Ch04

The appearance of flickering lights emerging from below the surface of swamps was noted by Plinius (van Brakel 1980) and Van Helmont recorded the emanation of an inflammable gas from decaying organic matter in the 17th Century. Volta is generally recognized as putting methane digestion on a scientific footing. He concluded as early as 1776 that the amount of gas that evolves is a function of the amount of decaying vegetation in the sediments from which the gas emerges, and that in certain proportions, the gas obtained forms an explosive mixture with air.

In 1804 - 1810 Dalton, Henry and Davy established the chemical composition of methane, confirmed that coal gas was very similar to Volta's marsh gas and showed that methane was produced from decomposing cattle manure. France is credited with having made one of the first significant contributions towards the anaerobic treatment of the solids suspended in waste water. In 1884 Gayon, a student of Pasteur, fermented manure at 35°C, obtaining 100 liters of methane per m of manure. It was concluded that fermentation could be a source of gas for heating and lighting. It was not until towards the-end of the 19th Century that methanogenesis was found to be connected to microbial activity. In 1868, Bechamp named the "organism" responsible for methane production from ethanol. This organism was apparently a mixed population, since Bechamp was able to show that, depending on the substrate, different fermentation products were formed. In 1876, Herter reported that acetate in sewage sludge was converted stoichiometrically to equal amounts of methane and carbon dioxide (Zehnder 1978, 1982).

As early as 1896, gas from sewage was used for lighting streets in Exeter, England, while gas from human wastes in the Matinga Leper Asylum in Bombay, India, was used to provide lighting in 1897. Then, in 1904, Travis put into operation a new, two-stage process, in which the suspended material was separated from the wastewater, and allowed to pass into a separate "hydrolyzing" chamber. In 1906, Sohngen was able to enrich two distinct acetate utilizing bacteria, and he found that formate and hydrogen, plus carbon dioxide, could act as precursors for methane.

On the applied side, Buswell began studies of anaerobic digestion in the late 1920s and explained such issues as the fate of nitrogen in anaerobic digestion, the stoichiometry of reaction, the production of energy from farm wastes and the use of the process for industrial wastes (Buswell and Heave 1930; Buswell and Hatfield 1936).

Barker's studies contributed significantly to our knowledge of methane bacteria, and his enriched cultures enabled him to perform basic biochemical studies (Barker 1956). Schnellen (1947) isolate two methane bacteria: Methanosarcina barker) and Methanobacterium formicicum which are still studied.

Heating digestion tanks made practical use of the methane produced by the anaerobic process. It is of interest to note that methane gas was collected in Germany in 1914-1923 and used to generate power for biological treatment of plants, as well as for the cooling water from the motors being used to heat the digestion tanks.

Numerous additional studies led to a better understanding of the importance of seeding and pH control in the operation of anaerobic digestion systems. Much of this work is still relevant today, and those who are developing biogas as an energy source would gain much from review of this earlier work.

Present interest in anaerobic digestion

There is an increased recognition, in both developing and industrial countries, of the need for technical and economical efficiency in the allocation and exploitation of resources. Systems for the recovery and utilization of household and community wastes are gaining a more prominent place in the world community. During the last years, anaerobic fermentation has developed from a comparatively simple technique of biomass conversion, with the main purpose of energy production, into a multi-functional system:

a) treatment of organic wastes and wastewaters in a broad range of organic loads and substrate concentrations;
b) energy production and utilization;
c) improvement of sanitation; reduction of odors;
d) production of high quality fertilizer.

R & D has shifted from basic studies on anaerobic fermentation of quasi-homogeneous substrates, with contents of organic solids in the range of about 5 - 10%, to the digestion of more complex materials that need modified digester designs. The main fields of R & D activities are:

a) fermentation at high organic loadings;
b) high rate digestion of diluted waste waters of agro-industries including substrate separation during fermentation; immobilization of the microorganisms;
c) fermentation and re-use of specific materials in integrative farming systems;
d) biogas purification;
e) simple but effective digested design/construction of standardized fermenters;
f) domestic waste water treatment.

Anaerobic digestion with high organic load can be performed when the concentration of methanogenic bacteria is kept at a high level. Pilot experiments, with mixtures of slaughterhouse waste water and cattle manure, succeeded in reducing retention times from about 20 to 8 - 10 days, by a specific mixing technique, which allowed the mixture from time to time to separate. By this method, the liquid phase is enriched with dissolved organic matter, which is brought into contact with solid material, containing a relatively high concentrations of active bacteria.

Dissolved organic compounds can normally be degraded much faster than solid materials in suspension. If the retention times for dissolved and suspended components can be adjusted separately, the overall process can be performed at higher rates. Similar techniques are under investigation or implementation into large scale application in most of the countries which perform biogas R & D activities and biogas promotion programs. For example, in the Netherlands intense work is done to reduce the concentration of organic matter in the digested materials, and to reduce the volume of liquid effluents of agricultural activities. In that country, with its intensive animal production, problems of soil and groundwater pollution become more and more severe - a situation similar to other countries with intensive agricultural production.

A two-step system is being developed and tested for agricultural solid wastes (greenhouse waste, organic fractions of municipal refuse, cannery waste, grass clippings). The first step is a batch type hydrolytic/acidic unit, in which percolation water is circulated. The percolation water is anaerobically treated in the second step, and recycled to the percolation unit. The retention time of the waste in the first step depends on the digestibility of the raw material, and can take several months.

Another system is being tested for treatment of the organic fraction of municipal refuse. After mixing with recirculated water and subsequent maceration, the waste is pumped into the first step reactor. Here the conditions (temperature 37C, hydraulic retention time 12 - 24 h) are such that a very efficient microbial population develops, that degrades cellulose. This population, in which ciliated play an important role, resembles the population in the paunch of ruminants. After passing the first step reactor, the mixture is mechanically drained, the liquid fraction is anaerobically treated (e.g. in an UASB-type reactor) and recirculated to the mixture tank. The solid fraction is partly forced back into the first step reactor, the remainder being discharged. To cope with water shortage and water pollution in the medium/long-term, a 6 year R & D project for water re-use and energy recovery by biogas production has been implemented in Japan since 1985, under the sponsorship of the Ministry of International Trade and Industry ("Aqua Renaissance 90"). The object of this project is to establish the technology to ensure a low cost treatment of industrial waste water, sewage etc. to enable re-use of treated water by utilizing a big-reactor - working at high concentrations - coupled with membrane-separation techniques, and which allows the efficient production of methane and other useful resources as well.

The main areas of this project are:

a) Microorganisms and big-reactors (fixed beds, fluidized beds, two phase binary tank, UASB-processes);
b) Membrane techniques: materials (organic polymers and ceramics) and modules;
c) Control and sensor system: a direct measurement of activity of microorganisms to control and optimize methane production;
d) Total water treatment system: a technology to integrate all the above methods.

By studying the structure of the hierarchy that promotes biogas digestion system in some of the Developing Countries, which is the key to the efficient and wide distribution of biogas plants in those countries, officials can profit for their own country. This review summarizes the latest developments in anaerobic digestion applicable to Developing Countries, as reported in English language publications up to the year 1990, and the lessons from newly developed systems can be applied in other countries. Sharing the new ideas and their economic benefits, especially for the uses of digested slurry, can be beneficial to most Developing Countries.

Although the problems of stratospheric ozone depletion, (the Greenhouse Effect) and climatic changes, resulting from deforestation and wrong treatment of the environment, have not yet reached the same level of public recognition as toxic waste treatment, more and more people are becoming aware of and concerned about them. These problems are dramatic new reminders that we live on a valuable planet, and we have to think and act in consort and deal in a global, integrated, way with all our organic wastes as well as the woods and the forests.

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