Anaerobic digestion of wastes

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  • Conservation & Recycling, Vol.S,No.l, pp.5-14, 1982 0361-3658/82/OlOOOS-10$03.00/O Printed in Great Britain Pergamon Press Ltd.

    ANAEROBIC DIGESTION OF WASTES

    L. van den BERG Division of Biolog-lcaZ Sciences

    National Research CounciZ of Canada, Ottawa, Ontario, CAN. KZA OR6

    ABSTRACT

    The paper discusses briefly the principles of the anaerobic digestion process as well as of the advanced, more efficient technologies develop- ed in the last decades. These advanced technologies include the anaero- bic filter, anaerobic fluidized or expanded bed reactor, upflow anaero- bic sludge bed reactor and anaerobic fixed film reactor. Some of these appear suitable for small scale operation in rural areas.

    1. INTRODUCTION

    Accumulation of organic material is an unavoidable and occasionally unpleasant by- product of human activity, and methanogenesis by anaerobic digestion has long been used for waste treatment, not least because the methane produced is easy to use and a relatively clean source of energy. In North America, where natural gas was abundant and used to be cheap, the emphasis has been on the waste treatment aspects of the process but in Europe and Asia the methane produced was a significant and valuable by-product.

    As an industrial process, anaerobic digestion came into its own as a result of the industrial revolution when centralized treatment of sewage became a necessity. Aerobic treatment of sewage resulted in the production of a fairly concentrated sludge (1 - 5% organic solids) which had a pollution load equivalent to 213 to 314 of the original sewage. This sludge, when left to itself, without exposure to air, would turn into a sour, foul and unhealthy liquid which in some instances would start to produce methane. Anaerobic digesters were therefore designed to accelerate and control the process, to eliminate, as far as possible, the odoriferous interme- diate stage.

    These digesters, as originally developed, are sealed tanks to eliminate oxygen and to trap the methane produced. The process was controlled by the rate of addi- tion of raw sludge. By adding l/50 to l/100 of the digester volume each day it was found that methane production was maintained and that the liquid removed from the digester was quite innocuous - it smelled relatively innocent, looked black, con- tained considerably less pathogensthan the raw sludge and was a good fertilizer. Further improvements in control were obtained by maintaining the temperature at

    5

  • 6 L. van den BERG

    about 35'C and providing as much mixing as was possible from a technical and econo- mic point of view. Many such units are in operation now in Asiatic and African countries for manure treatment.

    Microbiology and biochemistr~~ of the process It has been well established that the bacteria producing methane are capable

    only of the terminal steps of the reactions involved in the breakdown of complex organics:

    43 + HC03- + H+ + CH4 + 3H20

    CH 3 COO+ + H 2 0 -f CH 4 + HC03-

    Other small molecules (0 and CH30H, for example) can also for methane production, but these normally play only a minor gesters.

    serve as substrates role in anaerobic di-

    The relative importance of these two overall reactions can be appreciated from the breakdown of glucose:

    C6H1206 + 4H20 -f 2CH3COO- + 4H2 + 2HC03- + 4H+

    It is generally accepted that at least two thirds of the methane in anaerobic di- gesters comes from acetic acid. In fact, some recent evidence suggests that even more methane may be formed from acetic acid, because hydrogen and carbon dioxide may be converted first to acetic acid and then to methane.

    We know relatively little about the methanogens, particularly those that are cap- able of converting acetic acid. Of the types of bacteria that convert hydrogen and carbon dioxide to methane, at least ten have been isolated in pure culture and un- doubtedly many more will be found. Only one or two strains of bacteria that convert acetic acid to methane have been isolated in pure culture and at least two others are being studied in enriched cultures, one in laboratories at the National Research Council of Canada. It is only recently that techniques have been developed for their isolation in pure culture. Undoubtedly, many more examples will be isolated of this type of methanogen.

    Methanogens appear to have characteristics that put them in a group all by them- selves. First, their protein and nucleic acid structures are similar within the group, but distinct from most other groups of bacteria. In addition, methanogens contain unusual coenzymes such as coenzyme M, the cofactor involved in methyl trans- fer and the smallest of all known coenzymes. The composition of the cell wall and the structure of the membranes of methanogens are also quite different from those in other bacteria.

    There are several reasons why so little is known about the methanogens that con- vert acetic acid to methane. They grow extremely slowly, making it difficult to obtain them in pure culture. The acetic acid-converting bacteria play a minor role in the rumen because the animal competes efficiently for the acetic acid, hence, rumen microbiologists have paid little attention to acetate-converting methanogens.

    The slow growth of methanogens converting acetic acid is undoubtedly related to the low yield of energy for the bacteria (less than 47 kJ per mole of methane pro- duced). The slow growth is responsible for many of the practical problems of anae- robic digesters, yet the low yield of energy for the bacteria makes the process very efficient in recoverable energy; in anaerobic digesters generally only 5 - 10% of the available energy is required to maintain the microbial population. Also, because

  • ANAEROBIC DIGESTION OF WASTES 7

    of the low energy requirement of the methanogens, digesters can be left without substrate for long periods of time (months) and yet lose only a small fraction of their activity. An indication of the rate at which methanogens grow in digesters is presented in Fig. 1. The maximum obtainable growth rate in wastes is plotted against the COD/Protein N of the waste. The COD/Protein N ratio is the complexity of the waste (van den Berg and Lentz (10, 11) ).

    indication of

    I I , ,

    0 - WASTE WITHOUT YEAST EXTRAC

    0 ----WASTE WITH YEAST EXTRACT 0

    0.02 0.04 0.06 0.08

    Fig. 1. Effective growth rate, day -1.

    It is now becoming apparent that hydrogen-converting methanogens, while not the major methane producers, p lay a very important role in digester performance. They are capable of converting hydrogen to methane at hydrogen concentrations below 1 nano mole/ml (0.002 mgll). At this low hydrogen concentration many biochemical con- versions involving the production of hydrogen become thermodynamically advantageous while at higher hydrogen contents these reactions would require an input of energy. Examples of such reactions are the conversion of propionic, butyric and other fatty acids to acetic acid, the anaerobic breakdown of cellulose to mainly acetic acid and hydrogen and, it has been postulated, the conversion of acetic acid to cell mass by acetate-converting methanogens.

    The reactions involved in converting biopolymers, such as cellulose, lignin, hemi- celluloses, and fats to hydrogen and acetic acid are generally also poorly understood and it is likely that many hitherto unknown bacteria are awaiting discovery and study, For example, investigators at the National Research Council, who are studying anae- robic degradation of cellulose to hydrogen, acids and methane, have discovered new species of cellulolytic bacteria. These bacteria produce cellulases having proper- ties that are of potential commercial interest.

    One of the major stumblingblocks in converting wood products to methane is the inability of the anaerobic microorganisms to degrade lignin and lignin-encrusted

  • 8 L. van den BERG

    cellulose, and little progress has been made in this area.

    In spite of the complexity of the biochemical pathways, it has been found that there is a simple relation between the composition of organics and the amount of methane and carbon dioxide that can be produced:

    CnHaOb + [n - ; - $1 H20 + [; - ; f $1 co2 + [; +; - 21 CH4

    Another relation may be obtained based on the composition of the organics expres- sed in terms of the oxygen required to oxidize fully the organic material to carbon dioxide and water. This amount of oxygen is called the chemical oxygen demand (COD). The COD of methane is 2 moles oxygen per mole methane or 2.857 g/l. In other words 0.35 1 of methane is produced for every g COD that is removed from the waste during anaerobic digestion. Because of certain limitations the amount of methane produced is in practice close to 0.33 1 per g COD removed.

    Practical advantages and limitations of the anaerobic digestion process The anaerobic digestion process as a waste treatment process is attractive for

    several reasons. when comnared with aerobic treatment processes: 1)

    2)

    3)

    4)

    it can easily handle wastes to concentrated levels that aerobicprocesses cannot handle them at all, or only with great difficulties; only a small amount of the organic material is converted into biomass, limit- ing the problem of further disposal; there is no need for energy and equipment for transferring oxygen into the waste; moreover, the end product (methane) separates automatically because of its low solubility; methane is often a valuable source of energy, both for operating the anaero- bic digestion process and for other operations such as producing electricity and for heating and cooling.

    There are, however , practical limitations on the digestion process which are caused by the processes and equipment used, the properties of many of the bacteria involved, and by our lack of understanding of the bacteria.

    1) Wash-out of bacteria has to be prevented. Since it takes 10 - 30 days, or even longer, to double the number of some essential bacteria, the volume of feed that can be added daily to a digester is often only l/20 - l/30 of the digester volume. Even if the wastes contain 2% digestible organics, the loading rate is only 0.7 to 2.0 kg organics per m 3 digester volume per day with a methane production rate of 0.25 - 0.65 m3 digester volume per day.

    2) Since the rate of growth of some of the essential bacteria drops off rapidly at temperatures below 35OC, digesters have to be maintained at a relatively high temperature. During severetinter conditions this means that much of the methane produced may be required for heating. In addition, heat exchangers for anaerobic digesters tend to foul and are difficult to maintain.

    3) Mixing of digester contents is necessary to attain maximum performance. It serves several purposes:

    a) b)

    c)

    _ _ to mix bacteria intimately with their substrate; to prevent short circuiting (some of the substrate may otherwise leave with little or no treatment); and to maintain a uniform temperature. In large tanks (several thousand cubic meters) mixing is difficult and requires a lot of energy. In practice, most digesters are inadequately mixed: recent studies showed that in many municipal digesters only 10 - 25% of the volume is effectively used.

  • ANAEROBIC DIGESTION OF WASTES 9

    4) As already mentioned some organics such as cellulose and lipids are only slowly broken down. As a result these may pass through the digester without being utilized and converted to methane.

    Maximwn quantities of methune obtainable The organic material in biomass can generally be grouped into three classes,

    each with its own characteristic methane yield:

    Carbohydrates: 0.42 - 0.47 m3 methane/kg (6.5 - 7.5 ft3/lb)

    Protein: 0.45 - 0.55 m3 methane/kg (7 - 9 ft3/lb)

    Fats: up to 1 m3 methane/kg (15 ft3/lb)

    In actual waste not all of the carbohydrate, protein, and fats are available for digestion; some are indigestible in the time available for digestion. Data on the volume of methane that can at present be produced from human and animal wastes aregiven in Table 1.

    Table 1. Methane production per head from human and animal wastes.

    Organic material produced, kg/day

    Methane produced, m3fday

    Conversion %

    Human-sewage 0.1 - 0.2 0.02 - 0.03 25 - 40 Human-garbage 1.5 - 3.0 0.3 - 0.7 40 - 60 Chicken 0.01 - 0.02 0.004 - 0.008 50 - 70 Hog 0.3 - 0.5 0.09 - 0.14 40 - 60 Beef Steer 3.0 - 4.5 0.3 - 0.6 30 - 50 Dairy Cow 3.5 - 5.5 0.4 - 0.7 20 - 40 Horse 4.5 - 9.0 ? ?

    Principle of advanced technologies High rates of conversion of waste organic materials into methane have mostly

    been achieved by getting around the problem of slow growth of essential microorga- nisms, particularly those converting acetic acid to methane. Until now attempts to increase the growth rate of these bacteria have essentially been unsuccessful except by changing the digestion temperature into the thermophilic range (50 - 65'C) from the mesophilic range (350C optimum). By preventing bacteria from escapingin the effluent, the digestion process becomes eventually independent of growth rate. This way it is possible to reach high concentrations of bacteria and hence high rates of reaction in spite of very slow growth rates. This is the principle on which advanced technologies are based.

    It is interesting to contemplate the maximum amount of methane that could be produced per unit volume of fermenter if all of the bacteria could be retained in the digester. To do this, it is necessary to introduce the concept on specific activity of the bacterial mass, i.e. the volume of methane produced per kg bacterial dry mass per day. In anaerobic digesters a high value would be 0.35 m3fkgfday. A

  • IO L. van den BERG

    suspension of bacteria containing 10% dry solids would be the most concentrated that can adequately be mixed. Hence, the maximum possible "practical" rate of me- thane production has to be about 35m3im3.day. In fact the limit will be lower be- cause of the presence of other suspended solids, and foaming or flotation that oc- curs at such high rates of gas production. Nevertheless, this calculation shows the high rates of reaction thatarepossible with improved design and operation of di- gesters.

    Advanced technozogies avai lab Ze at present These are:

    1. The anaerobic contact process (Fig. 2) T'nis process was developed during the fifties and its performance depends on: a) degree of mixing of digester contents; b) extent to which the bacteria settle out in the sedimentation tank for return

    to the digester.

    The anaerobic contact process was applied industrially to some extent, and was studied in detail at the National Research Council in Ottawa. It will work with hydraulic retention times as short as 1 - 2 days (i.e. addition of 0.5 to 1 m3 of waste p m3 digester per day) and methane production rates of up to 1.5 - 3 m3/m3/day.

    DIGESTEF

    MIXER

    - DIGESTED LIQUID

    -4 r 3

    - DIGESTER LIQUID

    Fig. 2. Sketch of anaerobic contact process.

    Problems and limitations of the process are caused by difficulties of control: a) adequate mixing of large digesters is difficult; b) settling of microorganisms from the digester liquid is often unpredictable

    and slow; C) the process tends to be unstable at high methane production rates, causing

    long-term set backs.

    The anaerobic contact process requires a complicated set-up and close supervision.

  • ANAEROBIC DIGESTION OF WASTES II

    Even so, its advantages over conventional digesters in terms of greatly increased gas production and hence reduced capital investment justifies more widespread app- lication than it has had.

    t FEED

    2. Anaerobic filter The anaerobic filter was also developed in the fifties, especially for relatively

    dilute and soluble wastes. In this system, the digesters are filled with rocks or plastic shapes, to provide a multitude of random channels and a large surface area. Waste water is introduced at the bottom and allowed to overflow at the top. The bacteria are present in clumps in the channels near the bottom of the filter as well as attached to the surface provided.

    The filter is being used commercially to some extent. Its use is limited in that it is usable only for waste waters with soluble organics or with organics that are readily converted into soluble components; otherwise the channels will plug. Also growth eventually plugs many of the channels, necessitating regeneration of the filter. These drawbacks have limited the commercial application of the anaerobic filter. Rates of methane production up to 5 m3/m3/day have been reported.

    3. The anaerobic upflow sludge blanket reactor This reactor (Fig. 3) was developed in the Netherlands in the seventies after

    encountering the limitations of the anaerobic filter. The essential feature of this reactor is the presence of a very active sludge blanket in the bottom of the reac- tor. Other important features are means of removal of gas without interference with the settling of microorganisms and their return to the sludge blanket. The waste is introduced at the bottom of the reactor into the sludge bed where most of it is con- verted into methane and carbon dioxide. The gas formed causes sufficient agitation to keep the sludge bed particles moving around and the volume of the bed fully mixed. Some particles will be lifted up above the sludge blanket, but, as they lose the en- trapped gas, they settle back.

    - SEDIMENTATION AREA

    -DIGESTED LIPUID

    Fig. 3. Sketch of upflow anaerobic sludge blanket reactor.

  • 12 L. van den BERG

    The properties of the particles in the sludge blanket are of critical importance in the process - they are composed mostly of bacteria, but usually contain 10 - 20% inorganics to give them a reasonably high density so they will settle quickly and stay in the digester. Since the properties of the particles will depend on the waste, the process is likely to be waste dependent. Dilute soluble wastes are the preferred substrates so far. The process is being studied at the National Research Council with excellent results (up to 10 m3lm3fday).

    4. The fluidized bed process This process was also developed in the seventies and is somewhat similar to the

    Dutch process. However, the bacteria are grown on particles such as sand to give them a high settling velocity. By pumping liquid through at a high rate, the par- ticles remain suspended and high rates of mass transfer are obtained. So far the process is still in the laboratory stage, and the amount of information available on the process is limited. Methane production rates of up to 3 m3/m3/day are be- ing mentioned.

    5. Anaerobic fixed film reactor This reactor is being developed by the National Research Council in Ottawa. In

    these reactors the bacteria are grown on surfaces and relatively little suspended growth occurs or is retained. The latter is achieved by removing the effluent from the bottom rather than from the top as is the case in most anaerobic digesters (Fig.4).

    FEED--: :

    GAS

    J SUPPORT MATERIAL FOR BACTERIAL FILM DEVELOPMENT ARRANGED IN VERTICAL CHANNELS

    DIGESTED LlWl D

    Fig. 4. Sketch of anaerobic fixed film reactor.

    Since in these reactors the film area is fixed, the performance of these reactors depends on two factors: how much active bacterial mass is present per unit surface, and how much surface ispresent per volume. The product of these two factors gives the amount of microbial mass per unit volume and this in turn determines the volume- tric rate of methane production. This reactor or digester is capable of producing

  • ANAEROBIC DIGESTION OF WASTES

    3 to over 5 m3 methane/m3/day.

    This fixed film reactor with effluent removal from the bottom has been found to produce methane from wastes with high suspended solids contents. The wastes tested are a simulated sewage sludge which contains over 3% organic suspended solids, and pig manure. Some of this material is indigestible and would accumulate in many types of digesters and impair their efficiency. Another finding has been that the reactor changes readily from one waste to another with relatively little loss of capacity. The latter is important for installations where the character of the waste water changes rapidly due to the season or production schedules.

    A major advantage of the fixed film reactor over other advanced designs is its apparent simplicity. No mixing is required because the combination of vertical channels and gas production causes intense mixing. It is possible to remove the treated waste infrequently rather than continusouly, and infrequent addition of waste appears also feasible.

    A comparison of performance between a completely stirred tank reactor without biomass recycle, an anaerobic process reactor and a fixed film reactor, using two types of waste, is presented in Table 2. The fixed film reactors could handle the largest loading rate without appreciable loss of efficiency in removing COD.

    Table 2. Performance comparisons between different types of anaerobic digestersa'

    Fixed film Anaerobic Stirred reactor reactor contact with biomass

    process recycle reactor

    Bean blanching waste:

    Maximum loading rate, 16 7 1.2 kg/COD/m3/day

    COD-removal efficiency, % 86 88 70

    Simulated sewage sludge:

    Maximum loading rate, kg COD/m3/day

    12 10 6.5

    COD-removal efficiency, % 70 - 85 75 - 80 65 - 70

    aFor further details see papers by L. van den Berg, C.D. Lentz and D.W. Armstrong, (9, 10).

    2. CONCLUSION

    There has been a tremendous increase in interest in anaerobic methane production from waste. This interest has been translated into increased research and this in turn is leading to vast improvements in the process. Theoretical considerations indicate that further improvements are possible and in fact, within reach. At the

  • 14 I A. van den BERG

    present stage of development the emphasis is shifting towards more application of the results already obtained in the laboratory.. There should therefore, be exciting practical developments and better understanding in the field of methane production from waste in the next decade.

    REFERENCES

    1, Biogas Production from Animal Manure. Biomass Energy Institute Inc. P.O. Box 129, Postal Station "C", Winnipeg, Manitoba, Canada. R3M 3S7.

    2. W. J. Jewel. Future trends in digester design. Proceeding 1st Intermationai! Symposiwn on Anaerobic Digestion. 467-491 (1980).

    3. A.W. Khan. Anaerobic degradation of cellulose by mixed cultures. Can. J. M&rob. 23: 1700-1705". (1977).

    4. G. Lettinga, A.F.M. van Velzen, W. deZeeuw and S.W. Hobma. The application of anaerobic digestion to industrial pollution treatment. Proc. 1st International Symposiwn on Anaerobic Digestion. 167-186 (1980).

    5. P.L. McCarty. Anaerobic waste treatment fundamentals. Public Works, September, 107-112, October, 123-126, November 91-94 (l964).

    6. C.B. Patel, A.W. Khan, B.J. Agnew and J.R. Colvin. Isolation and characteriza- tion of an anaerobic cellulolytic micro organism, Acetovibrio celluloZyticus gen. nov., sp. nov. Int. J. Syst. Bacterial. 30. 179-185 (1981).

    7. L. van den Berg. Effect of temperature on growth and activity of a methanogenic culture utilizing acetate. Can. J. MicrobioZ. 23: 898-902*. (1977).

    8. L. van den Berg and C.P. Lentz. Methane production during treatment of food plant wastes by anaerobic digestion. Proc. 1977 Cornell Agriculture Waste Management Conference, 381-393*. (1977).

    9. L. van den Berg and C.P. Lentz. Effects of film area-to-volume ratio, film support, height and direction of flow on performance of methanogenic fixed film reactors. Proc. of the U.S. Dept. of Envir. Workshop Seminar on Anaerobic Filters. l-10 (1981).

    10. L. van den Berg and C.P. Lentz. Performance and stability of the anaerobic con- tact process as affected by waste composition, inoculation and solidsletention time. Proc. 35th Purdue Industrial Waste Conference. 1980.

    11. L. van den Berg and C.P. Lentz. Effect of waste, inoculum and solids retention time on methane production and stability of the anaerobic contact processin@dvance in Biotechnology,Moo-YoungandKobinson(Eds), PergamonPress,VollI, 257-262(1981).

    12. L. van den Berg, C.P. Lentz and D.W. Armstrong. Anaerobic waste treatment effi- ciency comparisons between fixed film reactors, contact digesters and fully mix- ed, continuously fed digesters. Proc. 35th Purdue Industrial Waste Conference. 1980.

    13. L. van den Berg, C.P. Lentz and D.W. Armstrong. Methane production rates of anaerobic fixed film fermenters as compared to those of anaerobic contact and fully mixedcontinuous fermenters, in Advances in Biotechnology, Moo-Young and Robinson (Eds), Pergamon Press, Vol. II, 251-256 (1981).

    14. C.D. Vogels. The global cycle of methane. Antony van Leeuwenhoek, 45: 347- 352. (1979).

    15. R.S. Wolfe. Methanogens: a surprising microbial group.Antony van Leeuwenhoek, 45: 353-364. (1979).

    16. J.C. Young and P.L. McCarty. The anaerobic filter for waste treatment. Proc. 22nd Purdue Ind. Waste Conf., 559-574. (1967).

    * Reprints are or will be available from the author.