Applied Energy 102 (2013) 1514–1521

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Applied Energy

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Comparative evaluation of raw and detoxified mahua seed cakefor biogas production

Aditi Gupta, Ashwani Kumar, Satyawati Sharma ⇑, V.K. VijayCentre for Rural Development and Technology, Indian Institute of Technology, Hauz Khas, Delhi 110 016, India

h i g h l i g h t s

" Mahua seed cake has the potential to produce biogas." Simple water treatments significantly removed the toxins from mahua cake." 50(Detoxified MC):50(CD) combination was found optimum for maximum biogas production." Biogas slurry produced possesses good manurial values.

a r t i c l e i n f o

Article history:Received 27 May 2012Received in revised form 4 September 2012Accepted 9 September 2012Available online 9 November 2012

Keywords:BiogasMahua cakeSaponinsDetoxificationBiomethanation

0306-2619/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apenergy.2012.09.017

⇑ Corresponding author. Tel.: +91 11 26591116; faxE-mail address: satyawatis@hotmail.com (S. Sharm

a b s t r a c t

Non-edible oils are progressively being utilized for production of bio-diesel around the world whichembraces the future assurance towards renewable energy. After the extraction of oil, 50–60% of the mate-rial, termed as de-oiled seed cake, goes waste due to the presence of toxins. The present paper evaluatesthe use of raw and detoxified (water treated; detoxified up to 75%) non-edible oil seed cake, Madhucaindica, for biogas production. Different treatments comprising of varying proportions of raw/detoxifiedmahua seed cake (MC) and cow dung (CD) were designed. Detoxified cake(s) produced significantly bet-ter results compared to raw cake. Combination of 50% hot water detoxified MC and 50% CD gave maxi-mum biogas production of 442 L/kg total solids with 58.5–60% methane content. This gave an increase of125% over CD, along with 33.15% and 34.05% reduction in total solids (TS) and volatile solids (VS), respec-tively. Significant reduction in celluloses (34.46%) and hemicelluloses (29.76%) and an increase in thenutrients (N,P,K) of the digested slurry were obtained for the same. Anaerobic digestion of mahua cake,detoxified by simple water treatments, offers one of the viable methods for waste to energy generation.

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1. Introduction

The growing worldwide concerns over environment, health andmonetary aspects have triggered a search for efficient and eco-nomic renewable sources of energy production. Agricultural sectorholds the potential for development of one of the major sources ofrenewable energy, i.e. bio-fuels [1,2]. Worldwide, more than 95% ofbio-diesel production takes place from edible oils that may resultin depletion of food supply leading to economic imbalance [3]. Asustainable solution to overcome this problem might be the useof non-edible oils for bio-fuel production [4]. Non-edible oil seedsare increasingly being cultivated to yield considerable quantity ofoil which can be used for the production of bio-diesel [5]. Afterthe expulsion of non-edible oil, approximately 60% is left as toxicde-oiled seed cake [1]. This generates a considerable amount ofbiomass, which can neither be used as a cattle feed nor as a good

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: +91 11 26591121.a).

quality fertilizer owing to its toxicity. Various applications are con-tinuously being explored to exploit the nutritive content of theseseed cakes [1,5–7]. Oil seed cakes contain high amounts of volatilesolids, proteins and starchy material, which make them a suitablefeedstock for biogas generation [8,9]. Recently, biogas is becomingan attractive source of energy in many nations across the globe be-cause it can be used to fuel a car or to power a city bus [10]. Anintegrated anaerobic waste valorization process is also an interest-ing option for energy generation from non-edible oil seed cakes[11]. Anaerobic digestion and composting of waste seed cake canbe considered as a sustainable solution to reduce the volume ofwaste along with reduction in emission of greenhouse gases intothe atmosphere [12]. Chandra et al. [4] recorded average specificmethane production potential of oil seed cake of jatropha (394 L/kg total solids) and karanja (427 L/kg total solids), respectively.

Madhuca indica, commonly known as mahua, is extensively cul-tivated in central and southern parts of India for its oil bearingseeds. The annual production of seed in the country is around0.50 million tons [1,5,6]. After the extraction of oil, much of the

A. Gupta et al. / Applied Energy 102 (2013) 1514–1521 1515

material goes as a waste. Presently, mahua seed cake (MC) findsapplication in dye removal, as a low quality cattle feed and fertil-izer, as a biopesticide, insecticide, and a piscicide [13–17]. The laterproperties are conferred by the presence of bitter saponins in theseed cake. However, their presence in the cake limits its extensiveusage [1,17,18]. MC is rich in volatile solids, sugars (35–40%) andproteins (28–30%) [19,20] and thus considered to be a good feedmaterial for biomethanation. However, not much work has beendone previously on this aspect [1,9]. The present paper deals withthe experiments performed to evaluate and compare the use of rawand detoxified mahua cake(s) for biogas production. Simple treat-ments with water were followed for detoxification and parallel setswith raw as well as detoxified cake(s) were maintained.

2. Materials and methods

2.1. Substrates

Fresh cow dung (CD) was collected from village katwaria Sarai,Delhi, India and mahua seed cake (MC) was procured from Pratap-garh, Uttar Pradesh, India. They were analyzed for their variousconstituents (Table 1). MC was identified by NISCAIR, CSIR, India.The toxins in the cake (saponins) were extracted with 80% metha-nol followed by partitioning into the butanol phase [21] and theirpercentage content was determined. Detoxification of the cake wasperformed using simple water treatments. MC was soaked over-night in tap water (room temperature; 25–27 �C) and hot water(100 �C), respectively, at a dilution ratio of 1:6 (cake:water). Thisdilution ratio was sufficient to soak the cake for a period of about12 h without making the mixture too thick and hence difficult toseparate [1]. The two treatments were designated as cold watertreated detoxified mahua seed cake (CW DMC) and hot water trea-ted detoxified mahua seed cake (HW DMC). The residual saponincontent in the two treated cakes was also determined by the sameprocedure as above.

2.2. Experimental design

The entire experiment was performed in 7 L bottles fabricatedat Biogas Lab, Centre for Rural Development and Technology(CRDT), IIT Delhi so as to collect the biogas by water displacementmethod and periodically withdraw the slurry samples from the di-gester for analysis (Fig. 1) [1]. Ten different treatments consistingof varying proportions of raw/detoxified mahua cake(s) (25%, 50%and 75%) along with CD were designed (Table 2). The total solids

Table 1Compositiona of cow dung (CD) and mahua seed cake (MC).

Constituents (%) CD MC

Moisture 80.07 ± 1.4 7.81 ± 0.23Total solids 19.93 ± 0.54 92.18 ± 2.8Volatile solids (%) (of TS) 87.14 ± 0.75 91.67 ± 1.67Total soluble sugars 19.48 ± 0.05 50.05 ± 1.23Protein 14.43 ± 0.36 19.68 ± 1.27Fat 0.35 ± 0.08 5.01 ± 0.7Celluloses 24.43 ± 1.14 61.6 ± 1.57Hemicelluloses 25.9 ± 2.3 29.3 ± 1.78Lignin 7.5 ± 0.58 4.3 ± 0.62Saponins (toxins) Nil 16.7 ± 4.28Carbon 38.0 ± 0.05 44.93 ± 0.08Nitrogen 1.4 ± 0.004 3.15 ± 0.02Phosphorous 0.25 ± 0.08 0.65 ± 0.07Potassium 0.18 ± 0.06 1.24 ± 0.04C/N ratio 27.14b

a Values indicated are average of three determinations.b Value of C/N ratio for MC is 14.26.

content and working volume of the digester, for each treatment,were maintained at 10% and 4 L, respectively.

The experiment was performed for a period of about 70 days ina room with its temperature maintained at 35 ± 2 �C, with the helpof a room heater thermostat. Each treatment was set up in tripli-cate and repeated twice. The volume of biogas produced was mea-sured by water displacement method and the correspondingcumulative volume was then calculated. Gas samples from eachset up were collected weekly and analyzed for their methane andcarbon dioxide content using a gas chromatograph (Agilent Tech-nologies 7890 A Agilent GC system) equipped with a thermal con-ductivity detector at 250 �C and a 274 cm � 3 mm Porapak-Qcolumn at 50 �C. Argon was used as the carrier gas at a flow rateof 30 ml/min.

2.3. Chemical analysis

Pure CD, MC and digester mixtures consisting of varying pro-portions of raw/detoxified mahua cake(s) and CD (as designed foreach treatment) were analyzed for their moisture, total solids(TS), volatile solids (VS), celluloses, hemicelluloses, lignin, total sol-uble sugars, protein, nitrogen, phosphorous and potassiumcontent.

Moisture content and TS were determined by first heating thesamples at 60 �C for 24 h and then at 103 �C for 3 h using a hotair oven. The percentage moisture content of the sample was thencalculated by using the following formula [5]:

% Moisture content ¼ ½ðWW �WDÞ=WW � � 100

Total solids ðTSÞ ¼ ½WD=WW � � 100

where WW is the weight of wet sample and WD is the weight of drysample

The above oven dried samples used for the determination of TSwere further dried at 550 ± 5 �C temperature for about 5 h in amuffle furnace. The volatile solids (VS) were then determined asfollows [5]:

Volatile solids ðVSÞ ¼ ½WD �WAÞ=WD� � 100

where WD is the weight of oven dried sample and WA is the weightof ash left after igniting the sample in muffle furnace.

Estimation of celluloses was done by acetolysis followed byhydrolysis to form glucose units. These glucose units were thendehydrated and reacted with anthrone to give a green coloredproduct, absorption of which was measured at 630 nm using a

Fig. 1. Fabricated biogas reactor. Source: [1]

Table 2Treatments designed for biogas production.

Treatment Total solidsTS (%)

Volatile solidsVS (of TS) (%)

C/N ratio

Cow dung (CD) control 10 ± 0a 87.39 ± 0.2a 27.1425 (MC):75 (CD) 10 ± 0a 88.29 ± 0.36c 23.0250 (MC):50 (CD) 10 ± 0a 89.60 ± 0.35e 21.4875 (MC):25(CD) 10 ± 0a 90.61 ± 0.29h 17.4525 (CW DMC):75 (CD) 10 ± 0a 87.96 ± 0.35b 23.8550 (CW DMC):50 (CD) 10 ± 0a 89.91 ± 0.36f 21.2775 (CW DMC):25 (CD) 10 ± 0a 90.89 ± 0.3i 17.1425 (HW DMC):75 (CD) 10 ± 0a 88.56 ± 0.55d 23.9450 (HW DMC):50 (CD) 10 ± 0a 89.67 ± 0.68e 21.3475 (HW DMC):25 (CD) 10 ± 0a 90.17 ± 0.57g 17.54

Data followed by a different superscript in each column are statistically differentfrom each other (P < 0.05) using DMRT.

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spectrophotometer [22]. Hemicelluloses and lignin were estimatedby determining neutral detergent fibre (NDF) and acid detergentfibre (ADF) [22]. Carbon and nitrogen contents were estimatedusing CHN analyzer (CHNOS Elementar, Vario EL III model) andthe crude protein was obtained by multiplying the nitrogen con-tent by a factor of 6.25 [1]. Flame photometer and spectrophotom-eter were used for the determination of available potassium (K)and available phosphorus (P), respectively [23]. Anthrone method[22] was used to determine the total soluble sugars. Oil contentof CD and MC was determined using soxhlet apparatus with hex-ane as the extracting solvent at 60–65 �C. pH of the slurry sampleswas measured periodically using a pH meter (Eutech InstrumentspH 510).

The periodical monitoring of volatile fatty acids (VFA) in theslurry samples was done using a gas chromatograph (5765 Nucongas chromatograph) equipped with a flame ionization detector anda 121 cm � 3 mm i.d. glass column packed with a 100–120 meshsize W-HP chromosorb. The temperature of the injector and detec-tor were 180 and 250 �C, respectively. Nitrogen was used as thecarrier gas at a flow rate of 30 ml/min. The injected sample volumewas 1 lL. In all the cases, average of three determinations wastaken and the final value was reported.

2.4. Statistical analysis

The data, collected in triplicates, was analyzed by one way anal-ysis of variance (ANOVA) using SPSS for windows (version 14.0).The significance of difference was determined according to Dun-can’s multiple range test (DMRT). P values <0.05 were consideredto be statistically significant.

3. Results and discussion

3.1. Analysis of the starting materials

MC was identified as M. indica by NISCAIR, CSIR, Delhi India. Avoucher specimen has been deposited there.

The chemical composition (moisture, TS, VS, total soluble sug-ars, fat, crude protein, celluloses, hemicelluloses, lignin and toxins,if any) of CD and MC has been reported in Table 1 (average of threedeterminations). The presence of considerable amount of sugars,proteins and VS in the seed cakes make them quite useful for bio-gas production [1]. Carbon from carbohydrates and nitrogen fromproteins, nitrates, etc. are the essential nutrients required for thegrowth of anaerobic bacteria. While carbon supplies energy, nitro-gen is needed for building up of the cell structure [24]. Neverthe-less, proper C/N ratio does play an important role in thebiomethanation process. C/N ratio ranging from 20:1 to 30:1 isgenerally considered suitable for anaerobic digestion [5]. A high

C/N ratio would mean low amounts of nitrogen which would beunfavorable because of deficiency of amino acids/proteins to thebacteria. On the other hand, a low C/N ratio would imply highamounts of nitrogen which would again be detrimental becauseof ammonia toxicity. However, it is also known that all forms ofnitrogen that enter the digester cannot be utilized by bacteria,especially methanogenic bacteria [24]. It can be seen from Table 2that C/N ratio for the CD control is 27.14 which is considered to beoptimum. However, for the treatments involving cake as one of theconstituent, the ratios are lower (Table 2). Addition of 25% and 50%cake to CD keeps the C/N ratio in the range of 23 ± 2 but increasingthe content to 75% further lowers the C/N ratio. Inspite of such lowC/N ratios, significant gas production occurs. This observation indi-cates that a part of nitrogen of raw/detoxified MC is not assimilatedby methanogenic bacteria which leads to an increase in the effec-tive C/N ratio. This explanation can be well supported by the workof Singh and Mandal [24] who reported a similar observation forother non-edible oil seed cakes used.

The percentage of crude saponins in MC was found to be16.7 ± 4.28% (average of three determinations). These mahua sapo-nins are known to possess insecticidal, fungicidal as well as bacte-ricidal properties [7,15,25,26]. Experiments were performed to seetheir effect, if any, on the bacteria involved in the biomethanationprocess.

The cake was subjected to simple water treatments and it wasfound that they could significantly reduce the saponin content.Table 3 shows the results of water treatments on MC and thechemical properties of so obtained cold water detoxified mahuaseed cake (CW DMC) and hot water detoxified mahua seed cake(HW DMC).

3.2. Batch experiments

3.2.1. Effect on biogas volumeThe effect of 70 day hydraulic retention time (HRT) gave varying

results for different treatments designed. It was observed that ini-tially biogas production was slow and after 17 days to about 45–50 days, it was significant [1]. Thereafter, the rate decreased be-cause of the reduction in nutrients, which might have affectedthe growth of bacteria thus affecting the biogas production(Fig. 2a) [24]. Fig. 2b depicts the total volume of biogas producedper kilogram TS for different treatments designed. Maximum bio-gas volume of 442 ± 5.5 L/kg TS was obtained with treatment 50(HW DMC): 50 (CD), i.e., combination containing 50% hot waterdetoxified mahua seed cake and 50% cow dung with TS maintainedat 10% in the digester. This led to an increase of around 125% in thebiogas volume produced over CD. This was followed by treat-ments; 75 (HW DMC): 25 (CD), 50 (CW DMC): 50 (CD), 25 (HWDMC): 75 (CD), 75 (CW DMC): 25 (CD), 25 (CW DMC): 75 (CD) inthe order. Varied volumes with other non-edible oil seed cakeshave been reported so far. Gupta et al. [1] obtained 379 L/kg TS bio-gas from combination containing 50% CW DMC and 50% CD over aperiod of 70 days [1]. Biogas production of 348 L/kg TS was re-ported from 10% TS in jatropha cake after an incubation of 40 days[27]. A combination containing 30% mustard oil cake and 70% cowdung gave 329 L/kg VS biogas (HRT; 30 days) [6]. 427 L/kg TS bio-gas was obtained with karanja cake [dilution ratio of 1:3.5 (cake:water)] and 50% CD over a period of 30 days [5] whereas a maxi-mum of 523 L/kg VS was observed with combination containing50% karanja cake and 50% CD in 41 days [24]. Different biochemicalcomposition of these seed cakes might be one of the important fac-tors affecting the total amount of biogas production from them[1,4,28]. Varied amounts of protein, viz., 24.06%, 29.60%, 34.37%;fat, viz., 8.3%, 12.80%, 7.2%; nitrogen, viz., 3.85%, 4.73%, 5.50%;ash, viz., 7.44%, 7.84%, 4.22%, etc. have been reported for jatropha,mustard and karanja, respectively [4,5,24,27]. Recently, Gunaseela

Table 3Properties of detoxified mahua seed cake.

Substrate % Decrease in saponins Protein (%) Total soluble sugars (%) C/N ratio

MC – 19.66 ± 0.10a 50.05 ± 0.12a 14.26CW DMC 50.23 ± 1.36a 23 ± 0.1b 18.23 ± 0.25b 12.81HW DMC 75.3 ± 1.27b 25.26 ± 0.08c 14.46 ± 0.8c 11.34

Data followed by a different superscript in each column are statistically different from each other (P < 0.05) using DMRT.

A. Gupta et al. / Applied Energy 102 (2013) 1514–1521 1517

[28] showed that amounts of proteins, fat, total carbohydrates, cel-lulose, ash, etc. strongly influence the methane yield from jatrophaand predicted a multiple regression model for the same.

The addition of seed cake(s) might enhance the availability ofnutrients in the digester thus promoting the growth of bacteriaand thereby leading to an increase in the biogas production. Itwas observed that in all the cases, detoxified cake(s) gave better re-sults as compared to raw MC. Also HW DMC (at 50% and 75% addi-tion) gave significantly higher biogas production compared to CWDMC. These findings suggest the possibility of saponins in the cakebeing toxic to bacteria involved in the biomethanation process; asmore the removal of saponins from the cake, better was the biogasproduction. Nevertheless, further microbial studies would be stillrequired to validate this observation.

The increase in the proportion of raw MC from 25% to 50% wasunfavorable as depicted by the decrease in biogas volume obtained(Fig. 2b). The same, however, was not the case with the detoxifiedcake(s). Infact, an increase of about 93% and 125% was observed byincreasing the proportion of detoxified cake from 25% to 50% forCW DMC and HW DMC, respectively. However, addition of 75%cake in all the cases led to a decrease in the biogas volume pro-duced compared to 50% addition. A decrease in the biogas yieldat higher concentrations of other oil seed cakes such as jatropha,safflower and olive has already been reported [24,29].

Prabhudessai et al. [30] observed that Quillaya saponins inhib-ited biogas production from food waste and cattle dung. Variousauthors have reported that saponins extracted from plants suchas sapindus and tea, decreased the number of methanogens and,hence methane production in rumen [31–35]. This reduced rateof methanogenesis might be attributed to diminished activity ofthe methane producing gene with/without changing the totalmethanogen population [35,36]. These research findings mightbe helpful in supporting our data as well because fermentationprocess in the rumen bear many similarities to biogas production,although rate of biodegradation occurring in anaerobic digestersremains lower than in animal guts [37,38]. Toxicity of mahua/otherplant saponins towards selected bacterial strains has also been re-ported [39–41]. Therefore, in the present study, decrease in biogasproduction at high (75%) cake concentration might be due to theinhibitory/toxic effect of saponins. However, in treatments involv-ing 50 (DMC):50 (CD), there might be a balance achieved between

Fig. 2a. Plot of biogas production from different treatments during the 70 day HRT. Econsecutive points. Vertical bars over each point indicate standard deviation (SD).

the amount of nutrients and saponins introduced by the detoxifiedcakes. Comparatively low biogas production is expected fromtreatments with 25% DMC because of limited supply of nutrientsas against from 50% and 75% DMC. The ratio of 50 (HW DMC):50(CD) was therefore considered as optimum for mixing cake andcow dung to obtain maximum production of biogas.

3.2.2. Effect on biogas qualityIt was observed that initially for about 11–15 days, the methane

content was low after which it starts increasing with time for 35–50 days, till the gas production automatically falls. Maximummethane content, 55–57%, was obtained for the combination of50 (HW DMC):50 (CD) and not much difference was observed be-tween the values obtained for other treatments involving detoxi-fied cakes. Satyanarayan et al. [6] reported a maximum of 60%methane content in biogas obtained from 30% mustard oil seedcake and 70% CD whereas 66 ± 2% methane has been obtained with10% and 15% jatropha oil seed cake [24]. On the other hand,Chandra [5] reported a maximum of 70% methane content fromkaranja cake [dilution ratio was 1:3.5 (cake:water)] and 50% CDas the substrate, and 68% from the combination containing jatro-pha oil seed cake [dilution ratio 1:4 (cake:water)] and 50% CD.

3.2.3. Effect on pHpH of the digester is one of the most important parameters

affecting its performance. The pH of an anaerobic digester initiallyfalls because of the production of volatile acids but as the metha-nogenic bacteria consume the volatile acids and alkalinity is pro-duced, the digester stabilizes itself. Most anaerobic bacteria,including methane forming bacteria, perform well within a pHrange of 6.8–7.2. A pH range of 6.5–7.5 is generally considereddesirable for optimum biogas production [5]. Therefore it is oftennecessary to periodically monitor the pH of digester to keep trackof the changes in alkalinity throughout the experiment. In ourstudy, pH of the control digester, i.e., digester containing CD andwater in the ratio 1:1, varied between 6.4 and 7.2. On the otherhand, digesters containing cake mixtures showed an initialdecrease in the pH (5.9–6.1) for about 6–9 days, indicating fasthydrolytic and acetogenic phase and then moved towards thealkaline range. The pH values obtained here are well in range tosupport the growth of the anaerobic bacteria [6,24]. It should be

ach data point on the plot represents the cumulative biogas production between

Fig. 2b. Biogas volume obtained from different treatments. Vertical bars over the histogram indicate standard deviation (SD). Bars followed by different letters arestatistically different from each other (P < 0.05) using DMRT.

1518 A. Gupta et al. / Applied Energy 102 (2013) 1514–1521

noted that MC is acidic in nature. Howsoever, increase in propor-tion of raw/detoxified MC from 25% to 75% in the digester didnot affect the performance of reactor much in terms of pH. Thiscan be explained by the fact that total solids (TS) in all the digesterswere maintained at 10% and that the cake contains higher amountsof nitrogen than CD [1]. Thus increase in concentration of cake inthe digester did increase the content of volatile acids, but thiswas compensated by the production of ammonia [5,6]. The excessammonia, not utilized by the bacteria, might have neutralized theexcess concentration of acids without being toxic [5]. This verywell maintained the stability of the reactor. The excess ammonia-cal nitrogen also assisted in greater digestion of carbohydrates, fatsand proteins present in the feed material [5]. The pH of digestereffluents of all the treatments varied within a range of 6.8–7.5which is congenial for the production of biogas. The results in thisregard can be corroborated by the work of Satyanarayan et al. [6]and Singh and Mandal [24].

3.2.4. Effect on TS, VS and VFA contentThe experiment was also monitored with respect to changes in

total solids (TS), volatile solids (VS) and volatile fatty acids (VFA)content in the biogas slurry. Fig. 3 gives the reduction in TS andVS for different treatments designed. It can be seen that maximumdecrease of 33.15 ± 0.5% in TS and 34.05 ± 1.7% in VS was observedfor the treatment 50 (HW DMC):50 (CD), for which the maximumbiogas volume was obtained. This definitely supported the correla-tion between volatile solids degraded and the biogas produced[6,29]. Table 4, lists the variation in VFA content during the70 day period, for different treatments designed. The content ofvolatile fatty acids in the digester mixture increased with the pro-portion of the raw/detoxified MC being added to CD. However, asmentioned above, this increased acidity does not affect the perfor-mance of the digester. The concentration of VFA in the mixture isan indicator of total amount of acetic acid being generated, whichon assimilation by the bacteria gets converted to methane in themethanogenic phase. As a result, their concentration initially in-creased to a maximum and thereafter decreased. This observationcan again be supported by the work of Chandra [5] and Ranadeet al. [42].

3.3. Degradation of complex molecules

MC contains large amount of sugars, proteins, celluloses andhemicelluloses compared to CD (Table 1). The presence of thesefavorable components in the seed cake (especially when freed oftoxins) makes them useful in the biomethanation process [6].

Celluloses, hemicelluloses and lignin were degraded to varyingextents during the biomethanation of raw/detoxified MC. Detoxifi-cation improved the microbial digestibility and biodegradabilityduring anaerobic fermentation. Higher degradability of organicmatter, celluloses and hemicelluloses was obtained for the detoxi-fied cakes. Maximum degradation of celluloses (34.46 ± 1%) andhemicelluloses (29.76 ± 0.25%) was observed for the treatment 50(HW DMC):50 (CD). This indicates that, more the easily degradableraw material available to the bacteria, better is the biogas produc-tion, provided all other favorable conditions of nutrients, pH, tem-perature, water content/total solids and non-toxicity persist.Lignin, being recalcitrant, was the component that was degradedthe least. Table 5 lists the percentage degradation of these complexmaterials in the digester slurry of all the treatments designed.These findings are well within the limits of the results of Dar andTandon [43] who reported an approximate 50% decrease in organicmatter, celluloses and total carbohydrates but a marginal decreasein lignin content of treated plant residues when compared to CDalone.

3.4. Manurial value of digested slurry

After degradation of complex materials in the digester feed,there was a significant increase in the percentage content of nitro-gen (N), phosphorous (P) and potassium (K), most pronouncedbeing observed for nitrogen (Fig. 4). Nitrogen content in thedigested slurry varied between 1.48% and 2.74%, phosphorous be-tween 0.23% and 0.37% and potassium between 0.38% and 0.58%. Itwas observed that manurial value (monitored in terms of N, P andK content) of the control digester slurry was significantly low ascompared to the slurry obtained by the addition of raw/detoxifiedcake(s). This can be well supported by the work of Chandra [5],Satyanarayan et al. [6] and Rajasekaran et al. [44] who observeda similar trend with the addition of different oil seed cakes toCD. Thus it can be concluded that with the addition of raw/detox-ified MC, there occurs a significant increase in the above men-tioned nutrients over a period of biomethanation.

It was found that saponins in the digested slurry of raw/detox-ified MC were also degraded to some extent over a period of bio-methanation. Percentage degradation with respect to their initialcontent varied between 9–21%, 19–27% and 22–30%, for treat-ments containing raw MC, CW DMC and HW DMC, respectively.Degradation of some steroidal saponins by the rumen microbeshas already been reported [45,46]. However, to the best of ourknowledge, no such studies have been performed on the residualtoxin content in the biogas slurry of non-edible oil seed cakes,

Fig. 3. Percentage reduction in total solids (TS) and volatile solids (VS) for different treatments. Vertical bars over the histogram indicate standard deviation (SD). Bars, for aparticular series, followed by different letters are statistically different from each other (P < 0.05) using DMRT.

Table 4VFA content (mg/L) in biogas slurry of different treatments.

Treatment 5th day 15th day 25th day 35th day 45th day 55th day 65th day

Cow dung (CD) control 328.8 ± 0.12a 1788.5 ± 0.14a 10746.6 ± 5.5a 9601.4 ± 0.92d 4002.1 ± 0.19a 1100 ± 0.11d 88.4 ± 0.19a

25 (MC):75 (CD) 504.2 ± 0.14c 4742.9 ± 0.19b 14543.3 ± 2.51b 8679.6 ± 0.75a 5070.9 ± 0.66b 1226.3 ± 0.53e 95.2 ± 0.31b

50 (MC):50 (CD) 495.7 ± 0.45b 8616.0 ± 0.15e 18953.3 ± 1.52e 10413.1 ± 0.67e 6453.6 ± 0.75h 890.8 ± 0.15a 228.2 ± 0.20g

75 (MC):25(CD) 833.1 ± 0.20g 10993.2 ± 0.6h 22691.6 ± 0.76h 12010.1 ± 0.95h 5326.9 ± 0.60d 1644.8 ± 0.42h 199.7 ± 0.11d

25(CW DMC):75 (CD) 524.2 ± 0.08e 4933.4 ± 0.09 c 15130 ± 2.64c 9028.9 ± 0.39b 5270.6 ± 0.70c 1277.6 ± 0.26f 98.2 ± 0.31bc

50(CW DMC):50 (CD) 514.8 ± 0.08d 8960.3 ± 0.08f 19703.3 ± 1.52f 10832.6 ± 0.81f 6711.1 ± 0.97i 927.5 ± 0.53b 236.7 ± 0.24h

75(CW DMC):25 (CD) 865.5 ± 0.11h 11431.7 ± 0.08i 23592.6 ± 0.65i 12488 ± 0.52i 5540 ± 0.79f 1711.2 ± 0.34i 208.4 ± 0.24e

25(HW DMC):75 (CD) 533.9 ± 0.18f 5028.1 ± 0.15d 15412.6 ± 0.64d 9202.9 ± 0.66c 5370.9 ± 0.37e 1302.7 ± 0.28g 101.3 ± 0.10c

50(HW DMC):50 (CD) 524.6 ± 0.11e 9132.9 ± 0.17g 20092.6 ± 0.64g 11040 ± 0.80g 6844.8 ± 0.55j 946.5 ± 0.37c 241.4 ± 0.12i

75(HW DMC):25 (CD) 882.5 ± 0.10ij 11651.5 ± 0.09j 24054.4 ± 1.2j 12731 ± 0.65j 5651 ± 0. 21g 1743.1 ± 0.28j 212.1 ± 0.11f

Data followed by a different superscript in each column are statistically different from each other (P < 0.05) using DMRT.

A. Gupta et al. / Applied Energy 102 (2013) 1514–1521 1519

when latter are used for anaerobic digestion. The presence of theseresidual saponins in the biogas slurry might not affect its manurialvalue as there are evidences of soil microbial degradation of sapo-nins by hydrolysis or non-biological degradation due to saponinadsorption to humic acid in the soil [47,48]. Nevertheless, mahuasaponins are known to possess biopesticidal properties against avariety of pathogenic bacteria, fungi and insects [39,40]. However,overall effect might depend upon the exact target species in thesoil rhizosphere which further opens new avenues for research.

4. Potential contribution of the present study to national fuelconsumption

The use of non-edible oil seed cakes for biogas production hasgained considerable attention over the past few years. As men-tioned above, oil cakes of jatropha, karanja, etc. have been exten-

Table 5Degradation of complex molecules in biogas slurry of different treatments.

Treatment Celluloses content Hem

Initial (%) % Decrease Init

Cow dung (CD) control 24.33 ± 0.30a 25.6 ± 0.52a 2525 (MC):75 (CD) 31.66 ± 0.15b 26.5 ± 0.5b 2650 (MC):50 (CD) 45.1 ± 0.45c 24.26 ± 0.25c 2775 (MC):25(CD) 47.56 ± 1.27d 18.53 ± 0.50d 2825(CW DMC):75 CD) 29.8 ± 0.72e 27.33 ± 0.30e 2650(CW DMC):50(CD) 42.26 ± 0.22f 30.43 ± 0.40f 27.575(CW DMC):25(CD) 49.43 ± 0.66g 28.76 ± 0.25g 28.325(HW DMC):75(CD) 29.33 ± 0.28e 29.76.5 ± 0.25h 2650(HW DMC):50(CD) 41.6 ± 0.52f 34.46 ± 1i 2775(HW DMC):25(CD) 47.37 ± 0.40d 33.93 ± 0.11i 28.4

Data followed by a different superscript in each column are statistically different from e

sively studied for their biogas potential. However, not muchwork has been done on the use of MC for anaerobic digestion.The present study highlights the potential of MC for biogasproduction. The cake was detoxified using simple water treat-ments. Use of these detoxified mahua cake(s) for biogas productionhas been reported for the very first time, to the best of ourknowledge.

The potential availability of MC is around 0.32 million tons/yearin India [5]. Using the results of maximum biogas production of442 L/kg TS from the present study, an average of 141 million cubicmeters biogas could be obtained per year from this abundantlygenerated waste in the country. Keeping in view, the commercialconsumption of 31.8 billion cubic meters of natural gas in 2008–2009 in India, biogas generated from (detoxified) MC holds the po-tential for 0.44% replacement of the total natural gas consumed.Thus biogas production from mahua cake, detoxified by simple

icelluloses content Lignin content

ial (%) % Decrease Initial (%) % Decrease

.9 ± 1.70a 18.6 ± 0.52a 7.48 ± 0. 13a 1.56 ± 0.05a

.8 ± 0.15bcd 20.6 ± 0.53 b 6.71 ± 0.07b 1.05 ± 0.05b

.8 ± 0.26def 18.86 ± 0.15 a 5.86 ± 0.05c 1.12 ± 0.05b

.6 ± 0.20f 14.43 ± 0.40 c 5.03 ± 0.05d 0.38 ± 0.10d

.4 ± 0.30b 23.16 ± 0.15d 6.68 ± 0.02b 1.38 ± 0.05e

3 ± 0.05cde 26.46 ± 0.45e 5.81 ± 0.02ce 1.55 ± 0.1a

6 ± 0.32ef 24.6 ± 0.52f 4.95 ± 0.05d 1.11 ± 0.02b

.6 ± 0.17bc 25.5 ± 0.5g 6.57 ± 0.24b 1.80 ± 0.06c

.6 ± 0.11cdef 29.76 ± 0.25h 5.63 ± 0.15e 1.12 ± 0.02b

5 ± 0.05ef 24.86 ± 0.23fg 4.86 ± 0.04d 1.65 ± 0.05c

ach other (P < 0.05) using DMRT.

Fig. 4. Manurial value of digested slurry. Vertical bars over the histogram indicate standard deviation (SD). Bars, for a particular series, followed by different letters arestatistically different from each other (P < 0.05) using DMRT.

1520 A. Gupta et al. / Applied Energy 102 (2013) 1514–1521

water treatments, offers one of the viable sources for renewableenergy production.

5. Conclusions

From the present study, it can be concluded that although MChas a potential to produce biogas, the presence of toxic saponinsinfluence its ability to do so. The detoxified cake(s) seem to per-form significantly better than raw MC. Nevertheless, further micro-bial studies would be still required to validate the recordedobservation.

The combination of 50:50 (detoxified cake:CD) was found to beoptimum for obtaining the best results. The degradation of TS, VS,and VFA further support and strengthen the reported results. Sig-nificant reduction in celluloses and hemicelluloses and an increasein the nutrient (N,P,K) content of the digested slurry could be ob-tained. In this study, we conclude that MC could be a suitable feed-stock for biogas production and also help in overcoming thedisposal problem of seed cake generated from biodiesel productionprocess.

Acknowledgements

The authors are thankful to CSIR and IIT Delhi for the financialsupport. The first author highly acknowledges the help given byMr. Amit Agarwal and Mr. Virendra Kumar, Biogas Lab, CRDT, IITDelhi for their immense support throughout the experiment.

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