effects of particle size on anaerobic digestion of tomato solid wastes

Agricultural Wastes 10 (1984) 285-295

Effects of Particle Size on Anaerobic Digestion of Tomato Solid Wastes

David J. Hills & Kouichi Nakano*

Agricultural Engineering Department, University of California, Davis, California 95616, USA

ABSTRACT

Laboratory digesters with operating volumes of 4 litres were used to assess the effect of particle size on methane gas production. Substrates consisted of tomato solid waste chopped to particle sizes of 1.3, 2.4, 3.2, 12.7 and 20 mm. These were Jed at 3 g Volatile Solids per litre of digester per day to mesophilic digesters with retention times of 18 days. Greatest gas production and Volatile Solids reduction occurred with the most finely chopped substrate, 1.3mm. This substrate produced 0"81 volume of methane per volume of digester per day (vol/vol/day) with a Volatile Solids reduction of 60.3 %. In contrast, the 20 mm substrate produced only 0.25vol/vol/day of methane and Volatile Solids reduction was 21.1%. The rate of methane gas production appears to be inversely linear to the product oJ the substrate's average particle diameter and its geometric description, sphericity.

I N T R O D U C T I O N

Vast quantities of tomato solid waste (peels, stems, seeds) are produced in California. Annually, approximately 450 000 tonnes are disposed of in sanitary landfills or spread on arable fields. In 1978 laboratory studies were conducted by Hills & Dykstra (1980) to assess the feasibility of converting this residue into methane gas via conventional mesophilic digestion. Their results were not encouraging; conversion rates were relatively low--approximately 0.15 litre of gas per gram of VS loaded. In a later study, Hills & Roberts (1982) endeavored to improve gas

* Present address: Daiki Gomu Engineering Co. Ltd, Kashima, Chiba, Japan. 285

Agricultural Wastes 0141-4607/84/$03-00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

286 David J. Hills, Kouichi Nakano

production through reduction in solids particle size by utilizing a pump with cutter blades for feeding tomato solid waste slurry into a 22 m 3 pilot- scale digester. The chopper pump had little effect on the tomato peel size and gas production rates were only slightly greater than in the previous study. In an attempt to quantify the benefits of particle size reduction for enhancement of gas production, a laboratory study was conducted in the summers of 1981 and 1982 using 4-1itre mesophilic digesters and tomato solid waste chopped to various particle sizes.

For solid wastes high in lignin content mechanical fine grinding has been shown to be beneficial in improving gas production (Cowling, 1963; Nelson et al., 1939). Both these studies support the hypothesis that the ligneous structure within an organic complex tends to shield the cellulosic materials from enzymatic hydrolysis. Lignin content for tomato solid wastes, however, is low compared with dairy, beef and horse manures-- 3.7 % versus approximately 16 % (Hills & Roberts, 1981). The benefit of particle size reduction for tomato solids is likely to result from a more rapid hydrolysis step as a result of increased surface area being available to extracellular enzyme activity. This hypothesis can be expressed in mathematical terms.

A standard kinetic expression hydrolysis is:

derived for steady-state enzymatic

k'ae v - (1)

K + e

where v is the product formation per unit time, k' is the rate constant of solid substrate hydrolysis, K is the half velocity coefficient equal to the enzyme concentration when the rate of conversion = k'a/2, a is the total number of enzyme adsorption sites per unit volume and e is the concentration of enzyme in the reaction mixture.

Reviews of the literature (Kirsch & Sykes, 1971; Kotze et al., 1969) indicate that enzymatic hydrolysis of solids appears to be the rate limiting step in methane generation from most agricultural residues. Eastman & Ferguson (1981) have, in fact, shown that solubilization of particulate organic carbon from domestic sewage plant sludge is the rate limiting step during the acid phase of anaerobic digestion. Equation (1) can therefore be expressed as:

dG k'ae dt K + e (2)

where dG/dt is the methane production per unit time.

Particle size and tomato waste digestion 287

For digesters with an established bacterial population, hydrolytic enzymes are usually in excess (DeWalle et al., 1978). An assumption can then be made that e ~> K and eqn. (2) is reduced to a zero order reaction:

dG = k 'a (3)

dt

The number of adsorption sites, a, is probably related to the available surface area per unit mass. The surface area per unit mass for uniform particle sizes can be expressed as:

A 6 m - ~sppDp (4)

where A is the total surface area, m is the total mass of the sample, pp is the density of the particles, Dv is the average particle diameter and Os is the sphericity of the particles. According to McCabe & Smith (1976), ~s varies between 1-0 for spheres, cubes and short cylinders and 0.28 for mica flakes. Assuming little change occurs in particle density through a chopping process, an approximation for the surface area per unit mass, A,., is:

f 1

Equations (5) and (3) can be combined:

dG 1 - k ( 6 )

dt ~sDp

where k is the overall proportionality constant and can be derived by plotting d G/d t versus 1/O~Dp. Equation (6) states that the rate of methane gas production is inversely proportional to the substrate average particle diameter when the sphericity of the particles is taken into account.

EXPERIMENTAL PROCEDURES

Six laboratory digesters having operating volumes of 41itres were fabricated from lucite plastic and were described earlier by Hills & Dykstra (1980). Mixing was performed by manually shaking the digesters for 1 min each day. All digesters were housed in a temperature controlled room adjusted to 35_+ I°C. The gas produced was collected in counterweight collectors filled with saline solution.


During start up the digesters were filled with effluent from a pilot plant dairy manure digester plus additional seed (5 700 dry weight basis) from a municipal digester. Over a 20-day period they were fed mixtures of dairy manure and tomato solids. The percentage of the dairy manure in the feed mixture was decreased over this period until the feed mixture was entirely tomato solids. Tomato solid waste was obtained from a cannery utilizing a 10 mesh (1.65-mm openings) vibrating screen for separating the solids from the liquid effluent. These solids were prepared for digester feed by chopping in a 'Rietz Disintegrator'. Screens on the disintegration discharge allowed for different particle sized feedstocks. Four different screens with the following opening sizes were used: 1.3, 2.4, 3.2 and 12.7mm. During the tomato season of 1981 two digesters were each operated with feedstocks from the 2.4, 3.2 and 12.7 mm screens. During the 1982 season four digesters were operated; two used feedstock from the 1-3 mm screen and two were fed unprocessed tomato solids as it came off the cannery's screen (referred to as 20 mm sized feedstock as measurement showed an average diameter of flat surfaces of 20 mm). After preparation, all feedstocks were bagged and kept frozen ( - 10 °C) until needed.

The digesters were operated under similar conditions during both seasons. They were each fed on alternate days, retention time was maintained at 18 days, and the loading rate was held constant at 3 g VS per litre of digester per day. The digesters were operated for 120 days each season.

Gas production rates were monitored daily for all digesters and the gases were analyzed periodically for methane content. All gas production values were corrected to 0 °C and one atmosphere pressure. The following parameters were assayed in the feeds and effluents: Total Solids, Volatile Solids and Chemical Oxygen Demand. Additionally, the feeds were analyzed for total phosphorus, potassium, and Kjeldahl-nitrogen and the effluents were assayed for ammonia nitrogen, pH, Volatile Acids and alkalinity. All analytical testing was performed according to the procedures in Standard Methods jor the Examination o f Water and Wastewater (Anon., 1980). Ammonia nitrogen was determined with an Orion Ammonia Electrode Model 95-10. Alkalinity was obtained by titration to pH 3.7 and Volatile Acids were determined by the Chromatographic Separation Method 504A. Total phosphorus was determined by using the perchloric acid method and potassium quantified by the flame photometric method. Gas samples were analyzed on a Hewlett-Packard Model 5730A gas chromatograph.


RESULTS AND DISCUSSION

A summary of the chemical characteristics of the tomato solid waste is presented in Table 1. Total Solids from the cannery's vibrating screen averaged about 12 ~ for the two seasons. Following the chopping process and just prior to digester feeding, the Total Solids content was lowered to 5.8 ~ by water dilution. The nitrogen content was quite high compared

T A B L E 1 Chemical Characterist ics of T o m a t o Solid Wastes Prior to Size

Reduct ion and Dilut ion for Digest ion Trials

Summer, 1981 Summer, 1982

Total Solids (~o) 11-9 12.3 Volatile Solids a 93.2 92.9 Chemical Oxygen Demand a 132.3 140.3 C O D / V S b 1-42 1.52 Kjeldahl-ni t rogen ~ 3.85 4.02 Tota l -phosphorus" 0.51 0.57 Potass ium a 0.49 0.50

a Uni ts in per cent of Total Solids (TS). b C O D / V S = Chemical Oxygen Demand/Vola t i l e Solids.

with other agricultural residues. The COD/N/P ratio of 300/8.6/1.2 for both seasons indicated that adequate nitrogen and phosphorus existed for good anaerobic digestion. Van der Berg & Lentz (1977) indicate that, for good methane production from food processing waste, a COD/N/P ratio of 300/5/1 is desirable.

The substrates used in this study are defined by the screen sizes which they each passed. For example, the tomato solids passing the screen with 3.2 mm openings were defined as having an average particle size, Dp, of 3.2 mm. Some particles passing this screen were most probably smaller or even dissolved, whereas others may have been larger, having passed through longitudinally. The estimation of the sphericity of these particles is listed in Table 2. A sphericity of 1.0 was assigned to the smallest sized particles which approached spherical or cubical configuration. At the other extreme, the unprocessed tomato peels resembled flat plates so a sphericity of 0"3 was assumed. Despite the fact that the substrates were not composed of definable uniform particles, the relative values of the


TABLE 2 Summary of Digester Feed and Operating Characteristics

Substrate particle size, Dp (mm) 1.3 2.4 3"2 12.7 20 ~

Loading rate b 3.0 3.0 3.0 3.0 3.0 Retention time (days) 18 18 18 18 18 Total Solids (~o) 5.82 5.80 5.80 5.80 5.82 Sphericity, • s 1.0 0.9 0-8 0.4 0.3

Tomato solid waste as obtained from cannery (i.e. no particle size reduction). b Loading rate in grams of VS per litre of digester per day.

average part icle d iamete r t imes the assumed spherici ty provide a good physical charac te r iza t ion of each substrate .

Anae rob ic digest ion o f the t o m a t o solid waste appeared very stable. Chemical character is t ics within the digesters are indicated by the effluent da t a of Tab le 3. The listed values are the mean values dur ing the last 60 days of the invest igat ion and are the average values for the two repl icated digesters for each substrate . The a m m o n i a concen t r a t i on was

greatest for the finest part icle sized feedstock. A l t h o u g h Kjeldahl- n i t rogen was no t mon i to r ed , possibly the greater gr inding exposed more o f the organic n i t rogen to eventual hydrolysis into am m o n ia . Th e a m m o n i a values listed in the Tab le at the respective p H values indicate tha t a m m o n i a gas toxici ty was no t a p roblem. The alkal ini ty and Volati le

TABLE 3 Summary of Mean Chemical Data of Digester Effluents During Steady-State

Periods

Substrate particle size, Dp (ram) 13 2.4 3"2 12.7 20

Total Solids (~o) 3.0 3.5 4.6 5.0 5.2 Volatile Solids (~o TS) 70-5 75.6 76.7 80.2 81.9 COD (g litre-1) 29.8 37.6 49.8 56.7 59.2 Ammonia-N a 820 680 680 540 520 Alkalinity ~ 3 340 3 040 3 100 2 800 2 700 Volatile Acids a 270 240 380 320 340 pH 7.0 7.1 7.0 7.1 7.0

a Units in mg litre-1


TABLE 4 Summary of Performance for Digesters Operating with Different Particle Sized Feeds

Substrate particle size, Dp (mm) 1.3 2.4 3"2 12.7 20

VS reduction (~ ) 60.3 51.1 35.2 26-2 21.1 COD reduction (~o) 46.1 41.4 31.5 22-5 17.2 Gas production ~ 1.36 1-15 0.76 0-48 0.40 Methane content (~o) 59.9 58.2 60.1 62-0 62.3 Litres of gas per gram TS loaded 0.45 0-38 0.25 0.16 0.13 Litres of gas per gram VS destroyed 0.75 0-75 0.72 0.61 0.63 Litres o fCH 4 per gram COD destroyed 0'39 0.38 0.34 0.31 0.32

Gas production corrected to 0°C and 1 atmosphere pressure--units in volume of gas per volume of digester per day.

Acid values were also within the range for stable digestion and no upsets occurred in any of the digesters.

A summary of digester performances is presented in Table4. Of significance are the relative reductions in Volatile Solids. The reduction was nearly three times greater for the 1.3 mm sized substrate than for unchopped 20 mm size substrate. The volume of gas produced for each gram of Volatile Solids destroyed ranged between 0.61 and 0.75 litres. Gas production rates are shown in Fig. 1. The data were corrected to 0 °C and one atmosphere pressure. Each point on the graph represents the mean daily values over a 1-week period and is the average of the replicated digesters. Daily fluctuations in gas production of up to 30 ~o were noted; however, the weekly mean values were quite stable during the investigation's last 60 days.

Gas production values in volume gas per digester volume per day (vol/vol/day) are listed in Table 4. They range between 0.40 for the unprocessed solids to 1.36 for the finely chopped, 1 "3 mm, material. This parameter is important since it represents monetary returns on capital investment. Generally, livestock manure digesters, operating under similar conditions, generate between 1 and 2 vol/vol/day. Of possibly more interest to a cannery is the quantity of gas produced per quantity of Total Solids loaded. The benefits of fine chopping to 1-3 mm compared with the use of unprocessed material are a threefold increase in gas production and a 50~o reduction in Volatile Solids remaining in the effluent for ultimate disposal.


>..

r ~

t X W

._l

Z 0 I--

a

0

0 .

J

I-- 0 I - -

F i g . 1 .

I I I I I I I I I I I

6 - - 1.3mm

~ ~ 12.7 mm 2 . . ~ . . ~ -

I

I I I I I I I I I I I I0 20 30 40 50 60 70 80 90 I00 I10

T IME , DAYS Gas production corrected to 0 °C and one atmosphere pressure. Points represent

weekly mean values for each particle sized substrate indicated.

The methane content in the gas decreased slightly with finer choppings, 62.3% to 59.9%. Methane gas productions for each particle sized substrate are plotted in Fig. 2. The methane increase with particle decrease was greater than first order. Particle size has received some attention from researchers interested in converting municipal solid waste into methane gas at sanitary landfills. DeWalle et al. (1978) observed that, by decreasing the particle size in garbage by a factor of 10, the rate of gas production increased by a factor of 4"4. This was an approximate linear relationship in the size range tested (250 mm-25 mm). The researchers felt that a shredding process for particle reddction greatly increases the available surface area of the substrate accessible for enzymatic attack. The shredding process, in contrast to grinding or pulverizing, causes a tearing of fibers at edges which may enhance capillary action and subsequent enzymatic attack. Moell (1978) found that the gas production rate was from 1.5 to 4 times higher for shredded refuse than for non- shredded refuse and that methane production in a pilot-scale landfill entered a steady-state rate within 6 to 18 months for shredded refuse but was still in an unsteady low rate after 24 months for the non-shredded refuse.

Figure 3 is a plot of methane gas production relative to the parameter


Fig. 2.

),.

r~

.._I 0 >

"1- ¢,..)

..J 0 >

Z o I-.- (,..)

C:J

0 n,- r,

u') <I

LLI Z

"r" I,--

W ~E

I l I l I

1.0

O.

0.6

0.4

0.2

I I I 1 I 0 4 8 12 IB 20

PARTICLE SIZE,Dp (ram)

24

Methane gas production, corrected to 0 ° C and 1 atmosphere pressure, as a function of particle sized substrate.

1/~fl)p. A straight line is drawn by using linear regression. The equation for this line is:

dG 1 - 0 . 9 6 O,---~ + 0.12 (7)

dt

with a good correlation coefficient of 0-97. The major difference between eqns (7) and (6) is the dG/dt intercept. Equation (6) predicts the line to pass through the origin. If the data point for the 1 "3 mm particle sized feed were dismissed, the straight line would intercept the dG/dt axis, close to the origin, at 0.04. Although the values selected for the sphericities, (I) s, and particle diameter, Dp, are estimates and are subject to conjecture, it does appear that, for tomato solid waste, methane gas production varies inversely with the product of particle diameter and sphericity.

The implementation of this technology at canneries depends on the economics of the system, taking into account that tomato solid waste is available only for about 4 months of the year. The results of this study


t~ _J o >

-'r"

Fig. 3.

v

r=

t9

.= -r I--

=E

1.2

1.0

0.8

0.6

0.4

0.2

1 1 dG 0.96

- - = - - + 0 . 1 2 d ! ~s Dp

r= 0 .97

I I

/

0 I I I I 0 0.2 0.4 0.6 0.8 1.0

I (mrn - I )

~s Dp Methane gas production data plotted according to eqn. (6).

could probably be applied to digesters operating on other food- processing substrates with chemical characteristics similar to those of tomato solid wastes.

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

This investigation was supported by the California Experiment Station.

A g r i c u l t u r a l

REFERENCES

Anon. (1980). Standard m e t h o d s / o r the examinat ion o / water and waste water (15th edn), Amer. Publ. Health Assn., Washington, DC.

Cowling, E. B. (1963). Structural features of cellulose that influence its


susceptibility to enzymatic hydrolysis. In: Advances in enzymatic hydrolysis of cellulose and related materials, Pergamon Press, London.

DeWalle, F. B., Chian, E. S. K. & Hammerberg, E. (1978). Gas production from solid waste in landfills. J. Environmental Engineering Division, Amer. Soc. Civil Engr., 104, 415-32.

Eastman, J. A. & Ferguson, J. F. (1981). Solubilization of particulate organic carbon during the acid phase of anaerobic digestion. J. Water Pollution Control Federation, 53, 352-66.

Hills, D. J. & Dykstra, R. S. (1980). Anaerobic digestion of cannery tomato solid wastes. J. Environmental Engineering Division, Amer. Soc. Civil Engr., 106, 257-66.

Hills, D. J. & Roberts, D. W. (1981). Anaerobic digestion of dairy manure and field crop residues. Agricultural Wastes, 3, 179-89.

Hills, D. J. & Roberts, D. W. (1982). Conversion of tomato, peach and honeydew solid waste into methane gas. Trans. Amer. Soc. Agr. Engr., 25, 820-6.

Kirsch, E. F. & Sykes, R. M. (1971). Anaerobic digestion in biological waste treatment. In: Progress in industrial microbiology, J.&A. Churchill, London, 156-237.

Kotze, J. P., Thiel, P. G. & Hattingh, W. H. J. (1969). The characterization and control of anaerobic digestion. Water Research, 3, 459-94.

McCabe, W. L. & Smith, J. C. (1976). Unit operations of chemical engineering (3rd edn), McGraw-Hill, New York.

Moell, C. E. (1978). Leach rate and gas production in municipal solid waste test cells. Paper presented at International Association of Hydrologists Seminar, Edmonton, Canada.

Nelson, G. H., Straka, R. P. & Levine, M. (1939). Effect of temperature of digestion, chemical composition and size of particles on production of fuel gas from farm wastes. J. Agr. Res., 58, 273-87.

Toerien, D. F. & Hattingh, W. H. J. (1969). The microbiology of anaerobic digestion. Water Research, 3, 385-416.

Van der Berg, L. & Lentz, C. P. (1977). Methane production during treatment of food plant wastes by anaerobic digestion. In: Food, jertilizer and agricultural residues (Loehr, R. C. (Ed.)), Ann Arbor Science, Ann Arbor.

effects of particle size on anaerobic digestion of tomato solid wastes

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