Dry digestion of crop wastes: Studies on dry anaerobic digestion with agricultural wastes

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  • Biological Wastes 20 (1987) 291-302

    Dry Digestion of Crop Wastes: Studies on Dry Anaerobic Digestion with Agricultural Wastes

    Guo-chao Sun, Yi-Zhe Wu, Shi-jin Sha & Ke-xin Liu

    Chengdu Institute of Biology, Chinese Academy of Sciences,

    PO Box 416, Chengdu, Sichuan, China

    (Received 3 October 1986; accepted 23 October 1986)

    A BS TRA C T

    This paper reports the requirements for batch dry-fermentation with agricultural wastes. The optimum initial solids content is 25-30%, with ratios of rice straw to manure (on a dry matter basis) of 2 or 1:1. The inoculum should be over 30% by volume. In the course of the fermentation, when biogas production rate is falling, two additions of inoculum and fresh pig manure lengthen the duration of a biogas production rate of above 0"3 m 3 m - 3 day- 1, to enhance the total biogas volume and to promote the degradation and utilization of the raw material

    This process of dry-fermentation with agricultural wastes gives relatively high total biogas yield, biogas production rate, and degradation rate of raw materials and needs a simple routine management. NH4HCO 3 can substitute for manure to mix with straw, as well as the nitrogen source, to give similar results.

    INTRODUCTION

    Many studies on batch dry-fermentation have been carried out (e.g. Cheremisinoff, 1976; Jewell et al., 1976; Jewell et al., 1978; Jewell, 1980; Jewell et al., 1980; Wujcik & Jewell, 1980; Jewell, 1981; Jewell, 1982). In China, the Sandong Energy Institute, the Fujian Institute of Microbiology and others have also carried out investigations. Our studies, since 1980, have been aimed at solving some important problems, such as acid accumulation, short duration of high biogas production, etc.

    291 Biological Wastes 0269-7483/87/$03.50 Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

  • 292 Guo-chao Sun, Yi-Zhe Wu, Shi-jin Sha, Ke-x& Liu

    METHODS

    Bench-scale experiments on batch dry-fermentation at ambient temperature were carried out.

    The inoculum was sludge from a digester at Chengdu sewage-treatment station incubated with an equal part of pig manure for 2 months. The TS content was 4.1-4.7% with VS, 60.54-64.54% of TS.

    Materials were crop stalks, pig manure and NH4HCO 3. Before loading, the stalks were pretreated. The rice straw and corn stover were weighed and cut into 2-3 cm diameter pieces. They were then put into a glass fermentor followed by the addition of a certain amount ofbiogas sludge, and incubated for 46 h at 35C before mixing for the main fermentation. Different ratios of the materials and different percentages of inoculum were used, but the C: N ratios for all experiments were within the range of 25-34:1.

    1. To determine the effects of different solids concentrations on biogas production there were six treatments--8%, 15%, 20%, 25%, 30% and 35%. The ratio of straw or stover to pig manure was 2:1, on a dry matter basis. The inoculum was 20% by volume.

    2. For effects of different ratios of straw to pig manure on biogas production, there were three treatments--1:1, 2:1 and 1:0. The initial solids content was 25%. The inoculum was 20% (v/v).

    3. For effects of different inocula on the biogas production and degradation of raw materials, there were five treatments--0%, 10%, 20%, 30%, 40% and 100% (v/v). The ratio of straw to pig manure was 2:1 on a dry matter basis. The initial solids content was 25%.

    4. For the effects of addition, in the course of fermentation, of influents composed of biogas sludge and fresh pig manure, with TS at 5.27%; VS at 79.63%, there were six treatments:

    (A) Rice straw: pig manure was 2:1, with no addition. (B) Rice straw: pig manure was 2:1, with addition. (C) Rice straw and 0"3% NH4HCO3 by volume of the fermentor

    (Sun & Liu, 1981) with addition. (D) Corn stover: pig manure was 2:1, with no addition. (E) Corn stover: pig manure was 2:1, with addition. (F) Corn stover and 0.3% NH4HCO 3 by volume of the fermentor,

    with addition.

    Apparatus

    The 1-1itre fermentors had 600ml effective volume. The biogas produced was collected by water displacement. The various raw materials were mixed

  • Dry digestion of crop wastes 293

    thoroughly and placed in the fermentor and inoculum added. Then water was added to give the required initial solids content. The fermentation was carried out at ambient temperature, 26-28C.

    Analysis

    The parameters, such as TS, VS, C, N, cellulose, CH 4 content of the biogas and pH values, were determined by standard methods given in Analysis of Biogas Fermentation, Beijing Technology Science Press (1984).

    RESULTS

    Experiments on different initial solids contents

    The results from fermentations of 150 and 200 days are given in Tables 1 to 4. The following results may be pointed out.

    Within the range of initial solids content of 8-30%, the higher the TS, the greater the biogas yield. However, for 35% initial solids content, the biogas

    TABLE 1 Biogas Yields, Degradation of Raw Materials and Biogas Compositions for Six Treatments

    with Rice Straw and Pig Manure as Feedstock

    Treatment 8% 15% 20% 25% 30% 35% Duration (days) 70 103 119 132 146 146 Total biogas yield (ml) 9 968 21 655 25 059 37 142 37 142 5 227 Mean biogas yield rates

    (ml ml- 1 day- ~) 0"23 0"35 0"36 0'38 0-42 - - Days with biogas yield rate

    above 0"2mlml- 1 day 1 34 63 60 84 103 Mean rates of biogas yield

    above 0"2 mlml- 1 day- 1 0-33 0"46 0"53 0"52 0'54 Biogas yield from TS

    (ml per gram of TS) 458 496 504 489 510 364 Biogas yield from VS

    (ml per gram of VS) 509 564 571 564 595 471 Degradation of TS 45'52 49" 13 42"98 41" 18 40"43 6"83

    55"56 58"65 51-44 48-26 48"37 6"99 48"84 50'20 41"51 38"65 35-40 13"30 57"60 62"31 56'54 53"25 52"27 23"95 63"8 61"8 59-1 59 58'5 - - 30"9 34"4 38"4 39 39-5 - -

    7.1-7.3 5-5-5'6

    Biogas analyses

    pH values

    VS C Cellulose cn4 COz

    - , Gas production effectively zero, so no analyses.

  • 294 Guo-chao Sun, Yi-Zhe Wu, Shi-jin Sha, Ke-xin Liu

    TABLE 2 Biogas Yields and Degradation Rates of Raw Materials for Six Treatments of Corn Stover

    and Pig Manure as Feedstock

    Treatments 8 % Duration (days) 78 Total biogas yield (ml) 11 221 Mean biogas yield rates

    (ml ml- 1 day- 1) 0.24 Days with biogas yield rate

    above 0"2 mlml- i day- l 45 Mean rates of biogas yield

    rate above 0"2mlm1-1 day- l 0"32 Biogas yield from TS

    (ml per gram of TS) 462 Biogas yield from VS

    (ml per gram of VS) Degradation of TS

    Biogas analyses

    pH values

    VS C Cellulose ci4, C02

    15% 20% 25% 30% 35% 156 156 168 198 198 23423 27 368 38 748 50490 5987

    0.25 0.29 0.35 0.43

    69 93 117 135

    0-41 0.40 0.46 0"55

    475 477 558 625 343

    548 548 545 628 703 374 50'56 59.56 47.77 46-29 44.89 8.32 52.35 57.04 50.00 48.93 47.32 9'03 55.73 59.56 51 .23 48 .61 47.34 20.23 61.19 63.11 59.91 57.10 56.66 23.35 63-8 63.1 62.2 60"8 60.2 9.1 33.0 35.1 39.4 40.1 36'6 53.4

    7.1-7.3 5.5-5.6

    a The corn stover was stalk over 1-year old.

    TABLE 3 Distribution of Biogas Yields from Different Initial Solids Contents for Rice Straw and Pig

    Manure as Feedstock

    TS contents Percentage of total biogas volume within Total biogas (%) (ml)

    First Second Third Fourth Fifth month month month month month

    8 60 35 5 9 698 15 42 42 12 4 21 655 20 49 30 13 8 25859 25 46 29 14 9 2 29969 30 42 29 14 9 5 37142 35 100 5227

  • Dry digestion of crop wastes 295

    TABLE 4 The Distributions of Biogas Yield of Different Initial Solids Contents for Corn Stover and Pig

    Manure as Feedstock

    TS Percentage of total biogas volume within Total contents biogas

    (%) First Second Third Fourth Fifth Sixth Seventh (ml) month month month month month month month

    8 53 37 11 11221 15 33 34 14 9 9 1 23432 20 26 30 24 10 9 1 27368 25 25 27 22 12 8 6 1 38748 30 22 29 22 11 8 5 2 50490 35 100 5987

    was produced for the first 15 days and then it stopped. Except for 35% TS, the duration ofa biogas yield rate above 0.2 ml ml- x day- 1 increased in line with the increase in initial solids content.

    The degradation and utilization of raw materials for 15% initial solids content was the highest; otherwise, for initial solids contents within the range 8-30%, degradation decreased along with the increasing initial solids content. The biogas yield per gram of TS and VS tended to increase with increase in initial solids content. This result agrees with that obtained by Jewell et al. (1982).

    The percentage of the total biogas produced within the first 2 months decreased, while the duration of biogas production increased, with increase in initial solids content. The CH 4 content of the gas fell, while CO2 content rose, with the increase in the initial solids content.

    Towards the end of the fermentations the pHs of the five treatments were 7-1-7-3 (within the normal range for biogas fermentation). However, that of the treatment with 35% initial solids content was abnormal, being 5.5-5.6.

    Experiments on different ratios of straw to manure

    From fermentations of 121,132 and 146 days, respectively, the results were as follows.

    The total biogas yield and the biogas yield rate decreased with the increase in the ratio. However, the duration of a biogas yield rate above 0.2 ml ml- 1 day-1 for the ratio 2:1 was longer than that for the ratio 1:1 (Table 5).

    The degradation and utilization of raw materials decreased with increase in the ratio, but the difference in the degradations between ratios 1 : 1 and 2:1 was slight. The biogas yield per unit of solids for the 2:1 ratio was slightly higher than that for the other two (Table 5). The percentage of the total

  • 296 Guo-chao Sun, Yi-Zhe Wu, Shi-jin Sha, Ke-xin Liu

    TABLE 5 Biogas Yields, Degradation of Raw Materials and pH Values for Three Ratios of Rice Straw

    to Pig Manure

    Rice Rice Rice straw straw straw to pig to pig only

    manure = manure = 1:1 2:1

    Duration of fermentation (days) Total biogas yield (ml) Mean biogas yield rates

    (ml ml- 1 day- 1) Days with biogas yield rate above

    0.2 ml- 1 day- 1 Mean rates (ml ml- 1 day- 1) of biogas yields

    above 0-2 ml ml- 1 day- 1 Biogas yield from TS

    (ml per gram TS) Biogas yield from VS

    (ml per gram VS) TS

    Degradation of raw VS materials (%) C

    Cellulose pH

    121 132 146 30300 29968 19758

    0"42 0'38 0'23

    61 84 67

    0"69 0"52 0"27

    464 489 427

    533 564 470 43-88 41'18 31'06 52"57 48'26 36'89 41'12 38"65 31"84 59'24 33"25 48"95 7'3 7"2 7'1

    Fig. 1.

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  • Dry digestion of crop wastes 297

    TABLE 6 Distribution of Biogas Yields and pH Values for Three Ratios of Rice Straw to Pig Manure

    Ratio of Percentage of total biogas volume within Total rice straw biogas

    to pig First Second Third Fourth Fifth (ml) manure month month month month month

    1 : 1 50 33 9 7 0.2 30 300 2:1 46 29 14 9 2 29 968 Pure straw 23 33 20 14 9 19 759

    biogas produced in the first 2 months increased with the decrease in the ratio (Table 6).

    Addition of more of the easily degradable pig manure increased the methane content of the gas (Fig. 1) and combustible biogas was produced earlier in the digestion than with straw alone. The pH values (Table 5) increased together with the increments of pig manure.

    Experiments on different amounts of inoculum

    From fermentations of more than 120 days, the results can be summarized as below.

    TABLE 7 Biogas Yields, Degradation Rates of Raw Materials and pH Values for Six Different

    Amounts of Inocula

    Amount of inoculum 0% 10% 20% 30% 40% 100% Duration of

    fermentation (days) 124 124 132 112 121 121 Total biogas yield (ml) 188 24 642 29 963 31 102 32 362 32 668 Mean biogas yield rate

    (mlml 1day l) 0-38 0"38 0"46 0"45 0"45 Days with biogas yield

    rate over 0"2 ml ml- l day- 1 55 84 92 92 92 Mean of biogas yield rate

    over 0-2 ml ml- 1 day- 1 0.61 0"52 0-53 0.54 0.54 Biogas yield from TS

    (ml per gram of TS) 415 489 436 451 460 Biogas yield from VS

    (ml per gram of VS) 500 564 503 528 526 Degradation of TS 3-05 39-56 41" 18 47"55 47"77 47"32

    VS 6"20 44.36 48"26 55"92 55'59 47.32 C 7"11 33"81 38'65 40"48 41"24 40"59 Cellulose 14"05 48"94 53'25 58"39 57"41 57"53

    pH values 5"5 7'2 7"3 7-3 7"3 7"3

  • 298 Guo-chao Sun, Yi-Zhe Wu, Shi-jin Sha, Ke-xin Liu

    On the whole, the biogas yield and duration of biogas yield rate above 0.2 ml ml- 1 day- 1 increased with increase in amount ofinoculum (Tables 7 and 8). With no inoculum, only 188 ml of CO2 was produced within the first 4 days of fermentation, then gas production stopped. With 10% inoculum, gas, chiefly CO2, was produced in the first 12 days, then it stopped until the 41st day, and after this rose gradually. The interval was, presumably, a period of enrichment of the microbes concerned in biogas fermentation.

    Degradation and utilization of the raw materials was promoted with increase of inoculum (Table 7). For inocula above 30%, the results were similar; the material biogas yield rose with the increase in inoculum. The temporal distributions of biogas yield for different amounts of inoculum above 20% were similar (Table 8). The start-up of methane fermentation was quicker with increased inoculum (Fig. 2).

    The pH values tended to increase with increasing amount of inoculum. The pH value with no inoculum was 5.5. However, the pH values for inoculated digests were 7.2-7.2, within the normal range for fermentation (Table 7).

    Experiments on the effects of adding portions of inoculum and fresh pig manure in the course of fermentation

    In the hope of lengthening the duration of fermentation and promoting the degradation and utilization of the raw materials, we added portions of inocula and fresh pig manure to the digester twice during the course of the fermentations, when the peak biogas yield rate was decreasing. The results were as follows.

    The total biogas yield was increased and the duration of the higher yield rate was lengthened considerably (Table 9). The biogas yields per gram of TS

    TABLE 8 Distribution of Biogas Yields for Six Different Amounts of Inoculum

    Amount of

    inoculum (%)

    Percentage of total biogas yield

    First Second Third Fourth Fifth month month month month month

    Total biogas (ml)

    0 100 10 4 16 63 16 1 20 46 29 14 9 2 30 48 31 14 6 40 47 30 14 9 0"2 50 47 30 14 9 0"2

    188 24642 29968 31108 32362 32668

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  • 300 Guo-chao Sun, Yi-Zhe Wu, Shi-jin Sha, Ke-xin Liu

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    and VS for the digesters with additions (B, E) were higher than those without additions (A, D) (Table 9). The temporal distributions of biogas production (Table 10) for the digesters with additions (B, E) were relatively even, while, without additions, gas production was chiefly concentrated within the first 2 months. Methane content (Table 9) of the biogas was similar in all experiments, at about 60%. The pH values of the digesters, both with and without additions, were similar, at 7.2-7-4 (Table 9).

    Experiments on NH4HCO 3 substituting for pig manure

    When 3 g of NH4HCO 3 per litre of digester was used to substitute for pig manure to mix with rice straw or corn stover, similar results were obtained (C and F in Tables 9 and 10).

    TABLE 10 Distribution of Biogas Yield in Per cent of Total for Adding and Not Adding Inoculum and

    Manure

    First Second Third Fourth Fifth Sixth Seventh Total month month month month month month month biogas

    (ml)

    A 4"8 37 14 65 368 B 2"3 18 24 19 15 1 135 134 C 21 19 16 21 15 8 145700 D 30 32 27 1 76 797 E 19 20 16 16 14 13 2 124 518 F 18 20 14 15 14 14 5 129417

  • Dry digestion of crop wastes 301

    DISCUSSION

    Dry fermentation has been said to have some drawbacks, such as a slow start, acid accumulation and short duration of maximum biogas production rate. In the present experiments it was shown that combustible biogas can be produced within one or two days, if suitable measures are adopted. These are 30-50% (dry matter) of pig manure in the feedstock, incubation for a short time after mixing and a large enough amount of inoculum (above 30%). These measures can supply not only sufficient easily degradable materials which can be converted to combustible biogas at the very beginning of fermentation, but also sufficient bacteria, especially methanogens, which can convert volatile acids into CH4 and CO 2, to avoid acid accumulation. The pig manure and inoculum have high nitrogen contents and non-ammonia N is converted in the course of fermentation to NH 4, which can modify the pH value as well as act as a nitrogen source for bacteria and prevent acid accumulation. The duration of high biogas production can be extended and the total amount of biogas produced can be increased by having 50-70% (dry matter) of rice straw in the initial feedstock.

    So, the optimum conditions for dry digestion of rice straw or corn stover and pig manure can be summarized as follows.

    (1) The initial solids content should be 25%-30%. (2) The ratio of rice straw or corn stover to pig manure (by dry matter)

    should be 2:1 or 1:1. (3) The amount of inoculum should be above 30% (v/v). (4) Additions of suitable portions of inoculum and fresh pig manure

    during the course of fermentation are key factors to boost total biogas yield, extend the duration of higher biogas yield rate and enhance the degradation and utilization rates of materials.

    (5) 0.3% NH4HCO 3 can substitute for manure to mix with rice straw or corn stover as the nitrogen source.

    ACKNOWLEDGEMENT

    Program costs for the studies were provided by the Energy Committee of the Chinese Academy of Sciences. In addition to the authors, the research team had three other members, Lian Li-wen, Guo Xue-min and Zhou Shao-ying, who performed laboratory analysis.

    REFERENCES

    Cheremisinoff, P. M. & Morresi, A. C. (1976). Energy from solid wastes. Marcel Dekker, New York.

  • 302 Guo-chao Sun, Yi-Zhe Wu, Shi-jin Sha, Ke-xin Liu

    Jewell, W. J. (1980). Future trends in digester design, In: Anaerobic Digestion (Stafford, D. A., Wheatley, B. I. & Hughes, D. (Eds)). Applied Science Publishers, London, pp. 467-92.

    Jewell, W. J. et al. (1976). Bioconversion of agricultural wastes for pollution control and energy conservation. Final Report No. TID-27164, US Energy Research & Development Administration, National Technical Information Service (NTIS), Springfield, VA, 321 pp.

    Jewell, W. J. et al. (1978). Anaerobic fermentation of agricultural residue: potential for improvement and implementation. US Department of Energy Final Report,, Vol. I, HCP/T2981-07, NTIS, Springfield, VA, 427 pp.

    Jewell, W. J. et al. (1980). Anaerobic fermentation of agricultural residue: potential for improvement and implementation. US Department of Energy Final Report, Vol. 2, Project No. DEAC02-76ET20051, NTIS, Springfield, VA, 599 pp.

    Jewell, W. J. et al. (1981). Crop residue conversion to biogas by dry fermentation. Paper No. 813573, American Society of Agricultural Engineers, St. Joseph, MI, 19 pp.

    Jewell, W. J. (1982). Dry fermentation of agricultural residues. SERI Annual Report No. XB-09038-1-7, NTIS, Springfield, VA, 533 pp.

    Sun, Gao-chao, Liu, Kezin et aL (1981). An approach to increase in biogas yield of a rural biogas digester. Acta Energeae Solaris Sinica, 2, 247-51.

    Wuj cik, W. J. & Jewell, W. J. (1980). Dry anaerobic fermentation. Biotechnology and Bioengineering Symposium, No. 10, Gatlingburg, TN. US Department of Energy, Oak Ridge, TN, pp. 93-107.