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Biogas Production in Batch andSemicontinuous Digesters UsingWater Hyacinth

S Vaidy anath an, K. M . Kavadia, K. C. Shroff, and S. P. M a h a j a n tDepartment of Chemical Engineering I. . T., Powai Bombay 400 076 India

Accepted for publication August 10 1984

INTRODUCTION

Th e rapidly exploding global population and th e ex-pansion of in dustr ies cre ate rapid depletion of naturalresources and tremendous increase in environmentalpollution. In rece nt yea rs attention has been paid bothto the dev elopm ent of renewable energy resourc es andto the problem of environ men tal pollution. A quatic

plants in gen eral and wate r hyacinth in particular havedraw n considerable interest as solutions to the currentproblems of energy and pollution.

Wa ter hyac inth exhib its prolific growth. S everal de-tailed investigations in the laboratoryas well as in thefield have be en m ade on the use of water hyacinth forpollution control.'-5 Shroff has studied the key questionof cost-benefit ratio in treating wastew aters from pes-t ic ide plan t by wate r hyac in th . W ~ l v e r to n ,~riramuluand Bhargava,' and Klass and Ghosh' have reportedthat w ater hyacinth could be utilized a s a sourceofmethane-rich fuel. Recovery of methane from waterhyacinth is a sound proposition in the current energy

crisis. Since scant d ata a re available in the li teratureon hyacinth gasification, preliminary exp erim ents in abatch digester were performed. However, scaling upthe batch data to a semicontinuous digester for a com-mercial plant might have disadvantages.

Therefore, bench scale experiments ina pilot sizesemico ntinuous digester were performed fo r7 months,and m ore quantitative data were developed and reported(this article).

MATERIALS AND METHODS

Water hyacinth employed in the present experimentswere from G oregaon and Amboli. T he water hyacinthwas ana lyze d for moistu re, total solids, volatile solids,organic carbon, total nitrogen, and potassium. Allsampling and analysis were done as per standardmethods and Jackson . All the gas data reportedwere converted to normal li ters (NL) a t 0C and Iatm. The seed material for digestion runs was from

t To whom the correspondence should be addressed.*Excel Industries Ltd., Bombay, India.

Biotechnology and Bioengineering, Vol. XXVII, Pp. 905-908 (1985)1985 John Wiley & Sons, Inc.

the batch laboratory digester using water hyacinth.Experim ents were performed at ambient tempe raturesof 29 f 2C and pH 6.9 0 .2 .

Sect ion 1

Tw o se ts of experiments were conducted in35-Lcarboys. Each carboy was fit ted with one two-holed

rubb er stoppe r and made air t ight using plaster of pa rkas cementing material. A thermometer was insertedin each carboy and a rubber tube was connected toa sealed container filled with salt solution. The totalvolume of gas produced w as measured a s the volumeof salt solution displaced. Chopped water hyacinthofsize 5-6.25 cm and gr oundw ater hyacinth of size0.6-1.2 mm were used. The contents of carboys wereshaken intermittently. The freshly chopped andgroundwater hyacinths were analyzed (see Tables Iand 11).

Sect ion 2The mild steel biogas digester used in this study

was 60 cm diameter and 75 cm deep . Two 7.6-cm-diameter pipes were used, one for feeding groundwaterhyacinth slurry into the digester and the other forwithdrawing. T her e was an inlet tank for charging andan ou tlet tank for receiving the processed sludge. Thecon ten ts of digeste r was stirred with four flat impellerblades of 20 cm diameter a t 30 rpm. The total volumeof the digester was 165 L, but the working volumewas 150 L. The digester was provided with a ther-mocouple. T he gas produce d was measured by a wetgas meter. The digester was fed daily with groundwaterhyacinth of size 0.6-1.2 mm and operated for7 months.Experim ents w ere performed at three different hydraulicdetention times of 16.67, 25, and 30 days; volatilesolids (VS) loading rates were varied between 0.886and 1.856 kg dry VS/m' day. When the digester sta-bilized to the above operating conditions, the datawere collected. Water hyacinths were analyzed forphysical an d chemical char acteristics and are shownin Table 111. The operating conditions and methaneyields a re tabulated in Table IV.

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Table I. Analysis of freshly chopped and groundwater hyacinthemployed in Section 1.

Test

Freshly chopped Freshly groundwate r hyacinth, water hyacinth,

total plant total plant

Mois ture , w/wTotal Solids, w/wOn dry basis, w/w

Volatile solids

Organic carbonNitrogenPhosphorousIronCalciumMagnesium

93.106.90

74.0032.40

1.200.500.501.401.10

92.807.20

74.0029.00

1.300.590.901.220.60

hyacinth at longer digester residence time of90 daysgave 44 mo re methane yield. The groundwater hy-acinth gave methane yield of 307 NL/k g VS over50day s residence time. How ever, th e maximum specificmethane production rate of6.15 NL/kg dry VS daywas obtained with groundwater hyacinth. The lesserspecific methane production rate of4.91 NL/kg dryVS day was obtained from chopped water hyacinth.Wolverton' reported that chopped wate r hyacinth ove r

a digester reside nce time of 126 days a t 25_t 5Cproduced biogas of 61 methane and obtained themaximum m ethane of 350 L/kg dry m ass. This valuewas comp arable with our results of 327 NL m ethane/kgdry mass with chopped water hyacinth.

RESULTS AND DISCUSSION

Section 1

Th e results of methane produced from chopped andgroundwater hyacinths are presented in TableXI

Grou ndw ater hyacinth produced 228 N L methane/kgdry mass. C hoppe d water hyacinth produced 327 N Lmeth ane/kg dry mass. Gro undw ater hyacinth gavebiogas of 77% methane and chopped water hyacinthgave biogas of 70% methane. For groundwater hyacinth,meth ane production ra te was 0.282 N L/ L day , whichwas 45 more than from chopped water hyacinth.Asexp ecte d, increased -digester residence time had theeffect of increasing methane yields. Chopped water

Section 2

Digestion of ground whole-w ater hyacinth plant wascarried out in a semicontinuous digester at29 k 2Ca n d p H 6.9 _C 0.2, and results of methane yields arepresented in Table IV. It can be se en that the methaneyield ranged from0.081 to 0.16 Nm3/kg dry VS addedov er volatile solids loading rateof from 0.763 to 1.856kg dry VS/m3 day. At a volatile solids loading rateof 1.619 kg dry VS/m3 day and longer hydraulic de-tention time of 30 days, maximum methane yield of0.16 Nm3/kg dry VS added and maximum volatilesolids conversion efficiency of44.2 were obtained.Klass an d Ghosh' condu cted experiments ina 2-dm3unit using Mississippi and Florida water hyacinth andreported m ethane yield of0.166 and 0.098 Nm3/kg dry

Table 11 0perating.conditions and methane yieldsof freshly chopped and groundwater hyacinths in batch digester.

Series No. Units Freshly chopped

~~

Freshly ground

A . Feed1. Weight of material2. Solids content3. Volatile solids conte nt

(on dry basis)4. C/N ra t io5. C/P ra t io6. Particle size

B. Digester1 Type2. Volume3. Materials of construction4. Mo de of mixing

C. Operating conditions

1. Tempera ture2. p H3. Duration4. Volatile solids loading

1. Total gas collected2. Methane content3. Methane production rate4. Methane

D. Methane yields

5 Specific methane production

Kilogramw/ww/w

Liter

C

Dayskg dry VS/rn'

Normal l i tersv /v

N L / L dayNL/kg dry VS

NL/kg dry VS day

206.90

74.00

26.2963.90

5-6.25 cm

Batch35

Polythene carboy sIntermittent

29 26.9 0.29029.18

96770

0.194442.00

(327 NL/kg dry so l ids)4.91

207.20

74.00

22.1050.40

0.6-1.2 mm

Batch35

Polythene carbo ysIntermittent

29i

26.9 .25

30.45

63877

0.282307.40

(228 NL/kg dry so l ids)6.15

a VS, volatile solids; NL, normal liters.

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Table 111.hyacinth used in Section 2.

Physical and chemical characterist icsof groundwater

Ultimate Analysis ( w/w)C 28.88N 1.88P 0.52Ca 1.32Fe 2.80N a K 5.85

Proximate A nalysis ( w/w)

Moisture 93.50Solid conten t 6.50Volatile matter (dry basis) 74.00Ash 26.00C/ N ratio 15.52C /P ratio 55.40

VS added and volatile solids conversion efficiency of29.2 and 17.0 , respectively, for a volatile solids loadingrate of 1.6 kg/m3 day and hydraulic detention time of12 days. For Mississippi and Florida water hyacinthsmethane production rates were 0.28 and 0.166 Nm3/m3

day, respectively, and the corresponding methane pro-duction ra te in our study was 0.451 Nm3/m3 day. Inthe present study water hyacinth slurry of 6.55 totalsolids (TS) was employed and longer hydraulic detentiontime was used. Klass and Ghosh employed dilute waterhyacinth slurry of 2.45 TS and used a shorter detentiontime of 12 days.

Specific methane production rate increased with in-creased volatile solids loading rate. In the presentstudy maximum specific methane production rate was

0.0063 Nm3/kg dry VS added per day at the highestvolatile solids loading rate of 1.856 kg dry VS/m3 dayand lowest hydraulic detention time of 16.67 days.For Mississippi and Florida water hyacinth Klass andGhosh reported specific methane production rate of0.014 and 0.0082 Nm3/kg dry VS added per day, re-spectively. Specific methane production rate and volatilesolids loading rate are important to design biogasdigesters using water hyacinth. The specific methane

production rate was correlated with volatile solidsloading rate as follows:

R = aebL

where R is the specific methane production rate,Nm3/kg dry VS added per day, and L is the volatilesolids loading rate, kg dry VS/m3 day. The correlationhad regression coefficient of 0.98. The value of a was0.00161 and that of b was 0.72. For a given biogasplant capacity one may either go for high volatile solidsloading rate and shorter hydraulic detention time orlow volatile solids loading rate and longer hydraulicdetention time. At high volatile solids loading rate

more methane is produced per volume of digester,resulting in a smaller digester and cheaper digestercost. But because of shorter hydraulic detention time,there is a smaller reduction in volatile solids and lessmethane per mass of feed. At longer hydraulic detentiontime and lower volatile solids loading rate, percentageof volatile solids destruction is increased but methaneper volume of digester is decreased; so there is greaterdigester volume and greater capital cost. Therefore,it is desirable to determine optimal size, hydraulic

Table IV. Operating condition and m ethane yieldsof groundwater hyacinth in sem icontinuous digester.

Run no.

Observation data 1 2 3 4 6 7 8 9 10

A . Feed1. Total solids, ' w/w2. Volatile solids, w/w

1 Temperature, C2. pH3. Hydraulic detention

time, days4. VS loading rate, kg

dry VS/m3 day

B. Digester

C . Gas Production

1 Methane content,

2. Methane production,N L / L day

3. Methane yield,Nm'/kg dry VS added

4. Specific methane pro-duction Nmz/kg dryVS added day

5. VS conversion effi-ciency, (experimen-tally determinedvalues)

v/v

5.673.10

29 26.9 0.2

16.67

1.856

61.8

0.315

0.105

0.0063

21.2

6.554.86

29 26.0 0.2

30

1.619

57.4

0.451

0.160

0.0053

44.2

5.123.59

29 26.9 t 0.2

25

1.436

54.0

0.260

0.103

0.0041

26.8

4.242.90

29 t 26.9 t .02

25

1.161

56.6

0.177

0.086

0.0035

21.9

4.093.13

29 26.9 t 0.2

30

1.043

57.9

0.181

0.100

0.0034

28.4

5.183.53

29 t 26.9 f 0.2

30

1.176

57.8

0.235

0.116

0.0039

29.4

5.103.59

29 -c 26.9 t 0.2

30

1.198

57.8

0.288

0.139

0.0046

36.4

4.432.66

29 26.9 0.2

30

0.886

65.4

0.123

0.091

0.003

20.3

4.56 4.143.10 2.29

2 9 ? 2 2 9 ? 26.9 f 0.2 6.9 0.2

30 30

1.033 0.763

64.0 63.2

0.168 0.098

0.104 0.081

0.0035 0.0027

26.25 16.70

a VS, volatile solids: N L , normal liters.

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dete ntion tim e, and volatile solids loading rate by max-imizing net operational revenue accrued from biogasplant.

CONCLUSlONS

Digestion experiments at ambient temperatureof29 2C conducted in a batc h digester using choppedand groundwater hyacinth showed higher methaneproduc tion rate; lowe r digester residence time are ob-tained with groundw ater hyacinth. The biogas producedfrom groundwater hyacinth had higher methane content,77%.

Semicontinuous digestion experiments at ambienttemperature conducted ina pilot size mixed digesterusing groundwater hyacinth showed that maximumspecific me thane production rate was obtained a t thehighest volatile solids loading rate an d lowest hydraulicdetention time. Specific methane production rate iscorrelated with volatile solids loading rate; this cor-

relation offers a simple tool to design semicontinuous

biogas digesters for commercial plants using waterhyacinth.

References

1.2.

3.4.

5.6.

7.

8.

9.

10

11.

R. Dinges, Amer. J Pub. Health 68 12 (1978).B. C. Wolverton and R. C. Mcdonald, Econ. Bot. 34 101(1980).R. M. Harrer and J. L. Fox, JWPCF 45, 1928 (1973).S. Vaidyanathan, K. M. Kavadia, M . G. Rao, S. Basu, andS. P. Mahajan, Znt. J Environ. Studies 23 183 (1983).B. C Wolverton and R. C. McDonald, J W P C F 51,305 (1979).K. C. Shroff, Paper Presented at All India Manufacturers As-sociation at the Seminar on Recovery and reuse of materialspollutants, Bombay, 1977.B. C. Wolverton, Bio Conversion of Water Hyacinth into Meth-ane, NASA Tech. Mem. TM-X-72725 1975.D. L. Sriramulu and H. D. Bhargava, Energy Management 5 ,17, Oct-Dec. (1980).D. L. Klass and S. Ghosh, Fuels fro m biomass and Wastes(Ann Arbor Science, Ann Arbor, MI, 1981), p. 129.M. A. Franson (ed.), Standard Methods for the Examinationof Wa fer and W aste Water 13th ed. (APHA-AWWA-WPCFPublication, John D. Lucas, Baltimore, 1971).M. L. Jackson, Soil and Che mical Analysis (Prentice-Hall, New

Delhi, India, 1967).

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