Effect of Pectin on Anaerobic Digestion of Cattle Dung

D. B. Madamwar and B. M. Mithal Birla Institute of Technology and Science, Pilani-333 03 1, Rajasthan, India

Accepted for publication June 17, 1985

Anaerobic digestion of waste matter resulting in pro- duction of biogas, which is a valuable source of energy, has received much attention recently. Although an- aerobic digestion has a long history of use, it has a reputation for poor process stability. Digesters are also susceptible to malfunctioning due to shock of loading, temperature, and variety of toxic substances. Mal- functioning manifests itself in terms of reduced gas production, reduced degradation of organic materials, and increase in acidity. It is therefore desirable to dis- cover procedures to increase the rate of digestion.

It has been reported that addition of powdered ac- tivated carbon results in an increase in total gas pro- duction with high methane content.14 Based on a re- view of the literature, it is evident that carbon is responsible for improved digestion. The surface of the activated carbon provides adsorption sites where sub- strate can accumulate, thereby providing high localized substrate concentration. These areas of adsorption provide a more favorable growth environment for bac- terial-substrate systems.’ No study, however, seems to have been made so far on the effect of other additives like pectin on anaerobic digestion of cattle dung.

Since pectin is also used as one of the adsorbents in many cases, it appears desirable to study the effect of pectin on anaerobic digestion of cattle dung with the ultimate aim of improving the production of gas with increased methane content. Experiments carried out with this object in view are reported in this paper. The impact of pectin on volatile acid, pH, and process sta- bility has also been examined. This paper presents the results of two sets of experiments involving pectin ad- dition to bench-scale digesters. One set of experiments is carried out at a controlled temperature of 38 2 1”C, while the other is carried out at ambient temperature- ambient temperature varied from 40 to 15”C, that is, from August to November.

MATERIALS AND METHODS

Several bench-scale anaerobic digesters were used. Each vessel consisted of a 10 L glass reaction bottle, having a working volume of 6 L and containing 7.5%

of the total solids. The digesters were continuously stirred with a magnetic stirrer. In one set all digesters were maintained at 38 2 1°C in a thermostat, and in the other set all digesters were exposed to ambient temperature. Gas was collected and measured by dis- placement of acidified saturated salt solution, making due corrections for atmospheric pressure and temper- ature. The digesters were fed on a semicontinuous ba- sis: once per day with a retention time of 10 days, which was found to be most suitable by other workers als0.~7~ Prior to feeding, an equal quantity of sludge was withdrawn from the bottom of the digester. Pectin was incorporated with feed sludge. Substrate samples were analyzed for pH, volatile acids, BOD, and COD as per “standard procedures.”’

RESULTS AND DISCUSSION

A trend of enhanced gas production with increased amount of commercial pectin is evident from Figure 1. Maximum enhancement (of over 150%) was achieved with the addition of 10 g/L pectin (Fig. 1, Table I). In addition to increasing total gas production, pectin was responsible for higher methane content in the digester gas. As shown in Figure I , as much as 65% methane was present in the total gas. A similar trend was also observed even in the case where the digester operated at ambient temperature. The averages of the data re- corded from August to November when the ambient temperature varied from 40 to 15°C are given in Table 11. Table I contains the data of controlled temperature (38 2 1°C).

Process stability as evidenced by lower volatile acidss consistently increased with increased levels of pectin (Table I). Average acid concentration ranged from 1248 mg/L in the digester with no pectin to 660 mg/L in the highest dosed digester. The effect of pectin on volatile acid concentration is found to be more prominent in the digester at ambient temperature, where volatile acid concentration is reduced to half. It indicates that volatile acids are consumed at a faster rate than that in the controlled experiments where no pectin is used. In studying the effect of retention time on the fermen-

Biotechnology and Bioengineering, Vol. XXVIII, Pp. 624-626 (1986) 0 1986 John Wiley 8t Sons, Inc. CCC 0006-3592/86~040624-03$04.00

G A S PROOUCTION - -

0 0 1000 3000 5000 7000 9000 11000 13000 15000

P E C T I N DOSE ( m g / l )

Figure 1. various doses of pectin in digester maintained at 38 c 1°C.

Average daily gas production and methane content at

tation of sludge at 35°C,9910 it is documented that pro- tein and carbohydrate fermenting bacteria grow rap- idly, and the substrates are rapidly degraded to fatty acids even at retention times of less than one day. However, the fermentation of fatty acids does not oc- cur until the retention time is extended to 5 days or

more due to the slow growth of the fatty acid fer- menting bacteria.

As indicated above, and from studies on growth rates of isolated fatty-acid-degrading bacteria,'+'2 the rate limiting step in methane fermentation often involves the degradation of fatty acids, which is related to the efficiency of H2 utilization by methanogenic bacteria. From the present study it appears that pectin did en- hance the methane forming step of the digestion process.

In both experiments pH is found to be more or less constant, that is, near the neutral region.

Process performance can also be judged by bio- chemical oxygen demand (BOD) and chemical oxygen demand (COD) values, which indicate extent of bio- deg rada t i~n . ' , ' ~ , ' ~ Pectin in both experiments gave low values of BOD and COD, indicating greater biodegra- dation, as shown in Tables I and 11. Even the addition of crude pectin (rind along with some inner part of Citrus media fruits; drying of rind was done at 65"C, and it was then powdered) resulted in increased meth- ane production and greater process stability.

In the present study it was found that pectin is not used much in the anaerobic digestion process; rather, its quantity remains constant, which is slightly less than the loaded amount. Therefore, it seems that pectin performs a function similar to that of powdered acti- vated carbon. Pectin may be providing adsorption sites where substrate can accumulate, thereby providing high localized substrate concentration. These areas of ad- sorption provide a more favorable growth environment for bacteria-substrate systems.

Another interesting observation was made in the sec- ond set of experiments, where digesters were operated at ambient temperature, the ambient temperature vary-

Table I. Summary of effluent data during steady-state periods of digester maintained at 38 % 1°C.

Total gas Volatile Dose production BOD COD acids

(mg/L) I/day/digester CH,% (mg/L) (mg/L) (mg/L) PH

Control 5.1

Commercial pectin 100 (obtained from BDH) 500

1000 2000 4000 6000 8000

10,000

5.1 6.4 7.2 8.5

10.1 12.5 12.7 12.8

58

59.0 60.5 62.0 63.5 64.5 65.0 65.0 65.0

10,333 25,600 1248 6.5

10,333 9666 7833 6166 4333 3333 3000 3000

24,000 21,200 17,600 15,200 14,000 13,000 12,200 11,800

1200 1056 744 696 696 672 660 660

6.5 6.7 6.75 6.7 6.6 6.65 6.7 6.7

Crude pectin (prepared 1000 5.9 58 10,333 25,000 from the rind of 2000 6.7 60 10,000 24,400 Citrus media fruits) 5000 8.8 61.5 8000 24,000

8000 10.2 62.5 6333 21,200 10,000 11.0 63 .O 5Ooo 19,400 12,000 11.3 63.0 4333 17,000 15,000 11.5 63 .O 4000 16,600

~~-

1248 I200 1116 1056 960 744 720

~

6.1 6.75 6.7 6.4 6.6 6.5 6.5

~ ~~~

COMMUNICATIONS TO THE EDITOR 625

Table 11. August to November, when the ambient temperature varied from 40 to 15°C.)

Summary of effluent data during steady-state periods of digester at ambient temperature. (The average of the data recorded from

Total gas Volatile Dose production BOD COD acids

(mg/L) 1 /da y/digester CH,% (mg/L) (mg/L) (mg/L) PH

Control 3.2 53 13,000 31,000 2640

Commercial pectin 100 (obtained from BDH) 500

1000 2000 4000 6Ooo 8000

10,Ooo 15,000

3.2 3.4 4.3 5.0 5.6 5.8 6.2 6.4 6.5

53 55 56.5 58.0 60.5 61 .O 62.0 62.5 62.0

12,666 11,666 11,000 10.000

8000 6333 5666 5000 5333

30,000 28,800 26,400 24,200 22,400 20,000 19,200 18,800 18,600

2592 2400 2208 2016 1608 1584 1500 1308 1296

6.5

6.5 6.4 6.45 6.55 6.7 6.75 6.6 6.65 6.7

-

ing from 40 to 15°C. As shown in Table 11, addition of pectin shows considerable increase in total gas pro- duction with high methane content. This clearly indi- cates that pectin also serves the function of shock ab- sorbent, so that gas production is not significantly affected by variation in temperature.

Thus, pectin could be used to increase the poten- tiality of existing digesters.

References

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