2.9　Relationship Between Different Biogas Microbes

There exist multiple kinds of species and a spectrum of complex microbes in biogas fermentation. Methane production is the result of such mutual actions and restrictions between various microbes of this microorganism's spectrum.

The mutual actions of biogas fermenting microbes include the actions between non-methane-producing bacteria and methane-producing bacteria, as well as the actions between non-methane-producing bacteria, among which the former is of the most importance.

In a biogas fermentation system, non-methane-producing bacteria and methane-producing bacteria depend upon each other and serve material basis, optimum conditions for the needs of their life span, but at the same time they also restrict each other.

This mode of relationship between them exhibits mainly the following features.

①Non-methane-producing bacteria provide substances required for the growth of methane-producing bacteria whilst the latter one gets rid of the feedback inhibition from the former.

Non-methane-bacteria make use of various complex organic materials to perform anaerobic fermentation, forming H2,CO2,NH3,CH3COOH, HCOOH, CH3-CH2COOH, CH3CH2CH2COOH, CH3OH, C2H5OH and so forth. CH3CH2COOH, CH3CH2CH2COOH, C2H5OH can be converted into H2, CO2, CH3COOH respectiuely by hydrogen-producing acetogenic bacteria microbes. Thus, non-methane-bacteria through their life processes provide materials and energy for continuous synthesis of cellular components,and precursors and energy needed by methanobacteria.On the other hand, the products yielded by non-methane-bacteria can inhibit the process of fermentation: the accumulation of acids inhibits acid-producing bacteria to yield acid subsequently and accumulation of hydrogen inhibits hydrogen production, as the products—acids, H2, and CO2are successively used by methanobacteria, the products enables non-methanobacteria to grow and metabolize normally. Owing to these mutual actions between them, a balance of acidproducing and CH4-forming mechanism is established in the process, which makes it proceeding normally.

②In early stages of biogas fermentation, some air can get into the biogas digester and accompanies the feeding of materials and water, which is certainly harmful to methane-producing bacteria. The oxygen removal must rely on the active degradation of non-methane-producing bacteria, such as aerobes or facultative anaerobes. The various anaerobic microorganisms can cause a continuous fall of the oxidation-reduction potentials in the media of fermentation,and provide an anaerobic condition for the growth of methanogen by their sequential adaptability, alternative growth, and metabolic activity.

③Toxic substances released by non-methane-producing bacteria for methane-producing bacteria.

Using waste water and wastes from industry as resources for fermentation, there usually exist toxic substances harmful to methane-producing bacteria, such as phenols, benzene, cyanides, long-chain fatty acids and heavy metals. Fortunately, those toxic substances can be released and utilized by many of non-methaneproducing bacteria, which favourites the process of fermentation. In addition, the toxic effects of heavy metals can be abolished by forming precipitates of metalsulfuric compounds (heavy metal ions +H2S released by non-methane-producing bacteria), thereby some toxic effects of heavy metals are avoided.

④Optimum pH is maintained by their mutual actions.

In the early stage of biogas fermentation, starch and sugar are first degraded by non-methane-producing bacteria and large amount of organic acids are produced. At the same time CO2yielded is dissolved in water, which cause a drop of pH of the fermenting media. Ammoniation, however, occurs in the system forming NH3, which neutralizes part of the acids, and furthermore because of CH3COOH, H2and CO2being successively used by methane-producing bacteria and formed NH3, pH is stabilized in a favourable range. Thus, in a normal setting-up biogas digester from the beginning to the end, the pH required is regulated automatically instead of artificially adjusted.