (1996) reported 39–67% COD removal, with a corresponding BOD removal of over 80%. The results suggested that the wastewater contained high concentration of refractile compounds; this, in turn, affected the microbial population in the sludge granules. Generally, the predominant genera of methanogens in granular sludge are Methanobacterium, Methanobrevibacter, Methanothrix and Methanosarcina (Bhatti et al., 1997); however, the predominance of  Methanothrix in granular sludge is most essential for the establishment of a high performance UASB process. In this study, abundance of  Methanosarcina sp. was observed whereas Methanothrix sp. was present to a lesser extent thereby indicating that the latter are more sensitive to refractile compounds (Harada et al., 1996). 4.1.1.2. Fixed bed reactor. This involves immobilization of microorganisms on some inert support to limit the loss of biomass and enhance the bacterial activity per unit of  reactor volume. Moreover it provides higher COD removal at 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)

UASB UASBa UASBb Thermophilic UASB Thermophilic UASB Thermophilic UASB Two-stage anaerobic treatment a Anaerobic filter UASB Downflow fixed film reactor Two-phase thermophilic process Acidogenesis Methanogenesis Diphasic (upflow) fixed film reactor (clay brick granules support) Diphasic (upflow) fixed film reactor (granular activated carbon support) Upflow anaerobic filter (UAF)a Hybrid baffled reactor Downflow fluidized bed reactor with ground perlite b Downflow fluidized bed reactor with ground perlite b Downflow filter Two-stage bioreactor(anaerobic) Ist stage (upflow sludge bed reactor) IInd stage (batch operated bioreactor, flocculator, precipitator) Anaerobic contact filter (in series) Granular bed anaerobic baffled reactor (GRABBR)a Upflow blanket filter

24 15 18 Up to 86.4 Up to 28 Up to 30

a

HRT (Days)

2.1

% COD reduction

% BOD reduction

75 90 90 60 39–67 87

4

0.3

Sa ´nchez Riera et al. (1985) Goodwin and Stuart (1994) Wolmarans and de Villiers (2002) Wiegant et al. (1985) Harada et al. (1996) Syutsubo et al. (1997) Blonskaja et al. (2003)

2.5–5.1 0.6–2.5 14.2–20.4

10–19 20–39 3.3–2.5

4.6–20.0 22

2 15.2 3

71.8

Seth et al. (1995)

21.3

4

67.1

Goyal et al. (1996)

20 20 17 kg TOC/m3 d

0.35

76 77 75–95% TOC

Tokuda et al. (1999) Boopathy and Tilche (1991) Garcı ´a-Bernet et al. (1998)

4.5

3.3–1.3

85

Garcia-Calderon et al., (1998)

8 7

54 93 85–97 65

80

4

References

60–73 85

Bories et al. (1988) Yeoh (1997)

55–85 71

86

Athanasopoulos (1987) Vlissidis and Zouboulis (1993)

73–98 82–90

90

Vijayaraghavan and Ramanujam (2000) Akunna and Clark (2000)

11 0.10

4 4.75 9–11

11–12

70

Bardiya et al. (1995)

Malt whisky wastewater. Winery wastewater.

b

shock loadings. In anaerobic contact filters, various packing materials, viz. polyurethane, clay brick, granular activated carbon (GAC), polyvinyl chloride (PVC) plastic media have been employed resulting in 67–98% reduction in COD (Bories et al., 1988; Seth et al., 1995; Goyal et al., 1996; Vijayaraghavan and Ramanujam, 2000). GAC as support media is relatively expensive but because of its adsorptive properties, it contributes towards improved process stability. The interference by sulfate, unionized sulfite and total hydrogen sulfide in anaerobic filters is reported to be negligible (Vijayaraghavan and Ramanujam, 2000). It was observed that the percentage sulfate removal increased with increasing HRT from 2 to 5 d. This may be due to the utilization of sulfate as a nutrient by microorganisms present in anaerobic contact filter and their conversion to sulfide by sulfate reducing bacteria (SRB) under anaerobic conditions. However at

higher sulfate concentration (426 mg/l), the removal decreased, possibly due to low SRB population in comparison to methanogens. Also the removal of sulfide was explained by stripping of hydrogen sulfide from liquid to vapor phase by the carbon dioxide and methane generated during the anaerobic process. In another study, Tokuda et al. (1999) performed anaerobic treatment of undiluted whisky pot ale using an upflow anaerobic filter (UAF) packed with special support type (Pelia 4555, Herding GmbH, Germany) which resulted in 76% COD removal. The pilot system also consisted of a decanter, dephosphatation or magnesium ammonium phosphate (MAP) (MgNH4PO4) reactor, denitrification reactor, nitrification reactor and sedimentation tank for the reduction of nitrogen and phosphate. Downflow filter using plastic PVC as support material has been employed for the treatment of beet molasses

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wastewater (Athanasopoulos, 1987). The system resulted in 55–85% reduction in COD. Also, though high sulfide concentration (4250 mg) was inhibitory to the system, it was not toxic at higher loadings (44kgCOD/m3 d) probably due to high stripping of H 2S. 4.1.1.3. Fluidized bed reactor. Fluidized bed reactors contain an appropriate media such as sand, gravel or plastics for bacterial attachment and growth. The reactors can be operated either in the upflow or downflow modes, with fluidization being realized by applying high fluid velocities, normally by effluent recycling. Studies on distillery effluent treatment in a downflow fluidized bed system using ground perlite (an expanded volcanic rock) resulted in 75–95% reduction in carbon content (Garcı ´aBernet et al., 1998). The utilization of ground perlite as a carrier material was advantageous in the downflow configuration because it requires low fluidization velocities which preclude the possibility of clogging. 4.1.1.4. Two-stage processes and hybrid reactors. A twostage process with an anaerobic filter followed by a UASB reactor was investigated by Blonskaja et al. (2003). The acidogenic and methanogenic phases were clearly separated ensuring better conditions for the methanogens. COD reduction was 54% and 93% in the first and second stage, respectively. In another study on a two-phase thermophilic system, 65% COD reduction combined with a three-fold increase in biogas yield over a single phase system was observed (Yeoh, 1997). Boopathy and Tilche (1991) studied the anaerobic digestion of 3–12 times diluted beet molasses wastewater, without pH adjustment, in a hybrid anaerobic baffled reactor (HABR). Additional nitrogen and phosphorus were provided in the form of urea (0.007 g/g of  COD) and diammonium hydrogen phosphate (0.0006 g/g of COD). The reactor consisted of three chambers and a final settler. 77% COD removal at a loading rate of  20 kg COD/m3 d was obtained. Several variations of the UASB reactor have been investigated for distillery wastewater treatment. In largescale operations, highly variable process wastewater flows makes it difficult to maintain suitable inlet UASB flow rate; further, prevention of the loss of low density granules is also important. To overcome these problems, Akunna and Clark (2000) used granular-bed anaerobic baffled reactor (GRABBR). The reactor consisted of 10 equal compartments, each of which was further divided into two with suitable baffles. Acidogenesis was found to be predominant in the compartments near the inlet and methanogenesis in those located near the outlet. 82–90% COD reduction was observed at a HRT of 4 d. Yet another modification is upflow blanket filter (UBF) in which the packing is limited to 5–10% height of the reactor. This configuration resulted in 70% COD removal in sugarcane molasses distillery spentwash (Bardiya et al., 1995). Vlissidis and Zouboulis (1993) have investigated the thermophilic anaerobic treatment of wastewater from the

processing of beet molasses. The process consisted of two stages: anaerobic digestion in upflow sludge bed reactor followed by coagulation–flocculation with lime. The HRT was 11 d in the bioreactor and 2.5 h in the flocculator–precipitator tank. On an average, the overall treatment scheme resulted in 86% BOD and 71% COD removal. Biogas produced in the anaerobic reactor had a methane content of 76%. This configuration was reported to be efficient in treating undiluted wastewaters. 4.1.2. Aerobic treatment The post-anaerobic treatment stage effluent still has high organic loading and is dark brown in color, hence it is generally followed by a secondary, aerobic treatment. Solar drying of biomethanated spentwash is one option but the large land area requirement limits this practice. Further, in India, solar drying beds become non-functional during the rainy season (Nandy et al., 2002). The other treatment options that have been demonstrated for biomethanated distillery effluent are described below. 



Aquaculture: Post-biomethanted effluent has been used for pisciculture near Chennai city in southern India. The biodigested effluent, which is a rich growth medium, is directed to bioconversion ponds after which it is spread in about 6 ha of fishponds. The BOD is reduced to nearly zero and the initiative yields about 50 tons per hectare per year of fish (Vorion Chemicals & Distilleries Ltd., 1999). Constructed wetlands (CWs): Billore et al. (2001) have demonstrated a four-celled horizontal subsurface flow (HSF) CW for the treatment of distillery effluent after anaerobic treatment. The post-anaerobic treated effluent had a BOD of about 2500 mg/l and a COD of nearly 14,000 mg/l. A pre-treatment chamber filled with gravel was used to capture the suspended solids. All the cells were filled with gravel up to varying heights and cells three and four supported the plants Typha latipholia and Phragmites karka respectively. The overall retention time was 14.4 d and the treatment resulted in 64% COD, 85% BOD, 42% total solids and 79% phosphorus content reduction. In another study, a laboratory scale CW employing T. latipholia was used to treat diluted distillery effluent (Trivedy and Nakate, 2000). A root zone of 1.5 Â 0.3 Â 0.3 m, filled with 75% sand and gravel and 25% soil was used and the diluted effluent was applied after 4 weeks of planting. The system resulted in 76% COD reduction in 7 d which increased marginally to 78% COD reduction in 10 d. The BOD reduction was 22% and 47% on days 7 and 10, respectively. In yet another instance, a distillery in northern India is presently employing CW for polishing the effluent prior to land discharge for irrigation in the surrounding paddy fields (M.K. Pilania, pers. comm.). The effluent is initially subjected to primary treatment which includes settling and anaerobic digestion in a structured media attached growth (SMAG)-type

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anaerobic reactor. The primary treated effluent, with a COD of 28,000–35,000 mg/l, is subjected to two-stage aeration to bring down the COD to 400 mg/l. Thereafter, it is directed to a CW before final discharge. Biocomposting: The spentwash, either directly, or after biomethanation is sprayed in a controlled manner on sugarcane pressmud. The latter is the filter cake obtained during juice clarification in the manufacture of sugar. Biocomposting is an aerobic, thermophilic process resulting in a product rich in humus which is thus used as a fertilizer. This is a popular option adopted by several Indian distilleries attached to sugar mills with adequate land availability.

The most common post-biomethanation step is the activated sludge process wherein research efforts are targeted at improvements in the reactor configuration and performance. For instance, aerobic sequencing batch reactor (SBR) is reported to be a promising solution for the treatment of effluents originating from small wineries (Torrijos and Moletta, 1997). The treatment system consisted of a primary settling tank, an intermediate retention trough, two storage tanks and an aerobic treatment tank. A start up period of 7 d was given to the aerobic reactor and the system resulted in 93% COD and 97.5% BOD removal. Another configuration that has been examined is the rotating biological reactor (RBR) (de Bazu´a et al., 1991). The system consisted of  300 l anaerobic fluidized bed reactor coupled with a 3000 l RBR. Both the reactors were tested with diluted raw vinasse with a COD of 60–70 g/l. At a HRT of 2 d and COD loading up to 20 kg COD m3/day, the anaerobic unit resulted in 70% COD removal while a lower 46% COD removal was obtained in the aerobic step. Further testing in the aerobic system was planned with the effluent from anaerobic reactor but no results were reported. The activated sludge process and its variations utilize mixed cultures. To enhance the efficiency of aerobic systems, several workers have focused on treatment by pure cultures. Further, aerobic treatment has also been examined as a precursor to anaerobic treatment. In studies on both beet spentwash and molasses, aerobic pretreatment of beet spentwash with Penicillium decumbens resulted in about 74% reduction in phenolics content and 40% reduction in color (Jime ´ nez et al., 2003). Anaerobic digestion without aerobic pre-treatment resulted in a sharp drop in COD removal efficiencies with decreasing HRT. The organic matter removal was marginally higher for beet molasses previously fermented with P. decumbens. The anaerobic reaction followed first-order kinetics and the rate constant decreased on increasing the organic loading with untreated molasses; however, it remained almost constant with pre-treated molasses (Jime ´nez et al., 2003, 2004). Geotrichum candidum is another species that resulted in partial elimination of phenolic inhibitors such as gentisic acid, gallic acid, quercetin, p-coumaric acid, etc., thereby

enhancing the effectiveness of anaerobic process (Borja et al., 1993). The following sections discuss pure culture studies on molasses distillery wastewater targeting both COD reduction and effluent decolorization. 4.1.2.1. Fungal treatment. White rot fungus secreting ligninolytic enzymes are capable of degrading xenobiotics and organopollutants. Phanerochaete chrysosporium and Trametes versicolor are the most widely studied among these (Gonzalez et al., 2000). P. chrysosporium JAG 40 resulted in 80% decolorization of diluted synthetic melanoidin (absorbance unit of 3.5 at 475 nm), as well as with 6.25% anaerobically digested spentwash (Kumar et al., 1998; Dahiya et al., 2001a). T. versicolor produces a 47 kDa extracellular enzyme identified as peroxidase which is involved in mineralization of melanoidins. The fungus resulted in 82% decolorization of 12.5% anaerobically–aerobically treated effluent. Of this, 90% color was removed biologically and the rest by adsorption on the mycelium (Dehorter and Blondeau, 1993; Benito et al., 1997). In addition, treatment by Trametes species I-62 (CECT 20197) detoxifies the effluent by degrading furan derivatives as observed by gas chromatography analysis (Gonzalez et al., 2000). Decolorization of melanoidin pigment has also been reported by extracellular H2O2 and peroxidase produced by Coriolus hirsutus (Miyata et al., 1998). For 6.25% anaerobically digested spentwash, this species showed 71–75% reduction in color and 90% reduction in COD (Kumar et al., 1998). Further, Coriolus versicolor Ps4a decolorizes the effluent by decomposition of melanoidins and not by the partial transformation of chromophores. The decolorizing activity was attributed to an intracellular enzyme which is induced in the presence of melanoidin pigment (Aoshima et al., 1985). Treatment of biodigested distillery wastewater by C. versicolor has also been investigated by Chopra et al. (2004). The fungus was able to reduce both COD and color up to 53% in 8 d; however, glucose and peptone were required as additional nutrient sources. In another study, Raghukumar et al. (2004) used a marine fungus, Flavodon flavus for the combined decolorization and detoxification of 10% molasses spentwash. It was suspected that Maillard reaction also resulted in the formation of pyrogenic compounds like polycyclic aromatic 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. The fungus was more effective in decolorizing raw molasses spentwash than the anaerobically and aerobically treated streams. This was possibly due to changes in the chemical structure of the melanoidin pigments during anaerobic and aerobic treatment. However, the oxygen demand of the fungus 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 the production of a series of extracellular hydrolytic enzymes makes it easier for them to grow on starch and cellulose substrates. 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% COD removal; also, the treated effluent enhanced the seedling growth in Zea mays (Angayarkanni et al., 2003; Miranda et al., 1996). Immobilized fungal isolate of  A. niger UM2 resulted in a 72% decolorization of diluted synthetic melanoidin (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-26 decolorized 75% of melanoidin pigment solution at 45 C. Gel filtration chromatography revealed that large molecular weight fractions of melanoidins, in particular, were degraded rapidly (Ohmomo et al., 1987). Apart from white-rot and filamentous fungi, yeast has also been investigated for distillery wastewater treatment. Yeast is characterized by quick growth and is less susceptible to contamination by other microorganisms; further, yeast produces biomass with high nutritive value (Friedrich, 2004). The yeast Citeromyces WR-43-6 resulted in high and stable removal efficiency in both color intensity and organic matter. The removal efficiencies for diluted spentwash (absorbance unit of 3.5 at 475 nm) were 75% for color intensity and 76% for BOD (Sirianuntapiboon et al., 2004a). Shojaosadati et al. (1999) optimized the growth conditions for single cell protein (SCP) production and COD reduction by the use of  Hansenula sp. in sugar beet stillage. They concluded that production of SCP from stillage is one of the most promising options. Besides, the yeast was also found to utilize lactate and acetate that are inhibitory to ethanol production. As a result, the treated effluent could be used as dilution water for fermentation thereby reducing the residual stillage volume by 70%. Another strain of  Hansenula anomala J 45-N-5 and I-44 isolated from soil, resulted in 74% reduction in total organic carbon (TOC) (Moriya et al., 1990). S. cerevisiae also provides promising results on a larger scale. The use of  pure culture of  S. cerevisiae resulted in 82.7% decolorization in the 10% anaerobically treated distillery effluent along with 84% reduction in COD (Selim et al., 1991; Rajor et al., 2002). It was also reported that the nitrogen present in the distillery effluent was sufficient for the growth of yeast. The dye decolorizing fungus G. candidum Dec 1 immobilized 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 experiments, Candida utilis and Trichoderma viridiae each showed less 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-based media with an initial COD of 40–75 g/kg. The fungus Mycelia sterilia D90 resulted in 91% decolorization of raw spentwash. The color intensity (in terms of absorbance unit 1

at 475 nm) was originally 47 for the raw spentwash but it was diluted to a value of 3.5 before use (Sirianuntapiboon et al., 1988). However, a lower color removal of 60–65% was obtained for the wastewater from anaerobic and aerobic ponds. This was possibly due to either the formation of some toxic compounds during anaerobic and aerobic treatment or the inability of the strain to attack the color causing compound due to a change in their structure during anaerobic and aerobic treatment. 4.1.2.2. Bacterial treatment. Treatment of distillery wastewater by the use of  Pseudomonas putida followed by Aeromonas sp. in a two-stage bioreactor resulted in COD as well as color reduction (Ghosh et al., 2002). P. putida produces hydrogen peroxide which is a strong decolorizing agent. Since the organism cannot use spentwash as a source of carbon, 1% w/v glucose supplement was provided along with 12.5% spentwash. Aeromonas sp. utilizes the carbonaceous compounds present in spentwash as the sole carbon source, thereby eventually reducing the effluent COD by 66% in a 24 h period. P. putida also resulted in 44% COD removal accompanied by 60% color reduction. In another study on predigested distillery effluent with Aeromonas 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 with longer incubation period of up to 96 h. P. fluorescence immobilized on porous cellulose carrier resulted in 66% color removal with non-sterile diluted spentwash (absorbance unit of 3.5 at 475nm) and 90% decolorization with sterile samples at 30 C o v e r a 4 d period (Dahiya et al., 2001b). The decolorization efficiency was further increased to 94% with cellulose carrier coated with collagen. These immobilized cells could be reused but the efficacy of color removal was reduced. In another study, three different bacterial strains Xanthomonas fragariae, Bacillus megaterium and Bacillus cereus were used both in free form as well as after immobilization on calcium alginate beads for the treatment of 33% predigested distillery effluent (Jain et al., 2002). B. cereus resulted in maximum COD (81%) and color (75%) reduction in free form. The reduction efficiencies increased marginally with immobilization. The decolorization activity of acetogenic bacteria has been reported for the first time by Sirianuntapiboon et al. (2004b). Acetogenic bacteria is capable of oxidative decomposition of melanoidins thereby removing low molecular weight compounds in untreated molasses spentwash and almost all the low and high molecular weight compounds in anaerobically treated molasses spentwash. Nearly 76% decolorization, which is possibly due to a sugar oxidase, has been observed. The nitrifying bacteria Nitrosococcus oceanus is capable of detoxifying the spentwash accompanied by a reduction in the chloride content (Arora et al., 1992). However, no explanation was 1

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provided for this observation. The treated wastewater leads to better growth of rice plant due to adequate nitrogen content and can therefore be used as a low cost fertilizer. In yet another investigation, two aerobic bacterial strains TA 2 and TA 4 have been isolated from sites contaminated with anaerobically treated distillery effluent (Asthana et al., 2001). These bacteria, which were identified to be Gramnegative and Gram-positive, respectively, resulted in 66% and 62% BOD reduction in anaerobically treated spentwash. However the reduction in BOD was found to be higher (80%) when the two were used together; further the combination resulted in 76% color removal after 72 h. More recently, the decolorization of four types of  synthetic melanoidins i.e., glucose–glutamic-acid (GGA), glucose–aspartic-acid (GAA), sucrose–glutamic acid (SGA), and sucrose–aspartic-acid (SAA), were investigated using three different isolates, viz. Bacillus thuringiensis, Bacillus brevis and Bacillus sp. (Kumar and Chandra, 2006). The degree of decolorization of the melanoidins separately by each isolate was in the 1–31% range; however, when used collectively, these isolates resulted in up to 50% decolorization due to the enhanced effect of  coordinated metabolic interactions. The results also indicated that the GAA polymer was the most recalcitrant among the melanoidins tested. The biodegradability of spentwash can be enhanced by enzymatic pre-treatment prior to the aerobic step (Sangave and Pandit, 2006a). After 24 h of treatment with Grampositive bacterium ASN6, the COD reduction of cellulase pre-treated spentwash was 28.8% in comparison to 18.3% for untreated effluent. This was explained by the fact that pre-treatment affected the metabolic value (microbial acceptability) by generating intermediate hydrolysis products from the parent cellulosic compounds present in the spentwash. The biodegradability was further enhanced by combined ultrasound and enzymatic pre-treatment resulting in 62.2% COD reduction after 36 h as compared to 39.4% COD removal for the untreated effluent (Sangave and Pandit, 2006b). The enhancement in biodegradability was attributed to molecular transformation of effluent constituents by ultrasound pre-treatment. 4.1.2.3. Algal treatment. The treatment of anaerobically treated 10% distillery effluent using the microalga Chlorella vulgaris followed by Lemna minuscula resulted in 52% reduction in color (Valderrama et al., 2002). In another study, Kalavathi et al. (2001) examined the degradation of  5% melanoidin by the marine cyanobacterium Oscillatoria boryana BDU 92181. The organism was found to release hydrogen peroxide, hydroxyl ions and molecular oxygen during photosynthesis resulting in 60% decolorization of  distillery effluent. In addition, this study suggested that cyanobacteria could use melanoidin as a better nitrogen source than carbon. Further, cyanobacteria also excrete colloidal substances like lipopolysaccharides, proteins, polyhydroxybutyrate (PHB), polyhydroxy-alkanoates (PHA), etc. These compounds possess COOÀ and ester

sulphate (OSOÀ 3 ) groups that can form complexes with cationic sites thereby resulting in flocculation of organic matter in the effluent. It was observed that the strain Oscillatoria resulted in almost complete color removal (96%) whereas Lyngbya and Synechocystis were less effective resulting in 81 and 26% color reduction, respectively (Patel et al., 2001). The consortium of the three strains showed a maximum decolorization of 98%. This was attributed to adsorption in the initial stages followed by degradation of organic compounds which dominated in the subsequent stages. 4.2. Physico-chemical treatment Sugarcane molasses spentwash after biological treatment by both anaerobic and aerobic method can still have a BOD of 250–500 mg/l (Mall and Kumar, 1997). Also, even though biological treatment results in significant COD removal, the effluent still retains the dark color (Inanc et al., 1999). The color imparting melanoidins are barely affected by conventional biological treatment such as methane fermentation and the activated sludge process (Migo et al., 1993). Further, multistage biological treatment reduces the organic load but intensifies the color due to re-polymerization of colored compounds (Pen ˜a et al., 2003). In this context, various physico-chemical treatment options have been explored. 4.2.1. Adsorption Activated carbon is a widely used adsorbent for the removal of organic pollutants from wastewater but the relatively high cost restricts its usage. Decolorization of  synthetic melanoidin using commercially available activated carbon as well as activated carbon produced from sugarcane bagasse was investigated by Bernardo et al. (1997). The adsorptive capacity of the different activated carbons was found to be quite comparable. Chemically modified bagasse using 2-diethylaminoethyl chloride hydrochloride and 3-chloro-2-hydroxypropyltrimethylammonium chloride was capable of decolorizing diluted spentwash (Mane et al., 2006). 0.6 g of chemically modified bagasse in contact with 100 ml 1:4 (v/v) spentwash:water solution resulted in 50% decolorization after 4 h contact with intermittent swirling. Significant decolorization was observed in packed bed studies on anaerobically treated spentwash using commercial activated charcoal with a surface area of 1400 m2/g (Chandra and Pandey, 2000). Almost complete decolorization (499%) was obtained with 70% of the eluted sample, which also displayed over 90% BOD and COD removal. In contrast, other workers have reported adsorption by activated carbon to be ineffective in the treatment of distillery effluent (Sekar and Murthy, 1998; Mandal et al., 2003). Adsorption by commercially available powdered activated carbons resulted in only 18% color removal; however, combined treatment using 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 carbon both in granular and powdered form, manufactured from paper mill sludge) and bagasse flyash have also been studied for this application. Ramteke et al. (1989) reported color removal up to 98% with pyorchar. However, to achieve the same level of color removal, larger doses of the indigenously prepared powdered and granular pyorchar were required in comparison to commercial activated carbon. Mall and Kumar (1997) compared the color removal using commercial activated carbon and bagasse flyash. 58% color removal was reported with 30 g/l of  bagasse flyash and 80.7% with 20g/l of commercial activated carbon. Since the bagasse flyash has high carbon content and the adsorbed organic material further increases its heating value, the spent adsorbent can be used for making fire briquettes. Yet another adsorbent that has been examined is the natural carbohydrate polymer chitosan derived from the exoskeleton of crustaceans. Lalov et al. (2000) studied the treatment of distillery wastewater using chitosan as an anion exchanger. At an optimum dosage of 10 g/l and 30 min contact time, 98% color and 99% COD removal was observed. 4.2.2. Coagulation and flocculation Inanc et al. (1999) reported that coagulation with alum and iron salts was not effective for color removal. They explored lime and ozone treatment with anaerobically digested effluent. The optimum dosage of lime was found to be 10 g/l resulting in 82.5% COD removal and 67.6% reduction in color in a 30 min period. These findings are in disagreement with those of  Migo et al. (1993) who used a commercial inorganic flocculent, a polymer of ferric hydroxysulfate with a chemical formula [Fe2 (OH) (SO4)3À /2] for the treatment of molasses wastewater. The treatment resulted in around 87% decolorization for biodigested effluents; however an excess of flocculent hindered the process due to increase in turbidity and TOC content. FeCl3 and AlCl3 were also tested for decolorization of biodigested effluent and showed similar removal efficiencies. About 93% reduction in color and 76% reduction in TOC were achieved when either FeCl 3 or AlCl3 was used alone. The process was independent of  chloride and sulfate ion concentration but was adversely affected by high fluoride concentration. However in the presence of high flocculent concentration (40 g/l), addition of 30 g/l CaO enhanced the decolorization process resulting in 93% color removal. This was attributed to the ability of  calcium ions to destabilize the negatively charged melanoidins; further, formation of calcium fluoride (CaF2) also precipitates the fluoride ions. Almost complete color removal (98%) of biologically treated distillery effluent has been reported with conventional coagulants such as ferrous sulfate, ferric sulfate and alum under alkaline conditions (Pandey et al., 2003). The best results were obtained using Percol 47, a commercial n

n

m

organic anionic polyelectrolyte, in combination with ferrous sulfate and lime. The combination resulted in 99% reduction in color and 87 and 92% reduction in COD and BOD, respectively. Similar findings have also been reported by Mandal et al. (2003). Coagulation studies on spentwash after anaerobic–aerobic treatment have also been conducted using bleaching powder followed by aluminum sulfate (Chandra and Singh, 1999). The optimum dosage was 5 g/l bleaching powder followed by 3 g/l of aluminum sulfate that resulted in 96% removal in color, accompanied by up to 97% reduction in BOD and COD. Non-conventional coagulants namely wastewater from an iron pickling industry which is rich in iron and chloride ions and titanium ore processing industry containing significant amounts of iron and sulfate ions have also been examined (Pandey et al., 2003). The iron pickling wastewater gave better results with 92% COD removal, combined with over 98% color removal. Though the titanium processing wastewater exhibited similar color removal levels, the COD and BOD reductions were perceptibly lower. 4.2.3. Oxidation process Ozone destroys hazardous organic contaminants and has been applied for the treatment of dyes, phenolics, pesticides, etc. (Pen ˜a et al., 2003). Oxidation by ozone could achieve 80% decolorization for biologically treated spentwash with simultaneous 15–25% COD reduction. It also resulted in improved biodegradability of the effluent. However, ozone only transforms the chromophore groups but does not degrade the dark colored polymeric compounds in the effluent (Alfafara et al., 2000; Pen ˜a, et al., 2003). Similarly, oxidation of the effluent with chlorine resulted in 497% color removal but the color reappeared after a few days (Mandal et al., 2003). Ozone in combination with UV radiation enhanced spentwash degradation in terms of COD; however, ozone with hydrogen peroxide showed only marginal reduction even on a very dilute effluent (Beltran et al., 1997). In another study, Sangave and Pandit (2004) employed sonication of distillery wastewater as a pre-treatment step to convert complex molecules into a more utilizable form by cavitation. Samples exposed to 2 h ultrasound pretreatment displayed 44% COD removal after 72 h of  aerobic oxidation compared to 25% COD reduction shown by untreated samples. These results are contrary to those of  Mandal et al. (2003) who concluded ultrasonic treatment to be ineffective for distillery spentwash treatment. A combination of wet air oxidation and adsorption has been successfully used to demonstrate the removal of  sulfates from distillery wastewater. Studies were done in a counter current reactor containing 25 cm base of small crushed stones supporting a 20 cm column of bagasse ash as an adsorbent (Gaikwad and Naik, 2000). The wastewater was applied from the top of the reactor and air was supplied at the rate of 1.0 l/min. The treatment removed

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57% COD, 72% BOD, 83% TOC and 94% sulfates. Wet air oxidation has been recommended as part of a combined process scheme for treating anaerobically digested spentwash (Dhale and Mahajani, 2000). The post-anaerobic effluent was thermally pre-treated at 150 C under pressure in the absence of air. This was followed by soda-lime treatment, after which the effluent underwent a 2 h wet oxidation at 225 C. 95% color removal was obtained in this scheme. Another option is photocatalytic oxidation that has been studied using solar radiation and TiO2 as the photocatalyst (Kulkarni, 1998). Use of TiO2 was found to be very effective as the destructive oxidation process leads to complete mineralization of effluent to CO 2 and H2O. Up to 97% degradation of organic contaminants was achieved in 90 min. Pikaev et al. (2001) studied combined electron beam and coagulation treatment of distillery slops from distilleries processing grain, potato, beet and some other plant materials. Humic compounds and lignin derivatives constitute the major portion of this dark brown wastewater. The distillery wastewater was diluted with municipal wastewater in the ratio of 3:4, irradiated with electron beam and then coagulated with Fe2(SO4)3. The optical absorption in UV region was decreased by 65–70% after this treatment. The cost was found to be less than the existing method wherein the effluent was transported about 20 km via pipeline to a facility for biological treatment followed by sedimentation. The treatment cost was 0.45–0.65 US$/m3 which dropped to 0.25 US$/m3 using combined electronic-beam and coagulation method. 1

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4.2.4. Membrane treatment Pre-treatment of spentwash with ceramic membranes prior to anaerobic digestion is reported to halve the COD from 36,000 to 18,000 mg/l (Chang et al., 1994). The total membrane area was 0.2 m2 and the system was operated at a fluid velocity of 6.08 m/s and 0.5 bar transmembrane pressure. In addition to COD reduction, the pre-treatment also improved the efficiency of the anaerobic process possibly due to the removal of inhibiting substances. Kumaresan et al. (2003) employed emulsion liquid membrane (ELM) technique in a batch process for spentwash treatment. Water–oil–water type of emulsion was used to separate and concentrate the solutes resulting in 86% and 97% decrease in COD and BOD, respectively. Electrodialysis has been explored for desalting spentwash using cation and anion exchange membranes resulting in 50–60% reduction in potassium content (de Wilde, 1987). In another study, Vlyssides et al. (1997) reported the treatment of vinasse from beet molasses by electrodialysis using a stainless steel cathode, titanium alloy anode and 4% w/v NaCl as electrolytic agent. Up to 88% COD reduction at pH 9.5 was obtained; however, the COD removal percentage decreased at higher wastewater feeding rates. In addition, reverse osmosis (RO) has also been employed for distillery wastewater treatment. A unit in

western India is currently processing effluent obtained after anaerobic digestion, followed by hold-up in a tank maintained under aerobic conditions, in a RO system (B.P. Agrawal, pers. comm.). 290 m3/d of RO treated effluent is mixed with 300 m3/d of fresh water and used in the process for various operations like molasses dilution (290 m3/d), make-up water for cooling tower (178 m3/d), fermenter washing (45 m3/d), etc. Yet another unit in southern India is employing disc and tube RO modules for direct treatment of the anaerobically digested spentwash (M. Prabhakar Rao, pers. comm.). The permeate is discharged while the concentrate is used for biocomposting with sugarcane pressmud. In a recent study, Nataraj et al. (2006) reported pilot trials on distillery spentwash using a hybrid nanofiltration (NF) and RO process. Both the NF and RO stages employed thin film composite (TFC) membranes in spiral wound configuration with module dimensions of 2.5 inches diameter and 21 inches length. NF was primarily effective in removing the color and colloidal particles accompanied by 80%, 95% and 45% reduction in total dissolved solids (TDS), conductivity and chloride concentration, respectively, at an optimum feed pressure of  30–50 bar. The subsequent RO operation at a feed pressure of 50 bar resulted in 99% reduction each in COD, potassium and residual TDS. 4.2.5. Evaporation/combustion Molasses spentwash containing 4% solids can be concentrated to a maximum of 40% solids in a quintuple-effect evaporation system with thermal vapor recompression (Bhandari et al., 2004; Gulati, 2004). The condensate with a COD of 280 mg/l can be used in fermenters. The concentrated mother liquor is spray dried using hot air at 180 C to obtain a desiccated powder with a calorific value of around 3200 kcal/kg. The powder is typically mixed with 20% agricultural waste and burnt in a boiler. The use of recirculating fluidized bed (RCFB) incinerator is recommended to overcome the constraints due to stickiness of spentwash and its high sulfate content (Alappat and Rane, 1995). Combustion is also an effective method of on-site vinasse disposal as it is accompanied by production of potassium-rich ash (Cortez and Pere ´z, 1997) that can be used for land application. 1

5. Discussion

A range of biological and physico-chemical methods have been investigated for the treatment of wastewater from molasses-based distilleries. Because of the very high COD, anaerobic treatment with biogas recovery is employed extensively as the first treatment step. Anaerobic lagoons are still used; however, most Indian distilleries employ high rate digesters wherein the HRT is decoupled from the SRT thereby retaining the slow growing anaerobic microorganisms in the reactor even at high wastewater flow. Biomethanation reduces the organic pollution load and brings down BOD to 80–95% of the

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original value; however, the biodigested effluent still contains BOD in the range of 5000–10,000 mg/l (Pathade, 1999). Moreover, anaerobic treatment is a slow process and typically requires long start-up periods. In addition, the problem of color associated with this effluent not only remains unsolved (Patil et al., 2003) but actually gets aggravated since the color causing melanoidin pigment intensifies under anaerobic conditions (Pen ˜a et al., 2003). Therefore anaerobically treated effluent is darker in color compared to untreated spentwash and needs several-fold dilution by fresh water prior to discharge. The current practice of using diluted, biodigested spentwash for irrigation (‘‘ferti-irrigation’’) by a large number of Indian distilleries is reportedly causing gradual soil darkening (Kumar et al., 1998). Yet another limitation is the need to dilute the spentwash prior to biomethanation itself. Most distilleries follow 1–3-fold dilution to ensure proper operation of the biomethanation plant. Since 8–15 l spentwash is generated per liter of ethanol, dilution increases the load on the effluent treatment plant besides consuming considerable amount of fresh water for this purpose. Biological treatment using aerobic processes like activated sludge, biocomposting etc. is presently practiced by various molasses-based distilleries. Due to the large volumes generated, only a part of the total spentwash gets consumed in biocomposting. Biocomposting utilizes sugarcane pressmud as the filler material; thus it is typically employed by distilleries attached to sugar mills. Since sugar manufacturing is a seasonal operation, pressmud availability is often a constraint. Further, biocomposting requires large amount of land; also, it cannot be carried out during the rainy season. Though aerobic treatment like the conventional activated sludge process leads to significant reduction in COD, the process is energy intensive and the color removal is still inadequate. Thus several pure cultures of fungi, bacteria and algae have been investigated specifically for their ability to decolorize the effluent as discussed earlier. In all instances, supplementation with either nitrogen or carbon source is almost always necessary because the microbial species are not able to utilize the spentwash as the sole carbon source. Further, high dilution (typically up to 1:10 fold for untreated spentwash and 1:16–1:2 fold for biomethanated spentwash) is required for optimal microbial activity. In addition, these studies are mostly limited to laboratory scale investigations and no pilot/commercial scale operations are reported as yet. Physico-chemical treatment, viz. adsorption, coagulation/flocculation, oxidation processes, membrane treatment have been examined with particular emphasis on effluent decolorization. Though these techniques are effective for both color removal as well as reduction in organic loading, sludge generation and disposal is a constraint in coagulation/flocculation and adsorption. Also, the cost of chemicals, adsorbents and membranes is a deterrent to the adoption of these methods (Rajor et al.,

2002). Membrane operations like microfiltration/ultrafiltration for spentwash treatment are characterized by significant membrane fouling that limits its applicability (Jain and Balakrishnan, 2004). Decolorization through chemical treatment with ozone and chlorine leads to temporary color reduction because of transformation of  the chromophore groups so these are not preferred solutions. Thus, solutions for effective management of molassesbased distillery wastewaters are still evolving. Some of the gaps are highlighted below: 

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Biomethanation of spentwash is well established commercially; however, there is scope for several operational improvements. These include adapting the system to treat the spentwash without any dilution, ensuring shorter start-up periods and degrading refractile components to improve the anaerobic treatment efficiency. Also, a better understanding of the re-polymerization of  color-causing compounds during anaerobic digestion would assist in the subsequent decolorization steps. Biocomposting with sugarcane pressmud is increasingly being adopted by a number of sugar complexes as a method for disposing the biodigested spentwash. However, pressmud availability is limited; thus, alternative materials like rice husk, wood chips, bagasse pith etc. have been suggested (CPCB, 1994). This requires exploring readily available local filler materials that can be utilized for this purpose. Also, the issue of  manure quality, possibly to match the requirements of  local 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 an appropriate process scheme for their removal. It is established that several microorganisms (bacteria, fungi, algae), especially in pure cultures, display a limited ability to decolorize the spentwash. A better understanding of the microbial enzymes/activities responsible for the degradation of melanoidins would contribute to enhancing the efficiency of the decolorization process. Investigations on pure culture aerobic systems that can result in both COD and color removal have been confined exclusively to laboratory scale set-ups. The issues of appropriate system design, including scale-up, have not been addressed. In this context, systems like membrane bioreactors that have lower sludge production can be considered. Also, issues like minimizing nutrient supplementation, avoiding feed dilution and operation under non-sterile conditions should be examined. These points are particularly significant in translating these studies into field applications. Adsorbents like activated carbon that result in almost complete decolorization are not cost effective for treating the enormous volumes of spentwash typically generated in a distillery. Thus, there is scope for

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examining low cost adsorbents, including wastes generated in other industrial processes/operations. In this context, appropriate sludge disposal methods should also be examined. Membrane processes like nanofiltration and RO can result in significant color removal thereby permitting reuse of the treated effluent. Since spentwash is a complex, multicomponent stream that is known to cause considerable fouling, an understanding of the components those are primarily responsible for this phenomenon would assist in appropriate feed pre-treatment, for the efficient operation of the membrane system.

6. Conclusion

The review indicates that a comprehensive treatment scheme for molasses distillery wastewater leading to effective removal of both organics and color is not currently available. The properties of the color-causing melanoidins and their transformation during anaerobic treatment are also not fully understood. Biological treatment, especially with pure cultures, appears promising and possibly cost-effective for color removal; however, the initiatives are mainly confined to laboratory trials. Physicochemical methods are capable of both color and organic load reduction; consequently, in spite of the cost, even high-end options like membrane filtration are being fieldtested. There is thus an urgent need to address the limitations in the existing methods and to develop integrated treatment processes that provide a complete solution to the treatment of wastewater from molassesbased distilleries. Acknowledgement

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