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
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.
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
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) ).
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
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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)
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