Biological Wastes 30 (1989) 231-237

Start-up of Anaerobic Digestion of Tomato-Processing Wastes for Methane Generation

R. Sarada & Krishna N a n d *

Microbiology and Sanitation Discipline, Central Food Technological Research Institute, Mysore 570013, India

(Received 28 May 1988; revised version received 25 February 1989; accepted 29 March 1989)

ABSTRACT

A good start-up of digestion of tomato-processing waste (TP W) was achieved by the stepwise addition of feedstock over 10-12 weeks of digestion. The stepped increases of TP W in the digester resulted in a steady-state gas yield of 0.597m3/kg VS added with a 72% methane content. Volumetric production of gas and methane remained high after the eighth week of digestion throughout the investigation. The volatile fatty acids concentration in this system was similar to a cowdung-fed digester and never exceeded more than 150mg/litre throughout the period

INTRODUCTION

Escalating energy costs and rapid depletion of fossil fuels in recent years have aroused interest in alternative sources of energy. Among these, biomethanation of agro-industrial wastes and biomass has worldwide application as it yields a good quality fuel gas and also fermented slurry which may be used as manure or soil conditioner. In addition it also helps to a great extent in the abatement of pollution.

Most of the installations for biogas production are based on the use of cattledung or sewage sludge. However, in recent years, efforts have been made to utilize other feedstocks such as crop residues, animal and poultry

* To whom correspondence should be addressed. 231

Biological Wastes 0269-7483/89/$03'50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

232 R. Sarada, Krishna Nand

manures and food-processing wastes, etc., which have a potential for contributing to the energy problem. The total gas and rate of methane generation are markedly affected by the nature of feedstocks employed (Hawkes, 1980; Radhika et al., 1983).

A key step in biogas production is the startup of the process (Clausen et al., 1979, 1981) using a starter containing a good number of methanogenic and non-methanogenic microorganisms. This report is concerned with labora- tory studies on the start-up of anaerobic digestion of tomato-processing waste (TPW) for methane generation.

METHODS

Feedstocks

Tomato-processing wastes were collected from Kissan Products Limited (Bangalore, Karnataka), Clean Foods (Madanapalle, Andhra Pradesh) and Globe Foods (Mysore, Karnataka), and stored either as such at - 4 ° C or shredded into fine pieces, air dried, powdered and stored at low temperature for further use.

Laboratory digesters

All the experiments were carried out in 5.5-1itre digesters at 30°C, operated in the semi-continuous mode unless otherwise indicated, and having 5-1itre working volume. In order to avoid growth of algae, digesters were wrapped with black paper to protect them from the light.

Inoculum

The starter was obtained from an active commercial digester (CFTRI, Mysore) which was regularly fed with animal house wastes (excreta of rabbits and rats, etc.).

Stabilization and operation of digesters

The digesters were first charged with 6% (dry weight basis) cowdung slurry and inoculated with 10% of an active starter culture as above and allowed to adapt for eight weeks by batch mode of digestion. The fourth digester, which served as a control, was fed with TPW instead of cowdung and inoculated

Start-up of tomato-waste digestion

TABLE 1 Protocol for Start-up of Digesters

233

Digester 1 Digester 2 Digester 3 Digester 4

A: CD alone a As digester 1 As digester 1 TPW alone

B: CD b TPW c stepped Feeding TPW at As digester 3 increase with the rate 6% (TS) decrease in CD after 8th week of every week in the starting the feed (6% TS) experiments

a Period A: Digesters operated in batch mode for 8 weeks. b Period B: Digesters operated in the semi-continuous mode of feeding by removing and adding daily 156ml of slurry maintaining a hydraulic retention time of 32 days. ¢ Feed proportion 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80. 10:90 and 0:100 at the rate of 6% (TS) of cowdung and tomato-processing wastes. d pH adjusted to 7"0 using sodium hydroxide. TPW, Tomato-processing wastes. CD, Cowdung.

with the starter culture as above while maintaining other conditions as described previously. The experimental design is shown in Table 1.

In the No. 2 digester, a procedure involving stepped decreases in cowdung and increases in TPW of the feed in the proport ion of 90:10, 80:20, 70:30, 60:40, 50: 50, 40: 60, 30: 70, 20: 80, 10:90 and 0:100 (CD :TPW) was followed every week. The feed was prepared by blending the wastes to get a homogeneous slurry. The slurry was prepared daily just prior to the feeding of the digester by mixing the desired quantity of feed with the required volume of water. Care was taken to maintain solids in suspension during feeding.

Analyses

The total gas production was measured daily by the water displacement technique and the gas composit ion was periodically determined by a Netel Gas Chromatograph using Poropak Q and molecular sieve columns with T C D detector, pH of the digester was monitored daily by a meter. Total and volatile solids, volatile fatty acids (VFA) and moisture, minerals and fat were determined by standard methods (AOAC, 1975). Ammoniacal N and total alkalinity were measured according to A P H A (1975) procedures. Total nitrogen was measured by the micro-Kjeldahl method. Cellulose and hemicellulose were estimated according to the method of Ericson and Elbein (1980). Gas volume was corrected to standard temperature and pressure (STP).

234 R. Sarada, Krishna Nand

RESULTS AND DISCUSSION

The proximate compositions of tomato-processing wastes and cowdungs are given in Table 2. Compared to cowdung, TPW contained high amounts of total volatile solids (VS), carbohydrates, proteins and fats, and they also had high C/N and C/P ratios. The major carbohydrates were hemicellulose and cellulose. The results of the startup experiments are summarized in Figs 1-3.

The runs for start-up studies were carried out for 12 weeks. Immediately after commencing the different methods of feeding, the gas production of digester 3 and digester 2 exceeded that of digesters 1 (fed with cowdung) and 4 (control) (Fig. 1). After the second week of digestion, the rates of gas and methane production in digester 3 remained fairly steady except at the sixth week. In the case of digester 2, which involved stepped increases of TPW, the rates of total gas and methane production were somewhat unsteady during the initial stages up to the fourth week, but thereafter these rates increased steadily and reached a steady-state after the eighth week, giving gas at 0.597m3/kg VS added, with 72% methane on average (Figs 1 and 2). Throughout this experiment the gas composition was steady and VFA content was low. The higher yield and efficient performance of digester 2 is attributed to the stepped increase of TPW in the feed.

The pH of digester 4 dropped rapidly to approximately 5 and thus affected the gas production. The pH was then adjusted to 7 by daily addition of 2.5 N NaOH. Digesters 1 and 2 maintained a pH around 6"8-7.0 and had low levels of VFA, indicating good digester stability; but digester 3 exhibited unstable digestion and progressive increase in the VFA content (Fig. 3). In digester 4,

TABLE 2 Proximate Compositions of Feedstocks (% dry wt basis)

Constituent Cowdung Tomato

Moisture 80 6.7 Volatile solids 79.48 95"45 Ether extract 3.77 17.0 Hemiceilulose 12.3 39'3 Cellulose 29.4 8-0 Lignin 13.0 6.0 Kjeldahl nitrogen 1"55 3"73 C/N ratio 24 13.4 Protein 9"68 23'3 Phosphorus 0"087 0'220 Sodium 0"116 0'151 Potassium 1.580 0.954 Calcium 0-100 0.028

Start-up o[ tomato-waste digestion 235

13 i i i '", '~ / / D ' 7 . ~ w "0- --I:~- .--0 "~o. o

> I

.1¢

E ~ O .

~ O .

0

a 0 . 1 o x~X--- x ~ X ~ x ~ x ~ x~x- ._x~x~x ~- ~ x

I I I I 1 I 2 ~ 6 8 10 1

O p e r a t i o n t ime ( w e e k s )

Fig. 1. Total gas production during the start-up of digesters in semi-continuous mode after batch digestion. - - O - - , Digester 1; - - C ) - - , d i g e s t e r 2; - - U ] - - , d i g e s t e r 3; - - × ,

digester 4.

acids rose as high as 12 g/litre and the proportion of methane and total gas produced was appreciably lower than in the other digesters. At 20-30 days HRT 0-1-0.2 g/litre of VFA in piggery wastes digesters was reported by Hobson et al. (1981). In our studies similar values were found in the case of cowdung and stepwise TPW-fed digesters (1 and 2). The concentration of VFA in digester 3 was always highest from the initial stages of start-up to the end of digestion (Fig. 3).

Fig. 2.

A I ~ 0.5

o

>~0-~,

~ 0-3 p/

. -

~ 0 . 2 / o d ol

o 0 . 1 e . ~ . . . . . . . . . _ - • ~ o . . . L ~ .

~¢ X - - X - - X " " ' X " ~ " X - - X - - X - - X ~ X ~ X - - - - X ~ X I I I ~ I I

0 2 ~ 6 1 0 1 2

Operat ion t ime (weeks)

Methane yield during the start-up of digesters. For symbols see Fig. 1.

236 R. Sarada, Krishna Nand

Fig. 3.

16000

E

1~ooo 0

o 8000

600 U 0

~00

0 200

x/ "--..~p--x--x \ x..~X~x

..a" \

,=" \

I ..d

0 2 ~ 6 8 10 12

Operotlon t i m e ( w e e k s )

Changes in VFA content of the digesters. For symbols see Fig. 1.

By stepped increases in TPW in the feed a rapid and stable anaerobic digestion could be achieved within 8 or 10 weeks. This method of start-up gave a low acid content, unlike the pH controlled and other digesters (Fig. 3), and finally led to a significant increase in the yield of gas and methane. The stepped-feeding offers a method that can be used on a large scale without much difficulty.

A C K N O W L E D G E M E N T S

This work was supported by the Department of Non-Conventional Energy Sources, Government of India, New Delhi. The authors are thankful to Dr B. L. Amla, Director of the Institute, for his keen interest in this investigation.

R E F E R E N C E S

AOAC (1975). OfficialMethods of Analysis, 12th edn, ed. W. Horwitz. Association of Official Analytical Chemists, pp. 135, 198, 606.

APHA (1975). American Public Health Association, Standard Methods for the Examination of Water and Waste Water, 14th edn. APHA and American Water Works Association and Water Pollution Control Federation, Washington, pp. 293, 440.

Clausen, E. C., Sitton, O. C. & Gaddy, J. L. (1979). Biological production of methane from energy crops. Biotech. Bioeng., 21, 1209-19.

Clausen, E. C., Ford, J. R. & Shah, A. H. (1981). Importance of start-up in the anaerobic digestion of crop materials to methane. Process Biochem., 16, 18-19.

Start-up of tomato-waste digestion 237

Ericson, M. C. & Elbein, A. D. (1980). Biosynthesis of cell wall polysaccharides and glycoproteins. In The Biochemistry of Plants, ed. J. Preiss. Academic Press, New York, 3, pp. 589-616.

Hawkes, D. L. (1980). Factors affecting net energy production from mesophilic anaerobic digestion. In Anaerobic digestion, ed. D. A. Stafford, B. J. Wheatly & D. E. Hughes. Applied Science Publishers Ltd, London, pp. 131-50.

Hobson, P. N., Bousfield, S. & Summers, R. (1981). Methane Production from Agricultural and Domestic Wastes, Applied Science Publishers, London, p. 269.

Radhika, L. G., Seshadri, S. K. & Mohandas, P. N. (1983). A study of biogas generation from coconut pith. J. Chem. Tech. Biotech., 33B, 189-94.