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biogas HANDBOOK narrow pH interval, from about 5,5 to 8,5 , with an optimum interval between 7,0-8,0 for most methanogens. Acidogenic microorganisms usually have lower value of optimum pH. The optimum pH interval for mesophilic digestion is between 6,5 and 8,0 and the process is severely inhibited if the pH-value decreases below 6,0 or rises above 8,3. The solubility of carbon dioxide in water decreases at increasing temperature. The pH-value in thermophilic digesters is therefore higher than in mesophilic ones, as dissolved carbon dioxide forms carbonic acid by reaction with water. The value of pH can be increased by ammonia, produced during degradation of proteins or by the presence of ammonia in the feed stream, while the accumulation of VFA decreases the pH-value. The value of pH in anaerobic reactors is mainly controlled by the bicarbonate buffer system. Therefore, the pH value inside digesters depends on the partial pressure of CO and on the 2 concentration of alkaline and acid components in the liquid phase. If accumulation of base or acid occurs, the buffer capacity counteracts these changes in pH, up to a certain level. When the buffer capacity of the system is exceeded, drastic changes in pH-values occur, completely inhibiting the AD process. For this reason, the pH-value is not recommended as a stand-alone process monitoring parameter. The buffer capacity of the AD substrate can vary. Experience from Denmark shows that the buffer capacity of cattle manure varies with the season, possibly influenced by the composition of the cattle feed. The pH-value of domestic animal manure is therefore a variable which is difficult to use for identification of process imbalance, as it changes very little and very slowly. It is, however, important to note that the pH-value can be a quick, relatively reliable and cheap way of registering system imbalance in more weakly buffered systems, such as AD of various wastewater types. 3.3.3 Volatilefattyacids(VFA) The stability of the AD process is reflected by the concentration of intermediate products like the VFA. The VFA are intermediate compounds (acetate, propionate, butyrate, lactate), produced during acidogenesis, with a carbon chain of up to six atoms. In most cases, AD process instability will lead to accumulation of VFA inside the digester, which can lead furthermore to a drop of pH-value. However, the accumulation of VFA will not always be expressed by a drop of pH value, due to the buffer capacity of the digester, through the biomass types contained in it. Animal manure e.g. has a surplus of alkalinity, which means that the VFA accumulation should exceed a certain level, before this can be detected due to significant decrease of pH value. At such point, the VFA concentration in the digester would be so high, that the AD process will be already severely inhibited. Practical experience shows that two different digesters can behave totally different in respect to the same VFA concentration, so that one and the same concentration of VFA can be optimal for one digester, but inhibitory for the other one. One of the possible explanations can be the fact that the composition of microorganism populations varies from digester to digester. For this reason, and like in the case of pH, the VFA concentration can not be recommended as a stand-alone process monitoring parameter. 26PDF Image | biogas HANDBOOK
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